On the effects of an electric field on the fracture toughness of poled piezoelectric ceramics

Central crack specimens have been used to study the effects of an applied electric field on the fracture toughness of poled soft lead zirconate titanate ceramics (PZT-5). The ultrasonic lapping technique was used to machine the central crack of the specimens used. The present experimental study illustrated that changing the field from negative to positive reduced the fracture toughness of a specimen subjected to an applied electric field. These experimental results were in agreement with the observations made by Park and Sun (J. Am. Ceram. Soc. 78 (1995) 1475) and Heyer et al. (Acta Mater. 46 (1998) 6615). Finite element results were employed to compare with the experimental data. Four fracture criteria, i.e. total energy release rate, mechanical energy release rate, local energy release rate and strain energy release rate, were compared with the experimental results. It was found the local energy release rate, which was defined based on a strip electric saturation model, was in broad agreement with the experimental observations. Moreover, it was found that the strain energy density criterion was unable to describe the effect of electric field on fracture toughness when a large electric field was applied.     

On the characteristic distance and minimum fracture toughness for cleavage fracture in a C-Mn steel

This study makes a further investigation on the characteristic distance, minimum fracture toughness and its temperature dependence for cleavage fracture in a C-Mn steel by the detailed finite element analysis combined with experimental observation and measurement. Results show that there is a minimum active zone for cleavage initiation, and the minimum fracture toughness of steel results from the minimum active zone necessary. Corresponding to the minimum fracture toughness, the cleavage fracture ahead of a crack tip can only initiate in a distance range from the minimum distance X_fmin determined by the lower boundary of the active zone to the maximum distance X_fmax determined by its upper boundary. The reason for the occurrence of the minimum active zone and the factors influencing it are analyzed. The temperature dependence of the characteristic distance and minimum fracture toughness and its mechanism are also discussed.     

陶瓷材料动态断裂韧性测试

为了测试陶瓷材料动态断裂韧性,利用Hopkinson压杆实验原理和改装的Hopkinson压杆装置,并将试件加工成单边切口梁进行了三点弯曲动态试验.利用改装的Hopkinson压杆装置可直接测得透射应力波,从而直接得到试件变形过程中作用在试件上的支反力.本文定义了无量纲挠度和挠度变化率,给出了几种陶瓷材料在不同挠度变化...     

片状氧化铝晶种对氧化铝陶瓷断裂韧性的影响

以片状氧化铝晶种作为第二相,采用无压烧结制备了氧化铝陶瓷,分析了片状氧化铝含量对氧化铝陶瓷微观结构的影响,采用扫描电子显微镜（SEM）观察分析试样的断口形貌;采用压痕法计算试样的断裂韧性（KIC）值;研究了不同含量的晶种引入量对氧化铝陶瓷断裂韧性的影响。结果表明烧结温度为1575℃时,相对致密度可以达到96.7%;片状氧化铝晶种的引入能够显著提高氧化铝陶瓷的断裂韧性;其片晶的裂纹偏转、片晶拔出效应等增韧机制发挥了主导作用;随着片状氧化铝含量的提高,氧化铝陶瓷的力学性能逐渐提高,当掺杂含量达到35%（质量分数）时,KIC达到6.4MPa.m1/2,当含量继续增加,KIC呈现逐渐降低的趋势。     

Formulation and implementation of strain rate-dependent fracture toughness in context of the phase-field method

The phase-field approach is a promising technique for the realistic simulation of brittle fracture processes, both in quasi-static and transient analysis. Considering fast loading, experimental evidence indicates a strong relationship between the rate of strain and the material's resistance against fracture, which can be considered by a dynamic increase factor for the strength of the material. The paper at hand presents a novel approach within the framework of phase-field models for brittle fracture. A rate-dependent fracture toughness is formulated as a function of the rate of crack driving strain components, which results in higher strength for faster loading. Beside the increased amount of energy necessary to evolve a crack at a high strain rate loading situation, the model incorporates quasi-viscous stress-type quantities that are not directly related to the formation of the crack and exist only in the phase-field transition zone between broken and sound material. The governing strong form equations for a transient simulation are derived and the relevant information for an implementation of the model into a finite element code is outlined in detail. The performance of the model is demonstrated for static and dynamic benchmark simulations and for a comparison to experimental findings.     

