Ductile failure behavior of polycrystalline Al 6061-T6 under shear dominant loading

Ductile failure in polycrystalline aluminum alloys under pure shear as well as with superposed tension and compression loading is explored through the modified Arcan shear experiments. Specimens obtained through tests interrupted at various stages of deformation and failure evolution are examined through quantitative microscopy to discern the mechanisms of failure and to evaluate the local strain evolution quantitatively. Fractographic observations are used to identify the onset and evolution of damage processes during deformation and failure of these aluminum alloys. Local strain levels are estimated from measurements of the change in grain size with deformation and used to indicate that the local values of failure strains are likely to be much larger than that estimated from strains averaged over characteristic specimen dimensions such as the gage length or the specimen diameter. Lower bound estimates of the failure strain in low triaxiality conditions are obtained from the experiments. It is shown that strain-to-failure decreases monotonically with stress triaxiality in stark contrast with recent works where a reverse behavior in low stress triaxiality levels was reported. Eventual failure that occurs through void growth and coalescence is shown to be restricted to a very small region within the localized deformation band.     

A finite element analysis of dynamic fracture initiation by ductile failure mechanisms in a 4340 steel

In some recent dropweight impact experiments [5] with pre-notched bend specimens of 4340 steel, it was observed that considerable crack tunneling occurred in the interior of the specimen prior to gross fracture initiation on the free surfaces. The final failure of the side ligaments happened because of shear lip formation. The tunneled region is characterized by a flat, fibrous fracture surface. In this paper, the experiments of [5] (corresponding to 5 m/s impact speed) are analyzed using a plane strain, dynamic finite element procedure. The Gurson constitutive model that accounts for the ductile failure mechanisms of micro-void nucleation, growth and coalescence is employed. The time at which incipient failure was observed near the notch tip in this computation, and the value of the dynamic J-integral, _J d, at this time, compare reasonably well with experiments. This investigation shows that J-controlled stress and deformation fields are established near the notch tip whenever J _d , increases with time. Also, it is found that the evolution of micro-mechanical quantities near the notch root can be correlated with the time variation of J _d .The strain rate and the adiabatic temperature rise experienced at the notch root are examined. Finally, spatial variations of stresses and deformations are analyzed in detail.     

On the deformation and failure of Al 6061-T6 in plane strain tension evaluated through in situ microscopy

We investigate deformation and failure of Al 6061-T6 in plane strain conditions through in situ scanning electron microscopy. The global behavior of the specimen, as well as the local deformation of the matrix material, second phase particles, and preexisting voids, is observed with a combination of high temporal/low spatial resolution images and low temporal/high spatial resolution images. It is found that the matrix dominates the deformation response, with the second phase particles and voids imparting little influence on the deformation under the moderate triaxiality levels encountered in this experiment. The initiation or nucleation of cracks is observed to occur by plastic slip.     

Investigations on different fatigue design concepts using the example of a welded crossbeam connection from the underframe of a steel railcar body

For thin-walled steel structures, welded joints often turn out to be the weak spots under cyclic loading. For the dimensioning of welded safety components in vehicle construction, strength concepts are necessary that are well-defined and well-reviewed, in terms of their predictive accuracy.Within the scope of the research results presented here, the applicabilities of the nominal, structural and notch stress approaches have been examined on the basis of different arc welded and cyclic loaded steel structures, taken from the railway sector. In detail, this is a crossbeam connection from the underframe of a railcar body. Different sheet thicknesses in the range 2.7 mm-4 mm in combination with a misalignment lead to an increase of load in the region of the weld seams. Several different components and specimens with critical regions of failure have been tested under cyclic loading with constant amplitude. With the help of strain gauges, the (technical) crack initiation has been determined. A short review of the possibility of obtaining information about the crack initiation by ultrasonic-burst-phase-thermography is also given.The specimens were the basis for the application and evaluation of the different concepts for the assessment of fatigue life. The numerical determination of the nominal, structural and notch stresses has been performed with finite-element models. For a local approach, according to the notch stress analysis, a submodelling technique has been used. The FE-models have been compared with the experimental data by means of optical 3D deformation analysis and strain measurements with strain gauges. Existing weld seam and specimen tolerances have been included through the use of parametric models. Finally the experimental and computational results have allowed the derivation of structural and notch S-N curves for the crack initiation and the rupture of the specimen.     

