failure initiation and effect of defects in structural ... - Hexcel.com

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Loading Figure 10. Micrographs of two crack locations showing initiation at the chip end. The surface chips are well aligned to the load direction. The crack at PO2-3-LP2 is a shallow surface crack. The crack at PO2-4-LP3 formed between chip ends and propagated through chips at an off angle from the load direction. Loading Figure 11. Micrograph showing a shallow surface crack. The crack likely started at the surface voids.
3.2 Relation of First Crack to Static Tensile Strength The stress level at which the first audible crack occurs in DFC is not an accurate predictor of ultimate properties, unlike a first ply failure in CFC. We tested 24 HexMC specimens (ten samples from two separate panels, and four samples from a third panel) in static or step wise load cycling. 12 specimens were tested following ASTM 3039. Another 12 specimens were step cycled to monitor modulus changes due to increasing level of cracks; see the test plan in the Experimentation section. For a given panel, the average tensile strength from statically tested specimens was used as the expected strength for step cycled specimens. An event button was used to record the first crack heard for both static and step cycled specimen. Figure 12 shows a scatter plot of all results. Statistically, there is not a correlation between the stress when the first audible crack occurs and the ultimate stress of the sample. The data has a Pearson correlation strength of 0.362 and P- value of 0.082. Many of the samples with high ultimate strength exhibited the first audible cracks later than lower strength specimens, but it was not consistently the case. At best, there is a wide envelope in which the first audible crack could indicate the ultimate strength level. Later on, we explored the failure envelope for samples monitored with acoustic emission. Figure 12. Scatter plot showing the ultimate strength in relation to the stress when the first sound occurred. Each samples was 3.17 cm wide, average thickness of 0.338 cm and 56.5% fiber volume. Overall correlation: Pearson correlation strength 0.362, P-value 0.082. Static samples correlation: Pearson correlation strength 0.263, P-value 0.408. Step Cycled sample correlation: Pearson correlation strength 0.530, and P-value 0.077 Since detecting audible cracks is operator subjective, we also monitored 75 tensile specimens using an acoustic emission transducer. Specimens were 3.81 cm wide with a nominal thickness of 3 mm. Acquisition parameters were 500 KHz rate with RMS averaging over a 10 millisecond period. Since many first cracks were observed to form in areas with local chip misalignment to the load direction; it stands to reason samples with higher levels of chip misalignment should have cracks form at earlier load levels, and thus generate a detectable sound. We tested eight control specimens and specimens with deliberate defects to simulate off-axis chip concentrations. Samples with defects included:
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Page 15 and 16: Grip tab Sample 5 Front Figure 17.

failure initiation and effect of defects in structural ... - Hexcel.com

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