Showing papers on "Fractography published in 2000"

On the mechanism of fatigue failure in the superlong life regime (N>107 cycles). Part 1: influence of hydrogen trapped by inclusions

Abstract: The fracture surfaces of specimens of a heat-treated hard steel, namely Cr-Mo steel SCM435, which failed in the regime of N= 10 5 to 5 x 10 8 cycles, were investigated by optical microscopy and scanning electron microscopy (SEM). Specimens having a longer fatigue life had a particular morphology beside the inclusion at the fracture origin. The particular morphology looked optically dark when observed by an optical microscope and it was named the optically dark area (ODA). The ODA looks a rough area when observed by SEM and atomic force microscope (AFM). The relative size of the ODA to the size of the inclusion at the fracture origin increases with increase in fatigue life. Thus, the ODA is considered to have a crucial role in the mechanism of superlong fatigue failure. It has been assumed that the ODA is made by the cyclic fatigue stress and the synergetic effect of the hydrogen which is trapped by the inclusion at the fracture origin. To verify this hypothesis, in addition to conventionally heat-treated specimens (specimen QT, i.e. quenched and tempered), specimens annealed at 300 °C in a vacuum (specimen VA) and the specimens quenched in a vacuum (specimen VQ) were prepared to remove the hydrogen trapped by inclusions. The specimens VA and VQ, had a much smaller ODA than the specimen QT. Some other evidence of the influence of hydrogen on superlong fatigue failure are also presented. Thus, it is concluded that the hydrogen trapped by inclusions is a crucial factor which causes the superlong fatigue failure of high strength steels.

Impact damage characterisation of carbon fibre/epoxy composites with multi-layer reinforcement

Abstract: Low-velocity impact tests were performed to investigate the impact behaviour of carbon fibre/epoxy composite laminates reinforced by short fibres and other interleaving materials. Characterisation techniques, such as cross-sectional fractography and scanning acoustic microscopy, were employed quantitatively to assess the internal damage of some composite laminates at the sub-surface under impact. Scanning electron microscopy was used to observe impact fractures and damage modes at the fracture surfaces of the laminate specimens. The results show that composite laminates experience various types of fracture; delamination, intra-ply cracking, matrix cracking, fibre breakage and damage depending on the interlayer materials. The trade-off between impact resistance and residual strength is minimised for composites reinforced by Zylon fibres, while that for composites interleaved by poly(ethylene-co-acrylic acid) (PEEA) film is substantial because of deteriorating residual strength, even though the damaged area is significantly reduced. Damages produced on the front and back surfaces of impact were also observed and compared for some laminates.

Fracture and impact behaviours of hollow micro-sphere/epoxy resin composites

Abstract: Fracture and impact behaviours of glass-hollow micro-sphere/epoxy resin composites are studied in terms of fracture toughness, fractography, flexural properties and impact force. Volume fraction of micro-spheres for the composites was varied up to 0.65. The addition of micro-spheres did not enhance the fracture toughness of the composites despite the presence of pinning mechanism. Performance of reducing the impact force was enhanced as the content of micro-spheres increased but at the expense of other properties. (a) For the covering entry of this conference, please see ITRD abstract no. E204495.

Evolution of defect size and strength of porous alumina during sintering

Abstract: The evolution of fracture strength with increasing density in ceramics, using alumina as a model system, is discussed in terms of the interplay between a defect serving as stress concentrator, a crack lying in its enhanced stress field and the fracture toughness of the porous ceramic. Introduction of crack-free fracture-causing artificial pores of various sizes allows detailed measurement of their shrinkage with ongoing densification, while fractography describes the location and type of fracture initiation. A fracture mechanics model, describing growth of a semicircular crack emanating from the pore until instability, yields good agreement with experiment. In particular, the result that the radius of the artificial, spherical defect in a size regime between 25 and 120 μm has only a small influence on fracture strength for samples with an average grain size smaller than 1μm, can be explained.

