Resulting Fracture Surfaces Research Articles

Fracture surface interference effects in mode III

Abstract In this experimental investigation, the effects of fracture surface interference upon the torsional properties of steel alloys were quantitatively characterized. A cyclic compression pre-cracking technique was employed to produce microscopically fine crack fronts in circumferentially notched cylindrical test specimens. The abrasion and deformation that the resulting fracture surfaces underwent during the application of torsional loading was studied in detail. The interference of the fracture surfaces was observed to enhance the measured torsional stiffness of a mode III specimen by as much as 70%. The most intense abrasion effects occur close to the crack tip where the separation between adjacent fracture surfaces is relatively small. Compliance techniques, which are considered standard practice in the construction of crack resistance curves in mode I loading applications, were applied to determine their accuracy in mode III loading applications. These techniques, in comparison with direct post-fracture crack length measurements, were found to produce unreliable results in mode III due to the effects of fracture surface interference.

Microstructural evolution and mechanical properties of a CrCr 2Hf alloy

The cast, homogenized and forged microstructures of a two-phase CrCr 2Hf alloy (Cr-6.5 at.% Hf) were characterized via optical, scanning and transmission electron microscopy. The as-cast microstructure was substantially broken down by the hot working operation and subsequent heat treatment led to further refinement in the second-phase morphology. In the forged condition, the eutectic Cr 2Hf had the C14 structure; in addition, precipitation on a fine scale was observed in the Cr phase due to solid state decomposition, and these fine precipitates were tentatively identified as C15 Cr 2Hf. Extended heat treatments at high temperatures (1273–1373 K) led to the metastable C14 eutectic phase transforming to the C36 phase. Compression specimens from the forging, tested in the temperature range 293–1473 K, exhibited a yield strength-temperature profile characteristic of BCC alloys. Four-point bend tests were conducted as a function of temperature and strain rate to obtain an estimate of the ductile-to-brittle transition temperature. Notched bend specimens tested as a function of temperature provided a toughness of ∼7 MPa√m at 293 K that increased almost linearly with temperature to ∼15 MPa√m at 873 K. The resulting fracture surfaces were examined in the scanning electron microscope. The measured properties were correlated with the observed microstructures; microstructural changes that accompany the thermal-mechanical processing of such an alloy were understood.

Fracture Toughness, Fracture Planes and BDT in Stoichiometric Feal and Nial Single Crystals

AbstractFracture toughness data of stoichiometric FeAl and NiAl single crystals were measured in four-point-bending tests and the crystallographic orientations of the resultant fracture surfaces were determined.For FeAl single crystals unexpectedly high fracture toughness values were measured even at low temperatures (17 MPa [001] at 77K). However, these crystals failed in a brittle manner for temperatures up to 300K. The crystallographic orientation of the fracture surfaces were strongly dependent on the environment due to hydrogen embrittlement.The NiAl specimens were precracked and the fracture toughness was measured in the weakest crystal orientation. The fracture surfaces were similar to those obtained in previous investigations. Although the room temperature (RT) KIC-value was the highest for pure weak-oriented NiAl (7.5 MPa [001]) the BDT temperature was found in the same region where it occurs for the hard orientation (673K to 693K). The onset of the BDT will be discussed.

ＳＵＳ３０４ステンレス鋼単結晶における水素ぜい化の引張軸方位依存性

Tensile tests were carried out on SUS304 Stainless steel single crystals, whose respective tensile axes were parallel to [001], [111] and 0.5 in Schmid factor, under cathodic charging in 0.5kmol/m3 H2SO4 solution with a small amount of NaAsO2 at room temperature, and the characterristics of hydrogen embrittlement of this material in tension were evaluated from the nominal stress-strain curve.The susceptibility to hydrogen embrittlement appeared prominently in the elongation and fracture morphology. The elongation decreased with increasing current density and the resulting fracture surface morphology changed from ductile to cleavage manner. The cracks produced by hydrogen initiated at the deformation bands and propagated perpendicular to the tensile axis, then connected each other and fractured.The orientation dependence of hydrogen embrittlement was recognized on the fracture morphology when the current density was between 4 to 10A/m2. The morphology of fracture surface under the present test condition was dependent on both amount of martensite induced by deformation and the concentration of hydrogen.

