Cone crack initiation induced by contact from cylindrical punch

The critical load for cone crack initiation in a brittle material indented by a rigid cylindrical punch is related to the fracture toughness of the material and the punch radius through the classical energy principles. The strain energy required to form an embryo cone crack on a flaw-free surface adjacent to the punch edge is formulated, from which the critical load for cone cracking is then determined. The present analysis shows that the stress singularity close to the sharp contact edge is akin to that a sharp crack tip. The results in this study can be used to set up a simple and practical technique for evaluating some strength-related properties of brittle materials such as the fracture toughness.     

Supersmall punch test to estimate fracture toughness JIc and its application to radiation embrittlement of 2.25Cr-1Mo steel

A supersmall punch test has been used to extract fracture strain information on irradiated 2.25Cr-1Mo steel from transmission electron microscopy disc specimens as small as 3 mm in diameter and 0.25 mm in thickness. The test is based on driving a steel ball punch through a clamped specimen. The size effect of biaxial equivalent fracture strain of various kinds of materials and irradiated steel has been demonstrated. The results of fracture strain obtained from specimens 3 mm in diameter has been related almost linearly to the fracture toughness J_Ic for elastic and plastic behaviour. The relationship between fracture strain and fracture toughness J_Ic has been verified for the irradiated nuclear pressure vessel steel 2.25Cr-1Mo so that large amounts of irradiation space in nuclear reactor could be saved.     

Small punch testing for determining the cryogenic fracture properties of 304 and 316 austenitic stainless steels in a high magnetic field

This paper examines the effect of magnetic field on the fracture properties of austenitic stainless steels at liquid helium temperature (4 K). Small punch tests were performed on cold-rolled 304 and 316 austenitic stainless steels. Previously proposed correlation for small punch and elastic-plastic fracture toughness test methods was applied to predict a small punch test-based fracture toughness from equivalent fracture strain.     

Predicting the J integral fracture toughness of Al 6061 using the small punch test

The 6000 series aluminium alloys (Al–Mg–Si systems) are commonly used as medium‐strength structural materials; in particular, the 6061 (Al–1Mg–0.6Si) alloy is widely utilized as a general‐purpose structural material due to its excellent formability and corrosion‐resisting capabilities. The objective of this study was to obtain a correlation between the small punch (SP) test estimated equivalent fracture strain (ɛ_qf) and fracture toughness (J_1C) property for 6061 aluminium, and determine its viability as a non‐destructive fracture toughness test technique for remaining life assessment of in‐service components. Samples of 6061‐T6 aluminium were cut from bulk plate, in both the longitudinal and transverse directions, for the as‐received condition as well as subjected to three different over‐ageing heat‐treatment schedules. A strong linear correlation between valid J_1C and SP estimated biaxial fracture strain ɛ_qf is presented for aluminium 6061 at room temperature.     

Quasibrittle fracture beneath a flat bearing surface

The problem of fracture of the quasibrittle and brittle material such as glass when subject to indentation by a relatively rigid, square-ended uniform punch is considered, using classical energy principles. The strain energy needed to form a crack adjacent to the punch edge is found, and from this the load needed to initiate fracture given. The calculation is of practical interest in the design rating of bearing blocks supporting structural glass. Additionally, it implies also the potential possibility to set up a very simple and practical technique for evaluating some strength-related properties of materials such as the fracture toughness using specimens without macro-pre-existing cracks.     

Biaxiality Dependence of the Fracture Toughness for Glass in Plane Stress State

The fracture behaviour of glass in biaxial stress state has been investigated. Fracture toughness of disk specimen with a straight-through crack was measured under biaxial tension and uniaxial tension loads respectively. The difference between them and the reasons for the difference are discussed. The influence of the stress parallel to crack on fracture of brittle material was demonstrated in theory and experiments. The results show that plane stress fracture toughness of glass is not a material constant. and that the fracture toughness measured in biaxial tension state is higher than that measured under uniaxial tension. The conventional fracture criterion upon the stress intensity factor is questioned in the case of biaxial stress problem, and the strain dependence of crack growth is discussed.     

Fracture toughness prediction of aged CF8M using small punch test

Following Ref. 1 , this technical note presents a method to predict the thermal ageing effect on fracture toughness of cast stainless steel CF8M from the small punch test using finite element (FE) damage analysis. A procedure is given to extract tensile properties and multi‐axial fracture strain locus of aged CF8M from the small punch test using FE analysis. It is further shown that fracture toughness of aged CF8M can be predicted from the small punch test using FE damage analysis. Comparison with experimental data shows good agreement.     