A numerical investigation of ductile fracture initiation in a high-strength low-alloy steel

In this work, static and drop-weight impact experiments, which have been conducted using three-point bend fracture specimens of a high-strength low-alloy steel, are analysed by performing finite-element simulations. The Gurson constitutive model that accounts for the ductile failure mechanisms of microvoid nucleation, growth and coalescence is employed within the framework of a finite deformation plasticity theory. Two populations of second-phase particles are considered, including large inclusions which initiate voids at an early stage and small particles which require large strains to nucleate voids. The most important objective of the work is to assess quantitatively the effects of material inertia, strain rate sensitivity and local adiabatic temperature rise (due to conversion of plastic work into heat) on dynamic ductile crack initiation. This is accomplished by comparing the evolution histories of void volume fraction near the notch tip in the static analysis with the dynamic analyses. The results indicate that increased strain hardening caused by strain rate sensitivity, which becomes important under dynamic loading, plays a benign role in considerably slowing down the void growth rate near the notch tip. This is partially opposed by thermal softening caused by adiabatic heating near the notch tip.     

Experimental study on damage evolution of rock under uniform and concentrated loading conditions using digital image correlation

Damage evolution and crack propagation in sandstone specimens have been observed by digital image correlation method. To investigate deformation and failure process of rock under different loading conditions, uniaxial compression and indentation tests were performed. Through the experiment, displacement and strain fields are simultaneously obtained that can visually display the distribution, mode and evolution of deformation and cracking in rock. Experimental results show that the damage distributes diffusely in rock at early loading stage, and the measured apparent strain increasingly concentrates with loading because of the nucleation of crack; propagation of the crack leads to the eventual failure of the specimen. Damage factor is calculated on the basis of deviation of apparent strain, and localization factor is presented to describe the level of deformation localization. The combined use of two factors can well represent the damage evolution of rock under compression.     

Local deformation at micro‐notches and crack initiation in an intermetallic γ‐TiAl‐alloy

The relation is studied between crack initiation from micro‐notches in a fully lamellar intermetallic γ‐TiAl alloy and the local strain field. These micro‐notches were introduced using femtosecond‐laser ablation and had dimensions below the average colony size. The specimen under investigation was then subjected to fatigue loading. Continuous monitoring using a travelling optical microscope allowed detecting microcracks at an early stage. Prior to fatigue loading, a sustained load was applied and the local strain field was determined using digital image correlation. This was supplemented by a Finite Element analysis of the notches and their neighbourhood. It was found that a crack was initiated from a notch causing high normal strains in lamella direction, whereas no crack was initiated from notches with high shear strains.     

Stellite12钴基合金热循环冲击前后拉伸断裂机理研究

对不同缺口的Stellite12钴基合金试样(700℃/20℃进行不同次数的热循环冲击和未冲击)进行原位拉伸,并结合试验数据的分析以及断口形貌的扫描电镜观察,分析了Stellite12钴基合金热循环冲击前后的拉伸断裂过程和断裂机理。结果发现:热循环冲击后不同半径试样的断裂过程略有不同,热循环冲击后的小圆弧缺口试样在缺口根部产生表面微裂纹,试样边缘及微裂纹两侧产生氧化微孔;原位拉伸时,该试样热冲击过程产生的裂纹先向试样厚度方向扩展,待厚度方向贯通,然后裂纹尖端的基体发生变形、黑相(白相)穿晶开裂、少量沿氧化微孔裂开,试样瞬间发生断裂;而经历热循环冲击后的大圆弧试样表面并未产生明显的裂纹,拉伸加载过程经历大圆弧根部基体变形、黑白相内开裂、边缘氧化微孔张开,试样突然断裂;对于未冲击试样,在加载过程中,试样的断裂过程经历基体变形、黑白相内部开裂,能量聚集到一定程度试样突然断裂。对于未热冲击的三种不同试样其断裂过程基本类似,仅仅是由于小圆弧半径的试样应力集中程度更大,从而使得其断裂应力低于平板以及大圆弧试样。     

A notch strain calculation of a notched specimen under axial-torsion loadings

In this paper, a notch analysis model is presented for the numerical prediction of multiaxial strains of a notched 1070 steel specimen under combined axial and torsion loadings. The proposed model is based on the notion of a structural yield surface and uses a small-strain cyclic plasticity model to describe stress–strain relations. A notch load–strain curve is calculated with Neuber’s rule and incremental nonlinear finite element analysis. The presented model is applied to simulate the notch root deformations of a circumferentially notched specimen under cyclic tension–compression–torsion loading histories. The model predictions are evaluated with strain measurements at the notch root of the specimen in a comprehensive set of cyclic tests. The computed strain loops were in accord with experimental data and matched qualitatively with measured shear–axial strain histories irrespective of loading path of the test. In proportional balanced torsion-axial loading, the nonlinear shear strain–axial strain loops were calculated properly. The modeling errors were determined to be a function of the loading path shape, and compared to shear strains, axial strain predictions were more accurate.     

