Showing papers on "Fractography published in 1987"

Static and Cyclic Fatigue Behavior of a Sintered Silicon Nitride at Room Temperature

Abstract: The static and cyclic fatigue behavior of sintered silicon nitride was investigated at room temperature. Flexure specimens, with an indentation-induced flaw at the center, were tested under a static or cyclic load applied by four-point bending. Sintered silicon nitride was shown to be susceptible to static and cyclic fatigue failure. Comparing the static and cyclic fatigue lifetimes at frequencies from 0.01 to 10 Hz, it was shown that minimum time to failure was almost the same, in spite of differences in loading mode or frequency. However, cyclic stress decreased the scatter in lifetime by reducing the upper limit. Moreover, the cyclic fatigue limit was significantly lower than the static fatigue limit. High-magnification fractography revealed a fatigue failure dominated by intergranular cracking with partial transgranular failure at perpendicularly elongated crystals. This suggests that the intergranular fatigue crack can be arrested at grain-boundary triplets, and also can be reactivated by subsequent cyclic loading. The crack growth rate, calculated from the fatigue lifetime, showed three characteristic regions having a plateau at 70% to 90% of the fracture toughness, which suggests a possible intergranular stress corrosion cracking mechanism resembling that in glass or alumina.

Strength, structure, and fracture properties of ceramic fibers produced from polymeric precursors. I: Base-line studies

Abstract: Classical brittle failure has been observed for ceramic fibers produced by pyrolysis from three different polymer precursors. Characterization by SEM fractography methods has shown typical flaw-mirror-mist-hackle morphology and such study has provided observations for understanding the fracture mechanisms. The gage length dependence of tensile strength combined with the fractography study showed the flaw-controlled failure mechanism of all the ceramic fibers. Systematic failure types have been identified for specific strength ranges. Complementary study of the microstructure and chemical structure by TEM, X-ray diffraction, and electron spectroscopy for chemical analysis (ESCA) has shown the nature of the bulk ceramic to be predominantly random and glassy even in the case of a fiber with microcrystalline β-SiC. Fracture mechanics calculations have shown the fibers all behave as glasses.

Fractography of Modern Engineering Materials: Composites and Metals

Abstract: The fractographic analysis of fracture surfaces in composites and metals is discussed in reviews and reports of recent theoretical and experimental investigations. Topics addressed include fracture-surface micromorphology in engineering solids, SEM fractography of pure and mixed-mode interlaminar fractures in graphite/epoxy composites, determination of crack propagation directions in graphite/epoxy structures, and the fracture surfaces of irradiated composites. Consideration is given to fractographic feature identification and characterization by digital imaging analysis, fractography of pressure-vessel steel weldments, the micromechanisms of major/minor cycle fatigue crack growth in Inconel 718, and fractographic analysis of hydrogen-assisted cracking in alpha-beta Ti alloys.

Neutral molecule emission from the fracture of crystalline MgO

Abstract: We present further measurements of the neutral molecule emission accompanying the fracture of single‐crystal MgO. Comparison is made of the intensities and species of this emission for basically two types of MgO: optically clear and so‐called ‘‘cloudy’’ MgO. The latter material contains voids and precipitates of micron and submicron dimensions. In addition to time resolved mass selected measurements of the released neutral species we also examine the fractrography of the fracture surfaces and infrared spectroscopy of the bulk material. These measurements strongly support a correlation between O2, CO, H2O, and CH4 emission intensities and the presence of dissolved H2O, microscopic voids, and precipitates in the MgO. Segregation of gases in zones near precipitates may be the major source of O2 emission.

Structure, mechanical properties and fracture of aluminium alloy A-356 modified with Al-5Sr master alloy

Abstract: The cooling rate during solidification of the aluminium-silicon alloy A-356 was found to have a pronounced effect not only on the structure and mechanical properties, but also on the modification process using Al-5 mass% Sr master alloy. Cooling rates in the range of 0.2 to 5 K sec−1 were conducted using different types of moulds. Decreasing the cooling rate increases the degree of modification obtained, and a cooling rate of 0.2 K sec−1 showed the optimum modification in the present study. The mechanical properties were significantly improved by modification. Ductility of the modified alloy was eight times greater than that for the non-modified one. Moreover, the mechanical properties of the modified alloys, in the present study, were higher than those modified with sodium, and additionally the ductility of the present modified alloy was higher than that for alloys modified with strontium. A tendency towards ductile rupture was observed. The transgranular type of fracture was optically detected, and a dimple-pattern was observed on the fracture surface by scanning electron microscopy.

