脆性涂层材料断裂韧性和残余应力压痕表征技术综述

多种多样的脆性涂层材料已成为航天航空、国防科技领域的核心关键材料,在国民经济中发挥了巨大作用。断裂韧性和残余应力是评估脆性涂层材料的重要力学性能指标。维氏压痕法是表征脆性涂层材料体系力学性能指标的有效方法之一,得到了广泛的应用。从涂层特点、压痕测试方法、力学模型、实验装备等方面综述了脆性涂层断裂韧性和残余应力压痕测试的研究进展,讨论了力学模型的特点和适用性,并对今后脆性涂层材料压痕表征研究进行了分析展望。     

仪器化压痕法单次加载数据绘制应力-应变曲线计算拉伸试验结果的方法初探

采用压痕仪自动采集的位移数据减去压痕仪的弹性变形量得到压痕深度,进而计算出压痕直径,再根据TABOR提出的关于压痕方法测试拉伸性能的系列关系式,计算出真应力、工程应力和应变,绘制出压痕法的连续应力-应变曲线和lg W-lg d曲线,根据两种曲线计算出抗拉强度、屈服强度和应变硬化指数(n值)。实测数据显示压痕法的连续应力-应变曲线与拉伸法的应力-应变曲线存在显著差异。对压痕法测得的抗拉强度、屈服强度和n值的测量不确定度进行了详细分析,结果表明:抗拉强度的测量不确定度小于抗拉强度值的10%,而n值的相对测量不确定度显著大于抗拉强度的相对测量不确定度,目前尚未找到可靠评估屈服强度测量不确定度的方法。     

一种可切削玻璃陶瓷压痕裂纹在残余应力驱动下的非平衡扩展特性

本文将Ｇｕｐｔａ等人提出的用压痕实验来测定脆性材料应力腐蚀指数ｎ的方法［１］进行了改进，使其可以直接测定压痕径向裂纹在压痕塑性区残余应力驱动下非平衡扩展的Ｖ－Ｋ１关系曲线，并用这一方法对一种可切削玻璃陶瓷及其原始玻璃，分别在水及空气介质中的应力腐蚀指数ｎ及Ｖ－Ｋ１曲线进行了测定，结果表明，水显著提高裂纹扩展速率，使Ｖ－Ｋ１曲线左移，ｎ值下降，而结晶化则起相反作用．     

新型压痕底模系统的设计与试验研究

建立了一种新型压痕底模系统的模型,并测试分析了不同压痕条件下纸板的抗弯应力曲线和最大抗弯应力值.以550g/m2纸板为例,试验结果表明,此新型压痕底模系统最佳压痕条件为:钢线规格1.05mm×24.0mm,压痕底模凹槽规格0.7mm×2.3mm;并且当压痕底模凹槽宽度越大,纸板最大抗弯应力也越大.     

一种可切削玻璃陶瓷压痕裂纹在残余应力驱动下的非平衡扩展特怀

本文将Ｇｕｐｔａ等人提出的用压痕实验来测定脆性材料应力腐蚀指数ｎ的方法＾（１）进行了改进，使其可以直接测定压痕径向裂纹在压痕塑性区残余应力驱动下非平衡扩展的Ｖ－Ｋ１关系曲线，并用这一方法对一种可切削玻璃陶瓷及其原始玻璃，分别在水及空气介质中的应力腐蚀指数ｎ及Ｖ－Ｋ１曲线进行了测定，结果表明，水显著提高裂纹扩展速率，使Ｖ－Ｋ１曲线左移，ｎ值下降，而结晶化则起相反作用。     

平头压痕试验确定薄膜弹塑性参数的研究

本文研究用平头压痕试验确定薄膜-基体材料中薄膜材料弹塑性参数的可行性,重点研究了薄膜材料的屈服强度和硬化模量的确定方法.利用有限元(FEM)进行了模拟计算,给出了平头压痕下典型的等应力分布,以及载荷-压入深度的曲线.通过对载荷-压入深度曲线的研究,给出了通过平头压痕试验确定薄膜屈服强度和薄膜硬化模量的方法.     

双扭法及其在脆性材料力学性能评定中的应用

本文介绍了一种新的脆性材料力学性能评定方法一双扭法,提出修正的双扭试样应力强度因子表达式.由于双扭试样裂纹尖端应力强度子与试样裂纹长度无关,因而对材料有广泛的适应性.其研究范围较广,可测定裂纹亚临界扩展、断裂韧性、高温或应力腐蚀条件下的力学行为等.本文比较了双扭法在电工陶瓷、结构陶瓷、硅酸盐玻璃、有机玻璃以及滚珠轴承钢等脆性材料中实际应用的结果,讨论了适用于双扭法应力分析的试样裂纹长度及其标定等问题.     

