Showing papers on "Fractography published in 1971"

Delay Effects in Fatigue Crack Propagation

Abstract: The fatigue crack propagation behavior resulting from variations in load is examined for 2024-T3 aluminum alloy, from both a macroscopic and a fractographic point of view. A peak load is found to cause retardation of the crack growth rate, which becomes more pronounced as the percentage overload or baseline stress intensity level or both is increased. The delaying effect of the overload is observed to exist for a calculated crack length increment equivalent to the plastic zone size formed during the peak load. Multiple overloads and high-low block loading sequences are found to result in additional retardation. It is observed that the macroscopic fracture surface appearance (that is, transition to plane stress) is a function of the crack growth rate. From fractographic examination it is found that the initiation of microvoid coalescence during fatigue occurs when plane stress conditions are achieved; this limits the extent of the stretch zone associated with an overload cycle. As a result, the stretch zone is found to be followed by striations in plane strain and by dimples under plane stress conditions. The size of the stretch band is observed to depend on the stress intensity level during the overload cycle. The usefulness of closure concepts in aiding the understanding of fatigue crack propagation under uniform and nonuniform loading conditions is considered. Evidence is given to demonstrate the general applicability of closure concepts for analysis of macroscopic and fractographic fatigue crack propagation results. /Author/

Fracture and fractography of metastable austenites

Abstract: External test variables such as rate and temperature, and changes in alloy composition are shown to have a number of effects on the fracture of high-strength, metastable austenitic steels. One rate-dependent phenomenon is an unusual fracture mode transition wherein a flat mode changes to a shear mode when the amount of transformation product in the vicinity of the crack tip is reduced by adiabatic heating. The point at which this happens in any one test is dependent upon the velocity of the slowly growing crack which in turn is dependent upon the crosshead rate. Because of this rate effect, the plane stress fracture toughness decreases by as much as 30 pct at higher crosshead rates. Fractographically, it was ascertained that at room temperature, both phases failed in a ductile manner, but at −196°C, martensite containing greater than about 0.27 wt pct C would cleave. This resulted in a “ductile-brittle” transition in metastable austenites at −196°C as a function of carbon content. Other compositional variations change the austenite stability which controls the amount of strain-induced marteniste occurring at the crack tip. It is shown that a plane stress fracture toughness (KC) approaching 500,000 psi-in.1/2 may be achieved by decreasing the stability of the austenite. The variation ofKc with austenite stability agrees qualitatively with a theoretical model for the invariant shear contribution to the fracture toughness of metastable austenites.

The relation between microstructure and toughness in 7075 aluminum alloy

Abstract: Electron microscopy, fractography, and notched tear tests have been used to investigate the effects of heat treatment upon the fracture behavior of aged 7075 aluminum alloy sheet. Toughness, as measured by crack propagation energy, decreases as the yield stress increases; the toughness of an overaged structure is inferior to that of an underaged structure at the same yield stress. The decrease of toughness with increased aging time is accompanied by a change in fracture mode from predominantly transgranular to intergranular. Transgranular fracture proceeds by dimple rupture and is facilitated by chromium-rich particles which are dispersed throughout the microstructure. Intergranular fracture proceeds by the fracture of grain boundary precipitate particles. The variation of fracture mode with aging time is attributed to a steady decrease of the intergranular fracture stress relative to the transgranular fracture stress, due to increasing grain boundary particle size. A possible explanation of this effect is discussed using the stress concentration due to colinear crack arrays as an analogy. The effects of quenching variations and two-step aging are discussed. It is shown that, in aged 7075, microstructural variables such as the width of precipitate-free zones and the nature of the matrix precipitate do not have a controlling effect on toughness.

Electron fractography and fatigue crack propagation in 7075-t6 aluminum sheet

Abstract: A centrally slotted $$\tfrac{1}{8}''$$ thick sheet of 7075-T6 aluminum alloy was cyclically loaded. Striation spacings and crack propagation rates on the specimen surface were measured and compared and the fractographs were examined. The average striation spacing is found proportional to ΔK 1.8; while the surface crack propagation rate is proportional to ΔK2 8, where ΔK is stress intensity factor range. Cleavage fractures of brittle particles appear to cause a difference between the overall surface crack propagation rate and striation spacings. In the lower ΔK region, there are fewer cleavage fractures, the striations are more distinct and the directions of the striations do not deviate much from the normal to the direction of the overall crack propagation. In the higher ΔK region, there are more cleavage fractures. In this region striations run in divergent directions and become less evident. The differences in fracture surface features result from the cleavage fracture of brittle particles.

