In an impact analysis, there is a curve that shows residual tensile strength of composites that has incurred damage due to impact. You can see this curve in attachment.
You see in impact testing two phenomenon occur simultaneously:
1. Since, initially, the specimen has some strength distribution, the weaker constituents
break first and the average strength of the rest increases.
2. The repeated impact loading enlarges the microdefects in the material (fatigue)
and weakens the specimens. This causes an overall decrease in strength towards the end of the process.
In practical terms, these phenomena indicate that strength is changing
during handling and conveying along the production line, although damage is not
visible at every step. On further loading an increase occurs due to achieving a stable hardened or stiffened state and remain constant. This is due to the occurence of a critical amount of plastic dislocations. See attached link which describes this phenomena in terms of an energetic CoR acc. to W. J. Stronge.
http://www.sciencedirect.com/science/article/pii/S0009250914001651#
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