Deformation and fracture of a particle-reinforced aluminum alloy composite: Part I. Experiments

Mechanical tests were performed on a powder-metallurgically processed 7093/SiC/15p discontinuously reinforced aluminum (DRA) composite in different heat-treatment conditions, to determine the influence of matrix characteristics on the composite response. The work-hardening exponent and the strain to failure varied inversely to the strength, similar to monolithic Al alloys, and this dependence was independent of the dominant damage mode. The damage consisted of SiC particle cracks, interface and near-interface debonds, and matrix rupture inside intense slip bands. Fracture surfaces revealed particle fracture-dominated damage for most of the heat-treatment conditions, including an overaged (OA) condition that exhibited a combination of precipitates at the interface and a precipitate-free zone (PFZ) in the immediate vicinity. In the highly OA conditions and in a 450 °C as-rolled condition, when the composite strength became less than 400 MPa, near-interface matrix rupture became dominant. A combination of a relatively weak matrix and a weak zone around the particle likely contributed to this damage mode over that of particle fracture. Fracture-toughness tests show that it is important to maintain a proper geometry and testing procedure to obtain valid fracture-toughness data. Overaged microstructures did reveal a recovery of fracture toughness as compared to the peak-aged (PA) condition, unlike the lack of toughness recovery reported earlier for a similar 7XXX (Al-Zn-Cu-Mg)-based DRA. The PA material exhibited extensive localization of damage and plasticity. The low toughness of the DRA in this PA condition is explored in detail, using fractography and metallography. The damage and fracture micromechanisms formed the basis for modeling the strength, elongation, toughness, and damage, which are described in Part II of this work. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Deformation and fracture of a particle-reinforced aluminum alloy composite: Part I. Experiments

Mechanical tests were performed on a powder-metallurgically processed 7093/SiC/15p discontinuously reinforced aluminum (DRA) composite in different heat-treatment conditions, to determine the influence of matrix characteristics on the composite response. The work-hardening exponent and the strain to failure varied inversely to the strength, similar to monolithic Al alloys, and this dependence was independent of the dominant damage mode. The damage consisted of SiC particle cracks, interface and near-interface debonds, and matrix rupture inside intense slip bands. Fracture surfaces revealed particle fracture-dominated damage for most of the heat-treatment conditions, including an overaged (OA) condition that exhibited a combination of precipitates at the interface and a precipitate-free zone (PFZ) in the immediate vicinity. In the highly OA conditions and in a 450°C as-rolled condition, when the composite strength became less than 400 MPa, near-interface matrix rupture became dominant. A combination of a relatively weak matrix and a weak zone around the particle likely contributed to this damage mode over that of particle fracture. Fracture-toughness tests show that it is important to maintain a proper geometry and testing procedure to obtain valid fracture-toughness data. Overaged microstructures did reveal a recovery of fracture toughness as compared to the peak-aged (PA) condition, unlike the lack of toughness recovery reported earlier for a similar 7XXX (Al-Zn-Cu-Mg)—based DRA. The PA material exhibited extensive localization of damage and plasticity. The low toughness of the DRA in this PA condition is explored in detail, using fractography and metallography. The damage and fracture micromechanisms formed the basis for modeling the strength, elongation, toughness, and damage, which are described in Part II of this work. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

The effect of hydrogen on the fracture toughness of alloy X-750

The effect of hydrogen on the fracture toughness behavior of a nickel-base superalloy, Alloy X-750, in the solutionized and aged condition was investigated. Notched bend specimens were tested to determine if the fracture process was stress or strain controlled. The fracture was observed to initiate at a distance between the location of maximum stress and maximum strain, suggesting that fracture required both a critical stress and strain. The effect of hydrogen was further investigated and modeled using fracture toughness testing and fractographic examination. The fracture toughness of the non-charged specimen was 147 MPa√m. Charging with hydrogen decreased the fracture toughness, K _Ic , to 52 MPa√m at a rapid loading rate and further decreased the toughness to 42 MPa√m for a slow loading rate. This is consistent with the rate-limiting step for the embrittlement process being hydrogen diffusion. The fracture morphology for the hydrogen-charged specimens was intergranular ductile dimple, while the fracture morphology of noncharged specimens was a mixture of large transgranular dimples and fine intergranular dimples. The intergranular failure mechanism in Alloy X-750 was a microvoid initiation process at grain boundary carbides followed by void growth and coalescence. One role of hydrogen was to reduce the void initiation strain for the fine intergranular carbides. Hydrogen may have also increased the rate of void growth. The conditions ahead of a crack satisfy the critical stress criterion at a much lower applied stress intensity factor than for the critical fracture strain criterion. A model based on a critical fracture strain criterion is shown to predict the fracture behavior.     