Acoustic Harmonic Generation Due to Thermal Embrittlement of Inconel 718

This paper describes an attempt to characterize the deterioration of a structural material's mechanical properties by nonlinear acoustics. In this particular case, the damage was caused by “thermal embrittlement” during which the material, here the nickel-based alloy Inconel 718, loses a significant fraction of its fracture toughness. Harmonic generation was the experimental method used to characterize the microstructural changes in the material as a function of exposure time at elevated temperatures. Tests were performed on two heats of Inconel 718 with slightly different chemistries, with one heat showing particular sensitivity of the fracture toughness to the elevated temperature exposure with corresponding higher changes in the nonlinearity parameter. As a mechanical measure of the fracture toughness deterioration, a small specimen punch test was used in which the ductility of a thin slice of material is determined. A clear difference between the two heats was noted in the metallographic examination, which is reflected in the harmonic generation as well as the punch test data. An explanation for the changes of the harmonic generation during the embrittlement process is speculative at the present time.     

Analysis of key factors for the interpretation of small punch test results*

In recent years, great advances have been made in non‐destructive or quasi‐non‐destructive test techniques for the determination of fracture toughness. One of the most interesting of these techniques is the one known as Small Punch, in which a punch acts on a small plane specimen, deforming it until fracture. From the continuous records of force and displacement of the punch taken during the test, it is possible to quantify the energy absorbed and then to determine the classic parameters of fracture mechanics such as ductile‐to‐brittle transition temperature or even the fracture toughness of the material. In this paper, fundamental aspects of the interpretation of the results of this kind of test have been studied. The effect of specimen thickness has been analysed. Moreover, a study has been carried out with the aim of clarifying the relation between the orientations in which it is possible to machine Small Punch specimens and the orientations of traditional specimens used in the determination of fracture toughness.     

铁素体钢断裂韧度尺寸效应的小冲孔试验

对六种不同厚度的试样进行了小冲孔试验,并进行了有限元模拟,此外还通过扫描电镜观察并分析了断裂面的微观结构。试验结果显示,试样的SP断裂变形能和断裂韧度随试样厚度的增大而增大。提出了裂纹起始遵循断裂应变准则及裂纹扩展遵循断裂能密度准则的观点。有限元模拟结果与试验结果基本吻合。断裂面呈典型的韧性断裂特征,试样变形后的半球状外表面布满微小的褶皱状突起和微裂纹。     

The nonlinear and biaxial effects on energy release rate, J-integral and stress intensity factor

A nonlinear finite element analysis is performed for a finite center-cracked specimen subjected to biaxial loading. A Ramberg-Osgood type stress-strain relation is used to characterize the material property. It is found that the energy release rate, J-integral, stress intensity factor, strain intensity factor depend not only on applied stress perpendicular to the crack but also on applied stress parallel to the crack. Biaxial effects on fracture toughness parameters increase as applied stress increases. The coupling between biaxial effects and material nonlinearity has been indicated.     

Multiscale analysis of stiffness and fracture of nanoparticle-reinforced composites using micromechanics and global–local finite element models

A combined micromechanics analysis and global–local finite element method is proposed to study the interaction of particles and matrix at the nano-scale near a crack tip. An analytical model is used to obtain the effective elastic modulus of nanoparticle-reinforced composites, then a global–local multi-scale finite element model with effective homogeneous material properties is used to study the fracture of a compact tension sample. For SiO₂ particle-reinforced epoxy composites with various volume fractions, the simulation results for effective elastic modulus, fracture toughness, and critical strain energy release rate show good agreement with previously published experimental data. It is demonstrated that the proposed parametric multi-scale model can be used to efficiently study the toughness mechanisms at both the macro and nano-scale.     

压杆法研究厚度对小圆片试样断裂韧性的影响

采用SP压杆实验方法,在常温下对六种不同厚度的小圆薄片的断裂韧性进行测试。实验结果表明,随着厚度的增加,断裂变形能和断裂韧性增加,断裂部分的外表面表现出微突起,微突起四周存在微小裂纹明显的特征。考虑弹塑性变形能,通过SP冲压断裂变形能计算得到断裂韧性,进而得到临界断裂变形能密度。采用塑性断裂应变作为裂纹起裂判据,临界断裂变形能密度作为裂纹扩展判据的断裂模型,对SP压杆实验进行模拟,得到与实验结果比较相符的模拟结果。采用GTN模型研究厚度对空洞增长的影响,得到厚度增加导致微突起明显的空洞率增加。     

