Showing papers on "Fractography published in 1977"
TL;DR: In this paper, the detailed microscopic processes which occur during the hydrogen embrittlement of pure Nb are examined using in situ SEM crack propagation studies using electron diffraction and ion probe methods, and the results show that the fracture process occurs in a stress induced NbH hydride phase which forms in front of the propagating crack.
106 citations
TL;DR: In this article, the surface on which the particles impinge is shown to play a decisive role with respect to the orientation of the grains within each particle and the position of the particle within the coating.
87 citations
TL;DR: Tin-induced temper embrittlement of 3.5 pct Ni, 1.7 pct Cr and 0.4 pct C was studied by means of notched-bar testing, scanning electron fractography, and Auger electron spectroscopy of isothermally aged specimens as mentioned in this paper.
Abstract: Tin-induced temper embrittlement of 3.5 pct Ni, 1.7 pct Cr steels containing 0.2 and 0.4 pct C was studied by means of notched-bar testing, scanning electron fractography, and Auger electron spectroscopy of isothermally aged specimens. The ductile-brittle transition temperature varied linearly with Sn concentration on grain boundaries at a rate which increased with hardness of the steel. The rate of approach to the steady state level of embrittlement was faster in the higher C steel. The grain boundary concentrations of Ni and Sn bear a unique relationship in a manner analogous to the behavior found previously in Sb-doped steels. The potency of Sn as an embrittling element is somewhat less than that of Sb, but considerably greater than that of P.
42 citations
TL;DR: In this paper, the mechanical properties of aluminium-copper solid-phase welds have been examined in the as-bonded condition, and after heat treatment to produce intermetallic layers of up to 50 μm in thickness at the weld interface.
Abstract: The mechanical properties of aluminium-copper solid-phase welds have been examined in the as-bonded condition, and after heat treatment to produce intermetallic layers of up to 50 μm in thickness at the weld interface. The layers were grown both by continuous annealing and by thermal cycling. The presence of the intermetallic layers led to a reduction in the tensile, shear, and impact strengths of the joint. The impact strength was observed to be negligible for layer thicknesses greater than 2 μm. Thermal cycling had a more detrimental effect than an equivalent heat treatment by continuous annealing, and this was attributed to the build-up of thermal strains. Fractography showed that the fracture propagated between the brittle CuAl2 and CuAl phases, and it was concluded that the existence of these brittle phases was principally responsible for the reduction in properties. Contrary to an earlier hypothesis, it was shown that porosity was not responsible for the poor joint properties after heat trea...
35 citations
TL;DR: The technique of ultrasonic fractography has been used to study crack interactions in a porous glass as discussed by the authors, which has great potential for the study of local changes in crack shape and velocity in brittle composite systems.
Abstract: The technique of ultrasonic fractography has been used to study crack interactions in a porous glass Observations indicate the initial attraction of a crack to the porosity, the change of crack shape as the crack segments to bypass the pores and the production of fracture surface steps as the crack breaks away This technique has great potential for the study of local changes in crack shape and velocity in brittle composite systems
28 citations
TL;DR: In this paper, the temperature dependence of embrittlement of Type 304L stainless steel saturated with hydrogen at high pressure was explored, and the initial fractographic and metallographic examination of the specimens was emphasized.
24 citations
19 citations
TL;DR: In this article, it was suggested that corrosion fatigue of 7075 Al alloy is essentially a hydrogen embrittlement phenomenon where the low diffusivity of hydrogen is counterbalanced by the fact that hydrogen need only be present in the alloy free surface for crack initiation and in the plastic zone of growing cracks for propagation.
Abstract: High cycle fatigue experiments have been performed on a 7075 Al alloy, principally in the T6 temper in dry air, distilled water 0.5N NaCl and 0.5N Na2SO4 solutions as functions of cathodic charging and catalyst poisoning of the hydrogen evolution reaction. All aqueous solutions appreciably lowered fatigue resistance with Cl-ion producing the greatest reduction in resistance and SO4=ion behaving essentially in the same manner as distilled water. Under cathodic charging conditions fatigue resistance is significantly reduced and both Cl- and SO4=solutions produce similar fatigue lives. A catalyst poison (As) added to Cl- solutions reduces fatigue resistance relative to neutral Cl- solutions. Fractography of specimens fatigued in aqueous environments shows that a significant amount of cleavage and quasicleavage occurs, the extent of these features being apparently a function of hydrogen available to the alloy free surface and to the tips of growing cracks. On the basis of these observations, it is suggested that corrosion fatigue of 7075 alloy is essentially a hydrogen embrittlement phenomenon where the low diffusivity of hydrogen is counterbalanced by the fact that hydrogen need only be present in the alloy free surface for crack initiation and in the plastic zone of growing cracks for propagation.
