PSI - Issue 19

Sara Eliasson et al. / Procedia Structural Integrity 19 (2019) 81–89 Sara Eliasson / Structural Integrity Procedia 116 (2019) 000 – 000

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Figure 7: Too high clamping force resulting in a crushed specimen.

Figure 8: Micrograph sample cut transverse to fibers, showing and inter fiber crack.

Figure 6: Typical unwanted failure in tab region of CFRP specimen.

Failure modes for the CFRP material from static testing is presented in Figure 6a. Failure modes from the fatigue testing using the final chosen specimen geometry is presented in Figure 6b. Majority of the fatigue specimens show a failure in the gauge length, however some specimens still show a questionable failure (e.g. 83% and 72% in Figure 6). It is of the author’s opinion that this is unavoidable. For a material such as UD CFRP there will always be test specimens with a questionable failure and each test needs to be evaluated if it is to be considered or if the test is invalid, e.g. the specimen for 72% failed at first in the gauge length and the energy released from failure probably triggered the tab failure. A failure close to the tab can be an effect of the Poisson’s ratio effect in the area creating stress concentrations.

Figure 9: To the left (a) failure modes for CFRP material in static tensile testing, and to the right (b) failure modes for the CFRP material in fatigue testing at different load presented in percentage of UTS, with R = 0.1 and f = 5 Hz.

6. Summary test procedure

Characterizing the CFRP materials with static testing is a first step before defining fatigue testing specimen parameters. There are several parameters to consider for a parallel-side-coupon specimen. Based on the testing performed in this study, developing a functioning test procedure for fatigue testing, a work scheme is proposed in Figure 10a. It is always a question of trial and error for each lab environment and the result differs depending on