THE DYNAMIC BEHAVIOR OF HYDROSTATIC-PRESSURE-SENSITIVE MATERIALS AT HIGH STRAIN-RATES BASED ON SPLIT HOPKINSON PRESSURE BAR TECHNIQUE

Abstract

It is still an outstanding issue that whether the strength enhancement of engineering materials with strain-rate can be attributed to only the strain-rate effect or it involves "structural" effects due to non-strain-rate factors. One of the non-strain-rate factors is the change of stress state with the increase of strain-rate in a split Hopkinson pressure bar (SHPB) test for hydrostatic-pressure-sensitive materials. The lack of understanding on the change of stress state with strain-rate in SHPB tests leads to a wide spread misinterpretation of SHPB testing data. This thesis intends to answer this problem. Based on the study of a wide range of experimental data on various hydrostatic-pressure-sensitive materials, transition strain-rate, which has been well accepted to represent the start of significant strain-rate dependence of the material compressive strength, can be identified for all these materials from SHPB testing data. Using numerical SHPB model and a "reconstitution method", this thesis provides solid evidences to show that the observed transition strain-rate represents the transition from uniaxial stress state to non-uniaxial stress state due to the inherent nature of the tested material and the limitation of SHPB technique. Contribution from this stress state transition to the apparent strength enhancement dominates the SHPB measurement of the material strength. The examples examined in this thesis include concrete-like materials, rocks, ceramics and polymers. Influences of other factors (pulse shapes, geometric dimensions, interface frictions, etc.) on the transition strain-rate are studied quantitatively. In addition to identifying the transition strain-rate and understanding the influences of other factors on the transition strain-rate, this thesis also made following observations,(a) The dynamic tensile strength enhancement measured by various dynamic tensile test methods is valid;(b) The real transition strain-rate based on the wing crack model is greater than or around the order of 104 s-1, which is higher than the upper limit of SHPB technique for brittle materials;(c) A complete procedure for constructing the dynamic macroscopic model of polymers is proposed and verified. This thesis offers strong supports to a limited number of viewpoints on the nature of the transition strain-rate. It has been largely ignored by material testers and structure modelers, who consider the strength enhancement from SHPB tests as a real strain-rate effect, which may cause non-conservative design of structures against dynamic loads. This thesis suggested important issues, that cannot be completed in the present program, for further investigation.

Date of Award	1 Aug 2010
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Qing Li (Supervisor)