In situ monitoring of particle fracture in aluminium alloys

Material damage at the microscale involves both initiation and interaction effects that are typically activated long before the appearance of macroscopically observable failure events. These early appearances of damage have been broadly classified as microstructure‐sensitive damage precursors or indicators. A particular class of such precursors which is of importance to aluminium and other precipitate‐hardened alloys is the focus of this article. Specifically, the hard, intermetallic particles in aluminium alloys fracture before significant failure occur in the surrounding matrix. In this investigation, an effort to directly assess particle fracture activity at the time and scale that it occurs is made by coupling mechanical testing inside a scanning electron microscope with nondestructive evaluation techniques including digital image correlation as well as real‐time acoustic emission monitoring. The use of a surface measurement technique along with a volumetric monitoring method at the microscope scale provides a way for coupling of fracture information at locations which are directly related to the particle activity. In this article, Al2024‐T3 specimens in the as‐received condition were subjected to tension as well as to tension‐tension cyclic loading. The obtained in situ results demonstrate, for the first time to the best knowledge of the authors, that particle fracture occurs early in the damage process which justifies its characterization as a material damage precursor. The overall approach provides datasets capable to detect particle fracture initiation, which could become useful in future structural health monitoring applications.     

Stress triaxiality effect on void nucleation in ductile metals

The stress triaxiality effect on the strain required for void nucleation by particle‐matrix debonding has been investigated by means of micromechanical modelling. A unit‐cell model considering an elastic spherical particle embedded in an elastic‐plastic matrix was developed to the purpose. Particle‐matrix decohesion was simulated through the progressive failure of a cohesive interface. It has been shown that the parameters of matrix‐particle cohesive interface are correlated with macroscopic material properties. Here, a simple relationship for the maximum cohesive opening at interface failure as a function of material fracture toughness and yield stress has been derived. Results seem to confirm that, increasing stress triaxiality, the strain at which void nucleation is predicted to occur decreases exponentially in a similar way as for fracture strain. This result has substantial implications in modelling of ductile damage because it indicates that if the stress triaxiality is high enough, ductile fracture can occur at plastic strain lower than that necessary to nucleate damage for moderate or low stress triaxiality regime.     

Controlling the onset of numerical fracture in parallelized implementations of the material point method (MPM) with convective particle domain interpolation (CPDI) domain scaling

The material point method is well suited for large‐deformation problems in solid mechanics but requires modification to avoid cell‐crossing errors as well as extension instabilities that lead to numerical (nonphysical) fracture. A promising solution is convected particle domain interpolation (CPDI), in which the integration domain used to map data between particles and the background grid deforms with the particle, based on the material deformation gradient. While eliminating the extension instability can be a benefit, it is often desirable to allow material separation to avoid nonphysical stretching. Additionally, large stretches in material points can complicate parallel implementation of CPDI if a single particle domain spans multiple computational patches. A straightforward modification to the CPDI algorithm allows a user‐specified scaling of the particle integration domain to control the numerical fracture response, which facilitates parallelization. Combined with particle splitting, the method can accommodate materials with arbitrarily large failure strains. Used with a smeared damage/softening model, this approach will prevent nonphysical numerical fracture in situations where the material should remain intact, but the effect of a single velocity field on localization may still produce errors in the post‐failure response. Details are given for both 2‐D and 3‐D implementations of the scaling algorithm. Copyright © 2015 John Wiley & Sons, Ltd.     

Failure of aluminium self-piercing rivets: An experimental and numerical study

The present paper investigates the fracture mechanisms of AA7278-T6 aluminium self-piercing rivets under compression during the riveting process. First, a microstructure investigation was conducted to disclose the grain structure and the particle distribution of the extruded aluminium alloy. Transmission electron micrographs revealed precipitate free zones along grain boundaries. Uniaxial tensile tests in three different directions with respect to the extrusion direction revealed anisotropy of the alloy in strength and ductility and a change in fracture mode with tensile direction. The behaviour of the alloy under compression was studied experimentally using upsetting tests and self-piercing riveting tests. Micrographs of the deformed specimens provided insight into the influence of the microstructure on the deformation and fracture of the alloy under compression. Second, numerical analyses were carried out using a 2-D axisymmetric model in LS–DYNA in an attempt to investigate the role of different physical variables on the final failure of the rivet. The numerical results revealed that constituent particles, precipitate free zones, and friction between the rivet and plates are important for strain localisation and fracture in the rivet.     

