Showing papers on "Fractography published in 2001"

Experimental study of porosity and its relation to fatigue mechanisms of model Al–Si7–Mg0.3 cast Al alloys

Abstract: The microstructure and fatigue properties of three model AS7G03 cast aluminium alloys containing artificial pores have been studied. Synchrotron X-ray tomography has been used to characterise in three dimensions the pore population in the alloys. The development of fatigue cracks in relation with local crystallography has been studied by means of electron back scattered diffraction (EBSD). Both the average number of cycles to failure and the lifetime scatter depend on the pore content specially at high stress level. The mechanism leading to the initiation of a crack from a pore has been identified. The crack propagation at high stress level appears to be quite insensitive to microstructural barriers and can be reasonably well described by a Paris type law. At low stresses, however, short cracks are often observed to be stopped at grain boundaries and the fatigue life is no longer predicted by a simple propagation law.

Fracture and impact behaviours of hollow micro-sphere/epoxy resin composites

Abstract: Fracture and impact behaviours of hollow-glass micro-sphere/epoxy resin composites are studied in terms of fracture toughness, fractography, flexural properties and impact force. Volume fraction of micro-spheres for the composites was varied up to 0.65. The addition of micro-spheres did not enhance the specific fracture toughness of the composites despite the presence of a pinning mechanism at relatively low volume fractions. Performance in reducing the impact force was enhanced as the content of micro-spheres increased, but at the expense of other properties such as specific fracture toughness and specific flexural strength, while specific flexural modulus marginally increased at some high volume fractions of micro-spheres.

Deformation and fracture in martensitic carbon steels tempered at low temperatures

Abstract: This article reviews the strengthening and fracture mechanisms that operate in carbon and low-alloy carbon steels with martensitic microstructures tempered at low temperatures, between 150 °C and 200 °C. The carbon-dependent strength of low-temperature-tempered (LTT) martensite is shown to be a function of the dynamic strain hardening of the dislocation and transition carbide substructure of martensite crystals. In steels containing up to 0.5 mass pct carbon, fracture occurs by ductile mechanisms of microvoid formation at dispersions of second-phase particles in the matrix of the strain-hardened tempered martensite. Steels containing more than 0.5 mass pct carbon with LTT martensitic microstructures are highly susceptible to brittle intergranular fracture at prior austenite grain boundaries. The mechanisms of the intergranular fracture are discussed, and approaches that have evolved to minimize such fracture and to utilize the high strength of high-carbon hardened steels are described.

The effect of environment on the fatigue of bonded composite joints. Part 1: testing and fractography

Abstract: In this work, the effect that test environment and pre-conditioning had on the fatigue behaviour of CFRP/epoxy lap–strap joints was investigated. It was shown that the fatigue resistance of the lap–strap joints did not vary significantly until the glass transition temperature, Tg, was approached, at which point a considerable reduction in the fatigue threshold load was observed. It was also noted that absorbed moisture resulted in a significant reduction in the Tg of the adhesive. This must be taken into account when selecting an adhesive to operate at elevated temperatures. The locus of failure of the joints was seen to be highly temperature dependent, transferring from primarily in the composite adherend at low temperatures to primarily in the adhesive at elevated temperatures. It was also seen that as the crack propagated along the lap–strap joint, the resolution of the forces at the crack tip tended to drive it into the strap adherend, which could result in complex mixed mode fracture surfaces.

Modeling the ductile fracture process of void coalescence by void-sheet formation

Abstract: Ductile fracture of engineering alloys frequently occurs by a mechanism of void coalescence in which void-sheets form between the primary voids. Based on the microstructural features that control failure of HY-100 steel, computational modeling has been performed to examine the deformation localization behavior between primary voids and to predict ductile fracture by the void-sheet coalescence mechanism. Elongated inclusion-initiated voids are simulated as two distinct, hole-like voids on a plane inclined to the stress axis based on the inclined nature of the fracture surface. Consistent with experimental behavior, the micro-mechanical model identifies a strong tendency for strain localization between the voids (and therefore void-sheet failure) but only at a high degree of stress triaxiality. Furthermore, based on the formation of a secondary void population, the analysis also predicts with reasonable accuracy both the magnitude and stress-state dependence of the experimentally determined failure strains.

