Effect of Induction-Heat Post-Curing on Residual Stresses in Fast-Curing Carbon Fibre Reinforced Composites

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Effect of Induction-Heat Post-Curing on Residual Stresses in Fast-Curing Carbon

Fibre Reinforced Composites

Mercedes Amelia Bettelli

Materials Engineering, master's level (120 credits) 2020

Luleå University of Technology

Department of Engineering Sciences and Mathematics

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Effect of Induction-Heat Post-Curing on Residual Stresses in Fast-Curing Carbon Fibre Reinforced

Composites

MASTER THESIS

by Mercedes Amelia Bettelli

Division of Materials Science

Department of Engineering Sciences and Mathematics Luleå University of Technology

Luleå, Sweden SE 97187

Supervisor;

Andrejs Pupurs Co- Supervisor;

Stephanie Nunes

Luleå, May 2020

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Preface

This project was realised as part of the master's in Advanced Materials Science and Engineering (AMASE) at Luleå University of Technology in collaboration with INHARESMO project. The experimental work was performed within the department of engineering sciences and Mathematics at Luleå University in Sweden during the period from January to June 2020.

I thank God for allowing me to perform this master and grow professionally and individually. Thanks for makings my dreams come true.

I would like to thank my parents, brothers and my all family who are my model role and are always encouraging me for their supporting and loving, they want the best for me and I would love to help them from the distance.

First of all, I would like to express my sincere gratitude to my supervisor's Professor Andrejs Pupurs and Stephanie Nunes for their patience, useful comments, remarks, assistance, sharing the knowledge and experience during this project.

Zainab Al-Maqdasi, thanks for your time, knowledge and recommendations, without you this would not have been possible.

In the same way my thanks to Lars Frisk and Erik Nilsson for his great contribution with laboratory equipment. My thanks to Liva Pupure, Patrik Femberg, Roberts Joffe, Nawres Al-Ramahi and PhD students for all their collaborations and knowledge.

It is a pleasure to thank the AMASE program to give the opportunities to study and meet with an amazing atmosphere and wonderful people.

Many thanks go to my friends and colleagues who have supported and encouraged me during this project such as my three beloved friends. Ira, Flor and Nerea, without you three and our craziness from the distance, AMASE students for help me move very heavy equipment, Benedict for your great help with the electronic problems on my computer.

Finally, but not least I would like to thanks Johannes for your words of support, help in times of stress and enjoy Luleå as it should be and makes my days happier, without you this would not be possible, I love you!

Luleå, June 2020

Mercedes Bettelli

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ABSTRACT

Manufacturing induced shape distortions is a common problem for composite materials. Due to the non-isotropic nature of carbon fibre reinforced polymers (CFRP) unavoidable deformations occur during part production. During fabrication of polymer composites, the material obtains its final shape at elevated temperatures. The curing process involves a transition from the liquid state to the solid, glassy state, allowing bonding between fibres and matrix. As the material cools the mismatch in thermal expansion coefficients and cure shrinkage obtained during the matrix polymerization leads to residual stresses on the mechanical level within composite part. There is a great interest from the aircraft and automotive industries, to increase the ability to understand development of shape distortions and residual stresses during the cure, since these deformations often lead to dissatisfaction of tolerances and it is essential to predict the deformations beforehand in order to compensate time and cost. In this context, a study of residual stresses during the curing process of thermosetting resin composites is presented. A methodology is proposed for predicting the formation and development of manufacturing- induced residual stresses. The present project reports on a comprehensive experimental study on the dependency of different short curing cycles on the build-up of residual stresses in a carbon fibre/fast-curing epoxy system and evaluate of post-curing methods through induction heating and oven post-curing with unidirectional [904] and unsymmetrical [9020] laminates. It includes characterization in thermo-elastic properties and degree-of-cure of the material by Thermal bending test, thermal expansion test, mechanical tensile test and Differential Scanning Calorimetry (DSC) in non-post-cured and post-cured laminates. The results showed slight variation in the thermal properties and not effect in the mechanical properties at different cure and post-curing conditions. Analytical data by Laminate Analysis program validated the experimental thermo-elastic data with analytical simulations. In addition, it is shown improvements in the temperature distributions in the post-curing by induction heating with different experimental set-ups, however, oven post-curing showed a more systematic system, higher heat efficient a low cure temperature, with more consistent mechanisms of shape distortions and residual stresses compared to induction heating. These findings are relevant for the future development of prediction methods for process induced deformations of Fast Curing Epoxy Resins (FCER).

Keywords: Carbon Fibres Reinforced Polymer Composite, epoxy resins, cure behaviour, residual stress and shape distortions.

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List of figures

Figure 1.1: Increase in the composite’s percentage within the airplane structure Figure 1.2: Cross- Linked Epoxy Network

Figure 1.3: Specific Strength of reinforced fibres

Figure 1.4: Classification of Composites Materials by geometry of the reinforcement Figure 1.5: a) Unidirectional Laminates, b) Orthotropic Laminate and c) Quasi-isotropic

laminates

Figure 1.6: Vacuum assisted resin transfer moulding (VARTM)

Figure 1.7: a) Vacuum bag preparation for autoclave cure of thermoset matrix composite, b) Vacuum bag sequence and tool plate placed in an autoclave

Figure 1.8: Residual stresses in carbon /epoxy at 100°C Figure 1.9: Thermal Deformation of an unsymmetrical lay-up Figure 1.10: Distorted angle section

Figure 1.11: Warpage on flat panel

Figure 1.12: Volume change during the manufacturing

Figure 1.13: Distortion due to volume fraction gradient through the thickness Figure 1.14: Shape distortions during the post-cure process

Figure 1.15: Induction Heating Process

Figure 1.16: Induction Process Techniques for Composites compared to others heating approaches

Figure 2.1: Experimental Plan

Figure 2.2: Differential scanning calorimetry machine Figure 2.3: Laminate manufacturing by vacuum bag system Figure 2.4: Curing of the CFRP by hot- press

Figure 2.5: Hot-press´s heating rate

Figure 2.6: Thermal edges alignments for sample manufactured at 150 °C, 10 minutes condition. a) Post-cured zone seen from the holder, b) Selected zone in laminate c) temperature distribution seen from digital thermal camera

Figure 2.7: Thermal bending test experimental set-up Figure 2.8: Determination of curvature in cylindrical strips Figure 2.9: Thermal expansion test set-up

