增韧环氧树脂的动态裂纹扩展研究

主要进行了环氧及增韧环氧树脂的断裂韧性及裂纹快速扩展的试验研究,试验过程中采用一ＧＬＣ－１型高速裂纹扩展测试仪来测试裂纹扩展速度,得到在裂纹扩展过程中裂纹扩展速度曲线,本文结合不同的计算公式及有限元分析方法,讨论了各个确定断裂韧性公式的准确程度,发现传统的静态断裂韧性的分析方法所得到的结果偏大,有一定的危险性,建议使用试验与数值计算相结合的方法,同时还发现增韧不仅可以提高材料的静态和动态怕裂性能,     

硝基胍发射药的断裂韧性和临界孔内外压差的研究

利用单边裂纹拉伸实验的方法对Ａ和Ｂ两种硝基胍发射药的断裂韧性进行了测试，分别用应力强度因子法和能量平衡法求出了这两种发射药的断裂韧性Ｋｃ值，比较了这两种方法的优缺点，讨论了这两种发射药之Ｋｃ值随温度和裂纹深度的变化规律，并对这两种发射药的临界孔内外压差进行了研究。     

陶瓷断裂韧性测试方法准确性和简便性比较分析

陶瓷是一种典型的脆性材料,如何能准确测量其断裂韧性是与陶瓷材料设计、制备和服役安全密切相关的一个研究重点。目前陶瓷断裂韧性测试方法有10余种,根据引发或诱导裂纹产生方式可分为压痕法、切口梁法、预裂纹梁法和其他方法。针对既能保障陶瓷断裂韧性测量结果的准确性,又能满足工程试验操作简便性的要求,综述了常用的压痕法、切口梁法、预裂纹梁法等3类测试方法的特点,结合所提出的新方法,对所述陶瓷断裂韧性测试方法的难点、优缺点及适用范围提出了几点建议。     

基于尖锐四棱锥压头的陶瓷材料断裂韧性测试方法

根据量纲分析建立陶瓷材料断裂韧性与压入响应参数以及裂纹开裂长度之间的无量纲函数关系,采用虚拟裂纹闭合法和有限元数值分析法实现HPC裂纹、RC裂纹以及过渡裂纹的裂纹尖端等KI开裂面几何的设计,通过对仿真数据的回归分析得到无量纲函数关系的通解,进而建立基于尖锐四棱锥压头的陶瓷材料断裂韧性压入测试方法.对三种陶瓷试样压入实验表明,测试方法精度较高,可以在较小载荷条件下产生裂纹,测试结果几乎不受压入载荷的影响.     

酞侧基聚芳醚酮冲击断裂韧性的研究

本文用仪器化冲击仪研究了酞侧基聚芳醚酮（ＰＥＫ－Ｃ）的冲击断裂韧性对温度的依赖性。依据线弹性断裂力学原理，通过１．５ＹＰＬ／ＢＤ２对ａ－１／２作图，所得直线斜率即为应力强度因子Ｋｃ；用Ｕ对ＢＤΦ作图，所得直线斜率即为扩展裂纹所需的能量Ｇｃ。在不同温度下测试，便可得到Ｋｃ－Ｔ，Ｇｃ－Ｔ的依赖关系图。温度对冲击断裂韧性的影响，是ｔａｎδ损耗和裂尖前缘热钝化共同作用的结果。在１００～１４０℃之间，断裂韧性的升高主要是由于β转变作用的结果。通常ｔａｎδ对能量吸收过程的影响，要比裂尖前缘热钝化的影响小，只有当裂尖前缘区域的有效温度Ｔｅ低于软化温度Ｔｓ时，ｔａｎδ才起作用。当Ｔ＝１８０℃，Ｔｅ＝２０２．４３℃，大于软化温度Ｔｓ时，裂尖前缘热钝化开始起作用，冲击断裂韧性随温度升高而升高     

