Ultra high molecular weight polyethylene deformation and fracture behaviour as a function of high strain rate and triaxial state of stress

The dynamic response and fracture characteristics of Ultra High Molecular Weight Polyethylene (UHMWPE) were investigated both experimentally and numerically. The strain rate sensitivity of the material was studied by carrying out tensile tests on smooth cylindrical specimens over a range of high strain rate conditions using the purpose built `flying wedge' testing machine at separation velocities up to 9 m/s. The effect of the initial stress triaxiality conditions on the material's ductility at different strain rates was studied using pre-notched cylindrical specimens with different notch radii. The true stress-strain results indicated that the tested material is highly sensitive to strain rate changes. Post-fracture geometric measurements of the fractured specimens indicated that the ductility of UHMWPE is strongly dependent on both the initial stress triaxiality conditions and the strain rate. Numerical simulations of the quasi-static and high strain rate tests were used to predict, for different notch radii, variation of the centre-most element radial strain and stress triaxiality factor with the average radial strain. Based on the combined numerical and experimental results, a simple relation for the ductile fracture of UHMWPE was derived as a function of stress triaxiality and strain rate.     

Influence of Triaxial Stress State on Ductile Fracture Strength of Polycrystalline Nickel

In this contribution, the effect associated with stress triaxiality on ductile damage evolution in high purity nickel has been investigated from both experimental and theoretical points of view. Tensile tests on smooth and notched round bar specimens were performed to calibrate the fracture strain in a wide range of stress triaxiality. The capability of the Gurson model to reproduce and predict physical failure behaviour was examined. It was shown that stress triaxiality played a major role on damage evolution as demonstrated by the progressive reduction of material ductility under increasing triaxial states of stress.     

新型耐火耐候钢材高温力学性能与本构模型研究

火灾下钢构件断裂破坏是导致整体结构连续性倒塌的主要原因之一,掌握钢材高温断裂性能是研究钢结构抗火承载性能和评估高温后结构安全性的基础。以Q355钢为研究对象,设计光滑圆棒和缺口圆棒试件,进行火灾全过程(升温段、降温段、高温后)拉伸断裂试验,研究复杂应力状态(应力三轴度)和温升历程(峰值温度和拉伸温度)对钢材工程和真实应力-应变曲线、断裂应变的影响,并通过电镜扫描研究其微观断裂机理,结合数值模拟对高温断裂模型进行参数标定。研究表明：火灾全过程下Q355钢材呈现韧性断裂特征,拉伸温度和应力三轴度对其断裂性能影响较大;拉伸温度和峰值温度越高,断裂应变越大,延性越好,高温后断裂性能与常温相似;应力三轴度影响材料断裂性能对温度的敏感性,温升历程影响真实应力-应变曲线塑性段斜率;SMCS断裂模型适用于预测Q355钢材火灾全过程断裂行为,需采用不同参数表征钢材升温段和降温段断裂性能。

     

Local approach to ductile fracture and dynamic strain aging

Experiments on smooth and notched round specimens on a C–Mn steel used in nuclear industry are performed at different temperatures under quasi-static loadings, revealing dynamic strain aging (DSA). The behavior is highly dependent on temperature and strain rate, and a drop in fracture strain is observed. Fracture surface observation on notched tensile specimens shows classical ductile fracture mechanisms with growth and coalescence of voids. The apparent strain hardening behavior at each temperature and strain rate is taken into account to compute the void growth with the Rice and Tracey model and with a damage law developed from unit cell computations. It is shown that the apparent strain hardening at large strains is of major importance to correctly predict fracture with the Rice and Tracey model, but its influence on the void growth law is of minor importance. In particular, the stress triaxiality ratio within the notch is increased due to the negative strain rate sensitivity. The ductility drop observed in DSA domain is then partly explained, but void nucleation and void growth in presence of strain bands should be included in the fracture modeling of such materials.     

