97
The advantage of the FEM model is the contour plots of the output variables. Figure 4.6 and 4.7 are plotted for MAXSCRT to identify the distribution of damages in matrix material for the 1.45 mm (0.057 in.) deformation and for the thin and thick matrix respectively. Contour plots for the 1.45 mm (0.057 in.) deformation are presented because according to Figure 4.5, 1.45 mm (0.057 in.) deformation shows higher damage in the matrix material. One zoomed in section is shown for each loading pattern so that the contour of damage can be seen clearly. The color of contours ranges from blue to red; blue means small damage and red means large damage. Comparing Figure 4.6 with Figure 4.7, there are more red color regions for thin matrix than thick matrix, since thin matrix have higher damaged locations than thick matrix. Most importantly both adhesive and cohesive damages are occurred in dry and wet conditioned samples for thin matrix but mostly cohesive damage observed for thick matrix. Surely, thick matrix is stronger than thin matrix and carries more deformation before damage.
It is difficult to quantify damages in matrix and identified it to adhesive and cohesive damage under the triangular pattern for thin matrix by only observing and comparing the Figure 4.6(a) and 4.6(b). Similar scenario is also true for Figure 4.6(c) and 4.6(d) under the sawtooth pattern for thin matrix. For the rectangular pattern, cohesive damage at the top of matrix and adhesive damage at the bottom of matrix and near the interface are clearly shown in the Figure 4.6(e) and 4.6(f) for the thin matrix. Both dry and wet samples show cohesive and adhesive damages but wet sample shows more damage at the bottom of the matrix. Damage initiates under the deformation loading zone and at the top of the matrix. Most of the elements at the top of matrix damaged just after applying deformation. After initiating damage at the top of the matrix, it progresses towards the
98
bottom of the matrix and near the interface region since interface region is weakest in the whole domain. Damage progressed towards the bottom of the matrix and continues progression until every element near interface region exposed to damage; when no element is capable of taking any stress at the interface region, than the damage progressed to the second bottom layer since those elements are the weaker in the domain. It is clear that matrix material near interface region is the weakest and prone to damage under both dry and wet conditions for the thin matrix. In addition, the strength and stiffness under dry condition is higher than under wet condition; for this reason, dry condition sustain more deformation and carry more stress than wet condition at the bottom of the matrix and shows less damage. The elements near to the left side boundary conditions do not show significant damages because according to Eq. (4.3) pure compressive stress will not cause any damage in the matrix. Indeed those elements are under pure compressive stress.
Figure 4.7 presents MAXSCRT for 0.057 in. deformation load for the thick matrix. According to Figure 4.7 (a) to 4.7(b) the maximum MAXSCRT value is 0.13 and 0.18 for the triangular pattern under the dry and wet conditions respectively. In Figure 4.7(c) and (d), the maximum MAXSCRT value is 0.21 and 0.30 for the sawtooth pattern under the dry and wet conditions respectively. The MAXSCRT value less than 1.0 means no element exposed to damage but will damage with changes in deformation duration time or higher deformation magnitude. Figure 4.7(e) and 4.7(f) shows the maximum MAXSCRT value 1.0 for the top elements, means cohesive damage occurred for the rectangular pattern. It should be noticed that the minimum value of MAXSCRT are showing zero but this is not zero rather very small; the values are showing zero since the
99
MAXSCRT values are rounded up to two decimal points. The Triangular and sawtooth patterns do not show damage but they definitely shows the path of the stress flow from top the surface to the bottom surface of matrix. This stress flow and path from the top to bottom of matrix is not clearly visible for thin matrix as shown in Figure 4.6. Damage progresses from the top of matrix layer to the bottom of matrix layer following the stress path as show in Figure 4.7(a) to 7(d). The locations of the maximum MAXSCRT for both triangular and sawtooth patterns are on the surface and near 10.16 mm (0.4 in.) from the left support. The region shows the stress concentration at the top of the matrix and perpendicular stress path from the top of the matrix to the bottom of the matrix. This location is important because for both thin and thick matrix the path is similar. For thin matrix, when the cohesive damage occurred at the top of the matrix than this perpendicular path at the end of the loading zone is followed to initiate and progress of the adhesive damage at the bottom of the matrix. For the rectangular pattern in the Figure 4.7(e) and (f), this path is not present, since the cohesive damage initiates and dominates at the top of the matrix for entire duration. This stress concentration path is more visible when the deformation is ramped up like triangular and sawtooth pattern but not for rectangular pattern when the deformation jumps to maximum intensity in a very short time. Stress distributed evenly when load increase gradually with step time like the triangular or the sawtooth pattern.