Figure 2.10: Mechanical tensile test set-up

Figure 2.11: Laminate Analysis Program. Calculations of thermal expansion coefficients using experimental data

Figure 2.12: Points selected in Post-cured samples by induction heating for the DSC evaluation

Figure 3.1: Thermogram: a) 130°C and b) 150 °C for different curing times

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Figure 3.2: Residual stresses on cross-plies laminates [0902]: a) flat [0902] laminate manufactured at 130 °C,1 minute and b) curved [0902] laminate at 150 °C for 5 minutes

Figure 3.3: Comparison of heating patterns of prepreg [0/90/0] after 19 seconds of applying same intensity of magnetic fields with a static coil

Figure 3.4: Shape distortions a) [9020] unsymmetrical and b) [904] Unidirectional lay-up, manufactured at 150 °C for 10 minutes

Figure 3.5: Temperature distributions for cured laminates: a) 130 °C, 3 minutes, b) 130

°C, 10 minutes, c) 150 °C, 3 minutes, d) 150 °C, 10 minutes, post-cured by induction heating at 160 °C, 30 minutes with a non-static coil

Figure 3.6: Curvature versus temperature at different heating treatments. a) Laminate not- post-cured, b) sample post cured by oven at 160°C for 30 minutes and c) sample post-cured by oven at 160°C for 60 minutes

Figure 3.7: Curvature versus temperature by induction heating at a) run I, b), run II and c) run III

Figure 3.8: Effects of the stress-free temperature T(s-f) at different heat treatments obtained by method I

Figure 3.9: Effects of the stress-free temperature T(s-f) between oven and induction post- curing by method II

Figure 3.10: Thermal expansion coefficient CTE a) longitudinal and b) transverse direction at different heat treatments by oven post-curing

Figure 3.11: Young´s modulus in a) longitudinal and b) transverse direction at different heat at treatments by oven post-curing

Figure 3.12: Major Poisson’s ratio at different post-curing treatments

Figure 8. 1: a) Heating rate of hot -press when it was heated up to 130 °C and 150 °C and b) heating rate in the mould when it was heated up to 130 °C and 150 °C with the hot- Press pre-heated

Figure 8. 2: Oven Termarks series 4000 calibration

Figure 8. 3: Thermal behaviour during thermal bending test at 130°C and 150°C for 3 and 10 minutes

Figure 8. 4: Climate chamber calibration

Figure 8. 5: Curve strain versus time at 150 °C, 10 minutes: a) [04] and b) [904] UD laminates by Thermal tensile test

Figure 8. 6: Curve stress versus strain at 130 °C, 3 minutes: a) [04] and b) [904] UD laminates by mechanical tensile test

Figure 8.7: Thermogram for the laminate manufactured at 130 °C, 3 minutes, with different thermal treatments

Figure 8. 8: Thermogram for the laminate manufactured at 130 °C, 10 minutes, with different thermal treatments

Figure 8. 9: Thermogram for the laminate manufactured at 150 °C, 3 minutes, with different thermal treatments

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Figure 8. 10: Thermogram for the laminate manufactured at 150 °C, 10 minutes, with different thermal treatments

Figure 8. 11: Curvature versus temperature for not post-cured samples at different runs

Figure 8.12: Behaviour of curvature versus time for not post-cured samples at different conditions separately with three repetitive runs

Figure 8.13 Curvature versus temperature for samples post-cured by oven at 160

°C, 30 min with different runs.

Figure 8.14: Curvature versus temperature for samples post-cured by oven at 160

°C, 60 min with different runs Figure 8.15: Tensile test failure modes

Figure 8.16 Curve weight versus time sample I

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List of Tables

Table 2.1: Curing Schedule for the samples manufactured Table 2.2: Curing cycles for unsymmetrical laminates Table 2.3 Notation of laminates manufactured

Table 2.4: Post-curing conditions to be applied on manufactured laminates Table 2.5: Sample dimensions

Table 3.1: Glass transition temperature (Tg) and heat of reaction (∆𝐻) at different curing schedules and heating techniques

Table 3.2: Sample curvature at 150 ° C for 5 minutes

Table 3.3: Glass transition temperature (Tg) and heat of reaction (∆𝐻) at at different heat treatments

Table 3.4: Analytical and experimental (αT-αL) data Table 3.5: Ultimate Tensile strength and strain

Table 8.1: Samples dimensions by thermal expansion and mechanical tensile tests Table 8.2: Samples dimensions by thermal bending test

Table 8.3: First, second and third character of failure modes Table 8.4 Fibre volume fraction data

Table 8.5 Fibre volume fraction of fast-curing carbon/epoxy prepregs

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Nomenclature List

The symbols and abbreviations used in this thesis are listed here.

Symbols List

EL Longitudinal modulus ET Transverse modulus E Young modulus ɛ Strain

σ Stress

ν12 Major Poison’s ratio ɛL Longitudinal strain ɛT Transversal strain

𝜀_𝑇^∗ The ultimate transverse strain 𝜎_𝑇^∗ The ultimate transverse strength

αL Longitudinal coefficient of thermal expansion αT Transverse coefficient thermal expansion Tg Glass transition temperature

∆ H Heat of reaction Ts-f Stress free temperature K The curvature

[90₂ 0] Cross-Ply

[0₄] Fibre direction unidirectional laminate [90₄] Transverse direction unidirectional laminate Abbreviations:

CFRP Carbon Fibre Reinforced Polymer UD Unidirectional laminates

FRP Fibre Reinforced Polymer

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viii FCER Fast Curing Epoxy Resins

GFRP Glass Fibre Reinforced Polymer EP Epoxy Resin

FEA Finite Element Analysis

DSC Differential Scanning Calorimetry DMTA Dynamic Mechanical Thermal Analysis PMCs Polymer Matrix Composites

IH Induction Heating

(VARTM) Vacuum Assisted Resin Transfer Moulding (CTE) Thermal expansion coefficient

FVF Fibre Volume Fraction

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1. Introduction

Nowadays, due to the rapid technological development and increase competition in the industry, there is a need to design lightweight structures in body frames [1]. Development of materials with different range and specific properties such as physical, chemical and mechanical properties help designers to use the right combination of materials and structures. shows the percentage increase of using the composite materials in aircraft structures during the last decades. It is obvious that the percentage of composites within the aircraft structures has increased very significantly and in some of the modern airplanes and the content of composites is used increasingly [2]. One example is the Boeing 787 which reduced by 20% the fuel consumption due to the decrease of overall weight by 50%, using 25% a composite of carbon ﬁbre-reinforced polymer (CFRP), while the Airbus A380 frame was reduced by 12%. The research and use of composite materials are highly important in many industries, such as for military and commercial air vehicles, robot arms, the automotive industry and especially for use in aviation and aerospace industry [3].

