Arief Yudhanto, Graduate School of System Design, Department of Aerospace Engineering
INTRODUCTION
Delamination in composites has been a major issue because it causes significant reduction of strength. Impact loading may give rise to the delamination. One technique to improve composite’s resistance to the delamination is stitching. Stitching is able to improve delamination resistance by increasing the interlaminar strength of composites. |
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| Figure 1. Laminate subjected to impact and stitched composite. |
OBJECTIVES
My research aims to study the in-plane strength of stitched composites experimentally and numerically. |
MATERIALS AND EQUIPMENT
The material used in my research is carbon/epoxy composites stitched with Vectran® (liquid crystalline polymer with excellent tensile properties and minimum moisture absorption). Static and fatigue tension-tension tests are conducted using Universal Testing Machine Instron 8802. Effect of hole is of particular interest because holes are very common in aircraft structure to facilitate bolted-joints. |
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| Figure 2. Vectran-Stitched CFRP and testing machine. |
RESULTS
♦Static test ► It is found that stitches reduce tensile strength of carbon/epoxy composites by 8.4% due to undulation (fiber waviness), resin pockets and fiber breakage. However, stitched composite has better notched properties (open hole tension strength) than unstitched composites (Figure 3) because the stress amplification, that is usually endured by single hole, may partially be borne by stitch holes. |
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| Figure 3. Test piece and comparison on static tensile strength. |
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► In 2D composites containing circular holes, thickness increase generally results in a strength reduction. However, present investigation reveals that slight improvement of OHT strength in stitched composites (Figure 4). It turns out that thicker stitched composite has lower stress concentration factor and slower damage formation around the hole rim. These two factors are considered as the reasons underlying the OHT strength improvement. |
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| Figure 4. Thickness of test piece and static tensile strength. |
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♦Fatigue test ►
Tension-tension fatigue test is performed at various stress levels to plot the fatigue life (Wöhler or S-N curve) of stitched composites. The test is conducted at the stress ratio (R) of 0.1 and frequency of 2 Hz. Figure 5 shows that the fatigue life of stitched composites is comparable with that of unstitched composites. |
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Figure 5. S-N curve of stitched composites. |
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► It is commonly known that during fatigue test the stiffness of composites is gradually reduced. This is quite different with composite under static test, which undergoes minor stiffness reduction. The reason for the stiffness reduction is that some damage has been formed within composites. Ultrasonic C-scan is employed to observe the formation of delamination during early cycle (up to 10,000 cycles). Figure 6 shows that stiffness reduction in stitched composites is also accompanied with the formation of delamination. |
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| Figure 6. Stiffness reduction and formation of delamination. |
FUTURE WORKS
Future works would deal with computer simulation to predict in-plane stiffness and strength of stitched composites at microscale and mesoscale using homogenization method. Computational methodology is also developed to predict fatigue life of stitched composites under compression. |
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Figure 7. Microscale and mesoscale analysis. |
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