X-ray and electron microscope examination of hardened steel in bending: impact, rising and constant load

Impact bending fracture of hardened steel is examined by X-ray measurements and electron microscopic fractographic data. They are compared to those for bending with rising and constant amplitude load.Obtained are residual stresses and diffraction lines the width of which are integrated with reference to the depth under the failure surface of the hardened steel. The characteristics of the plastic zone formed under the fracture surfaces are determined for the three different types of loading. In particular, the zones of initial failure and final rupture are analyzed by electron microscopy. The results are quantified by the texture of microstructure surface appearance with reference to local regions failured in a brittle and ductile fashion.     

The brittle and ductile behavior of clay samples containing a crack under mixed mode loading

The mechanics of propagation of a single crack in brittle and ductile samples of clay as well as their mode of failure were investigated. The crack in the brittle and in the ductile samples was subjected to a mixed-mode (Mode I and Mode II) type of loading. In the brittle and ductile samples, secondary cracks developed from the tips of the original crack. The secondary cracks did not follow the plane of the original crack but formed an angle α with this plane. The angle of crack propagation α was greater in the brittle than in the ductile samples. The brittle samples failed when the secondary cracks reached the edges of the samples. Their mode of failure was a typical tensile failure. In the ductile samples, the secondary cracks extended for a limited distance from the tips of the original crack and did not influence the failure of the samples that was recorded to be in shear. The Maximum Tangential Stress criterion predicted well the direction of crack propagation in the brittle clay samples. The direction of crack propagation in the ductile samples of clay was found to be a function of their water content. From the laboratory results, a relationship from which to obtain the angle of crack propagation α in the ductile samples is presented.     

A Mixed Criterion of Delayed Creep Failure Under Plane Stress

The paper discusses delayed creep failure criteria and their experimental justification. These criteria allow transition from the strength characteristics under uniaxial stress to the strength characteristics under plane stress. The criterion is chosen in the form of a mixed invariant that relates two stress components responsible for brittle and ductile failure. The limit characteristics take the effect of the principal stresses into account. The criterion was tested for isotropic metallic materials subjected to internal pressure, internal pressure with tension, pure torsion, and tension with torsion     

Experimental studies on the yield behavior of ductile and brittle aluminum foams

Specimen size effects are a major cause of the unreliability of foam models in finite element codes. Here, the modified Arcan apparatus is used to investigate the biaxial yielding of ductile and brittle Al foams. This apparatus subjects a central section of a “butterfly-shaped” specimen to a uniform state of plane stress. The stresses have local maxima at the central section, thus ensuring that yielding occurs there. A yield envelope, which directly relates to the crushing process, can then be determined. Size effects are introduced when using conventional methods such as tri-axial or plate-shear tests. In such tests, averages of stress and strain are measured. These measures do not represent the actual yield event, because foam's internal structure is inhomogeneous and so is the deformation field. Strain localization and failure can occur at any weak layer of cells in the bulk. In this study, we have performed a series of biaxial tests on isotropic Alporas and anisotropic Hydro closed-cell Al foams of approximately equal densities. Alporas failed locally by a ductile phenomenon of progressive crushing of cells. It also possessed uniaxial strength asymmetry. Hydro specimens parallel and perpendicular to ‘foam rise’ were investigated. The Hydro foam developed a local, characteristic brittle crack at loads in the vicinity of the yield point. Phenomenological yield surfaces, which incorporate these features are obtained for the foams, and show dependence on both the deviatoric and hydrostatic stresses. We also provide expressions for the shear and hydrostatic strengths in terms of the uniaxial strengths. Finally, the size-independence of the yield surface is verified using the uniaxial compression of tapered specimens.     

Ductile and brittle fracture analysis of surface flaws using CTOD

A reference stress method is used to analyze both brittle and ductile fracture in structures containing surface flaws. Crack-tip opening displacement (CTOD) is used as the fracture-toughness input, althoughJ-based reference stress analyses are also possible. Both detailed and simplified analyses for brittle and ductile fracture are described. A brittle fracture analysis which takes account of stress concentrations, secondary stresses and stress gradients is presented, together with a complete ductile tearing analysis which utilizes a single CTOD value measured at maximum load. In addition, two simplified approaches are proposed: a yield-before-break criterion for brittle fracture and a critical learing modulus for ductile fracture.     