Deformation of polycrystalline Ti2AlC under compression

Abstract: Deformation of polycrystalline Ti2AlC under room and high temperature compression was investigated. The results demonstrated that Ti2AlC was damage tolerant at room temperature and the samples were shear fractured upon failure. At high temperatures Ti2AlC deform plastically. The brittle-to-ductile-transition temperature (BDTT) of Ti2AlC was between 1000 °C and 1050 °C. The microstructure and fracture surfaces were examined using scanning electron microscopy. Due to insufficient number of dislocation systems, the room-temperature deformation was constitute of kinking and delaminating of laminated Ti2AlC grains, basal plane dislocation slip, formation of voids and cavities in the vicinity of main crack. At high temperatures below BDTT, the deformation was a combination of cavities formation and intergranular sliding. At temperatures above BDTT, the deformation was mainly plastic flow.

Microstructure and ductile–brittle transition of as-cast Zr-based bulk glass alloys under compressive testing

Abstract: This paper investigates mechanical properties and fracture mechanisms of Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 alloys with various volume fractions of quenched-in crystalline. The alloys with various volume fractions of quenched-in crystalline were prepared by controlled oxygen content of alloys and overheating of the pouring. The phase structure, particle size and volume fraction of all samples were identified by X-ray diffraction, differential scanning calorimeter (DSC) curves and scanning electron microscopy (SEM) photographs. The mean sizes of crystalline increased from 0.3 to 1.3 μm with increasing volume fraction of crystalline from 4 to 13%. The compressive mechanical tests show a ductile–brittle transition with significant decrease in the fracture stress and ductility. Detailed observations in the flow deformation and fracture surface illustrate the relationship between the quenching-in crystalline and the mechanical behavior. The full bulk amorphous Zr-based alloy exhibits typical ductile deformation and fracture behavior. The torn shear bands form the typical vein patterns on the fracture surface. The effects of quenching-in crystalline on the flow deformation and fracture behavior depend on the nature, size, volume fraction and distribution. The particle size of the crystalline in the sense of the width of shear bands is critical. When the size is larger than the width of the shear bands the particles induce an obvious inhomogeneity of the flow deformation and more microcracks by the separation of the interfaces. Nano-scale particles, on the other hand, may increase the viscosity of the flow but do not form microcracks, resulting in particle strengthening of the metallic glass. Increasing the volume fraction of large-scale particles is favorable to leaking the microcracks and brittle fracture. With increasing particle size and volume fraction up to two times the width of the shear band and 10% vol., respectively, the ductile fracture of bulk amorphous alloy completely transforms to brittle fracture under compressive testing.

S-N Property and Fractography of High Carbon Chromium Bearing Steel over Ultra Wide Life Region under Rotating Bending.

Abstract: In order to clarify the fatigue behavior in a wide life region, fatigue tests in the life region of N=0.5a(10)9 were performed for high strength steel of SUJ2, by means of a multi-type testing machine developed here. Thus, it was found that the S-N curve in the medium life region can be straightforwardly extrapolated to the low cycle region and connected to the static strength which provides the fatigue strength at N=0.5. It is another finding that S-N property of this steel in such a wide life region was successfully explained as duplex S-N characteristics corresponding to the surface induced fracture and interior inclusion induced fracture with a fish-eye, respectively. Characteristic fracture surface so-called fine granular area (FGA) was formed in the vicinity at the inclusion at the center of the fish-eye. Consequently, the long life fatigue fracture of this steel was caused through three different processes of (1) formation of FGA, (2) crack propagation to form the fish-eye and (3) rapid crack propagation to cause the catastrophic fracture.

R-Curve characterization of the fracture toughness of nanocrystalline nickel thin sheets

Abstract: The fracture resistance curves of nanocrystalline nickel and carbon doped nanocrystalline nickel for different annealing temperatures have been generated and studied. The results indicate that crack growth resistance of pure nanocrystalline nickel is very sensitive to annealing temperatures. The crack growth resistance decreased with increasing annealing temperature for the nanocrystalline nickel. Carbon doping greatly reduces crack growth resistance of nanocrystalline nickel. However, the crack growth resistance of carbon-doped nanocrystalline shows improvement through annealing processing. A cluster model was used to explain the crack growth resistance behavior of nanocrystalline nickel.

In situ stir cast Al–TiB2 composite: processing and mechanical properties

Abstract: Elemental Ti and B powders of stoichiometric composition were mixed and added to molten aluminium. In situ TiB2 particles were formed in the aluminium melt. On casting, an Al–TiB2 composite was produced. Despite the presence of the Al3Ti phase associated with the Al–Ti–B ternary system, the in situ TiB2 particles, with sizes of 1–3 µm formed in the composite was able to yield an improvement of 57% increase in tensile strength, 66% in yield strength and 22% in modulus in an Al–15 vol.-%TiB2 composite. The extent of improvement in these properties depended on the volume fraction in the composite. Fractography showed a texture of dimples seated with hexagonal TiB2 particles indicating retention of high ductility in the composite, despite the fact that the predominance of the coarse Al3Ti in the composite had led to premature rupturing.

