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Showing papers on "Fractography published in 1998"


Journal ArticleDOI
TL;DR: In this article, the effects of changes in the notch root radius from 250 m to a fatigue precrack on the fracture toughness of a bulk amorphous glass were determined, and it was shown that the average toughness obtained from 6 fatigue precracked specimens was 18.4 {+-} 1.4 MPa {radical}m, while the notch toughness obtained on specimens with notch root radii ranging from 65 microm-250 microm were in the range of 101--131 MPa{ radical}m.

226 citations


Journal ArticleDOI
TL;DR: In this article, the fracture path of plane-strain fracture-toughness specimens of 7050 alloy (a typical alloy of the 7XXX series) is quantitatively characterized as a function of degree of recrystallization, specimen orientation, and aging condition.
Abstract: The fracture toughness of Al-Zn-Mg-Cu-based 7XXX aluminum alloys decreases with an increase in the extent of recrystallization. In this contribution, the fracture path of plane-strain fracture-toughness specimens of 7050 alloy (a typical alloy of the 7XXX series) is quantitatively characterized as a function of degree of recrystallization, specimen orientation, and aging condition. The fracture path is quantitatively correlated to fracture toughness, and the bulk microstructural attributes estimated via stereological analysis. In the companion article, these quantitative data are used to develop and verify a multiple-fracture micromechanism-based model that relates the fracture toughness to a number of microstructural parameters of the partially recrystallized alloy plate.

131 citations


Journal ArticleDOI
TL;DR: In this article, the effects of Y, Sr, and Nd additions on the microstructure and microfracture mechanism of the four squeeze-cast magnesium alloys based on the commercial AZ91 alloy were investigated.
Abstract: This study aims to investigate the effects of Y, Sr, and Nd additions on the microstructure and microfracture mechanism of the four squeeze-cast magnesium alloys based on the commercial AZ91 alloy. Microstructural observation, in situ fracture tests, and fractographic observation were conducted on the alloys to clarify the microfracture process. Microstructural analyses indicated that grain refinement could be achieved by small additions of alloying elements, although the discontinuously precipitated Mg17Al12 phases still existed on grain boundaries. From in situ fracture observation of an AZ91-Sr alloy, it was seen that coarse needle-shaped compound particles and Mg17Al12 phases located on the grain boundary provided easy intergranular fracture sites under low stress intensity factor levels, resulting in the drop in toughness. On the other hand, the AZ91-Y and AZ91-Nd alloys showed improved fracture toughness, since deformation and fracture paths proceeded into grains rather than to grain boundaries, as the planar slip bands and twinnings actively developed inside the grains. These findings suggested, on the basis of the well-developed planar slip bands and twinnings, that the small addition of Y or Nd was very effective in improving fracture toughness.

116 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of ternary additions such as V, Cu, Nb, Zr and Mn as well as the deviation from the stoichiometric composition to the Al-rich composition on mechanical properties of TiAl was studied at room temperature (RT) in air, and RT, 873 and 1073 K in vacuum.

91 citations


Journal ArticleDOI
TL;DR: The creep properties of specimens taken from the core of AZ91D magnesium alloy ingots were investigated in the temperature range 120-180°C and stress range 40-115 MPa.
Abstract: Creep properties of specimens taken from the core of AZ91D magnesium alloy (9% Al–1% Zn) ingots were investigated in the temperature range 120–180°C and stress range 40–115 MPa. Creep tests were performed under constant and varying loads, solution treated creep specimens were also tested. Observed creep rates are about three orders of magnitude lower than those of pure magnesium and elongations to fracture are at least twice those of pure magnesium. No steady-state stage is observed, however a minimum creep rate is reached after approximately two thirds of the creep life of the specimens. The stress exponent is ≈11 and is independent of temperature and stress. The activation energy decreases with increasing temperature, from 220 to 94 kJ mol −1 . Fractography studies show that the fracture is intergranular.

88 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of the non-uniform distribution of the SiC particle volume fraction and aspect ratio in the matrix is considered for the evaluation of the stress-strain relationship and the damage.

