Frequency and wave-form effects on the fatigue crack growth behavior of alloy 718 at 298 K and 823 K
Frequency and wave-form effects on the fatigue crack growth behavior of alloy 718 at 298 K and 823 K
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DOI：
10.1007/bf02646402
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发表时间：
1978-04
期刊：
影响因子：
--
通讯作者：
M. Clavel;André Pineau
中科院分区：
文献类型：
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作者：
M. Clavel;André Pineau
在298和823 K温度下，对718合金CT型试样进行了疲劳裂纹扩展速率（FCGR）的测定。在823 K下，使用正弦波形信号，在5 - 10- 3 Hz和20 Hz之间的范围内研究了频率的影响。随着温度从298 K升高到823 K，频率在823 K降低，FCGR显著增加，特别是在低应力强度水平下。在升高的温度下，同样地研究了循环应力波形的影响，使用产生相同频率5.10- 2 Hz的三角波和方波波形信号。三角形负载导致比方波形式更高的FCGR。此外，在与方形负载相关的最大和最小负载下的10 s保持时间对FCGR无显著影响。用电子显微镜观察疲劳裂纹前的亚结构。这些观察结果表明，在某些情况下，塑性变形是由被确定为孪晶的平面带的传播进行的。在室温下，孪生被发现是丰富的，只有在阈值制度。在823 K时，在较高的FCGR域中观察到孪生，特别是在低频下。采用断口分析方法研究了裂纹扩展的微观机理。在823 K。随着频率的降低，发生晶间裂纹。对低周疲劳中形成的亚结构与扩展裂纹塑性区的亚结构进行了比较。讨论了在823 K下加载速率降低时，平面变形和孪晶对沿晶开裂和加速FCGR的重要性。
The fatigue crack growth rate (FCGR) of Alloy 718 was measured on CT type specimens at 298 and 823 K. At 823 K, the influence of frequency was studied in the range between 5 – 10-3Hz and 20 Hz, using a sinusoidal wave form signal. A substantial increase in FCGR occurred, particularly at low stress intensity levels, as the temperature was increased from 298 to 823 K and as the frequency was decreased at 823 K. At elevated temperature, the effect of cyclic stress wave form was equally investigated, using triangular and square wave form signals producing the same frequency of 5.10-2Hz. The triangular load led to higher FCGR than the square wave form. In addition the hold time of 10 s both at the maximum and the minimum load associated with the square load had no significant effect on the FCGR. Electron microscopy was used to observe the substructures that developed ahead of fatigue cracks. These observations showed that in certain circumstances plastic deformation proceeded by the propagation of planar bands which were identified as twins. At room temperature, twinning was found to be abundant only in the threshold regime. At 823 K, twinning was observed in the domain of higher FCGR, particularly at low frequencies. Fractography was carried out to study the micromechanisms of crack propagation. At 823 K. intergranular cracking occurred as the frequency was decreased. The comparison between the substructures formed in low cycle fatigue and those associated with the plastic zones of propagating cracks is made. The importance of planar deformation and twinning on intergranular cracking and on the acceleration of FCGR when the loading rate is decreased at 823 K, is discussed.