Fracture toughness, fracture morphology, and crack-tip plastic zone of a Zr-based bulk amorphous alloy

Abstract: The elastic constants, fracture toughness, fracture morphology, and crack-tip plastic zone of a bulk amorphous alloy (bulk metallic glass), with the composition Zr-10Al-5Ti-17.9Cu-14.6Ni (at. pct), were investigated. The room-temperature fracture toughness reached values as high as 69 MPa&mrm. However, it showed considerable scatter, which is, at least in part, due to microcrystalline regions in the castings. Controlled crack propagation could not be obtained in chevron-notched specimens. The fracture-surface morphologies of chevron-notched specimens varied as the crack advanced, and this effect is probably related to differences in the crack propagation rate. Controlled fracture resulting in featureless fracture surfaces was observed during in-situ transmission electron microscope (TEM) fracture experiments. The plastic zone of a fatigue-precracked bulk flexure specimen was examined in situ in an atomic-force microscope (AFM). Shear displacements up to 2 &gmm were found. The AFM observations did not reveal any cracks associated with the shear steps. Sectioning of shear steps using a focused ion beam (FIB) with a diameter of 5 nm also did not reveal any cracks.

Effects of microstructural morphology on quasi-static and dynamic deformation behavior of Ti-6Al-4V alloy

Abstract: The effects of microstructural morphology on quasi-static and dynamic deformation behavior of a Ti-6Al-4V alloy were investigated in this study. Quasi-static and dynamic torsional tests were conducted using a torsional Kolsky bar for Widmanstatten, equiaxed, and bimodal microstructures, which were processed by different heat treatments, and then, the test data were analyzed in relation to microstructures, tensile properties, and fracture mode. Quasi-static torsional properties showed a tendency similar to tensile properties and ductile fracture occurred in all three microstructures. Under dynamic torsional loading, maximum shear stress of the three microstructures was higher and fracture shear strain was lower than those under quasi-static loading, but the overall tendency was similar. In the Widmanstatten and equiaxed microstructures, adiabatic shear bands were found in the deformed region of the fractured specimens. The possibility of the adiabatic shear band formation under dynamic loading was quantitatively analyzed, depending on how plastic deformation energy was distributed to either void initiation or adiabatic shear banding. It was found to be most likely in the equiaxed microstructure, whereas it was least likely in the bimodal microstructure.

Dependency of fracture toughness on the inhomogeneity of coarse TiN particle distribution in a low alloy steel

Abstract: An investigation of the effect of the coarse TiN particle distribution on the fracture toughness of a steel, as determined by crack-tip opening displacement (CTOD), was carried out using a range of samples from a Ti-treated steel that had been thermally cycled to simulate a coarse grained heat-affected zone (CG HAZ) microstructure. Experimental results from tests carried out at room temperature showed that the inhomogeneous spatial distribution of the coarse TiN particles in the microstructure ahead of the fatigue precrack caused the samples to fail with significantly different CTOD values. Detailed fractographic investigation showed that, with an increased number of overall fracture initiation sites (FISs) and number density of local cleavage initiation sites (CISs) caused by coarse TiN particles, the fracture toughness CTOD values generally decreased. The increase in FIS number and CIS number density has been related to the inhomogeneous coarse TiN distribution ahead of the fatigue precrack and so the sampling of microstructural areas with a high number density of coarse TiN particles. The mechanism by which the coarse TiN particles cause cleavage fracture initiation is discussed.

Combined application of electron backscatter diffraction and stereo-photogrammetry in fractography studies

Abstract: The main aim of this paper is to report on recent experimental developments that have succeeded in combining electron back-scatter diffraction (EBSD) with stereo-photogrammetry, compared with two other methods for study of fracture surfaces, namely visual fractography analysis in the scanning electron microscope (SEM) and EBSD directly from facets. These approaches will be illustrated with data relating to the cleavage plane orientation analysis in a ferritic and C-Mn steel. It is demonstrated that the combined use of EBSD and stereo-photogrammetry represents a significant advance in the methodology for facet crystallography analysis. The results of point counting from fractograph characterization determined that the proportions of intergranular fracture in C-Mn and ferritic steels were 10.4% and 9.4%, respectively. The crystallographic orientation was determined directly from the fracture surface of a ferritic steel sample and produced an orientation distribution with a clear trend towards the [001] plane. A stereo-photogrammetry technique was validated using the known geometry of a Vickers hardness indent. The technique was then successfully employed to measure the macroscopic orientation of individual cleavage facets in the same reference frame as the EBSD measurements. Correlating the results of these measurements indicated that the actual crystallographic orientation of every cleavage facet identified in the steel specimens is [001].

