FORMATION OF HYDROGEN-ASSISTED INTERGRANULAR CRACKS IN HIGH STRENGTH STEELS

Abstract— The fracture behaviour of high strength steels under the influence of hydrogen was studied, with special emphasis on the critical condition for the formation of intergranular (IG) cracks. Mechanical tests were carried out on cathodically charged specimens subjected to both a constant load and a constant displacement under a variety of hydrogen-charging conditions. Experimental analyses show that a high local hydrogen concentration plus a high stress intensity at a quasi-cleavage (QC) crack tip are required to initiate intergranular cracking. The condition for continued intergranular crack propagation, leading to micro-void coalescence (MVC), is determined by the combined effect of the speed of crack propagation together with the rate of hydrogen diffusion. A quasi-cleavage crack triggers the onset of intergranular crack growth irrespective of the test control parameters, i.e. constant loading or constant displacement conditions.
A fracture map is proposed relating to the boundary conditions between QC, IG and MVC cracking, which will assist further research.     

基于微孔贯通细观损伤模型的金属韧性断裂分析

韧性材料断裂过程通常可看作是材料内部微孔洞的形核、扩展及相互贯通的积累。经典的Gurson- Tvergaard (GT)模型能够很好地模拟具有变形均匀、各向同性的孔洞的萌生及扩展过程;但无法模拟由孔洞贯通而引起的局部变形过程,因此需要对其修正,引入相应的孔洞贯通准则。该文采用两种贯通准则对经典GT模型进行修正,即Thomason的塑性极限载荷准则和临界等效塑性应变准则。借助用户自定义程序UMAT将采用这两种贯通准则修正的GT本构关系嵌入至商用有限元软件ABAQUS中,从而可通过对金属材料应力状态和断裂机理的分析控制孔洞的贯通。以一组含有不同缺口根半径的圆棒拉伸试验件为例,分析了该类金属构件自孔洞萌生至最终断裂的整个损伤演化过程,并与试验数据进行了对比,验证了该模型的有效性。该文还讨论了金属断裂过程中应力三轴度对微裂纹萌生与扩展的影响。     

The influence of stress intensity and microstructure on fatigue crack propagation in ferritic materials

New and published fatigue crack growth data for a wide range of steels have been categorized in terms of different growth mechanisms, namely striation formation, microcleavage, void coalescence and intergranular separation. General principles emerged concerning the influence of mean stress, specimen thickness, flow stress and toughness on rates of fatigue crack propagation through their effect on growth mechanism.

Crack propagation rates associated with striation formation were insensitive to changes in mean stress (except at very low stress intensities) and specimen thickness. Increase in flow stress resulted in a small decrease in growth rate, although the path of a crack through complex structures like welds was, nevertheless, strongly influenced by plastic relaxation. Crack propagation rates increased when deformation led to net-section yielding (general yielding) and the increase was related to specimen thickness and geometry. It has been shown that simple relationships between the rate of propagation and alternating stress intensity are adequate for describing fatigue crack growth by the striation mechanism.

Departures from exclusively striation formation to include micro-cleavage, void coalescence or intergranular separation were found to result in accelerated growth rates. Where growth occurred by combined striation formation and microcleavage, the increase in fatigue crack growth rate was dependent on the maximum tensile stress and hence on the mean stress and specimen thickness. Similarly, when fatigue involved the void coalescence mechanism the rate was increased by raising the mean stress. The role of microstructure and fracture toughness in promoting the different growth mechanisms is discussed. Modification of the simple growth law is necessary in order to describe the observed results.     

Growth of tensile and fatigue cracks in metal foils

The extension of cracks in thin foils of copper, brass, and tantalum has been investigated for both monotonic and cyclic loading using optical microscopy and scanning electron microscopy. Crack growth in both tension and fatigue is found to occur primarily by a void coalescence mechanism, as previously reported for aluminum foils. The particular morphology of void formation exhibited by a given material is determined by its purity level and stacking fault energy, the latter being important as it affects the slip character—varying from planar to wavy—of the metal.Fatigue crack growth without void formation has also been observed in brass and tantalum. The resulting fracture surfaces are flat; a different growth mechanism, attributed to plane strain conditions at the crack tip rather than plane stress, accounts for this mode of propagation.     

