LAUSR

It is well known that metallic materials exhibit worse fatigue damage tolerance as they behave stronger in strength and softer in modulus. This raises concern on the long term safety of the recently developed biomechanical compatible titanium alloys with high strength and low modulus. Here we demonstrate via a model alloy, Ti-24Nb-4Zr-8Sn in weight percent, that this group of multifunctional titanium alloys possessing nonlinear elastic deformation behavior is tolerant in fatigue notch damage. The results reveal that the alloy has a high strength-to-modulus (σ/E) ratio reaching 2% but its fatigue notch sensitivity (q) is low, which decreases linearly from 0.45 to 0.25 as stress concentration factor increases from 2 to 4. This exceeds significantly the typical relationship between σ/E and q of other metallic materials exhibiting linear elasticity. Furthermore, fatigue damage is characterized by an extremely deflected mountain-shape fracture surface, resulting in much longer and more tortuous crack growth path as compared to these linear elastic materials. The above phenomena can be explained by the nonlinear elasticity and its induced stress relief at the notch root in an adaptive manner of higher stress stronger relief. This finding provides a new strategy to balance high strength and good damage tolerance property of metallic materials.摘要金属材料的抗损伤容限能力一般随着强度的提高及模量的降低而显著弱化. 对于高强度低模量医用钛合金, 这可能引发长期使用安全性的担忧. 本文以Ti-24Nb-4Zr-8Sn合金为模型材料研究了缺口疲劳行为, 结果表明: 非线性弹性合金具有优异的抗疲劳缺口损伤能力, 其强度与模量的比值高达2%时仍然具有较弱的疲劳缺口敏感性, 两者关系远优于常规线弹性金属材料, 如不锈钢、 钛合金、 铝合金和镁合金等; 该合金的疲劳断口呈现极度偏折的山峰状形貌, 显著抑制了裂纹的萌生与扩展. 依据非线性弹性变形理论, 本文给出了一种抑制缺口疲劳损伤的自适应机制: 非线性弹性有效缓解了缺口根部的应力集中, 并且这种效果随着应力的提高而显著增强. 因此, 非线性弹性变形行为是一种实现高强度金属材料耐疲劳损伤的重要手段.