Experimental and numerical assessment of fracture toughness of dual-phase austempered ductile iron

The effects of the microstructure topology on the fracture toughness of dual-phase austempered ductile iron are studied in this paper by means of finite element modelling and experimental testing. To this end, specimens with matrix microstructures ranging from fully ferrite to fully ausferrite were studied and the preferential zones and phases for crack propagation were identified in every case. The effectiveness of the ausferrite phase as a reinforcement of the ferritic matrix via the encapsulation of the brittle and weak last-to-freeze (LTF) zones was confirmed. The toughening mechanism is consequence of the increment in the crack path longitude as it avoids the encapsulated LTF zones. Besides, the presence of small pools of allotriomorphic ferrite increase the crack propagation resistance of the ausferrite-ferrite matrices.     

平面应变断裂韧度评定中临界载荷研究

针对新版断裂韧性测试规范扩大的裂纹长度适用范围,利用有限元精细分析用于平面应变断裂韧度KIC评定中割线法确定临界载荷PQ的相对割线斜率ΔS的合理性,提出0.45≤a/W≤0.7范围内的相对割线斜率ΔS的表达式。结果表明:现行规范推荐的相对割线斜率ΔS不再适合于确定0.55a/W≤0.7范围内的临界载荷PQ,其最大相对误差已近8%。     

Fatigue fracture toughness of metals

The paper considers the peculiarities of fatigue crack propagation and final fracture of metals under cyclic loading. It is shown that the value of the fatigue fracture toughness of steels in an embrittled state is appreciably lower than that of the fracture toughness under static loading. A model of the transition from stable to unstable fatigue crack propagation is justified.     

Energy analysis on fracture of ferroelectric ceramics

In the spirit of the Griffith's theory on mechanical fracture, a fracture criterion of ferroelectric ceramics in terms of energy analysis is proposed in this paper. The energy criterion is compared with the local energy release rate proposed by Gao et al. and the strain energy release rate of Park and Sun. In addition, the criterion can be used to explain why a positive electric field promotes cracking while a negative electric field can retard crack propagation.     

Fracture toughness of the interface between Ni–Cr/ceramic,alumina/ceramic and zirconia/ceramic systems

Few studies have focused on the interface fracture performance of bi‐layered structures, which have an important role in dental restorations, using ceramic materials. The purpose of this study is to evaluate the fracture mechanics performance of the Ni–Cr/ceramic, alumina/ceramic and zirconia/ceramic interfaces by investigating the propagation of an interfacial crack under a wide range of mode mixities. The effect of the mechanical properties of the base materials and the interface, on the crack initiation and crack path, will also be studied. The finite element method (FEM) was used to calibrate the production of the experimental specimens, allowing to obtain the minimum dimensions and amounts of material needed to correctly characterize the fracture event. The specimens were tested until failure using a three‐point bending test machine. The interface fracture parameters were obtained using the FEM. For all specimens, the cracks propagated into the ceramic. The results suggest that, in Ni–Cr/ceramic, alumina/ceramic and zirconia/ceramic bi‐layered structures, the ceramic is weaker than the interface, which can be used to explain the clinical phenomenon that the ceramic chipping rate is larger than interface delamination rate. Consequently, a ceramic material with a larger fracture toughness is needed to decrease the failure rate of ceramic restorations.     

Experimental research on the compressive fracture toughness of wing fracture of frozen soil

It is commonly found that not only bending fracture but also compressive fracture occur frequently in compression, furthermore, in some specific conditions, compressive fracture sometimes has dominant effect on frozen soil. Therefore, it is extremely necessary to study the mechanical characteristics of the compressive fracture of frozen soil and to investigate the damage and fracture mechanism of frozen soil based on the previous research on frozen soil damage in compression. This study draws on the ideas and methods used in compression fracture research on ice that is very similar to frozen soil, and specific clay in Shenyang region was adopted as the experimental material, to make compressive specimens containing tilted wing crack of different angles, and uniaxial unconfined compression fracture experiments were conducted at different temperatures and loading rates. The fracture toughness K_IC and K_IIC of the main crack tip of the specimens are calculated with obtained experimental results and the law of K_IC and K_IIC changing with tilted angles, temperatures and loading rate is obtained to gain an insight to damage mechanism of frozen soil in compression. This paper presents a meaningful attempt for the research on compressive fracture of frozen soil, so as to better solve practical engineering problems.     