Application of fatigue precracking methods and their influence on fracture toughness values

The purpose of this work is to compare three fatigue precracking techniques for SENB specimens containing residual stresses. These are local compression, the use of a high R-ratio in the fatigue cycle and reverse bending. The possible effects of these techniques on the values of fracture toughness are assessed. The main conclusion is that local compression reduces the measured fracture toughness up to 50 % and various materials have different sensitivities to 1 % plastic strain. Local compression produces also greater scatter of values. Reverse bending does not affect the fracture toughness but requires further research and development. A disadvantage caused by the use of a high R-ratio is the essential increase in the total precracking time. Uniform fatigue crack front profiles can be obtained by all three methods.     

The toughness of high hardness laminar composite steel as influenced by specimen and crack orientation

The toughness behavior of high hardness laminar composite steel (high carbon, ∼ Rc 60, hard layer metallurgically bonded to a medium carbon ∼Rc 50, softer layer) was investigated. The effort focused on the effect of test temperature, specimen orientation and crack location on toughness. Charpy V-notch specimens with the notch extending through both the hard and soft layers were tested over a series of temperatures to provide transition curves for both the longitudinal and transverse direction. These transition curves are compared to those obtained from specimens that were surface notched on either the hard or soft side. By precracking similarly oriented specimens, information on the fracture toughness (K _Q andW/A) was obtained over approximately the same temperature range. These data show the effect of the interface between the hard and soft layer on the various toughness parameters. Lastly, stress corrosion cracking was investigated andK _ISCC values provided.     

The toughness of high hardness laminar composite steel as influenced by specimen and crack orientation

The toughness behavior of high hardness laminar composite steel (high carbon, ∼Rc 60, hard layer metallurgically bonded to a medium carbon ∼Rc 50, softer layer) was investigated. The effort focused on the effect of test temperature, specimen orientation and crack location on toughness. Charpy V-notch specimens with the notch extending through both the hard and soft layers were tested over a series of temperatures to provide transition curves for both the longitudinal and transverse direction. These transition curves are compared to those obtained from specimens that were surface notched on either the hard or soft side. By precracking similarly oriented specimens, information on the fracture toughness (K _Q andW/A) was obtained over approximately the same temperature range. These data show the effect of the interface between the hard and soft layer on the various toughness parameters. Lastly, stress corrosion cracking was investigated and K_ISCC values provided.     

The effect of hydrogen on the fracture toughness of alloy X-750

The effect of hydrogen on the fracture toughness behavior of a nickel-base superalloy, Alloy X-750, in the solutionized and aged condition was investigated. Notched bend specimens were tested to determine if the fracture process was stress or strain controlled. The fracture was observed to initiate at a distance between the location of maximum stress and maximum strain, suggesting that fracture required both a critical stress and strain. The effect of hydrogen was further investigated and modeled using fracture toughness testing and fractographic examination. The fracture toughness of the non-charged specimen was 147 . Charging with hydrogen decreased the fracture toughness, K _lc, to 52 at a rapid loading rate and further decreased the toughness to 42 for a slow loading rate. This is consistent with the rate-limiting step forthe embrittlement process being hydrogen diffusion. The fracture morphology for the hydrogen-charged specimens was intergranular ductile dimple, while the fracture morphology of noncharged specimens was a mixture of large transgranular dimples and fine intergranular dimples. The intergranular failure mechanism in Alloy X-750 was a microvoid initiation process at grain boundary carbides followed by void growth and coalescence. One role of hydrogen was to reduce the void initiation strain for the fine intergranular carbides. Hydrogen may have also increased the rate of void growth. The conditions ahead of a crack satisfy the critical stress criterion at a much lower applied stress intensity factor than for the critical fracture strain criterion. A model based on a critical fracture strain criterion is shown to predict the fracture behavior.     