Fracture toughness behaviour of ferritic ductile cast iron

Abstract: The static rate fracture toughness of a series of eight heats of ductile cast iron has been measured. Samples from each heat were tested in a heat treated condition which produced a fully ferritic matrix. The dominant influence of carbide (primarily in e pearlitic form) in controling the fracture toughness was thus eliminated in this study. The chemical composition and the microstructural feature size has also been measured directly from each specimen tested. A multiple linear regression method was used to establish a simple mathematical relationship between fracture toughness and the composition and microstructure. Fracture toughness was found to be strongly associated with the spacing (or size) of the graphite nodules in these fully ferritic ductile cast irons. Other features, including the composition, the ferrite grain size, or the amount of graphite (over the ranges examined), did not strongly influence the fracture toughness. Fracture toughness also did not correlate with tensile properties (i.e. strength or ductility) in these alloys. The results of this work can be used to develop an appropriate quality control program for applications which require assurance against fracture toughness related failures.

Fracture-toughness of some metallic glasses

Abstract: The critical stress intensity factor of Fe40Ni40B20, Fe30Cr10Ni40B20, Ni80Si10B10 and Ni80Si5B15 metallic glass ribbons was measured. Stressing by ultrasound, a new method for preparation a sharp crack in a metallic glass, was used. Only shear rupture failures was investigated for all alloys.

Pressure-induced improvement in interfacial bonding between graphite and the aluminium matrix in graphitic-aluminium particle composites

Abstract: Etude et mise en evidence des liaisons interfaciales entre le graphite et la matrice d'aluminium par microscopie

A Fractographic Investigation of the Influence of Stacking Sequence on the Strength of Notched Laminated Composites

Abstract: The fracture behavior of T300/5208 CFRP laminate panels with 12 different combinations of ply orientation and stacking sequence is investigated experimentally, using optical microscopy, SEM, and X-ray radiography to characterize the notch-tip damage zones and fracture surfaces of center-cracked tension specimens subjected to tensile loading at constant crosshead displacement rate 20 micron/s. The results are presented graphically and analyzed in detail. Significant differences in notched strength are found for different ply fiber orientations and stacking sequences; the laminates with few major delaminations had a greater percentage of fracture due to broken fibers and also higher notched strength.

Dynamic fracture instability in glassy polymers as studied by ultrasonic fractography

Abstract: Ultrasonic fractography studies were performed on poly(methyl methacrylate) of high molecular weight. The transient fracture velocity change at the slow-to-fast transition during discontinuous propagation has been measured precisely. Fast fracture starts with a characteristic velocity which falls in a narrow range between 90 to 150 m/s, nearly independent of the loading speeds and the specimen temperature from −50 to 40°C. Parallel double-cantilever-beam specimens exhibited stick-slip type propagation whose velocity change was also evaluated. In these specimens, the fast fracture abruptly slows down to speeds on the order of 10° m/s. These intermediate velocities have never been obtained in the slow-to-fast transition. Velocity measurements under hydrostatic pressure have shown that fracture velocities decrease significantly with increasing pressure, and that the slow-to-fast transition tends to disappear at a pressure between 5 and 10 MPa. Models have been presented concerning the mechanism of the slow-to-fast transition, crazing and cracking under superposed cyclic stress field, and the relationship between dynamic toughness and fracture velocity in this material.

Low-temperature fracture toughness of some iron, nickel-based metallic glass ribbons

Abstract: The critical stress intensity factor of Fe40Ni40B20, Ni80Si10B10 and Ni80Si5B15 metallic glass ribbons was measured at temperatures 42 to 300 K The low-temperature embrittlement for Ni-Si-13 glasses was not investigated The low-temperature embrittlement of Fe40Ni40B20 glass depends on the structural relaxation and is connected with a change from vein pattern fracture surface morphology through chevron morphology up to cleavage

The Differential Displacements and the Failure in Solvent-Welded Lap Joints

Abstract: A recent analytical model for adhesive joints proposed by Yue and Cherry for analysing and predicting the strength of solvent-welded lap joints is examined. The experimental verification of an important assumed basis of the applicability of this model to solvent-welded joints is considered. The differential strain in the composite adhesive layer of the solvent-welded joint was shown to be approximately equal to the differential strain in its final adhesive layer. The differential strain and hence the stress concentration was largest near the edge of the overlap. Fractography suggested that failure of the joint initiated at the edge of the overlap.