Multiscale modeling of crack initiation and propagation at the nanoscale

Fracture occurs on multiple interacting length scales; atoms separate on the atomic scale while plasticity develops on the microscale. A dynamic multiscale approach (CADD: coupled atomistics and discrete dislocations) is employed to investigate an edge-cracked specimen of single-crystal nickel, Ni, (brittle failure) and aluminum, Al, (ductile failure) subjected to mode-I loading. The dynamic model couples continuum finite elements to a fully atomistic region, with key advantages such as the ability to accommodate discrete dislocations in the continuum region and an algorithm for automatically detecting dislocations as they move from the atomistic region to the continuum region and then correctly “converting” the atomistic dislocations into discrete dislocations, or vice-versa. An ad hoc computational technique is also applied to dissipate localized waves formed during crack advance in the atomistic zone, whereby an embedded damping zone at the atomistic/continuum interface effectively eliminates the spurious reflection of high-frequency phonons, while allowing low-frequency phonons to pass into the continuum region.The simulations accurately capture the essential physics of the crack propagation in a Ni specimen at different temperatures, including the formation of nano-voids and the sudden acceleration of the crack tip to a velocity close to the material Rayleigh wave speed. The nanoscale brittle fracture happens through the crack growth in the form of nano-void nucleation, growth and coalescence ahead of the crack tip, and as such resembles fracture at the microscale. When the crack tip behaves in a ductile manner, the crack does not advance rapidly after the pre-opening process but is blunted by dislocation generation from its tip. The effect of temperature on crack speed is found to be perceptible in both ductile and brittle specimens.     

Crushing of particles in idealised granular assemblies

Four idealised assemblies of equally sized spherical particles are subjected to a range of macroscopic compressive principal stresses and the contact forces on individual particles are determined. For each set of contact forces the stress fields within individual particles are studied. A failure criterion for brittle materials is imposed and indicates that crushing (or rupture) occurs when the maximum contact force reaches a threshold particle strength value, irrespective of the presence and magnitude of other lesser contact forces acting on the particle and the material properties of the particle. Combining the crushing mechanism with an assembly instability mechanism enables failure surfaces to be drawn in the three-dimensional stress space. A simple spatial averaging technique has been applied to the failure surfaces to remove the effects of assembly anisotropies. Sections of the failure surfaces on π planes have similarities to those commonly used in sand modelling.     

Crack tip blunting and cleavage under dynamic conditions

In structural materials with both brittle and ductile phases, cracks often initiate within the brittle phase and propagate dynamically towards the ductile phase. The macroscale, quasistatic toughness of the material thus depends on the outcome of this microscale, dynamic process. Indeed, dynamics has been hypothesized to suppress dislocation emission, which may explain the occurrence of brittle transgranular fracture in mild steels at low temperatures (Lin et al., 1987). Here, crack tip blunting and cleavage under dynamic conditions are explored using continuum mechanics and molecular dynamics simulations. The focus is on two questions: (1) whether dynamics can affect the energy barriers for dislocation emission and cleavage, and (2) what happens in the dynamic “overloaded” situation, in which both processes are energetically possible. In either case, dynamics may shift the balance between brittle cleavage and ductile blunting, thereby affecting the intrinsic ductility of the material. To explore these effects in simulation, a novel interatomic potential is used for which the intrinsic ductility is tunable, and a novel simulation technique is employed, termed as a “dynamic cleavage test”, in which cracks can be run dynamically at a prescribed energy release rate into a material. Both theory and simulation reveal, however, that the intrinsic ductility of a material is unaffected by dynamics. The energy barrier to dislocation emission appears to be identical in quasi-static and dynamic conditions, and, in the overloaded situation, ductile crack tip behavior ultimately prevails since a single emission event can blunt and arrest the crack, preventing further cleavage. Thus, dynamics cannot embrittle a ductile material, and the origin of brittle failure in certain alloys (e.g., mild steels) appears unrelated to dynamic effects at the crack tip.     

轻质合金压印接头质量判断及其力学性能研究

在结构轻量化的进程中,新型薄板材料被大量使用,新兴的压印连接技术可以实现这些材料的连接．以钛合金为主要材料进行压印连接实验,结果显示材料的母材性能对连接性能、接头强度、失效形式均有一定的影响．压印接头的拉伸-剪切失效形式为颈部断裂时,拉伸-剪切实验过程中载荷位移曲线有两次明显的下降过程,分别是由于圆形压印点的上半部分颈部被拉断,圆压印点的下半部分颈部被拉断造成的．微观分析显示TA1-TA1压印接头断口呈现类解理穿晶断裂,5052-TA1压印接头断口出现拉长韧窝特征,属于塑性断裂,1420-TA1接头断口呈现大面积平面及少量冰糖状花样,属于沿晶脆性断裂．     

Micro Scale Tensile Behaviour of Thin Bitumen Films

The tensile behaviour of two types of viscoelastic bituminous films confined between mineral aggregates or steel as adherends, was investigated in the brittle and ductile regimes. Uniaxial specimens were fabricated employing a prototype set up allowing construction of micro-scale thin films and visualization of failure phenomena. The effect of key parameters, namely, temperature (23°C and −10°C), binder type (straight run and polymer modified), adherend type (stainless steel and mineral aggregate), and water conditioning were investigated sequentially. The results show that water sensitive aggregate-binder combinations in macro (150 mm diameter) and mega (in service) scales also displayed reduced tensile strength in the micro scale when water conditioned. At 23°C ductile failure and at −10°C brittle fracture were observed. At 23°C phenomena, such as formation of striations during tensile mechanical loading, void nucleation and growth, filamentation and large ductile flow before fracture could be witnessed. When using proper surface preparation procedures, in all types of specimen investigated at 23°C only cohesive failure and at −10°C predominantly adhesive-cohesive failure were found.     