Modelling brittle and semi-brittle fracture processes

Abstract: Recent advances in the microscopic modelling of fracture processes are summarised and discussed in this paper. Examples are taken from ab-initio simulations and empirical atomistic modelling of brittle fracture which show that the production of metastable fracture surfaces or directional cleavage anisotropy are readily anticipated consequences of the discrete nature of the bond breaking at the crack tip. Dislocation simulations are necessary to predict the dependence of fracture toughness on predeformation, temperature or loading rate in semi-brittle materials below the brittle-to-ductile transition. By comparison of fracture experiments on tungsten single crystals with simulations, it is shown that dislocation nucleation is the limiting factor at low temperatures, while the dependence on loading rate at intermediate temperatures can only be understood if dislocation mobility takes control. Furthermore, it is shown that the entire temperature and strain rate dependence of the fracture toughness can be scaled onto a master curve with one unique activation energy.

Effect of Bond Thickness on Fracture Behaviour in Adhesive Joints

Abstract: To study the effects of bond thickness on the fracture behaviour of adhesive joints, experimental investigation and finite element analysis have been carried out for compact tension (CT) and double-cantilever-beam (DCB) specimens with different bond thickness Fractography and fracture toughness exhibited apparent variations with bond thickness Numerical results indicate that the crack tip stress fields are affected by bond thickness due to the restriction of plastic deformation by the adherends At the same J level, a higher opening stress was observed in the joint with a smaller bond thickness (h) Beyond the crack tip region, a self-similar stress field can be described by the normalized loading parameter, J/hσ0 The relationship between J and crack tip opening displacement, δ, is dependent on the bond thickness The strong dependence of toughness upon bond thickness is a result of the competition between two different fracture mechanisms For small bond thickness, toughness is linearly propo

Temperature, strain rate, stress state and the failure of HY-100 steel

Abstract: The influence of temperature and strain rate on the deformation and failure behavior of HY-100 steel has been examined as a function of stress state using notched and un-notched axisymmetric tensile specimens. Behavior over the range of temperatures/strain rates from −85°C and 1 s −1 to 27°C and 10 −3 s −1 shows an equivalence of decreasing test temperature or increasing strain rate on deformation behavior in a manner that can be predicted by the thermally activated flow theory. Over the entire range of temperatures/strain rates, the influence of stress state on failure is such that two void coalescence mechanisms control failure; at low stress triaxialities, relatively equiaxed voids grow to impingement, while at high triaxialities, a void-sheet process intervenes linking elongated MnS-initiated voids by a shear instability. The failure strains decrease rapidly with increasing stress triaxiality ratio in a similar manner for all temperatures and strain rates except for an intermediate stress triaxiality condition where the void-sheet mode of failure extends to lower stress triaxialities under cryogenic test conditions.

Hot ductility and fracture mechanisms of a C-Mn-Nb-Al steel

Abstract: Hot-ductility tests of a C-Mn-Nb-Al steel were performed in a tensile machine at different strain rates of 1×10−4, 3×10−4, 1×10−3, and 3×10−3 s−1 and at temperatures of 650 °C, 710 °C, 770 °C, 840 °C, 900 °C, 960 °C, and 1020 °C, which are close to the continuous casting conditions of steel. Fracture surfaces were examined using a scanning electron microscope. It was found that low strain rates and coarse austenitic grains decrease hot ductility. At all test temperatures, when the strain rate decreases, the hot ductility also decreases because the void growth mechanism predominates over void nucleation, giving time for nucleated cracks to grow. This leads, finally, to the catastrophic failure. The minimum hot ductility was found at 900 °C for all strain rates, and the fracture was intergranular. Fractographic evidence showed that the voids formed during the deformation surrounded the austenite grains, indicating that the deformation was concentrated in ferrite bands located in the same places when the testing temperature was in the two-phase field.