FATIGUE CRACK PROPAGATION AND CRACK CLOSURE BEHAVIOR IN POLYCARBONATE AND FIBRE REINFORCED POLYCARBONATE

Abstract— Fatigue crack propagation was investigated in polycarbonate and glass fibre reinforced polycarbonate and the effect of stress ratio and glass fibre content determined. The addition of glass fibre increases the tensile strength, but does not always contribute to an increase in fatigue crack propagation resistance. For polycarbonate the effect of stress ratio can be partly explained by using crack closure concepts as other researchers have suggested, but for glass fibre reinforced polycarbonate this was not possible. Fractography revealed a void growth process, which occurred by decohesion at the interface of the glass fibres and the base material, which was dependent on the maximum stress intensity factor. The process of linking the voids and the main crack growth behavior depended on the stress intensity factor range, Δ K. A proposed crack propagation model can explain the effect of stress ratio on crack propagation in fibre reinforced polycarbonate.     

Micromechanisms of fatigue-crack advance in PVC

The fatigue-crack propagation characteristics in poly(vinyl chloride) (PVC) are examined in terms of fracture mechanics concepts where the crack growth rate is related to the applied stress intensity factor range. The microscopic details of fatigue crack extension are examined with the aid of light optical, scanning and transmission electron microscopes. The mechanism of crack advance is found to be that of void coalescence through craze material generated in advance of the crack tip. While the craze is shown to grow continuously with cyclic loading, the crack is found to grow discontinuously in several hundred cycle increments.     

Investigation on fracture mechanisms of metals under various stress states

Fracture mechanisms for widely used metal materials are investigated under various loading conditions. Several specimens and different loading methods are deliberately designed to produce various stress states. The stress triaxiality is used to rate the level of tension and compression under various stress states. The stress triaxiality increases with adding a notch in the specimen under tension loading and decreases by changing the loading from tension to compression. Scanning electron microscopes are used to observe the microscopic features on the fracture surfaces. The fracture surfaces observed in the tests indicate that with the decreasing stress triaxiality the fracture mechanism for a given metal material includes intergranular cleavage, nucleation, growth, void coalescence, and local shear band expansion. With the fracture mechanisms changing from intergranular cleavage to nucleation, growth, and coalescence of voids, and expansion of a local shear band, four possible fracture modes can be observed, which are quasi-cleavage brittle fracture, normal fracture with void, shear fracture with void, and shear fracture without void. Quasi-cleavage brittle fracture and normal fracture with void are both normal stress-dominated fracture modes; however, their mechanisms are different. Shear fracture with and without void are both shear stress-dominated fracture, and shear fracture with void is also influenced by the normal stress. To a certain metal material, under high stress triaxiality, quasi-cleavage brittle fracture and normal fracture with void tend to occur, and under low stress triaxiality, shear fracture with and without void tend to occur. In addition, the critical positions and fracture criteria adapted to each fracture mode will also be different.     

APPLICATION OF LOCAL DAMAGE MODELS TO THE NUMERICAL ANALYSIS OF DUCTILE RUPTURE

Abstract— Two damage models were implemented into the finite element program ADINA to study the correlation between microscopical damage and macroscopical material failure. In the first model, based on the Gurson yield function the nucleation, growth and the coalescence of voids were incorporated into the constitutive relations. In the second model the void growth was determined according to the Rice and Tracey model using the von Mises yield function, and material failure was simulated by eliminating the elements where the critical void growth ratio was exceeded. The numerical results for the local and global behaviour of the specimens were compared with experiments. The generality of the damage parameters was checked by investigating several specimen geometries. Both damage models deliver qualitatively consistent results with regard to the influence of the stress triaxiality on the void growth and on the beginning of the material failure. However, the Gurson model gives a more accurate numerical simulation because the damage development and the stress drop continue after the onset of void coalescence while the critical void growth model causes less convergence problems in the simulation of large crack extension. The J _n-curve was estimated on the basis of both models.     

Experimental approach to dimple fracture mechanisms under short pulse loading

Dimple fracture mechanisms are discussed for three kinds of aluminum alloys on the basis of an experimental approach and a finite element (FEM) analysis. The void growth and coalescence process was observed by an optical microscope and a scanning electron microscope. The fractographic observation for aluminum alloys 7075-T651 and 6061-T651 showed that several large voids called a dominant void are nucleated at inclusion sites or the second-phase particles ahead of the crack tip and followed by fine voids initiation leading coalescence of the dominant voids with the crack tip. On the other hand, in aluminum alloy 2017-T3, voids are nucleated very close to the crack tip and directly coalesce with the crack tip. FEM computation results suggested that the void nucleation and growth process is closely related to the triaxial stress state ahead of the crack tip.     