ANALYSIS OF CAST STEEL FRACTURE MECHANISMS FOR DIFFERENT STATES OF STRESS

The process of fracture in a low-carbon cast steel was studied for different states of stress. As a result of heat treatment, two different microstructures have been obtained: ferritic-pearlitic and bainitic. The triaxial states of stress were realised by tensile tests on specimens with various notch configurations and on smooth specimens subjected to different hydrostatic pressures.
During tensile tests carried out under triaxial stress states, the following quantities at fracture were determined: the effective strain, effective stress, stress state components, mean stress and stress triaxiality factor. Fractography of the specimens was carried out to observe the fracture mechanisms and relate them to the state of stress. The fracture mechanism depended on the state of stress and microstructure. With a decreasing stress triaxiality factor, the failure mechanism changed from ductile to shear. The fracture mechanism changed across the diameter of the sample and also depended on the microstructure. The small, smooth samples fractured at a higher stress than the larger samples. Ductile fracture in the ferritic-pearlitic microstructure was controlled by cracking of the matrix–precipitate boundary. Samples with the bainitic microstructure fractured by shear, and fracture depended mainly on the effective stress, although void growth (which is controlled by stress triaxiality) reduced the critical effective stress at positive values of mean stress.     

Effects of the stress state on plastic deformation and ductile failure: Experiment and numerical simulation using a newly designed tension‐shear specimen

The stress state is one of the most notable factors that dominates the initiation of ductile fracture. To examine the effects of the stress state on plasticity and ductile failure, a new tension‐shear specimen that can cover a wide range of stress triaxialities was designed. A fracture locus was constructed in the space of ductility and stress triaxiality for two typical steels based on a series of tests. It is observed that the equivalent plastic strain at failure exhibits a nonmonotonic variation with increasing the value of stress triaxiality. A simple damage model based on the ductility exhaustion concept was used to simulate the failure behaviour, and a good agreement is achieved between simulation results and experimental data. It is further shown that consideration of fracture locus covering a wide range of stress triaxialities is a key to an accurate prediction.     

Fracture of austenitic steel subject to a wide range of stress triaxiality ratios and crack deformation modes

The stress triaxiality ratio (hydrostatic pressure divided by von Mises equivalent stress) strongly affects the fracture behaviour of materials. Various fracture criteria take this effect into consideration in their effort to predict failure. The dependency of the fracture locus on the stress triaxiality ratio has to be investigated in order to evaluate these criteria and improve the understanding of ductile fracture.This was done by comparing the experimental results of austenitic steel specimens with a strong variation in their stress triaxiality ratios. The specimens had cracks with varying depths and crack tip deformation modes; tension, in-plane shear, and out-of-plane shear. The crack growth in fracture mechanics specimens was compared with the failure of standard testing specimens for tension, upsetting and torsion. By associating the experimental results with finite element simulations it was possible to compare the critical plastic equivalent strain and stress triaxiality ratio values at fracture. In the investigated triaxiality regime an exponential dependency of the fracture locus on the stress triaxiality ratio was found.     

Micromechanics of room and high temperature fracture in 6xxx Al alloys

The micromechanics of ductile fracture has made enormous progress in recent years. This approach, which was mostly developed in the context of structural integrity analysis, is becoming a key tool for materials scientists to optimize materials fracture properties and forming operations. Micromechanical models allow quantitatively linking fracture properties, microstructure features at multiple lengths scales, and manufacturing conditions. After briefly reviewing the state of the art, this paper illustrates the application of the micromechanics-based methodology by presenting the results of an investigation on the damage resistance of 6xxx Al produced by extrusion.The presence of coarse, elongated, particles is the key microstructural feature affecting the fracture behaviour of 6xxx Al. The detrimental elongated β-type particles are transformed into rounded α-type particles by heat treatment. In situ tensile tests revealed that, at ambient temperature, the α particles and the β particles oriented with the long axis perpendicular to the main loading direction undergo interface decohesion, while the β particles oriented perpendicular to the loading direction break into several fragments. At high temperatures, only interface decohesion is observed. Uniaxial tensile tests on notched and smooth round bars were performed on two different alloys, at different temperatures ranging between 20 °C and 600 °C, under different loading rates, while systematically varying the content in β versus α particles. The ductility increases with decreasing amount of β particles, increasing temperature and strain rates, and decreasing stress triaxiality.A viscoplastic extension of the Gurson model has been developed for capturing the complex hierarchy of damage mechanisms, coupled with viscoplastic and stress state effects. Three populations of voids are modelled while accounting for the different void nucleation mechanisms leading to different initial void aspect ratio. Proper modelling of the initial void aspect ratio and of its evolution with void growth was the key to predict the effect of the β → α conversion on ductility. The void coalescence criterion takes into account the presence of secondary voids resulting from particle fragmentation. The characteristics of particles entering the model were all measured experimentally. The temperature and rate dependent flow properties of the matrix material have been obtained by inverse modelling. The only fitting parameters are the critical stresses for void nucleation. The model is validated by comparing the predictions to the experimental data involving different relative proportion of α and β particles, temperature, loading rate and stress triaxiality. This type of model opens the path for an “alloy by design” strategy which relates end-use properties to upstream manufacturing operations.     