Composite materials can be defined as a combination of two or more types of materials with distinct phases. Each material complements the other with a characteristic that the others do not have, generating new material with physical, mechanical, chemical properties, etc that are better than those of the individual constituents working separately, transmitting loads from the matrix to the fibres through shear loading at the interface. There are many reinforcements in the market but carbon fibres are usually used in high-performance applications. It provides strength and stiffness, and is in most cases harder, stronger and stiffer than the matrix. The reinforcement can be in layers, but also as yarn or woven or as short fibres without specific organisation [4, 5]. The advantages in comparation to conventional materials are: lower weight, corrosion resistance, improvement fatigue life, the number of parts could be reduced and fewer assembly operations could reduce the costs for acquisition, possible tailor the lay up for optimum strength and stiffness and less maintenance, and so on. However there are disadvantages such as high cost for fabrication and assembly, effects of both temperature and moisture, high cost for raw material, more difficult to repair composites compared with metallic structure, susceptibility to impact damage and delamination, etc [5].

The structural polymer composites have been used widely in several industries for a long time. During the last decades process models have been developed and verified for mould filling/consolidation and cured for several manufacturing methods [6].

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2 Figure 1. 1:Increase in the composite’s percentage within the airplane structure [2].

The high-performance composites can be manufactured by different heating process that can use convection oven, laser, hot-plate, induction heating and so on; which the consolidation of the fibres and matrix is done at the same time. Carbon Fibre reinforced polymer, specifically carbon fibre with epoxy matrix it is economically advantageous to minimise the curing time by increasing the cure temperature to produce the necessary properties in less time.This event called curing usually requires an elevates temperatures to start and maintain the curing reaction [7].

The high-quality components in composites material depend on manufacturing factors such as voids, dry spots, high temperature peaks, cracking, residual stresses and shape distortions.

During cure of thermoset composite structures residual stresses and shape distortions are always present. A number of factors affect manufacturing induced shape distortions and one of the most sources of residual stresses and shape distortions is thermal contraction. [6] Residual stresses and shape distortions are developed during the manufacturing of thermoset composite and it can affect the product quality. Another responsible factor is cure shrinkage, and it is conformed for two components such as thermal shrinkage that occur when the material is cooled from cure temperature to room temperature and chemical shrinkage occurs from the transformation of thermoset matrix from liquid to solid material, this means, that the shape distortions increase with increasing degree of cure. Residual stresses and shape distortions are phenomena that must be monitored during the manufacturing of the product to ensure its quality and avoid strange failure even prior to demolding and deform a component [7].

The purpose of this project is to gather more knowledge about carbon fibre reinforced polymer and issues that may occur when they have residual stresses during the manufacturing.

In addition, the objective of this study is to gather more knowledge about the usage and manufacturing of CFRP in order to find the first findings for the future development of robust prediction methods for process induced deformations of fast curing epoxy resins (FCER).

The main and specific objectives of this project are:

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This study is focused on the dependency of different short curing cycles on the build-up of residual stresses in a carbon/fast-curing epoxy system and it is addressed to analyse the change in residual stresses due to post-curing cycle applied on the studied composites through electromagnetic induction heating. The specifics objectives of this project are:

➢ To evaluate the effect of curing and post-curing parameters on the thermo- mechanical properties.

➢ To evaluate the post-curing methods through induction heating and oven-post- curing.

The project is limited to focus on CFRP, especially with epoxy. Carbon fibre reinforced epoxy is one of the most promising options thanks to its excellent properties such as electromagnetic properties, high strength and low weight, and would be a good choice for many applications in the industry.

This thesis is divided by different chapters. In chapter one, composite materials definition, fibre types, matrix type, Carbon Fibre Reinforced Polymer (CFRP), classification according to its spatial orientation, processing of thermoset composite, residual stresses during manufacturing for CFRP are part of the literature study in this section. The second chapter, materials, description of experimental work and characterization are presented. The results from the tests and discussion are shown in chapter 3. Chapters (4-6) include the conclusion, recommendations and future work. References and appendices can be found in chapters 7 and 8 respectively.

1.2 Reinforced Polymeric Matrix Composites

The composite material is formed by a matrix and a second phase called reinforcement, it improves or reinforces the mechanical properties of the matrix. Usually the reinforcement is harder, stronger and more rigid than the matrix, however, matrix can be made of different materials such as ceramics, polymers, pure or mixed materials with other additives to improve its properties [8].

1.2.1Constituents 1.2.1.1 Fibres

Fibres reinforcement provides high stiffness and strength for the structural performance required of a product. With high modulus polymer composites, three major classes of fibres are in use:

a) Glass Fibres: Glass fibres are manufactured by melting of the constituents (mainly sand) and drawing of the fibres through orifices and subsequent cooling. They have high tensile strength, hard resistant to chemical attack, flexibility and low price [9].

b) Aramid Fibres: These fibres are made of aromatic polyamides and possess remarkably high Young's modulus, more than 20 times higher than conventional polyamide fibres. Kevlar and Nomex belong to this group. These fibres offer excellent physical and chemical properties

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4 at high temperatures. They are considered high-performance fibres and command relatively high prices [9].

c) Carbon Fibres: Carbon fibres are made from pre-stretched polyacrylonitrile fibres which are stretched further and heat-treated .From intermediate cycle polymer, graphitization treatment lead to the formations of turbo-static graphite microstructure producing strong covalent bonds in the longitudinal direction of the fibre (high stiffness) and lack of covalent bonds in the radial direction (low stiffness) . This material is considered one of the most used nowadays and was used on this project because its relevant mechanical properties. However, it present some disadvantages as skin irritation, dangerous to human health without proper handling [10] and high price, aspect considered important in some applications (Bledzki &

Gassan, 1999) [11].Carbon fibres have different type of carbon fibres based on modulus and strength, [12]:

• Ultra-high-modulus, type UHM (modulus >450 GPa).

• High-modulus, type HM (modulus between 350-450 GPa).

• Intermediate-modulus, type IM (modulus between 200-350 GPa).

• High strength fibres, HS (modulus between 230-280 GPa).

• Low modulus and high-tensile, type HT (modulus < 100 GPa, tensile strength> 3.0 GPa).

