Federica De Magistris* and Lennart Salmén STFI, Swedish Pulp and Paper Research Institute Inc.
Stockholm, Sweden e−mail: fde@stfi.se Department of Mechanics
Kungl Tekniska Högskolan, Stockholm, Sweden
ABSTRACT
Summary The Iosipescu method is a method used for testing of unidirectional composite material in shear. In our case tests using a modified Iosipescu shear-compression fixture were performed on wood.
To test the reliability of the modified device a Finite Elements Analysis of the test, first in shear, was carried out and the results compared with the ones from experiments done on a particle board. The comparison will in the future be continued to incorporate the combined case of shear and compression.
1 Introduction
In mechanical pulping, wood fibers are repeatedly sheared and compressed at high temperatures in order to make them suitable for use as a papermaking pulp. The idea of this project is to study a combination of shear and compression of wood in order to determine under what conditions improvement of fiber collapse could be gained.
Figure 1. New test setting for the Iosipescu shear test device
In order to be confident on the results obtained with this new device, Fig 1, and to better understand the results a Finite Element model of the sample testing has been made and will be developed along with the tests. Shear test have been performed on particle board and the results have been compared with the solutions of the FE models
2 Finite element analysis
To investigate if proper deformation fields were reached by wood in the modified combined shear-compression Iosipescu device a Finite Element analysis has been performed. The software ABAQUS 5.8 was used. Three different 2D models [3,4,5], proposed in the literature to represent the Iosipescu device, with different load and boundary conditions, were analysed in order to choose the one that best represented the mechanical behaviour of the sample. For all models homogeneous and orthotropic materials were analyzed.
Figure 2. Models for the Finite Elements Analysis of the Iosipescu test (force couple, left; symmetric loadings block, right; asymmetric loading blocks, bottom)
3 Material and methods
The first step of the FE model has been to modelize a wood sample subjected to simple shear. The material model considered was wood viewed as an orthotropic material. The material constants were extrapolated from experimental data assigned as “orthotropic particle board” and are reported in Tab. 1.
From experimental data GRT has been determined while all the other caracteristic constant were, for the moment, estimated from this value in relation to typical characteristic of wood. The particle board was used in order to have a model of an orthotropic wood material without the annual ring structure of wood but with comparable material properties.
Table1. Characteristic constant chosen for the FE analysis. Young and shear moduli are given in units of MPa
ER ET EL GRT GRL GLT νRT νRL νLT
FE model (orthotropic particles board )
11250 562,5 900 80 803 755 0,43 0,36 0,52
Different load and constrain conditions were analyzed and compared for the three different models. To check the results of these models, shear tests on an “orthotropic particle board” wood material was performed. Four samples were tested twice in shear to a load of 30 N both in “RL” and “RT” direction, the denomination was used to characterized the different cutting orientation of the sample from the particle board. The displacements where registered by Electronic Speckle Photography (ESP). By spraying black paint in a fine pattern on the wood previously painted in white the local displacements were possible to measure.
4 Results and discussion
Exy in particle board sample (30 N)
-0,006 -0,005 -0,004 -0,003 -0,002 -0,001 0
l3 l4 l5 l6 l7 l8 l9 l10 l11 l12 l13
1 (RT ) 2 (RT ) 3 (RL)
4 (RL)
1 (2)
Figure 3. Strain field of the “particle board”, samples tested at 30 N
In the following figures shear values from the experiments and from the analysis are shown, Fig. 3 and Fig. 4 respectively. For all the cases the shear values obtained at a load of 30 N are shown. The shear values are calculated in the section between the notches.
Figure 4. Calculated strain field from Finite Elements Analysis of the “particle board” samples at a displacement of 0.04 mm. (RT orientation, left and RL orientation, right)
As seen from the calculation of Fig. 4 the shape of the curves as well as the difference in shear between the RT and RL orientation are in agreement with the experimental results of Fig. 3. However the FE calculation are not in good agreement with the experimental data. This can bee due to the fact that the characteristic constants of the material are extrapolated from the shear modulus obtained from the test.
Thus more experimental characteristic of the particle board are needed.
The model that represents the behaviour of the sample during the shear test in the best way was the one with asymmetric loadings blocks, see Fig. 2; in this model in fact theshear filed has the same shape that is founded in literature [5]. For particle board the results of the shear at a load of 30 N are shown in Fig.
6.
Figure 6. Shear field for the model with asymmetric loading blocks at 30 N 5 Conclusions
A Finite Element model with asymmetric loading blocks showed a solution closest to what is expected in on Iosipescu shear test, both in the shape of the strain field and in its absolute values. This model is chosen as a base model and will be further developed in a 3D model with introduction of a material model for the wood microstructure.
