Bearing strength and failure behavior of hybrid composite laminates.

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Bearing strength and failure

behavior of hybrid composite

laminates.

-Master thesis performed at RISE SICOMP AB, Linköping

Author: Hanasoge Saraswathi Deepthi Prasad (hanpr569)

Supervisor (LiU): Mohamed Sahbi Loukil

(RISE): Mats Bergwall

Examiner: Mikael Segersäll

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List of figures

Figure 1: Usage of composite materials in aircraft (S.Rana, 2016) ... 1

Figure 2: Orientation of composite layups in [0/90]s direction ... 5

Figure 3: Sequence of composite part manufacturing process using pre-pregs (Björnsson, 2017) 6 Figure 4: Common damage types in composite laminates (Tual, 2015) ... 7

Figure 5: Bearing test assembly (Anon., 2013) ... 8

Figure 6: Macroscopic failure modes of composite joints (Ekh, 2006) ... 10

Figure 7: Schematic drawing of the bearing test specimen ... 12

Figure 8: Struers mounting set-up... 13

Figure 9: Tegramin polishing machine ... 14

Figure 10: Area looked at the microscope for damage ... 15

Figure 11: Types of joints used in the transition zone ... 16

Figure 12: Bearing stress/strain curve of 100% T80 loaded at 70%, 80% and 100% of its ultimate load. ... 19

Figure 13: Onset strength of all layups ... 19

Figure 14: Ultimate bearing strength of all layups ... 20

Figure 15: Ratio of onset strength to ultimate bearing strength for all the layup ... 20

Figure 16: Bearing failure modes in material 50% T80 loaded up to 80 % of its ultimate load .. 22

Figure 17: Fractography of 50% T160 and the damages caused at 14.6kN with stress of 808MPa ... 22

Figure 18: Fractography of 100% T160 and the damages caused at 16.4kN and stress of 825MPa ... 23

Figure 19: Normalized results of fiber kinking in all the layups which were loaded up to 77-80% of its ultimate load ... 24

Figure 20: Normalized results of fiber kinking in all the layups which were loaded up to 70% of its ultimate load ... 24

Figure 21: Normalized results for delamination of all layups which were loaded up to 70% and 80% ... 25

Figure 22: Tensile stress/ strain curve for layups with butt joint ... 26

Figure 23: Microstructure of the specimen with edge reinforcement ... 27

Figure 24: Specimens that were broken after the tensile tests and failure position illustrated with red line ... 27

Figure 25: Yield force of all the layups ... 37

Figure 26: Ultimate bearing force of all the layups ... 37

Figure 27: Fractography of 50% T80 and the damages caused at 15.9kN and stress of 779MPa 38 Figure 28: Fractography of 100% T80 and the damages caused at 18.2kNand stress of 750MPa39 Figure 29: Fractography of 100% UD and the damages caused at 15.7kN and stress of 825MPa ... 39 Figure 30: Fractography of 50% T80 and the damages caused at 14.4kN and stress of 703MPa 40 Figure 31: Fractography of 100% T80 and the damages caused at 16kN and stress of 660MPa . 41

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Figure 32: Fractography of 50% T160 and the damages caused at 12.7kN and stress of 696MPa

... 41

Figure 33: Fractography of 100% T160 and the damages caused at 14.5kN and stress of 734MPa ... 42

Figure 34: Fractography of 100% UD and the damages caused at 13.7kN and stress of 722MPa ... 42

Figure 35: Normalized microstructure of 50% T80 ... 43

Figure 39: Normalized microstructure of 100% UD ... 45

Figure 40 : Normalized microstructure of 50% T80 ... 46

Figure 44: Normalized microstructure of 100% UD ... 48

Figure 45: Onset force of all layup before normalization ... 53

Figure 46: Onset strength of all layup before normalization ... 53

Figure 47: Ultimate bearing force of all layup before normalization ... 54

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List of tables

Table 1: Yield force, strength, UBF and UBS values from the tests……….17

Table 2: Theoretical and experimental values of calculated fiber volume fraction………..19

Table 3: Tensile stress and maximum load values obtained from the tensile test……….26

Table 4: Values of the specimen used prior and during matrix burn test………..31

Table 5: Calculation of fiber volume fraction……….. 31

Table 6: Layup sequence, orientation and the layup used in 100% T80………...32

Table 7: Layup sequence, orientation and the layup used in 50% T80……….33

Table 8: Layup sequence, orientation and the layup used in 100% UD………...34

Table 9: Layup sequence, orientation and the layup used in 100% T160………..………...35

Table 10: Layup sequence, orientation and the layup used in 50% T160……….36

Table 11: Layup sequence and orientation used in transition zone using lap joint………...49

Table 12: Layup sequence and orientation used in transition zone using butt joint………..50

Table 13: Layup sequence and orientation used in transition zone using edge reinforcement…..51

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Abstract

Composite layups have been continuously used over many years in various applications. It is necessary to optimize its composition by studying various parameters influencing the mechanical properties and studying the failure behavior. In this master thesis, the objective was to test five different plies manufactured using thick and thin plies and various combinations of thick and thin laminates called hybrid laminates. Bearing tests are performed for five layups with each layer has its thickness varying from 40µm to 130µm, and a combination called hybrid laminate, and the results from the tests are investigated. The resulting system has a good performance with onset damage above 700 MPa and an ultimate failure above 1130 MPa, using fibers' full potential. Also, the different failure modes like fiber kinking, matrix crack, delamination, and their effect on the layup's strength are investigated using fractography. This paper also investigates the influences of the thickness of the laminate on the strength of hybrid composites jointed using different mechanisms, and its failure modes are checked. Results from this experiment are used to validate in the form of FEM model, which is a part of an internal project at RISE SICOMP AB. This thesis is suitable for an engineering student in mechanical engineering, material science interested in composite materials and fractography. This work will be published and submitted to the journal of composite structure.

