Flat tapered tensile methodology for stress corrosion cracking threshold determination in biodegradable magnesium implant alloys

Biodegradable magnesium (Mg) alloys are emerging as a transformative class of materials for temporary orthopaedic implants; however, their widespread clinical adoption is impeded by pronounced susceptibility to stress corrosion cracking (SCC) in chloride-rich physiological environments. While existing SCC assessment methods for Mg alloys provide useful susceptibility indices via slow strain rate testing, these are primarily limited to comparative ranking and fail to provide the quantitative design parameters needed for robust implant design. This study addresses this gap by developing a flat tapered tensile–constant extension rate tensile methodology, in which a tapered tensile specimen is tested under constant extension rate in simulated body fluid to generate a continuous local stress gradient, followed by post-test SEM-based last-crack mapping to estimate a quasi-static SCC threshold. The method is demonstrated using the widely investigated WE43 magnesium alloy in simulated body fluid at 37 °C. By applying a framework to locate the last crack along the varying stress distribution of the gauge length via scanning electron microscopy, a stress threshold is determined for each strain rate. To identify the SCC threshold corresponding to a constant load, these values are extrapolated to a quasi-static limit, yielding an estimated threshold of 178 MPa. Independent validation via constant-load exposure below this threshold for 72 hours produces no detectable cracking, supporting the threshold interpretation under the present test conditions. The proposed methodology provides a practical route for moving beyond comparative SCC susceptibility ranking toward a quantitative, design-relevant stress parameter for biodegradable Mg alloys.