18 citations
16 citations
TL;DR: In this article, the effect of thickness on retardation behavior was investigated by using 12.6mm thick single-edge notched specimens of 7075 alloy in the T6 and T73 conditions and 2024 alloy in T3 and T8 conditions.
Abstract: The effect of specimen thickness on retardation behavior was investigated by using 12.7, 6.4, and 1.6-mm thick single-edge notched specimens of 7075 alloy in the T6 and T73 conditions and 2024 alloy in the T3 and T8 conditions. Single overload cycles were used with two different overload ratios, and interferometry was used to measure the surface plastic-zone size. The cyclic hardening exponents for all four materials were determined by obtaining cyclic stress-strain curves using incremental strain-cycling tests on 6.4-mm diameter cylindrical specimens. Test results indicated that the amount of retardation decreases with increasing specimen thickness. However, the relative decrease was more predominant in the 7075 alloy and at the higher overload ratio, 2.0. The effect of thickness on the retardation behavior of the 2024-T8 alloy was not conclusive. It appeared that thickness changes have only a minor effect on the retardation behavior of the 2024-T8 alloy. Fractography was used to study the micromechanisms of crack growth. Well-defined striations and changes in the striation spacings after an overload cycle were not seen in all specimens studied, and no striations were observed immediately following the overload cycle. The lack of discernable striations could be due to: abrasion, difficulty in resolving striations associated with relatively low crack-growth rates, change in crack-propagation mode, or a combination of all of these factors. However, the agreement between the measured da/dN and the striation spacings was good for all the specimens in which striations were discernable.
12 citations
TL;DR: In this paper, the fracture surfaces of over 50 fibers tested at Hughes or ITT re-search Laboratories were examined on the scanning electron microscope (SEM) and the sources of failure in these fibers were determined to be either mechanically induced sharp cracks, cracks resulting from inclusions or foreign particles, bubbles or inclusions.
Abstract: : The fracture surfaces of over 50 fibers tested at Hughes or ITT reasearch Laboratories were examined on the scanning electron microscope (SEM). The sources of failure in these fibers were determined to be either mechanically induced sharp cracks, cracks resulting from inclusions or foreign particles, bubbles or inclusions. It was suggested that many of these sources of failure can be eliminated by changing production procedures. For example, some larger inclusions can be eliminated by using synthetic, high grade silica, starting material. Another example for eliminating low strength defects is by using high quality silica tubing in the CVD process. The fracture surface demarcations known in glass as mirror, mist, hackle, and crack branching were identified and related to the stress, at failure. In all cases but one, fracture surface analysis agreed with measured stresses during testing within experimental accuracy. Thus, fractography can be used to determine the stress and source of unexpected service failures.
TL;DR: In this article, the authors measured the crack velocity as a function of crack tip stress intensity factor (K), temperature and hydrogen content, and compared the results with recent models of hydrogen embrittlement.
Abstract: The delayed failure of cold-worked Zr-2.5Nb pressure tube material has been studied using static load tests on compact-tension specimens containing hydrogen within the range ∼ 10 to 400 μg/g. The experimental approach wasto measure crack velocity (V) as a function of crack tip stress intensity factor (K), temperature and hydrogen content, relate these data to fractographic and metallographic observations, and compare the results with recent models of hydrogen embrittlement. Slow crack growth was observed at all temperatures between 25 and 325°C and at K values between ∼ 10 and 50 MPa m. Below 250°C, the V-K relationships exhibited two-stage behavior; at K > 15 to 20 MPa m, the crack velocity was only weakly dependent on stress intensity, whereas at smaller K values, the crack velocity decreased rapidly with K, an indication of a threshold value of K ∼ 5 to 10 MPa m. The crack velocity increased with increase in temperature, although because of scatter in the data, this could not be expressed quantitatively. At 250°C and above, slow crack growth was not reproducible except after a thermal cycle. The thermal cycle produced a region of reoriented hydrides concentrated at the crack tip which significantly reduced the incubation period for crack growth and confirmed the important role of the hydride phase in the fracture process. Fractography showed that the features of the slow growth fracture were similar at all temperatures studied. The main observations were of ductile striations, or stretch zones, parallel to the crack front, with brittle, plate-like regions, some of which contained cleavage features, between the striations. A fracture mechanism is suggested which involves the repeated precipitation of hydride at the crack tip, followed by crack advance through this embrittled region, and crack arrest in the more ductile matrix, leading to discontinuous crack growth. This general mode of crack growth has been considered in a recent model for embrittlement in hydride-forming materials, the predictions of which show good agreement with the results from this study.