80 citations


Journal ArticleDOI
Abstract: The translaminate fracture behavior of carbon/epoxy structural laminates with through-penetration notches was investigated to develop a residual strength prediction methodology for composite structures. An experimental characterization of several composite materials systems revealed a fracture resistance behavior that was very similar to the R-curve behavior exhibited by ductile metals. Fractographic examinations led to the postulate that the damage growth resistance was primarily due to fractured fibers in the principal load-carrying plies being bridged by intact fibers of the adjacent plies. The load transfer associated with this bridging mechanism suggest that a progressive damage analysis methodology will be appropriate for predicting the residual strength of laminates with through-penetration notches. A progressive damage methodology developed by the authors was used to predict the initiation and growth of matrix cracks and fiber fracture. Most of the residual strength predictions for different panel widths. notch lengths, and material systems were within about 10 percent of the experiemntal failure loads.

70 citations


Journal ArticleDOI
TL;DR: A reciprocating extrusion process has been developed for producing hypereutectic Al-20wt.% Si alloys with fine and uniform microstructures and superior properties as discussed by the authors.
Abstract: A reciprocating extrusion process has been developed for producing hypereutectic Al–20wt.% Si alloys with fine and uniform microstructures and superior properties. Rapidly-solidified layers and cast billets are used as the starting materials. The results show that the interfaces between the layers of the rapidly-solidified alloys could be fully eliminated, and strength and ductility are both improved as the number of extrusion passes increases. For the ingot-processed alloys, the plate-like Si phase in the eutectic and enormous primary Si crystals are refined to a large extent and thus both strength and ductility are increased by 9 and 67%, respectively. Because of having finer Si particles, the rapidly-solidified alloys exhibit a superior combination of mechanical properties to that of the ingot-processed alloys. Fractography demonstrates that the larger Si particles in the ingot-processed material increased the amount of cleavage failure processes.

70 citations


01 Jan 1998
TL;DR: In this article, the authors used fractographic evidence to analyze prediction problems offatigue crack growth under variable-amplitude (VA) loading and found that the predicted growth rate per spectrum block can still agree with the experimental value.
Abstract: Fatigue crack growth tests were carried out on 2024-T3 and 7075-T6 central cracked specimens. Variable-amplitude (VA) load spectra were used with periodic overload (OL) cycles added to constant-amplitude (CA) cycIes. The fatigue fracture surfaces were examined in the SEM to obtain more detailed information on crack growth contributions of different load cycles. The striation pattems could be related to the load histories. SEM observations were associated with delayed retardation, the effect of 10 or a single OL on retardation, crack growth during the OL cycIes, and crack growth arrest after a high peak load. Fractographs exhibited local scatter of crack growth rates and sometimes a rather tortuous 3d geometry of the crack front. Indications of structural sensitive crack grwoth under VA loading were obtained. Fractography appears to be inadmissable for the evaluation of fatigue crack growth prediction models in view of similarities and dissimilarities between crack growth and V A and CA loading. Nomenclature a BL cycles CA-loading daidN OL cyc1es VA loading A~ff ASeff Introduction crack length base line cyc1es constant-amplitude loading crack growth rate overload cycle variable"amplitude cycles effective stress intensity factor effective stress range The problem addressed in this paper is the potential usefulness of fractographic evidence to analyze prediction problems offatigue crack growth under variable-amplitude (VA) loading. Verifications of prediction models can be made on different levels. The most global one is a comparison of predicted and experimental crack growth lives. If they do agree, it is possible that an underprediction in the fust part of the crack growth life is compensated by an overestimation in the second part. In other words, in the first part the crack growth rate daidN is overestimated, whereas daidN is underestimated in the second part. The prediction model is apparently not accurate. A more precise verification of a prediction model requires that the crack growth rate is correctly predicted for the entire crack growth life. Such verifications are made by comparing predicted and experimental da/dNvalues as a function ofthe crack length a. Ifthe predicted relation and the experimental one are similar, the prediction model could be supposed to be accurate. However, da/dN values as obtained from crack growth records are calculated slopes. Such da/dN values still have a kind of a global nature. Fig.l shows the load spectra applied in the present test series. The spectra contain small base line (BL) cycles and intermittent larger overload (OL) cycles. Ifthe crack rate during the large cycles is higher than predicted by a cycleby-cycle prediction model, and the crack rate during the small cycles is lower than predicted, then the predicted growth rate per spectrum block can still agree with the experimental value. Local under-predictions can he canceled by local over-predictions. The prediction model might appear to he acceptabie, but it is physically still not correct. Prediction models should he verified by still more detailed crack growth measurements. Ultimately, the crack rate should he measured in each individual cycle. Experimentally it implies that striation spacings have to be measured. Fractographic observations in the electron microscope are essential for that purpose. Experiments were carried out on 2024-T3 (bare) and 7075-T6 (clad) specimens with simple VA load histories (Fig.l). Simple load spectra were chosen to be sure about striation pattems which might be vi si bie in the scanning electron microscope (SEM). The experimental conditions and load histories are described first, followed by results, a discussion and some conclusions. It is not the the aim as yet to compare the observations with results of aprediction model. Experimental conditions Crack growth fatigue tests were carried out in an electro-hydraulic fatigue machine equipped with MTS TestStar computer controlled load monitoring. The tests occurred in normal lab air. Central cracked tension specimens were used, see Fig.2. The specimens were produced from 4 mm thick plates. A lower thickness might have given shear lips in a too early stage ofthe crack growth. Plates of2024-T3 bare and 7075-T6 Clad material were used as they were available