Damage of ductile materials deforming under multiple plastic or viscoplastic mechanisms

This work presents a model to represent ductile failure (i.e. failure controlled by nucleation, growth and coalescence) of materials whose irreversible deformation is controlled by several plastic or viscoplastic deformation mechanisms. In addition work hardening may result from both isotropic and kinematic hardening. Damage is represented by a single variable representing void volume fraction. The model uses an additive decomposition of the plastic strain rate tensor. The model is developed based on the definition of damage dependant effective scalar stresses. The model is first developed within the generalized standard material framework and expressions for Helmholtz free energy, yield potential and dissipation potential are proposed. In absence of void nucleation, the evolution of the void volume fraction is governed by mass conservation and damage does not need to be represented by state variables. The model is extended to account for void nucleation. It is implemented in a finite element software to perform structural computations. The model is applied to three case studies: (i) failure by void growth and coalescence by internal necking (pipeline steel) where plastic flow is either governed by the Gurson–Tvergaard–Needleman model or the Thomason model, (ii) creep failure (Grade 91 creep resistant steel) where viscoplastic flow is controlled by dislocation creep or diffusional creep and (iii) ductile rupture after pre-compression (aluminum alloy) where kinematic hardening plays an important role.     

Effect of adhesive thickness,adhesive type and scarf angle on the mechanical properties of scarf adhesive joints

The effects of adhesive thickness, adhesive type and scarf angle, which are determined as the main control parameters by the dimensional analysis, on the mechanical properties of a scarf adhesive joint (SJ) subjected to uniaxial tensile loading are examined using a mixed-mode cohesive zone model (CZM) with a bilinear shape to govern the interface separation. Particularly, the adhesive-dependence of the vital cohesive parameters of CZM, which mainly include initial stiffness, total fracture energy and separation strength, is introduced emphatically. The numerical results demonstrate that the ultimate tensile loading increases as the adhesive thickness decreases. Cross the ultimate tension, the joint loses the load-bearing capacity when adopting the brittle adhesive but sustains partial load-bearing capacity while selecting the ductile adhesive. In addition, for the joint with the ductile adhesive, the maximum applied displacement until the complete failure of it is directly proportional to the adhesive thickness, which is different from the case using the brittle adhesive. Taking the combination of the ultimate loading and applied displacement into account, failure energy is employed to evaluate the joint performances. The results show that the failure energy of the joint with the brittle adhesive increases as the adhesive thickness decreases. Conversely, the situation of the joint using the ductile adhesive is vice versa. Moreover, the effect of the adhesive thickness becomes more noticeable with decreasing the scarf angle owing to the variation of the proportion of each component of the mixed-mode. Furthermore, all the characteristic parameters (the ultimate tensile loading, the maximum applied displacement and the failure energy) that adopted to describe the performances of SJ increase as the scarf angle decreases. Finally, the numerical method employed in this study is validated by comparing with existing experimental results.     

Thermodynamically consistent nonlocal theory of ductile damage

In this paper a thermodynamically consistent, weakly nonlocal theory of ductile damage is presented. The theory is based on the classical dynamical balance laws of forces and couples in the physical space and dynamical balance laws of material forces on evolving defects and on the first and second law of thermodynamics formulated for physical and material space. Assuming general constitutive equations their frame-invariant and thermodynamically admissible form is determined. It is shown that physical and material forces and stresses consist of two parts, a nondissipative part derivable from a free energy potential, and a dissipative part, which can be obtained from a dissipation pseudo-potential, if such a pseudo-potential exists.The theory can be considered as a framework with gradient elastoplasticity, isotropic and anisotropic brittle and ductile gradient damage at finite strain as special cases.     

Analysis of failure modes in an impact loaded thermoviscoplastic prenotched plate

Kalthoff observed experimentally that the failure mode in a prenotched maraging steel plate impact loaded on the notched side changes from brittle to ductile with an increase in the impact speed. Here we numerically investigate the effect on the failure mode transition speed of the shape of the notch-tip and the presence of a hole ahead of a circular notch-tip. The shape of the notch-tip is varied by changing the ratio, a/b, of the principal axes of an elliptic notch. For a circular notch-tip, we also investigate the effect, on the failure mode transition speed, of the presence of a circular void ahead of the notch-tip and situated either on or away from the axis of the notch. The Bodner–Partom thermoviscoplastic relation is used to model the strain hardening, strain-rate hardening and thermal softening response of the material of the plate. The transient plane strain thermomechanical deformations of the plate are analyzed by the finite element method. It is found that for a/b=2.0 and 10.0, the brittle failure preceded the ductile failure for the six impact speeds studied herein, and for a/b=0.4 and 1.0, the failure mode transitioned from the brittle to the ductile with an increase in the impact speed. The presence of a circular void ahead of the circular notch-tip shifts towards the axis of the notch the point on the the notch-tip surface where a shear band initiates.     