Tempering effects on high temperature brittle intergranular fracture in Mn–Mo–Ni steel

AbstractThe influence of tempering on high temperature brittle intergranular fracture has been examined in the simulated heat affected zone microstructures of three Mn–Mo–Ni steel alloys, namely, the base alloy and phosphorus doped or phosphorus–sulphur doped base alloy. Crack growth behaviour was determined as a function of stress intensity at 550 and 450°C in high vacuum conditionsfor material as quenched and after tempering at 615°C. The resulting fracture surfaces were examined in detail and analysis of crack tip and general grain boundary chemistry was performed using scanning Auger electron spectroscopy. Microstructural and rheological changes induced by tempering were evaluated via electron microscopy and hardness tests. At 550°C, tempering largely eliminated high temperature brittle intergranular fracture, leaving intergranular microvoid coalescence as the dominant fracture mode. Crack growth rates were attenuated by tempering in alloys susceptible to high temperature brittle intergranular fractur...

Effects of matrix characteristics on diamond composites

A series of cobalt-matrix diamond composites was fabricated by hot pressing, and their microstructure, physical properties, transverse rupture strength and resulting fracture surface were studied in detail. Segments of the diamond composites were manufactured, and a one-segment circular sawblade was used for the evaluation of the sawing performance. Results show that the fracture surface of composites containing a cobalt matrix exhibits an excellent ductile appearance, while the fracture surface of composites containing an additive of tin powder in the cobalt matrix displays a less ductile behaviour due to the existence of a tin-rich brittle phase. It is also found that a diamond composite having low porosity, high hardness, and less surface attack of diamond particles will result in a low value of radial sawblade wear.

Room Temperature Fracture of FeCo

FeCo is a B2 intermetallic compound which undergoes an order-disorder transformation. In this paper, the effects of changes in both constitutional and thermal disorder on the room temperature fracture of FeCo are presented. Tensile tests were performed on three compositions of FeCo, Fe[sub 30]CO[sub 70], Fe[sub 50]CO[sub 50] and Fe[sub 70]CO[sub 30]. The resulting fracture surfaces were examined by SEM and the grain boundary chemistry was investigated by Auger electron spectroscopy, AES. Ordered Fe[sub 50]CO[sub 50] and Fe[sub 70]CO[sub 30] were very battle, fracture occurring before yielding, with intergranular fracture occurring in most grains. In contrast, ordered Fe[sub 30]CO[sub 70] showed about 18% elongation and exhibited a dimple-type fracture. It was also found that disordering improved the ductility of each composition but had little influence on the fracture mode. AES showed that a low level of sulfur segregation was present at the grain boundaries, suggesting that sulfur segregation alone was not responsible for the brittle behavior of the ordered alloys.

The effect of microstructure on localized melting at separation in Ti-6Al-4V tensile samples

The phenomenon of localized melting during the separation process of tensile samples has been found in a number of different titanium alloys. In this study, the effect of microstructure on the mechanism responsible for localized melting found on the fracture surfaces of Ti-6Al-4V tensile samples has been investigated. Tensile samples with three different microstructures were strained to fracture at quasi-static applied strain rates, and the resulting fracture surfaces were investigated and characterized using stereo scanning electron microscopy. Selected fracture surfaces were also nickel-plated and sectioned to reveal the subsurface microstructures and to get an idea of the nature of deformation in the region of the fracture surface. Samples which had an equiaxed α microstructure as a result of mill annealing and heat treating showed large regions of concentrated shear and considerable localized melting. By contrast, samples with a finea plate colony microstructure displayed much higher fracture surface tortuosity and no localized melting. The connection between large-scale shear and localized melting implies that a shear instability, such as adiabatic shear, is quite likely a necessary factor for achieving the melting temperature at final separation.

Relationships between microstructure and properties of unalloyed compacted graphite cast irons

Austempered cast irons have been the subject of much attention in recent years because of their excellent mechanical properties. The hardness, ultimate tensile strength and dynamic elastic modulus are presented for a commercially available unalloyed compacted iron (C.E. 4.31) and correlated with different matrix microstructures (as-cast, ferritized, normalized and austempered). For this study, two isothermal temperatures for the austempering treatment were chosen: 400°C and 300°C. The influence of a ferritizing treatment prior to normalizing and austempering has been evaluated, the results indicating that no advantages are obtained with this additional treatment. The influence of microstructure on properties and on the resulting fracture surfaces in tensile tests are discussed.

