An insight into three approaches for determining fatigue delamination resistance in DCB tests on composite laminates

Fatigue delamination resistance G(cf), defined as the strain energy release rate during the fatigue crack growth at a certain crack length, was proposed in literature to characterize the real changing resistance of composite laminates against the fatigue delamination growth. Determining G(cf) is important for an accurate interpretation of the fatigue delamination behavior when the normalization method is used to evaluate the fatigue delamination growth rate. In this paper, a re-fatigue loading approach is firstly proposed for judging the similarity/difference of the damage states between the fatigue and static delaminations. Combining with other two approaches established in previous studies, a systematic experimental investigation on these approaches is performed by designed tests. The researches show the damage state of the fatigue and static delaminations in composite laminates can be the same or different, depending on the pre-crack location and first-applied fatigue loading, which is the fatigue loading applied to achieve the desired pre-crack length. The re-fatigue approach is suitable for judging the similarity/difference of the damage states between the fatigue and static delaminations. The G(cf) can be further effectively determined by the compliance approach. However, the re-static loading approach is not applicable due to the difficulty in identifying the re-growing point.