The Mechanism of Brittle Fracture in a Microalloyed Steel: Part I. Inclusion-Induced Cleavage

Abstract: The cleavage resistance of two microalloyed steels (steels A and B) was studied using several tests, including the instrumented precracked Charpy and Charpy V-notch (CVN) techniques. Ductile-to-brittle transition temperatures were measured for the base-metal and simulated heat-affected zone (HAZ) microstructures. Steel B showed inferior cleavage resistance to steel A, and this could not be explained by differences in gross microstructure. Scanning electron fractography revealed that TiN inclusions were responsible for cleavage initiation in steel B. These inclusions were well bonded to the ferritic matrix. It is believed that a strong inclusion-matrix bond is a key factor in why TiN inclusions are potent cleavage initiators in steel. Strong bonding allows high stresses in a crack/notch-tip plastic zone to act on the inclusions without debonding the interface. Once an inclusion cleaves, the strong bond allows for transfer of the TiN crack into the ferritic matrix. It was estimated that only 0.0016 wt pct Ti was tied up in the offending inclusions in steel B. This indicates that extended times at high temperatures during the casting of such steels could produce TiN-related toughness deterioration at even modest Ti contents.

Mechanical Properties of La1‐xSrxCo0.2Fe0.8O3 Mixed‐Conducting Perovskites Made by the Combustion Synthesis Technique

Abstract: This paper examined the room-temperature mechanical properties of a mixed-conducting perovskite La 1-x Sr x Co 0.2 Fe 0.8 O 3 (x = 0.2-0.8). Powders were made by the combustion synthesis technique and sintered at 1250°C in air. Sintered density, crystal phase, and grain size were characterized. Young's and shear moduli, microhardness, indentation fracture toughness, and biaxial flexure strength were determined. The Young's and shear moduli slightly increased with increasing strontium content. Young's modulus of 151-188 GPa and shear modulus of 57-75 GPa were measured. Biaxial flexure strength of ∼160 MPa was measured for lower strontium content batches. Strength greatly decreased to ∼40 MPa at higher strontium concentrations (x = 0.6-0.8) because of the formation of extensive cracking. Indentation toughness showed a higher value (∼1.5 MPa.m 1/2 ) for low strontium (x = 0.2) content and a lower value (∼1.1 MPa.m 1/2 ) for the other batches (x = 0.4-0.8). Materials with fine and coarse grain size were also tested at various indent loads and showed no dependence of toughness on crack size. In addition, fractography was used to characterize the critical flaw and fracture mode.

Effects of martensite morphology and volume fraction on quasi-static and dynamic deformation behavior of dual-phase steels

Abstract: The effects of martensite morphology and volume fraction on the quasi-static and dynamic deformation behavior of dual-phase steels were investigated in this study. Quasi-static and dynamic torsional tests were conducted using a torsional Kolsky bar for four steel specimens, which had different martensite morphology and volume fraction, and then the test data were compared via microstructures, tensile properties, and fracture mode. In the intermediate quenched (IQ) steel specimens, very fine fibrous martensites were well distributed in the ferrite matrix, but bulky martensites were mixed with ferrites in the step quenched (SQ) specimens. Quasi-static torsional properties were similar to tensile properties, and fracture occurred in a ductile mode in IQ specimens, whereas cleavage fracture was predominated in SQ specimens. Under a dynamic loading condition, the fracture mode of SQ specimens was changed from cleavage to ductile fracture, whereas IQ specimens had a ductile fracture mode, irrespective of loading rate. These phenomena were analyzed using a shear lag model, phase continuity, and the thermal softening effect of martensite.

The high cycle fatigue and fracture behavior of aluminum alloy 7055

Abstract: This paper highlights the results of a study on the high-cycle fatigue, deformation and fracture behavior of aluminum alloy 7055. Specimens of the alloy, in the T7751 temper, were cyclically deformed over a range of stress amplitudes at both ambient and elevated temperatures. While an increase in test temperature was found to have a detrimental influence on cyclic fatigue life of the transverse orientation specimens, little influence was found on the longitudinally oriented specimens. The macroscopic fracture mode was essentially identical regardless of the orientation of the test specimen with respect to the wrought rolled plate. Cyclic fatigue fracture, on a microscopic scale, revealed features reminiscent of locally ductile and brittle mechanisms. The microscopic fracture behavior was a function of test temperature. The mechanisms governing cyclic fatigue life and fracture behavior are discussed in light of the mutually interactive influences of microstructural effects, matrix deformation characteristics and test temperature.