压痕诱发GaAs和Si晶体塑性、损伤与断裂

对压痕诱发脆性材料塑性、损伤与断裂研究进行总结, 并结合与之有关学科研究进展予以评述. 主要结果：微压痕诱导硅和砷化镓晶体的纳米和非晶转变, 并发现这一转变的临界应力; 转变过程是由切应力, 并非静水压力控制; 电子辐照诱导非晶晶化, 并发现晶化临界条件; 晶化速率与电流密度有关; 压痕诱发的裂纹尖端不是原子尖的, 其萌生与扩展伴随位错的产生, 并由此引发点阵的畸变, 并产生1～2nm宽非晶带; 裂纹扩展沿非晶带发生, 而非裂端前方原子键相继断裂的结果; 经傅立叶变换和逆变换发现, 裂纹尖端变形显示出各向异性.     

应用纳米压痕法测量电沉积镍镀层残余应力的研究

应用纳米压痕法测量残余应力的2种理论模型对5种电沉积镍镀层中的残余应力在不同压痕深度处进行了测量,并与X射线衍射法的测量结果进行了比较.结果表明,压深位于薄膜/基底界面处的2种压痕法测量结果与X射线衍射法的测量结果相近,且Yun-Hee模型与其符合得更好.     

氧化锆层状复合陶瓷表面压应力与相变增韧的关系

利用维氏硬度仪和X射线应力分析仪、X射线衍射仪等手段分别对单层和层状氧化锆陶瓷进行了力学性能测试和分析,研究结果表明,在ZrO2层状复合陶瓷中,压痕裂纹的形成除了因塑性区体积变化产生的残余应力外,还与相变应力和表面压应力有关,表面压应力对表面裂纹具有较大的抑制作用.层状陶瓷断裂韧性提高,主要是通过表面压应力对压痕裂纹区应力强度因子的贡献、提高断裂相变量,强化相变增韧效果、细化晶粒等几个方面来实现的.     

The tensile behaviour of polyethylene terephthalate

Measurements of the load/extension curves of polyethylene terephthalate (PET) over a wide range of temperatures showed four regions of behaviour. These were brittle fracture, ductile, cold-drawing, and uniform extension. A particular study was made of the transitions between brittle fracture and ductile failure, and between ductile and cold-drawing, since these define the limits of the three regimes of failure observed at temperatures below the glass transition and softening range.The effects of molecular weight and crystallinity were examined. The brittle strengths measured in tension at low temperatures showed a very large scatter. There was evidence, in spite of this scatter, that the brittle strength falls with decreasing molecular weight. The yield behaviour was not affected, so that the brittle/ductile transition moves to higher temperatures.Crystallinity affects both brittle strength and yield behaviour. The brittle strength falls with increasing crystallinity, whereas the yield stress rises. Both effects combine to raise the temperature of the brittle/ductile and the ductile/cold-drawing transitions.Stress/temperature curves were also constructed for notched specimens. Notching raises the effective yield stress and reduces the brittle strength so that the brittle/ductile transition is moved to higher temperatures. The observed effects are in qualitative agreement with theoretical predictions of the plastic constraint at the tip of a notch, and thus the latter gives a satisfactory qualitative explanation of notch sensitivity. Notching leads to brittle failure at room temperature, and in notched specimens the brittle strength rises as the temperature is decreased. The brittle strength of the lowest molecular weight sample was again significantly less than that of the higher molecular weight samples.Joint appointment between ICI Ltd and Bristol University.     

Modeling particulate self-healing materials and application to uni-axial compression

Using an advanced history dependent contact model for DEM simulations, including elasto-plasticity, viscosity, adhesion, and friction, pressure-sintered tablets are formed from primary particles. These tablets are subjected to unconfined uni-axial compression until and beyond failure. For fast and slow deformation we observe ductile-like and brittle softening, respectively. We propose a model for local self-healing that allows damage to heal during loading such that the material strength of the sample increases and failure/softening is delayed to larger strains. Local healing is achieved by increasing the (attractive) contact adhesion forces for those particles involved in a potentially breaking contact. We examine the dependence of the strength of the material on (a) the damage detection sensitivity, (b) the damage detection rate, and (c) the (increased) adhesion between healed contacts. The material strength is enhanced, i.e., the material fails at larger strains and reaches larger maximal stress values, when any of the parameters (a)–(c) is increased. For very large adhesion between the healed contacts an interesting instability with strong (brittle) fluctuations of the healed material’s strength is observed.     

Fracture toughness and evaluation of coating strength with an initial residual stress field

The effect of residual elastic stresses on the geometry of cracks which arise with contact and spontaneous failure of brittle coatings made of high-strength compounds is studied. Conditions are established for the correctness of fracture toughness K_Ic tests with indentation of a standard Vickers pyramid as applied to surface layers with an inhomogeneous structure and an initial residual stress field. Taking account of the anisotropy of fracture toughness established by experiment a reliable approach is suggested for evaluating the brittle strength of coatings in the presence of residual stresses.Translated from Problemy Prochnosti, No. 1, pp. 51–61, January, 1994.     

