Thermal shock fracture of piezoelectric materials

This paper deals with a case study for the piezoelectric materials suddenly exposed to an environmental medium of different temperature. The problem is idealized to a plate containing an edge crack or an embedded crack. The stress and electric displacement histories in an uncracked plate are calculated. These stresses and electric displacements are then added to the crack surface tractions and electric displacements with opposite sign to formulate a mixed boundary value problem. The cracking problem is thus reduced to a singular integral equation of Cauchy type, which is then solved numerically. Both impermeable crack assumption and permeable crack assumption are considered. The results for stress and electric displacement intensity factors are computed as a function of normalized time and crack size. Lower bound solutions are obtained for the maximum thermal shock that the material can sustain without catastrophic failure according to the two distinct criteria: (i) The maximum local tensile stress equals the tensile strength of the medium. (ii) The maximum stress intensity factor for the pre-existing representative crack equals the fracture toughness of the medium. The parameters that control the transient thermal stress and electric displacement are also identified. The method can be used to explore susceptibility to thermal fracture in piezoelectric materials containing pre-cracks.     

Atomistic simulation of microtwinning at the crack tip in L12 Ni3Al

The mechanisms of deformation at the crack tip in L1₂ Ni₃Al have been studied by molecular dynamics simulations. The stress-induced microtwinning is found to occur at the crack tip when a sufficiently high stress concentration exists. The formation mechanism of the microtwinning is discussed. It is found to be achieved by the emission of Shockley partial dislocations from the crack tip and then slip of the Shockley partial dislocations on adjacent {111} planes. Furthermore, the mechanism of the microtwinning is also discussed from the standpoint of stress.     

Role of hydrogen in stress corrosion cracking of austenitic stainless steels

Effects of hydrogen charging on the stress corrosion cracking (SCC) behavior of 304 and 310 stainless steels under sustained load were investigated in boiling 42% MgCl₂ solution. The cracking was accelerated by the incorporation of hydrogen into the steel without altering the crack growth mechanism. The fact that the active dissolution is almost unaffected by the hydrogen charging and tensile stress indicates that the phenomenon of hydrogen-promoted SCC is unlikely a result of hydrogen-facilitated active dissolution. In contrast, hydrogen significantly promotes anodic dissolution in the potential range where the active-to-passive transition occurs. The electrochemical noise detected in the SCC process implies that the crack propagation process is discontinuous and hydrogen charging can raise the frequency of film breakdown at the crack tip. These observations suggest that the hydrogen-promoted SCC may result from the hydrogen-induced passivity degradation.     

Very-High-Cycle-Fatigue of in-service air-engine blades, compressor and turbine

In-service Very-High-Cycle-Fatigue(VHCF)regime of compressor vane and turbine rotor blades of the Al-based alloy VD-17and superalloy GS6K,respectively,was considered.Surface crack origination occurred at the lifetime more than 1500 hours for vanes and after 550 hours for turbine blades.Performed fractographic investigations have shown that subsurface crack origination in vanes took place inspite of corrosion pittings on the blade surface.This material behavior reflected lifetime limit that was reached by the criterion VHCF.In superalloy GS6K subsurface fatigue cracking took place with the appearance of flat facet.This phenomenon was discussed and compared with specimens cracking of the same superalloy but prepared by the powder technology.In turbine blades VHCF regime appeared because of resonance of blades under the influenced gas stream.Both cases of compressor-vanes and turbine blades in-service cracking were discussed with crack growth period and stress equivalent estimations.Recommendations to continue aircrafts airworthiness were made for in-service blades.     

邻苯二甲酸二酯系玻璃材料中裂纹愈合效应研究

用最近发明的一种能够实时检测玻璃材料中应力诱导裂纹(stress cracking)和裂纹愈合(crack-healing)的新方法(RMS-L-CH)对邻苯二甲酸二甲酯、邻苯二甲酸二乙酯、邻苯二甲酸二丁酯和邻苯二甲酸二辛酯的系列玻璃材料中应力诱导裂纹和裂纹愈合效应进行了测量,对裂纹愈合效应的现象及其与玻璃化转变的关系进行了深入的研究与讨论.结果进一步验证了RMS-L-CH方法的有效性,并且表明该方法可能提供一种表征玻璃化转变过程和研究玻璃化转变机理的新手段.     

