Fracture Surface Markings Research Articles

Fractography of hardmetal dies used for the manufacture of polycrystalline diamond

Some WC/Co hardmetal dies used for the high-temperature, high-pressure fabrication of polycrystalline diamond have been subjected to fractographic investigation after service failure. In each die a number of flat plate-like fragments have been found showing an unusual fountain-like appearance to the fracture surface markings. Despite extensive probing, discrete fracture origins could not be found. Instead, from the evidence of the microcracking found in the die bore region and the recognised development of deformations during a campaign, it was concluded that microstructural damage was developed under the complex non-equitriaxial compression stresses which are developed during the duty cycles. When the propagation of this damage reached the axial tensile zone that exists in the cooler regions of the die, the plate-like, more-brittle failure pattern developed.

Near-surface rock stress orientations in alpine topography derived from exfoliation fracture surface markings and 3D numerical modelling

The fractographic analysis of plumose axes of exfoliation fracture surfaces from a preceding study in the Grimsel region of the Swiss Alps suggested complex directional trends of near-surface (i.e., within ~100m below ground surface) maximum principal stress (σ1) trajectories within the investigated inner-trough valleys (i.e., U-shaped valleys). The stress trajectories describe a pattern governed by local topographic variations. In situ stress measurements from the region are scarce, locally scattered, based on different methods, and, thus, difficult to interpret at regional scale. In this study, we inferred that plumose structure axes form parallel to compressive σ1, and improve our knowledge of the near-surface three-dimensional stress field in alpine settings with complex topography. We investigated near-surface stress tensors utilising three-dimensional, elastic numerical models. Our models account for morphological details of the Grimsel area at the decametre scale. We used two models with vertical boundaries aligned N–S/W–E (0°-model) and NW–SE/SW–NE (45°-model). These models allowed investigating gravitational stresses and superimposed isotropic and anisotropic compressive stresses arising from (active) tectonic shortening and/or stresses induced by exhumation (remnant stresses). Our model results illustrate that the superposition of gravitational stresses and realistic horizontal strains reveals complex near-surface stress trajectories that widely follow the patterns of exfoliation fracture plumose axes. Our models demonstrate large variations of stress orientations within the shallow subsurface, including depth levels where exfoliation fractures formed. These variations cannot be captured by a small number of local stress measurements. Our study reveals that directional data from exfoliation fracture plumose axes of Middle to Upper Pleistocene ages can be used to constrain geologically recent and current maximum principal stress directions of the shallow subsurface of up to a few hundred metres below ground.

Experimental and numerical analysis of dynamic rupture of steel pipes under internal high-speed moving pressures

This paper reports the experimentation and finite element analysis (FEA) of dynamic ductile rupture of steel pipes subjected to high-speed internal moving pressures. The experimentation included the detonations of tiny explosive cords inside small segments of ordinary gas pipes. A number of specific features of the detonation-driven fracture of cylindrical tubes such as; formation of special fracture surface markings due to cyclic crack growth, flap bulging, and crack curving/branching adjacent to the bulged area were identified. In the analysis part, the overall transient dynamic response of the pipe to detonation loading, the detonation-driven crack growth, the cyclic bulging of the crack flaps, and the resultant crack branching were simulated. The blast simulation was performed using a multi-material arbitrary Lagrangian–Eulerian (MMALE) formulation. The fluid–structure interaction (FSI) was simulated using a coupling algorithm that treated the air as a static media and the pipe as a deformable Lagrangian mesh. The accuracy of the FEA results was verified using analytical solutions and data collected from the tested pipes. The experimentation and analysis clearly showed that the self-similar propagation of the initial axial cracks in the pipe was the incremental cyclic growth governed by the structural waves.

Investigation of a Boeing 747 wing main landing gear trunnion failure

A loud noise was heard from the vicinity of the port wing landing gear during pushback of a Boeing 747-300 from the terminal at Sydney (Australia) airport. Inspection showed that one of the wing landing gear trunnion fork assemblies had failed. Detailed investigation revealed that the trunnion had failed by fatigue cracking. Deep machining grooves were found at the root of an internal radius that had not been shot-peened as required, and a chemical surface process during manufacture had resulted in shallow intergranular attack at the bottom of these grooves. It is probable that the critical cracking started from some of these grooves. In addition, the wall thickness at the failure location was significantly less than the minimum required in the drawings.Since the deep machining grooves, the lack of peening and the intergranular attack were all consequences of manufacturing, the fatigue cracking probably started shortly after the component entered service. This implies that fatigue cracking was present during all the trunnion overhauls, but was not detected by non-destructive inspections during the overhauls. Quantitative fractography was used to produce a crack growth curve based on fracture surface markings thought to represent the overhaul timings. The crack growth curve suggested that the fatigue cracking was large enough to be detected by inspection during the last overhaul, if not the one before. However, it was probably not easy to detect the cracking. This investigation therefore highlights the difficulties that can be encountered when inspection is the last (or only) line of defence against failure owing to unexpected manufacturing deficiencies.