Fractographic and acoustic emission of mullite-alumina-zirconia composites prepared by reaction sintering

Abstract: The fracture behaviour of a mullite-alumina-zirconia material prepared by reaction sintering has been studied by fractography and acoustic emission during indentation tests. The acoustic emission characteristics of the mullite composite are compared with those obtained from tetragonal zirconia polycrystal and aluminium oxide tested under the same conditions.

Influence of particulate fillers on the indentation hardness of a glassy cross-linked polymer

Abstract: The indentation hardness of a glassy cross-linked network was decreased to a minimum value by a low volume fraction (0.03 to 0.05) of each of six fillers having rigid particles varying in size and surface. This “minimum” effect was eliminated after specimens were more highly cross-linked by prolonged exposure to γ-rays. These results are consistent with an earlier suggestion that filler particles act as stress concentrators which may cause increased localized plastic deformation and hence a decreased indentation hardness. In the case of larger particles, morphological evidence of localized plastic deformation was obtained by fractography.

The Ductile-to-Brittle Transition Behavior of Martensitic Steels Neutron Irradiated to 26 dpa

Abstract: Charpy impact tests were conducted on specimens made of HT-9 and 9Cr-1Mo in various heat treatment conditions which were irradiated in EBR-II to 26 dpa at 390 to 500/sup 0/C. The results are compared with previous results on specimens irradiated to 13 dpa. HT-9 base metal irradiated at low temperatures showed a small additional increase in ductile brittle transition temperature and a decrease in upper shelf energy from 13 to 26 dpa. No fluence effect was observed in 9Cr-1Mo base metal. The 9Cr-1Mo weldment showed degraded DBTT but improved USE response compared to base metal, contrary to previous findings on HT-9. Significant differences were observed in HT-9 base metal between mill annealed material and normalized and tempered material. The highest DBTT for HT-9 alloys was 50/sup 0/C higher than for the worst case in 9Cr-1Mo alloys. Fractography and hardness measurements were also obtained. Significant differences in fracture appearance were observed in different product forms, although no dependence on fluence was observed. Failure was controlled by the preirradiation microstructure.

Temperature-Dependent Toughening in Whisker-Reinforced Ceramics

Abstract: The mechanical response of whisker-reinforced ceramics, which is dominated by interfacial chemistry-arid stress, is expected to vary as thermal mismatch stresses diminish and girain boundary phases soften with increasing temperature To examine the temperature dependence of the toughening process two silicon carbide whisker-reinforced systems have been studied: alumina and mullite The systems represent a range of thermal mismatch conditions and potential crack-whisker interactions including crack bridging, whisker pull- out and crack deflection For both systems, fracture toughness and hardness are measured as a function of temperature The fracture toughness of both materials shows little change to 1100°C However, post-mortem fractography indicates that limited pullout occurs in the mullite/SiCw system at elevated temperatures Only whiskers oriented normal to the fracture plane are pulled out without fracture due to the additional bending stresses on the whiskers In the alumina/SiCw system, the mode of fracture remains invariant with temperature because of high whisker compressive stresses These observations indicate that for randomly-oriented whiskers substantial toughening by pull-out is contingent upon low interfacial stresses coupled with high strength whiskers which can sustain large bending stresses

Cyclic fatigue behavior of ceramics at room temperature.

Abstract: The cyclic fatigue behavior of different kinds of ceramics was investigated at room temperature. Flexural specimens, with an indentation induced flaw at the center of the reduced gage section, were tested under a cyclic load applied by four-point bending. All ceramics tested were shown to be susceptible to cyclic fatigue failure. The cyclic fatigue degradation compared to its flexural strength increased in this order: hot-pressed AlN, sintered SiC, hot-pressed Si3N4, sintered Si3N4, and sintered Al2O3. The apparent fatigue limit was about 80% of the average flexural strength for AlN, and about 60% for sintered Al2O3. There existed a correlation between the scatter in fatigue and flexural strength. The scatter in fatigue strength was smaller than the scatter in flexural strength. SEM fractography revealed a dominant intergranular fracture in Al2O3 and Si3N4, which suggests a possible stress corrosion cracking mechanism in the grain boundary glassy phase. On the other hand, hot-pressed AlN showed a mixed mode fracture of intergranular and transgranular cracking, and SiC exhibited transgranular cracking only.