Toughness anisotropy and damage behavior of plasma sprayed ZrO2 thermal barrier coatings

Abstract: This paper shows how the fracture properties of thermal barrier coatings (TBCs) can be determined for different crack orientations, and demonstrates the complex interaction between these properties during coating failure. Atmospheric plasma-sprayed ZrO2 coatings removed from the substrate were broken in three-point bending using micro-bending test equipment. Linear elastic fracture mechanics was used to calculate the toughness of a macroscopic through-thickness crack as a function of crack length. A strong R-curve was identified. The problem of using linear elastic fracture mechanics is addressed. Additionally the work of fracture of delamination cracks which propagate parallel to the interface was measured. Comparison of the work of fracture of delamination and through-thickness cracks showed a strong anisotropy. Coatings were annealed at different temperatures to investigate aging effects on the properties. The critical energy release rate of through-thickness cracks increases with annealing temperature while the work of fracture of delamination cracks decreases. A finite element calculation was performed to simulate the state of stress in a coating system for a typical gas turbine application. Using time-dependent safety maps the interaction between through-thickness cracking and coating delamination is shown. If the ratio between the critical energy release rates for the two species of cracks is favorable, segmentation of coating takes place prior to delamination, which thus can be prevented by the reduction of strain energy in the coating. The influence of aging effects and creep deformation on coating failure is discussed.

Fatigue crack-growth in shape-memory NiTi and NiTi-TiC composites

Abstract: An experimental study was conducted to examine the room-temperature fatigue crack-growth characteristics of shape-memory NiTi matrix composites reinforced with 10 and 20 vol.% of TiC particles. Microstructural characterization of these hot-isostatically-pressed materials shows that the TiC particles do not react with the NiTi matrix and that they lack any texture. Overall fatigue crack-growth characteristics were found to be similar for the unreinforced and reinforced materials. However, a slight increase in the threshold for fatigue crack initiation was noted for the composites. The fracture toughness, as indicated by the failure stress intensity factor range, was found to be similar for all materials. Neutron diffraction studies near the crack-tip of the loaded fracture NiTi specimen detected no significant development of texture at the crack-tip. These results are explained by recourse to fractographic observations. Finally, a comparison is made between the micromechanisms of fracture of metal matrix composites, which deform by dislocation plasticity, and those of the present NiTi–TiC composites, which deform additionally by twinning.

Deformation and fracture of a particle-reinforced aluminum alloy composite: Part I. Experiments

Abstract: Mechanical tests were performed on a powder-metallurgically processed 7093/SiC/15p discontinuously reinforced aluminum (DRA) composite in different heat-treatment conditions, to determine the influence of matrix characteristics on the composite response. The work-hardening exponent and the strain to failure varied inversely to the strength, similar to monolithic Al alloys, and this dependence was independent of the dominant damage mode. The damage consisted of SiC particle cracks, interface and near-interface debonds, and matrix rupture inside intense slip bands. Fracture surfaces revealed particle fracture-dominated damage for most of the heat-treatment conditions, including an overaged (OA) condition that exhibited a combination of precipitates at the interface and a precipitate-free zone (PFZ) in the immediate vicinity. In the highly OA conditions and in a 450 °C as-rolled condition, when the composite strength became less than 400 MPa, near-interface matrix rupture became dominant. A combination of a relatively weak matrix and a weak zone around the particle likely contributed to this damage mode over that of particle fracture. Fracture-toughness tests show that it is important to maintain a proper geometry and testing procedure to obtain valid fracture-toughness data. Overaged microstructures did reveal a recovery of fracture toughness as compared to the peak-aged (PA) condition, unlike the lack of toughness recovery reported earlier for a similar 7XXX (Al-Zn-Cu-Mg)-based DRA. The PA material exhibited extensive localization of damage and plasticity. The low toughness of the DRA in this PA condition is explored in detail, using fractography and metallography. The damage and fracture micromechanisms formed the basis for modeling the strength, elongation, toughness, and damage, which are described in Part II of this work.

A study on fatigue crack growth in dual phase martensitic steel in air environment

Abstract: Dual phase (DP) steel was intercritically annealed at different temperatures from fully martensitic state to achieve martensite plus ferrite, microstructures with martensite contents in the range of 32 to 76%.Fatigue crack growth (FCG) and fracture toughness tests were carried out as per ASTM standards E 647 and E 399, respectively to evaluate the potential of DP steels. The crack growth rates (da/dN) at different stress intensity ranges (DK) were determined to obtain the threshold value of stress intensity range (DKth). Crack path morphology was studied to determine the influence of microstructure on crack growth characteristics. After the examination of crack tortuosity, the compact tension (CT) specimens were pulled in static mode to determine fracture toughness values. FCG rates decreased and threshold values increased with increase in vol.% martensite in the DP steel. This is attributed to the lower carbon content in the martensite formed at higher intercritical annealing (ICA) temperatures, causing retardation of crack growth rate by crack tip blunting and/or deflection. Roughness induced crack closure was also found to contribute to the improved crack growth resistance at higher levels of martensite content. Scanning electron fractography of DP steel in the near threshold region revealed transgranular cleavage fracture with secondary cracking. Results indicate the possibility that the DP steels may be treated to obtain an excellent combination of strength and fatigue properties.

