Tampere University, Faculty of Engineering and Natural Sciences, P.O.Box 589, FI-33014, Tampere, Finland; Orton Orthopaedic Hospital and Research Institute Orton, FI-00280, Helsinki, Finland.
Royal Institute of Technology, Surface and Corrosion Science, P.O.Box 10044, SE-16440, Stockholm, Sweden.
J Mech Behav Biomed Mater. 2019 Nov;99:93-103. doi: 10.1016/j.jmbbm.2019.07.004. Epub 2019 Jul 10.
Miniature laser welds with the root depth in the range of 50-300 μm represent air-tight joints between the components in medical devices, such as those in implants, growth rods, stents and various prostheses. The current work focuses on the development of a fatigue test specimen and procedure to determine fatigue lives of shear-loaded laser welds. A cobalt-chromium (CoCr) alloy is used as a benchmark case. S-N graphs, damage process, and fracture surfaces are studied by applying x-ray analysis, atomic force microscopy, and scanning electron microscopy both before and after the crack onset. A non-linear material model is fitted for the CoCr alloy to run finite element simulations of the damage and deformation. As a result, two tensile-loaded specimen designs are established and the performance is compared to that of a traditional torque-loaded specimen. The new generation specimens show less variation in the determined fatigue lives due to well-defined crack onset point and, therefore, precise weld seam load during the experiments. The fatigue damage concentrates to the welded material and the entire weld experiences fatigue prior to the final, fracture-governed failure phase. For the studied weld seams of hardened CoCr, a regression fatigue limit of 10.8-11.8 MPa, where the stress refers to the arithmetic average shear stress computed along the region dominated by shear loading, is determined.
微型激光焊缝的根部深度在 50-300μm 范围内，可实现医疗设备组件（如植入物、生长棒、支架和各种假体）之间的气密连接。目前的工作重点是开发疲劳试验试样和程序，以确定承受剪切力的激光焊缝的疲劳寿命。钴铬（CoCr）合金被用作基准情况。通过在裂纹起始之前和之后应用 X 射线分析、原子力显微镜和扫描电子显微镜，研究 S-N 图、损伤过程和断裂表面。为 CoCr 合金拟合了非线性材料模型，以进行损伤和变形的有限元模拟。结果，建立了两种受拉载的试样设计，并将其性能与传统的扭矩加载试样进行了比较。新一代试样由于明确的裂纹起始点和实验过程中精确的焊缝载荷，确定的疲劳寿命变化较小。疲劳损伤集中在焊接材料上，整个焊缝在最终的断裂控制失效阶段之前经历疲劳。对于研究的硬化 CoCr 焊缝，确定了 10.8-11.8MPa 的回归疲劳极限，其中应力是指沿主要受剪切载荷区域计算的算术平均剪切应力。
