On the Changes of Residual Stresses Produced by Shot Peening Due to Repeated Stressing
On the Changes of Residual Stresses Produced by Shot Peening Due to Repeated Stressing
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论喷丸强化后残余应力随重复应力的变化
DOI：
10.2472/jsms1952.8.607
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发表时间：
1959
期刊：
影响因子：
--
通讯作者：
Y. Murakami
中科院分区：
文献类型：
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作者：
S. Taira;Y. Murakami
喷丸强化广泛应用于弹簧和其他汽车零件，以提高其疲劳强度。事实上，通过这种处理，他们的耐力极限大大提高，他们的疲劳耐久性也明显增加。这些改善的原因被认为是两个：（1）加工硬化和附近的表面层和（2）在表面层中产生的残余压应力。与这些有利的影响相反，由射击产生的压痕被认为是不利的，对试样施加缺口效应。为了研究残余应力对疲劳强度的贡献，首先必须弄清楚由于重复应力引起的残余应力的变化。这次使用低碳钢（S40 C），并在离心式喷丸机上对厚度为3.2mm的板状试样进行喷丸处理。四个应力幅，两个低于和其他两个高于疲劳极限，在重复应力和循环次数选择尽可能相同的不同的应力幅。如前所述，通过蚀刻方法测量残余应力，但这一次，考虑到砂金石表面，重量测量也用于确定蚀刻深度。根据其他研究人员的说法，喷丸的有利效果会因加热而大大降低。为了检验这一点，喷丸试样分别在100°、200°、300°、400°和500°C下加热1小时，然后空气冷却。测量了这些试样的残余应力和显微维氏硬度。结果表明：（1）喷丸强化产生的残余应力属于热应力类型，表面为压应力，心部为拉应力，表面残余应力的值与加工硬化后材料的屈服强度相比可能达到很高的值。(2)残余应力的衰减在重复循环的早期阶段（约2×104次循环）是明显的，此后逐渐进行。这种早期衰退称为残余应力衰退的第一阶段，随后的阶段称为第二阶段。(3)表面残余应力在重复千万次循环后不会完全消失。在70%疲劳极限的应力幅值下，它们接近初始值的65%，在95%疲劳极限的应力幅值下，它们接近初始值的55%。(4)相同循环次数后，应力幅越大，表面残余应力的衰减越大。(5)绘制表面残余应力比σr/σ0（其中σr为反复加载后的表面残余应力，σ0为初始状态下的表面残余应力）与循环比n/N的对数的关系图，得到各应力幅值的近似直线关系。这些直线的梯度对于更高的应力幅是更大的。假设该梯度与应力幅成正比，得到了第二阶段表面残余应力衰减的经验公式：当应力幅小于疲劳极限时，σr/σ0=1-0.0156σa-0.0035σa·log 10 n/N（σa<0.7σw）σr/σ0=1.088-0.02σa-（0.01+0.003σa）log 10 n/N（σw≥σa≥0.7σw）
Shot peening is widely used for springs and other automobile parts in order to improve their fatigue strengths. In fact, their endurance limits are considerably raised and their fatigue durabilities are also noticeably increased by this treatment. Two causes are considered for these improvements: (1) work hardening at and near the surface and (2) residual compressive stresses produced in the surface layers. In contrast to these favourable effects, the indentations produced by shots are assumed to act unfavourably, exerting notch effects for the specimens. To investigate the contribution of residual stresses on the fatigue strength, their changes due to repeated stressing must, first of all, be made clear. This time, mild steel (S 40C) was used and the plate specimens with the thickness of 3.2mm were shot peened by the centrifugal type machine. Four stress amplitudes, two being below and the other two above the endurance limit, were employed in repeated stressing and the number of cycles were selected to be the same as much as possible for different stress amplitudes. Residual stresses were measured by the etching method as before, but this time, considering the aventurine surfaces, weight measurements were also employed in determining the depths of etch. According to other investigators, the favourable effects of shot peening are reduced considerably by heating. To examine this, shot peened specimens were heated at 100°, 200°, 300°, 400°C and 500°C for 1hr respectively and then air-cooled. Residual stress and Micro-Vickers hardness were measured on these specimens. The results obtained were as follows:(1) Residual stresses produced by shot peening are of the thermal stress type, being compressive at the surface and tensile in the core and the value of surface residual stress may reach a very high amount as compared to the yield strength of the material as the result of work hardening.(2) The fading of residual stresses is noticeable during the very early stage of repetition of cycles (about 2×104 cycles) and thenceforward proceeds gradually. This early fading is called the first stage of fading of residual stresses and the following one the second stage.(3) Surface residual stresses do not fade completely after the repetition of ten-million cycles. They have nearly 65% of the initial value under the stress amplitude of 70% of the endurance limit and 55% under the stress amplitude of 95% of the endurance limit.(4) The fading of surface residual stresses after the same number of cycles is larger for the higher stress amplitudes.(5) Plotting the ratio of surface residual stress σr/σ0, where σr is the surface residual stress after repeated stressing and σ0 that of the initial state, against the logarithm of cycle ratio n/N, nearly straight line relation was obtained in each stress amplitude. The gradients of these straight lines are larger for the higher stress amplitudes. Assuming that this gradient is proportional to the stress amplitude, the following empirical formulas concerning the fading of surface residual stresses in the second stage were obtained;For the stress amplitudes less than the endurance limitσr/σ0=1-0.0156σa-0.0035σa·log10n/N (σa<0.7σw)σr/σ0=1.088-0.02σa-(0.01+0.003σa)log10n/N (σw≥σa≥0.7σw)