• Super high-tensile, type SHT (tensile strength > 4.5 Gpa).

Depending on the application, these types of fibres can be used, e.g HM fibres have high stiffness but low strain to failure. These fibres are primarily used in space structures only stiffness and thermal expansion coefficient are important whereas HS fibres are used in aircraft and sport applications.

1.2.1.2 Matrices

The matrix can be made of different materials such as ceramics, polymers, pure o mixed materials, nevertheless, the most common advanced composites are elaborate from polymeric matrix composites (PMC) which consist of a polymer (phenolic resins, polyester, vinyl ester and polyamide).Although they provide low cost, high strength, low density and good toughness, entirely different processing need to be used and commercial use is therefore still limited [8].

Thermoset matrices such as epoxy resins is one the most common matrices in polymeric matrix composites. They are considered high performance material since their high physical and chemical properties, usually used in all sectors such as aeronautical, nautical, automotive and industrial [13].

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5 Epoxies are thermosets undergo a chemical reaction when initially heated and cured to form a three-dimensional cross-linked network. Once these materials are cured, this structure is locked in place and the materials cannot be reformed or reprocessed. An epoxy is the reaction between a Diepoxide Monomer and a Diamine. The epoxide rings contained in the diepoxide can be opened by an active hydrogen on the end of the diamine to produce chemical bonds between the epoxide monomer and the hardener with heat [14].The hardener (diamine) has four active hydrogens that are capable of reacting with four different epoxide monomers to chemically cross-link them together ( figure 1.2).

Figure 1. 2 :Cross- linked epoxy network. [14]

The main function of an epoxy resin as matrix in composite materials is to distribute the stresses between the fibres through the interface through cohesive and adhesive forces. It also distribute the fibres in the desired geometric arrangement, avoiding fibre bearing to compression forces .Their high modulus with low shrinkage makes them widely used as a matrix for composites applications [2, 15].

In this project the resin used is an epoxy matrix that belongs to the group known as a “fast curing resins”. The curing cycle and heating process takes less than an hour while traditional adhesives often take days to complete the curing cycle. It can be cured in only 3 minutes with a temperature above 150°C, for hot-press process [16]. For instance, resin HexPly M10 is cured in one hour at 120°C at 0.3-5 bars, however, there are many epoxy resins can be cured for short time e.g Hexply M77 epoxy resin. This resin is fully cured in 1.5 minutes at 150°C, at pressure between 1-10 bars [17].

One important aspect in epoxy resins is the glass transition temperature (Tg). The Tg is the temperature range the polymer transitions from a glassy to rubbery state which the mobility of the polymer chains increases significantly. A cross-linked polymer chains having multiple degrees of freedom and modes of movement in response to any applied thermal energy. High cross-link density will require very large amounts of thermal energy to provide the systems with enough mobility to transition to a rubbery state. However, the ultimate Tg is determined by a number of factors: the chemical structure of the epoxy resin, the type of hardener and the degree of cure [13].

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6 Some researchers have looked at the effect of the cure cycle on the impact properties of epoxy resin or FRP composite materials containing epoxy resin. Segovia et al [18] used epoxy resin reinforced with a hybrid fabric of aramid and glass fibres to investigate impact damage influence by the cure cycle. Kumar and Radhakrishna [19], investigated the post-cure effect on the impact properties of a Glass Fibre Reinforced Plastic (GFRP) composite and reported an optimum post-cure temperature of 85°C for four hours. Similarly. Tucker et al [20] also reported that an epoxy vinyl ester resin mode I interlaminar fracture toughness is improved by post- curing but not at a too long duration. In addition, simulation has been performed e.g Zhang et al [21] proposed a Finite Element Analysis (FEA) model to investigate the process of temperature variation in epoxy resin during the cure cycle.

1.2.2 Carbon Fibre Reinforced Polymer (CFRP)

Carbon fibres are usually combined with a polymer matrix, and they are usually called carbon fibre reinforced polymers (CFRP). CFRP obtains high-performance material with a weight reduction of more than 50 per cent compared to high strength steel. Generally, the properties of carbon fibre reinforced composites are determined for many aspects such as the carbon fibre content, fibre length, fibre orientation, fibre matrix adhesion, but also by the inherent characteristics of the polymer matrix. Carbon fibre-reinforced exhibit excellent in- plane properties, but they typically show poor out-of-plane performance that is dominated by the polymer matrix [8]. The composite material used on this project is Carbon fibre/epoxy matrix, these material usually have high specific strength (the ratio of the material strength to the material density) and high specific modulus (the material Young’s modulus per unit material density) as it is shown in figure 1.3; characteristic very desirable in the aeronautical and aerospace industry. [5, 8, 22].

Figure 1. 3: Specific strength of reinforced fibres [8].

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7 1.2.3 Classifications by the Geometry of the Reinforcement

As was mentioned above the composite material is formed by a matrix and a second phase called reinforcement. The reinforced can be distributed in the matrix as fillers, discontinuous and continuous fibres changing the composite properties enhancing strength as it is shown in figure 1.2. Discontinuous fibres are referred to as fillers, can be of various shapes such as flakes platelets, short fibres, whiskers or microspheres [23].

Continuous fibres usually come in form of prepreg that consist of fibres impregnated in unpolymerized resin in semi-liquid state, serving as binder of fibres [2]. This type of material is used in structural applications called laminated formed from the addition of multiply layers of prepregs arranged unidirectionally; common term for fabric reinforcements. For this type of laminate, each fibre runs continuously and straight from one of the laminates of the other with a fibre content typically of 45-65% by volume, proving high stiffness and strength. However, the processing method are slow and sometimes not suited for high-volume production [22].

Figure 1. 4: Classification of composites materials by geometry of the reinforcement [8].

On the other hand, continuous fibres can be classified into three categories:

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8 -Unidirectional laminates, they are formed by stacking laminates, which fibres are oriented in the same direction (Figure 1.5 (a)). Alternatively, the reinforcement can be in woven fibres to form a layer or a thin layer of randomly oriented fibres.

-Crossply laminates are laminates with orthotropic behaviour, which are laminated 0/90°

manufactured from the stacking laminates in two orthogonal directions (Figure 1.5 (b)) -Quasi-isotropic laminates, are laminates that are oriented in different directions, e.g [0, 90, 45, -45]s ( Figure 1.5 (c)).

On this project, the lay-up used to analyse residual stresses and shape distortions were cross ply [9020] and unidirectional [904] laminates made with a fast-curing carbon/epoxy prepreg.

