Compression Stress Relaxation Test Research Articles

Time-Temperature Dependence of Compressive Behavior of Polypropylene Foams

The time-temperature dependence of the compressive behavior of polypropylene (PP) foam was investigated to make predictions about what sort of behavior for wide ranges of temperature and strain rate. Compressive stress relaxation tests were conducted at 213 K and 373 K. Compression tests were also conducted. The strain rate was 2×10-3 1/s at 213 K and 373 K. The compressive stress-strain curves were roughly linear and dependent on temperature until the maximum stress was reached. The maximum stress occurred at 5% strain regardless of temperature. The plateau stresses decreased as temperature increased. By plotting compressive behavior of the PP foam at the master curve of the stress relaxation modulus, its temperature dependence could be explained by the thermo-viscoelastic properties. Therefore, the behavior of PP foam at different strain rates could be approximately predicted from the stress relaxation modulus with the timetemperature equivalence principle.

Age‐associated changes in viscoelastic properties of the bovine temporomandibular joint disc

To test the hypothesis that compressive properties of the temporomandibular joint (TMJ) disc change with age, we investigated its viscoelastic properties and stress-relaxation behavior under compression. Compressive stress-relaxation tests were performed in different regions of bovine discs of various ages. For each disc, specimens were derived from three different regions (anterior, central, and posterior). For four strain levels (5, 10, 15, and 20%), a stress-relaxation test was conducted over a 5-min period. Values of the instantaneous modulus, E(0), appeared to be larger in the anterior than in the posterior region of the disc, irrespective of age. The E(0) value increased with age, especially in the central region. Values of the relaxed modulus, E(R), also increased significantly with age. There were no regional differences in values of the relaxed modulus. Under stress-relaxation, the relaxation time became longer with age, especially in the posterior region. The results suggest that the compressive properties, instantaneous and relaxed moduli, increase with age, while the relaxation time becomes longer. This implies that the TMJ disc becomes harder with age. Furthermore, the compressive properties of the TMJ disc are region-specific. As a result of the harder disc, it is likely that the TMJ becomes more vulnerable to secondary damage, such as fracture and tissue degradation.

Mechanical behavior of calcified plaques: a summary of compression and stress-relaxation experiments.

This paper summarizes the results from mechanical testing of atherosclerotic plaques performed in the Cardiovascular Mechanics Laboratory and the Laboratory for Implantable Materials at UMBC. The motivation for our work is that balloon angioplasty, stenting, and roto-ablation are mechanical processes that are designed to permanently alter the shape of an occluded arterial lumen. The mechanisms of permanent plaque deformation are not known. Therefore, to study the mechanical behavior of plaques, we performed mechanical tests on atherosclerotic lesions with different compositions and investigated differences in the materials' mechanical responses. Atherosclerotic plaque specimens were subjected to two main types of loading: multiple cyclic compression and stress-relaxation. The multiple-cycle test protocol was two fifteen-cycle loading phases that were separated by a 10-15 minute unloaded "rest" period. The compressive stress-relaxation test protocol was a series of three consecutive loadings (called phases I, II, and III). Each phase consisted of a 25% compression that was achieved in less than 1 second, a 10 minute relaxation period, and a 10 minute unloaded "rest" period between loadings. In the multiple cycle compressive loading, plaques exhibited three distinct types of behavior, which corresponded to the plaque compositions. Calcified plaques showed behaviors distinct from other plaque types and healthy vessels. In contrast to the cyclic compression results, plaque types could not be distinguished solely on the basis of stress relaxation behavior. Calcified and fibrous plaques had similar behavior, and therefore histology was used for definite identification. Calcified plaques have unique mechanical properties, and therefore interventions like angioplasty, roto-ablation, and stenting may require protocols specific for calcified lesions. The optimum protocols for calcified plaques may be quite different from plaques with other compositions. It is essential to learn more about the mechanical behavior of all plaque types to increase the success rate of occlusive atherosclerosis treatments.

Compressive and shear properties of alginate gel: effects of sodium ions and alginate concentration.

The equilibrium and viscoelastic properties of alginate gel crosslinked with Ca2+ were determined as a function of alginate concentration and duration of exposure to physiologic concentrations of NaCl. Compressive and shear stress relaxation tests and oscillatory shear tests were performed to measure the material properties at two time periods after storage in NaCl compared to no NaCl exposure. The effect of concentration was determined by testing 1-3% alginate gel in a bath of physiological NaCl and CaCl2. After 15 h of exposure to NaCl, the compressive, equilibrium shear, and dynamic shear moduli decreased by 63, 84, and 90% of control values, respectively. The material properties exhibited no further changes after 7 days of exposure to NaCl. The loss angle and amplitude of the relaxation function in the shear also decreased, indicating less viscous behaviors in both dynamic and transient configurations. All moduli, but not the loss angle, significantly increased with increasing alginate concentration. The observed decrease in compressive and shear stiffness for alginate gel after exposure to Na+ was significant and indicated that physiological conditions will soften the gel over a time period of up to 7 days after gelation. The alginate gel retains significant solid-like behaviors, however, as measured by a loss angle of approximately 3 degrees. This study provides the first available data for material properties of alginate gel tested in physiological saline.

A transversely isotropic biphasic model for unconfined compression of growth plate and chondroepiphysis.

Using the biphasic theory for hydrated soft tissues (Mow et al., 1980) and a transversely isotropic elastic model for the solid matrix, an analytical solution is presented for the unconfined compression of cylindrical disks of growth plate tissues compressed between two rigid platens with a frictionless interface. The axisymmetric case where the plane of transverse isotropy is perpendicular to the cylindrical axis is studied, and the stress-relaxation response to imposed step and ramp displacements is solved. This solution is then used to analyze experimental data from unconfined compression stress-relaxation tests performed on specimens from bovine distal ulnar growth plate and chondroepiphysis to determine the biphasic material parameters. The transversely isotropic biphasic model provides an excellent agreement between theory and experimental results, better than was previously achieved with an isotropic model, and can explain the observed experimental behavior in unconfined compression of these tissues.