基于材料全程本构关系对延性断裂韧性的数值模拟方法与应用

利用ANSYS APDL编程语言,采用FAT (Finite-element-analysis Aided Testing)方法获取26NiCrMoV11-5汽轮机转子材料、A508-III压力容器钢和304不锈钢材料全程单轴拉伸本构关系曲线。借助ABAQUS软件中GTN (Gurson-Tvergaard-Needleman)损伤模型,采用有限元逆向法对3种材料漏斗试样单轴拉伸试验过程进行数值仿真,标定相应的GTN模型参数,获得了较好的结果。在此基础上将3种材料GTN模型参数用以模拟紧凑拉伸(Compact Tension,CT)试样的准静态裂纹扩展,基于载荷分离法实现了对26NiCrMoV11-5、A508-III和304不锈钢3种材料J阻力曲线的良好预测。     

Experimental and numerical investigation of thickness effect on ductile fracture toughness of steel alloy sheets

Effect of thickness on ductile fracture toughness of plates made of steel alloy GOST 08Ch22N6T is investigated experimentally. Multiple specimen tests for determining fracture toughness have been conducted using compact tension (CT) specimens with thicknesses of 1.25, 1.64 and 4.06 mm according to standard test method ASTM E813. The results show the significant effect of thickness on fracture toughness. It is observed that in low thickness, J_c increases with the thickness increase until it reaches a maximum; however, further increase in the thickness causes the J_c-value to decrease. Two-dimensional finite element analysis is also performed to reproduce the experimental results. The comparison shows a very good agreement.     

Development of the Euro fracture toughness dataset

Ten European laboratories have generated the Euro fracture toughness dataset in order to provide an experimental data base sufficiently large to study specimen size and temperature effects on cleavage fracture toughness in the ductile-to-brittle fracture transition regime. The Euro fracture toughness dataset quantifies the fracture behaviour of the quenched and tempered pressure vessel steel DIN 22NiMoCr37. This material is frequently used in nuclear power plants. About 800 fracture toughness tests were performed using compact tension specimens with a size range from 1/2T to 4T.In the lower shelf temperature regime, no significant specimen size effects on cleavage fracture toughness scatter was observed. At higher temperatures, the lower tails of the toughness scatter bands are not significantly effected by the specimen size but with decreasing specimen size the toughness scatter increases due to the fact that the upper part of the scatter band is extended. The presence of a specimen size effect on fracture toughness scatter coincides with the appearance of single cleavage initiation sites at the fracture surface. At the lower shelf temperature both, cleavage initiation sites and size effects are not observed whereas at higher test temperatures both phenomena are present. The specimen size effect trends and the corresponding fracture surface morphology support a weakest-link type cleavage fracture mechanism in the ductile-to-brittle transition regime. A unique correlation between the amount of ductile tearing and cleavage fracture toughness was observed for the steel investigated. This result offers the possibility to determine cleavage fracture toughness from post-test fracture surface examinations.Due to the large number of tests and the wide range of testing conditions, the Euro fracture toughness dataset gives a comprehensive insight into specimen size effects and temperature effects on ductile-to-brittle transition fracture. The Euro fracture toughness dataset includes a large set of raw test data such as load versus load line displacement curves and raw tensile test data for deriving stress-strain curves. The Dataset can be downloaded from the internet via the address ftp://ftp.gkss.de/pub/eurodataset.     