Figure 1. 5: a) Unidirectional laminates, b) Orthotropic laminate and c) Quasi-isotropic laminates [2].

1.2.4 Processing of continuous fibres composites using thermoset polymer matrix For continuous fibre composites, heat and pressure are fundamental parameters for the laminate manufacturing. Heat and pressure are first applied to the laminate to reduce the viscosity of the polymer matrix and achieve full vitrification from rubbery to glassy of the laminate. The application of heat to the laminate is governed by the laws of heat transfer and time. Further, the pressure in the laminate is shared by the polymeric matrix and the fibres.

These parameters must be well controlled to obtain high quality products and avoid residual stresses and shape distortions.

Some process used in CFRP are:

a) Resin Transfer Moulding

Resin transfer moulding of composite laminates is a process wherein the dryfibre perform is infiltrated with a liquid polymeric resin, and the polymer is advanced to its final cure after the impregnation process is complete (Figure 1.6). The flow may be assisted by a vacuum (vacuum assisted (VARTM)) [24].

a) b) c)

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9 Figure 1. 6: Vacuum assisted resin transfer moulding (VARTM) [24].

b) Autoclave moulding

The process consists of fibrous reinforcement and matrix are laid down on a tool in the desired orientation and spot welded to make sure that the stacked plies do not move between them (Figure 1.7a)). The laid-up part is vacuum bagged and consolidated under vacuum and cured in an autoclave (pressurised oven) at around 120-180°C and 2-6 bars (Figure 1.7 b)). This process is similar to the hot press technique used on this study, the only difference between them is the method of applying pressure and heat [24].

a) b)

Figure 1. 7: a) Vacuum bag preparation for autoclave cure of thermoset matrix composite, b) Vacuum bag sequence and tool plate placed in an autoclave [24].

1.3 Factors affecting shape distortions and residuals stresses

Residual stresses are developed during the manufacturing composites and have a direct influence on the product quality that can lead to reduction of structural strength, cracking or delamination, dissatisfaction of tolerance and so on. Residual stresses and their influence on the structure had been considered at various levels [25, 26, 27, 28] .There are a number of factors causing residual stresses and shape distortions, divided into intrinsic and extrinsic factor [29].

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10 Intrinsic factors include anisotropic thermal contraction and expansion, resin shrinkage, matrix type, ply-level, fibre type and fibre volume fraction. Extrinsic factors consist of cure schedule, structure shape and tool part interaction [7, 30, 31]. Some of these factors will be described below:

a) Differences in thermal expansion.

One of the main sources of the shape distortion is free expansion-contraction of the material. For most composite system the expansion coefficient of the polymer matrix is usually higher than of the fibres. Carbon Fibres have very low or negative expansion coefficients in the fibre direction, but higher expansion values in the transverse direction, producing tension in the matrix in the fibre direction [7].This leads to residual stresses at the microscale during cooling even in unidirectional materials. As it shown in Figure 1.8, hopps tensile stresses arise in the matrix as it shrinks around the fibres. There are also radial stresses which after compressive when the fibres are close each other or tensile when they are furthest apart. These local stresses can affect the mechanical properties of the materials but it cannot produce shape distortion [32].

Figure 1. 8:Residual stresses in carbon /epoxy at 100°C [32].

b) Ply-Level

The difference in ply-level thermal expansion coefficients in the fibre and transverse directions causes in-plane stresses in the laminate. As was mentioned before, composite material consists of unidirectional plies with different or equal orientations that are stacked together [7].

A unidirectional ply [ 90°/90°] has much thermal expansion in the transversal direction and low stiffness than in the axial direction due to difference in thermal expansions between fibres and matrix [7].

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11 A balanced laminate implies that shear strain and shear force are uncoupled from normal strain and force, that means, forces and midplanes strain are uncoupled from moments and bending. However, if the lay-up is unsymmetrical for example [0°/90°] a bend or warped shape can be expected due to the large difference in thermal expansion between transverse and axial directions. This phenomenon can be observed in figure 1.9; it is called stable shape or spring- in, when there is higher through- lay-ups expansion coefficients (matrix dominated) compared with in-plane values (fibre-dominated), and it depends on the laminate size; depending on the size of the laminate, a saddle shape (small laminate) or single curved shape (large laminates) can be presented [7].

Figure 1.9: Thermal deformation of an unsymmetrical lay-up [7].

Carbon fibre reinforced composites have residual stresses due to coefficient of thermal expansion (CTE) between the fibres and the resin matrix. As the material is cured, it develops an equilibrium state at elevated temperature. As the material cools, stresses are developed within the part [33].

c) Anisotropy

During the curing process of the composite, part of the matrix experiences considerable shrinkage whereas the fibre lengths remain nearly constant. It causes small strains in the fibre direction and higher strains in transverse direction. This produces a phenomenon called Spring- in; thickness reduction in curved part areas whereas the arc length in the curved remains constant and after demoulding an unavoidable decrease of the enclosed angle Ө (see figure 1.10). The cure shrinkage is the main driver of this mechanism and it consist of two parts:

Chemical shrinkage which cause by the formation of cross-linked and thermal shrinkage which develop during cooling from cure temperature to room temperature [34].

Figure 1.10:Distorted angle section [7].

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12 Spring-in is manifest in angled structures whereas in figure 1.11 is shown another phenomenon called warpage that occurs in curve/flat parts and can happen in corner section with smaller angle in comparison to spring-in which the angle formed is higher [29].

Figure 1. 11: Warpage on flat panel [35].

d) Cure Shrinkage

Cure shrinkage is one of the factors responsible for shape distortions after curing, affecting the quality product contribute to failure. Shrinkage is conformed in two components:

(a) shrinkage due to volumetric reduction resulting from cross link formation and (b) thermal contraction when the laminate is cooling from cure temperature to room temperature. Both occur together during the manufacturing and depending on the state of the polymer the contribution of each components will develop residual stresses within the composite [24].

Thermosetting resin are in a fluid state and are called monomers or prepolymers. They are solidified by a chemical reaction which molecules of monomers or prepolymers are linked together to form polymer networks [36]. During the cure there are two transformation from the resin, the first one is the gelation when the degree of cure wherein the polymer is transformed from liquid state to the rubbery state. At this point the resin consist of linear polymer chains and the glass transition temperature (Tg) is lower than the cure temperature. When the cure process proceeds the Tg increase and eventually exceeds the cure temperature, in addition, the polymer chain turns on cross-linked. This second transformation is called the vitrification and at this point the resins transform from rubbery state to the glassy state, with a dramatic change in mechanical properties [7].