Acknowledgments

I am sincerely grateful for the opportunity provided to write my master thesis at RISE SICOMP AB in Linköping as a part of a research project for SAAB AB and other reputed companies. I want to thank my supervisors Mohamed Sahbi Loukil from LiU and Mats Bergwall from RISE SICOMP, for continuous support and successful mentoring. I also like to thank my examiner Mikael Segersäll for his support and recommendations. My gratitude to Rodger Romero Ramirez, the Engineering materials division staff at Linköping University, for his help during the sample preparation for the microstructure. I would also like to thank my opponent Manoj Dammur for his feedback and recommendations.

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Abbreviations and terms

LiU-Linköping University

RISE SICOMP AB- Research Institute of Sweden AB 100% T80-100% thin plies of Textreme 80gsm layup

50% T80-50% thin plies+50% thick plies of Textreme 80gsm layup 100% T160-100% thin plies of Textreme 160gsm layup

50% T160-50% thin plies+50% thick plies of Textreme 160gsm layup 100% UD-100% Uni-directional thick plies of Oxeon layup

YS- Yield stress/strength also called as onset of damage UBS-Ultimate bearing stress/strength

Higher loading %-specimens loaded up to 77% and 80% of the total load used in the test.

Lower loading %-specimens loaded up to 60% and 70% of the total load used in the test.

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1 Introduction

The aerospace industry's unrelenting passion for improving the commercial and military aircraft performance is continuously driving the development and production of new, high-performance structural layups (Adam Quilter, 2020). Out of all the layups, composite layups are one such class that plays a significant role in current and future aerospace components. Composite layups are particularly attractive to aviation and aerospace applications because of their exceptional strength and strength-to-weight ratios and superior physical properties (Adam Quilter, 2020). There are also other advantages like the excellent corrosion resistance, wear resistance, low thermal and expansion and conductivity, which add more weightage for its usage in various applications (Anon., 2020). Although there has been continuous usage of composite layups in various applications with the mentioned advantages, it is essential to optimize its composition by studying various parameters influencing the mechanical properties and by studying the failure behavior (S.Rana, 2016). Figure 1 shows the highest usage of materials in aircraft is composite layups due to many advantages.

Figure 1: Usage of composite materials in aircraft (S.Rana, 2016)

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1.1 Background

The increasing use of composite layups in the aviation industry and the importance of the research in composite joints strength has been brought out due to its popularity and also as the joint decides the actual efficiency of the structure.The bonded and bolted joints are the primary type of composite joints in a ircraft structures (Yeole, 2006). Bolted joints are used vastly in many applications using composite layups due to various obvious reasons. For the joints to be connected, holes are to be drilled in the structure, which causes high-stress concentration, which reduces the material strength. The reliability of the bolted joints depends on the bearing strength of the local laminate. Hence, there are many laminates stacked up in different orientations with various thicknesses. The bearing test is done to find the best combination of hybrid laminates and the failure modes (Yeole, 2006).

1.2 Project objectives

The master thesis includes activities like:

1. The first objective is to investigate the influence of thin ply composite and its effect on the bearing strength of different composite layups and characterize the mode of failures.

2. As a second objective of this project, failure modes of hybrid laminates with overlap joint, butt joint and edge reinforcement will be studied to investigate the strength of the transition specimens from thick to thin plies

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2 Theory

2.1 Composite materials

Composite materials are the “combination of two or more materials with different properties”. When combined, different physical and chemical properties result in better properties with characteristics better and different from the individual components. The two constituents are matrix, which surrounds and binds together a cluster of fibers or reinforcements and then transfers the load to the fibers (Campbell, 2010).

There are different types of composites based on the nature of the matrix and reinforcements. This thesis concentrates on polymer matrix composites instead of ceramic and metal matrix composites, which are the other two types. Among the polymer matrix composites, thermoset matrix composites are more predominant than thermoplastic composites.

There are many advantages of using composites in any application: (Anon., 2020) (Gautam, 2020)

• The primary explanation for the selection of composites is their improved specific strength and stiffness.

• Composites have low density when compared with other bulk layups.

• Applications, mainly marine, require rust-free materials, and the composite layups are one of them.

• Composites are perfect thermal insulators.

• Composite's design allows for freedom of architectural form.

With many advantages of composite layups, there are few drawbacks of it (Clyne T W, 2019):

• The price of raw layups and the fabrication process are comparatively high. • Composite materials are brittle when compared with the wrought metals. • They have low properties through the thickness.

• It is often difficult to recycle most of the time because of the difficulty in separation of matrix and fiber.

• Repair of the composites is difficult as the structure loses its integrity once it is disintegrated.

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2.2 Composite laminates and its orientation

Composite laminate, also called composite plies, are the assembly of layers of composite layups, which consists of high-modulus, high-strength fibers in polymeric, metallic or ceramic matrix layup.

Based on the thickness, the standard lamina can be divided into 2 categories. • Thick ply

• Thin ply

The standard plies typically have a thickness of ~125μm and areal weight of ~125g/m2_{, or higher. Any standard ply can be considered thick when the thickness}

and the areal weight of the fiber (the weight of the fiber per unit area) exceeds ~200μm and ~200g/m2_{(Galos, 2020).}

Thin plies can be broadly defined as those, whose thickness <100μm and ply areal weights <100 g/m2_{. Most applications of thin-ply composites have been in}

relatively thin aerospace structures and high-performance sport structures and in other various applications (Galos, 2020).