On the formation of ridges and burnished debris along internal fatigue crack propagation in 42CrMo4 steel

A detailed microscopic analysis of fracture surfaces of 42CrMo4‐hardened steel after ultrasonic fatigue testing revealed globular and cylindrical particles located in ridges along the crack propagation direction. Observed particles could be easily taken for non‐metallic inclusions; however, chemical analysis showed that they are of the same composition as the steel matrix. The formation of such round‐shaped debris was found to be a result of cutting out the matrix fragments by ‘en passant’ cracks interaction and their subsequent fretting burnishing. Possible correlation of the parameters of ridges and debris with cyclic plastic zone width and martensite structure is discussed.     

A mesh-free approach for fracture modelling of gravity dams under earthquake

Fracture is a major cause of failure for concrete gravity dams. This can result in the large-scale loss of human lives and enormous economic consequences. Numerical modelling can play a crucial role in understanding and predicting complex fracture processes, providing useful input to fracture-resistant designs. In this paper, the use of a mesh-free particle method called smoothed particle hydrodynamics (SPH) for modelling of gravity dam failure subject to fluctuating dynamic earthquake loads is explored. The structural response of the Koyna dam is analysed with the base of the dam being subjected to high-intensity periodic ground excitations. The SPH prediction of the crack initiation location and propagation pattern is found to be consistent with existing FEM predictions and experimental results from physical models. The transient stress field and the resulting damage evolution in the dam structure were monitored. The amplitude and frequency of the ground excitation is shown to have considerable influence on the fracture pattern and the associated energy dissipation. The fluctuations in the kinetic energy of the dam wall and its fragments are found to vary with different frequencies and amplitudes as the structure undergoes progressive fracture. The dynamic responses and the fracture patterns predicted establish the strong potential of SPH for fracture modelling of dams and similar large structures.     

Hafnium carbide growth behavior and its relationship to the dispersion hardening in tungsten at high temperatures

The growth behavior of hafnium dispersed carbide particles and its relationship the strength of a tungsten-rhenium alloy at temperatures above 2200 K have been examined with transmission electron microscopy. From 2200 to 2600 K, HfC particles grow homogeneously with a relatively slow growth rate. In this temperature region, the activation energy for HfC particle growth is determined to be 50.2 ± 1.0 kcal mol⁻¹. The coarsening of HfC particles is a bulk diffusion-controlled process and hafnium is the less stable element in the HfC decomposition and growth process. Rapid particle growth occurs at temperatures above 2600 K. A bi-modal particle size distribution is observed at 2800 and 3000 K and selective particle growth takes place along dislocation lines and at grain boundaries, indicating that dislocation and boundary enhanced diffusion mechanisms are involved in the HfC particle coarsening process above 2600 K. Based upon the studies on the growth behavior of HfC particles, An Ashby-Orowan dispersion strengthening model is applied to the HfC hardened tungsten. Good agreement is obtained between the calculated resolved shear stress and experimental results.     

Precipitate growth activation energy requirements in the duplex size γ′ distribution in the superalloy IN738LC

The precipitate growth features in the duplex size (fine and coarse) precipitate distribution, obtained by quenching the alloy from the holding temperature of 1140°C, was studied by treating the alloy for various times in vacuum at selected temperatures in the range 800–1100°C. It was found that both the fine and the coarse precipitate particles grow with time, apparently aided by the particle movement in the matrix and coalescence, by the so-designated Precipitate Agglomeration Mechanism (PAM). At 1100°C the fine particles grew to the size of the coarse particles in about 100 hours and a near single coarse size of about 840 nm was obtained in the matrix. The activation energies for the growth of both the fine and coarse particles were calculated using particle sizes at different temperatures after 25 h of aging and at 1040 and 1100°C for different aging times. These were found to decrease continuously with an increase in size of the particles, meaning that the coarse particles grow more easily than the fine particles, requiring less activation energies. This behavior could be attributed to the greater attractive force with which the coarser particles would attract the finer particles.     

Fracture of Al-4% Cu-0.1% Fe single crystals

The fracture behaviour of heat-treated Al-4% Cu-0.1% Fe single crystals was studied in tension at room temperature. Three heat-treatment conditions were examined: quenced, fully hardened and overaged. Slip lines, shear bands and fracture surfaces were studied to yield information on the plastic deformation processes occurring prior to fracture. The presence of stable Al₇Cu₂Fe particles was found to be an important factor in fracture formation. In the as-quenced condition two fracture planes of different topography were formed. Large shear zones together with scattered shallow dimples were observed in both planes due to stable particles. In the fully hardened condition fracture occurred without necking and usually by shearing along the conjugate slip system. The presence of shear zones and dimples of different sizes was observed. Finally, in the overaged condition fracture took place by void coalescence after strong necking, as in polycrystalline samples. No shear zones were observed on the fracture surface of these samples.     

