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环境断裂研究进展 总被引:5,自引:2,他引:3
本文介绍了环境断裂近年来的研究进展.第一部分是功能材料的环境断裂.研究发现,铁电陶瓷如PZT和Ba-TiO3在有水或无水环境中,应力能使压痕裂纹发生滞后扩展(即存在应力腐蚀).恒电场能引起铁电陶瓷的畴变,不协调畴变会产生内应力,电场和应力场对环境断裂存在耦合作用,因此,恒电场下环境断裂的本质是内应力引起的环境断裂;对磁致伸缩材料如(ThDy)Fe2,应力和磁场均能引起畴变,卸载压痕裂纹在湿空气中的滞后扩展以及恒磁场引起的滞后畴变及滞后开裂均能发生.第二部分是关于氢压裂纹(白点)的再认识.氢压裂纹形核前是一个内壁光滑的空腔,微裂纹从空腔壁产生,而后连接形成白点.白点断口和含白点试样的断口概念不同,对车轮钢,前者为准解理的穿晶断裂,和氢致滞后开裂断口相同,但后者则依赖断裂方式和试样厚度.钢中白点除了产生二次裂纹外,对各种断口形貌均没有影响.车轮钢的滞后断裂由原子氢引起,与白点无关. 相似文献
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通过在硅油中加恒电场实验,研究了PZT-5H铁电陶瓷Vickers压痕裂纹的扩展行为,探讨了电场、残余应力以及介质间的耦合作用.结果表明,残余应力不足以使压痕裂纹在硅油中发生滞后扩展,只有外加恒电场E>0.2 kV/cm,电场、残余应力和介质的耦合才能使压痕裂纹在经过一个孕育期tp后发生滞后扩展.由于有效应力强度因子随裂纹扩展而下降,故压痕裂纹扩展10-30 μm后就将止裂.压痕裂纹在硅油中滞后扩展的门槛电场强度EDp=0.2 kV/cm.如外加电场大于临界电场Ep=5.25 kV/cm,电场和残余应力的耦合可使压痕裂纹瞬时扩展;保持恒电场,裂纹能继续扩展,然后止裂.如外加电场大于12.6 kV/cm,不需要残余应力协助,电致裂纹也能在光滑试样上形核、长大、连接,导致试样断裂.试样发生电致滞后断裂的门槛电场EDF=12.6 kV/cm,发生瞬时断裂的临界电场EF=19.1 kV/cm. 相似文献
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通过对预先切向极化和纵向极化的PLZT铁电陶瓷试样表面引入维氏压痕,在压痕引入前后以及外加电场作用下原位记录维氏压痕裂纹尖端处的Raman光谱变化。结果表明:裂纹尖端的Raman光学模的强度与90°畴变、试样的预先极化方向以及裂纹尖端的空间分布位置密切相关。当试样的极化方向与外加电场方向垂直时,极化试样的维氏压痕裂纹尖端发生90°畴变,抑制裂纹的生长,此时观测到的Raman光谱强度变化最显著。当试样的极化方向与外加电场方向平行时,几乎不发生90°畴变,将导致裂纹尖端的裂纹生长。 相似文献
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《Acta Materialia》2007,55(17):5758-5767
In this paper, the interactions between the crack propagation and corresponding domain switching in ferroelectric single crystal under mechanical loading were investigated. An experimental setup with a polarized light microscope (PLM) was designed and constructed to in situ observe the crack propagation in poled BaTiO3 specimens subjected to three-point-bending loading. The observed domain switching was stimulated by the intensive stress field near the crack tip, and the theoretical R-curve taking into account the domain switching toughening agrees well with the experimental results quantitatively. It is confirmed from the actual switched zone that the 90° domain switching is the major mechanism of the fracture toughening, and the apparent fracture toughness increases by 150% in the BaTiO3 single crystal specimen. 相似文献
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The crack propagation in ferroelectric single crystals subjected to electric fields was studied experimentally and theoretically. An in situ observation of crack propagation and domain switching near the crack tip in a poled PMN–PT62/38 single crystal was carried out using polarized optical microscopy. It was found that a pure negative electric field leads to a larger domain switching zone near the crack tip than a positive one does. A negative electric field below the coercive field can cause crack propagation, while no crack growth was observed for a positive electric field far larger than the coercive field. A fracture model based on energy analysis was developed which indicates that the energy variation due to the domain switching provides the thermodynamic driving force for the crack propagation under pure electric loading. The critical electrical loading for the crack growth determined by this model agrees well with experiments. 相似文献