Multiscale approach to micro/macro fatigue crack growth in 2024-T3 aluminum panel

When two contacting solid surfaces are tightly closed and invisible to the naked eye,the discontinuity is said to be microscopic regardless of whether its length is short or long.By this definition,it is not sufficient to distinguish the difference between a micro-and macro-crack by using the length parameter.Microcracks in high strength metal alloys have been known to be several centimeters or longer.Considered in this work is a dual scale fatigue crack growth model where the main crack can be micro or macro but there prevails an inherent microscopic tip region that is damaged depending on the irregularities of the microstructure.This region is referred to as the"micro-tip"and can be simulated by a sharp wedge with different angles in addition to mixed boundary conditions.The combination is sufficient to model microscopic entities in the form of voids,inclusions,precipitations,interfaces,in addition to subgrain imperfections,or cluster of dislocations.This is accomplished by using the method of"singularity representation"such that closed form asymptotic solutions can be obtained for the development of fatigue crack growth rate relations with three parameters.They include:(1)the crack surface tightness*represented by o/=0.3-0.5 for short cracks in region I,and 0.1-0.2 for long cracks in region II,(2)the micro/macro material properties reflected by the shear modulus ratio*(=micro/macro varying between 2 and 5)and(3)the most sensitive parameter d*being the micro-tip characteristic length d*(=d/do)whose magnitude decreases in the direction of region I II.The existing fatigue crack growth data for 2024-T3 and 7075-T6 aluminum sheets are used to reinterpret the two-parameter da/dN=C(K)nrelation where K has now been re-derived for a microcrack with surfaces tightly in contact.The contact force will depend on the mean stress m or mean stress ratio R as the primary parameter and on the stress amplitude a as the secondary parameter.     

钢中脆硬粒子裂纹形成机理

钢中的脆硬粒子对钢的解理脆断有直接的影响，解理断裂源大都发生在脆硬粒子上.根据微裂纹形成的热力学模型，利用钢中脆硬粒子开裂时所释放的弹性应变能、位错塞积弹性能，所产生的表面能，对脆硬粒子裂纹形成机理进行分析.模型计算表明，正应力和位错塞积力都是脆硬粒子开裂的必要条件，这与实验事实相符；同时给出脆硬粒子开裂的临界条件计算方法，计算发现，脆硬粒子临界开裂应力不仅取决于脆硬粒子尺寸及表面能，而且与晶粒直径有一定的相关关系，当晶粒直径较小时，这种关系与实验测定的材料解理断裂应力与晶粒尺寸的关系一致，说明整体失稳解 关键词： 解理断裂 裂纹形核 脆硬粒子     

Effect of interfacial debonding and matrix cracking on mechanical properties of multidirectional composites

In composites, debonding at the fiber–matrix interface and matrix cracking due to loading or residual stresses can effect the mechanical properties. Here three different architectures — 3-directional orthogonal, 3-directional 8-harness satin weave and 4-directional in-plane multidirectional composites — are investigated and their effective properties are determined for different volume fractions using unit cell modeling with appropriate periodic boundary conditions. A cohesive zone model (CZM) has been used to simulate the interfacial debonding, and an octahedral shear stress failure criterion is used for the matrix cracking. The debonding and matrix cracking have significant effect on the mechanical properties of the composite. As strain increases, debonding increases, which produces a significant reduction in all the moduli of the composite. In the presence of residual stresses, debonding and resulting deterioration in properties occurs at much lower strains. Debonding accompanied with matrix cracking leads to further deterioration in the properties. The interfacial strength has a significant effect on debonding initiation and mechanical properties in the absence of residual stresses, whereas, in the presence of residual stresses, there is no effect on mechanical properties. A comparison of predicted results with experimental results shows that, while the tensile moduli E ₁₁, E ₃₃and shear modulus G ₁₂ match well, the predicted shear modulus G ₁₃ is much lower.     

Evaluation on the interfacial fracture toughness of fiber-reinforced titanium matrix composites by push out test

The models for single-fiber push out test are developed to evaluate the fracture toughness G_IIc of the fiber/matrix interface in titanium alloys reinforced by SiC monofilaments. The models are based on fracture mechanics, taking into consideration of the free-end surface and Poisson expansion. Theoretical solutions to G_IIc are obtained, and the effects of several key factors such as the initial crack length, crack length, friction coefficient, and interfacial frictional shear stress are discussed. The predictions by the models are compared with the previous finite element analysis results for the interfacial toughness of the composites including Sigma1240/Ti-6-4, SCS/Ti-6-4, SCS/Timetal 834, and SCS/Timetal 21s. The results show that the models can reliably predict the interfacial toughness of the titanium matrix composites, in which interfacial debonding usually occurs at the bottom of the samples.     