55 citations


Journal ArticleDOI
TL;DR: In this paper, a multiple micromechanisms-based model is developed to quantitatively relate the fracture toughness of partially recrystallized 7XXX aluminum alloys to their fracture surface morphology.
Abstract: A multiple micromechanisms-based model is developed to quantitatively relate the fracture toughness of partially recrystallized 7XXX aluminum alloys to their fracture surface morphology. The model is verified using the experimental data on partially recrystallized 7050 alloy reported in the companion article. It is then used to obtain a quantitative relationship between the fracture toughness and microstructural attributes. The model relates fracture toughness to microstructural parameters such as degree of recrystallization, grain size of recrystallized grains, thickness of recrystallized regions, total surface area of the constituent particles per unit volume, and microstructural anisotropy. The model predicts the changes in the fracture toughness with the specimen orientation.

51 citations


Journal ArticleDOI
TL;DR: In this article, the microstructure and mechanical properties of BaTiO3 and composites containing nanosized SiC particulates were investigated, and fracture behavior and fractography were discussed from the fracture surface observation by scanning electron microscopy and by the controlled surface flaw method.
Abstract: Microstructure and mechanical properties of BaTiO3 and BaTiO3-based composites containing nanosized SiC particulates were investigated. Fracture behaviour and fractography were also discussed from the fracture surface observation by scanning electron microscopy and by the controlled surface flaw method. The added SiC particulates were uniformly distributed within the matrix BaTiO3 grains, with some larger particulates located at the BaTiO3 grain boundaries. The microstructure of BaTiO3 was modified by incorporation of the SiC particulate. Mechanical properties, particularly the fracture strength, were remarkably improved by adding the SiC particulate, owing to grain-size reduction and/or crystal structure change by incorporating the SiC. From the fracture surface observations, it was confirmed that the subcritical crack growth phenomenon of BaTiO3 was also improved by the nanosized SiC dispersions.

Journal ArticleDOI
TL;DR: In this article, the effects of fiber/matrix adhesion and residual strength of notched polymer matrix composite laminates (PMCLs) and Fibre Reinforced Metal (FRMLs) were investigated.
Abstract: Effects of fibre/matrix adhesion and residual strength of notched polymer matrix composite laminates (PMCLs) and fibre reinforced metal laminates (FRMLs) were investigated. Two different levels of adhesion between fibre and matrix were achieved by using the same carbon fibres with or without surface treatments. After conducting short-beam shear and transverse tension tests for fibre/matrix interface characterisation, residual strength tests were performed for PMCLs and FRMLs containing a circular hole/sharp notch for the two composite systems. It was found that laminates with poor interfacial adhesion between fibre and matrix exhibit higher residual strength than those with strong fibre/matrix adhesion. Major failure mechanisms and modes in two composite systems were studied using SEM fractography. The effective crack growth model (ECGM) was also applied to simulate the residual strength and damage growth of notched composite laminates with different fibre/matrix adhesion. Predictions from the ECGM were well correlated with experimental data.

