Usually, an impact test is conducted to the point of fracture, so that the fracture energy and other fracture properties of a material under impact loading conditions can be assessed. To understand the fundamentals of and gain insight into impact phenomena, it would be valuable to monitor and quantify the deformation mechanics in a material leading to fracture and to relate the deformation profile history to the resulting impact traces. This task is accomplished in the present paper by carrying out impact tests at varying depths of penetration using a Rheometrics impact tester. The materials under investigation are a rubber-modified polystyrene [high-impact polystyrene (HIPS)] and a high-density polyethylene (HDPE).
The force-displacement traces of partial impacts of these two materials are presented and compared with those at fracture. The effects of impact speed and clamp ring size are also discussed. The two polymers, being ductile in nature, exhibit indentation deformation before complete puncture occurs. The deformation profile/impact force/ram displacement relationship is established.
The present study also includes a computational analysis of the impact response in polymeric materials. The impact model makes use of a finite-difference wave mechanics code. A wave mechanics approach is used in order to include the effects of inertia in the development of deformation and stress states under impact conditions. It incorporates a critical stress criterion for incipient damage and a damage assessment on the basis of stress-time accumulation. Comparisons are made between the calculations made at high impact speeds and those from the impact experiments conducted at low speeds. The computer simulation and experimental results are in qualitative agreement.