ZL-130TCII Fracture Markers Solution for Full-Scale Fatigue Test

ZL-130TCII Fracture Markers Solution for Full-Scale Fatigue TestIn the context of PZL-130TC-II full-scale fatigue test, several strategies of fatigue loadings that create fracture surface markings were considered. One block of spectrum is made of 200 flights. By reordering those flights, a block which should create a fracture marker, was developed. It was very important that reordering the load spectrum or adding overloads or underloads did not change spectrum severity. Pilot tests of aluminium alloys specimens were carried out to finalise appropriate marker intervals before commencing full-scale tests. The experiment was conducted with the MTS machine with 810.23 system. The results and conclusions are presented within this paper.

Formation of nanowaves in compressive fracture of a less-brittle bulk metallic glass

Formation of periodical fracture surface marks in the dynamic fracture can only be observed, mainly in ideally brittle amorphous materials and under mode I loading conditions. Here the authors report on the observation of elastic-wave patterns on nanoscale that form in the compressive fracture process of a Ni-based alloy which is a less-brittle amorphous metal with an appreciable compressive ductility. The formation of nanowaves is evidenced via the creation of microbranching instability as well as the interaction of a crack front with the material inhomogeneities, i.e., microcracks.

The debonding and fracture of Si particles during the fatigue of a cast Al-Si alloy

Constant-amplitude high-cycle fatigue tests (σmax=133 MPa, σmax/σy=0.55, and R=0.1) were conducted on cylindrical samples machined from a cast A356-T6 aluminum plate: The fracture surface of the sample with the smallest fatigue-crack nucleating defect was examined using a scanning electron microscope (SEM). For low crack-tip driving forces (fatigue-crack growth rates of da/dN 1 × 10−6 m/cycle), silicon particles ahead of the crack tip were fractured, and the crack subsequently propagated through the weakest distribution of prefractured particles in the Al-Si eutectic. Only small rounded silicon particles were observed to debond while the fatigue crack grew at high rates. Using fracture-surface markings and fracture mechanics, a macroscopic measure of the maximum critical driving force between particle debonding vs fracture during fatigue-crack growth was calculated to be approximately K max tr ≈6.0 MPa √m for the present cast A356 alloy.

Evaluation of Crack Growth in Glass by Using Stress‐Wave Fractography

Subcritical crack growth in glass shows three characteristic regions, depending on crack velocity. Among three regions, in region III, where water does not affect crack‐growth behavior, the slope of the crack‐growth curve can be correlated with the intrinsic nature of glass fracture. In this study, to measure the crack velocity in region III, a periodic stress wave was applied to produce fracture‐surface markings in the double‐cleavage‐drilled compression specimen. The crack‐growth data obtained were compared with results obtained via direct observation of the crack front. As a result, this method, by using stress‐wave fractography, was found to be effective to obtain the crack‐growth curve in region III.

On the role of microcracks in the dynamic fracture of brittle materials

The dynamic fracture behavior of brittle materials is investigated. The morphology of the fracture surface is examined in detail in four polymers: polymethylmethacrylate, Solithane-113, Homalite-100 and poly-carbonate. The fracture surface markings are examined to determine the micromechanisms of fracture. This examination reveals clearly that the operative micromechanism that governs dynamic fracture in brittle materials is the nucleation, growth and coalescence of microcracks. Following a quantitative characterization of the microcracking patterns, a very simple nucleation and growth model is then put forward. Imposing nucleation and growth criteria based on the experimental observations, the simulation recreates the experimental observations, not only of the microscope surface features, but also of the macroscopic behavior such as the constancy of the crack speed.

Fracture surface markings in Liassic limestone at Lavernock Point, South Wales

Abstract A study of fracture surface markings (FSM) on three joint sets, striking at 190, 335 and 280°, cutting a 2.5 m thick slice of interbedded limestones and mudstones of lower Liassic age, yields evidence which enables determination of the orientation of the palaeostress field at the time of joint formation. A specific element of the FSM, namely the border planes, or the twist hackle, has been used to determine the orientation of σ 1 , in that these twist-hackle structures are considered to be developed normal to the remote tensile stress σ 3 . In the case of the joint sets striking at 335 and 190°, the sense of rotation of the twist plane would suggest that for both sets σ 1 was aligned N-S and almost parallel to bedding. Other FSM features suggest that the 190° set propagated rapidly with S-type plumose markings being the most common. The small angle of border plane rotation (10–15°) suggests that all of the joint sets propagated under relatively low confining pressures. The sense of border plane rotation on the 335 and 190° sets suggests that there is a possible conjugate relationship between them. However, the other main joint set which strikes at 280° cannot be accounted for by simultaneous development in a N-S aligned σ 1 which might have caused the development of the 335–190° joint sets.

Fracture surface markings in Liassic limestone at Lavernock Point, South Wales

Fracture surface markings in Liassic limestone at Lavernock Point, South Wales

Transmission electron microscopy detection of cyclic-deformation-induced f.c.c.-to-h.c.p. transformation in a cobalt-based prosthetic device material

The fatigue deformation substructure beneath a “modulated” fracture surface structure in porous coated F-75 alloy was examined using transmission electron microscopy techniques. An f.c.c.-to-h.c.p. transformation was observed on discrete planar bands. Correlation of the spacing of h.c.p. platelets with fracture surface markings indicated a transformation influence on fracture surface morphology.

