Showing papers on "Fractography published in 1970"

The fractography of thermoplastic textile fibres

Abstract: The use of a scanning electron microscope in studying the fractography of fibres is described and its advantages explained. A common pattern of break is shown for thermoplastic fibres at extension rates up to 8×10−2 sec−1. This involves a slow crack which becomes catastrophic at the critical crack length. The critical crack length depends upon the rate of extension, and possibly other undetermined factors. Under some conditions other forms of break are observed in thermoplastic fibres. A qualitative interpretation of the fracture phenomena is proposed.

Fractographic analysis of the low energy fracture of an aluminum alloy

Abstract: A study of the fracture process in a high strength aluminum alloy, 2014T6, was undertaken to identify the void nucleating particles in this material, to determine their composition, and to suggest means by which they might be eliminated without loss of strength.

The effects of particles on fracture processes in magnesium alloys

Abstract: The influence of coherent and noncoherent particles on the mechanism of ductile fracture has been studied in two magnesium base alloys: Mg-3.7 pct Th-0.4 pct Zr and Mg-0.6 pct Zr. The ternary alloy, was tested in the solution treated condition, and as a function of aging time at the temperature of peak hardening, 325°C; the binary alloy was tested in the solution treated condition and after hydriding to form e-ZrH2 particles. Fractographic studies were carried out on smooth-bar and notched tensile samples after testing at −196°, 25°, and 210°C. Fractographic features observed in the solution treated alloys were explainable in, terms of the operative slip systems in magnesium. Features observed in aged Mg−Th−Zr and hydrided Mg−Zr were related to particle size, morphology, and distribution, with emphasis on dimple-particle relation-ships. Coherent, plate-like particles (250A long) produced dimples in Mg−Th−Zr, and did larger, noncoherent particles. However, many cases of dimple formation without associated particles also were noted. Microvoid sizes observed by light microscopy were 20X dimple sizes revealed by electron fractography. When submicroscopic voids (<0.5μ) associated with cracked particles were present, lateral growth of these voids did not occur prior to fracture. It was concluded that microvoid coalescence as classically pictured, was not the failure mechanism in these alloys nor was it the mechanism of formation of dimples.

STRESS-CORROSION CRACKING BEHAVIOR OF AN 18% Ni MARAGING STEEL

Abstract: Stress-corrosion cracking of an 18 pct Ni maraging steel in aqueous solutions was studied using precracked cantilever beam specimens. By appropriate heat treatments, six different structures having the same yield strength were obtained. Although significantly different plane strain fracture toughness values (KIc) resulted, it was found that the threshold plane strain stress intensity (KIscc) was the same for all structures.KIscc had the same value in 3 pct NaCl at various pH values, in 1N H2SO4, and in distilled water. Specimens tested in 3 pct NaCl under both anodic and cathodic applied potentials also exhibited this sameKIscc value. Fractographic inspection of the crack surfaces revealed no apparent differences due to changes in solution, pH, or applied potential. The crack path was intergranular in all cases. However, specimens austenitized at 1500°F exhibited crack branching, whereas in specimens austenized at much higher temperatures branching no longer occurred. Aging time and temperature seemed to change only the time to failure. The mechanism most consistent with all observations appears to be hydrogen cracking.

Microstructural and fractographic studies of boron carbide subjected to ballistic impact and static flexural loading

Abstract: : Microstructural and fractographic studies were conducted on hot- pressed boron carbide ceramics subjected to ballistic impact by caliber .30 AP M2 projectiles and static flexural loading. A variety of observational techniques were employed, including conventional metallographic and low-power optical examinations, electron microprobe analysis, scanning electron microscopy, and replication electron microscopy. Results suggest that macroscopic topography of fracture-exposed surfaces is indicative of stress states occurring during the fracture event whereas microscopic topography is determined by microstructural constituents, particularly secondary phases, pores, and grain boundaries.