Effect of thickness and orientation on fatigue crack growth rate in 4340 steel

Fatigue crack growth rate in 4340 is evaluated from the viewpoint of orientation and specimen thickness. It is shown that orientation has little effect on the relation between crack growth rate and stress intensity factor. However, increasing the specimen thickness from 1/16 to 1/2 in. caused a significant increase in the value ofm in the relationda/dN α (ΔK) ^m , viz. 2.6 to 5.4. This is believed to show an effect of stress state in that, in the thicker specimens, fracture is flat and the stress state tends more toward plane strain. The data show that applying crack growth data from thin specimens to larger structural members can be misleading.     

Critical fracture stress and fracture strain models for the prediction of lower and upper shelf toughness in nuclear pressure vessel steels

Critical fracture stress and stress modified fracture strain models are utilized to describe the variation of lower and upper shelf fracture toughness with temperature and strain rate for two alloy steels used in the manufacture of nuclear pressure vessels, namely SA533B-1 (HSST Plate 02) and SA302B (Surveillance correlation heat). Both steels have been well characterized with regard to static and dynamic fracture toughness over a wide range of temperatures (−190 to 200°C), although validJ _Ic measurements at upper shelf temperatures are still somewhat scarce. The present work utilizes simple models for the relevant fracture micromechanisms and local failure criteria to predict these variations in toughness from uniaxial tensile properties. Procedures are discussed for modelling the influence of neutron fluence on toughness in irradiated steel, and predictions are derived for the effect of increasing fluence on the variation of lower shelf fracture toughness with temperature in SA533B-1. An erratum to this article is available at .     

Instability of plastic flow in the directions of pure shear: II. Experimental

Two aspects of the phenomenon of plastic instability in direction of pure shear are examined, namely that the condition dσ = 0 (maximum in true flow stress) is necessary for localization of flow along characteristics as defined in continuum plasticity, and that fracture is initiated and propagates along characteristics. Two types of sheet specimens were employed, the standard-type flat sheet specimens, and specimens simulating both plane stress and plane strain. Grids were placed on gage sections and photographs were taken successively in the plastic range to enable strains to be calculated and instabilities to be observed and recorded. The principal variable in the flat specimen test was theW/T ratio (width to thickness). In the plane* strain specimens, both the gage length (constantW/T) and the strength level of the material (quenched and tempered AISI 4340 steel) were varied. A maximum in true flow stress is found consistently at the onset of instabilities. Fracture propagated consistently along the instability band-matrix interface. Variations in specimen geometry produces significant changes in stress state, directions of characteristics, and ductility. For a given specimen geometry, plane strain is more closely approached the higher the strength level of the material. In mixed mode fracture paths slant fracture is associated with the more embrittling stress state. Formerly with Metals and Ceramics Laboratory, Aerospace Research Laboratories, Wright-Patterson AFB     

Fracture toughness of polycrystalline NiAl from finite-element analysis of miniaturized disk-bend test results