THE HIGH STRAIN-RATE FRACTURE CHARACTERISTICS OF DUCTILE METALS AT ELEVATED AND SUB-AMBIENT TEMPERATURES

Abstract— High strain-rate tensile tests have been carried out on pre-notched specimens of OFHC copper and Remko iron at both elevated and cryogenic temperatures. When properly expressed as a function of stress triaxiality at the centre of the notch (as predicted by numerical simulations of the experiment), the ductility of copper was found to be independent of temperature over a range from —190°C to 300°C. The specially-processed Remko iron was found to undergo a ductile-to-brittle transition at a temperature dependent on the stress triaxiality and the particular batch of the material. Otherwise the fully ductile strains-to-failure (when expressed as a function of stress triaxiality) for iron were found to decrease with increasing temperature up to 400°C; this being the maximum temperature tested.     

Behavior and failure of uniformly hydrided Zircaloy-4 fuel claddings between 25 °C and 480 °C under various stress states,including RIA loading conditions

The anisotropic plastic behavior and the fracture of as-received and hydrided Cold-Worked Stress Relieved Zircaloy-4 cladding tubes are investigated under thermal–mechanical loading conditions representative of Pellet–Clad Mechanical Interaction during Reactivity Initiated Accidents in Pressurized Water Reactors. In order to study the combined effects of temperature, hydrogen content, loading direction and stress state, Axial Tensile, Hoop Tensile, Expansion Due to Compression and hoop Plane Strain Tensile tests are performed at room temperature, 350 °C and 480 °C on the material containing various hydrogen contents up to 1200 wt. ppm (hydrides are circumferential and homogeneously distributed). These tests are combined with digital image correlation and metallographic and fractographic observations at different scales. The flow stress of the material decreases with increasing temperature. The material is either strengthened or softened by hydrogen depending on temperature and hydrogen content. Plastic anisotropy depends on temperature but not on hydrogen content. The ductility of the material decreases with increasing hydrogen content at room temperature due to damage nucleation by hydride cracking. The plastic strain that leads to hydride fracture at room temperature decreases with increasing hydrogen content. The influence of stress triaxiality on hydride cracking is negligible in the studied range. The influence of hydrogen on material ductility is negligible at 350 °C and 480 °C since hydrides do not crack at these temperatures. The ductility of the material increases with increasing temperature. The evolution of material ductility is associated with a change in both the macroscopic fracture mode of the specimens and the microscopic failure mechanisms.     

Gurson model parameters for ductile fracture simulation in ASTM A992 steels

This study is concerned with the modelling the ductile fracture in ASTM A992 steels using the Gurson‐Tvergaard‐Needleman (GTN) model for high stress triaxiality regime. The GTN model for ASTM A992 structural steels is calibrated from the experiments performed on axisymmetrically notched tensile specimens. The experiments are designed to obtain a range of stress triaxiality and different fracture initiation locations. The non‐uniqueness in the constitutive parameters of the GTN model is illustrated in this study. The choice of a unique set of GTN constitutive parameters is made by choosing the nucleation strain (?_N) as a material constant. The process of estimating this material specific nucleation strain is provided. All the other GTN model parameters corresponding to the material specific nucleation strain (?_N) are evaluated to best fit the experimental results. The calibrated GTN model is shown to predict the load displacement behaviour, ductility and fracture initiation locations in the notched specimens. The calibrated GTN parameters are used to successfully predict the ductility of structural components: (a) bars with a hole; (b) plate with reduced section and (c) plate with holes; that are typically found in structural engineering applications.     