Notch effect on creep–fatigue life for Sn–3.5Ag solder

This paper studies the creep–fatigue crack initiation and failure lives of Sn–3.5Ag solder notched specimens focused on the multiaxial strain at the notch root. Push–pull creep–fatigue tests were performed using three circumferential notched specimens using four kinds of creep–fatigue strain waveforms. Multiaxial strains at the notched section were calculated by finite element (FE) analysis under four kinds of creep–fatigue loading. Creep–fatigue damage laws were applied for evaluating the crack initiation and failure lives using the multiaxial strains obtained by the FE analysis. von Mises equivalent strain at the notch root estimated the crack initiation lives with a large scatter as well as the failure lives. Instead, the mean value of von Mises equivalent strain over the cross section of the notch root estimated the crack initiation and failure lives with a small scatter.     

Effect of martensite transformation on fracture behavior of shape memory alloy NiTi in a notched specimen

In this paper, the finite element calculation of the stress–strain distribution in front of a notch tip were carried out for two materials. One is a shape memory alloy NiTi with the stress-induced martensite transformation, and another is a fully transformed martensite NiTi without the transformation. Based on the results obtained, and combining a model of the fracture process zone, effect of martensite transformation on the fracture behavior of the shape memory alloy NiTi in a notched specimen of plane stress state is comparably analyzed. The results show that the martensite transformation increases the load to produce plastic deformation in the transformed martensite at the notch tip and decreases the maximum normal stress and plastic strain near the notch tip, and tends to suspend the crack nucleation and propagation in the fully transformed martensite in front of the notch tip, and thus increases the fracture load and improves the toughness. A quantitative analysis based on the model of the fracture process zone shows that the martensite transformation in the SMA NiTi causes about 47% increase in the apparent fracture toughness.     

The effects of ageing condition on shear localization from the tip of a notch in maraging steel

An experimental investigation of shear localization at the tip of a notch in several material states of 300 maraging steel is reported. Side-impacted edge-notched plates were tested in the under-aged, peak-aged, and over-aged conditions, and the critical impact velocity above which shear localization occurs at the notch tip is reported for a notch root radius of 175 m. It is found that the critical minimum impact velocity required for shear localization is independent of the ageing condition. Next, the propagation of shear localization in two of the three ageing conditions is recorded using high speed photography at framing rates of 480 000 frames per second. It is seen that, within the uncertainty of the experimental method, shear localization initiates at approximately the same time and stress intensity factor for the two materials. A heavy dependence of the shear band propagation speed and final length upon the ageing condition is seen. For the conditions examined shear failure is seen to propagate at an average velocity of 1000 m s-1 in peak-aged material and 300 m s-1 in under-aged material. The peak-aged material fails fully by shear while the shear failure in under-aged material arrests and is followed by tensile failure. This result demonstrates the effect of material strength, roughly independent of the strain hardening characteristics for these materials, on shear localization propagation. The effects of the specimen and projectile geometry on the results are examined qualitatively using elastodynamic finite element analysis of the stationary notch tip loading. © 1998 Chapman & Hall     

An analysis of dynamic,ductile crack growth in a double edge cracked specimen

Dynamic crack growth is analysed numerically for a plane strain double edge cracked specimen subject to symmetric impulsive tensile loading at the two ends. The material behavior is described in terms of an elastic-viscoplastic constitutive model that accounts for ductile fracture by the nucleation and subsequent growth of voids to coalescence. Two populations of second phase particles are represented, including large inclusions or inclusion colonies with low strength, which result in large voids near the crack tip at an early stage, and small second phase particles, which require large strains before cavities nucleate. The crack growth velocities determined here are entirely based on the ductile failure predictions of the material model, and thus the present study is free from ad hoc assumptions regarding appropriate dynamic crack growth criteria. Adiabatic heating due to plastic dissipation and the resulting thermal softening are accounted for in the analyses. Different prescribed impact velocities, inclusion spacings and values of the inclusion nucleation stress are considered. Predictions for the dynamic crack growth behavior and for the time variation of crack tip characterizing parameters are obtained for each case analyzed.     