The controlled-flaw method in conjunction with the miniaturized disk-bend test (MDBT) was implemented to determine the fracture toughness of polycrystalline NiAl. This procedure was previously used to measure the fracture toughness of completely brittle materials, so the present research extends the method to a material that exhibits a small amount of ductility prior to failure. The controlled-flaw method is based on the placement of a Vickers indentation in the center of the tensile side of the disks. In the MDBT, the specimens are disks 3 mm in diameter, and in this investigation, the disks ranged from 194 to 367 μm in thickness. Fracture initiated at the corners of the indentations for indentation loads exceeding 39 N. The fracture toughness was determined from an analysis of the dependence of fracture stress, σ _f , on indentation load. In brittle materials, σ _f can be calculated from the measured load at fracture, but this is not possible when the specimen deforms plastically prior to failure. The finite-element program NIKE2D was therefore used to calculate the stress during plastic deformation, using data on the tensile behavior of NiAl to model its deformation as an inelastic cylindrically symmetric plate. The fracture toughness of polycrystalline NiAl was measured as 6.41±1.75 MPa√m, which agrees well with independently measured values for similarly processed material. The relatively large uncertainty is associated with scatter in the experimentally measured yield stresses. The results of this investigation demonstrate that the controlled-flaw method can be used in conjunction with the MDBT and finite-element modeling to provide a reasonable estimate of the fracture toughness of a material with limited ductility, provided fracture initiates at the corners of the indentation.     

The influence of the stress state on the plasticity of transformation induced plasticity-aided steel

The influence of the stress state on the plastic deformation of CMnSi, CMnSi(Nb), and CMnAlSi transformation induced plasticity (TRIP)-aided steel has been analyzed. Imposing hydrostatic pressures up to 800 MPa during tensile deformation made it possible to change the stress state of the tensile testing specimens. It was found that the ratio of normal to shear stresses has a pronounced effect on the evolution of the microstructure, the austenite volume fraction change during straining, and the fracture surface morphology. The CMnAlSi TRIP steel, which has the largest uniform elongation and the smallest equivalent strain at fracture in the absence of the hydrostatic pressure, had a more pronounced improvement of all plastic characteristics at increasing hydrostatic pressure. An increased austenite stabilization, brought about by the high hydrostatic pressure, was clearly observed. The austenite stabilization results in a decrease of 20 °C to 25 °C of M _s for an increase of 100 MPa of the hydrostatic pressure. The implications of the observations could be far-reaching for new sheet forming technologies, such as hydroforming, as the full transformation potential is available for crashsensitive structural parts by avoiding the formation of the martensite during forming operations.     

Part I-the effects of pre-dissolved hydrogen on cleavage and grain boundary fracture initiation in metastable beta Ti-3Al-8V-6Cr-4Mo-4Zr

The effects of electrochemically pre-dissolved hydrogen on room-temperature fracture initiation in Beta-C titanium (Ti-3Al-8V-6Cr-4Mo-4Zr wt pct) have been investigated using circumferentially notched tensile specimens. Finite element-based analysis of notch stress fields was used to define relationships between the local threshold stress for crack initiation vs total internal hydrogen concentration. The as-received, solution heat treated (ST, σ_y.2 pct=865 MPa) and the ST + peak-aged conditions (STA, σ _y.2% pct=1260 MPa) were compared after defining the relationships between the fracture process zone hydrogen concentration, hydrogen-metal interactions (i.e., hydrostatic stress field occlusion, trapping, hydriding), and the resulting fracture initiation behavior of each. Solutionized + peak-aged (β+α) Beta-C fractured intergranularly above total hydrogen concentrations of ∼1000 wt ppm. (5.1 at. pct). A fracture mode consistent with cleavage occurred at ∼2100 wt ppm. (10.7 at. pct). Solutionized Beta-C resisted hydrogen-assisted cracking (e.g., did not crack intergranularly) but was not immune; cleavage cracking was provoked at ∼4000 wt ppm. (20.4 at. pct). Coldworked ST Beta-C (CW, σ _y.2 pct=1107 MPa) did not crack intergranularly; fracture initiation behavior was similar to the ST condition regardless of specimen orientation. This suggests that high yield strength alone does not account for the susceptibility to intergranular cracking observed in the STA β+α condition. Stroke-rate studies and X-ray diffraction investigation of H partitioning suggests that equilibrium hydriding and/or irreversible trapping does not singularly control intergranular fracture initiation of the STA condition. Fractographic evidence and finite element results show that a finite plastic zone exists prior to intergranular fracture of the STA condition. This suggests that a criterion for fracture that incorporates plastic strain and stress should be considered.     