J integral as a fracture criterion of rubber-like materials using the intrinsic defect concept

Using the fracture mechanics framework, a fracture criterion based upon the intrinsic defect concept was developed to predict the failure of rubber parts under biaxial monotonic loading. This fracture criterion requires as input data the fracture toughness of the material in terms of critical value of the J integral, the constitutive law of the material and the breaking stretch of a smooth specimen under uniaxial tension. To develop this criterion a generalized expression of the J integral under biaxial loading is proposed on the basis of finite element calculations on a RVE containing a small circular defect. The estimated failure elongations were found in very nice agreement with experimental data on two kinds of rubber materials. Moreover, we have also shown that this criterion could be extended to the failure analysis of thermoplastic polymers.     

The effect of material defects on the fatigue behaviour and the fracture strain of ABS

The structural integrity and durability of a construction are highly dependent on the material quality. Poly(Acrylonitrile-Butadiene-Styrene) Copolymer (ABS) is a material which is preferably chosen for high performance products, because of its superior toughness. The toughness of ABS is revealed by its high fracture strain in a tensile test, and high notched Izod impact fracture energy. However, the fatigue resistance of ABS is less favourable. This investigation is mainly devoted to the fatigue behaviour of ABS and to the fracture strain and the notched Izod fracture energy. Various mechanical tests, performed in conjunction with scanning electron microscope investigations of the fracture surfaces, demonstrate that fracture initiates from small defects which are abundantly present in the material. Especially the fatigue fracture surfaces show numerous cracks which had initiated from the defects. The fracture strain in tensile tests is high, but shows a large scatter. It is demonstrated that the fracture strain is also related to the presence of defects. A pre-fatigue load up to 40% of the anticipated fatigue life, followed by a tension test shows a significant reduction of the fracture strain as compared with a tension test on non damaged as-moulded material. Microscopic investigations show that this fracture strain reduction is caused by the presence of small cracks which initiated from the defects, during the preceding fatigue load. A similar but much smaller effect of pre-fatigue was observed for notched Izod tests. Finally it is concluded that the fatigue behaviour of ABS is dominated by the growth live of microscopic small cracks from material defects.     

Effect of rare-earth oxides on fracture properties of ceria ceramics

The influences of the sintering additive content of rare-earth oxide (Y₂O₃, Gd₂O₃, Sm₂O₃) on microstructure and mechanical properties of ceria ceramics were investigated by scanning electron microscopy and small specimen technique. A small punch testing method was employed to determine the elastic modulus and biaxial fracture stress of the ceria-based ceramics, and the fracture toughness was estimated by Vickers indentation method. Grain growth in the rare-earth oxides doped ceria ceramics was significantly suppressed, compared to the pure ceria ceramics. However, the elastic modulus, fracture stress and fracture toughness were decreased significantly with increasing additive content of the rare-earth oxides, possibly due to the oxygen vacancies induced by the rare earth oxides doping. The experimental results suggest that the change in the mechanical properties should be taken into account in the use of ceria-based ceramics for solid oxide fuel cells, in addition to the improvement of oxygen ion conductivity.     

The effect of aspect ratio on toughness in composites

Much information now exists on factors affecting toughness in composites. Theoretical expressions for fracture energy also abound, in response to the many factors that have been identified as contributing to toughness in fibre reinforced materials. This material is reviewed from the point of view of the effect of aspect ratio on toughness. Expressions relating fracture energy to aspect ratio are derived and compared with experimental data. It is shown that in many cases aspect ratio should be as large as possible. There are a few cases, however, where the aspect ratio should be as close to the critical value as possible.     

A micromechanical model for the prediction of the temperature fracture behaviour dependence in metallic alloys

In the present paper, a micro-mechanical model based on energetic considerations is developed to simulate the effect of environmental temperature on the fracture toughness of metallic alloys. By considering a reference elementary volume (REV) with the same composition of the real material, the stress-strain field inside such a volume and the corresponding strain energy due to a temperature variation is determined. The energy balance to determine the material fracture toughness is generalised in order to take into account the temperature effects. The proposed micro-mechanical model is governed by few parameters which can be simply estimated, and allows us to determine the fracture toughness for any temperature below the room temperature. Such a model is applied to three metallic alloys which show a ductile-brittle transition temperature: ASTM A471, Carbon Steel D6ac, Steel S275 J2. From the comparison of theoretical results with experimental data, it can be concluded that the model seems to be able to correctly predict the fracture toughness at low temperatures.