Each laminate is oriented in a direction where the laminate property in the whole is enhanced (Campbell, 2010). The fiber orientation directly impacts the mechanical properties; it seems logical to orient as many layers as possible in the load-carrying direction. There are usually four directions of orientations 0°, 90°, 45°, -45°.It can also be combination of either of those directions. Figure 2 shows the orientation of fibers in multi-direction.

In this case, the direction of the fibers in the outer plies is selected as the direction of reference, the direction, and the middle ply, which is

almost twice as thick, whose fibers are oriented to the direction of reference. The laminate is marked as [0/90]S, where 'S' is symmetric.

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Figure 2: Orientation of composite layups in [0/90]s direction

2.3 Manufacturing of composite layups

There are many ways of manufacturing the composites, depending on their application. This thesis work mainly concentrates on manufacturing the composite structures using prepregs. Prepreg is a term for a reinforcing fiber that has been pre-impregnated with resin as a matrix system. Prepreg layups are prepared under very highly controlled conditions that yield high-quality prepregs and excellent control over the fiber volume fraction, i.e. the number of fibers in relation to the matrix. The fiber volume fraction has a significant effect on the layup properties of a manufactured product, and since the fibers provide the strength in a composite material (Björnsson, 2017).

Once the fibers are impregnated in the resin, they are cut and stacked upon each other in certain orientation as required by the application and is then cured in a process called autoclave, which is the open molding process where the molded part is cured by the application of the vacuum and heat. A vacuum bag is built around the layup and sealed, and then they are kept in an oven at a specific temperature for a couple of hours, dependent on the requirements. Figure 3 shows the entire process used in the manufacturing process of prepregs.

Prepregs are mainly used in applications where the structural parts require excellent properties.

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Figure 3: Sequence of composite part manufacturing process using pre-pregs (Björnsson, 2017)

2.4 Composite failure modes

Any layup for that matter would have defects like voids and porosity, while or prior using them during the manufacturing process or in the service life of the component (Ghobadi, 2017). Following are the most seen defects. Figure 4 shows the common damages in composite layups.

A matrix crack typically found where there is a high stress concentration or is associated with thermal shrinkage during manufacture (Ghobadi, 2017)

.

Debonding is observed when an adhesive stop adhering to a substrate layup. Debonding occurs in the typical cases where the physical, chemical or mechanical forces that hold the bond together are broken.

Delamination is a failure in a laminate of a composite material, which leads to separation of the layers of plies. Delamination failure can be observed as several types, such as fracture within the adhesive or resin, a fracture within the

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7 reinforcement or debonding of the resin from the reinforcement (Greenhalgh, 2009).

A void is a result of imperfection from the processing of the layup and it is generally undesirable. Because a void is non-uniformity in a composite layup, it can affect the mechanical properties and lifespan (Greenhalgh, 2009).

Fiber kinking is the broken fiber in compression, and it is the initial failure mode. Porosity is caused by volatile entrapment during the curing of the resin.

Figure 4: Common damage types in composite laminates (Tual, 2015)

2.5 Bearing strength

In addition to mechanical tests used to determine layup properties of composite layups, tests to determine structural properties are commonly performed on multidirectional composite laminates. A subset of these are bearing tests, which is similar to the tensile tests and is mostly performed to determine the bearing stress at which failure occurs in a composite laminate, or the bearing strength, of mechanically fastened composite joints (Dzenan Hozic, Sofia Stenberg, 2018). Bearing strength is defined as the maximum stress load that the unit can bear before the structure fails (Anon., 2013). The bearing stress of the composite laminate at each data point is calculated using the equation below.

𝝈 = 𝑷

𝒌∗𝑫∗𝒉 (1)

Where σ is the bearing stress, P is the bearing force, k is the force per hole factor, D is the hole diameter, h is the thickness of the test specimen.

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8 Ultimate bearing strength F is calculated according to equation 2 below

𝑭 = 𝑷𝒎𝒂𝒙

𝒌∗𝑫∗𝒉 (2)

Where Pmax is the maximum applied bearing force before bearing failure (Anon.,

2013).

2.6 ASTM D5961, test for bearing response

Any mechanical testing process follows a particular standard. Here, two ASTM standards are available for bearing testing of the composites. ASTM D953, initially developed for plastics and ASTM D 5961 developed explicitly for polymer matrix composites. The thesis concentrates on the bearing strength assessment performed according to ASTM D 5961. This standard consists of 4 different procedures A, B, C, D. For this assessment, the bearing strength is performed according to procedure A, double-shear tensile loading shown in the Figure 5. According to this procedure, a flat, rectangular cross-section specimen with a centerline hole located at the end of the specimen is loaded at the hole in the bearing. The bearing force is added by loading the assembly in tension in a testing machine (Anon., 2013).

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2.7 Bearing failure modes in composite joints

Once the bearing test rig is loaded with samples up to a specific load, the specimens break, and there are few principal failure modes, which are also macroscopic modes of the bolted joints (Figure 6).