01 Jan 1977
TL;DR: In this article, the authors described and evaluated the current models for SCC in terms of existing fractographic evidence and concluded that the film-rupture model, which is considered to advocate propagation by continuous anodic dissolution, is consistent with the fractography of the majority of intergranular failures but cannot be reconciled with that of transgranular SCC.
Abstract: The current models for SCC are described and evaluated in terms of existing fractographic evidence. It is concluded that the film-rupture model, which is considered to advocate propagation by continuous anodic dissolution, is consistent with the fractography of the majority of intergranular failures but cannot be reconciled with that of transgranular SCC. The latter appears to occur by discontinuous cleavage and is best accounted for by a bulk-embrittlement mechanism such as hydrogen embrittlement or the model based on selective dissolution. The fractographic evidence suggests that certain intergranular failures may also occur discontinuously, again indicating that a bulk-embrittlement model may be operative. In its present form, the tunnel model does not appear to be consistent with the fractographic observations, nor does the adsorption model seem to have wide application. The tarnish-rupture model, proposed for the brass-ammonia system, is also discarded on the basis of the fractographic evidence.
TL;DR: In this paper, large slip-band facets of an age-hardened Al-Zn-Mg alloy, produced by fatiguing notched single-crystal specimens, were studied in detail using optical and electron fractographic methods.
Abstract: To gain a better understanding of the mechanisms operating during the first stage of fatigue-crack propagation in aluminium alloys, large slip-band facets of an age-hardened Al-Zn-Mg alloy, produced by fatiguing notched single-crystal specimens, were studied in detail using optical and electron fractographic methods. The features observed on the fracture surfaces were found to be a function of (i) crack propagation velocity, (ii) orientation of the crystallographic deformation modes with reference to macroscopic crack-propagation direction, and (iii) moisture level in the test environment. The observations have led to the following conclusions: (1) crack propagation in crystallographically favoured slip directions is energetically favoured; (2) nucleation and propagation of secondary cracks ahead of the main crack front plays an important role during slipband cracking, propagation of secondary cracks occurring parallel to the slip band preferentially in crystallographically favoured slip direction...
TL;DR: Human intact tibiae were tested using the static bending method to learn about the relationship between the fracture surface and the failure mode, and the mechanical properties of the fractured bone can be estimated to some extend by considering the direction of the break line and theFailure mode.
Abstract: Human intact tibiae were tested using the static bending method to learn about the relationship between the fracture surface and the failure mode. The bending test was applied to test pieces and to whole bones. The fracture surface was observed by scanning electron microscopy. The bone fracture is closely related to the architecture of the bone substance, especially to the direction of the Haversian canals and the lamellae. The failure mode and the sequence of the break line of the bone can be found out by the observation on the fracture surface. Hardly any crushing effects caused by the compressive force is seen. The mechanical properties of the fractured bone can be estimated to some extend by considering the direction of the break line and the failure mode. The strength calculated by the simple beam formula for elastic materials can not be obtained directly because of the plastic deformation of the bone. The results of the tensile test may be applied to the fracture using the static bending moment.
Dissertation•
01 Jan 1977
TL;DR: In this article, the authors present an approach for the design of a two-dimensional lattice with respect to a single lattice, which is known as a lattice lattice-free lattice.
Abstract: Thesis. 1977. M.S.--Massachusetts Institute of Technology. Dept. of Materials Science and Engineering.
Journal Article•
01 Feb 1977
TL;DR: In this article, it is proposed that stress-corrosion cracking and liquid-metal embrittlement in aluminium alloys (and possibly other materials) can be explained on the basis that chemisorption of environmental species facilitates nucleation of dislocations at crack tips.