Dependence of ductile crack formation in tensile tests on stress triaxiality, stress and strain ratios

An experimental and numerical study on ductile crack formation in tensile tests was conducted. Five different specimens including flat specimens, smooth round bars, notched bars (two types) and flat-grooved plates were investigated. Von Mises equivalent strain to crack formation, stress triaxiality, and stress and strain ratios at critical locations, were obtained. Accuracy of the Bridgman formulas for stresses in necked round bars, and McClintock's model for flat-grooved plates, were studied. A relationship between the stress triaxiality and equivalent strain to crack formation was determined in a high stress triaxiality range for Al 2024-T351. More importantly, it was found that equivalent strain and stress triaxiality are the two most important factors governing crack formation, while stress and strain ratios cause secondary effects. It appears possible to make a good prediction of crack formation with equivalent strain and stress triaxiality.     

Study of low‐temperature effect on the fracture locus of a 420‐MPa structural steel with the edge tracing method

Quasi‐static tensile tests with smooth round bar and axisymmetric notched tensile specimens have been performed to study the low‐temperature effect on the fracture locus of a 420‐MPa structural steel. Combined with a digital high‐speed camera and a 2‐plane mirror system, specimen deformation was recorded in 2 orthogonal planes. Pictures taken were then analysed with the edge tracing method to calculate the minimum cross‐section diameter reduction of the necked/notched specimen. Obvious temperature effect was observed on the load‐strain curves for smooth and notched specimens. Both the strength and strain hardening characterized by the strain at maximum load increase with temperature decrease down to ?60°C. Somewhat unexpected, the fracture strains (ductility) of both smooth and notched specimens at temperatures down to ?60°C do not deteriorate, compared with those at room temperature. Combined with numerical analyses, it shows that the effect of low temperatures (down to ?60°C) on fracture locus is insignificant. These findings shed new light on material selection for Arctic operation.     

Stress state dependent fracture behaviour of porous PM steels

Ductile fracture of metals is a result of void nucleation, growth and coalescence. Various criteria have been proposed to model the ductile fracture strain as a function of the stress triaxiality that greatly influence the fracture process. In the present investigation, the well-known Rice and Tracey approach (with a re-evaluation conducted by Huang) was used to model the ductile fracture behaviour of two porous steels, produced by Powder Metallurgy (PM): a ferritic–pearlitic Fe–0.4%C PM steel and a high-strength steel produced by using diffusion-alloyed Fe–4%Ni–1.5%Cu–0.5%Mo–0.5%C powder. Tensile, compressive and bending tests were carried on un-notched and notched specimens. The experimental curves were used as a reference for the Finite Element (FE) modeling of the tests aimed at evaluating the equivalent fracture strain at fracture and the correspondent stress triaxiality for each geometry. The results obtained for the Fe–0.4%C PM steel proved the suitability of the modified Rice and Tracey relationship to successfully obtain a simple fracture criterion. However, in the case of high-strength steel, a mixed ductile/brittle fracture behaviour was observed because of the microstructural heterogeneity of the alloy. Because of this, the Rice and Tracey model overestimates the experimental equivalent fracture strains and has to be accordingly corrected.     

Effect of Strain Rate,Adiabatic Heating and Phase Transformation Phenomena on the Mechanical Behaviour of Stainless Steel

Abstract: The influence of strain rate on the stress–strain behaviour of an AISI 304 austenitic stainless steel sample was investigated. For this purpose, uniaxial tensile tests were performed at room temperature for different strain rates. Microstructural measurements of transformed martensitic phase as a function of plastic strain, and thermal analyses of the specimens were carried out as well. It was found that increasing the strain rate from 10^?4 to 10^?1 s^?1 leads to a 25% improvement in uniform elongation. Moreover, a ‘curve‐crossing’ phenomenon was observed for the hardening behaviour measured at different strain rates. These results were rationalized in terms of martensitic phase transformation suppressed by a temperature increase in the specimens deformed with high strain rates.     