Dynamic fracture initiation behavior of an HY-100 steel

An investigation was conducted into the effects of test temperature and loading rate on the initiation of plane strain fracture of an HY-100 steel. Fracture toughness tests were conducted using fatigue precracked round bars loaded in tension to produce a quasi-static stress intensity rate of ·K₁ = 1 MPa√m/s and a dynamic rate of ·K₁ = 2 × 10⁶ MPa√m/s. Testing temperatures covered the range from -150 °C to 200 °C, which encompasses fracture initiation modes involving quasi-cleavage to fully ductile fracture. The results of toughness tests show that the lower-shelf values of fracture toughness were substantially independent of loading rate, while the dynamic values exceeded the quasi-static values by about 50 pct on the upper shelf. In analyzing these results, phenomenological fracture initiation models were adopted based on the requirement that, for fracture to occur, a critical strain or stress must be achieved over a critical distance. In separate tests, the observation of microfracture processes was investigated using fractography and anin situ scanning electron microscope (SEM) fracture technique. The layered ppearance of the fracture surfaces was found to be associated with a banded structure which generally contains many MnS inclusions, probably resulting in a reduction of the fracture toughness values.     

A comparison of toughness of

The low-temperature toughness of C-Mn weld steel with different grain sizes was investigated with notched and precracked specimens. The results indicated that the fine grain steel, evaluated by notched specimens (Charpy V-notch and 4 point bending specimens), is tougher than that of the coarse grain steel over a temperature range from -196 °C to -30 °C. On the other hand, the coarse grain steel, evaluated with precracked specimens, has a remarkably greater plane strain fracture toughness compared to the fine grain steel. The microstructural analysis revealed that the fracture toughness of both the fine grain and the coarse grain steel is not directly related to the distance of the fracture initiation site from the precrack tip or the size of the ferrite grain. The behavioral discrepancy can be explained in terms of the ratio of local fracture stress to yield stress,i.e., σ _f f/σ _y . The fine grain steel had a higherσ _f /σ _y in the notched specimens but a lower value in the precracked specimens compared to the coarse grain steel. The scatter of toughness data can be mainly attributed to the probabilistic distribution of the weakest particle. We suggested thatσ _f /σ _y may be a useful parameter for the engineering evaluation of toughness.     

The influence of internal stresses on the fracture toughness of α/β titanium alloys

The influence of internal stresses on fracture toughness was investigated in α/β titanium alloys. It was shown that the direct linkage of K _IC to various microstructural parameters was not so conclusive, since the different parameters act simultaneously on fracture toughness. On the contrary, the metallurgical parameters change the plastic strain incompatibilities inside the material. Thus X, which is a macroscopic measurement of these incompatibilities, is the relevant parameter to account for the different metallurgical influences on toughness. The influence on X of the α-phase percentage, aspect ratio, and the secondary α-phase percentage was determined, and it was established that low internal stresses could provide high fracture toughness. The present work also showed the ability of a modified Gurson-Tvergaard (GT) model to calculate the experimental K _IC value.     

Failure characteristics of 6061/AI2O3/15ρ and 2014/AI2O33/15ρ composites as a function of loading rate

The effects of loading rate on the toughness and fracture mechanisms of two cast 6061/Al₂O₃/15p and 2014/Al₂O₃/15p composites under the as-worked (AW) and AW + T6 conditions have been examined. The quasistatic bending and high-rate impact tests were conducted over strain rates from 5 X 10^-4 to 1 X 10³ s^-1 using screw-driven or servohydraulic high-rate systems. The results showed that the peak loadP _max, specimen deflectiond, specimen lateral expansion fraction Δw, crack initiation energyE _i, propagation energyE _p, total fracture energyE _t and deformation zone all tended to increase with increasing strain rate. Under quasistatic loading, the composites failed predominantly by ma-trix/reinforcement interface decohesion. As the loading rate increased, reinforcement failure became the major failure mechanism. Differences in the effect of matrix microstructure and stress state on the fracture properties also are discussed. In comparing the fracture modes in the AW and AW + T6 specimens, the latter showed a higher tendency toward particle cracking. Based on mechanical data, the degree of specimen deflection and expansion and fracture modes, the AW composites exhibited a higher strain-rate dependence. The T6 specimens, due to their intrinsicly more brittle nature, appeared to be less influenced by loading rate over the strain-rate range examined.     

