Showing papers on "Fractography published in 1984"

Failure modes and fracture mechanisms in flexure of Kevlar-epoxy composites

Abstract: The results of testing in three-point bending of aramid fibre-reinforced epoxy composites are described. This loading mode has been chosen in order to increase the variety of failure modes and of fracture mechanisms. The main failure modes observed are tensile and delamination, with a transition at a fibre volume fraction of about 46%. This mode transition is detectable by monitoring various mechanical properties and acoustic emission data against the fibre volume fraction. The tensile mode comprises a fracture mechanism of fibre splitting and pull-out and the delamination comprises fibre bending, tearing off of fibre skin and shearing of individual filaments. Other effects such as the shifting of the neutral axis and the compressive failure at the compression side are also reported.

Effect of temperature and strain rate on the strength of a PET/glass fibre composite

Abstract: Constant strain-rate mechanical testing and surface fractography were used to characterize the failure behaviour of a PET/glass injection-moulding compound and of its unfilled matrix material. Parameters for this investigation were temperature and strain rate. The matrix material exhibited a viscous-brittle transition between room temperature and 60° C. Low temperature failure occurred by craze growth, followed by slow and rapid crack propagation. The composite material likewise behaved as a viscous solid at superambient temperatures. Failure at low temperatures and/or high deformation rates occurred by brittle matrix fracture and fibre pull-out. Under these conditions, mechanical properties improved, relative to those at room temperatures. At intermediate temperatures and/or low strain rates, failure occurred via matrix crazing and crack propagation near the fibre ends. An observed serration of the fracture path at high strain rates is suggested to be due to the need for high shear stresses at the fibre-matrix interface.

Effect of oxidation kinetics on the near threshold fatigue crack growth behavior of a nickel base superalloy

Abstract: The influence of oxidation kinetics on the near threshold fatigue crack growth behavior of a nickel base precipitation hardened superalloy was studied in air from 427° to 649 °C. The tests were conducted at 100 Hz and at load ratios of 0.1 and 0.5. The threshold ΔK values were found to increase with temperature. This behavior is attributed to oxide deposits that form on the freshly created fracture surfaces which enhance crack closure. As determined from secondary ion mass spectrometry, the oxide thickness was uniform over the crack length and was of the order of the maximum crack tip opening displacement at threshold. Oxidation kinetics were important in thickening the oxide on the fracture surfaces at elevated temperatures, whereas at room temperature, the oxide deposits at near threshold fatigue crack growth rates and at low load ratios were thickened by an oxide fretting mechanism. The effect of fracture surface roughness-induced crack closure on the near threshold fatigue crack growth behavior is also discussed.

A microstructural investigation of the origin of brittle behavior in the transverse direction in Mo-based alloy bars

Abstract: The mechanical properties of commercial bars of Mo and a Mo alloy have been investigated. Although longitudinal mechanical properties are good, completely brittle behavior is invariably observed in samples tested in the transverse direction. The microstructures of these materials have been examined using optical metallography of bar stock, scanning electron microscopy of fracture surfaces, and Auger electron spectroscopy ofin situ fractured samples. A number of deleterious microstructural features has been identified; these include a large grain size in the transverse direction as a result of a 〈110〉 fiber texture and fractured carbide-crack stringers on grain boundaries. Oxygen segregation to grain boundaries has been shown not to be a factor contributing to brittle behavior. The origin of the carbides and the development of the associated cracks are described. A thermomechanical processing route, based on a recrystallization and forging procedure, has been developed to manufacture disc-shapes from which components having greatly improved mechanical properties can be produced.

The fracture characterization of highly irradiated Type 316 stainless steel

Abstract: The fracture characteristics of 20% cold worked Type 316 stainless steel irradiated at 377–400°C to a fluence of 11 × 1022n/cm2 (E > 0.1 MeV) were evaluated based on SEM fractography andJ-integral fracture toughness data. Compact tension specimens were tested at temperatures ranging from 232 to 649°C. A significant reduction in fracture toughness after irradiation was observed, which correlated well with the fracture morphology of the material. It was found that irradiation induces a shift in the transgranular-intergranular fracture transition temperature to a lower temperature. Irradiated specimens exhibited transgranular channel fracture from 232 to 538°C and intergranular fracture at 649°C, while unirradiated specimens failed in a transgranular ductile fracture mode up to 549°C. SEM fractographs revealed plate-like, faceted fracture surfaces of irradiated specimens. The faceted fracture appearance is associated with flow localization and dislocation channeling, phenomena that undoubtedly contribute to the decrease in fracture toughness. The possibility that ordered γ′-phase particles are partially responsible for the reduced toughness as well as for irradiation hardening is discussed.