Service life of a belt chamber-plasticity aspects

Abstract

The life time of the belt compound vessel is solved from the point of view of a fracture mechanics. It is shown that the existence of plastic deformations created in the matrix of the vessel during working state has practically no influence on the behaviour of the dominant crack and that for estimation of the critical state of the vessel the knowledge of the corresponding stress intensity factor is sufficient. The dominant crack grows under small scale yielding conditions until final fracture of the matrix.     

Low-cycle fatigue small crack initiation and propagation behaviour of cast magnesium alloys based on in-situ SEM observations

In-situ observations on the initiation and propagation behaviour of low-cycle fatigue small cracks in cast magnesium–aluminium alloys (AM50 and AM60B) were carried out with scanning electron microscopy (SEM) to elucidate the resistance to fatigue cracking and to evaluate the fatigue small crack growth rate accurately and quantitatively. The results indicate that the fatigue small cracks formed preferentially on β-phase (Mg₁₇Al₁₂) boundaries at room temperature. In addition, the effects of the parameters of stress levels in low-cycle fatigue and temperatures as well as microstructure on fatigue small crack propagation behaviour are revealed. The variation of crack open displacement (COD) with stress levels and cycles at elevated temperature shows that it is unsuitable to estimate the fatigue small crack growth rate of cast magnesium alloys using conventional measurement methods such as the plastic-replica technique due to the obvious difference between microscopic cracks in the open and closed states. Stabilized crack propagation behaviour is limited to cases where the physical crack length is less than 1?mm in low-cycle fatigue.     

Dynamic cracking resistance of metallic materials during rapid propagation of a self-similar crack

A model is proposed to determine the dynamic cracking resistance K _ID of metals and alloys for the case of a rapidly moving fractal or self-affine crack. The values of this characteristic correlate with the fractal dimension D _f of the future contour of a crack surface profile. K _ID is lower or higher than K _IC depending on the fractal dimension.     

Cohesive zone modelling of anodic dissolution stress corrosion cracking induced by corrosion product films

Damage mechanics based on the cohesive zone model were applied to study the anodic dissolution stress corrosion cracking (SCC) in flat and U-shaped edge-notched specimens. The simulation results show that corrosion product films (CPFs) facilitate crack initiation in SCC due to the CPF-induced stress and CPF rupture. In the flat specimen, SCC susceptibility increases with the CPF thickness and CPF Young’s modulus, while it decreases with CPF fracture strength. For the U-shaped edge-notched specimen, the normalised threshold stress intensity factor K_ISCC/K_IC decreases with the CPF thickness and notch depth.     

In situ observation and analysis of multiple cracking phenomena in thin glass layers deposited on polymer films

The multiple cracking phenomena in thin SiO_x films deposited on 12 μm-thick polyethylene terephthalate (PET) substrates during the tensile test are investigated. Thicknesses of SiO_x films ranged from 43 to 320 nm. The multiple cracking progress is observed in situ by optical microscopy and from which the crack density in SiO_x films is measured. The predicted crack density by the shear lag analysis including residual strains, explains reasonably well the experimental results. The critical energy release rate, G_c, for the first film cracking is also evaluated from simple energy balance arguments. Although it depends on the analytical model, G_c is estimated to be a constant value of about 1.0 J/m² regardless of the thickness.     

A general model for hydrogen trapping at the inclusion-matrix interface and its relation to crack initiation

Abstract

The role of non-metallic inclusions in hydrogen-induced failure of structural materials has been a controversial topic for many years. In this paper, hydrogen trapping and its relation to the crack initiation at the inclusion-matrix interfaces are studied by considering the interfacial structure and the interaction between the dissolved hydrogen atoms and the elastic strains produced by lattice matching and misfit dislocations. A model is proposed to analyse the change of interfacial structure with inclusion size and its relation to hydrogen trapping. Hydrogen accumulation at the interfaces is quantitatively analysed. The obtained results are in good agreement with the experimental observations. The multiple factors, such as interfacial structure, chemical composition, elastic properties of matrix and inclusions, crystallographic relationship between inclusions and matrix, inclusion morphology and size, simultaneously control hydrogen trapping. In addition, the mechanism of hydrogen-induced crack initiation at the interface is investigated. A criterion is proposed to determine critical conditions for crack initiation. For the first time, the inherent relationship between hydrogen trapping and hydrogen-induced cracking at the interface is clarified. This work paves a way for an in-depth understanding of the effects of inclusions on hydrogen-induced degradation of mechanical properties.     