• Bearing failure • Tension failure • Shear-out failure • Cleavage failure

Bearing failure is developed by the forces in compressive direction acting between the hole surfaces and the fastener. It is a crushing mode that involves several micromechanical failure modes, usually starting with matrix cracking followed by a buckling of destabilized fibers. As loading continues to increase, delamination and kink bands start to occur. Kink bands leads to shear cracks progressing from the center of the laminate towards the surface at a 45◦ angle (Ali Najafi, 2009). Net-section or tension failure is governed by the tensile forces in the laminate and the stress concentration created by the fastener hole. Because of this, a crack is initiated at the hole edge, which usually leads to a sudden catastrophic failure of the joint (Ali Najafi, 2009).

The shear-out mode is associated with in-plane failure resulting in matrix cracking, fiber-matrix cracking and fiber breakage, which is the same as tension failure. Joints that fail in shear-out mode usually deform substantially before totally shearing off. (Ali Najafi, 2009)

The cleavage failure mode is often triggered from an incomplete net-tension failure and occurs in laminates with inadequate end distances and too few transverse (90°) plies. (Michael McCarthy, 2003).

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Figure 6: Macroscopic failure modes of composite joints (Ekh, 2006)

2.8 Bearing strength improvement methods

The bolted connections are commonly used in preference to the other joining techniques because they allow greater freedom in the assembly and repair and are unavoidable in many applications. But, when the bolted joints are introduced into a laminate, it introduces failure modes against the relatively weak laminate. (A Crosky, 2006) To improve the composite bearing strength, many alternatives are being used, which have been proved to be useful for many applications at present (lBülent Muratİçten, 2002). Usage of thin plies is one of the best ways to improve the strength. The “ply-size effect” is linked directly to complete suppressing of damage initiation and growth of transverse crack and delamination. This in turn improves the ultimate strength and much improved onset of damage (Robin Amacher, 2015). It has been observed that the bearing strength composite plate was significantly influenced by different parameters, which included the fiber orientation, the fastener and the layup parameters, the geometric parameters like width (W), edge distance (E) and the hole diameter (D) (lBülent Muratİçten, 2002). By taking the advantage of size effect and broader design aspects offered by thin plies, composite designers have an advantage of achieving much lighter structures, which is a key factor in a high-value market like aerospace.

2.9 Matrix burn off method

There are many uses of hybrid composite layups in aerospace, automotive applications because of its various advantages. The hybrid composites use a

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11 minimum of two different fibers to reinforce the resin, thus having the advantages of using both the fiber properties. Obtaining the best properties with those layups makes it very important that fibers are mixed correctly. Features like fiber mix ratio, the fiber and void content are to be monitored, but unfortunately, there is no easier way to determine all these features without doing the matrix burn off test (International, 2020).

The current burn off test is described in test method I-procedure G of the ASTM D3171 standards. Usually, the burns off specimens are measured, weighed and then placed in a pre-heated muffle furnace at around 565°C ± 30°C for not more than 60 minutes burns off the matrix and leave the reinforcement/ fibers behind (International, 2020). The fibers are then weighed. The required calculation for the fiber volume fraction, which is the percentage of fiber volume in the entire volume of a fiber-reinforced composite layup, is carried with the equation 3 (Foulds, 2019).

𝐕𝐟 =

𝐯𝐟

𝐯𝐜

(3)

Where, Vf= is the fiber volume ratio, vf=the volume of fibers, vc=volume of the

composites.

2.10 Transition zones of hybrid laminates and the

joints

There are many advantages of thin plies due to apparent reasons like better design of freedom, strength, etc. With thin plies in any applications, it takes a lot of time to manufacture, and this process becomes very expensive. Hence to reduce the usage of thin plies, a transition zone is designed with the combinations of thin and thick plies, also called a hybrid laminate with three different joining methods like a butt joint, overlap joint and edge reinforcement.

Generally, a butt joint is a technique used when two plies are joined by placing the ends together without overlapping. Overlap joint is one of the common types in which the composite ply edge overlaps over the other. Edge reinforcement is another method used for joining the thick and thin plies together.

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3 Methodology

3.1 Layups and manufacturing

In this study, three different materials, Textreme 80, Textreme 160, UD 100% with the combination of thick and thin plies counting to 5 layups in total and each measuring 135mm*36mm is used ( the layup, combination thin and thick plies, along with layup is mentioned in Annex A,I). Before a specimen is tested and examined under the microscope, a series of preparation stages are followed. Lay-up is a process where the base layLay-up is stacked Lay-upon each other in the required order and orientation. The process is continuously repeated until all the layers are assembled. Air bubbles are removed by flattening them using a rolling pin at each stage. Once it is assembled, it is cured using the auto-clave process for a couple of hours. Curing is done to meet performance requirements such as longevity, corrosion and fatigue resistance, low odor and more. Once it is cured, the specimens are cut according to the ASTM D5961 standards.

3.2 Specimen geometry

Each layup is initially cut with 300mm*300mm for the layup, and again each layup has about 7-9 specimens cut with the dimension of 36mm*135mm for the testing purpose after the layup is cured (Figure 7). There are five different layups with different fiber orientations in this thesis, manufactured with the process mentioned above.

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3.3 Testing of layups

The specimens that are cut, the bearing tests are performed according to ASTM D5961 procedure A, double-shear tensile loading, and the scope of the test method is to check the bearing and fastener failure modes. According to procedure A Figure 5), a flat constant rectangular cross-section test specimen (Figure 7) is loaded at the hole in bearing. The bearing force is created by loading the assembly in tension in a test machine. (Anon., 2013) . Each layup had up to 9 specimens tested to achieve statistically good values.