Failure Analysis of Induction Hardened Automotive Axles

Rollover accidents in light trucks and cars involving an axle failure frequently raise the question of whether the axle broke causing the rollover or did the axle break as a result of the rollover. Axles in these vehicles are induction hardened medium carbon steel. Bearings ride directly on the axles. This article provides a fractography/fracture mechanic approach to making the determination of when the axle failed. Full scale tests on axle assemblies and suspensions provided data for fracture toughness in the induction hardened outer case on the axle. These tests also demonstrated that roller bearing indentions on the axle journal, cross pin indentation on the end of the axle, and axle bending can be accounted for by spring energy release following axle failure. Pre-existing cracks in the induction hardened axle are small and are often difficult to see without a microscope. The pre-existing crack morphology was intergranular fracture in the axles studied. An estimate of the force required to cause the axle fracture can be made using the measured crack size, fracture toughness determined from these tests, and linear elastic fracture mechanics. The axle can be reliably said to have failed prior to rollover if the estimated force for failure is equal to or less than forces imposed on the axle during events leading to the rollover.     

A discrete element model to describe failure of strong rock in uniaxial compression

A bonded particle model is investigated by means of an extended Discrete Element Method with respect to failure of strong rock in uniaxial compression. A coordination number based inflation scheme is presented, which generates isotropic sphere packings that are featuring a higher average coordination number than conventional procedures. A progressive failure model is proposed, which promotes crack propagation and localization and allows adjusting brittleness of fracture. Failure of the granular solid is discussed in detail. The fracture process is studied in dependence of the introduced failure models. The influence of particle size and bond strength distributions, particle numbers, particle layering in finite sphere packings and end constraints is addressed. Comparison to published experimental results reveals that many of the observed features of rock failure are reproduced.     

Importance of interface chemistry in hipping diffusion bonding of precipitation hardened martensitic stainless steels

Abstract

The effect of processing route on the microstructure and properties of hot isostatically pressed diffusion bonds, manufactured from precipitation hardened martensitic stainless steels, is addressed in this paper. The quality of the diffusion bond was assessed using a range of analytical techniques and a programme of mechanical testing. It is shown that the microstructure and properties of the diffusion bond are strongly influenced by the prebonding processing route used in its manufacture. Using conventional encapsulation techniques the interface is contaminated by oxide particles, whose composition and morphology depends upon the precise chemistry of the steel. These particles prevent grain growth across the interface and frequently result in tensile fracture at the interface. Refinements to the prebonding processing route result in lower particle densities and grain growth across the interface. However, these measures result in the interface becoming weakened by the precipitation of copper at the interface.     

Particle Size and the Mechanical Properties of Uganda Clay

This paper describes a study of the effects of particle size on the porosity, microstructure, and mechanical properties of compacts fabricated under uniaxial compression with a rectangular die. Four ranges of different particle sizes were investigated to determine the dependence of fracture toughness; porosity and strength of the compacts on particle size. Thermal shock effects on the compacts were also investigated by determining the number of cycles to failure, when the compacts were cold shocked at four different temperatures. Compacts made from powders of larger particle size tended to have larger number of cycles to failure than those made of finer particle sizes. Contrary to this, compacts made from smaller particles size exhibited higher fracture toughness compared to those made from larger particle size.     

Influence of surface condition on the fatigue behaviour of specimens made of a SAE 5115 case-hardened steel

The effect of different surface conditions on the fatigue properties of cyclically loaded bending specimens of the case‐hardened steel SAE 5115 (Material Number 1.7131) was investigated. The aim of the investigations was to achieve further knowledge of the processes that finally lead to the failure of case‐hardened and cyclically loaded bending specimens. Surface roughness, microstructure and residual stress distribution were regarded as parameters that govern the fatigue process. On the basis of a well‐defined adjustment of different surface conditions the effect of internal oxidation, surface roughness and residual stresses on fatigue crack initiation and growth were assessed. The effect of different parameters on the endurance limit has been quantified by the application of a fracture mechanics and a weakest‐link approach. The calculations contributed to a deeper insight into the complex interaction between the parameters governing the fatigue process.     