The Shrinkage depends on degree cure and the type of the thermoset. During curing process, the pre-gelation state of the polymer in flowing liquid state, offering little resistance, and cure shrinkage occurs without stresses. At gelation, the liquids transform to a solid; the shrinkage result in micro-stresses in a composite owing to the constraint imposed by the fibre [32].In the rubbery state, the stiffness is significantly low than that after vitrification. Further, the viscous of the polymer prior vitrification may also allow residual stresses to release over time [24]

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13 Figure 1. 12: Volume change during the manufacturing [24].

In figure 1.12, can be shown how the composites´s volume is affected during the thermal cycle. At the beginning there is an increase in volume along a-b when the polymer is heated up from a room temperature (To) to cure temperature (Tcure), due to the thermal expansion. It is held at this temperature until cure is complete. In addition, it is pointed out a reduction in volume (b–c) due to cross link formation. On cooling stage, the polymer contraction exhibits two events; the first rate before (c–d) occurs when the polymer is in the rubbery state and a second rate after (d–e), when it is in the glassy state in the region; passing through the glass transition temperature Tg at d point. Further, the Tg is a function of the degree of cure of the polymer; and shrinkage is related to the extent of cure that it has achieved [24].

Chemical shrinkage is very important in the cure modelling of thermoset material.

Mergheim et al [37], modelled the curing shrinkage of a thermoset showing damage in the material due to the volumetric shrinkage and the produced stresses. Other researcher like Kaspar et al [38] and Sadeghinia et al [39] have investigated the measurement of chemical shrinkage in thermoset composites, which affect the amount of residual stresses produced in these structural parts after curing. In addition, An optimal cure process of composite structures was proposed by White and Hahn [40, 41], whose objective is to decrease residual stresses during the curing process and models have been successfully applied to Carbon Fibre reinforced polymer (CFRP) [42, 43].

e) Tool/ part interaction (Stress Gradient)

Residual stresses and shape distortion can be obtained during the manufacturing process due to the differential strains between the part and tooling. During cure, as the prepreg is heated under pressure, the fibres are closer to the mould and are clamped against the surface by the processing pressure, therefore, can be constrained by the mould. Metals tools have much higher expansion coefficients than composite material, and tend to cause small stresses at the tool interface causing tension in the ply-up laminates. These residual stresses are frozen; therefore, gradients of in-plane stresses increase, causing bending when the stresses are released after the cooling [32].

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14 Research reported that prepreg- tool interaction has an important effect in residual stresses and shape distortions. Twigg et al [44] studied the shear stress development at the interface between the tool and laminate. They evaluated strains oriented longitudinally and transversely and demonstrated that the cure cycle parameters can influence the amount of residual stress and the final shape.

On the other hand, the mould surface has an impact in the shape distortion. Fernlund et al [45], in their research about spring-in of C-channels manufactured by carbon-epoxy prepregs [0n/90n] on aluminum smooth tool and a rubbery tool showed that, laminate plates manufactured by aluminum smooth surface presented largest shape distortion than the rubbery tool, due to the slippage effect between the rubbery tool and the laminate, being largest in the rubbery tool and lowest in the smooth aluminum tool, this means, the gradients of in-plane stresses decrease, avoiding bending in the laminate.

The effect of the mould it is very important on shape distortions. During mould-in cure the material is constrained by the mould and shape distortions are formed by released of residual stresses during demoulding. However, if the specimens are free standing during cure, shape distortions maybe formed without development of macroscopic residual stresses [7].

f) Other parameters

Fibre Volume fraction and distribution of fibre on the matrix can affect the shape distortion.

Low fibre content implies a high amount of heat per unit mas generated during the cure and high temperature peak in the middle of the laminate causing gradients in expansional strains and shape distortions in thick laminates (see figure 1.13). In addition, inhomogeneous distribution of the fibre in the matrix means gradients in the fibre content through the thickness, causing heat gradients and affecting mechanical properties even in flat unidirectional laminates.

Figure 1.13: Distortion due to volume fraction gradient through the thickness [32].

The laminate thickness is another important factor must be considered. During the curing process a considerable amount of reaction energy is released from the polymer, which it could cause gradients in temperature and degree of cure leading to shape distortions. Thick laminates have poor heat transfers, which gradients are developed through the thickness and chemical shrinkage does not occur uniformly due to variation in the degree of cure [7]. Ali et al [46]

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15 showed that a laminate with thickness variations have different deformation behaviour in comparison to uniform thickness part. Causses et al [47] concluded that the manufacturing quality has a significant impact on the distortions through different mechanism. The variation of the thickness changes locally fibre volume fraction which modify coefficient of thermal expansion. A resin rich zone constitutes regions of higher coefficient of thermal expansion creating stress concentration on one side of the curve area.

In figure 1.14, shows a summary of some micro-mechanism of shape deformation mentioned above, developed during the manufacturing. It can be observed as residual stresses in different times and proportions are released when the material is heated by free-standing post-curing from room temperature to cure temperature such as thermal expansion in the glassy and rubbery state, chemical and thermal shrinkage.

Figure 1.14: Shape distortions during the post-cure process [31].

Line 1 and 2 corresponds to decrease in shape distortion due to matrix expansion during the glassy and rubbery state. An increase in residual stresses is perceived due to chemical and thermal shrinkage during the post-curing, illustrated by line 3 and line 4 respectively. The development of cross-linking network in the rubbery state and the volumetric contraction during the cooling from the cure temperature to room temperature cause the matrix shrink, obtaining residual stresses and a decrease of shape deformation. In addition, it is demonstrated that after the post-curing treatment, the shape deformation decreases, indicating this treatment is a way for the material to release stress obtained during the manufacturing.

1.4 Induction Heating of Continuous- Carbon- Fibre

New mechanism for manufacturing composites laminates is by induction heating which, is efficient, precise, clean and cost-effective. It is a well-known for the welding of thermoplastic and curing thermosets combines a high energy density with the ability to heat selectively and intrinsically. This phenomenon occurs, due to induced eddy currents flowing along conductive loops in the conductive material, as shown in figure 1.15. In each conductive loop, heating

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16 occurs wherever there is a voltage drop due to electrical resistance. The heating of this method is volumetric and dependent on intrinsic properties of the material [48].