A new method to estimate the plane strain fracture toughness of materials

Although the testing method for fracture toughness K_IC has been implemented for decades, the strict specimen size requirements make it difficult to get the accurate K_IC for the high‐toughness materials. In this study, different specimen sizes of high‐strength steels were adopted in fracture toughness testing. Through the observations on the fracture surfaces of the K_IC specimen, it is shown that the fracture energy can be divided into 2 distinct parts: (1) the energy for flat fracture and (2) the energy for shear fracture. According to the energy criterion, the K_IC values can be acquired by small‐size specimens through derivation. The results reveal that the estimated toughness value is consistent with the experimental data. The new method would be widely applied to predict the fracture toughness of metallic materials with small‐size specimens.     

铁电陶瓷中电畴翻转研究进展

电畴为铁电陶瓷固有的独特微观组织特征之一,铁电陶瓷的许多性能均与其密切有关.综述了铁电陶瓷中的电畴结构,系统介绍了电场、机械作用引起的电畴翻转,概述了电畴翻转对铁电陶瓷断裂韧性的影响及其研究进展.     

The lower bound toughness procedure applied to the Euro fracture toughness dataset

The validity of a statistical method for estimating an engineering lower bound fracture toughness in the ductile-to-brittle transition region is investigated using the Euro fracture toughness dataset generated in the European SM&T Project “Fracture Toughness of Steel in the Ductile-to-Brittle Transition Regime”. The lower bound method is based on the empirical evidence that, in the low probability regime, the cumulative failure probability function tends to be a straight line rather than a curve as is the case for Weibull distributions. The investigation demonstrates that the engineering lower bound toughness values as predicted by the method are related to a cumulative cleavage failure probability lower than 2.5%. Such bound predictions can be achieved on the basis of a small number of cleavage toughness values measured at the temperature of interest. The results confirm the validity of the method.     

A modified specimen for evaluating the mixed mode fracture toughness of adhesives

This article introduces a specimen geometry that allows the separation of fracture energy release rates G _I and G _II in adhesively joined beams made of disparate materials. The analysis is based upon a Green's functions formulation for shear deformable beams and circumvents the need to employ finite element computations. The current method results in a system of non-singular integral equations, that can be discretized and reduced to a system of algebraic equations which may be solved by common numerical techniques. The analysis accounts for the dimensions and properties of the adhesive and provides results for a wide range of G _I, G _II and their ratio. Those results agree with finite element computational values to within less than 4%.     

An experimental cum numerical technique to determine dynamic interlaminar fracture toughness

A method combining experimental and finite element analysis is developed to determine interlaminar dynamic fracture toughness. An interlaminar crack is propagated at very high speed in a double cantilever beam (DCB) specimen made of two steel strips which are bonded together by epoxy with a precrack of about 40 mm length. The face of the front cantilever is bonded to a large solid block and a special fixture is designed to apply impact load to the rear cantilever through a load bar. In the load bar, a compressive square shaped elastic stress pulse is generated by impacting it with a striker bar which is accelerated in an air gun. The rear cantilever is screwed to the load bar; when the incident compressive pulse reaches the specimen, a part of the energy is reflected into the load bar and the rest of it passes to the specimen. By monitoring the incident and the reflected pulses in the load bar through strain gauges, deflection of cantilever-end is determined. The crack velocity is determined by three strain gauges of 0.2 mm gauge length bonded to the side face of the rear cantilever. Further, the first strain gauge, bonded very close to the tip of the precrack, and the crack velocity determine the initiation time of crack propagation.

The experimental results are used as input data in a finite element (FE) code to calculate J-integral by the gradual release of nodal forces to model the propagation of the interlaminar crack. The initiation fracture toughness and propagation fracture toughness are evaluated for an interlaminar crack propagating with a velocity in the range of 850 to 1785 m/s. The initiation toughness and propagation toughness were found to vary between 90–200 J/m² and 2–13 J/m², respectively.     

A new method for determining the fracture toughness of main pipeline steels

One of the fundamental aims of fracture mechanics is to define fracture toughness K_IC of a material. Hence, the ASTM E399 standard was developed. However according to the standard, large‐sized specimens are required to determine the fracture toughness of low alloy carbon steels. ASTM E1921 standard was developed on the fracture toughness of ferritic steels. In this study, a new method was proposed to determine the fracture toughness of ferritic steels. The purpose of the present paper is to compare the results of the method with the experimental results. Two steels that are used in gas and oil main pipelines were investigated in this study.