In a typical Whipple shield for protecting spacecraft from hypervelocity impact (HVI), the debris cloud, formed by the shattered material of the outer bumper layer, commits multitudinous, disorderedly scattered pitting craters and cracks over a wide region in the rear wall layer. Material degradation due to the pitting damage is a precursor of structural fragmentation and system failure. In this study, microscopic material degradation of the rear wall layer in a typical dual-layered Whipple shield, initiated and intensified by the debris cloud-engendered pitting damage, is characterized using metallographic analysis. Diverse microstructure changes (e.g., refined grains, dislocation, micro-voids and micro-cracks) are observed, accompanying the generation of visible macro-scale pitting craters, and intensification of material plasticity and nonlinearity. In addition to the material nonlinearity in the vicinity of pitting craters, micro-voids and micro-cracks are also developed which distort propagation of probing guided ultrasonic waves GUWs, thereby triggering acoustic nonlinearity. Targeting at the evaluation and monitoring of this sort of pitting damage, an insight into the generation of high-order modes in GUWs is achieved, and then validated via experiment. On this basis, a monitoring and evaluation framework based on the lead zirconate titanate (PZT) network, in conjunction with the use of the developed nonlinear damage indices, is developed, whereby the hypervelocity debris cloud-induced pitting damage can be depicted and characterized quantitatively and precisely.