Continuum damage mechanics modelling incorporating stress triaxiality effect on ductile damage initiation

Micromechanical modelling of void nucleation in ductile metals indicates that strain required for damage initiation reduces exponentially with increasing stress triaxiality. This feature has been incorporated in a continuum damage mechanics (CDM) model, providing a phenomenological relationship for the damage threshold strain dependence on the stress triaxiality. The main consequences of this model modification are that the failure locus is predicted to change as function of stress triaxiality sensitivity of the material damage threshold strain and that high triaxial fracture strain is expected to be even lower than the threshold strain at which the damage processes initiate at triaxiality as low as 1/3. The proposed damage model formulation has been used to predict ductile fracture in unnotched and notched bars in tension for two commercially pure α‐iron grades (Swedish and ARMCO iron). Finally, the model has been validated, predicting spall fracture in a plate‐impact experiment and confirming the capability to capture the effect of the stress state on material fracture ductility at very high stress triaxiality.     

A fracture locus for a 50 volume-percent Al/SiC metal matrix composite at high temperature

The effect of the stress state on the fracture locus function of the 50 vol.% Al/SiC metal matrix composite at high temperature is studied. The value of fracture locus function is quantitatively characterized by the amount of shear strain accumulated prior to the moment of failure. Nondimensional invariant parameters are used as characteristics of the stress state, namely, the stress triaxiality k and the Lode-Nadai coefficient μ _σ showing the form of the stress state. Besides conventional testing for tension, compression and torsion of smooth cylindrical specimens, the complex of mechanical tests includes a new type of testing, namely, that for bell-shaped specimens. These kinds of testing enable one to study fracture strain under monotonic deformation in the ranges μ _σ ?=?0?…?+?1 and k?=???1.08...0 without using high-pressure technologies. The stress–strain state during specimen testing is here evaluated from the finite element simulation of testing in ANSYS. The tests were performed at a temperature of 300 °C and shear strain rate intensity Η?=?0.1;?0.3;?0.5 1/s. The test results have offered a fracture locus, which can be used in models of damage mechanics to predict fracture of the material in die forging processes.     

Anisotropic ductile failure in free machining steel at quasi-static and high strain rates

Leaded Free Machining Steel (FMS) specimens were tested in tension at quasi-static and high strain rates in both the longitudinal and transverse directions with respect to the axis of the bar material. For the quasi-static tests, a high degree of anisotropy of fracture behaviour was observed for both plain (unnotched) and notched specimens. However the difference in fracture strains for longitudinal and transverse directions was significantly reduced for the high stress triaxiality conditions produced by the sharper notches. Plain specimens tested at dynamic strain rates (10³ s⁻¹) failed at somewhat higher strains than those tested quasi-statically. For the notched specimens tested dynamically, there was a transition to a brittle mode of failure and there was no statistically significant anisotropy in the very low strains to failure recorded. These experimental results were linked to numerical predictions of the local stress, strain and strain rate conditions in the specimens carried out using a modified Armstrong–Zerilli constitutive model for the FMS. Changes in the percentage area and aspect ratio of the lead inclusions which act as sites for void growth under ductile failure conditions were measured for both longitudinal and transverse directions of loading. It was found that the apparent area of inclusions increases with degree of deformation due to void growth but that the aspect ratio decreases due to the inclusions/voids becoming more spherical. This effect was greater for loading in the transverse direction indicating that voids grow more readily from inclusions when the latter are aligned perpendicular to the direction of loading.     

Tests on impact behaviour of micro-concrete-filled steel tubes at elevated temperatures up to 400°C

Concrete-filled tubular (CFT) columns are being more and more utilized in construction of tall buildings and bridges. The CFT column system, which has been proved to have excellent load carrying capacity and ductility, by static and simulated seismic loading tests, also has good dynamic impact behaviour. The impact resistance of small-size micro-concrete-filled steel tubes under axial impact load at elevated temperatures up to 400°C was experimentally studied by using a spilt Hopkinson pressure bar. The stress and strain time history curves of the tested specimens were recorded to analyze the impact behaviour of CFT at elevated temperatures. The failure patterns and the influence of temperature on the impact resistance of CFT are discussed. The test results show that CFT has an excellent impact resistant capacity at elevated temperatures and the dynamic behaviour of core concrete under high temperatures was discovered. A simplified calculation method to determine the impact resistant capacity of CFT at elevated temperatures is presented, which is validated by the tested results.