3.4 Mounting specimen for microscopy

Fragile layups, multiple samples, samples with surface features and thin samples require support to ensure a flat planar surface. Hence, mainly to obtain a planar surface and hold the specimens to obtain microstructure, epoxy resin, in this case, is used to impregnate and preserve the features of the damaged area. EpoFix is the layup that is being used in the Struers set-up, as shown in Figure 8. They are transparent, have good adhesion and are duroplastic when hardened. Initially, the resin and the curing agent/hardener are measured with the required consistency, mixed and then introduced into the cleaned mold to obtain suitable mounting. The mounting cups/molds with the specimen are placed inside a vacuum chamber, and the resin is poured using a manual controller. Once the mold is filled, it can cool at room temperature for about 12 hours as epoxy has a very low curing rate.

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3.5 Polishing

After mounting, polishing is performed. Polishing is done to create a flat, defect-free surface to examine the composite's microstructure under a microscope. This was done using the machine "Struers Tegramin-30", Figure 9, where the mounted specimens are subsequently polished using sandpaper varying from the size of 320-4000. It is made sure that the specimens are scratch-free to get pictures of good quality from the light optical microscope of further analysis.

Figure 9: Tegramin polishing machine

3.6 Microstructure

All the specimens which were encapsulated in the resin after polishing is looked at under Lecia microscope. The main objective is to check for the damages and its types and analyze the strength of the layups and characterize the modes of failure present in the tested specimens. Figure 10 shows the area where the microstructure is taken of the specimens to see the damage present.

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Figure 10: Area looked at the microscope for damage

3.7 Matrix burn off method

Since there was an issue with the thickness tolerances in some layup, each layup had an extra amount of resin. Hence, to solve this issue, a matrix burn test was conducted to cross-check the amount of fibers' present in each specimen. It was done by cutting the specimen, measuring its dimension. Its weight was measured before and after burning them in the muffle furnace with enough space for ventilation so that the harmful gases produced during the process escaped.

3.8 The transition zone of hybrid laminates

Usage of thin plies in any application takes a lot of time to manufacture as each ply measures ~40μm, reducing the overall efficiency of the process. Hence transition zones of both thick and thin plies are designed so that the thin plies are used only in places where more strength is required. This is done using different types of joints, as shown in Figure 11. Each layup had up to 7 specimens tested to achieve statistically good values.

In this case, towards the left is the thin ply and towards the right is the thick ply, as seen in Figure 11, and the layup sequence is showed in ANNEX H. All the layup is tested for tensile strength.

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16 b) Butt joint

c) Edge reinforcement

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4 Results and discussion

This section explains the bearing test results, the microstructure's damages, and the normalized comparison between all the layups.

4.1 Bearing test

The thesis's main aim is to investigate the bearing strength of all the five layups mentioned earlier in this report.

Layups used Onset force

(kN)

Onset

strength

(MPa)

Ultimate

bearing force

(kN)

Ultimate

bearing

strength

(MPa)

50% T80

13.7

602.6

22.1 1102.9

100% T80

10.9

704

20 1134.1

100% UD

8.9

515

15.9 1040

100% T160

11.3

595.4

20.3 1068.7

50% T160

9.3 _508.4

18.7 _909.1

Table 1: Onset force, strength, UBF and UBS values from the tests

Table 1 shows the result value of onset force, onset strength done on 9 specimens, ultimate bearing force and ultimate bearing strength done on 7 specimens of all the five layups. Out of all the five layups, 100% UD is kept as a reference or the baseline and then is compared with other layups. From the table 1 it is seen that the layup 100% T80 has higher ultimate bearing strength which is 1134 MPa and the highest onset strength, with 704 MPa when compared with the reference and other layups. Annex B shows the graph of yield force and ultimate bearing force for further reference.

All materials work best when used within its material tolerance. As discussed earlier, the value of the volume of fiber deviated from the expected tolerance and this difference can be seen in table 2. Hence the volume of fiber was experimentally measured, and a factor was used in the calculation of final results. This was done by measuring the specimen and weighing them before putting the specimens in the oven and again after removing them. Once the specimen was removed out of the oven, the matrix was burnt, and this left behind the fibers. The volume of the fiber is calculated using equation 1.

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18 Materials used Fiber volume

fraction (theoretical) Fiber volume fraction (experimental) Factor (experimental)

50% T80

51.61%

53.4%

0.9 100% T80

47.3%

48.9%

0.83 50% T160

51.61%

59%

1 100% T160

54.5%

59.2%

1 100% UD

56.9%

58.8%

1

Table 2: Theoretical and experimental values of calculated fiber volume fraction

Figure 12 shows the example of bearing stress/ strain curve where the test is done on one of the samples loaded up to 70%, 80% and 100% of each layup's ultimate load/strength. This difference between 70% and 80% was done to see the difference between the dip in the stress/strain curve. Unfortunately, it was not possible. Figure 13 and Figure 14 show the graph of onset strength (MPa) and ultimate bearing strength (MPa), respectively of all five layups, used obtained after the compensation made due to matrix burn off test as shown in ANNEX A. It is seen from the compiled data that the values of onset strength and the UBS of 100% T80, with completely thin plies, is the highest of all. Although 100% T160 is made of completely thin plies, the value obtained for onset strength and the UBS is lesser than that of 100% T80; this is because of the orientation and the thickness of each ply in ANNEX B.

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Figure 12: Bearing stress/strain curve of 100% T80 loaded at 70%, 80% and 100% of its ultimate load.

