Fulfilled R&D projects

Finite element meso-scale stress analysis of stitched glassfiber UD-laminates

Keywords composite material, glassfibre, stitching, bundle waviness, local volume factor, stress concentration, homogenization
Programs in use ANSYS, SolidWorks

CompMechLab in collaboration with Research Center YTI, Mikkeli University of Applied Science by request of Ahlstrom company carried out finite element analysis of stress concentration in stitched glassfiber composites.

The global aim of the study is to compare the fatigue behavior differences of glassfibre composites with different stitching parameters. Research included a FEM simulation (CompMechLab), an analytical study and an experimental study (YTI) for three laminates with different stitching parameters.
 
The object under research is a laminate made of stitched unidirectional E-glass NCF reinforcement, 0°/90°/M and epoxy resin. The heavy 0° yarns form the actual reinforcing layer, while the lightweight 90°- and CSM layers exist to increase handling stability. Stitching yarn combines these three layers. The laminate consists of two plies of this material, CSM layers being placed outwards while 0° yarns are in the middle. The laminate is vacuum infused in stiff mould with epoxy resin. The stitching can be made dense or sparse with stitching yarn tight or loose. These stitching parameters affect not only the handling and manufacturing comfort but also the fibre bundle waviness and orientation. The research scope is whether this has effect on the strength of the laminate. The effect of these parameters was studied both with a FEM analysis and laminate fatigue tests.
 
     
Fig. 1. Solid model of composite Fig. 2. Photo of the bundle Fig. 3. Bundle solid model
 
Finite element analysis is performed on meso-level: fiber bundles and matrix are modeled, but not separate fibers. Bundle waviness, variable thickness and volume fraction are taken into consideration. Direct FE Homogenization procedure is performed for the glassfibre bundle. Effective elastic properties are defined based on periodicity cell of composite with hexagonal lattice.
 
     
Fig. 4. Composite cross section image Fig. 5. Hexagonal structure Fig. 6. 1/4 of the periodic cell

To account for bundle waviness, elastic coefficients tensor for each layer of finite elements along Z-axis was rotated. The meso-scale model also accounts for the effect of higher local fibre volume fraction by variation of local elastic moduli values. Bundle thickness is variable along specimen length because of different value of chopped strand tension. Variation of the bundle cross-section area leads to variation of glassfibre volume factor in the bundle. So, effective orthotropic properties of each layer of finite elements along Z-axis are calculated. Homogenization procedure is carried out for different fibre volume factors, and approximation with non-linear curves for all components of elastic coefficient tensor are obtained.

   
Fig. 7. Model with non-regular bundle geometry Fig. 8. Ez variation in Z-axis direction

Automated procedure (APDL-macros) of local elastic coefficients computation based on cross-section area and interpolation of homogenization results is developed and used for structural analysis. In total, over 400 materials with different orthotropic parameters are used in simulation.

   
Fig. 9. 3-D FE model with variable material properties (bundles only) Fig. 10. 3-D FE model fragment (interface bundle-matrix)

 FE model used for analysis contains around 5 mln degrees of freedom. Such a fine mesh is used to account for stress concentration due to bundles waviness and provide smooth variation of elastic coefficients along length.

Stress field obtained from simulation showed significant stress concentration due to geometry waviness and variable bundle thickness. The results were verified with fatigue tests. Both methods indicate that the less the fibre bundles geometry is disturbed, the better the fatigue properties will be.
 
   
Fig. 11. Stress concentration in teh fibre bundles Fig. 12. Stress concentration in matrix
 
Acknowledgements
The authors thank Ahlstrom Glassfibre and Miktech Oy for backing the research.
Other materials on topic:
Composite materials, composite structures