Journal ArticleDOI
TL;DR: In this paper, a series of fracture mechanics tests were conducted at temperatures of 650 C and 704 C in air, using Inconel 719, and the results establish that linear elastic conditions dominate the near-crack-tip displacements and strains at 650 C during crack growth, and confirm that K{sub 1} is a viable continuum-based fracture parameter for creep crack growth characterization.
Abstract: A series of fracture mechanics tests were conducted at temperatures of 650 C and 704 C in air, using Inconel 719. A noncontacting measurement technique, based on computer vision and digital image correlation, was applied to directly measure surface displacements and strains prior to and during creep crack growth. For the first time, quantitative comparisons at elevated temperatures are presented between experimentally measured near-crack-tip deformation fields and theoretical linear elastic and viscoelastic fracture mechanics solutions. The results establish that linear elastic conditions dominate the near-crack-tip displacements and strains at 650 C during crack growth, and confirm that K{sub 1} is a viable continuum-based fracture parameter for creep crack growth characterization. Postmortem fractographic analyses indicate that grain boundary embrittlement leads to crack extension before a significant amount of creep occurs at this temperature. At higher temperatures, however, no crack growth was observed due to crack tip blunting and concurrent stress reduction after load application.

Journal ArticleDOI
TL;DR: In this article, a strain-rate-dependent viscosity equation is presented which describes well the behavior of silicate glasses and a mechanism for the formation of mirror, mist and hackle regions in the fractography of fractured surfaces.
Abstract: Results of recent research are reported in the areas of: (1) non-linear viscous flow in glass, (2) molecular dynamics of brittle fracture and (3) optical properties of semiconductor–glass quantum composites. In all three areas, a fundamental understanding of the underlying behavior of materials has emerged from key experiments or computer simulations. These are described here: (1) The non-linear viscosity studies have established the behavior of viscosity at high strain rates in silicate glasses. Results of measurements show marked shear thinning associated with changes in the glass structure at high shear rates. A strain-rate-dependent (SRD) viscosity equation is presented which describes well the behavior of silicate glasses. (2) Molecular dynamics computer simulations of amorphous and crystalline silica have examined brittle fracture under applied uniaxial strain. Their results begin to reveal the underlying atomic processes that take part in free surface formation and fracture. For example, an examination of the atomic dynamics at the crack tip reveals that cracks form by the coalescence of pre-existing voids in the glass structure and that fracture surfaces are formed as silicon ions are shielded from the surface by oxygen ions resulting in a predominance of oxygen ions on the fracture surface. The results also suggest a mechanism for the formation of mirror, mist and hackle regions in the fractography of fractured surfaces. (3) Experimental studies of the optical properties of semiconductors formed in a glass matrix were conducted to examine the effect of size confinement on the electronic bandgap of the semiconductors. The results reveal differences in behavior between direct-gap semiconductors (CdS, CdSe and CdTe) and indirect-gap semiconductors (Si and Ge). An analysis of the data shows that the expected blue shift in bandgap energy of direct-gap semiconductors that results from reduced crystal sizes saturates at very small sizes due to the mixing of states between conduction band side valleys and the central valley. This effect does not appear to dominate indirect-gap semiconductors allowing their size-induced bandgap blue shift to cover the entire range of visible wavelengths. This effect suggests the use of quantum confined indirect-gap materials such as Si and Ge in stacks of films with different bandgaps to produce solar cells with increased theoretical efficiency.

Journal ArticleDOI
TL;DR: In this article, the microstructure and fracture properties of AZ91 Mg matrix composites fabricated by the squeeze-casting technique, with variations in the reinforcement material and applied pressure, were investigated.
Abstract: The present study aims to investigate the microstructure and fracture properties of AZ91 Mg matrix composites fabricated by the squeeze-casting technique, with variations in the reinforcement material and applied pressure. Microstructural and fractographic observations, along with in situ fracture tests, were conducted on three different Mg matrix composites to identify the microfracture process. Two of them are reinforced with two different short fibers and the other is a whisker-reinforced composite. From the in situ fracture observation of Kaowool-reinforced composites, the effect of the applied pressure on mechanical properties is explained using a competing mechanism: the detrimental effects of fiber breakage act to impair the beneficial effects of the grain refinement and improved densification as the applied pressure increases. On the other hand, for the composites reinforced with Saffil short fibers, microcracks were initiated mainly at the fiber/matrix interfaces at considerably higher stress intensity factor levels, while the degradation of fibers was not observed even in the case of the highest applied pressure. This finding indicates that the higher applied pressure yields better mechanical properties, attributable to the Saffil short fibers having relatively high resistance to cracking. Although an improved microstructure was obtained by accommodating the appropriate applied pressure in the short fiber-reinforced composites, their mechanical properties were far below those of conventional A1 matrix composites. In this regard, the Alborex aluminum borate whisker is suggested as a replacement for the short fibers used in the present investigation, to achieve better mechanical properties and fracture toughness.

