多冲接触载荷下涂层零件低应力宏观塑性行为

对高强度Co基、Ni基合金为表面涂层的中碳钢、不锈钢试样进行低应力多冲试验和测试,结果表明,在远低于材料屈服强度的低应力多冲接触载荷下,涂层和其下部一段基体材料将发生塑性变形,平均每次冲击造成的塑性变形量随冲击周次增加先大后小,累积可出现宏观塑性变形并伴随材料硬化现象。塑性变形量和变形区域大小与冲击应力值和材料强度有关。形变硬化程度由表及里衰减,并且只在冲击接触表面及以下一段长度上发生,形成一"易变形区"。分析认为多冲低应力变形是一种累积疲劳损伤。当峰值应力相同时,材料在多冲接触载荷下比静载荷和静疲劳载荷下吸收的能量多,且吸收量与离冲击点的距离成反比。多次冲击使原子易动性增强,临界切应力下降,位错易于启动和增殖。

LOW STRESS MACROSCOPIC PLASTICBEHAVIOR OF COATED PART UNDER REPEATED IMPACT CONTACT LOAD

Low stress repeated impact experiments and tests are done on medium carbon steel specimens and stainless steel specimens who are coated with high strength Co or Ni alloy. The results show that plastic deformation happen in the coat material and a part of substrate material near the coat. The mean plastic deformation of every impact, along with the increment of impact frequency, is getting large at first and than turn small. And under accumulating, there is macroscopic plastic deformation along with the material hardening phenomenon. The value of total deformation and the size of deformation area are related to stress value of impact and materials strength. The degree of deformation hardening is attenuating from surface to inner, and this phenomenon only occurs at the surface of impact contact and the next following length, forming an "easy deformation area". The analysis considers that repeated impact deformation is a kind of cumulative fatigue damage. When the peak stress is same, the absorbed energy of material under the repeated impact contact load is more than under the static fatigue load. And absorbability is in inverse proportion with the distance from the impact point. Repeated impact can intensify the mobility of atom, reduce critical shear stress, and make the enablement and multiplication of dislocation easy.