A new approach to the ring test is presented in this paper. Three-dimensional elastoplastic finite element modeling with contact has been performed to analyze the stress and strain distribution in the ring, to optimize the ring testing system, and to investigate the effect of friction between the ring specimen and the fixture. Based on the numerical results, a new design of a holding device that creates a uniaxially stressed zone is proposed in order to determine transverse behavior of tubular products, such as the modulus and the stress-strain curve. A case study is presented on nuclear cladding tubes in Zircaloy. The approach shows promise for testing various kinds of materials in structural tubing, including ductile or brittle materials, metals, composites, or polymers.
Knowledge of the Young's moduli is necessary for piezoelectric ceramics in order to separate the total deformation into an elastic and a plastic contribution. The methods for the determination of the Young's moduli, well known from conventional ceramics, are much more complicated due to the special electrical boundary conditions. The main aim of the report is to discuss the question in which mechanical tests the conditions of a constant electrical field are fulfilled and in which type of test constant dielectric displacements occur. It will be illustrated that the results from tests with homogeneous stress states are much more trustworthy than results from bending tests.
Linear reciprocating pin-on-plate-type wear testing has been a standard technique for the screening of orthopaedic implant materials since the early 1980s. This investigation compares a wear screening technique based on linear motion with a modern hip joint simulator based on multi-axial motion. Two groups of differently sterilized UHMWPE samples were tested. The first group of samples was sterilized by ethylene oxide (EtO) gas that caused no structural changes in the UHMWPE. The second group of samples was sterilized in nitrogen by gamma-irradiation and then subjected to a stabilization treatment that resulted in a significant level of crosslinking in the UHMWPE. When tested on the linear reciprocating wear machine, the EtO sterilized specimens (non-crosslinked linear polyethylene) showed an approximately 30% lower wear rate than the gamma-irradiated and stabilized specimens (crosslinked polyethylene). When tested on the hip simulator, the EtO sterilized specimens exhibited two to three times higher wear rates than the gamma irradiated and stabilized specimens. The ranking of wear resistance obtained with the hip simulator was strikingly different than that obtained with the linear reciprocating wear machine. This study indicates that screening wear machines based on linear motion do not correlate with multi-axial joint simulators and may produce misleading results in the prediction of clinical wear performance of UHMWPE bearing materials.
The experimental investigation of a new approach to the ring test is presented in this paper. A new design of an optimized ring specimen geometry and ring testing system was used, which was described in an earlier paper. Based on the three-dimensional elastoplastic finite element modeling with contact, a procedure for determining the stress from the global force and the strain from the global displacement has been proposed. The influences of different parameters, such as yield strength, hardening rate, Young's modulus, and friction coefficient on these curves has been examined. A comparison has been made between the finite element modeling results and the ring test results on two materials. One is a model material (aluminum-silicon, an aluminum alloy reinforced by silicon particles) that has been widely investigated in our laboratory and the second is a material (Zircaloy) used in the nuclear industry.
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