Journal ArticleDOI
TL;DR: In this paper, the authors used optical and scanning electron microscopy (SEM) examinations for characterization of the material microstructure and fracture behavior and found that the fracture surface is dominated by the ductile fracture features, that is, dimples.
Abstract: Metal matrix composites (MMC) were manufactured using hot pressing followed by hot extrusion of aluminum (Al) powder reinforced by alumina (AI2O3) particles. Under tensile as well as compressive loads, a strength improvement of 64 to 100 % compared to the matrix material strength was obtained. The percent elongation to fracture ranged from 20 to 30%, which indicates good ductility as compared to the ductility of MMC manufactured by other techniques. Optical as well as scanning electron microscopy (SEM) examinations were used for characterization of the material microstructure and fracture behavior. Porosity retained in the microstructure was very limited in the case of pure aluminum billets. Microstructural examination revealed uniform distribution of Al2O3 particles in the Al-matrix. Under tensile loads, voids opened by decohesion between the matrix and reinforcement. Such behavior led to a decrease in strength properties of the MMC as a function of reinforcement volume fraction. The fracture surface is dominated by the ductile fracture features, that is, dimples. Voids were found to initiate at retained porosity sites at the AI/AI2O3 interface or in the matrix close to the interface due to stress concentration. The SEM revealed the formation of a complex fine subgrain structure. Such a polygonized structure is a major source of strengthening.

Journal ArticleDOI
Mingyuan Gu1, Yanping Jin1, Zhi Mei1, Zengan Wu1, Renjie Wu1 
TL;DR: In this paper, the microstructures and mechanical properties of artificially oxidized and as-received silicon carbide particle (SiCp) reinforced pure aluminum and 2024Al composites were investigated.
Abstract: The microstructures and mechanical properties of artificially oxidized and as-received silicon carbide particle (SiCp) reinforced pure aluminum and 2024Al composites were investigated. It was shown that surface oxidation of SiCp increased the tensile strength and fracture strain of the pure Al based composite. Diffusion of aluminum into SiO 2 during fabrication of the composite, which increased the diffusion bonding between the SiC particle and the aluminum matrix, is thought to be responsible for the increases in strength and ductility. However, the strength and ductility of the oxidized SiCp reinforced 2024Al composite were much lower than those of the as-received SiCp reinforced 2024Al composite. Based on the experimental results from DSC, TEM, EDX and SEM fractography analyses, it is suggested that the interfacial reactions related to SiO 2 depleted the magnesium in the matrix and subsequently decreased the amount of the age strengthening phase containing magnesium. The lower level of strengthening in the matrix and the formation of a thick interfacial reaction layer, which was continuous and brittle, finally led to the lower strength and ductility of the artificially oxidized SiCp reinforced composite.


Journal ArticleDOI
TL;DR: In this paper, the fracture path was examined by observing the side surface in a partially ruptured specimen and the relationship of the fracture facets to the microstructures was established by simultaneously observing the fracture surface and the adjacent side surface simultaneously.
Abstract: The fracture mode and crack propagation behavior of brittle fracture at 77 and 4 K in an 18Cr-18Mn-0.7N austenitic stainless steel were investigated using optical and scanning electron microscopy. The fracture path was examined by observing the side surface in a partially ruptured specimen. The relationship of the fracture facets to the microstructures was established by observing the fracture surface and the adjacent side surface simultaneously. Three kinds of fracture facets were identified at either temperature. The first is a smoothly curved intergranular fracture facet with characteristic parallel lines on it. The second is a fairly planar facet formed by parting along an annealing twin boundary, a real {111} plane. There are three sets of parallel lines on the facet and the lines in different sets intersect at 60 deg. The third is a lamellar transgranular fracture facet with sets of parallel steps on it. Fracture propagated by the formation of microcracks on a grain boundary, annealing twin boundary, and coalescence of these cracks. The observation suggests that the ease of crack initiation and propagation along the grain boundary and the annealing twin boundary may be the main reason for the low-temperature brittleness of this steel. A mechanism for grain boundary cracking, including annealing twin boundary parting, has been discussed based on the stress concentration induced by impinging planar deformation structures on the grain boundaries.