New features of the low temperature ductile shear failure observed in bulk amorphous alloys

Abstract: Fractographic studies of ductile shear failure under the uniaxial compression for rod–like samples of the Zr41.2Ti13.8Ni10Cu12.5Be22.5 and Cu50Zr35Ti8Hf5Ni2 bulk amorphous alloys at temperatures 300 and 77 K are presented. The mechanisms of shear deformation and failure appeared to have characteristics in common with other amorphous alloys prepared in the form of thin ribbons. However, there were a number of new fractographic features observed due to the bulk character of the samples and to the large supercooled liquid region of these alloys.

Microstructural effects on fracture toughness in AA7010 plate

Abstract: The influence of recrystallization and quench rate after solution treatment on the fracture toughness of 7010 aluminum plate has been studied in longitudinal-transverse (L-T) and short-longitudinal (S-L) orientations for T76-type heat treatments. Extensive fractographic analysis was carried out to identify the failure mechanisms, including simultaneous scanning electron microscope (SEM) observation of fracture surfaces and underlying microstructures. A slow quench rate was strongly detrimental because it modified the dominant failure mode from a relatively high energy primary void growth mechanism to lower energy transgranular shear and grain boundary ductile failure in the L-T and S-L orientations, respectively. Low energy failure was associated with coarse ν precipitation during the quench in both L-T and S-L orientation tests, with intragranular and intersubgranular particles contributing to L-T quench sensitivity, and intergranular particles contributing to S-L sensitivity. Partial recrystallization was generally detrimental, with recrystallized grains being shown to be a preferential crack path. The commonly supposed susceptibility of recrystallized grains to intergranular failure did not explain this behavior, particularly in fast quench materials, as recrystallized grains primarily failed by transgranular void growth from the large intermetallics with which they were intrinsically associated. Exceptional S-L orientation quench sensitivity was observed in unrecrystallized material and attributed to a synergistic interaction between heterogeneous boundary precipitation and the specific location of coarse intermetallics along grain boundaries in the unrecrystallized condition. Quantitative assessment of individual contributions to overall fracture resistance is discussed for cases where multiple failure mechanisms occur, highlighting the importance of interacting and noninteracting mechanisms.

Influence of nonmetallic inclusion characteristics on the mechanical properties of rail steel

Abstract: An extensive investigation has been carried out on six commercial heats of pearlitic rail steel to study the influence of nonmetallic inclusion characteristics on the tensile, fatigue, and fracture toughness properties. The steels investigated were made through the basic oxygen furnace (BOF)-continuous casting route and rolled in the rail and structural mill into 90 kg/mm2 ultimate tensile strength (UTS) grade rails. While tensile properties (yield strength [YS], UTS, and elongation) of the rail steels investigated were found to be insensitive to inclusion type and volume fraction at their present level (0.23 to 0.45%), the fracture toughness and high-cycle fatigue properties were found to be inclusion sensitive. The fracture toughness values of the steels were found to range between 42.33 and 49.88 MPa √m; higher values, in general, were obtained in heats exhibiting lower volume fractions (0.15 to 0.19%) of sulfide inclusions. The high-cycle fatigue limit, i.e., stress corresponding to 107 cycles, was found to be higher in cleaner steels, particularly in those with lower volume fractions of oxide inclusions. This phenomenon was corroborated by scanning electron microscopy (SEM) observations of fracture surfaces, where oxide inclusions in particular were found to be instrumental in crack initiation. Although fatigue life did not show any direct correlation with the volume fraction of sulfides, elongated MnS inclusions were sometimes observed at crack initiation sites of fatigue-tested specimens.

Effect of stress ratio on fatigue crack growth in TiAl intermetallics at room and elevated temperatures

Abstract: The fatigue crack growth behavior of γ-based titanium aluminides (TiAl) with a fine duplex structure and lamellar structure has been investigated by scanning electron microscope (SEM) in situ observation in vacuum at 750°C and room temperature. For the duplex structured material the fatigue crack growth rates are dominated by the maximum stress intensity, particularly at 750°C. The threshold stress intensity range for fatigue crack growth at 750°C is lower than that at room temperature for any corresponding stress ratio. The fatigue crack growth rate at 750°C is affected by creep deformation in front of the crack tip. The severe crack blunting occurs when the stress ratio is 0.5. For the lamellar structured material the scatter of fatigue crack growth data is very large. Small cracks propagate at the stress intensity range below the threshold for long fatigue crack growth. The effects of microstructure on fatigue crack growth are discussed.