Mechanical characterization of Ti–5Al–2.5Sn ELI alloy at cryogenic and room temperatures

An experimental campaign consisting of tensile and fracture tests at cryogenic and room temperatures has been conducted on a Ti–5Al–2.5Sn extra-low-interstitial (ELI) alloy. It has been assessed that, at decreasing testing temperature: Young’s modulus slightly increases; yield and failure strengths increase significantly; fracture toughness decreases. Since a ductile void growth to coalescence micromechanism always governs failure in the spanned temperature interval, crack growth is simulated by allowing for material nonlinearities in the process zone, where ductile tearing takes place. Numerical results have been obtained by modeling the response of the process zone through either a cohesive model or Gurson’s constitutive law for porous-ductile media. It is shown that the latter approach can accurately describe the failure mechanism at any test temperature and for any specimen geometry, whereas the former one is not able to account for stress triaxiality at the crack tip and therefore requires a new calibration anytime the specimen geometry is varied.     

Microstructural effects on ductile fracture in heterogeneous materials. Part I: Sensitivity analysis with LE-VCFEM

Ductile failure of heterogeneous materials, such as cast aluminum alloys and discretely reinforced aluminums or DRA’s, initiates with cracking, fragmentation or interface separation of inclusions, that is followed by propagation in the matrix by a ductile mechanism of void nucleation and growth. Damage localizes in bands of intense plastic deformation between inclusions and coalesces into a macroscopic crack leading to overall failure. Ductile fracture is very sensitive to the local variations of the microstructure morphology. This is the first of a two part paper on the effect of microstructural morphology and properties on the ductile fracture in heterogeneous ductile materials. In this paper the locally enhanced Voronoi cell finite element method (LE-VCFEM) for rate-dependent porous elastic–viscoplastic materials is used to investigate the sensitivity of strain to failure to loading rates, microstructural morphology and material properties. A model is also proposed for strain to failure, incorporating the effects of important morphological parameters.     

Effect of residual stresses on ductile crack growth resistance

It is well known that residual stresses influence the ductile fracture behaviour. In this paper, a numerical study was performed to assess the effect of residual stresses on ductile crack growth resistance of a typical pipeline steel. A modified boundary layer model was employed for the analysis under plane strain, Mode I loading condition. The residual stress fields were introduced into the finite element model by the eigenstrain method. A sharp crack was embedded in the center of the weld region. The complete Gurson model has been applied to simulate the ductile fracture by microvoid nucleation, growth and coalescence. Results show that tensile residual stresses can significantly reduce the crack growth resistance when the crack growth is small compared with the length scale of the tensile residual stress field. With the crack growth, the effect of residual stresses on the crack growth resistance tends to diminish. The effect of residual stress on ductile crack growth resistance seems independent of the size of geometrically similar welds. When normalized by the weld zone size, the ductile crack growth resistance collapses into one curve, which can be used to assess the structural integrity and evaluate the effect of residual stresses. It has also been found that the effect of residual stresses on crack growth resistance depends on the initial void volume fraction f₀, hardening exponent n and T-stress.     

Dynamic ductile crack growth and transition to cleavage – a cell model approach

The fracture behavior of ferritic steel in the transition regime is controlled by the competition between ductile tearing and cleavage. Many test specimens that failed by catastrophic cleavage showed significant amounts of ductile tearing prior to cleavage fracture. The transition from ductile tearing to cleavage has been attributed to the increase in constraint and sampling volume associated with ductile crack growth. This work examines the role of dynamic ductile crack growth on the fracture mode transition by way of a cell model of the material. The cell model incorporates the effects of stress triaxiality and strain rate on material failure characteristics of hole growth and coalescence. Loading rate and microstructure effects on the stress fields that evolve with rapid (ductile) crack growth are systematically studied. The stress fields are employed to compute the Weibull stress which provides probability estimates for the susceptibility to cleavage fracture. A center-cracked panel subjected to remote tension is the model problem under study. The computational model uses an elastic-viscoplastic constitutive relation which incorporates enhanced strain rate hardening at high strain rates. Adiabatic heating due to plastic dissipation and the resulting thermal softening are also accounted for. Under dynamically high loading rate, our model shows the crack speed achieves its peak value soon after crack initiation and quickly falls off to slower speeds with further crack growth. Remarkably, the Weibull stress follows a similar pattern which suggests that the transition to the cleavage fracture is most likely to occur, if at all, at the peak speed of ductile crack growth. Key words: Dynamic fracture, ductile tearing, crack growth, transition regime, cleavage fracture, cell model, finite element.     