Effect of microstructure, strength, and oxygen content on fatigue crack growth rate of Ti-4.5AI-5.0Mo-1.5Cr (CORONA 5)

Abstract: Fatigue crack growth rate behavior in CORONA 5, an alloy developed for applications requiring high fracture toughness, has been examined for eight material conditions. These conditions were designed to give differences in microstructure, strength level (825 to 1100 MPa [120 to 160 ksi]), and oxygen content (0.100 to 0.174 wt pct), in such a manner that the separate effects of these variables could be defined. For all eight conditions, fatigue crack growth rates (da/dN) are virtually indistinguishable over the full spectrum of stress-intensity range (ΔK) examined,viz., 8 to 40 MPa√m (7 to 36 ksi√in). Concomitantly, it is noted that over the sizable solution annealing range studied (830° to 915 °C [1525° to 1675 °F]), the primary α-phase morphology was substantially invariant. Eachda/dN curve exhibits a bilinear form with a transition point (ΔKT) between 16 and 19 MPa√m (15 and 17 ksi√in). A change in microfractographic appearance occurs at ΔKT, as extensive secondary cracking along α/β interfaces is observed at all hypertransitional levels ofAK, but not for AK < ΔKT. For each material condition, the mean length of primary α platelets is approximately the same as the cyclic plastic zone size at ΔKT. Accordingly, locations ofAKT (and their similarity for the different material conditions) are rationalized in conformance with a cyclic plastic zone model of fatigue crack growth. Finally, the difference in behavior of CORONA 5, as compared to conventional α/β alloys such as Ti-6A1-4V, is rationalized in terms of crack path behavior.

Indentation Fractography: A Measure of Brittleness.

Abstract: L'indentation est l'une des plus puissantes techniques de test permettant l'etude systematique des deformations et des fissures dans les materiaux fragiles. Les indentations peuvent etre utilisees pour evaluer les parametres mecaniques critiques (module elasticite, durete) d'une facon simple et tres precise

Precise evaluation of fast fracture velocities in acrylic polymers at the slow-to-fast transition

Abstract: Transient fast fracture velocities at the onset of the slow-to-fast transition in polymethylmethacrylate (PMMA) have been measured precisely in single-edge-notched specimens of various geometries by using ultrasonic fractography. Little sign of the fracture velocity of the order of 10° m sec−1 have been detected on the instability onset. The fast fracture starts from a point source almost instantaneously. The initial velocity has been shown to fall in a small range, 90 to 150 m sec−1, almost independent of the loading speeds from 0.1 to 100 mm min−1 and specimen temperature from −50 to 40° C, with exceptional cases for specimens loaded slowly (0.1 mm min−1) at a low temperature (−50° C). As the final minimum velocity of an arresting crack, a value of 42 m sec−1 has been obtained under room temperature. Crack propagation in low molecular weight PMMA has been shown to be more temperature, as well as loading rate, dependent than in higher molecular weight PMMA.

Effect of thermal processing variations on the mechanical properties and microstructure of a precipitation hardening HSLA steel

Abstract: Twenty-four variations in the heat treatment of ASTM A710 Grade A, Class 3 steel were performed to determine how sensitive this alloy is to deviations from the recommended heat treatment and to form a basis for understanding property variations in the heat-affected zone of weldments. Tensile, impact, and hardness properties were measured for each heat treatment. Fractography and metallography were carried out to correlate microstructure with properties. It was found that smallangle neutron scattering was extremely sensitive to small changes in the microstructure and also correlated extremely well with properties.

High energy radiation effects on ultimate mechanical properties and fractography of nylon 6 fibres

Abstract: Nylon 6 fibres were irradiated with up to 60 Mrad of cobalt-60 gamma radiation in air or up to 76μC of 4.5 MeV cyclotron-accelerated protons in vacuum. The products of the irradiations were then characterized by the resulting changes in ultimate mechanical properties and in fracture morphology when tested at 21° C under selected conditions of fibre moisture content. In general, the results show a progressive deterioration of tensile properties, an increase in torsional brittleness, and a decreasing flex life, with increasing dose, and display some dependence on fibre moisture content. The effects are most pronounced in the first interval of dose; i.e. 10 Mrads gamma irradiation and 12μC of high energy protons. The rate of deterioration of these mechanical properties generally slows with subsequent dose increases. An annulus/core fracture morphology was found in the gamma-irradiated material which was not in evidence for material irradiated with protons in vacuum. The morphology of the proton-irradiated tensile breaks are characteristic of a brittle material; flex fatigue fracture surfaces were often characterized by step-like formations reminiscent of kink bands. Discussion of the data in terms of radiochemical processes, structure-property relations, and implications for accelerated ageing programs are included.

Dependence of dynamic fracture resistance on crack velocity in tungsten: Part 1. single crystals

Abstract: The dependence of dynamic fracture resistance on crack propagation velocity on (100) in tungsten has been examined. A correlation is obtained between the measured local crack velocity with the surface and subsurface deformations. Based on the experimental results on one pass, two passes, and prestrained, electron beam zone refined single crystals, a discussion is given on the slip modes activated at the crack tip, the contributions to the dynamic fracture resistance from dislocations and surface features, and from the preexisting deformed microstructure.

Techniques for determination of the crystallographic characteristics of environmentally induced brittle fractures.

Abstract: Five methods for determination of the crystallography of transgranular, environment-assisted fractures are described. The application of these methods is illustrated in order to obtain the macrocrystallography and microcrystallography of fractographic features.

On the fatigue crack propagation behavior of superalloys at intermediate temperatures

Abstract: Two superalloys used in gas-turbine disks, Rene 95 and IN-100 were tested in several forms at 0.33 Hz in air, and the results were compared with earlier data on Astroloy to gain a better understanding of the effects of grain size, strength, and alloy composition on the fatigue crack propagation behavior. In addition, selected forms of Rene 95 were tested at 0.33 Hz in vacuum and in air using a cycle with a 120-sec tensile dwell to evaluate the effects of environment and creep. Results of the study emphasize the beneficial effect of large grain size on the fatigue and crep-fatigue crack growth resistance of the superalloys in the temperature range corresponding to the operating temperatures of aircraft gas-turbine engine disk rims.