Journal ArticleDOI
TL;DR: In this article, spray atomization and co-deposition was used to synthesize layered 6061/SiCp MMCs with different values of inter-layer spacing, and fractographic examinations indicated that the higher SiC volume fraction layers were the preferential regions for crack generation.

Journal ArticleDOI
TL;DR: In this paper, the fracture surfaces of FeAl/TiC composites containing 70% TiC were investigated and a systematic correlation of the fracture mode to the ligament thickness was performed, which clearly showed that FeAl ligaments thicker than about 1 − 2 μ m fracture by cleavage and those smaller in size fracture predominantly in a ductile manner.

Journal ArticleDOI
TL;DR: In this paper, the effect of the orientation and strain rate on low-cyclic fatigue was investigated on single crystals of nickel-based superalloy DD3 with different cyclic strain rates at 950°C.
Abstract: Fully reversed low-cyclic fatigue (LCF) tests were conducted on [001], [012], [-112], [011], and [-114] oriented single crystals of nickel-based superalloy DD3 with different cyclic strain rates at 950°C. The cyclic strain rates were chosen as 1.0×10−2, 1.33×10−3 and 0.33×10−3 s−1. The octahedral slip systems were confirmed to be activated on all the specimens. The experimental result shows that the fatigue behavior depends on the crystallographic orientation and cyclic strain rate. Except [001] orientation specimens, it is found from the scanning electron microscopy (SEM) examination that there are typical fatigue striations on the fracture surfaces. These fatigue striations are made up of cracks. The width of the fatigue striations depends on the crystallographic orientation and varies with the total strain range. A simple linear relationship exists between the width and total shear strain range modified by an orientation and strain rate parameter. The nonconformity to the Schmid law of tensile/compressive flow stress and plastic behavior existed at 950°C, and an orientation and strain rate modified Lall-Chin-Pope (LCP) model was derived for the nonconformity. The influence of crystallographic orientation and cyclic strain rate on the LCF behavior can be predicted satisfactorily by the model. In terms of an orientation and strain rate modified total strain range, a model for fatigue life was proposed and used successfully to correlate the fatigue lives studied in this article.

Journal ArticleDOI
TL;DR: In this article, a variety of infrared and visible optical methods and high-speed photography are used to study dynamic crack initiation in ductile steels (Ni-Cr steel and 304 stainless steel) at different loading rates and to establish appropriate dynamic failure criteria.
Abstract: The goal of the work presented here is to study dynamic crack initiation in ductile steels (Ni–Cr steel and 304 stainless steel) at different loading rates and to establish appropriate dynamic failure criteria. A variety of infrared and visible optical methods and high-speed photography are used in this study. Precracked steel specimens are subjected to dynamic three-point bend loading by impacting them in a drop weight tower. During the dynamic deformation and fracture initiation process the time history of the transient temperature in the vicinity of the crack tip is recorded experimentally using a high-speed infrared detector. The dynamic temperature trace in conjunction with the HRR solution is used to determine the time history of the dynamic J-integral J^d(t), and to establish the dynamic fracture initiation toughness, J^d_c. The measurements made using high-speed thermography are validated through comparison with determination of J^d(t) by dynamic optical measurements of the crack tip opening displacement (CTOD). Finally, the micromechanisms of fracture initiation are investigated by studying the fracture surface using scanning electron microscopy.

