Dynamic Compression Testing of Soft Materials

Abstract

Low-strength and low-impedance materials pose significant challenges in the design of experiments to determine dynamic stress-strain responses. When these materials are tested with a conventional split Hopkinson pressure bar, the specimen will not deform homogeneously and the tests are not valid. To obtain valid data, the shape of the incident pulse and the specimen thickness must be designed such that the specimens are in dynamic equilibrium and deform homogeneously at constant strain rates. In addition, a sensitive transmission bar is required to detect the weak transmitted pulses. Experimental results show that homogeneous deformations at nearly constant strain rates can be achieved in materials with very low impedances, such as a silicone rubber and a polyurethane foam, with the experimental modifications presented in this study.

Applicability of the time–temperature superposition principle in modeling dynamic response of a polyurea

“…For the present purposes we have explored pulse shapers of different materials, for example copper, various polymers and wood, but obtained unsatisfactory results for reaching stress equilibrium when loading forces were smaller than about a hundred Newtons. Chen et al (2002) encountered the same problems but were able to ignore these issues because they were minor perturbations on the larger deformation results that were of interest to them. Through a trial-and-error process we found that employing a composite pulse shaper consisting of a combination of polyurea (1 to 2 mm Fig.…”

“…The desire or need to deal with a specimen in equilibrium generates a significant challenge that derives from the large impedance mismatch between the specimen and the aluminum bars. To deal with a similar situation involving materials of relatively low rigidity, Chen et al (2002) introduced the pulse shaping technique, which seems to work quite well for polymeric materials under large deformations. For the present purposes we have explored pulse shapers of different materials, for example copper, various polymers and wood, but obtained unsatisfactory results for reaching stress equilibrium when loading forces were smaller than about a hundred Newtons.…”

High Strain Rate Mechanics of Polymers: A Review

“…Materials which have been well-studied in the literature are silicone elastomers [13,66,151], plasticized PVC [152,153] and polyureas [39,40,72,[154][155][156][157][158][159] and polyurethanes [160][161][162]. The rate dependence of these materials depends strongly on the glass transition, and in particular whether this transition affects the room temperature response at strain rates of interest.…”

“…If this is the case, then the force supported by the specimen is equal to both F 1 and F 2 and the stress-strain relationship in the specimen can be calculated using the forces and bar velocities. It is necessary to ensure that polymeric, or other low wave speed specimens, are in equilibrium [66]. This is typically done by comparing the two forces.…”

“…Furthermore, many elastomers must be deformed to large strains to fully characterize the mechanical response, especially for hyperelastic materials. However, rubbers were some of the first materials to be characterized in split Hopkinson bar experiments [1,2], and more recently a number of authors have proposed techniques to address these difficulties through modifications to the Hopkinson bar or other similar systems [57,66]. These include pulse shaping [62], low impedance Hopkinson bar materials to increase the transmitted force [76,77,81,82,148] or use of more sensitive force gauges to directly measure the force at the specimen bar interface [62][63][64].…”