Mesoscopic mechanisms in fatigue crack initiation in an aluminium alloy

The mechanistic aspects of process of initiation of a mode‐I fatigue crack in an aluminium alloy (AA 2219‐T87) are studied in detail, both computationally as well as experimentally. Simulations are carried out under plane strain conditions with fatigue process zone modelled as stress‐state–dependent cohesive elements along the expected mode‐I failure path. An irreversible damage parameter that accounts for the progressive microstructural damage due to fatigue is employed to degrade cohesive properties. The simulations predict the location of initiation of the fatigue crack to be subsurface where the triaxiality and the opening tensile stresses are higher in comparison with that at the notch surface. Examination of the fracture surface profile of fracture test specimens near notch tip reveals a few types of regions and existence of a mesoscopic length scale that is the distance of the location of highest roughness from the notch root. A discussion is developed on the physical significance of the experimentally observed length scale.     

Investigation on low cycle fatigue of nickel‐based single crystal turbine blade in different regions

Low cycle fatigue experiments of nickel‐based single crystal superalloy miniature specimens were carried out at 760 °C/1000 MPa and 980 °C/750 MPa. According to testing results, low cycle fatigue life is dependent on sampling position of turbine blade under same test conditions. Fracture surface morphology and longitudinal profile microstructure indicated that the fracture mechanism transformed from cleavage fracture to ductile fracture with the changing of medium temperature to high temperature due to the particle cutting at yield stress intensity. The scanning electron microscopy observation of original material demonstrated that the smaller precipitate size of samples have a shorter fatigue life. Meanwhile, the constitutive model considering size effect was built based on the crystal plastic theory. The finite element analysis demonstrated that the smaller precipitate size could dramatically reduce the plastic deformation suffering the same cycle loading.     

颗粒弥散和核-壳共存的TiCN基金属陶瓷的制备

为了制备具有良好综合力学性能的TiCN基金属陶瓷，研究了烧结温度对TiCN-HfN陶瓷微观结构和力学性能的影响，构建了颗粒弥散和核-壳共存的微观结构模型，揭示了材料的致密化机制、增硬机制、增韧补强机制。结果表明:在1 500℃下所制备的TiCN-HfN材料具有颗粒弥散与核-壳共存的微观结构，其中弥散的颗粒为HfN，核为TiCN，壳主要为(Ti, Hf, Mo)CN固溶体；材料具有较好的性能，其相对密度为99.7%、硬度为20.6 GPa、抗弯强度为1 682.5 MPa、断裂韧度为8.5 MPa·m1/2；其致密化机制主要为颗粒和金属液相填充到烧结颈实现致密化，增硬机制主要为致密化和颗粒钉扎强化增硬，增韧补强机制主要为颗粒弥散和颗粒钉扎增韧、骨架结构和颗粒钉扎增强。      

Fracture measurements on cement paste

This paper describes an investigation into the fracture behaviour of hardened cement paste. Notched specimens of the material were tested to failure in flexure and tension. In the initial flexural tests on beams of fixed overall depth, the stress intensity factor at failure as calculated from linear-elastic fracture mechanics appeared to be a material constant. However, further investigation showed that this factor varied with specimen size, and suggested that linear-elastic fracture mechanics and the concept of fracture toughness are not readily applicable to hardened cement paste, which would appear to be a relatively notch insensitive material whose strength is not greatly reduced by the presence of flaws. A “tied crack” model explains semi-quantiatively the observed behaviour.     

Effects of SiCp size and volume fraction on the high cycle fatigue behavior of AZ91D magnesium alloy composites

High cycle fatigue tests (i.e., stress-controlled, axial) were conducted on monolithic AZ91D and AZ91D magnesinm alloy composites processed via squeeze casting and extrusion to contain either 15 gm or 52 gm size SiC particles, at both the 20% and 25% volume fraction reinforcement level. The effects of changes in SiC particle size and volume fraction on the high cycle fatigue behavior have been determined. In addition, the number of cracked particles on the fatigue fracture surfaces, as well as the level of damage beneath the fatigue fracture surfaces were quantified in order to determine the effects of particle size on the evolution of damage during fatigue and during overload failure. Commercial purity Mg specimens containing a large grain size were also tested in fatigue for comparison with the alloy and composite data.     

The influence of temperature on the mechanical and fracture properties of a 20 vol% ceramic particulate-reinforced aluminium matrix composite

The influence of test temperature on the mechanical and fracture properties of a 20 vol% alumina particulate-reinforced 6061-aluminium matrix composite, in the peak-aged condition, was investigated in the temperature range 25–180 °C. Strength and stiffness were found to decrease but elongation to failure increased with increasing test temperature. However, the fracture toughness was relatively constant over this temperature range. The failure mechanism, the reaction zone around reinforcing particles, the number of debonded particles and void sizes were all significantly influenced by temperature. The role of the matrix/particle interface in the fracture process was also investigated.On leave at the Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.