The following paper is published in the Engineering Fracture Mechanics journal, Volume 325, August 25, 2025, as an open access work. Findings were developed through an Auto/Steel Partnership project.
Abstract: An in-plane bend test fixture was developed to produce uniaxial tensile and compressive stress states in sheet metals without need for anti-buckling devices while enabling optical strain measurement. An in-plane bend fixture and specimen geometry were developed with emphasis placed upon edge strain measurement and determination of appropriate fracture detection metrics. The proposed specimen geometry was found to reach edge strains of approximately 0.70 and could characterize R-values in uniaxial tension and compression. A so-called arc length method that is based upon geometric changes during in-plane bending was proposed and applied at the convex edge where cracks initiated. The arc length method strains showed excellent agreement with digital image correlation (DIC) and exhibited only a mild sensitivity to the optical lengthscale. Various fracture detection methods such as the peak force and bending moment, and metrics based upon a theoretical bending stress and the local strain rate were considered for a third generation 980GEN3 and an MS1500 steel. The force and bending moment were only valid in specific cases while the bending stress and strain rate methods were in close agreement with onset of visible cracking. The 980GEN3 in-plane bending failure limits were in excellent agreement with the conical hole expansion test suggesting a convergence in the uniaxial fracture strains between in-plane and out-of-plane loading when necking is suppressed. Finally, a host of advanced high strength steels with different strengths and thicknesses were tested to assess the sensitivity of the sample geometry to buckling. The gauge-height-to-thickness aspect ratio was the primary factor influencing geometric instability with an aspect ratio of 4.0 or lower recommended to mitigate buckling.