The separation of the fracture energy in metallic materials

Abstract: The total plastic strain energy which is consumed during fracture of a plain-sided CT specimen is separated into several components These are the energies required for deforming the specimen until the point of fracture initiation, for forming the flat-fracture surfaces, for forming the shear-lip fracture surfaces, and for the lateral contraction and the blunting at the side-surfaces, W lat Characteristic crack growth resistance terms, R flat and R slant, are determined describing the energies dissipated in a unit area of flat-fracture and slant-fracture surface, respectively R flat is further subdivided into the term R surf, to form the micro-ductile fracture surface, and into the subsurface term, R sub, which produces the global crack opening angle Two different approaches are used to determine the fracture energy components The first approach is a single-specimen technique for recording the total crack growth resistance (also called energy dissipation rate) Plain-sided and side-grooved specimens are tested The second approach rests on the fact that the local plastic deformation energy can be evaluated from the shape of the fracture surfaces A digital image analysis system is used to generate height models from stereophotograms of corresponding fracture surface regions on the two specimen halves Two materials are investigated: a solution annealed maraging steel V 720 and a nitrogen alloyed ferritic-austenitic duplex steel A 905 For the steel V 720 the following values are measured: J i=65 kJ/m2, R surf=20 kJ/m2, R flat=280 kJ/m2, R slant=1000 kJ/m2, W lat=30 J For the steel A 905 which has no shear lips, the measured values are: J i=190 kJ/m2, R flat=1000 kJ/m2, and W lat=45 J Apart from materials characterization, these values could be useful for predicting the influence of specimen geometry and size on the crack growth resistance curves Key words: Elastic-plastic fracture mechanics, fracture energy, energy dissipation rate, fracture surface analysis

Fracture toughness of high-chromium white irons: Influence of cast structure

Abstract: The fracture toughness of two high-chromium white iron alloys in the as-cast condition has been investigated. Fracture toughness test pieces were extracted at various orientations relative to the columnar macrostructure. The toughness of a 27 Cr white iron alloy was very sensitive to orientation, and the toughness was much larger when the crack propagated in a direction perpendicular to the long dimension of the eutectic carbides. The toughness of the 15-3 Cr-Mo white iron was insensitive to the orientation. The different fracture behaviour of the two alloys was related to the anisotropy of the eutectic carbide structures in the as-cast material. The limitations in applying quantitative data on the eutectic carbide structure to models of fracture toughness in white iron alloys were illustrated.

Influence of nanometre‐sized notch and water on the fracture behaviour of single crystal silicon microelements

Abstract: The influence of a notch and a water environment on the quasi-static and fatigue fracture behaviour was investigated in single crystal silicon microelements The tests were conducted in smooth and notched microcantilever beam samples Smooth specimens were prepared by micromachining (photo-etching) of (110) silicon wafers For some specimens, a nanometre-sized notch was machined 100 pm away from the sample root by using a focused ion beam system A machining condition was optimized, and the V-shaped notch was successfully introduced The radius of curvature of the notch, measured by an atomic force microscope (AFM), decreased with an increase in notch depth, and ranged from about 20 to 100 nm Single-crystal Si microelements deformed elastically until final failure, which was of a brittle nature The maximum fracture strength of a smooth microcantilever specimen reached about 77 GPa, which was higher than that obtained in millimetre-sized single crystal Si samples However, the fracture strength decreased with an increase in notch depth, even though the notch depth was of the order of a nanometre, This means that a nanometre deep notch, which is often regarded as surface roughness in ordinary-sized mechanical components, caused a decrease in the fracture strength of Si microelements The fracture initiated at the notch, and then the {111} crack propagated in the direction normal to the sample surface Fatigue tests were also conducted in laboratory air and in pure water at a stress cycle frequency of 01 Hz and a stress ratio of 01 In laboratory air, no fatigue damage was observed even though the surface was nanoscopically examined by an AFM However, when the fatigue tests were conducted in pure water, the fatigue lives in water were decreased Crack formation on the {111} plane was promoted by a synergistic effect of the dynamic loading and the water environment Atomic force microscopy was capable of imaging the nanoscopic cracks, which caused failure in water