17 th European Conference on Fracture 2-5 September , 2008 , Brno , Czech Republic Fatigue damage in copper polycrystals subjected to ultrahigh-cycle fatigue below the PSB threshold
17 th European Conference on Fracture 2-5 September , 2008 , Brno , Czech Republic Fatigue damage in copper polycrystals subjected to ultrahigh-cycle fatigue below the PSB threshold
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2008
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通讯作者：
A. Weidner;D. Amberger;F. Pyczak;Bernd;Schönbauer;S. Stanzl-Tschegg;H. Mughrabi
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作者：
A. Weidner;D. Amberger;F. Pyczak;Bernd;Schönbauer;S. Stanzl-Tschegg;H. Mughrabi
目前，人们越来越关注于理解在约10至10个循环以上的超高周疲劳（UHCF）或甚高周疲劳（VHCF）范围内发生的特定损伤和失效机制。目前大多数研究都集中在含有微观结构不均匀性的高强度材料的UHCF行为上，其中内部疲劳失效通常是由疲劳裂纹引起的，疲劳裂纹起始于内部不均匀性，如夹杂物，比较[1]。相比之下，目前的工作是纯延性单相面心立方（fcc）材料，如铜的UHCF行为的实验研究的一部分。在这些材料中，最常见的高周疲劳（HCF）失效模式源于持续滑移带（PSB）中的循环应变局部化。一般认为，PSB中循环应变局部化的必要前提是必须超过所谓的PSB阈值振幅[2]。然而，在早期的工作中已经假设，即使在非常低的振幅下，循环滑移仍然保留了一个小的但不可忽略的不可逆分量，该分量在UHCF状态下以随机方式在非常大量的循环中积累，可以导致表面粗糙化（以及位错子结构的不可逆变化），并且最终可能导致PSB形成和疲劳裂纹萌生。在该图中，可以预期甚至低于PSB阈值[3]的UHCF状态下会产生疲劳损伤，如图1所示（：应力，垂直轴）。为了检验这一假设，开始了一个项目，其中详细研究了经过10次以上超声疲劳的商业纯铜多晶的UHCF行为。在这项工作中，表面特征进行了详细的研究，通过扫描电子显微镜（SEM）和原子力显微镜（AFM）。这一早期工作的最重要的结果已经发表[4，5，6]，并总结如下：“传统的”轴向PSB阈值应力幅被发现是ca。63 MPa [4，5，6]。远低于PSB阈值，在非常大量的循环之后，在强烈的滑移带中发生显著的循环滑移局部化，并且随着循环次数的增加而增强[4，5]。
There is currently an increased interest in understanding the specific damage and failure mechanisms which occur in the UltraHigh-Cycle Fatigue (UHCF) or Very High Cycle Fatigue (VHCF) range above about 10 to 10 cycles. Most current studies have focused on the UHCF behaviour of high-strength materials containing microstructural heterogeneities, in which internal fatigue failure frequently is caused by fatigue cracks which are initiated at internal heterogeneities such as inclusions, compare [1]. In contrast, the present work is part of an experimental study of the UHCF behaviour of pure ductile single-phase face-centred cubic (fcc) materials such as copper. In these materials, the most common high-cycle fatigue (HCF) failure modes originate from cyclic strain localization in persistent slip bands (PSBs). The general belief is that a necessary prerequisite for cyclic strain localization in PSBs is that the so-called PSB threshold amplitude [2] must be exceeded. However, it had been postulated in earlier work that, even at very low amplitudes, cyclic slip still retains a small but non-negligible irreversible component which, accumulated in a random fashion over a very large number of cycles in the UHCF regime, can lead to surface roughening (and irreversible changes of the dislocation substructure) and, ultimately, perhaps to PSB formation and fatigue crack initiation. In that picture, fatigue damage can be expected to develop in the UHCF regime even below the PSB threshold [3], as illustrated in Fig. 1 ( : stress, axis vertical). In order to test this hypothesis, a project was started in which the UHCF behaviour of commercial purity copper polycrystals that had been ultrasonically fatigued up to more than 10 cycles was investigated in some detail. In this work, a detailed study of the surface features was conducted by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The most important results of this earlier work have been published [4,5,6] and are summarized as follows: The “traditional” axial PSB threshold stress amplitude was found to be ca. 63 MPa [4,5,6]. Well below the PSB threshold, marked cyclic slip localization occurred in intense slip bands after very large numbers of cycles and intensified with increasing numbers of cycles [4,5].