Figure 13: Onset strength of all layups

0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00 900.00 1000.00 0.0 1 0.3 1 0.5 6 0.8 6 1.0 8 1.2 9 1.4 9 1.7 0 1.9 0 2.1 9 2.5 2 3.0 9 3.4 4 3.7 5 4.0 7 4.5 0 4.9 4 5.2 8 5.6 8 6.1 6 6.5 1 6.9 0 7.2 7 7.6 7 8.1 4 B e ar in g st re ss (M Pa) Bearing strain (%) 100% 80% 70% 602 704 508 595 515 0 100 200 300 400 500 600 700 800 O ns et s tre ng th ( M P a ) 50% T80 100% T80 50% T160 100% T160 100% UD Ultimate bearing strength

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Figure 14: Ultimate bearing strength of all layups

Figure 15 shows the graph of the ratio of onset strength to ultimate bearing strength, and ANNEX C shows the graphs of yield force and the ultimate bearing force. It is seen that 100% T80 has the higher ratio i.e., the onset strength of the layup is 62% of the ultimate strength, which is an excellent value as the layup can take more load within the elastic limit when compared to the other layups and this is one of the most significant advantages of using the thin plies. All other Textreme layups, 50% T80, 50% 160, 100% T160, fall under the same group as they have the onset strength 55% of its ultimate strength. This percentage reduces when the thick plies are added, in the case of 50% T80 and T160.

Figure 15: Ratio of onset strength to ultimate bearing strength for all the layup

1102 1134 909 1068 ₁₀₄₀ 0 200 400 600 800 1000 1200 Ult im a te bea ring s tre ng th ( M P a ) 50% T80 100% T80 50% T160 100% T160 100% UD 54.64% 62.08% 55.92% 55.71% 49.52% 0.00% 10.00% 20.00% 30.00% 40.00% 50.00% 60.00% 70.00% O ns et /UB S ( %) 50% T80 100% T80 50% T160 100% T160 100% UD

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4.2 Composite damages

The microstructure is analyzed using a light optical microscope. To observe the internal damage near the hole in detail, the specimens were cut along the centerline of the hole, and the images are taken through the thickness sections of two specimen which were not loaded up to its ultimate load. As seen in Figure 10, the samples that end at 70% and 80% are the point where it is looked for damage. Figure 17 and Figure 18 show the different types of damages present in the microstructure.

For the comparison, the specimens were loaded with respect to the ultimate load. Therefore, the amount of damage studied also relates to each laminate’s ultimate strength. The actual load refers to 70 or 80% (77% in one case) of the loads in Table 26, ANNEX C. Since this work has shown that the onset of damage level varies for different laminates, shown in Figure 15, it should be noted that the comparison in this chapter relates to the ultimate strength, not adjusted to each laminate’s onset of damage level.

The significant type of bearing failure modes witnessed in this thesis is bearing failure and the net tension failure as shown in Figure 16. Apart from these, another typical composite damage present in all the layups after testing is fiber kinking, marked in Figures 17. Fiber kinking/ fiber pull-out/fiber fracture is one of the failure mechanisms towards the final failure in any fiber-reinforced composite materials. It is a crucial mode of damage. It is caused due to fiber breakage. It slowly causes the composite to fail through buckling. The second significant defect present is delamination, also marked in Figure 17 and Figure 18, mainly because of fracture within the reinforcement or debonding of the resin from the reinforcement. The other damage present is the matrix crack, which is usually caused due to high-stress concentration. There are significantly fewer voids or porosity found in the microstructure. ANNEX D and E show the damages present in all other layups loaded at different percentages of its ultimate loads.

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a) Bearing failure

b) Net tension failure

Figure 16: Bearing failure modes in material 50% T80 loaded up to 80 % of its ultimate load

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Figure 18: Fractography of 100% T160 and the damages caused at 16.4kN and stress of 825MPa

4.3 Normalized microstructure results

Figure 19 and Figure 20 show the results of fiber kinking, which are normalized by the number of zero layers. It simply depicts the average number of fiber kinks present in each layup as it goes away from the hole.

The microstructure of all the material layup is divided into seven zones, which is an equal distance of 0.5 mm each, as shown in ANNEX F and G. The number of fiber kinks in each zone is counted. The graph is plotted with the sum of total kinks/ 0° layers versus the equidistant zones as it goes away from the hole.

It is seen in the graph that every layup follows a typical pattern where there is a downfall in the number of kinks present as it goes away from the hole. The Textreme material 100% T80 has a smaller number of kinks when compared to other layups.

Although there is no considerable difference between the specimens loaded up to 70% and 80% of its ultimate load, Figure 20 also follows the same pattern as in Figure 19. It also has to be noted that in few specimens like 50% T160 in figure 18,

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24 few fibers are missing in the top and bottom layer, which gives a flat line in the curve up to a certain distance, and this does not mean that 50% T160 is better than 100% T80 which is wholly made of thin plies.

Figure 19: Normalized results of fiber kinking in all the layups which were loaded up to 77-80% of its ultimate load

Figure 20: Normalized results of fiber kinking in all the layups which were loaded up to 70% of its ultimate load

0 0.2 0.4 0.6 0.8 1 1.2 0.5 1 1.5 2 2.5 3 3.5 N fk/N 0 ° 50%T80 100% T80 50% T160 100% T160 100% UD

Length from the hole (mm)

0 0.2 0.4 0.6 0.8 1 1.2 0.5 1 1.5 2 2.5 3 3.5 Nfk/N 0 ° 50%T80 100% T80 50% T160 100% T160 100% UD

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25 The Figure 21 shows the result of delamination that was normalized for all the layups loaded up to 70% and 80% of its ultimate loading. The normalized values of delamination are calculated using equation 4 which is obtained by counting the total length of delamination over the span of 3.5mm of each microstructure as mentioned in ANNEX F and G times number of layers in each specimen.