Figure 1. 15: Induction heating process [48].

Polymers usually do not beat the inherent potential for inductive heating (IH) because they are not electrically nor electromagnetic conductive materials.In order to transfer energy from an electromagnetic field into polymer induction structures, conducting materials or materials that absorb the radiationare required. These conducting material are called susceptor materials and are able to transfer the available electromagnetic energy into heat. The most common are carbon fibres fabric and metallic additives [49].

The most common susceptors in different fields of polymer processing and synthesis varying depending on the frequency. For high-frequency applications (kHz or MHz), metallic additives, metal meshes, ferro or ferrimagnetic nano particles and semiconducting carbon fibre fabrics can be used. Besides, susceptors can be applied in low-frequency high magnetic field pulse to induce current in thin metallic loops and to trigger a chemical polymer reactions such as Polycarbonate (PC) and Polyvinylamine (PVAm) [50].

According to Faraday´s works [51, 52, 53] , the induced current is correlated to the frequency electromagnetic fields. In contrast to metals, polymers materials are not affected by electromagnetic effects. However, depending on the frequency applied an unwanted temperature can be reached that could degrade the material [50].

In CFRP, there are mechanisms which lead to volumetric heating in conductive fibre materials such as joules losses, dielectric heating at fibre junctions and contact resistance at junctions. Carbon Fibre can generate heat due to their intrinsic resistance and junctions (fibres from adjacent plies overlap) which can generate heat due to dielectric hysteresis by a small polymer gap, or contact resistance heating if the fibres are in direct contact. Depending of prepreg quality and processing conditions all mechanisms may occur to varying degrees.For

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17 this reason, pressure, temperature and stacking sequence dictate the degree of contact between adjacent plies and determine the through-thickness electrical behaviour of the composite [48].

Influences of ply number, interface thickness and ply thickness on induction heating behaviour have been investigated by Fink [54].

In this project, this technique is used for carbon fiber reinforced polymer, specifically carbon fiber with epoxy matrix due to its electromagnetic properties. This is a new technique used as direct heating laminate in CFRP through a post cure treatment [55]. Induction heating of CFRP is possible to get high volumetric heating rates, leading to higher throughput, compared to conventional manufacturing. IH makes it an interesting technology for traditionally engineered materials, especially for temperature-sensitive substrates [56, 57, 58], such as paper, wood, hydrogels, plastics, and natural fiber reinforced composites.

The ability for internally applied and localized heat, with high gradients, allows for processing at room temperature and also for thick samples. Traditional manufacturing process such as convection or radiation heat transfer through the thickness of composite, requiring time for the composite to equilibrate at the desired process temperature. However, induction heating occurs leading to higher heating rates and velocities (see figure 1.16), high productivity, reduction of cycles time, environmental sustainability, the reduction or complete exclusion of solvents for the curing of coatings and axing the use of a catalyst. For this reason, induction heating is used for carbon fibre reinforced composite material in multi-ply process [48].

Figure 1.16: Induction process techniques for composites compared to others heating approaches [49].

Several applications have been used for the curing thermosets by reaction activation via inductive heating, replacing conventional oven and autoclave. The main objective is to get a significant reduction of curing time and this method allows a solution for quick repairs since the intrinsic heating in a local area. The heating by induction concentrates mainly on small scale application due to this system use a finite coil that is placed on a good conductive material and its geometry will be depicted by the emerging temperature pattern in the workpiece due to the limitation of the electromagnetic field [48, 49].

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18 Heat transfer and conventional cure kinetics analysis in composite manufacturing has been studied by Tzeng and Loos [59]. Worrall et al [60] ,invented a new method for improving the control of induction heating welding thermoplastic composites without additional susceptors, inserting a thin electrically-insulating layer (gauze) between adjacent layers containing nonaligned carbon fibres. This research concluded that, this technique has the advantage that the heat generated is concentrated at the joint interface, which avoids excess of heat from the surface of the composite to avoid thermal damage. In addition, some research has been done regarding this CFRP. Lundström et al [61] showed that the fibre type and fibre volume fraction are of great impact in induction heating, pointing out that fibre volume fraction affects the temperature distribution thought the lay-up. Exceeding 60% of fibre volume is necessary to achieve a fairly isotropic resistivity and uniform induction heating pattern.

.

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19

2. Experimental work

2.1 Material

The analysis was carried out using one type of fast-curing Carbon/epoxy prepregs. CFRP material used in this project, is a product attractive due to epoxy matrix belongs to the group known as a “fast curing resins” and the curing cycle and heating process takes less than an hour.

2.2 Experimental set up

An initial material characterization had been performed in order to analyse which conditions should be used during the manufacturing of fast-curing Carbon/epoxy prepregs. The objective was to build up a range of curing temperatures with laminates with different degree of cure in order to study the residual stresses obtained during the manufacturing. Based on these characterizations, the new study parameters for curing and post-curing conditions have been decided and a work plan was elaborated which is graphically depicted in figure 2.1.

Two [90₂0], and [904] laminates were manufactured. Residual curvatures samples related to the magnitude of residual stresses were studied in unsymmetrical cross-ply laminates.

Degree of cure was analysed with DSC technique for different curing history, thermal expansion coefficients and mechanical properties were characterised with thermal expansion test and tensile test for [904] configuration in longitudinal and transverse direction. In the same way, oven and induction heating post-curing were performed and material samples with different degrees of cure and different residual stresses. The change in residual stresses and degree of cure were experimentally determined by studying thermal bending test, DSC test, thermal expansion and mechanical tensile test analysis. Fibre volume fraction test was performed to have more information about the material and its effect with the residual stresses generated during manufacturing

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20 Figure 2.1: Experimental plan.

a DSC: Differential Scanning Calorimetry.

2.2.1 Initial Characterization

2.2.1.1. Curing samples with different curing cycles

Small singles layer of carbon/epoxy prepreg with size equal to 50 x50 mm² were cured separately. The layer was placed between two layers of vacuum bag and cured with different curing cycles by hot-press with a very moderate level of pressure (ensuring the layer is in contact with the hot-press) and no vacuum. The curing started as soon as the small pre-preg sample was placed in the hot-press and the press was closed. After the curing cycle, the hot-

160 °C, 30 min and 160 °C,60 min 160 °C, 30 min

Fibre Volume Fraction Thermal

Expansion Test

Hot-Press Processing Composite Manufacturing

Post-Cure

[90₂0] [90₄]

Oven Induction

Heating

Characterisation Techniques

Thermal Bending

Test

Mechanical Tensile

Test Initial Characterization

DSC^a

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21 press was opened and the samples are immediately removed from it. In table 2.1, shows the curing schedule used for the samples.

