Fatigue Damage Mechanics of Notched Graphite-Epoxy Laminates

Abstract

A modeling approach is presented that recognizes that the residual properties of composite laminates after any form of loading depend on the damage state. Therefore, in the case of cyclic loading, it is necessary to first derive a damage growth law and then relate the residual properties to the accumulated damage. The propagation of fatigue damage in notched laminates is investigated. A power law relationship between damage growth and the strain energy release rate is developed. The material constants used in the model have been determined in independent experiments and are invariant for all the layups investigated. The strain energy release rates are calculated using a simple finite element representation of the damaged specimen. The model is used to predict the effect of tension-tension cyclic loading on laminates of the T300/914C carbon-fiber epoxy system. The extent of damage propagation is successfully predicted in a number of cross-ply laminates of the form (90i/0j)ns and the quasi-isotropic laminate (90/+45/-45/0)s. The dependence of damage on load amplitude and specimen size is also well described. Residual strength is calculated as a function of damage dimensions for (90/0)s specimens using a stress-based failure criterion in conjunction with a Weibull dependence of the 0° ply strength on the volume under stress.

Ultra High Resolution Computed Tomography of Damage in Notched Carbon Fiber—Epoxy Composites

“…The fiber breaks in the 08 ply cascade along the position of the intralaminar crack in the À458 ply which is tangential to the circular notch. The dominant role of these tangential intralaminar cracks in determining inter-laminar delamination has been previously noted [20].…”

Physical modelling of damage development in structural composite materials under stress

“…For example, using a physical model, the prediction of a composite's toughness based on its microstructural characteristics and modes of failure can be built into an empirical fatigue damage law (a Paris or power law), (Poursartip et al, 1982a(Poursartip et al, ,b,c, 1984Spearing et al, 1991. This follows the path of physical model-informed empiricism.…”

“…Under mechanical loading in tension, a variety of polymeric-based composite systems containing a notch or hole, sustain damage in the vicinity of the stress concentrator (see, for example, Kortschot et al, 1990aKortschot et al, ,b, 1991Kortschot and Beaumont, 1993;Spearing et al, 1991Otunga, 1995;Dimant, 1994;Dimant et al, 1997Dimant et al, , 2002. Examination of these damage zones shows that cracking in a variety of composites involves similar components of failure from one material system to the next (Fig.…”

“…Also, there have been no curve-fitting parameters to improve the fit of the data! 13.7.3 Example 6: A model of damage and fatigue strength A physical model for the tensile strength of a fatigue damaged composite is based on the observation that no further damage, beyond that observed by load cycling, occurs in the material that may subsequently be loaded monotonically to catastrophic fracture (Spearing et al, 1991.…”