Showing papers on "Fractography published in 1985"

Delamination and Debonding of Materials

Abstract: The general topics consist of stress analysis, mechanical behavior, and fractography/NDI of composite laminates. Papers are presented on a dynamic hybrid finite-element analysis for interfacial cracks in composites, energy release rate during delamination crack growth in composite laminates, matrix deformation and fracture in graphite-reinforced epoxies, and the role of delamination and damage development on the strength of thick notched laminates. In addition, consideration is given to a new ply model for interlaminar stress analysis, a fracture mechanics approach for designing adhesively bonded joints, the analysis of local delaminations and their influence on composite laminate behavior, and moisture and temperature effects on the mixed-mode delamination fracture of unidirectional graphite/epoxy.

Mechanical properties and biomechanical compatibility of porous titanium for dental implants.

Abstract: Titanium powder with a granule diameter of 420–500 μm was prepared and porous titanium specimens were made from this powder. The mechanical properties of these specimens were examined. The compressive strength and low cyclic compressive fatigue strength were 182 and 40 MPa, respectively. Fractography was also observed by scanning electron microscopy. Typical fatigue characteristics of the bonding areas of the powder were observed. In addition, porous-titanium-coated dental implants with pure titanium cores were prepared. The compressive strength of the material used was 230 MPa, fatigue strength not being improved. Biomechanical stress calculations using the finite element method were made using a model that employed the use of the material implanted in alveolar bone. Shear stress at the implant–bone interface as well as compressive stress concentrations in the bone was calculated. The most suitable elastic modulus for the dental implant was then estimated from these calculations. Finally, based on these results, the use of porous titanium for dental implants was assessed.

Stress corrosion cracking of stainless steels

Abstract: The similarities and differences in the stress corrosion cracking response of ferritic and austenitic stainless steels in chloride solutions will be examined. Both classes of materials exhibit a cracking potential: similar transient response (to loading) of the potential in open circuit tests or the current in potentiostatic tests and similar enrichment of chromium and depletion of iron in the film associated with localized corrosion processes. The ferritic steels are more resistant to localized corrosion than are the austenitic steels, which is responsible for the difference in the influence of prior thermal and mechanical history on cracking susceptibility of the two types of steel. Similarities in the fractography of stress corrosion cracks and those produced by brittle delayed failure during cathodic charging of the ferritic steels indicate that hydrogen embrittlement is involved in the failure process.

Effect of silicide precipitation on tensile properties and fracture of alloy Ti-6Al-5Zr-0.5Mo-0.25Si

Abstract: The effect of silicon in solid solution and in the form of suicides has been studied on the tensile properties and fracture behavior of alloy Ti-6Al-5Zr-0.5Mo-0.25Si (alloy 685). The heat treatment to hold silicon in solid solution consists of solutionizing at 1323 K for 0.5 hour under vacuum (∼10-5 MPa), followed by water quenching, and the treatment to precipitate suicides involves subsequent aging of the solutionized and water quenched specimens at 1073 K for 24 hours. There is only marginal effect of the aging treatment on strength values; however, the ductility parameters are found to be drastically reduced. There are marked differences in the fracture behavior of the alloy in the as-quenched and the quenched and aged conditions. While the fracture surface of the unaged specimen shows characteristic dimples, there is a large number of facets on the fracture surface of the aged specimen. The facets in the central region are relatively smaller in size than those in the peripheral zone. The central facets show fluted features at higher magnifications; however, the peripheral facets are usually featureless. The faceted fracture in the aged condition is attributed to enhanced tendency for heterogeneous planar slip. The fracture characteristics correlate with the observed differences in the ductility in the two conditions.

Fractography of the interlaminar fracture of carbon-fibre epoxy composites

Abstract: The failed surfaces of interlaminar fracture (mode I) specimens of AS4/3501-6 were examined using scanning electron microscopy. The principle fracture features were fibre pull-out (bundles and single fibres), hackle markings and regions of smooth resin fracture. Considerable (30 to 50%) relaxation of the deformed resin occurred when the specimens were heated above the matrix T g. This relaxation was taken as evidence of extensive shear yielding of the resin during the fracture process. Some of the fractography features are discussed in terms of transverse tensile stresses and peeling stresses acting on the fibres. In some instances these localized stresses focus failure close to the resin-fibre interface which can be mistakenly interpreted as interfacial failure and low fibre-resin adhesion.

Stress corrosion cracking of carbon steel in caustic aluminate solutions—crack propagation studies

Abstract: Stress corrosion cracking of a commercial 0.19 pct C steel (SA-516 Grade 70) was studied in hot (92 ‡C) caustic solutions of NaOH and NaOH plus aluminate (AlO− 2) species. Potentiostatically controlled tests were conducted near the active-passive transition, using fracture mechanics testing techniques and fatigue precracked double cantilever beam specimens. Crack propagation rates (Ν) were determined for a range of stress intensities (K l). In simple NaOH solutions, Region I (K1-dependent) and Region II (K 1-independent) cracking behavior were observed. Increasing the concentration of NaOH from2m to8m decreasedK ISCC and displaced Region I and the onset of Region II to lowerK 1 levels. The presence of AlO− 2 produced a comparable effect, with Region II being extended to lowerK I -Ν levels relative to simple NaOH solutions of similar hydroxyl anion concentration. The overallK I -Νv behavior and fractography were consistent with a dissolution mechanism of crack advance based on the general principles of the film rupture-dissolution model. The effect of environment composition uponK I — Ν behavior was attributed to changes in repassivation kinetics.

Effect of Crystallites on Subcritical Crack Growth and Strain‐Rate Sensitivity of Strength of Cordierite Glass‐Ceramics

Abstract: A study was conducted of the subcritical crack growth in a series of cordierite glass-ceramics, including the original glass, by means of the double-torsion method and measurement of the strain-rate sensitivity of the strength of specimens with controlled indentation cracks subjected to biaxial flexure. The double-torsion data showed that the same rate of crack growth required a stress intensity factor for the fully crystallized glass-ceramic about 3 times higher than for the original glass, accompanied by a simultaneous increase in the stress intensity exponent (N). The suppression in the rate of crack growth in the crystallized material at any value of stress intensity factor was attributed to enhanced crack deflection around the crystallites. Limited observations suggested that the increase in N on crystallite formation may be due to a greater degree of crack deflection at the lower crack velocities than at the higher velocities. For strength specimens of the as-received and crystallized glass with large surface flaws, the N values obtained by the strain-rate measurements and the double-torsion test showed good agreement. However, for the fully crystallized material with surface flaws too small to interact significantly with the coarser crystallites, the N value for the strain-rate data was well below the value for the double-torsion method. Indentation-fracture data of the double-torsion specimens confirmed the existence of crack-size dependent fracture toughness. Limited fracture toughness data coupled with SEM fractography indicated that with increasing crystallite size toughening occurs by crack deflection as well as by microcrack toughening counteracted by fracture through the crystallites.

Fatigue fracture of short‐glass‐fiber‐reinforced poly(vinyl chloride): A crack layer approach

Abstract: The fatigue behavior and fracture toughness of injection molded short-glass-fiber-reinforced poly(vinyl chloride) (sgfr-PVC) were investigated using the Crack Layer approach and fractography, Fatigue crack propagation (FCP) experiments in single-edge-notched (SEN) specimens were conducted concurrently with microscopic observations Fracture was observed to propagate as a main crack surrounded by a layer of damage The magnitude of damage was controlled by the content of glass fiber, which in turn controlled crack reduced acceleration and fracture toughness FCP behavior was successfully described by the Crack Layer theory, which accounts for the damage associated with crack propagation In absence of significant interfacial bonding, mechanical fiber/matrix interlocking provided the main resistance to crack propagation Fiber-induced matrix deformation and fiber pull-out appeared to be the dominant energy absorbing mechanisms

3-dimensional image analysis of fatigue fracture surface.

Abstract: Computer image processing technology has been applied to fracture surface analysis, and the generalized software for 3-dimensional fractography image analysis has been developed. By this software, it became possible to obtain 3-dimensional fracture surface topography from a stereo-pair of SEM fractograph with a sufficient precision. The fatigue fracture surface roughness of a hightensile strenght steel HT 55 in air and in vacuum was calculated, and the root mean square roughness of fracture surface in vacuum was made clear to be greater than that in air at R=-1, compared with those at R=0.1 and 0.5. The wedge effect induced by corrosion products was also clearly demonstrated on a matching-pair of corrosion-fatigue fracture surface topography of HT 55.

Effect of aging on internal stress and fatigue fracture of poly(4-methyl pentene-1)

Abstract: The fatigue fracture behaviour of injection-molded poly(4 methyl pentene-1) has been investigated. Crack growth rate experiments have been conducted on notched specimens after room temperature aging, after annealing at 175°C, and after prolonged cyclic loading prior to notching. Residual stress profiles have been determined for specimens in each of these states, and the crack growth data have been discussed with reference to differences in residual stresses. Annealing caused the reversal of the sense of the residual stresses, and a significant change in stress distribution was found for specimens freshly removed from prolonged storage at −18°C. Scanning electron microscopy of fracture surfaces revealed fibrillar features essentially similar to those found with other semicrystalline polymers.

Relationship between fracture topography and fracture toughness of a high strength steel

Abstract: The relationship between fracture topography and the plane strain fracture toughness of Comsteel En25 tempered at nine different temperatures and with crack planes in the R-L, R-C and L-R orientations has been studied. The results show that fracture toughness is qualitatively relatable to the fracture morphology. Due to the shape and alignment of the elongated MnS inclusions, fracture toughness in the L-R orientation was found to be 1.8 times those in the other two orientations. Terrace-type fracture prevailed in the R-L orientation, but this frequently was observed to change to zigzag type fracture in the R-C orientation. However, both these fracture mechanisms were absent in the L-R orientation.

Electron‐beam‐induced fracture of polymers

Abstract: When a notched polymeric material is stressed, the notch opens into a wide crack tip, exposing a region of high stress concentration. The consequences of electron bombardment of the tip of such a stressed material under vacuum are explored here for the first time. Evidence is presented for electron-induced crack growth at stress far below that needed for crack growth due to stress alone. The electron current densities used in these experiments are sufficiently small that thermal heating of the zone near the crack tip is minimal. To provide information on the phenomena involved, we present simultaneous measurements of electron current, gas pressure, and sample load in response to periodic bombardment of the sample. Experiments involving the bombardment of un-notched polymers under stress are also described. Fractography of the unique structures obtained by fracture due to the combination of electron bombardment and stress are presented and interpreted in terms of a crosslinking mechanism.

Relationship of the spacing of the fatigue striations to the range in the stress intensity factor

Abstract: The results of investigations of the spacing of the fatigue striations were studied for the purpose of establishment of agreement of the fatigue crack growth macrorate with the microrate. It was established that for steels, aluminum, titanium, and magnesium alloys and other materials the range of agreement of the fatigue crack growth macro- and microrates is limited to rates of (10-/sup 7/-10/sup -6/) in the majority of cases.

Cyclic fatigue of sintered silicon nitride at room temperature.

Abstract: The cyclic fatigue behavior of sintered silicon nitride was investigated at room temperature. Flexure specimens, with an indentation induced flaw at the center of the reduced gage section, were tested, where the cyclic load was applied by four-point bending up to frequencies of 10Hz using an electrohydraulic testing machine.Sintered silicon nitride showed a susceptibility to cyclic fatigue, and its lifetime decreased with increasing maximum applied stress. An apparent fatigue limit was about 70% of the flexural strength. A couple of crack arrest position marks around the indentation were detected in the fracture surfaces of both flexure and fatigue test specimens, and the size of each mark well corresponded with the analytical result based on a contact residual stress. Frequency did not affect the lowest limit of the fatigue lifetime, although a scatter of the data and the highest limit of the life-time increased as the frequency was lowered from 10Hz to 0.01Hz. High magnification fractography revealed an intergranular dominant fatigue failure with partial transgranular failure at perpendicularly elongated crystals. It suggests that the intergranular fatigue crack can be arrested at grain boundary triplets, and also can be reactivated by subsequent cyclic loadings.The crack growth rate calculated from the fatigue lifetime showed a three region characteristics having a plateau at 70 to 90% of the fracture toughness, which suggests the possible intergranular stress corrosion cracking mechanism resembling that in glass or alumina.

Variation of the fracture mode in temper embrittled 2.25 Cr-1 Mo steel

Abstract: Temper embrittled 2.25 Cr-1 Mo steel was tested by slow bending of notched specimens at various temperatures, and the fracture mode was examined by SEM fractography. Comparison of the local fracture mode with the load-displacement curves showed that intergranular fracture occurred most prominently in the region where cracking initiated, but that the fracture mode tended to change to cleavage as the cracking propagated and accelerated. When the area fraction of intergranular fracture was plotted as a function of test temperature, a maximum appeared, and the temperature of this maximum tended to increase with specimen hardness. It is argued that the gap between the cleavage fracture stress (σFCL) and that of intergranular fracture (σFIG) was greatest at some particular temperature, allowing a maximum amount of grain boundary fracture. However, the gap (σFCL-σFIG) diminished as cracking accelerated, and the fracture mode tended to switch to cleavage. The contrast in behavior between temper embrittled CrMo and NiCr steels is discussed.