Journal ArticleDOI
TL;DR: In this paper, the response to mechanical loads of unidirectional commingled warp knitted and woven glass fibre reinforced polyethylene terephthalate laminates has been characterized.
Abstract: The response to mechanical loads of unidirectional commingled warp knitted and woven glass fibre reinforced polyethylene terephthalate laminates has been characterized. The mechanical properties of the two materials were determined under tension, in-plane shear and flexure. The flexural fatigue properties were determined for the woven laminates by means of three-point bending tests with a loading ratio of R=0.1 at stress levels of 50–90% of the ultimate static strength. The Mode I, Mode II and mixed mode (Mode I : II ratios 4 : 1, 1 : 1 and 1 : 4) interlaminar fracture toughnesses of the laminates were determined by means of the double cantilever beam and mixed mode bending tests, respectively. The main fractographic features, as determined by a scanning electron microscopy examination, of the Mode I dominated failures were a brittle matrix failure and larger amounts of fibre pull-out. As the Mode II loading component increased, the amount of fibre pull-out was reduced and the features of the matrix appeared to be more sheared. Cusps were found on the fracture surfaces of specimens tested in pure Mode II and mixed mode I : II=1 : 4. Cusps are normally not found in thermoplastic matrix composites. © 1998 Kluwer Academic Publishers

Journal ArticleDOI
TL;DR: In this article, the effects of changes in R ratio on the fatigue crack growth behavior of a Nb-10 at. pct Si composite as well as bulk Nb 1.24 at.pct Si were determined.
Abstract: The effects of changes in R ratio on the fatigue crack growth behavior of a Nb-10 at. pct Si composite as well as bulk Nb-1.24 at. pct Si were determined. Fatigue crack growth experiments were performed over a range of ΔK levels at R ratios of 0.1 and 0.4. Qualitative and quantitative scanning electron microscopy studies were performed to characterize the fatigue fracture features of the composites and alloys, in order to determine the factors controlling these fracture features. The results of this work indicate that increases in R ratio reduce the observed threshold stress intensities in both materials. Somewhat higher fatigue thresholds were observed in the Nb-Si(ss) compared to pure Nb in the literature. In contrast to the bulk Nb-Si(ss) alloy, which exhibited no evidence of cleavage fracture in fatigue at any R ratio or ΔK level, the Nb-Si(ss) constituent in the Nb-10 at. pct Si composite exhibited a distinct fracture mode transition from ductile tearing near threshold and low ΔK to cleavage fracture with an increase in ΔK and Kmax. Possible reasons for such observations are provided.

Journal ArticleDOI
TL;DR: In this paper, the effect of specimen thickness on fracture toughness of a powder metallurgically processed 7093 Al/SiC/15p composite was evaluated in different microstructural conditions.
Abstract: The effect of specimen thickness on the fracture toughness of a powder metallurgically processed 7093 Al/SiC/15p composite was evaluated in different microstructural conditions. The fracture toughness in the underaged condition increased significantly with a decrease in specimen thickness, even at thicknesses well below the value specified by ASTM-E 813 for a valid J Ic test. The influence of thickness was considerably lower in the peak-aged (PA) condition. This relative insensitivity is believed to be due to the low strain to failure associated with severe flow localization in the PA condition. The effect of precracking on the fracture toughness of discontinuously reinforced aluminum (DRA) was also evaluated. The dependence of fracture toughness on specimen thickness and precracking is explained in terms of the hydrostatic stress, which has a strong influence on the plastic straining capability of the DRA material. The fracture toughness was modeled using a critical strain formulation, with the void growth strain dependent on hydrostatic stress through the Rice and Tracey model. The predicted toughnesses for the thick and thin specimens were in good agreement with the experimental data.

Book ChapterDOI
01 Feb 1998
TL;DR: In this paper, the effects of stress ratio (R) and crack opening behavior on fatigue crack growth rates (da/dN) for aluminum alloy (AA) 2024-T3 were investigated using constant-delta K testing, closure measurements, and fractography.
Abstract: The effects of stress ratio (R) and crack opening behavior on fatigue crack growth rates (da/dN) for aluminum alloy (AA) 2024-T3 were investigated using constant-delta K testing, closure measurements, and fractography. Fatigue crack growth rates were obtained for a range of delta K and stress ratios. Results show that constant delta K fatigue crack growth for R ranging from near 0 to 1 is divided into three regions. In Region I, at low R, da/dN increases with increasing R. In Region II, at intermediate R, fatigue crack growth rates are relatively independent of R. In Region III, at high R, further increases in da/dN are observed with increasing R.