Compressive maximum shear crack initiation and propagation

A maximum shear crack γ (a Mode II shear crack along the maximum shear direction associated with the crack tip shear displacement) was produced successfully in a so-called compressive maximum shear (CMS) specimen. This specimen was specificially designed to produce a compressive maximum shear failure which is one of two mechanisms widely believed to be responsible for limiting bearing fatigue life in rolling contact. The fracture initiation stress (or crack nucleation stress) σ_c and the upward crack propagation rate (toward the loading surface) $\text{dli}\text{dσi}$ per unit cyclic compressive stress increment were determined for the 52100 steel. These parameters were measured at two cleanness levels (DE and CEVM) [DE: basic electric arc furnace melted plus vacuum degassed. CEVM: Consumable electrode vacuum melted] and two tempered hardness levels, RC61 and 51. The possibility of determining K₁₁ for ith cycle was also elucidated. The formation of tail cracks and parallel multiple cracks as fine structure of CMS cracks can be well expounded by the concept of restoring tensile stresses and the residual shear stress relaxation at the CMS crack tip. The fracture mechanism advanced here can explain the formation of similar tail cracks and parallel multiple cracks frequently observed along the inclined shear cracks existing in the subsurface regions of rolling     

3D modeling of subsurface fatigue crack nucleation potency of primary inclusions in heat treated and shot peened martensitic gear steels

A computational strategy is developed to characterize the driving force for fatigue crack nucleation at subsurface primary inclusions in carburized and shot peened C61^® martensitic gear steels. Experimental investigation revealed minimum fatigue strength to be controlled by subsurface fatigue crack nucleation at inclusion clusters under cyclic bending. An algorithm is presented to simulate residual stress distribution induced through the shot peening process following carburization and tempering. A methodology is developed to analyze potency of fatigue crack nucleation at subsurface inclusions. Rate-independent 3D finite element analyses are performed to evaluate plastic deformation during processing and service. The specimen is subjected to reversed bending stress cycles with R = 0.05, representative of loading on a gear tooth. The matrix is modeled as an elastic–plastic material with pure nonlinear kinematic hardening. The inclusions are modeled as isotropic, linear elastic. Idealized inclusion geometries (ellipsoidal) are considered to study the fatigue crack nucleation potency at various subsurface depths. Three distinct types of second-phase particles (perfectly bonded, partially debonded, and cracked) are analyzed. Parametric studies quantify the effects of inclusion size, orientation and clustering on subsurface crack nucleation in the high cycle fatigue (HCF) or very high cycle fatigue (VHCF) regimes. The nonlocal average values of maximum plastic shear strain amplitude and Fatemi–Socie (FS) parameter calculated in the proximity of the inclusions are considered as the primary driving force parameters for fatigue crack nucleation and microstructurally small crack growth. The simulations indicate a strong propensity for crack nucleation at subsurface depths in agreement with experiments in which fatigue cracks nucleated at inclusion clusters, still in the compressive residual stress field. It is observed that the gradient from the surface of residual stress distribution, bending stress, and carburized material properties play a pivotal role in fatigue crack nucleation and small crack growth at subsurface primary inclusions. The fatigue potency of inclusion clusters is greatly increased by prior interfacial damage during processing.     

Ductile failure behavior of polycrystalline Al 6061-T6

Ductile failure in polycrystalline aluminum alloys is explored through uniaxial tension and notched tension experiments. Specimens obtained through tests interrupted at various stages of deformation and failure evolution are examined through microscopy to discern the mechanisms of failure and to evaluate the local strain evolution quantitatively. Fractographic observations are used to identify the onset and evolution of damage processes during deformation and failure of these aluminum alloys. Local strain levels are estimated from measurements of the change in grain size with deformation and used to indicate that the local values of failure strains are likely to be much larger than that estimated from strains averaged over characteristic specimen dimensions such as the gage length or the specimen diameter. Lower bound estimates of the failure strain at moderate triaxiliaties are obtained from the experiments.     

