Chapter 4 Experimental Results
4.4 Evolution of Strain Increment Ratio at Four Points of Interest
4.4.1 Strain increment ratio
The SIR at four points of interest as a function of elongation is given in Figure 4.7. This face is subjected to an additional compressive stress after the onset of wrinkling due to the surface curvature, elucidating a more negative minor principal strain.
(a) (b)
(c) (d)
Figure 4.7 Strain increment ratio at point of interest on the concave surface of (a) Curv® [0°/90°]; (b) Curv® [45°/-45°]; (c) Twintex® [0°/90°]; (d) Twintex® [45°/-45°] samples.
Generally, the evolutions of SIR for all the nodes of interest in all samples follow a constant and similar slope before the onset of wrinkling. The initial SIR reflects the effective mechanical properties of each material system. With a higher effective young’s modulus or a smaller Poisson’s Ratio, the initial SIR will become less negative. For Curv® [0°/90°] and Curv® [45°/-45°] samples, the initial SIR for node 1 appears to be around -0.504 and -0.599, respectively.
The evolutions of SIR are constant at initial SIR until the onset of wrinkling. The onset of wrinkling is indicated by the abrupt change in SIR, elucidating the abrupt change in the slope of strain path. The SIR for node 1, 2 and 3 shows an abrupt decrease, because the additional compression caused by the wrinkling behaviour is added to the minor principal strain. The evolution of SIR for node 4 sees no abrupt change. This may attributable to that only a negligible amount of strain is developed in the nearby region, which fails to trigger the wrinkling instability.
During the post-wrinkling stages for wrinkled samples, SIR for node in wrinkled region (typically node 1, 2 and 3) continues to decrease to a more negative value. Then, SIR for different nodes stables at different values. Typically, the node 1 exhibits the most pronounced change in SIR during the onset and the growth of wrinkling compared to other nodes of interest. For example, the initial SIR for Curv® [0°/90°] samples at node 1, 2 and 3 are around -0.504, while the SIR after the abrupt change at node 1, 2 and 3 are stable at around -2.193, -1.799 and -1.220, respectively. The differences between the initial and final value of SIR for these three nodes are 1.689, 1.295 and 0.716 respectively. The result suggests the central region undergoes the highest state of compressive strain, has the most severe wrinkling and also exhibits the most
pronounced change in SIR. There seems to be a clear relationship between the magnitude of the change in SIR and the wrinkling severity. Therefore, in addition to indicate the onset of wrinkling, it is also possible to use the abrupt change in SIR as a metric to examine the severity of wrinkling during the post-wrinkling process.
The point elucidating the onset of wrinkling for each node falls into the pre-defined elongation period of wrinkling initiation. Abrupt change for different nodes occurs at similar elongations but not at the exactly the same elongations. It seems that SIR at the node of interest exhibits the abrupt change in a sequential manner, indicating that different node starts to wrinkle at different elongation. For example, in Curv® [0°/90°] specimen, the abrupt drop for nodes 1 and 2 occurs at 0.7mm and 0.8mm, respectively; while the drop for node 3 started at 0.9 mm. This sequential drop may be attributable to the wrinkling propagation, for wrinkling should initiate at the central region which subjected to the highest state of compressive stress before growing to less stress- concentrated regions.
For Twintex® [45°/-45°] samples, SIR seems to become a slightly less negative after the abrupt decrease during the post-wrinkling process, while the SIR for Curv® samples continue to decrease to a more negative value. This may attributable to the different size of woven structure and different consolidating process, leading to different compressive strain behaviour. For this reason, evolutions of SIR may not necessarily follow the same path for different materials; however, it is clear that no matter in which material system, an abrupt change in SIR is observed when the samples start to wrinkle.
Figure 4.8 gives the information about the evolution of SIR at four points of interest on the opposite face of the sample. The minor principal stress on this face becomes less
negative after the onset of wrinkling, because the induced compression is partially released due the onset of tensile effect caused by the surface curvature due to wrinkling.
(a) (b)
(c) (d)
Figure 4.8 Strain increment ratio at point of interest on the convex surface of (a) Curv® [0°/90°]; (b) Curv® [45°/-45°]; (c) Twintex® [0°/90°]; (d) Twintex® [45°/-45°] samples.
For wrinkled samples, the evolutions of SIR remain constant before exhibiting an abrupt increase, indicating the onset of wrinkling. Unlike the evolutions of SIR on the opposite face becoming more negative after the abrupt drop, the abrupt increase leads to a less
negative or even a positive SIR at large elongation. Due to the surface curvature introduced by wrinkling behaviour, the induced compression on this face of the sample is partially released, exhibiting a less negative minor principal strain and an abrupt rise in SIR. These results clearly illustrated that using the abrupt change in SIR for wrinkling indication can indicate the onset and the propagation of wrinkling on either face of the sample.
For the unwrinkled Twintex® [0°/90°] sample, the evolution of SIR does not exhibit any pronounced change, indicating no wrinkling starts before failure initiates. Owing to that it seems that using the abrupt change in SIR as a metric is capable of indicating wrinkling.
4.5
Summary
This chapter has studied the wrinkling behaviour on woven thermoplastic composite materials through Yoshida tests experimentally, with a particular focus on the onset of wrinkling. The elongation period for the onset of wrinkling is defined through analysing the out-of-plane displacement. It is found that the abrupt change in principal strain path can indicate the onset of wrinkling. SR and SIR are used as the metric to measure the abrupt change in strain path. Compared to SR, developing a wrinkling indicator based on SIR can capture more subtle change in strain path, offering an elevated robustness of the indication of the onset of wrinkling. This metric of using the abrupt change in SIR to predict the onset of wrinkling is the fundamental contribution of the present work.