Fatigue testing coupons

At the undulation of warp and fill threads, delaminations develop in the interior, being initiated at intersections of the transverse and longitudinal cracks, where local tensile stresses are provided by the transverse warp threads. As soon as delaminations occur, the stiffness of a test coupon decreases considerably and enters stage III, that is, the last 50% of fatigue life, producing final failure. 

The results of all fatigue tests are summarized in Table 5.1. They show that there is a strong dependence on the test frequency, independent of the laminate type. At a frequency of 30Hz, even on the lowest maximum stress level, rupture occurs within a few load cycles, with a temperature rise of more than 80 K, which means at more than 100°C. At a frequency of 3 Hz, even at the highest maximum stress level, only a minor temperature increase can be observed, and there is no rupture of the test coupons. 

This is now becoming routine in coupons and components undergoing fatigue testing as it enables both the damage accumulation to be monitored and a failure criterion to be established. The example shown in Figure 12.7 is one in which the change in stiffness has been monitored. 

Figure 12.7 Degradation plot of a coupon undergoing fatigue testing normalized to the initial stiffness (S/5o) and the number of cycles to failure (N/Nf) (Bach/ECN).

Coupons through compression and tension/compression fatigue testing. Spars by fatigue testing in flapwise mode. 

Some subtly different definitions of fatigue cycles are used for material sample coupon fatigue test and engine test in the aviation industry. The... 

Specific creep tests have not been developed for composites at the coupon level. Data have been obtained using several of the standard test geometries described above, but creep is normally considered less of a problem than fatigue. The creep tests for plastics. ISO 899 [31], can be used for guidance with the standard composite specimens. 

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