Fatigue damage in a centrally notched composite laminate due to two-step spectrum loading

Two‐stage fatigue loading of woven carbon fibre reinforced laminates

“…Eventually, even the difference in the stress level between the first and the second stage seems to have no clear influence on the deviation of the damage sum from the linearity, as can be recognized from the comparison among our data and those reported by Kobayashi et al 7 and Jen et al 8 This is quite in contrast with that reported by Broutman and Sahu 22 for glass-reinforced laminates; in which they found that the deviation from the linearity of the damage summation is much higher as the difference in stress level between the first and second stage increases. The influence of the loading sequence has also been recognized in terms of the split growth rate for notched specimens.…”

“…Investigating the behaviour of a notched quasi‐isotropic [0/90/ ± 45] 2s T800/5208 graphite‐epoxy Jen and co‐workers 8 found, for constant amplitude tests, a greater delamination area for the low stress level than for the high stress, while in the latter case the damage was more concentrated through the thickness. During two‐stage fatigue loading on the same material, they also found for the low‐to‐high loading sequence a greater extent of the damage associated with a higher decrease of the normalized stiffness, at the same fractional life.…”

“… Longitudinal split growth rate for (a) unidirectional 11 and (b) quasi‐isotropic 8 carbon‐fibre epoxy laminates in presence of notches. …”

“…A common theme in the available data, both for carbon and glass reinforced materials, is that damage accumulation follows a nonlinear law. The assessment of the fatigue life with the Palmgren-Miner linear damage rule 32,33 can in fact give very dangerous results either for simple 2,4,7,8,13,18,21,22,26,28 or complex block and real spectrum loading conditions. 1,3, 6,9,12,19,30,31,34 It is worth noting, however, that in a few cases the linear damage rule turned out to be effective 9,10,12,27 (in Refs 9 and 12 only for all-tension or all-compression block loading).…”

2D woven fabric composites under fatigue loading of different types and in different environmental conditions

“…To understand variable loading fatigue of composites, a lot of experimental studies have been attempted, for example, by Sahu (1972), Jen, Kau, andWu (1994), Otani and Song (1997), Yang and Jones (1980), Lee and Jen (2000), Gamstedt and Sjogren (2002), and Mohandesi and Majidi (2009). However, an opposite effect of load sequence on variable loading fatigue of composites (Found & Quaresimin, 2003;Van Paepegem & Degrieck, 2002), depending on the sources of data, has been observed not only in simple stepwise amplitude change fatigue tests but also in realistic block loading fatigue tests.…”