Tan Kwek Tze, Graduate School of System Design, Department of Aerospace Engineering
| Composite materials possess a unique property of being extremely strong and yet amazingly light. This makes composite materials hugely employed in modern aircraft structures, so as to improve fuel economy and extend flight range. However, composite materials have a major weakness which is their susceptibility to impact damage, resulting in cracks between interlaminar plies, commonly known as delamination. In our research, we overcome this major weakness by using an effective technique called stitching. Stitching technology is basically insertion of a strong thread fibre in the through-thickness direction of the composite materials, so as to suppress delamination growth. Stitched composites have huge potential for stronger yet lighter aircraft materials. However, the effect of stitching on the behavior of composite materials is poorly understood. There are three main objectives in this study. They are to understand the effectiveness of stitching in increasing interlaminar strength, enhancing impact resistance and improving compression after impact strength. |
Increased Interlaminar Strength
We conducted Double Cantilever Beam test (Figure 1 left) to measure the energy needed to create ply opening crack in a material. We also performed Interlaminar Tension Test to understand the fracture behavior of a single stitch thread. Computer simulation was then used to model the performance of stitched composites and to validate experimental results. In our computer simulation, we developed a novel stitch model that captured the behavior of stitches during ply opening crack. Results have shown that with increasing stitch thread thickness and closer stitch space, interlaminar strength, GIC, which is a measure of energy needed to create delamination per unit area, is increased significantly in stitched composites (Figure 1 right). |
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| Figure 1. Double Cantilever Beam Test (left); Increase in Interlaminar Strength (right) |
Enhanced Impact Resistance
We used drop weight test to impact specimens and analyzed their dynamic responses. From the impact force-time graphs, we examined the different dynamic responses due to different stitch parameters. We then employed ultrasonic C-scan analysis and x-ray radiography to detect and compare the size of impact damage hidden inside the specimens (Figure 2). We also used micro x-ray computed tomography system to understand the impact resistance mechanisms of stitched composites. We discovered that delamination is suppressed by crack arresting and crack bridging mechanisms. |
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| Figure 2. Ultrasonic C-scan Analysis (left); Delamination Reduction in Stitched Composite (right) |
Improved Compression After Impact Strength
Compression after impact (CAI) strength is an important design requirement for aircraft materials. The impacted specimens were subjected to compression test using Instron test machine (Figure 3 left). We studied and analyzed the compression response curves. We have successfully measured and confirmed the vast improvement of CAI strength due to stitching (Figure 3 right). C-scan and x-ray were also used to study fracture mechanisms of CAI specimens. |
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| Figure 3. Compression After Impact Test (left); Improvement in CAI Strength (right) |
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In this study, we have presented our results in 8 international and domestic conferences, as well as revealed the findings in 8 manuscripts published in international journals. From the good response received, we are convinced that stitched composites will make aircraft materials lighter yet stronger. I believe our research work has great importance in the advancement of composite research. |
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