Characterization of Loading Rate Effects on the Interactions between Crack Growth and Inclusions in Cementitious Material

The microcapsule-enabled cementitious material is an appealing building material and it has been attracting increasing research interest. By considering microcapsules as dissimilar inclusions in the material, this paper employs the discrete element method (DEM) to study the effects of loading rates on the fracturing behavior of cementitious specimens containing the inclusion and the crack. The numerical model was first developed and validated based on experimental results. It is then used to systematically study the initiation, the propagation and the coalescence of cracks in inclusion-enabled cementitious materials. The study reveals that the crack propagation speed, the first crack initiation stress, the coalescence stress, the compressive strength and the ultimate strain increase with the loading rate. The initiation position, the propagation direction, the cracking length and the type of the initiated cracks are influenced by the loading rates. Two new crack coalescence patterns are observed. It is easier to cause the coalescence between the circular void and a propagating crack at a slow loading rate than at a fast loading rate.     

Void coalescence in a porous solid under dynamic loading conditions

Void coalescence in ductile voided solids subjected to dynamic loading is investigated numerically. Finite element simulations of an axisymmetric unit cell, taking inertia and finite strain effects into account, are used to describe the coalescence process in a porous material containing a periodic distribution of initially spherical voids. The numerical results suggest that inertia yields a stabilizing effect and slows down the necking of the ligaments between neighbouring voids. Besides, for sufficiently high stress triaxiality and loading rate, coalescence is found to occur by direct impingement, instead of ligament necking. This result correlates with experimental observations in spall fracture and dynamic crack propagation.     

Nanoscale void nucleation and growth and crack tip stress evolution ahead of a growing crack in a single crystal

A constrained three-dimensional atomistic model of a cracked aluminum single crystal has been employed to investigate the growth behavior of a nanoscale crack in a single crystal using molecular dynamics simulations with the EAM potential. This study is focused on the stress field around the crack tip and its evolution during fast crack growth. Simulation results of the observed nanoscale fracture behavior are presented in terms of atomistic stresses. Major findings from the simulation results are the following: (a) crack growth is in the form of void nucleation, growth and coalescence ahead of the crack tip, thus resembling that of ductile fracture at the continuum scale; (b) void nucleation occurs at a certain distance ahead of the current crack tip or the forward edge of the leading void ahead of the crack tip; (c) just before void nucleation the mean atomic stress (or equivalently its ratio to the von Mises effective stress, which is called the stress constraint or triaxiality) has a high concentration at the site of void nucleation; and (d) the stress field ahead of the current crack tip or the forward edge of the leading void is more or less self-similar (so that the forward edge of the leading void can be viewed as the effective crack tip).     

Dynamic vs. quasi-static shear failure of high strength metallic alloys: Experimental issues

Ductile fracture of metals by void nucleation, growth and coalescence under positive stress triaxiality is well admitted. This is not the case when metals are submitted to negative stress triaxiality. The present work aims at contributing to a better understanding of the competition between micro-mechanisms at the origin of failure of metals when submitted to shear-pressure loading at low and high strain rates. With this aim in view, experiments were carried out on Ti–6Al–4V shear-compression samples involving a stress triaxiality range comprised between −0.2 and −0.5. Results show that the material failure is the consequence of a void growth induced process. At high strain rate, due to the localization of the deformation within adiabatic shear bands, the failure of the material occurs earlier, leading to maximum shear strain smaller at high strain rate than at low strain rate. Impact tests were also carried out on Kalthoff and Winkler type double notched plates. They showed that the interaction between tension and shear waves leads to a complex Mode I–Mode II crack propagation.     

Ductile crack propagation by plastic collapse of the intervoid ligaments

In the present study ductile crack initiation and propagation is investigated by means of a micro-mechanical model under small-scale yielding conditions. Voids are resolved discretely in the fracture process zone where steep gradients occur during the loading history and are taken into accounted by a homogenized porous plasticity law elsewhere. The size of the region of discrete voids is not set a priori but is determined consistently. The results show that effective crack growth occurs by plastic collapse, i.e. purely geometric softening of the intervoid ligaments without incorporating material separation. Due to this mechanism a limit load exists coinciding with the maximum fracture toughness. In addition, it turns out that the shielding due to the growth of voids around the crack plane has a considerable influence on the computed R-curves compared to models neglecting this effect. Depending on the void arrangement a diffuse softening zone or even crack branching is observed. A comparison with experimental data from literature indicates that plastic collapse and the formation of diffuse zones of void growth are realistic mechanisms of ductile crack propagation.