Stanford University

MSC Software Aids in Stanford University's Study of Non-Homogeneous Composite Materials
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Traditional modeling of composite plies and laminates is almost always based on some degree of homogenization. A ply is often assumed to be homogeneous when constituents like fiber and matrix are smeared and replaced by a homogeneous ply. The body is then an orthotropic or transversely isotropic material in 2D or 3D.

By the same token, laminates consisting of multidirectional plies are smeared and replaced by a homogeneous body of orthotropic and anisotropic material. But this idealization is not so straightforward because the traditional stiffness matrix in [A], [B] and [D] can be individually orthotropic or anisotropic which do not have counter parts in a homogeneous material in 3D.

Such differences created among different models are being studied by using MSC software at Stanford University. One such approach is the use of micro- and meso-mechanics models that recognize local heterogeneity. With appropriate physical and geometric models, we were in the process to resolve new formulations of transformation properties and failure criteria which can no longer follow the traditional formulation. A new set of structural deformation and failure processes may emerge.

Of current practical significance is a novel bi-angle NCF (non crimp fabric) tape that can reduce automated layup process time by 62 percent. This phenomenal advancement results from 2-axis layup that is possible with the bi-angle tape as compared with the traditional 4-axis layup. Such panels however have discontinuous seams in their off-axis plies. It turned out that these seams have made the resulting composite laminate heterogenous. The traditional smearing of laminates cannot be applied directly. Classical transformation and appropriate failure criterion have to be treated with different formulation than the traditional ones for homogenous material. The meso-mechanical model using MSC software is expected to lead to an appropriate theory that will be expected to match the available strength data.

Specifically, we create 2D shell and 3D solid models of composite laminates incorporating the potential in-ply seam configurations. Both MSC.PATRAN and MSC.MENTAT are used for pre-processing the models. Thanks to specialized composite analysis capabilities within the MSC solver options, namely NASTRAN and MARC, we are able to address the failure characteristics of the laminates with discontinuities. Figure 1 represents the strength calculations based on MSC NASTRAN first ply failure (FPF) analyses.  Figure 2 is an example of 3D stress distribution. Our preliminary results indicate significant gain in strength recovery with staggering when the seams may be inevitable as in use of multiaxial NCFs.  Moreover, progressive failure analyses via MARC confirm the strength beyond the first ply failure load despite in-ply seams and discontinuities (Figure 3).

These sophisticated analyses and predictions are also essential and helpful to reduce extensive and expensive testing programs and better design the testing configurations.

Professor Tsai had this to say about MSC Software's effective capabilities. "We have found that MSC software has the combination of technical depth and easy of use. They made our challenge solvable. We were very pleased to be able to learn more about our problem and will continue to explore the next step," he said.

Figure 1: In-ply seam/discontinuity effect under uniaxial tension: Laminate failure index contours predicted (2D shell model solved in MSC.NASTRAN)
Figure 2: In-ply seam/discontinuity effect under shear: Stress distribution contours blow-out view of the continuous plies (3D solid shell model solved in MSC.MARC)
Figure 3: Progressive failure load-displecement curve under uniaxial tension (2D shell model solved in MSC.MARC)

About Professor Stephen W. Tsai
Professor Stephen W. Tsai is a Professor Research Emeritus in the Aeronautics and Astronautics department at Stanford University. He holds both a B.E. degree and D. Eng. Degree in Mechanical Engineering from Yale University. Professor Tsai is also part of the Stanford University Structures and Composites Laboratory and his research interests include the process and product development of composite materials that leads to improved design practice and commercialization. He has written two introductory texts on composite materials and two books on composites design and is known for the pioneering effort in promoting the use of spreadsheets as a design tool. He is also a member of the Nation Academy of Engineering, the American Society of Mechanical Engineering, and the Society of Aerospace Materials and Process Engineers and is an active participant in the International Conference on Composite Materials.

Dr. Stephen W. Tsai, Professor Emeritus, University of Stanford

Stanford University Structures and Composites Laboratory: