Experimental and numerical study on dynamic fracture behaviour of AISI 1045 steel for compressor crankshaft

Dynamic fracture behaviour of AISI 1045 steel for compressor crankshaft was studied by experimental and numerical methods. True stress–strain relations of the material under different strain rates were measured, and dynamic constitutive model with consideration to strain‐hardening and strain‐rate hardening was proposed. Dynamic fracture tests loaded by Hopkinson pressure bar were carried out, and fracture toughness was determined using a finite element method with the combination of ABAQUS and Zencrack software. Loading states of the specimen and determination methods of the dynamic fracture toughness were discussed. By comparing the fracture behaviours under quasi‐static and dynamic conditions, it was found that the fracture modes exhibited a transition from ductile to brittle fracture with the increasing loading rate, and the dynamic fracture toughness value was less than the quasi‐static one.     

Design and characterization of a lamellar nanostructure in a low C steel

A fully lamellar ferrite/cementite nanostructure was designed in a low C steel by using a specific thermal treatment. The strengthening of such microstructure has been investigated as a function of prestrain by rolling up to a deformation of 300%. As in usual pearlitic structure, its work-hardening shows no saturation and its elongation to fracture remains rather constant instead of decreasing drastically as conventional steels. The hardening by a similitude effect is thus not the privilege of pearlitic steels. Nevertheless, its lower initial work-hardening rate at low strain compared to an equivalent pearlitic steel and a lower hardening potential at high strain let us suspect major differences in the nature and the behaviour of ferrite channels in relation to the morphogenesis of the microstructure. This study opens a new way to obtain low carbon ultra-high strength steel by a nanostructuration process using severe plastic deformations.     

Application of model for work hardening behaviour of SiC reinforced magnesium based metal matrix composites

Abstract

An understanding of the work hardening behaviour of particulate reinforced metal matrix composites is crucial in optimising the parameters for deformation processing of these materials. In the present study, SiC reinforced magnesium metal matrix composites were produced using a liquid phase process. The microstructure of the composite was characterised and the mechanical properties were determined. The results of the ambient temperature tensile testing on the extruded Mg and Mg/SiC specimens revealed that an increase in the weight percentage of SiC particulates in pure magnesium increases the elastic modulus, does not affect the 0·2% yield strength, and reduces the ultimate tensile strength and ductility. A modified continuum model was applied to relate the work hardening behaviour of the composites to microstructural parameters and to predict the fracture strain of the composites. The model is shown to predict the fracture strain of the composites quite accurately for all the three weight fractions of reinforcements evaluated in the present study.     

Influence of austempering temperature on the mechanical properties of a low carbon low alloy steel

In this investigation, a new low alloy and low carbon steel with exceptionally high strength and high fracture toughness has been developed. The effect of austempering temperature on the microstructure and mechanical properties of this new steel was examined. The influence of the microstructure on the mechanical properties and the fracture toughness of this steel was also studied.Test results show that the austempering produces a unique microstructure consisting of bainitic ferrite and austenite in this steel. There were significant improvement in mechanical properties and fracture toughness as a result of austempering heat treatments. The mechanical properties as well as the fracture toughness were found to decrease as the austempering temperature increases. On the other hand, the strain hardening rate of steel increases at higher austempering temperature. A linear relationship was observed between strain hardening exponent and the austenitic carbon content.     

On the mechanical properties of heat-treated expanded perlite–aluminium syntactic foam

In this paper, a syntactic foam is fabricated by counter-gravity infiltrating packed bed of expanded perlite particles with A356 aluminium alloy. The samples are subjected to a T6 heat treatment. The impact of heat treatment on microstructure characteristics, mechanical properties, deformation behaviour, and cell wall fracture mechanism are investigated. The compression stress–strain curves of the heat treated foams showed the three stages of elasticity, stress plateau and densification. Heat treatment resulted in a significant increase in plateau stress and absorbed energy. It is found that the effect of density on mechanical properties after heat treated conditions is more significant in comparison to untreated conditions. Under compression, the heat treated foams shows more uniform deformation. The improvement in compression characteristics by heat treatment is found to be a result of refined microstructure and higher ductility of the cell walls. Heat treatment reduces the deleterious impact of the columnar dendritic structure of the cell wall and the casting defects on mechanical properties. It limits the crack propagation by increasing the aspect ratio and interparticle distance of the Si particles in the Al matrix.     

Micromechanics of fracture in notched samples of heavily drawn steel

This paper analyzes the consequences on fracture of the combined effects of triaxial stress states generated by notches of very different geometries and microstructural evolution produced by a heavy cold drawing when eutectoid high-strength prestressing steels are manufactured. The anisotropic fracture behaviour of these materials with high level of strain hardening is rationalized on the basis of the markedly oriented pearlitic microstructure of the drawn steels which influences the operative micromechanism of fracture in this case.     

Elastic Wave Scattering by an Interface Crack Between a Piezoelectric Layer and an Elastic Substrate

This paper analyzes the consequences on fracture of the combined effects of triaxial stress states generated by notches of very different geometries and microstructural evolution produced by a heavy cold drawing when eutectoid high-strength prestressing steels are manufactured. The anisotropic fracture behaviour of these materials with high level of strain hardening is rationalized on the basis of the markedly oriented pearlitic microstructure of the drawn steels which influences the operative micromechanism of fracture in this case.     

A NOTE ON THE INFLUENCE OF REST PERIODS ON THE FATIGUE ENDURANCE OF A TITANIUM ALLOY

S-N curves have been determined for continuous cycling and for interrupted cycling using commercial purity titanium, and also a Ti-6Al-4V alloy in four different states of heat treatment. The incorporation of rest periods during the fatigue tests resulted in increased lives to fracture to an extent which appeared to be independent of specimen composition or microstructure, and hence primarily a function of the titanium itself. Failures from internal crack origins were found at longer lives from some specimens for both continuous and interrupted tests. It is suggested that the observed effects are due to localised hardening and strain ageing as a consequence of oxygen and/or nitrogen pick-up from the testing environment, and also as a result of oxygen and/or nitrogen present in the starting material.     

Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 1—Mechanical behaviour

Abstract

A split Hopkinson bar is used to investigate the effects of prestrain and strain rate on the dynamic mechanical behaviour of 304L stainless steel, and these results are correlated with microstructure and fracture characteristics. Annealed 304L stainless steel is prestrained to strains of 0·15, 0·3, and 0·5, then machined as cylindrical compression specimens. Dynamic mechanical tests are performed at strain rates ranging from 10² to 5 × 10³ s^-1 at room temperature, with true stains varying from 0·1 to 0·3. It was found that 304L stainless steel is sensitive to applied prestrain and strain rate, with flow stress increasing with increasing prestrain and strain rate. Work hardening rate, strain rate sensitivity, and activation volume depend strongly on the variation of prestrain, strain, and strain rate. At larger prestrain and higher strain rate, work hardening rate decreases rapidly owing to greater heat deformation enhancement of plastic flow instability at dynamic loading. Strain rate sensitivity increases with increasing prestrain and work hardening stress (σ-σ_y). However, activation volume exhibits the reverse tendency. Catastrophic fracture is found only for 0·5 prestrain, 0·3 strain, and strain rate of 4·8 × 10³ s^-1. Large prestrain increases the resistance to plastic flow but decreases fracture elongation. Optical microscopy and SEM fracture feature observations reveal adiabatic shear band formation is the dominant fracture mechanism. Adiabatic shear band void and crack formation is along the direction of maximum shear stress and induces specimen fracture.     

Analysis of the unstable fracture behaviour of a high strength low alloy steel weldment

Local brittle zones (LBZ) cause the unstable fracture behaviour of weld metals. This threatens the safe service of welded structures and makes structural assessment procedures difficult. Therefore, the unstable fracture behaviour of an overmatched high strength low alloyed steel weldment was experimentally investigated. It showed that any interaction between two adjacent weld metal matrix and soft weld metal inclusions produces LBZ, causing local unstable fracture behaviour. The formation of a low hardness region is attributed to the multipass welding reheating process between Ac₁ and the self-tempering temperature. The presence of partly solid metallic inclusions with a high content of alloying elements and pro-eutectoid ferrite microstructure were found to be additional causes for the local unstable fracture behaviour of the weld metal. Local strength mis-match induced the yielding and strain hardening in the soft weld metal inclusions, contributing significantly to unstable fracture behaviour. Thus, a significantly different scatter of experimental results can be obtained. In the cases of specimens with through-the-thickness crack, not only is the scatter significantly lower, but the toughness itself.     

Comparative study of fracture toughness of austempered ductile irons with upper and lower ausferrite microstructures

Abstract

A ductile iron was austempered at 302 and 385°C for various times to get lower and upper ausferrite microstructures respectively. The microstructures were characterised by optical microscopy and X-ray diffraction. Plane strain fracture toughness was determined under all heat treatment conditions. While the austempered ductile iron with lower ausferrite microstructure showed higher fracture toughness, the one with upper ausferrite microstructure exhibited higher tensile toughness and strain hardening coefficient. A model was developed relating fracture toughness to the yield strength (σ_y) volume fraction of retained austenite (X_y) and the carbon content of the retained austenite (C_y). Experimental results showed excellent agreement with the prediction of the model that <disp-formula><graphic href="splitsection9-m1.tif"/></disp-formula> is proportional to σ_y(X_yC_y)^½.     

贝氏体型冷作强化非调质钢的氢致延迟断裂行为

采用电化学充氢和慢拉伸应变速率实验(SSRT)研究了形变和时效处理对一种低碳-Mn-B-Ti系贝氏体型冷作强化非调质钢氢致延迟断裂行为的影响规律。结果表明,无论热轧态还是拉拔态试样,充氢后试样的耐延迟断裂性能显著降低。在拉拔变形初期,减面率γ<20%时,耐延迟断裂性能降低的幅度较大;而随着拉拔量的继续增加,耐延迟断裂性能降低的趋势变缓。实验料拉拔后进行时效处理有助于其耐延迟断裂性能的改善,当时效温度提高到200℃以上时这种改善作用比较明显。     

Isothermal nanocrystallisation behaviour of melt spun Al86Ni9 Mm5 (Mmmischmetal)amorphous alloy

Abstract

The structure and mechanical properties of melt spun Al₈₆Ni₉Mm₅ alloy ribbons in both as solidified amorphous and heat treated nanocomposite conditions were investigated using DSC, XRD, TEM, and Vickers microhardness techniques. Primary crystallisation of the amorphous alloy resulted in the formation of fine nanocrystalline fcc-Al particles embedded in an amorphous matrix forming a nanocomposite. The growth behaviour of the primary fcc-Al particles under isothermal conditions was investigated. The hardness ofthe composite varied with the solute content in the amorphous phase and the microstructure after heat treatment. The hardening in these nanocomposites was quantitatively explained using a rule of mixtures model based on the volume fraction of the amorphous matrix and the Al particles. The nanometre sized particles were treated as perfect materials and the matrix was treated as an amorphousmaterial, in which the solute concentration increased as the volume fraction of the Al particles increased. The calculated results for the heat treated specimens using the rule of mixtures based on the isostress model have been found to be in good agreement with the experimentally obtained results.     

Cyclic stress–strain characteristics of two microalloyed steels

Abstract

The cyclic stress–strain behaviour of two microalloyed steels with different microstructures has been characterised at room temperature under strain controlled low cycle fatigue. The cyclic stress–strain curve in the double logarithmic plot shows a linear relation for both steels. A transition of the cyclic stress–strain curve from softening to hardening with increasing strain amplitude has been observed with respect to the corresponding tensile curve. The strain amplitude for the onset of cyclic softening to hardening transition has been found to be dependent on grain size. The strain lifetime behaviour, estimated from modified universal slopes equation, shows similar trends as Nb or V bearing microalloyed steels. The cyclic characteristics of the two microalloyed steels have been compared with corresponding predeformed state carried out under stress controlled conditions. While, cyclic saturation was observed in case where the extent of predeformation was within the Lüders strain, cyclic softening occurred when it exceeded the Lüders strain. It has been attempted to provide a mechanistic understanding of the differences in the cyclic behaviour of the two steels owing to the microstructure and predeformation.     

Stochastic uncertainty analysis of damage evolution computed through microstructure–property relations

Uncertainties in material microstructure features can lead to uncertainty in damage predictions based on multiscale microstructure–property models. This paper presents an analytical approach for stochastic uncertainty analysis by using a univariate dimension reduction technique. This approach is used to analyze the effects of uncertainties pertaining to the structure–property relations of an internal state variable plasticity–damage model that predicts failure. The results indicate that the higher the strain the greater the scatter in damage, even when the uncertainties in the material plasticity and microstructure parameters are kept constant. In addition, the mathematical sensitivity analysis results related to damage uncertainty are consistent with the physical nature of damage progression. At the beginning, the initial porosity and void nucleation are shown to drive the damage evolution. Then, void coalescence becomes the dominant mechanism. And finally, when approaching closer to failure, fracture toughness is found to dominate the damage evolution process.     

氢处理对铸造钛合金低周疲劳寿命及断裂韧性的影响

本文研究了氢处理对铸造钛合金的低周疲劳寿命及断裂韧性的影响,发现经氢处理后铸造钛合金粗大的魏氏组织转变成细小的等轴组织,其应变低周疲劳寿命大大延长。在低周疲劳过程中,应变量较小时,试样出现循环硬化现象;而在应变量较大时,出现循环软化现象。氢处理后材料的断裂形式由脆性断裂转为韧性断裂;断裂韧性提高。     

Beitrag zur Bestimmung des Schädigungszustandes von kriechbeanspruchtem CrMoV-Stahl

A Contribution to the Problem of the Determination of the Creep Damage for a CrMoV-Steel0 The lifetime of materials subjected to creep is limited by the accumulated damage. The estimation of the creep damage is difficult, especially in cases where there is uncertainty about the thermal history of a specific component. Creep damage usually consists of a reversible and an irreversible part. The reversible part can be recovered by a heat treatment and is mainly due to changes in the microstructure (dislocation configuration, state of precipitation etc.). Irreversible damage is usually a consequence of necking or of cavitation. In the present investigation a CrMoV-steel (0.18 C/0.6 Mn/0.4 Cr/1.1 Mo/0.4 V) was creep tested at 550 °C and a stress of 245 N/mm². After certain times or strains the specimens were removed and their microstructural state was determined. From the removed specimens new samples with smaller dimensions were machined and again tested under the same conditions. Between the first and the second creep test a new heat treatment was applied for some of the samples, in order to remove the reversible creep damage and to gain more information about the influence of the irreversible damage on creep. The creep behavior of the investigated CrMoV steel depends very strongly on the initial microstructure. For the same testing conditions the time to fracture can vary from 740 hours (40% upper bainite/60% ferrite) up to almost 10000 hours (100% coarse grained upper bainite). This result indicates that the type and the progress of the creep damage could also be influenced by the microstructure. In fact with the ferritic-bainitic structure cavities mainly develop on ferrite-bainite grain boundaries and are usually not linked together. Within the purely bainitic microstructures, the cavities are found on former austenite grain boundaries and have very often a crack-like appearance. The behavior of newly machined samples after the first creep test without a new heat treatment is dictated by the internal damage of the sample. After the removal of the scale and the macroscopic inhomogeneities the creep curves of the second test can be considered as a continuation of the first creep (curves figure 10 in German text). A new heat treatment ( 1000 °C/l h/air + 725 °C/4 h/air) between te two creep tests leads to a partial recovery of the original properties. For the mixed bainitic-ferritic microstructure, even for fractured samples, a considerale increase in time to fracture is observed, e.g. 1156 hrs for the first test, 6821 hrs for the second test after a new heat treatment. The new heat treatment produced a much more creep resistant microstructure and recovers therefore the influence of any creep damage, especially in the case of the originally ferritic-bainitic microstructure. In order to reveal the effect of creep damage on the second test, the creep properties of the microstructure produced by the intermediate heat treatment has to be considered as a reference (100% upper bainite). This fully bainitic steel exhibited in its new state mean values of time to fracture t_f = 8554 hrs, and secondary creep rate \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \varepsilon _{\rm S} $\end{document} = 2 × 10^?10 sec^?1. The experimentally measured creep data of the second creep test and the mean values form a specific ratio as a measure of the damage during the course of the first creep test. This behavior is shown schematically in Fig. 14. In this figure the creep strain is plotted as a function of the ratio between the mean time to fracture and the time to fracture in the second creep test (or secondary creep rate). The experimental results show that it needs of a minimum creep strain to initiate irreversible creep damage. The amount of strain depends also on the microstructure. In a relatively brittle steel containing e.g. coarse rained upper bainite, creep damage commences early in the secondary creep stage, whereas in more ductile steel the onset of tertiary creep coincides with the begin of irreversible damage.     

Finite Element simulation coupled to optimisation stochastic process to assess the effect of heat treatment on the mechanical properties of 42CrMo4 steel

The present study aims at studying the influence of heat treatment on the mechanical performance of 42CrM04 steel. Heat treatment is conduced in a way to affect significantly the microstructure, more precisely the tempered martensite structure. Tempering temperature is varied between 200 and 600° C. Experimental testing includes the determination of tensile properties and hardness scores. The identification of the mechanical parameters associated to elastic–plastic behaviour is undertaken using Finite Element computation to quantify the effect of heat treatment. The predictions show that characterisation of the material behaviour can be approached using a bilinear hardening model. The tempering temperature has a significant effect on the yield stress and tangent modulus.     

Predicting damage and failure under low cycle fatigue in a 9Cr steel

Experimental data have been generated and finite element models developed to examine the low cycle fatigue (LCF) life of a 9Cr (FB2) steel. A novel approach, employing a local ductile damage initiation and failure model, using the hysteresis total stress–strain energy concept combined with element removal, has been employed to predict the failure in the experimental tests. The 9Cr steel was found to exhibit both cyclic softening and nonlinear kinematic hardening behaviour. The finite element analysis of the material's cyclic loading was based on a nonlinear kinematic hardening criterion using the Chaboche constitutive equations. The models’ parameters were calibrated using the experimental test data available. The cyclic softening model in conjunction with the progressive damage evolution model successfully predicted the deformation behaviour and failure times of the experimental tests for the 9Cr steels performed.     

Tensile and bending behaviour of a strain hardening cement-based composite: Experimental and numerical analysis

IFSTTAR has developed a Multi-Scale Cement Based Composite (MSCC). This composite material is strain hardening in tension and exhibits ultra-high strengths as well in both compression and tension. The main research objectives for the present paper are the determination of the strain hardening properties of the material: using a newly developed tensile test in conjunction with a finite-element-based inverse analysis, the input parameters of an (adapted) numerical model can be identified. Therefore, numerical simulations can be performed to describe the bending behaviour of a thin slab having a thickness representative of the corresponding industrial application.The main conclusions of this study are:

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The studied material clearly exhibits strain hardening in tension with a uniaxial tensile strength of about 20 MPa and a modulus of rupture of about 50 MPa.

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Elasto-plastic behaviour with strain hardening is a relevant mechanical model (for the studied material) for designing (by the finite element method) structural elements behaving principally in bending.