Microstructural Alterations in Alumina Ceramics Associated with Ballistic Impact Events

Abstract: : Surface topography and defect structures associated with ballistic impact in a commercial 94 percent alumina ceramic armor are described. Fracture surfaces were examined by scanning electron microscopy, augmenting previous replication electron fractography studies. Direct transmission electron microscopy was employed in characterizing defect structures in extracted fracture fragments on replicas and in thin foils taken from bulk polycrystalline alumina in undamaged and damaged states. A variety of dislocation interactions and tangled masses of dislocations with densities as high as 2 x 10 to the 11th power/sq cm have been observed near intersecting surfaces in thin fracture fragments. Similar deformation textures within the bulk have been observed in impact-damaged alumina for a distance of at least 20 micrometers beneath a fracture surface. Localized plastic flow is most prominently evidenced in the case of cracks which are intersecting at acute angles to create fracture fragments, and in worked zones abutting propagating cracks in more massive material. These observations strongly support the concept that portions of the high stresses concentrated at the tips of propagating cracks during an impact event are relaxed by plastic deformation processes, thereby absorbing excess energy and/or delaying the fracture process to some degree. (Author)

Subcritical crack growth in several titanium alloys

Abstract: : Studies of low-cycle fatigue crack propagation in air and in salt water environments and salt water stress-corrosion cracking (SCC) were conducted on several intermediate-to-high strength, very low interstitial oxygen, titanium alloy plate materials. The alloys studied included one sample each of Ti-7Al- 2Cb-1Ta and Ti-7Al-2.5Mo, and two samples each of Ti-6Al-4V and Ti-6Al-6V-2Sn. The yield strengths of these alloys ranged from 110 to 150 ksi (77.3 to 105.5 kgf/sq mm) and included both as-received and heat-treated conditions. Fatigue and SCC data were obtained from single-edge-notched (SEN) cantilever specimens, and the results were analyzed in terms of crack tip stress-intensity parameters. Most of the alloys investigated exhibited low resistance to low-cycle fatigue crack propagation. The most favorable results were obtained from the Ti-6Al-4V alloys. However, several alloys were highly resistant to environmentally- accelerated crack growth in salt water. Several alloys also showed a correlation between fatigue crack growth behavior in salt water and the threshold stress- intensity for SCC to occur (K sub Iscc). Examination of fatigue surfaces by electron fractography revealed that the predominant mode of separation was microvoid coalescence, regardless of environment.

Brief note—microstructure of the fatigue fracture surfaces of steel elements submitted to load of very high frequency

Abstract: Extreme complexity of microrelief of the fatigue fractures caused by high frequency loading is determined by equally complex mechanisms of fracture development. However, many of the observed fracture areas do not include pronounced plastic deformation. There are dominating smooth, brittle areas, running along grain boundaries, having detached cementite, particles, showing traces of slip or cleavage. Here lies the essential difficulty in connecting the fractography of these areas with the kinetics of fracture development, and additional experiments with different materials are required.

Electron Fractography of Weld-Reheat Cracking in CrMoV Steel

Abstract: Our attention has been drawn to a recent paper by Boniszewski and Eaton in which reference is made to our work on 12% CrMoVNb steels. They have made a valuable contribution to the problem of reheat cracking in CrMoV steels but we feel that their understanding of creep-rupture ductility would be enhanced if they gave more consideration to the time variable.

Physical Parameters Affecting Fracture Strength and Fracture Mechanisms in Ceramics.

Abstract: : The fracture of any material may be separated into (at least) two stages. The first stage is the initiation of a crack at an otherwise flaw-free surface (or in a flaw-free interior volume). The second stage consists of propagation of this flaw, causing eventual mechanical failure of the structural element. Separate and different testing procedures are used to determine the fracture initiation resistance and the fracture propagation resistance. (Author)

The monotonic and cyclic mechanical response of the Ni-Ni3Nb eutectic composite

Abstract: Monotonic and cyclic mechanical response of nickel-nickel niobium eutectic alloy to tensile, compressive and fatigue testing