典型陶瓷岩板的力学性能研究

陶瓷岩板旨在跨界应用于三维家居领域,然而,在如切割或钻孔等后期加工过程中可能会由于裂纹失稳扩展而导致碎裂。断裂韧性是评价陶瓷材料抵抗裂纹扩展能力的关键指标,建陶行业应引入断裂韧性来评价陶瓷岩板的力学性能。为了可靠评价陶瓷岩板的断裂韧性并探究物相组成及显微结构对其影响,首次采用基于超尖V型切口的改良SEVNB法准确测得四种典型陶瓷岩板的断裂韧性值,其范围为1.04～1.35 MPa·m^1/2,并估算出这四种陶瓷岩板的临界裂纹尺寸为79.1～119.4μm。通过物相定量分析发现较多的刚玉和较少的玻璃相有利于提高其断裂韧性;通过显微结构分析发现四种陶瓷岩板的断面不完全致密,气孔分布不均匀,而切割裂岩板断面存在较大尺寸的游离石英晶粒和长度约为100μm的大尺寸裂纹,接近其临界裂纹尺寸估算值108μm,在低应力作用下,裂纹容易失稳扩展导致材料断裂,这是导致陶瓷岩板切割裂的主要原因。     

陶瓷材料断裂韧性的测量标准刍议

现行的陶瓷材料断裂韧性测试方法有多种，其中常用的是单边预裂纹梁法（SEPB）和单边V型切口梁法（SVENB）。本文比较了采用SEPB法和SEVNB法测量断裂韧性的三种标准（国际标准化组织ISO 23146、美国材料与试验协会ASTM C1421和中国国家标准GB/T 23806）的异同。三种标准在试样规定、加载测试和结果计算方面差异不大，不同形状因子计算相对差值低于1%。现行国标因采用非国际基本单位mm会造成断裂韧性计算结果错误，需要经过校正系数k修订后，才能获得正确的断裂韧性值（MPa·m0.5）。     

压痕残余应力对氮化硅基复合材料阻力曲线行为的影响

通过压痕小裂纹直接量测法获得了ＴｉＣ，ＴｉＮ／Ｓｉ３Ｎ４复合材料的阻力曲线，采用更具合理性的指数经验公式拟合处理实验数据，初步探讨了Ｋ∞，Ｋ＾＊Ｒ，ΔＣ，等参数的物理意义。对压痕残余应力消除前 实验结果进行比较，发现压痕残余应力的消除，提高了材料的极限断裂韧性值Ｋ＾＊Ｒ，却大大减少了裂纹稳态生长的容限，使得材料的脆性行为更为突出。     

Si3N4陶瓷材料高温力学性能和断裂行为定量化实验研究

陶瓷构件常常服役于高温极端环境，易发生脆性断裂破坏。然而，关于陶瓷材料定量化的高温微观断裂研究报道较少。本文基于硅钼棒加热技术开展了Si₃N₄陶瓷材料高温维氏压痕实验，测试了惰性环境下，材料从室温至1200℃间的维氏硬度和断裂韧性，定量化表征了典型温度下的裂纹偏转和穿沿晶行为。研究表明：Si₃N₄陶瓷材料的维氏硬度、断裂韧性和杨氏模量在高温下有一定的降低，实验温度下的沿晶裂纹比例远大于穿晶裂纹比例，而裂纹偏转角分布随温度的变化不大。     

单边V切口梁法测量结构陶瓷断裂韧性的研究进展

断裂韧性作为评价陶瓷材料脆性断裂的重要指标,在陶瓷断裂力学上起着关键性的作用。本文介绍单边V切口梁法测量结构陶瓷断裂韧性的发展及相关研究成果,重点讨论基于飞秒激光加工超尖V型切口的SEVNB法。采用飞秒激光在结构陶瓷试样上可获得尖端曲率半径小于0.5 μm的理想切口,经实验证明,这种改良后的SEVNB法可以准确、可靠测得结构陶瓷材料的断裂韧性。同时,论文中阐述了飞秒激光加工的超尖V型切口的尖端曲率半径、切口尖端显微结构及飞秒激光产生的热应力对断裂韧性测试结果的影响,为单边V切口梁法准确、可靠地测试结构陶瓷断裂韧性提供依据。     

Effect of Precrack "Halos" on Fracture Toughness Determined by the Surface Crack in Flexure Method