Journal ArticleDOI
TL;DR: In this article, the effect of Co and Ni additions on secondary hardening and fracture behavior of the martensitic steels bearing W and Cr was analyzed, where W exhibits a much weaker effect on the secondary hardness compared with Mo and Cr can contribute to an increase in toughness even though the Cr additions weaken the secondary hardness in Mo and W steels.
Abstract: The purpose of this study was to analyze the effect of Co and Ni additions on secondary hardening and fracture behavior of the martensitic steels bearing W and Cr, where W exhibits a much weaker effect on the secondary hardening compared with Mo and Cr can contribute to an increase in toughness even though the Cr additions weaken the secondary hardening in Mo and W steels. The chemical compositions of the alloys used in this study are presented in Table 1. Impact specimens were austenitized in a flowing argon atmosphere at 1,200 C for 1 hour and then water- or oil-quenched. Austenitized specimens were aged in a neutral salt bath at 400 C and 650 C for 1 hour and then water-quenched. Fractography was conducted on fracture surfaces cut off the impact-tested samples, while samples for hardness measurements were obtained from a second cut 5 to 10 mm below the fracture surface. Hardness was measured using the Rockwell C scale (HRc), and average values of five readings are reported. In order to observe martensitic substructures (M{sub 2}C and M{sub 3}C carbides), thin foils were examined in a JEOL transmission electron microscope (TEM), operated at 120 kV. The variations in hardnessmore » for 6W, 3W-3Cr, 3W-3Cr-14Co, and 3W-3Cr-14Co-10Ni steels are shown.« less

Journal ArticleDOI
TL;DR: In this article, the effects of ternary alloying with Ti on the fatigue and fracture behavior of a new class of forged damage-tolerant niobium aluminide (Nb3Al-xTi) intermetallics are presented.
Abstract: The results of a recent study of the effects of ternary alloying with Ti on the fatigue and fracture behavior of a new class of forged damage-tolerant niobium aluminide (Nb3Al-xTi) intermetallics are presented in this article. The alloys studied have the following nominal compositions: Nb-15Al-10Ti (10Ti alloy), Nb-15Al-25Ti (25Ti alloy), and Nb-15Al-40Ti (40Ti alloy). All compositions are quoted in atomic percentages unless stated otherwise. The 10Ti and 25Ti alloys exhibit fracture toughness levels between 10 and 20 MPa√m at room temperature. Fracture in these alloys occurs by brittle cleavage fracture modes. In contrast, a ductile dimpled fracture mode is observed at room-temperature for the alloy containing 40 at. pct Ti. The 40Ti alloy also exhibits exceptional combinations of room-temperature strength (695 to 904 MPa), ductility (4 to 30 pct), fracture toughness (40 to 100 MPa√m), and fatigue crack growth resistance (comparable to Ti-6Al-4V, monolithic Nb, and inconnel 718). The implications of the results are discussed for potential structural applications of the 40Ti alloy in the intermediate-temperature (∼700 °C to 750 °C) regime.

Journal ArticleDOI
TL;DR: In this article, the fracture toughness was found to be less sensitive to the loading velocity, having values of around 25 MPa √m, which is believed to be attributed to the high strain rate experienced at the crack tip.
Abstract: A γ-base TiAl alloy with duplex microstructure of lamellar colonies and equiaxed γ grains was prepared with a reactive sintering method. Tensile tests and fracture toughness tests at loading velocities up to 12 m/s (strain rate for tensile tests up to 3.2×102/s) were carried out. The micro-structure of the alloy before and after tensile deformation was carefully examined with a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The fractography of the tensile specimens and fracture toughness specimens was studied. The experimental results demonstrated that the ultimate tensile strength (UTS) and yield strength (YS) increase with increasing strain rate up to 10/s and subsequently level off. The UTS and YS exhibited similar strain rate sensitivity. The strain rate sensitivity exponent at strain rates lower than 10/s is about 1.5×10−2 and at higher strain rates is almost zero. In this study, fracture toughness was found to be less sensitive to the loading velocity, having values of around 25 MPa √m, which is believed to be attributed to the high strain rate experienced at the crack tip. The predominant deformation mechanism for the strain rates used in this study was found to be twinning. However, in the low strain rate range, the dislocation motion mechanism was operative at the initial deformation stage and twinning dominated the later stage of the deformation process. In the high strain rate range, the entire deformation process was dominated by twinning. The interaction between deformation twinning and grain boundaries resulted in intergranular fracture in the γ grains and delamination of α 2/γ interfaces in the lamellar colonies.