Fatigue crack propagation in microlayer composites of polycarbonate and poly(styrene–acrylonitrile)

AbstractThe effect of layer thickness on fatigue crack propagation (FCP) in microlayer composites of polycarbonate (PC) and styrene–acrylonitrile copolymer (SAN) has been studied. Variation in layer thickness was achieved by increasing the number of layers from 49 to 776 while keeping the overall sheet thickness and composition the same. It was found that the 776 layer composite had a larger number of fatigue cycles from crack initiation to fracture, a longer stable crack length and higher value of critical strain energy release rate J1C. Microscopic characterization of the damage zone and resulting fracture surface revealed a transition from SAN crazing to shear deformation as layer thickness decreased from 18 to 1 μm; the resulting plastic deformation and ductile fracture of the 776 layer composite was responsible for the enhanced FCP resistance. The ductile fracture mechanism produced a measurable temperature rise at the crack tip that revealed the stop–start nature of crack propagation.

In-situ observations of microvoid coalescence: Stacking fault energy effects

Although it has been well established that microvoid coalescence occurs during static or quasi-static fracture in ductile materials, the exact mechanism for microvoid formation is still unclear. It has been argued that microvoids initiate and grow from second phase particles. However this argument cannot be used to explain the existence of microvoids on the fracture surfaces of "pure" materials. An alternative mechanism for their formation in "pure" materials is that they initiate and grow along dislocation cell walls. If this premise is true; then the nature and extent of microvoid coalescence should be related to the stacking fault energy (SFE) of the material since the latter is a controlling parameter in the formation of dislocation cells. The relationship between microvoid coalescence and stacking fault energy may have some basis since absolute cell dimensions are of the same magnitude as the observed dimple sizes. The present study examines the effect of dislocation cell structures on the formation of microvoids as a function of the stacking fault energy of a given material through direct observation of the void formation and growth process within the TEM. The fundamental aspects of the work is to correlate the dislocation substructures, void initiation, growth, and coalescence to the resulting fracture surfaces.

The fractal nature of a fracture surface

A kinetic model is presented which simulates the propagation of cracks in solids composed of discrete atoms. The resulting fracture surface is found to be a fractal, the dimension of which depends on the elastic constants of the material.

Intermetallic and void formation in gold wirebonds to aluminum films

Gold ball bonds attached to either pure Al films or Al films with Cu and Si additions were annealed at temperatures in the range 77–277°C for periods of up to 3,000 h and then shear tested. The resulting fracture surfaces were observed to change progressively with time and temperature of annealing. The intermetallic phases and voids present after each annealing treatment were characterized by microscopic examination of the bond cross sections. Shear test results are discussed in terms of the observed microstructures.

Interfacial shear strength studies using the single‐filament‐composite test. I: Experiments on graphite fibers in epoxy

AbstractThe failure of the interface in a carbon fiber‐epoxy system was studied for six different epoxy blends using the single‐filament‐composite technique. The blends were formulated to yield a wide range of stiffnesses, and their effect on interfacial failure was examined. Specimens were made from Hercules IM6‐G carbon fiber and the different blends of epoxy, and then strained to obtain a distribution of fiber fragment lengths. Birefringence patterns near the fiber breaks were observed and recorded. Some of the specimens were strained until they failed and the resulting fracture surfaces were observed under a scanning electron microscope to determine fracture patterns and the existence of debonding. The fragment length distributions were interpreted using a Monte‐Carlo simulation of a Poisson/Weibull model for fiber strength and flaw occurrence. The results were used to calculate an effective interfacial shear strength. From this analysis we conclude that one cannot accurately predict the interfacial properties of a composite based solely upon conventional single fiber and bulk matrix properties. Local matrix properties and fiber/matrix interactions, on a microscale, play a key role in composite strength.