In situ TEM observations of high-strain-rate deformation and fracture in pure copper

The plastic deformation of polycrystalline metals at high strain rates is controlled by the way defects (dislocations and twins) nucleate, propagate, and interact in the microstructure. To-date, the role of these defects has been estimated based on dynamic mechanical measurements coupled with ex situ investigations of the deformed microstructure. However, such investigations are fundamentally limited in their ability to characterize transient mechanisms. Here, we present for the first time direct, experimental observations of the nucleation, motion, and interaction of defects and cracks during deformation of pure copper at strain rates between 10³ and 10⁴ s⁻¹. These observations are enabled by coupling a custom-built in situ high-rate straining stage with nanosecond-resolution dynamic transmission electron microscopy. The results show that while twins play only a minor role in the deformation of copper at quasi-static strain rates, the twin nucleation rate increases markedly at high strain rates. The preferred nucleation sites for twins also change, and the new twin interfaces become preferential paths for crack propagation, facilitating fracture through the original grains.     

Deformation und Versagen von StE460 und AlMg4,5Mn bei mehrachsig-proportionalen Beanspruchungen mit konstanten und variablen Amplituden

Deformation and failure behaviour of FeE460 and AlMg4.5Mn under multiaxial proportional loading with constant and variable amplitudes To calculate the fatigue life-to-crack initiation of engineering components under combined cyclic loading, experimentally secured knowledge on the cyclic deformation and failure behaviour of the materials used under the certain multiaxial cyclic stress and strain conditions are required. To obtain this, strain-controlled fully reversed experimental tests at tensional, torsional and combined loading with constant and variable amplitudes have been conducted using thin-walled tube specimens of FeE460 and AlMg4.5Mn. Experimental tests on standard uniaxially loaded hourglass specimens have also been conducted to study specimen form effects. Cyclic deformation behaviour can be uniformly described by the stabilised cyclic σ-ε-curve, if stresses and strains are expressed as equivalent values according to the von Mises criterion. Failure behaviour at constant and variable amplitude loading is characterized by the initiation and growth of short cracks at right angle to the direction of the greatest principal stress (mode I) in the case of tensional or combined loading and by short crack growing in both shear stress directions (mode II+III) in the case of torsional loading. At fully reversed constant amplitude loading, all three types of load can be described by one constant amplitude strain life-to-crack initiation curve. At variable amplitude loading (notch strain simulation with gaussian spectrum, H₀=10⁵), the experimental fatigue life-to-crack initiation values are lower than estimated values based on Miner-calculations using an equivalent stress-strain supported P_SWT-N-curve. The question of mean stresses and their evaluation is discussed.     

Analysis of unidirectional (0°) fiber-reinforced laminated composite double cantilever beam specimen using higher order beam theories

Mathematical models, for the stress analysis of unidirectional (0°) fiber-reinforced laminated composite double cantilever beam (DCB) specimen using classical beam theory, first and higher order shear deformation beam theories, have been developed to determine the mode I strain energy release rate (SERR) for unidirectional composites. In the present study, appropriate matching conditions at the crack tip of the DCB specimen have been derived by using variational principles. SERR has been calculated using compliance method. In general, the performance of shear deformation beam models of DCB specimen with variationally derived matching conditions at the crack tip is good in determining the SERR for medium to long crack lengths. Performance of higher order shear deformation beam model (having quadratically varying transverse displacement over the thickness) of DCB specimen, with non-variationally derived matching conditions at the crack tip, is good in determining the SERR for all the crack lengths in comparison with the available theoretical and finite element solutions in the literature. Higher order shear deformation beam theories having varying transverse displacement over the thickness are more appropriate to analyze DCB specimen as they predict the appropriate nature of the interlaminar normal stress at the crack tip and its distribution ahead of the crack tip.     

Nucleation and growth of small fatigue cracks in filled natural rubber under multiaxial loading

This paper presents and discusses observations of crack nucleation and small crack growth in a filled natural rubber compound subjected to multiaxial loading. A hollow cylindrical specimen was used in which simultaneous axial and shear strains are produced. The loading path types investigated include axial, torsion, proportional axial-torsion, and non-proportional axial-torsion loadings. It is shown that cracks appear and grow in a particular orientation during a typical fatigue test. The nature and evolution of these cracks with applied cycles were studied by direct observation. The observed failure plane behavior is compared to predictions based on the calculated cracking energy density. Effects of crack closure, load phase angle, crack density, and crack face shearing (mode II), as well as aspects that distinguish between “nucleation” and “growth” processes in rubber are discussed.