The surface crack in flexure method, which is used to determine the fracture toughness of dense ceramics, necessitates the measurement of precrack sizes by fractographic examination. Stable crack extension may occur from flaws under ambient, room-temperature conditions, even in the relatively short time under load during fast fracture strength or fracture toughness testing. In this article, fractographic techniques are used to characterize evidence of stable crack extension, a "halo," around Knoop indentation surface cracks. Optical examination of the fracture surfaces of a high-purity Al₂O₃, an AlN, a glass-ceramic, and a MgF₂ reveal the presence of a halo around the periphery of each precrack. The halo in the AlN is merely an optical effect due to crack reorientation, whereas the halo in the MgF₂ is due to indentation-induced residual stresses initiating crack growth. However, for the Al₂O₃ and the glass-ceramic, environmentally assisted slow crack growth is the cause of the halo. In the latter two materials, this stable crack extension must be included as part of the critical crack size to determine the appropriate fracture toughness.     

A New Method for Precracking Beam for Fracture Toughness Experiments

A simple and versatile precracking method using a triangular notch as a crack starter in limited bending was developed, which is suitable for both brittle ceramics and quasi-plastic materials that are difficult to precrack by the conventional bridge-indentation technique. Slow growth of large crack in brittle or quasi-brittle ceramics was controlled and observed in situ in this way. The precracking tests performed on various ceramics exhibited high reliability and feasibility. The precracked specimens were subsequently used to measure the fracture toughness, and the resultant data showed that the fracture toughness determined by using the precracked specimens reflected the minimum value of the toughness measured in single edge-notched beam (SENB) tests.     

Methodology for measuring fracture toughness of ceramic materials by instrumented indentation test with vickers indenter

Virtual crack closure technique and elastoplastic finite element method were employed to calculate the stress intensity factors (SIF) of ceramic materials on the tip of both half‐penny crack (HPC) and radial crack (RC) induced by Vickers indenter and the value of fracture toughness (K_IC) was extracted by the design of equi‐SIF contour of HPC and RC crack front. Through dimensional theorem and regressive analysis, a functional relationship between instrumented indentation parameters, crack length of Vickers impression and fracture toughness of ceramic materials was established, thus a novel methodology has been presented for measuring fracture toughness of ceramic materials by instrumented Vickers indentation. Both numerical analysis and experiments have indicated that this methodology enjoys higher measurement precision compared with other available indentation methods. The methodology is universally suitable for HPC, RC as well as transition cracks and capable of determining fracture toughness and elastic modulus in a single indentation test. In addition, it saves the effort of measuring the diagonal length of Vickers impression in case that the impression remains unclear.     

Double Torsion testing of thin porous zirconia supports for energy applications: Toughness and slow crack growth assessment

Thin, porous zirconia-based ceramic components are of high interest in energy application devices where they are used as structural ceramics. Mechanical reliability of such devices is not only dependent on the fracture toughness of the ceramic components, but also on their sensitivity to slow crack growth (SCG). In this work, the fracture toughness and SCG behavior of porous (4.5–45.5%) and thin (∼ 0.25 mm) 3Y-TZP ceramics are investigated using the Double Torsion method. The analysis of the double torsion data, previously developed for dense materials, was here assessed and adapted. The compliance of the samples was observed to change linearly with crack length and the measured stress intensity factor was dependent on crack length, as for dense materials. This dependency decreased by increasing the sample porosity. For all materials, the ratio of the SCG threshold to fracture toughness was of 0.56 ± 0.06.     

Experimental Study of the Fracture Toughness of a Ceramic/Ceramic-Matrix Composite Sandwich Structure

A hybrid experimental–numerical approach has been used to measure the fracture resistance of a sandwich structure consisting of a 304 stainless steel/partially stabilized zirconia ceramic-matrix composite crack-arresting layer embedded in a partially stabilized zirconia ceramic specimen. The mode I fracture toughness increases significantly when the crack propagates from the ceramic into the ceramic-matrix composite region. The increased toughening due to the stainless steel particles is explained reasonably well by a toughening model based on processing-induced thermal residual stresses. In addition, several experimental modifications were made to the chevron-notch wedge-loaded double cantilever beam specimen to overcome numerous problems encountered in generating a precrack in the small, brittle specimens used in this study.     