Abstract: : Metallographic and fractographic observations of sub-critical crack growth in a precipitation-hardened AlZn-Mg alloy in liquid-metal, aqueous, and water-vapour/air environments suggest that stress-corrosion cracking and liquid-metal embrittlement in this alloy involve a common mechanism. Sub-critical crack growth in all these environments can produce entirely dimpled fracture surfaces and, in liquid-metal and aqueous environments, crack growth can occur extremely rapidly. It is proposed that stress-corrosion cracking and liquid-metal embrittlement in aluminium alloys (and possibly other materials) can be explained on the basis that chemisorption of environmental species facilitates nucleation of dislocations at crack tips. Such a process would produce sub-critical crack growth with less blunting at crack tips than in inert environments and, hence, would explain observations that dimples on fracture surfaces after SCC are smaller and shallower than those on overload fractures. The results suggest that neither dissolution nor hydrogen-embrittlement processes occurred during stress-corrosion cracking in aluminium alloys. (Author)
TL;DR: In this article, the properties of dispersion-strengthened materials produced by the high-rate physical vapor deposition technique were studied, focusing on the copper-alumina system.
01 Feb 1977
TL;DR: In this article, it is proposed that chemisorption of liquid-metal atoms facilitates the nucleation of dislocations at crack tips, which would produce subcritical crack growth, change the distribution of slip around cracks and reduce the extent of blunting at crack tip, but would not generally influence bulk flow characteristics.
Abstract: : Metallographic and fractographic studies of fracture in single crystals of aluminum and zinc, and in polycrystalline aluminum, cadmium and tin in liquid-metal environments and in air are described. The commonly suggested mechanism for liquid-metal embrittlement--that chemisorption of liquid-metal atoms facilitates tensile rupture of stressed interatomic bonds at crack tips--is not supported by the present observations. Rather, observations of extensive slip around crack tips and dimpled fracture surfaces suggest that crack growth in embrittling liquid-metal environments occurs by plastic flow, but with considerably less blunting occurring at crack tips than in air. It is proposed that liquid-metal embrittlement can be explained on the basis that chemisorption of liquid-metal atoms facilitates the nucleation of dislocations at crack tips. Such an effect would produce sub-critical crack growth, change the distribution of slip around cracks and reduce the extent of blunting at crack tips, but would not generally influence bulk flow characteristics. The relevance of these results to hydrogen-embrittlement, stress-corrosion cracking, and ductile versus brittle behaviour in general is discussed. (Author)
TL;DR: In this article, the general appearance of fracture patterns in a row of boreholes blasted in a simulated infinite medium is analyzed and compared, and the effect of changing the experimental conditions of hole spacing and ecoupling ratio on fracture surface morphology, the point of crack initiation, and crack propagation, has been studied.
TL;DR: In this paper, a study of fracture development in Fe-31%Ni-0.1%C alloy has been made, where samples of this alloy, which show deformation-induced martensite transformation, were given pretreatments of shock loading and grain refining.
TL;DR: In this article, a scan electron microscopy was performed on fracture surfaces of failed AISI 4027 steel and the corresponding stress-life curve was determined, showing that a fatigue crack nucleates at the external surface of the carburized case, and propagates inward until it becomes large enough to cause rapid failure in bending.
TL;DR: In this paper, the extent of fracture of the 0-deg layers of fibers is evaluated in terms of a fatigue crack growth rate law based on fracture mechanics, and it was shown that the fiber fracture may be significantly less than the visible crack.
Abstract: Laminated boron/aluminum sheets of 0, ′60, 0, ′60, and 0-deg layer orientation were notched and subjected to tension-tension (T-T) fatigue loading. Fatigue cracks were monitored visually and the fatigue cycling terminatedat various amounts of fatigue crack growth. Crack growth rates were evaluated in terms of a fatigue crack growth rate law based on fracture mechanics. Some specimens were then loaded to fracture and the fracture surfaces examined by scanning electron fractography. Other similar specimens were examined by progressively removing the aluminum matrix by a selective etching technique. Each layer of fibers was thus exposed and could be examined in the scanning electron microscope (SEM). The results obtained show that the extent of fracture of the 0-deg layers of fibers is the same as the extent of the visible fatigue crack on the surface of the specimen. For the 60-deg layers, the extent of fiber fracture may be significantly less than the extent of the visible crack. The extent of fatigue crack growth in the matrix material in the interior of the specimen exceeds the length of the surface crack by several fiber diameters.