Showing papers on "Fractography published in 1973"

Influence of Microstructure on Fracture Propagation in Rock

Abstract: Influence of Microstructure on Fracture Propagation in Rock This paper describes the results of research to correlate the fracture resistance with microstructural features of Salem limestone and Berea sandstone. Tests were conducted on wedge-loaded double-cantilever-beam specimens containing machined slots as crack starters. The fracture resistance of Salem limestone measured in terms ofR, the energy dissipated per unit area of projected surface, was found to increase in the initial stage of crack extension but finally reached a constant value which was strongly dependent on crack orientation with respect to the bedding plane. For this limestoneR ranges from about 50 joules/m2 to 230 joules/m2 (1 joule/m2 = 104 erg/cm2). The sandstone which is a more compliant rock exhibited similar fracture behavior while theR values ranged from 465 joules/m2 to 1580 joules/m2. In addition, tests in liquid nitrogen which were aimed at eliminating plastic deformation in the rocks during fracture showed little difference inR for the limestone but a substantial reduction inR for the sandstone which, in the latter case, may be caused by differential thermal expansion between the quartz grains and the calcite cement. Acoustic emissions were detected in both rocks at very early stages in the tests indicating the occurrence of microcracking near the initial slot tip at low loads. The mode of fracture and the fracture path in both materials were identified by fractography over a broad range of magnifications. The evidence gained from this work strongly points to the existence of an extensive array of microcracks produced in a region surrounding the main crack tip and which advances with it. The energy dissipated in fracturing of rock is associated with the creation of the large amount of surface area contained in this microcrack array. This picture provides a self-consistent explanation for the puzzling discrepancy between the measured tensile strength of rocks and the strengths predicted from measuredR values.

Influence of inclusion content on fatigue crack

Abstract: Fatigue crack growth rates were measured at room temperature in dry air for three 7075-T6 aluminum alloys with different inclusion content. Volume fractions of inclusions were determined for each alloy by the point count method with two different automated systems. Plots of the fatigue crack growth rate (da/dN) vs the stress-intensity-factor range (ΔK) show a well defined change of slope at the transition between plane strain and plane stress fracture. This transition is associated with a marked increase in the amount of fracture by void growth around inclusions. The volume fraction and mean spacing of voids within the cyclic plastic zone have been determined as a function of ΔK by quantitative fractography. Fracture by voids is important when the mean spacing of such voids is approximately equal to the width of the cyclic plastic zone in the plane of the crack. It is concluded that the inclusion content increases the fatigue-crack growth rates only within the plane stress range, that is for values of the stress-intensity-factor range ΔK \s> 20 kpsi√in.

S-200 Grade Beryllium Fracture Toughness Properties

Abstract: S-200 grade vacuum hot pressed beryllium fracture toughness properties for both longitudinal and transverse orientations were generated in the temperature ranges between −320°F to 500°F and −320°F to 75°F, respectively. Limited data are presented for observations of fatigue behavior, fractography, dynamic and static tensile properties, and dynamic fracture toughness. Statistical bounds are given for fracture toughness data including a comparison of orientation effects. For longitudinally oriented specimens, fracture toughness increased linearly with temperature up to 300°F, and the 500°F results indicated a rapid increase above 300°F. Transverse fracture toughness was significantly higher and increased more rapidly with temperature than the longitudinal fracture toughness. Fatigue precracking experimentation revealed that beryllium does fatigue, but that current ASTM fatigue precracking procedures are impractical for general testing. In comparison to previously published notched specimen data, fatigue precracked specimens yielded significantly lower fracture toughness properties.

Studies related to the fracture of partial dentures--fractography of cobalt-chromium alloys.

Abstract: The fracture surfaces of specimens of a cobalt-chromium alloy fractured in a conventional testing machine have been compared with similar specimens fractured by fatigue. Notched bar impact tests were also carried out and the surfaces of the specimens compared to those fractured in tension and by fatigue. Results show a brittle failure in all cases, although some change in the rate of crack propagation was evident in the fatigue specimens. Clinical evidence suggests that failure of partial dentures is due to bad castings, but some evidence is produced to show that fatigue cannot be eliminated as a cause of failure.

Hydrogen Embrittlement and Stress Corrosion Cracking in Ti-Al Binary Alloys

Abstract: The fracture characteristics of hydrogen embrittlement (HE) in three binary Ti-Al alloys (2.5, 5, and 8 wt % Al) were investigated using electron fractography and X-ray diffraction. The de...

Stress-corrosion cracking of Ti-8Al-lMo-lV in aqueous environments: 2. Plastic zones, crack morphology, and fractography

Abstract: Optical and electron metallographic studies of stress-corrosion cracks in Ti-8Al-lMo-lV have verified that the principal crack extension mechanism is cleavage of theα grains. There are two distinct crack morphologies which correspond to the two regimes of subcritical crack velocity. At low stress intensities(a ∞ K I) the microscopic crack front consists of small cleavage facets approximately 1 to 4α grain diameters in size, and ligaments of material which fracture by ductile rupture and corrosion. At high stress intensities (a ≅ constant), the crack front consists of large cleavage “fingers”, 20 to 50α grain diameters in length, separated by regions which fracture by a combination of cleavage (on a much smaller scale), ductile rupture, and corrosion. The transition from Stage I to Stage II crack propagation apparently occurs when the strain-energy release rate is sufficient to support two crack branches,i.e., KI≥ √2K Iscc. Thereafter, the diameter of the plastic zone at the crack tip remains constant, suggesting that the effective stress intensity at the tip of each branch is also invariant. The slip within the plastic zone is markedly nonhomogeneous, and trenches are often observed along the slip steps.

