Cyclic Plasticity and Low Cycle Fatigue of an AISI 316L Stainless Steel: Experimental Evaluation of Material Parameters for Durability Design

AISI 316L stainless steels are widely employed in applications where durability is crucial. For this reason, an accurate prediction of its behaviour is of paramount importance. In this work, the spotlight is on the cyclic response and low-cycle fatigue performance of this material, at room temperature. Particularly, the first aim of this work is to experimentally test this material and use the results as input to calibrate the parameters involved in a kinematic and isotropic nonlinear plasticity model (Chaboche and Voce). This procedure is conducted through a newly developed calibration procedure to minimise the parameter estimates errors. Experimental data are eventually used also to estimate the strain–life curve, namely the Manson–Coffin curve representing the 50% failure probability and, afterwards, the design strain–life curves (at 5% failure probability) obtained by four statistical methods (i.e., deterministic, “Equivalent Prediction Interval”, univariate tolerance interval, Owen’s tolerance interval for regression). Besides the characterisation of the AISI 316L stainless steel, the statistical methodology presented in this work appears to be an efficient tool for engineers dealing with durability problems as it allows one to select fatigue strength curves at various failure probabilities depending on the sought safety level

Multi-spectroscopic analysis of high temperature oxides formed on cobalt-chrome-molybdenum alloys

Thanks to their thermal stability, resistance to oxidation and mechanical strength, cobalt -chrome molybdenum alloys are considered an ideal alloy for high temperature applications. The surface oxide layer evolves as a function of time and temperature, changing its chemical structure and increasing its thickness from a few nanometers to various microns. Making use of various diffractographic and spectroscopic techniques, namely X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and glow-discharge optical emission spectroscopy, coupled with complementary analysis, this work gives new insights on the chemical bonding, crystallographic structure, thickness and elemental composition of the oxide layers as a function of both time and temperature of oxidation. Results show that the initial nanometric passive layer of Co3O4 evolves into a metastable, sub-micrometric CoCrO4 structure and finally stabilizes into a micrometric Cr2O3 at the highest temperatures. This paper fills a fundamental gap in the understanding of the chemistry and stability of Cobalt-based alloys used for high temperature applications, such as in poppet or exhaust valves, aerospace components or hot gas turbines. Once calibrated, this innovative, complete surface characterization approach can be ideally extended to other metallic alloys. (C) 2022 The Author(s). Published by Elsevier B.V

Wear behavior of medical grade PEEK and CFR PEEK under dry and bovine serum conditions

Wear is among the main factors affecting the performance of artificial couplings in joint arthroplasty. In the attempt of investigating the applicability of PEEK and CFR PEEK in such applications, wear behavior of three medical grade PEEK formulations (unfilled PEEK, 30% wt. pitch carbon fiber reinforced PEEK and 30% wt. PAN carbon fiber reinforced PEEK) have been tested under dry and bovine serum lubricated conditions in a pin on flat test against Al2O3 1/8\u2033 spheres, replicating physiological conditions whereas possible. The crystallinity and the resulting mechanical properties of the three selected material were differentiated by annealing treatments performed at 200, 250, and 300\u202f\ub0C. All the specimens were tested under both lubrication regimes. The observed coefficient of friction variations, as well as the calculated volumetric wear rate, displayed an improved wear behavior of the reinforced formulations in both dry and bovine serum. Correlation among the modified material properties by annealing and the wear behavior was possible. The formation of a lubricating film was identified as responsible for an improved wear performance of PEEK and CFR PEEK formulations