Transgranular Stress Corrosion Cracking of Solution Treated and Qeenched Mg-8.6 Al Alloy

Abstract: Transgranular stress corrosion cracking of notched, solution treated and quenched Mg-8.6 wt % Al alloy rods tested in both salt solution and distilled water environments at 20 C (68 F) has been studied by direct electron diffraction techniques conducted on actual stress corrosion fracture surfaces, optical fractography, scanning electron fractography, and replica electron fractography. The studies revealed that considerable deformation twinning accompanied the formation of a stress corrosion fracture surface and that opposite faces of a stress corrosion crack contained matching faceted (stepped) topography. The detailed faceted topographical patterns were consistent with their origin being associated with underlying deformation twins formed prior to the passage of the crack tip and, frequently, deformation twins were observed adjacent and parallel to the stress corrosion fracture surface. All observations were consistent with a stress corrosion crack propagation process involving the formation of...

Fracture Marks Associated with Transition‐Region Behavior of Slow Cracks in Glass

Abstract: Techniques of fractography employed in a study of slow‐crack behavior in edge‐cracked glass sheets have led to the discovery of previously unreported fracture marks. The physical appearance of these marks and the mechanism of their formation is postulated to be a result of mixed stress‐corrosion dependent/stress‐corrosion independent effects in the transition region of slow‐crack propagation in glass.

The Effect of Microstructure on Fatigue Crack Propagation in Ti-6Al-6V-2Sn Alloy.

Abstract: : The fatigue crack growth behavior of Ti-6Al-6V-2Sn was investigated for five different microstructures, using contoured double-cantilever beam specimens The fatigue crack growth behavior was compared with the mechanical strength, toughness, and ductility properties for each microstructure The fatigue fracture suffaces were examined with a scanning electron microscope Annealed structures consisting of equiaxed primary alpha have the lowest crack growth resistance, while the acicular alpha microstructure resulting from beta annealing has a growth resistance four times greater in the intermediate growth regime No simple model of crack growth rate using gross mechanical properties describes the relative magnitude nor the ralative order of crack resistance among the different microstructures (Author)

Electron fractography of stress-corrosion fracture surfaces

Abstract: Stress-corrosion is the cause of a large number of service failures in high strength materials. Consequently, it became necessary to make an inventory of the features of stress-corrosion fracture surfaces for investigating service failures in general, especially because such conspicuous features as intergranular cracking are not limited to stress-corrosion cracking and also because stress-corrosion cracking may even proceed transgranularly. In this report the results of such an inventory involving a number of aircraft materials, is presented.

The mechanical properties and failure of mild steel in tension

Abstract: 1. The temperature range of blue brittleness is 100° wider in fine-grained mild steel than in coarsegrained, and extends approximately from +100 to +300°C. 2. With all three grain sizes investigated maximum ductility occurs in the temperature range from −100 to +20°C. Maximum permanent elongation occurs at −50°C, and maximum relative transverse contraction after failure occurs in the range from −100 to +50°C. In this range transverse contraction is slightly sensitive to temperature. 3. The methods of optical and electron fractography have revealed, even in tests at very low temperatures, traces of plastic deformation in the fracture, with patterns typical of slip and twinning. The appearance of cleavage tongues is evidently due to the intersection of a cleavage crack with deformation twins generated by a high-stress zone at the tip of a moving crack.

Fractography of High Boron Ceramics Subjected to Ballistic Loading

Abstract: Fractographic studies of B4C and AlB12 ceramics subjected to ballistic impact are presented. Results suggest that macroscopic textures of fracture-exposed surfaces are indicative of stress states occurring during the fracture event, whereas microscopic topography is strongly influenced as well by microstructural features. Variation observed as a function of distance from the impact axis allows some differentiation between the strain rate sensitivity of the fracture processes of the two materials.

The mechanical response of the Ni−Ni3Nb eutectic composite: Part I. Monotonic behavior

Abstract: To understand the mechanical behavior of the Ni−Ni3Nb eutectic composite, it was necessary to determine the operative deformation and fracture mechanisms in the Ni3Nb intermetallic phase. It was found that Ni3Nb deforms primarily by twinning along {112} planes and {011} planes when tension and compression, respectively, are applied parallel to the [100] growth direction. The {112} twins were observed to serve as crack nucleation sites with cracks forming along the twin boundaries. The monotonic response of the Ni−Ni3Nb eutectic composite was investigated with tension and compression tests, metallography, and electron fractography. Room temperature tensile testing of the Ni−Ni3Nb composite revealed this material to be capable of sustaining tensile strains in excess of 11 pct. This large composite ductility was associated with extensive {112} twinning of the Ni3Nb lamellae and subsequent twin boundary cracking. When amassed in sufficient numbers in a given cross-section, these {112} twin boundary fissures initiated composite rupture. The room temperature ultimate tensile and compressive strengths of the alloy were found to be 109 and 235 ksi, respectively.