𝑻𝒐𝒕𝒂𝒍 𝒅𝒆𝒍𝒂𝒎𝒊𝒏𝒂𝒕𝒊𝒐𝒏 𝒅𝒊𝒔𝒕𝒂𝒏𝒄𝒆

𝑻𝒐𝒕𝒂𝒍 𝒍𝒆𝒏𝒈𝒕𝒉 𝒐𝒇 𝒕𝒉𝒆 𝒎𝒊𝒄𝒓𝒐𝒔𝒕𝒓𝒖𝒄𝒕𝒖𝒓𝒆∗ 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒍𝒂𝒚𝒆𝒓𝒔 (4)

Although there is not much of difference between 70% and 80% of the loading, It is visible that specimens loaded up to 70%, 80% of 100% T80 has better values, i.e., less delamination when compared to all other layups and 100% T160 has higher delamination of all layups.

Delamination is a failure, which basically leads to separation of the layers of reinforcement or plies. This is avoided by having a better manufacturing process and better quality of matrix that are used.

(for a better comparison, ANNEX J represents the results obtained before normalization.)

Figure 21: Normalized results for delamination of all layups which were loaded up to 70% and 80%

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 50% T80 100% T80 50% T160 100% T160 100% UD N o rm al ize d d e lam in atio n len gth ( m m ) 80% 80% 80% 80% 80% 70% 70% 70% 70% 70%

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4.4 Transition zone of hybrid laminates

All the three specimens with the transition zone shown in the Figure 11 is loaded into the tensile testing rig and tested for its tensile strength. Out of seven specimens, five were loaded up to its ultimate load and the rest were loaded up to 70% of its ultimate load to check for the damages. Table 3 shows the results of tensile test and it is clearly seen that the layup with edge reinforcement has comparatively very high tensile strength of 843 MPa when compared to other layups. Figure 22 shows the tensile stress/ tensile strain graph of one of the joints and is clearly seen that the specimens do not show any onset of damage hence it was difficult to see the damage growth. Figure 23 shows that there is no evident damage present in the specimen as there is no onset of damage. Also, because there were no evident damages found, from the specimens that were broken during testing phase, it can been seen in Figure 24 that it is weak near the joints for butt and lap joint and for edge reinforcement, it is weak where there are thick plies. Materials used Maximum load

(kN)

Tensile stress at max load (MPa)

Total thickness of layup (mm) UD 128+ T160, lapjoint 38.8 502 3.1 UD 128+ T160, buttjoint 38.19 501 3.04 UD 128+ T160, edge reinforcement 32.73 843 1.55

Table 3: Tensile stress and maximum load values obtained from the tensile test

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Figure 23: Microstructure of the specimen with edge reinforcement

a) Lap joint

b) Butt joint

c) Edge reinforcement

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5 Conclusion

The rectangular test specimens with different layup sequences were loaded in bearing through the fastener using a double lap shear loading setup. It was done on four different Textreme layups 50% T80, 100% T80, 50% T160, 100% T160 and with one Oxeon layup 100% UD as reference/base layup. Comparing all five layups, 100% T80 outperformed other layups with the highest onset strength and the ultimate bearing strength.

Specimens manufactured with the transition zone using UD and T160 material with edge reinforcement used as a joining method has the highest tensile strength of 843MPa.

All the layups were designed to be as close as possible with the same areal weight and total thickness, but from the value of strengths obtained, it can be believed that the strength of hybrid composites mainly depends on the amount of reinforcement and their orientations. And it is also clear that layup with thin plies gives much better results; hence it can be concluded that usage of thin plies improves the elastic limit of the specimen.

Although the manufacturing of thin plies for aerospace applications is more expensive, it can be designed in a way such that, there is a transition from thick to thin plies i.e., use the thin plies where there is more stress concentration due to joining using bolts.

Apart from the strengths obtained from the bearing test, fractography also reviled that there are less defects in 100% T80 and the common types of damages present in the microstructure is limited to fiber kinking, delamination and the matrix cracks. Other damages like voids and porosity is very minimal which indicates the manufacturing process used is a good method. Also, from the stress/strain curve, it is seen that there are no different failure modes.

This work's merit includes the nature of laminate used, method of hybridization, and parameters selected for characterization. It will undoubtedly pay the way for superior performance in the usage of any application.

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6 Reference

A Crosky, D. K. e. a., 2006. Improvement of bearing strength of laminated composites. Adam Quilter, 2020. Composites in aerospace applications, CO, USA: IHS Corporate . Ali Najafi, M. G. F. A., 2009. Failure Analysis of Composite Bolted Joints in Tension. p. 8.

Anon., 2013. ASTM D 5961. Standard Test Method for Bearing Response of Polymer Matrix Composite

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Available at: https://www.mar-bal.com/applications/advantages-of-composites/ Anon., 2020. Composites UK/Introduction. [Online]

Available at: https://compositesuk.co.uk/composite-materials/introduction

Björnsson, A., 2017. Automated layup and forming of prepreg laminates, Linköping: Linköping university. Campbell, F. C., 2010. Structural composite materials/Introduction to composite materials, s.l.: s.n. Clyne T W, D. H., 2019. An Introduction to Composite Materials. 3rd edition ed. s.l.:Cambridge university press.