Fracture Toughness Evaluation of Small Thin Ceramic Specimens

A test method to evaluate the fracture toughness, K_IC, of thin, small, precracked ceramic specimens is described. The method is applicable for thin plates, wafers, self-supported layers, etc., especially when a large amount of material is not available for testing. The method consists of bonding a small, thin single-edge notched beam on one side of a metallic beam. A stress-free precrack with a square root singularity is achieved when the assembly is deformed in three-point bending.
The fracture toughness of a thin, alumina single-edge precracked beam was evaluated experimentally using this method, and compared with that obtained for similar specimens having a 0.3-mm-wide machined notch. Comparison with previously reported fracture toughness values suggests that even a very sharp machined notch overestimates the evaluated fracture toughness.     

Fracture toughness of glasses and hydroxyapatite: A comparative study of 7 methods by using Vickers indenter

Numerous methods have been proposed to estimate the indentation fracture toughness Kic for brittle materials. These methods generally uses formulæ established from empirical correlations between critical applied force, or average crack length, and classical fracture mechanics tests. This study compares several models of fracture toughness calculation obtained by using Vickers indenters. Two optical glasses (Crown and Flint), one vitroceramic (Zerodur) and one ceramic (hydroxyapatite) are tested. Fracture toughness and hardness are obtained by using instrumented Vickers indentation at micrometer scale. Young's moduli are obtained by instrumented Berkovich indentation at nanometer scale. Fracture toughness is calculated with models involving crack length measurements, and by models free of crack length measurements by considering critical force, chipping, pop-in. Finally, method based on the cracking energy, commonly employed for coated materials is also used.The aim of this work is to compare seven methods, which enable the facture toughness determination, on four brittle materials. To do so, it was necessary to determine some specific constant in the case of Vickers tip use.On the one hand, results show that methods using crack length, critical force, edge chipping or pop-in lead to comparable results, and the advantages and drawbacks are highlighted. On the other hand, the indentation energy method leads to underestimated results of about 20%.     

Fabrication and mechanical properties of Al2O3-SiCw-TiCnp ceramic tool material

Whiskers and nanoparticles are usually used as reinforcing additives of ceramic composite materials due to the synergistically toughening and strengthening mechanisms. In this paper, the effects of TiC nanoparticle content, particle size and preparation process on the mechanical properties of hot pressed Al₂O₃-SiC_w ceramic tool materials were investigated. The results showed that the Vickers hardness and fracture toughness of the materials increased with the increasing of TiC content. The optimized flexural strength was obtained with TiC content of 4 vol% and particle size of 40 nm. The particle size has been found to have a great influence on flexural strength and small influence on hardness and fracture toughness. It was concluded that the flexural strength increased remarkably with the decreasing of the TiC particle size, which was resulted from the improved density and refined grain size of the composite material due to the dispersion of the smaller TiC particle size. SEM micrographs of fracture surface showed the whiskers to be mainly distributed along the direction perpendicular to the hot-pressing direction. The fracture toughness was improved by whisker crack bridging, crack deflection and whisker pullout; the TiC nanoparticles in Al₂O₃ grains caused transgranular fracture and crack deflection, which improved the flexural strength and fracture toughness with whiskers synergistically. Uniaxial hot-pressing of SiC whisker reinforced Al₂O₃ ceramic composites resulted in the anisotropy of whiskers’ distribution, which led to crack propagation differences between lateral crack and radical crack.     

Effect of polycarbonate molecular weight on precrack hysteresis energy in determining its ductile-brittle transition

The distinctive ductile-brittle transition behavior of pseudoductile polymeric materials such as polycarbonate (PC) has been discovered to be closely related to the precrack hysteresis loss energy. The higher molecular weight (MW) PC with higher ductility also results in higher precrack hysteresis energy and therefore greater precrack plastic volume under condition of constant loads. If the precrack plastic volume exceeds a critical value, crack initiation thereafter will propagate within the domain of the plastic zone and results in ductile fracture. The deformation displacement is closely related to the precrack plastic volume, and the critical displacement actually determines the critical plastic volume. The higher molecular weight polycarbonate with higher entanglement density is able to withstand earlier crack initiation more effectively. Toughening plastics, such as rubber modification, is simply the result of delaying or retarding the crack initiation and allows the precrack plastic zone to grow over its critical value. A model of crack criterion based on precrack plastic zone is proposed to interpret the ductile-brittle transition phenomenon.