Table 2. 1: Curing Schedule for the samples manufactured.

Nr Curing

Temperature(°C) Curing Time (°C)

1 130 1

2 130 2

3 130 3

4 130 5

5 130 10

6 150 1

7 150 2

8 150 3

9 150 5

10 150 10

2.2.1.2 Analysis of curing properties by Differential Scanning Calorimetry (DSC) Samples manufactured in section 2.2.1.1, DSC scan was performance in order to have idea about the degree of the degree of cure through temperature of glass transition Tg and heat of reaction (∆H) at different curing schedules. In figure 2.2 below shows a DSC821 from Mettler Toledo used. A small amount of sample between 5-10 mg was weighed by PG5002-S.

Delta Range Mettler Toledo balance, tolerance 0,001g. The samples were taken on top of an individual heating disc and an identical empty reference aluminium pan was placed on a second heating disc. Energy was supplied separately to the sample and the reference in order for each of them to exactly match the heating rate of a predetermined temperature profile. The DSC test was performed under an inert nitrogen atmosphere with a ﬂow rate of 80 ml/min, consisted of two repeats scans from 20-200 °C with the heating/cooling rate of 20 °C per minute according to ASTM D3418 [62].

Figure 2.2: Differential scanning calorimetry machine.

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22 2.2.1.3 Fast study of residual stresses in unsymmetrical cross-ply laminates

Two unsymmetrical cross-ply laminates of size 50 x 50 mm² were manufactured. The lay-up of laminates was [0/902] and the manufacturing process were performed in the same way as it was mentioned in the section 2.2.1.1, whose manufacturing schedule can be detailed in table 2.2. The objective of this task is to perform a quick study of residual stresses in laminates depending on the curing cycle. The results can be found in 3.1.2 section.

Table 2.2: Curing cycles for unsymmetrical laminates.

2.2.1.4 Manufacturing of unsymmetrical cross-ply laminates

An unsymmetrical cross-ply laminate [0 90 0] with dimensions of 200 x 200 mm² was manufactured. The manufacturing was through vacuum bag system, whose experimental set up is detailed below:

• The first step in the composite manufacturing was to remove the prepreg roll from the freezer one hour before working and allow it to reach room temperature. Afterwards, a hair dryer was utilized to heat the prepreg as it was rolled and flattened out to avoid bending that could cause the fibres to break. 3 pieces of CFRP were cut and the prepreg was placed in the freezer again.

• Two prepreg pieces were placed next to each other, the protective film was removed and the surface was heated with the hair dryer to activate the properties of the adhesive until gets a shiny surface. The layers were placed one on top of the other with the desired orientation and number of layers desired. A metal rod was used each time to press the layers and avoid possible air between the layers. The rod must be passed in the direction of the fibres to avoid damage.

• Next, pieces of release film were cut to cover the laminate on both sides. The laminates were wrapped many times (three times at least), avoiding wrinkles, it is to prevent the substrate from adhering to the mould.

• A metal plate (450 X 320 mm²) was used and the composite laminate was placed on it directly. A piece of breath film was placed on the metal plate by the vacuum system held with special tape (high temperature resistant), the material was fixed on this plate and a Teflon plate (300 x 250 mm²) was placed on the top (figure 2.3 a) and b)). The metal plate must be previously cleaned with release agent avoiding the edges, this in order that the mould does not stick to the plate.

Laminate Lay-up Curing

Temperature (°C)

Curing time (min)

1 [0 902] 130 1

2 [0 902] ¹⁵⁰ ⁵

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23

• Sealant tape was placed on the edges of the metal plate and the mould was covered with enough breathe film avoiding it touch the sealant tape (figure 2.3c)). Finally, the whole system was covered with vacuum bag and the vacuum pump was connected and turned on checking for leaks (the pressure must be less than zero). The vacuum provides with a uniform pressure on the laminate and draw out volatiles created during the cure, avoiding voids in the laminate.

(figure 2.3 d)).

Figure 2. 3:Laminate manufacturing by vacuum bag system.

• Finally, all system was taken to the Hot-press (PHI. Pasadera Hydraulic, Inc) (figure 2.4) under vacuum and a pressure of 3 bars. The test conditions were at 140 °C for 7 minutes, with a previous hold mould time of 5 minutes to guaranty the mould reach the cure temperature, before the curing process.

Figure 2.4:Curing of the CFRP by the hot- press.

a) b)

c) d)

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24 2.2.1.5 Induction Heating (IH)

The laminate manufactured in the previous step, a post-cure by induction heating was performed by Lund University in collaboration with Corebon AB using an in-house experimental. An induction heater was used, it was consisted of an electromagnet and an electronic oscillator that passed a high-frequency alternating current (AC) through the electromagnet. The current circulating in the static coil was 58 A, a frequency of 55 kHz with heating time of 3 seconds.

The laminates used for this performance were plates of size of 200 x 200 mm². The measurements of actual temperature distribution during the induction heating were measured and recorded using a thermal camera. The surface of the plates was sprayed with a graphite powder to facilitate more uniform distribution of temperature as well as for improving accuracy of the temperature measurements with a thermal camera. The temperature distribution in the laminate can be detailed in section 3.1.3.

This pre-characterization though DSC data and curved laminates, gave an idea of the parameters that must be used to produce residual stresses in the material. Induction heating perform was based on DSC data and used for necessary modifications to improve the process.

The data obtained helped to select the new design parameters and steps for this study which can be detailed in the follow sections:

2.2.2 Hot-Press Processing

During the manufacturing, the composite material was in direct contact with the metal plate (figure 2.3 (a)), for this reason, a calibration of the hot-press was essential to control the curing parameters, obtaining the desired degree of curing through the laminate and avoiding overheating in the material. For this process, the Picolog 6 Beta device was used, which consists of measuring the temperature as a function of time through the use of a thermocouple (see figure 2.5) whose objective was determined three conditions: First, the time that the Hot Press takes to reach from room temperature to the cure temperature (130 ° C and 150 ° C). Second, the time the mould takes to reach the cure temperature when the Hot Press was preheated. Finally, verify if the temperature indicated in the Hot-Press thermometer corresponds to the real temperature.

The behaviour and analysis of the results can be detailed in appendice 8.1.1.