Mechanism of detrimental effects of carbon content on cleavage fracture toughness of low-alloy steel

The variation in fracture toughness of low-alloy base steels and weld steels with carbon contents of 0.08 and 0.21 wt pct was investigated using notched and precracked specimens tested at low temperatures. The attention is focused on the mechanism associated with detrimental effects on cleavage fracture toughness resulting from increasing carbon content. Analyses reveal that, in the case of constant ferrite grain sizes with increasing carbon content, the yield stress σ _y increases and the local fracture stress σ _f remains constant for notched specimens. For precracked specimens, the σ _y increases, whereas the σ _f decreases. In both cases, the ratio σ _f /σ _y decreases; this ratio is one of the principal factors inducing the deterioration in the cleavage fracture toughness of the higher carbon steels. Analyses also reveal that the critical strain for initiating a crack nucleus, which decreases with increasing carbon content and impurity elements, appears to be another principal factor that has a negative effect on the fracture toughness in both notched and precracked specimens. The results of the fracture toughness measured for weld metal with various grain sizes further support the predominant effect of grain size on the toughness of notched specimens.     

Linear elastic fracture mechanics of duralumin

Abstract

One desirable improvement of high-strength aluminum alloys often showing considerable anisotropy is the increase of their notch toughness in certain grain orientations. Although we know that material properties such as yield stress, fracture toughness, fatigue or stress corrosion behaviour generally have a strong directional dependence, the characteristics of yielding and particularly of fracture, are not well documented. These two aspects are specially studied in this paper.

The basis forms the presentation of various methods for the testing of the fracture toughness parameter K _1C . The most suitable technique is then adopted and the toughness evaluated, covering a range of temperatures which are of particular interest in typical engineering applications. From a three-point bend test performed on a Charpy-type specimen provided with a fatigue crack and sidenotched, a fracture toughness parameter can be derived. Influence of the strain rate on fracture and its implications arising for certain technical problems are investigated in an impact test. This is believed to be the first application of an instrumented Charpy specimen in the toughness investigations of a high-strength Al alloy and in the determination of crack movements through the section. Effects of the orientation are then evaluated and a connection with some metallurgical factors indicated. Evidence is presen ted showing the fracture mechanics approach applied in the analysis of a part-through crack in a thin-walled cylinder.

Résumé

Les alliages d’aluminium à haute résistance présentant généralement une grande anosotropie et une amélioration de leurs propriétés consiste à augmenter leur résilience pour certaines orientations des grains. Même si nous savons que les propriétés des matériaux, comme la limite élastique, la résistance à la rupture, leur comportement en fatigue ou leur corrosion sont fortement anisotropes, les caractéristiques de l’écoulement plastique et de la rupture sont, de ce point de vue, mal connues. Cet article se penchera particulierement sur ces deux aspects.

En premier lieu sont présentés différentes méthodes pour l’évaluation du facteur de concentration des contraintes K _1C . La technique la plus appropriée est alors adoptée et la résistance mesurée dans un domaine de températures d’intérêt particulier pour les ingénieurs. Le facteur K _1C peut être obtenu à partir d’un essai de flexion fait sur une éprouvette de type Charpy ayant une fissure de fatigue et entaillée sur ses faces latérales. L’influence du taux de déformation sur la rupture et ses implications dans certaines applications techniques sont recherchées dans l’essai de résilience. Nous croyons que ce soit là la premiére application d’une telle éprouvette pour l’étude des duralumins et pour connaître le déplacement des fissures dans la section. L’effet de la texture est alors évalué et l’on indique des corrélations avec d’autres facteurs métallurgiques. On montre aussi une approche de la mécanique de rupture appliquée à l’analyse d’une fissure dans un cylindre à paroi mince.