The mechanical behavior of wood is not jet fully understood and to make a proper model of its behavior, precise data for the FE analysis on simple deformation modes of wood will be needed.
REFERENCES
1. D. F. Adams and D. E. Walrath. Further Development of the Iosipescu Shear Test Method. Exp. Mech., 27 (2): 113-119, (1987).
2. J. A. Barnes, M. Kumosa and D. Hull. Theoretical and Experimental Evaluation of the Iosipescu. Shear Test. Comp. Sci. & Technol., 28: 251-268, (1987).
3. H. Ho, M. Y. Tsai, J. Morton and G. L. Farley. Numerical Analysis of the Iosipescu Specimen for Composite Material. Comp. Sci. & Technol., 46: 115-128, (1993).
4. M. Grédiac, F. Pierron and A. Vautrin. The Iosipescu In-plane Shear Test Applied to Composites: A New Approach Based on Displacements Field processing. Comp. Sci. & Technol., 51: 409-417, (1994).
5. Y. M. Xing, C. Y. Poon and C. Ruiz. A Whole-field Strain Analysis of the Iosipescu Specimen and evaluation of Experimental Errors. Comp. Sci. & Technol.,47: 251-259, (1993).
Modelling of Adhesive Joints in Timber Engineering
Erik Serrano and Per Johan Gustafsson Division of Structural Mechanics
Lund University, Lund, Sweden e−mail: erik.serrano@byggmek.lth.se
ABSTRACT
Summary The work presented concerns the modelling of adhesive joints in timber engineering applications. Numerical studies on the strength of various types of joints were performed. To obtain input data for the theoretical models used, experimental studies on the fracture behaviour of wood adhesive bonds where performed. The results show that wood adhesive bonds can be considered as being quasi-brittle, showing softening behaviour. Furthermore, strength, stiffness, fracture energy and shape of the softening curve are all important parameters for the strength of wood adhesive bonds.
Wood-adhesive joints play an important role in modern timber engineering. In order to add value to the raw material, several highly engineered wood-based products have been developed.
Often these involve the use of adhesive joints. Typical examples of such reconstituted materials are glued laminated timber (glulam) and laminated veneer lumber (LVL), Figure 1. In each of these, adhesive joints are used both for lengthwise splicing and for interlaminar bonding.
Another example of an adhesive joint application in timber engineering is that of glued-in rods which allow stiff and strong beam-to-column connections or column foundations to be obtained.
Figure 1: Phenol-resorcinol adhesives (left) are often used in the production of glulam (right).
In order to fully understand and model the behaviour of such structural elements as glulam beams, one must also understand the behaviour of their adhesive bond lines. Although adhesive bond lines often represent only a very small part of a structural component, they are often crucial parts for the strength and the reliability of the structural component. A typical adhesive bond line in timber engineering has a thickness in the range of 0.1–1 mm This is several orders of magnitude smaller than the scale of the structural components, one of approximately 0.1–
10 m.
The work presented is an overview of a recently completed research project on wood adhesive joints [1] and concerns experimental and numerical studies of mechanical behaviour on both the above-mentioned scales. Also, methods for bridging the gap between the two scales, making it possible to incorporate knowledge of the mechanical behaviour of a thin bond line into analysis on the structural-component-size scale are discussed. Applications such as finger-joints, glued-laminated timber and glued-in rods are considered, Figure 2. The experimental studies include the testing of the fracture characteristics of wood-adhesive bonds, including both wood-to-wood bonds and glued-in rods of either steel or glass fibre reinforced polyester. The numerical studies relate to the strength of finger-joints, laminated beams and glued-in rods for timber structures.
Figure 2: A finite element model of a glued-in rod.
The complete stress-displacement response of small specimens, particularly their fracture softening behaviour beyond peak stress, was recorded experimentally. Such responses were subsequently used as input to constitutive models based on nonlinear fracture mechanics and damage mechanics. The numerical analyses performed with these models show that the load-bearing capacity of wood-adhesive joints is highly influenced, not only by the local strength of the bond, but also by the material stifnesses involved, the geometrical shape of the adherends, by the fracture energy of the bond line and by the shape of the nonlinear stress-displacement relation.
REFERENCES
[1] Serrano, E. Adhesive joints in timber engineering. Modelling and testing of fracture properties. Doctoral Thesis. Report TVSM-1012. Division of Structural Mechanics, Lund University, Lund Sweden, 2000.