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The present paper demonstrates the effect of switching stresses on domain evolution and fracture toughening during quasi-static crack growth in elastically isotropic ferroelastic single crystals with transversally isotropic ferroelastic strains. With a simple switching algorithm and crack propagation procedure, domain evolution is simulated in an exemplary material with semi-infinite crack under mode I loading, starting from a mono-domain configuration. Domain reorientation is found to be strongly affected by switching stresses, which therefore have to be considered in the context of domain evolution modelling and fracture toughening. Before the onset of crack growth a needle-like domain is formed at the tip of the stationary crack, but this does not effect the crack tip stress intensity factor. Elongation of this domain during the onset of crack growth causes a large increase of the fracture toughness. Domain separation in a later stage results in toughness reduction. The subsequent domain evolution indicates a periodic formation of needle-like domains as observed in soft ferroelastic materials. 相似文献
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为探究淬火残余应力对铝合金厚板疲劳裂纹扩展的影响规律,建立7075铝合金厚板表面三维裂纹数值仿真模型。采用顺序热力耦合法求解淬火残余应力场,将残余应力场作为初始载荷条件求解裂纹应力强度因子,并与无残余应力场的应力强度因子值进行对比,研究两种条件下应力强度因子的分布规律和两者之间的异同;通过分析在初始淬火残余应力条件下不同半径裂纹受不同均匀拉应力荷载作用时的裂纹应力强度因子随裂纹位置角的演变曲线,探究淬火残余应力对裂纹扩展趋势的影响规律。结果表明,淬火残余应力的存在改变了铝合金厚板应力强度因子的分布规律和裂纹的扩展趋势,淬火残余应力使表层附近的裂纹扩展受到遏止,裂纹易于在厚度方向优先扩展。 相似文献
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《Acta Materialia》2000,48(16):4099-4113
The fracture resistance curves (R-curves) of BaTiO3 and commercial PZT–PIC 151 were measured with compact tension specimens under the influence of an electric field applied parallel to the crack front. A strong influence of the electric field on the starting and plateau value was found as well as on the length of the R-curve. Generally a toughness increase was detected with increasing electric field. The toughening effect is estimated from the change in crack tip stress intensity induced by ferroelastic domain switching near the crack tip using the weight function formalism developed for stress-induced transformation toughening of zirconia ceramics. In order to obtain a quantitative prediction of toughening, ferroelastic and ferroelectric properties were measured. 相似文献
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《Acta Materialia》2007,55(1):301-312
A series of experiments was performed with through-thickness cracks in ferroelectric double cantilever beam (DCB) specimens. Cyclic electric fields of different amplitudes were applied which resulted in cyclic crack propagation perpendicular to the electric field direction. Crack propagation was observed optically and three regimes were identified: a pop-in from a notch, steady-state crack growth and a decrease of the crack growth rate with increasing cycle number. Crack growth only occurred if the applied field exceeded the coercive field strength of the material. Furthermore, the crack extended during each field reversal and the crack growth rate increased with increasing field. Based on the experimental observations, a mechanistic understanding was developed and contrasted with a nonlinear finite element analysis which quantified the stress intensity in the DCB specimens. The driving forces for crack formation at the notch and subsequent fatigue crack growth were computed based on the distribution of residual stresses due to ferroelectric switching. The finite element results are in good agreement with the experimental observations and support the proposed mechanism. 相似文献