Fracture behaviour of selected mixed-ceramic tool materials up to 1200°C

The fracture behaviour of selected mixed-ceramic tool materials up to 1200°C has been studied. The materials examined were Al 2O 34wt.%ZrO 2, Al 2O 330wt.%Ti(N,C), β-(SiAlON). Fracture studies included measurement of load-bending displacement behaviour in four-point bend of rectangular bars pre-cracked using bridge indentation. The resulting fracture surfaces were characterized fractographically by scanning electron microscopy. The transformation state of the fracture surface of the ZrO 2-containing ceramic was examined by X-ray diffraction. At displacement rates of 60 and 600 μ m min −1, all materials exhibited linear elastic behaviour up to 1000°C. The room temperature K Ic values were 4.2 MN m − 3 2 , 3.5 MN m − 3 2 , 4.5 MN m − 3 2 and 4.3 MN m − 3 2 for the four materials respectively and these did not change significantly up to 1000°C. At 1200°C, subcritical crack growth was observed. The β′-(SiAlON) exhibited an increase in toughness, whereas a slight decrease was observed for the other materials tested.

Fracture toughnesses and surface energies of binary CaO- and MgO-phosphate glasses

The fracture of binary CaO- and MgO-phosphate glasses was studied by measuring the fracture toughnesses and calculating the resulting fracture surface energies. Application of the ionic potential concept yields a linear relationship describing both the normal - and the abnormal - structure glasses on the same surface energy/ionic potential plot. Trends are explained on the basis of the modifying cation to nonbridging oxygen bond density in the glass structure.

Bonding of Fibrillated Polypropylene Fibers to Cementitious Materials

ABSTRACTResults of Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX) studies on the fracture of polypropylene fiber-hydrated cement interface are presented.The fiber reinforced concrete specimens were tested in direct tension. The resulting fracture surfaces were observed, including surface adhesion and mechanical bond between the fibers and the hydrated cement paste.The nature of the fiber fracture was also studied. Mechanical bond was achieved due to the surface and shape of the fibers. Adhesion between the fibers and the cement paste was observed microscopically and confirmed by (EDX) analysis. The nature of the fiber reinforcement was observed at the tensile fracture.

Destructive testing of defective weldments: Fracture mechanics predictions of failure loads and defect criticalities compared with experimental results

Thirty welded test plates containing discrete artificial defects were tested to destruction under controlled four point bending, and the failure loads recorded. The defect parameters, length, height and depth, were measured from the resulting fracture surfaces. Prior to destructive testing, the defective welds had been ultrasonically inspected, in two separate UT programmes, by a number of independent operators. Both actual and predicted defect sizes were used in conjunction with the Design Curve approach of British Standard PD 6493 to calculate failure stresses for each of the test plates. In addition, the Failure Assessment Diagram of the Central Electricity Generating Board (UK) R-6 document was used to establish defect criticality, of both the actual and the predicted defect parameters. The results obtained indicate that there appeared to be some tolerance in both assessment routes to the significant variability observed in the ultrasonic defect size predictions. However, precise predictions of failure stress levels and defect criticalities are only possible when defects are accurately sized.

Initiation and growth of microfractures along adiabatic shear bands in Ti–6AI–4V

An experimental study has been made of the manner in which microfractures initiate and grow along adiabatic shear bands formed in the titanium alloy Ti–6AI–4V by the normal impact of hard steel spheres at velocities up to 340 m s−1. It is suggested that a critical shear strain must be exceeded along the shear bands for microvoids to nucleate, or to cause significant local thermal softening in the bands, leading to the formation of single voids or arrays of voids and smooth-sided cracks when the stress state became predominantly tensile. The final shape of the micro fractures within the shear bands and the morphology of the resultant fracture surfaces are explained in terms of the density of void nucleation sites and the tensile-stress state across the shear bands.MST/179

Initiation and growth of microfractures along adiabatic shear bands in Ti–6AI–4V

An experimental study has been made of the manner in which microfractures initiate and grow along adiabatic shear bands formed in the titanium alloy Ti–6AI–4V by the normal impact of hard steel spheres at velocities up to 340 m s−1. It is suggested that a critical shear strain must be exceeded along the shear bands for microvoids to nucleate, or to cause significant local thermal softening in the bands, leading to the formation of single voids or arrays of voids and smooth-sided cracks when the stress state became predominantly tensile. The final shape of the micro fractures within the shear bands and the morphology of the resultant fracture surfaces are explained in terms of the density of void nucleation sites and the tensile-stress state across the shear bands.MST/179