Scanning electron fractography of lap‐shear joints

Abstract: Fracture surfaces of Epon 901/B-3 bonded aluminum alloy joints in the lap-shear configuration were studied using scanning electron microscopy. Major differences in the appearance of the fracture surface from those reported (8) for tensile loaded joints at 23°C are produced either by cyclic loading at 23°C or a change in test temperature to −196°C. Fracture in tensile loaded joints at −196°C is a brittle single step process in the opening mode in which rapid crack extension occurs throughout the joint with very little adhesive flow. Tensile fatigue fracture at 23°C is in the opening mode but crack extension is complicated by extensive adhesive flow throughout the entire joint.

Electron fractography of stress corrosion fracture surfaces

Abstract: Introduction Visual inspection or observation at low magnification with an optical microscope has been of great help in analysing fracture surfaces. Optical microscopy, however, has distinct limitations, such as low resolution and small depth of field. The transmission electron microscope does not have these limitations.

Fractography of Stress Corrosion in Ti-8A1 Tested in Fatigue

Abstract: A fractographic examination was made of unnotched aged and solution treated Ti-8Al specimens subjected to fluctuating tension fatigue in air, 3% aqueous NaCl, and methanol-HCl. Crack initi...

Polymer fractography and fracture kinetics

Abstract: The fractographic method makes it possible to determine the test temperature and time at which anomalies appear in the temperature-time dependence of the strength of polymethyl methacrylate and polycaprolactam by finding the conditions of disappearance of specular zones from the fracture surfaces of these polymers. For PMMA these values are −40°C and 10−2 sec, for PCL −120°C and 10−7 sec, respectively.

Crack velocity measurements in the toughness testing of polymers

Abstract: Crack velocity measurements have been made on five polymers in dynamic toughness tests. The crack velocities have been studied in relation to the test variables, temperature, crack extension and strain rate and correlations attempted with fracture toughness values (K Ic) and with the results of electron fractography.

A mechanical property and stress corrosion evaluation of 431 stainless steel alloy

Abstract: The mechanical properties of type 431 stainless steel in two conditions: annealed bar and hardened and tempered bar are presented. Test specimens, manufactured from approximately 1.0 inch (2.54 cm) diameter bar stock, were tested at temperatures of 80 F (+26.7 C), 0 F (-17.8 C), -100 F (-73 C), and -200 F (-129 C). The test data indicated excellent tensile strength, notched/unnotched tensile ratio, ductility, shear, and impact properties at all testing temperatures. Results of the alternate immersion stress corrosion tests on stressed and unstressed longitudinal tensile specimens 0.1250 inch (0.3175 cm) diameter and transverse C-ring specimens, machined from 1.0 inch (2.54 cm) diameter bar stock, indicated that the material is not susceptible to stress corrosion cracking when tested in a 3.5 percent NaCl solution for 180 days.

Microstructural Analysis and Fractography

Abstract: The examination of fracture surfaces either by means of replicas which are subsequently studied using transmission electron microscopy or by direct observation using a scanning electron microscope has become a routine procedure in many metallurgical laboratories, and the techniques used in conducting such examinations are well documented, e.g., (1). Considerable literature has been devoted to the relationships existing between features found on the fracture surfaces and the microstructure as determined by optical or electron microscopy, in the latter case using either thin foils or replicas prepared from polished and etched specimens of the material. It is not intended that the present paper should cover this ground again, but rather it is proposed to discuss the use of fractographic techniques as an aid to microstructural analysis, as distinct from the more usual situation already noted where prior knowledge of the microstructure is used to interpret fractographic features.

Correlation of Diffusion Bonded Ti-6A1-4V Properties with Acoustic Emission, Pulse-Echo and Fractographic Data

Abstract: A series of 6Al-4V titanium pieces was diffusion bonded utilizing hot retort-cold press and vacuum hot press techniques. The time, temperature, and pressure parameters were varied with each specimen. The bonded specimens were a) examined metallographically in the bond area before destructive testing, b) ultrasonically before testing, c) monitored for acoustic emission during tensile pulling, and d) examined in detail using optical and electron fractography techniques, as well as metallography at the bondline, after failure.

Failure analysis of aluminum alloy components

Abstract: Analysis of six service failures in aluminum alloy components which failed in aerospace applications is reported Identification of fracture surface features from fatigue and overload modes was straightforward, though the specimens were not always in a clean, smear-free condition most suitable for failure analysis The presence of corrosion products and of chemically attacked or mechanically rubbed areas here hindered precise determination of the cause of crack initiation, which was then indirectly inferred from the scanning electron fractography results In five failures the crack propagation was by fatigue, though in each case the fatigue crack initiated from a different cause Some of these causes could be eliminated in future components by better process control In one failure, the cause was determined to be impact during a crash; the features of impact fracture were distinguished from overload fractures by direct comparisons of the received specimens with laboratory-generated failures

Estimation of the impact strength of steels at cryogenic temperatures

Abstract: 1. The character of the stress state in the vicinity of notches in impact specimens, which is determined by the notch, has a substantial influence on the γ → α transformation taking place as a result of cooling and deformation of steels with metastable structures; this leads to changes in the ductility of steel. 2. Increasing the notch depth in impact specimens of metastable austenitic steels produces a decrease in their impact strength, especially in the temperature range of intense γ → α transformation. 3. The reduction produced in the impact strength of structurally stable steels by increasing the notch depth is much smaller than in the case of metastable steels, since it is associated only with a change in the stress state. 4. The energy of crack propagation in steels of various strength and with various structures can be determined by the Gulyaev and Drozdovskii methods in the entire range of cryogenic temperatures. 5. Electron fractography of fracture surfaces of impact specimens is quite an effective method of estimating the impact properties of cryogenic steels.