In situ nanomechanics of GaN nanowires

The deformation, fracture mechanisms, and the fracture strength of individual GaN nanowires were measured in real time using a transmission electron microscope-scanning probe microscope (TEM-SPM) platform. Surface mediated plasticity, such as dislocation nucleation from a free surface and plastic deformation between the SPM probe (the punch) and the nanowire contact surface were observed in situ. Although local plasticity was observed frequently, global plasticity was not observed, indicating the overall brittle nature of this material. Dislocation nucleation and propagation is a precursor before the fracture event, but the fracture surface shows brittle characteristic. The fracture surface is not straight but kinked at (10-10) or (10-11) planes. Dislocations are generated at a stress near the fracture strength of the nanowire, which ranges from 0.21 to 1.76 GPa. The results assess the mechanical properties of GaN nanowires and may provide important insight into the design of GaN nanowire devices for electronic and optoelectronic applications.     

Crack extension and arrest in contact stress fields

Sudden crack extension and arrest is observed when indenters are pressed into the surface of brittle materials. The energetics of this system are examined. Crack extension is defined by a condition of decreased free energy (after A. A. Griffith) and crack arrest is defined by a condition of increased free energy with a further increase in crack size. The analysis shows that the critical stress required for crack extension depends on the dimension of the stress field and other factors, viz., crack size and material properties, usually associated with Griffith's fracture equation. The dependence on the dimension of the stress field explains Auerbach's empirical law which shows that the apparent strength of a brittle material increases with the decreasing size of the contact stress field. Experimental observations for hot-pressed Si₃N₄ and SiC are presented to examine this size effect and its predicted relation to material properties.     

聚氨酯高聚物注浆材料抗压强度测试与模拟

针对密度处于0.08~0.5g·cm~(-3)区间的聚氨酯高聚物,在扫描电镜下观察材料微观形貌特征,记录单轴受压下材料的应力-应变曲线,通过提取扫描电镜图中微泡孔的尺寸,基于几何参数拟合材料抗压强度。试验结果表明:随密度增大,材料的泡孔由接近多边形转变为圆球形,平均孔径与泡孔间接触面积均减小,高密度试件的泡孔分布特征遵循能量最小原理,以保证体系稳定。材料的受压响应有三阶段特征:弹性阶段(应变小于5%)高密度试件应变发展较快:屈服平台阶段(应变5%~50%)的应力可以作为抗压强度的表征,致密阶段(应变大于50%)高密度试件发生脆性破坏。基于泡孔几何参数的Gibson-Ashby模型可以较好地拟合材料强度,该强度受密度影响显著,不同密度的材料可以灵活应用于多种工程场合。     

Strength and elastic modulus of a porous brittle solid: An acousto-ultrasonic study

The strength and elastic modulus of a porous brittle solid such as gypsum have been studied using an acousto-ultrasonic technique. Acousto-ultrasonics has been found to be a sensitive indicator of strength and porosity which are linearly related to some powers of a stress wave factor. New equations for porosity dependence of ultrasonic velocity, elastic modulus and strength of brittle solids have been proposed.     

Evaluation of the fracture toughness of Nb-40Al-8Cr-1W-1Y-0.05B intermetallic material by indentation techniques

The fracture toughness of an Nb-40Al-8Cr-1W-1Y-0.05B intermetallic material was evaluated by indentation techniques at room temperature. Two widely used indentation methods, crack size measurement and indent strength, yielded excellent agreement with a conventional fracture toughness technique using straight-through precracked specimens, despite the occasional formation of poorly configured cracks. However, the modified indentation technique, using dummy indent flaws, resulted in a low fracture toughness compared to that evaluated by the other methods. The material did not exhibit rising R-curve behaviour, as evaluated from the indentation strength data. These results indicate that indentation fracture principles are applicable to this brittle intermetallic material without modification of the residual contact stress term originally calibrated for ceramic materials.     

Critical stress diagrams for brittle materials with defects of the cusped void/crack type

An analysis of the strength of a brittle solid with defects of the hypocycloid void/crack type in plane stress state is presented. Based on the tenets of the theory of equilibrium cracks, critical stress diagrams for a brittle material with defects of this type are constructed and compared with published experimental data. It is shown that, using a model of a brittle material with defects of the cusped void/crack type, it is possible to describe laws governing the fracture of certain materials.     

Effect of fiber modulus of elasticity on the long term properties of micro-fiber reinforced cementitious composites

Fibers for reinforcing cementitious composites are typically short and randomly dispersed in the matrix. Consequently, most of the fibers are inclined to the cracks that develop in the cement matrix and suffer from bending stress as these cracks open. For brittle fibers, such as carbon fibers, the bending stress may lead to flexural fiber rupture before the fiber attains its full capacity in direct tension. As a result, the efficiency of these fibers may be reduced. This phenomenon is not expected to occur in ductile fibers, which can yield locally rather than rupture. Predictions of a theoretical model show that the bending stress increases as the matrix becomes denser and suffer (an event which occurs as the matrix ages or due to the addition of silica-fume) and decreases for fibers of lower modulus of elasticity. Therefore, a reduction in strength with time in composites with dense matrices is expected for very brittle fibers of high modulus, moderate or no reduction for low modulus brittle fibers, and no reduction in strength is expected for ductile fibers. The long term properties of cementitious composites reinforced with various microfibers was studied to validate the model; PAN and Pitch type carbon fibers represented brittle fibers of high and low modulus, respectively; polypropylene and polyacrylonitrile fibers represented ductile fibers. The results showed good agreement with the theoretical model.