Influence of temperature on cyclic stress response, strain resistance, and fracture behavior of aluminum alloy 2524

Abstract: In this paper, the cyclic stress response, strain resistance, and cyclic fracture behavior of aluminum alloy 2524 is examined. Test specimens of the alloy was cycled at both the ambient and elevated temperatures using, tension-compression loading (load-ratio of −1.0), under total strain control, over a range of plastic strains giving less than 10 4 cycles to failure. The alloy displayed combinations of hardening and softening in both the longitudinal and transverse directions of the wrought plate. The alloy followed the Coffm–Manson relation, and exhibited a single slope for the variation of cyclic plastic strain amplitude with reversals to failure. Fracture of the alloy samples was predominantly transgranular for both orientations, with microscopic crack propagation along the grain boundaries. The stress response characteristics, and fracture behavior of the alloy are discussed in terms of competing and synergistic influences of cyclic plastic strain amplitude, response stress, intrinsic microstructural effects, and dislocation-microstructure interactions during cyclic straining.

Effect of bond thickness on fracture behaviour in adhesive joints

Abstract: To study the effects of bond thickness on the fracture behaviour of adhesive joints, experimental investigation and finite element analysis have been carried out for compact tension (CT) and double-cantilever-beam (DCB) specimens with different bond thickness. Fractography and fracture toughness exhibited apparent variations with bond thickness. Numerical results indicate that the crack tip stress fields are affected by bond thickness due to the restriction of plastic deformation by the adherends. At the same J level, a higher opening stress was observed in the joint with a smaller bond thickness (h). Beyond the crack tip region, a self-similar stress field can be described by the normalized loading parameter, J/hσ0. The relationship between J and crack tip opening displacement, δ, is dependent on the bond thickness. The strong dependence of toughness upon bond thickness is a result of the competition between two different fracture mechanisms. For small bond thickness, toughness is linearly propo...

High-frequency metal fatigue: the high-cycle fatigue behavior of ULTIMET® alloy

Abstract: ULTIMET® alloy is a relatively new commercial Co–26Cr–9Ni (wt.%) alloy, which exhibits good resistance to both wear and corrosion. A state-of-the-art high-frequency, 1000-Hz, material test system was used to study the high-cycle fatigue behavior of ULTIMET alloy up to 109 cycles. Fatigue experiments were conducted at high (1000 Hz) and conventional (20 Hz) frequencies in air at room temperature. The effects of the test frequency, the temperature increase during fatigue, and the change of crack initiation sites from the surface to subsurface on fatigue life are discussed. Although the fatigue life was comparable at test frequencies of 1000 and 20 Hz, the equilibrium temperature at 1000 Hz was considerably higher than that at 20 Hz. The fractographic study showed different morphologies of fracture surfaces at various frequencies. The high-cycle fatigue behavior of ULTIMET alloy at both high- and low-frequencies exhibited a typical two-stage fatigue-crack-growth process, i.e., (a) stage I fatigue-crack initiation in which the cracks formed on those planes most closely aligned with the maximum shear–stress direction in the grains of the fatigue specimen; and (b) stage II fatigue-crack growth in which the maximum principal tensile stress controlled crack propagation in the region of the crack tip.

Fracture Analysis of Hydrogen-Charged Nickel-Titanium Superelastic Alloy

Abstract: Superelastic Ni–Ti alloys are widely used as engineering and medical materials. However, the alloy is susceptible to environmental embrittlement in a corrosive atmosphere. Accelerated tests of hydrogen embrittlement of the Ni–Ti alloy were carried out. The effect of electrolytic charging hydrogen on the properties of the alloy was measured. The rupture process in tension of the alloy with charged hydrogen was discussed on the basis of the tensile stress-strain curve, hardness in a cross-sectional area and fractography. The results of the measurements suggested that hydrogen concentration on the surface of the alloy and immersion time in an electrolytic bath had different effects on embrittlement. This will be related with the distribution of the hydrogen concentration, which was determined from the diffusion equation and boundary conditions.

An experimental study of the influence of fibre–matrix interface on fatigue tensile strength of notched composite laminates

Abstract: The objective of this study was to investigate the effect of fibre–matrix interfacial adhesion on fatigue residual strength of polymer matrix composite laminates containing a circular hole. Composite laminates were manufactured using surface-treated and -untreated carbon fibres, and the interfacial adhesion was quantified by measuring the transverse flexural strength of the two material systems. Tensile–tensile cyclic fatigue experiments were conducted at three load levels. Residual strength of notched laminates, subjected to cyclic loading was then measured for the two composite systems. Damage mechanisms were analysed using C-scan and SEM fractography and correlated with notched residual strength.