Numerical experiments on the rupture of brittle solids––variation of microstructure,loading and dimensions

The significance of microstructural parameters for the damage and rupture of brittle materials is investigated by a numerical model that accounts for separation of grain interfaces. The microcracking approach refers to a specified material structure; synthetic random sampling allows for fluctuations. Thus, damage progression is simulated and the strength of material specimens is estimated by employing fracture mechanics within a microstructure that exhibits statistical variability. The issues addressed regard failure behaviour, strength level and sensitivity to microstructural parameters. Also discussed is the effect of specimen dimensions in connection with rupture.     

内聚力模型的形状对胶接结构断裂过程的影响

内聚力模型被广泛应用于粘接结构的断裂数值模拟过程中,为深入分析不同形状内聚力模型与胶黏剂性质和粘接结构断裂之间的关系,本文分别采用脆性和延展性两种类型胶黏剂,对其粘接的对接试件进行了单轴拉伸、剪切实验,以及其粘接的双臂梁试件进行了断裂实验.3种类型的内聚力模型(抛物线型、双线型和三线型)分别模拟了以上粘接结构的断裂过程,并与实验结果进行对比.结果发现:双线型的内聚力模型适用计算脆性胶黏剂的拉伸与剪切的断裂过程;指数型内聚力模型较适合计算延展性胶黏剂的拉伸和剪切的断裂过程,临界应力、断裂能和模型的形状参数是分析拉伸和剪切的重要参数;双臂梁试件的断裂过程模拟结果发现,断裂曲线与胶黏剂性质有关,内聚力模型形状参数也有影响.通过实验与计算结果分析,双线型内聚力模型更适合脆性胶黏剂粘接的双臂梁断裂计算,而三线型更适合计算延展性胶黏剂粘接的双臂梁断裂过程,此研究结果对胶黏剂的使用和粘接结构的断裂分析有很重要意义.     

Mechanics of formation and rupture of human aneurysm

The mechanical response of the human arterial wall under the combined loading of inflation, axial extension, and torsion is examined within the framework of the large deformation hyper-elastic theory. The probability of the aneurysm formation is explained with the instability theory of structure, and the probability of its rupture is explained with the strength theory of material. Taking account of the residual stress and the smooth muscle activities, a two layer thick-walled circular cylindrical tube model with fiber-reinforced composite-based incompressible anisotropic hyper-elastic materials is employed to model the mechanical behavior of the arterial wall. The deformation curves and the stress distributions of the arterial wall are given under normal and abnormal conditions. The results of the deformation and the structure instability analysis show that the model can describe the uniform inflation deformation of the arterial wall under normal conditions, as well as formation and growth of an aneurysm under abnormal conditions such as the decreased stiffness of the elastic and collagen fibers. From the analysis of the stresses and the material strength, the rupture of an aneurysm may also be described by this model if the wall stress is larger than its strength.     

Path dependency in hybrid structures with simultaneous ductile and brittle connections and materials

This article considers a steel–stud–concrete hybrid structure with a Fibre Reinforced Polymer plate adhesively-bonded to the steel member. Owing to the combination of ductile and brittle materials and connections present, the failure behavior of such a structure can be influenced by residual stresses, which in turn depend on the plasticity-inducing load paths previously experienced by the structure. Plasticity of only the stud connections generates a different residual stress pattern from plasticity of only the steel member, and an understanding of the mechanics of residual stress generation in each case is fundamental to the development of a framework of ideas on path dependency in such structures. Measurements of deformation do not necessarily permit faithful reconstruction of residual stress profiles, as these measurements typically give total (elastic plus plastic) deformations from the times that the measurements start, while the residual stresses are related to the elastic components of deformation from the times that the structural components were manufactured. Numerical work is thus needed to determine residual stresses. To that end, a verified finite element program is here used to investigate residual stress patterns in the above hybrid structure due to plasticity of either the studs or the steel member. For yield of the steel, the effects on the residual stresses of initial self-equilibrating stresses in the steel member are investigated. Crucial to the success of the analyses are curvilinear or multi-linear loading/unloading constitutive relationships not only for the materials, but also for the connections. The residual stress profiles from the steel yield and stud yield analyses are examined and compared, and ideas for extension of the work are suggested.