Dzenan Hozic, Sofia Stenberg, 2018. Bearing strength assement of composite material, Linköping: s.n. Ekh, J., 2006. Multi-Fastener Single-lap Joints in Composite Structures, Stockholm: s.n.

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30 Robin Amacher, W. S. e., 2015. New design opportunities using thin-ply composites, s.l.: JEC composite magazine.

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ANNEX A

This section shows the table and the values used for fiber volume calculation. layup Thickness (m) Length (m) Width (m) Volume of composite (m3₎ weight before burn(kg) weight of fiber after burn (kg) 50 % T80 0.00333 0.03586 0.02519 3.00803E -06 0.004515 0.0028963 100% T80 0.003915 0.03596 0.02571 3.61954E -06 0.005337 3 0.00319 50% T160 0.002932 0.03597 0.02611 2.75367E -06 0.004282 0.0029247 100% T160 0.003158 0.03603 0.02492 2.83547E -06 0.004388 0.0030254 100% UD 0.003022 0.036 0.02557 2.78181E -06 0.004375 7 0.0029455

Table 4: Values of the specimen used before and during matrix burn test

density of composite(kg/m3₎ Volume of composite (m3₎ volume of fiber (m3₎ ratio =Vf=vf/vc

50% T80 1.35813E-08 3.00803E-06 1.60906E-06 53.49% 100%

T80

1.93186E-08 3.61954E-06 1.77222E-06 48.96% 50%

T160

1.17912E-08 2.75367E-06 1.62483E-06 59.01% 100%

T160

1.2442E-08 2.83547E-06 1.68078E-06 59.28% 100%

UD

1.21724E-08 2.78181E-06 1.63639E-06 58.82%

Table 5: Calculation of fiber volume fraction

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ANNEX B

This section shows the Layup sequence of each layup used and its orientation.

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ANNEX C

This section shows the graphs of onset and ultimate bearing force of all the layups.

Figure 25: Yield force of all the layups

Figure 26: Ultimate bearing force of all the layups

13753 10951 9300 11355 8944 0 2000 4000 6000 8000 10000 12000 14000 16000 Y iel d f or ce (N) 100% T80 50% T80 100% UD 100% T160 50% T160 22153 20055 18773 20361 15986 0 5000 10000 15000 20000 25000 U lti m ate b ear in g for ce (N) 100% T80 50% T80 100% UD 100% T160 50% T160

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ANNEX D

This section shows the microstructure of all the specimens with the different kinds of damage present and which were loaded up to 77-80% of its ultimate strength. (the other pictures loaded up to 80% of the load are in figure 17 and 18)

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Figure 28: Fractography of 100% T80 and the damages caused at 18.2kNand stress of 750MPa

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ANNEX E

This section shows the microstructure of all the specimens with the different kinds of damage present and which were loaded up to 70% of its ultimate strength.

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Figure 31: Fractography of 100% T80 and the damages caused at 16kN and stress of 660MPa

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Figure 33: Fractography of 100% T160 and the damages caused at 14.5kN and stress of 734MPa

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ANNEX F

Normalized microstructure results of specimens which were loaded up to 80% of its ultimate load and the microstructure was divided into zones of 0.5 mm to check for damages like fiber kinking and delamination.

Figure 35: Normalized microstructure of 50% T80

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ANNEX G

Normalized microstructure results of specimens which were loaded up to 70% of its ultimate load and the microstructure was divided into zones of 0.5 mm to check for damages like fiber kinking and delamination.

Figure 40 : Normalized microstructure of 50% T80

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ANNEX H

This section shows the layup sequence of all the layups and its orientation for transitions zones of hybrid laminates.

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Table 12: Layup sequence and orientation used in the transition zone using butt joint

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ANNEX I

This section shows the layup sequence, compositions and thickness of each plies used. Materials 100% T80 50% T80 100% T160 50% T160 100% UD Layers (no.) 40 30 20 21 24 Measured thickness (mm) 3.95 3.33 3.15 2.93 3.02 Compositions 100% T80 50% T80+ 50% UD 100% T160 50% T160+ 50% UD 100% UD The thickness of ply (mm) ~40 µm ~80 µm ~128 µm Layup sequence [((0/90)/(4 5/-45))10]s [((0/90)/45/(0/ 90)/-45/(0/90))3]s [((0/90)/(4 5/-45))5]s [((0/90)/45/-45/(0/90)/45 /-45/(0/90))1, 5]s [(0/90/45 /-45)3]s

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ANNEX J

This section represents the results of onset force, strength, UBF, UBS of all the layups obtained before normalization for more reference.

Figure 45: Onset force of all layup before normalization

Figure 46: Onset strength of all layup before normalization

10951 13753 8944 11355 9300 0 2000 4000 6000 8000 10000 12000 14000 16000 Onset for ce (N) 50% T80 100% T80 50% T160 100% T160 100% UD 548 586 510 600 515 460 480 500 520 540 560 580 600 620 On set st re n gth ( M Pa) 50% T80 100% T80 50% T160 100% T160 100% UD

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Figure 47: Ultimate bearing force of all layup before normalization

Figure 48: Ultimate bearing strength of all layup before normalization

20,055 22,153 15,986 20,361 18,773 0 5000 10000 15000 20000 25000 Ul tim ate b e ar in g fo rc e (N ) 50% T80 100% T80 50% T160 100% T160 100% UD 1003 944 912 1077 1040 800 850 900 950 1000 1050 1100 Ul tim ate b e ar in g st re n gth (M Pa) 50% T80 100% T80 50% T160 100% T160 100% UD

Bearing strength and failure behavior of hybrid composite laminates.