Investigations on the growing, cracking and spalling of oxides scales of powder metallurgy Rene95 nickel-based superalloy

The surface and cross-sectional morphologies of powder metallurgy (PM) Rene95 nickel-based superalloy after 100 h oxidation in the temperature range of 700-1100 °C were investigated. It is shown that oxides nucleate first on the surface of the alloy and form an oxides scale. Afterwards, oxides scale endures decohesion, rumpling, cracking and finally spalling owing to the weak cohesive strength of the scale/alloy interface. The XRD and EDS analyses confirmed that the oxides scale of PM Rene95 superalloy is mainly composed by Cr₂O₃ at 800 °C and NiCr₂O₄ is the main spinel at 1100 °C. The subsequent analysis of internal stress verified that cracking and spalling are caused by growth stress and promoted by thermal stress. On these bases, improvement of the cohesive strength of the scale/alloy interface is considered to be the main way to increase the oxidation resistance of PM Rene95 superalloy.     

Nucleation of cracks near the free surface in deformed metallic nanomaterials with a bimodal structure

A theoretical model that effectively describes the nucleation of cracks in stress fields of dislocation pile-ups near the free surface in metallic nanomaterials with a bimodal structure has been developed. The dependences of the critical shear stress τ_c (for the formation of a crack with an equilibrium length of 10 nm on a dislocation pile-up near the surface) on the size d of a grain containing the dislocation pile-up have been calculated for copper with a bimodal structure. Theoretically, it has been found that the critical shear stress τ_c for the nucleation of a crack near the free surface in a nanomaterial with a bimodal structure is approximately 30% higher than that for the crack nucleation within the nanomaterial at a distance from the free surface.     

Combined mode-I and mode-II fracture of ceramics with crack-face grain and/or whisker bridging

Crack-face grain and/or whisker bridging in ceramics was investigated under combined mode-I and mode-II loading. A novel technique for analysing the stress shielding at the crack tip caused by the bridging was proposed, in which the critical values of the local mode-I and mode-II stress intensity factors were numerically derived from an azimuthal angle at the onset of noncoplanar crack extension using the mixed-mode failure criteria. The wedging effect, which induced local mode-I crack opening at the tip, was identified under the combined-mode loading on polycrystalline alumina as well as an alumina matrix composite reinforced with silicon carbide whiskers. The effect was accelerated with the increase in the mode-II component of nominally applied loading and the decrease in the bridging zone length. It was also found that the stress shielding due to the whisker bridging was not only effective for mode-I but also for mode-II crack opening.     

Fracture mechanics of propagating 3-D fatigue cracks with parametric dislocations

Abstract

Propagation of 3-D fatigue cracks is analyzed using a discrete dislocation representation of the crack opening displacement. Three dimensional cracks are represented with Volterra dislocation loops in equilibrium with the applied external load. The stress intensity factor (SIF) is calculated using the Peach–Koehler (PK) force acting on the crack tip dislocation loop. Loading mode decomposition of the SIF is achieved by selection of Burgers vector components to correspond to each fracture mode in the PK force calculations. The interaction between 3-D cracks and free surfaces is taken into account through application of the superposition principle. A boundary integral solution of an elasticity problem in a finite domain is superposed onto the elastic field solution of the discrete dislocation method in an infinite medium. The numerical accuracy of the SIF is ascertained by comparison with known analytical solution of a 3-D crack problem in pure mode I, and for mixed-mode loading. Finally, fatigue crack growth simulations are performed with the Paris law, showing that 3-D cracks do not propagate in a self-similar shape, but they re-configure as a result of their interaction with external boundaries. A specific numerical example of fatigue crack growth is presented to demonstrate the utility of the developed method for studies of 3-D crack growth during fatigue.