The analysis of entrapped debris provides a useful complementary method of investigating the laser ablation mechanism in laser processing of polycrystalline metal samples using a femtosecond laser (Clark MXR, CPA2001). Morphological investigations of the laser- processed areas, for a range of laser fluences and pulse number, were recorded using optical and scanning electron microscopies (SEM) and white light interferometry. Data obtained on ablation rates, ejected particle sizes, and crater morphologies prove that ablation changes from a smooth to an explosive process at high fluences, as identified with changes in the material removal mechanisms. The build-up of laser-induced mechanical stresses, due to the heating and cooling of the samples between successive laser shots, plays an important role in the material modification process, leading to the observed dependence of ablation threshold on shot number. The strength of the dependence is governed by the incubation coefficient, S, which has been measured for all materials studied. In this paper, additional insight is derived from the analysis of the debris generated for metal samples, which can be attributed to laser ablation mechanisms based on vaporization, spallation, phase explosion, and fragmentation.