Relationships among crack sharpness, crack velocity and fracture surface marks in plastic sheet specimens under tension. Results in UP and PMMA resins.

The crack velocities of compact-tension type sheet specimens having various crack-tip radii are experimentally investigated under tension using UP and PMMA plastics. The crack velocity increases rapidly to the maximum value and decreases slowly as the crack propagates in a specimen. This velocity change is approximated by the existing velocity equation for the double-cantilever-beam type specimen. The mean velocity decreases as the crack-tip radius sharpens. The density of the fracture surface marks which appeared in the fracture surface changes with respect to the relative crack length C1/C0, in a manner similar to the way the crack velocity changes in each specimen. Fracture strength and the fracture surface marks also decrease as the crack-tip radius sharpens, showing that the velocity and the number of marks tend to be smaller the lower the fracture strength. The relationships among fracture strength, the number of fracture surface marks and the mean velocity are discussed.

Relationships among central crack length, crack velocity and fracture surface marks in a center-cracked specimen of PMMA under tension.

Relationships among central crack length, crack velocity and fracture surface marks in a center-cracked specimen of PMMA under tension.

Discussion of a scalelike fracture surface mark in fine ceramics.

Scalelike marks are usually observed in the fracture surface of a Si3N4 specimen tensioned to fracture at 20°C and 500°C. They are the regularly shaped marks, ellipselike and parabolalike ones along with other irregularly shaped marks. The regularly shaped marks are carefully observed and analyzed in the same manner of defining an ellipselike mark (both ends closed mark, BECM) and a parabolalike mark (one end opened mark, OEOM) observed in unsaturated polyester resins (UP) and PMMA, as reported previously. the marks in a smooth specimen are mostly the OEOMs where the crack velocity often reached about 4500m/s at maximum and the BECMs are dominated in the fracture surface of a single edge-notched tensile specimen in which the crack velocity reached the maximum value that is less than the above. The variation of the marks and the mechanism of forming these marks seem to be the same with the cases in UP and PMMA, and they are discussed in detail from the points of fracture strength and the crack velocity change. The shapes of the marks are approximated by the equation of fracture surface marks.

Analysis of the proximity-effect of a before mark on the behind one on the fracture surface of a brittle plastic sheet.

A fracture surface mark is known to be formed by an interference between a primary crack front and a secondary crack front. We have shown so far that many such marks have been observed on a fracture surface of a brittle plastic specimen. The primary crack front passing through a before mark produces the succeeding behind mark as it propagates in the material, and the effect of the before mark often appears in the behind one. The cusp of the latter usually extends further as the two mark5 are closer together in front and in the rear on a fracture surface. For example, the experimentally measured chordal length at the nucleus of the behind mark increases as they close. Here, we define the proximity p* to express the linear distance between the nuclei of the two marks in front and in the rear, and we modify the previously reported equation of fracture surface marks to express those changes as a function of the proximity of the two. This expression, of course, gives better parabola-like configuration of the observed behind mark than the previous one.

The variation of fracture surface marks in brittle plastic sheet specimens during a primary crack propagation. In the case of an intermediate-length central crack.

We have reported that fracture surface marks in brittle plastic sheet specimens having short (≤ 5mm) and long (≥ 40mm) central primary cracks are well expressed by equations under the fracture conditions of a constant load (or stress) and a constant deflection (or strain), respectively. Here, we discuss the velocity and the marks in the specimens having an intermediate-length central crack. The crack velocity is measured and expressed by an equation where the velocity is presumed to be composed of both constant stress and strain components. In this case, a fracture starts with a constant stress condition which changes gradually to a constant strain condition as the crack propagates, and we call this a fracture in a mixed condition. This velocity expression, combined with the existing relation between crack velocity and a dynamic stress-intensity factor, gives the variations of the dynamic stress-intensity factor and a dynamic strain energy release rate both of which excellently predict the experimental variations of the fracture surface marks in those specimens such as the number of marks, the relative interference appearing distance (e/Co2) and the relative critical distance (d/Co2).

Variation of fracture surface marks due to static and dynamic strain rate.

Fracture tests of unsaturated polyester resin on large, single-edge notched-bend specimens were conducted under static and dynamic bend tests at room temperature. The velocity of a loading point in the three-point bending was changed from 8.3×10-6mm/s to 7×105 times its value. The two types of fracture marks of the both-ends-closed mark (like ellipses) and the one-end-opened mark (like parabolas) were observed and they were expressed by the equation of fracture marks. A rocket mark, defined here, was also often observed in the dynamic bending test and was well-approximated by the equation. The experimental number, percentage, relative interference appearing distance and the calculated relative critical distance of the marks changed with the crack propagation distance and the testing speed. The velocity of a primary crack propagation was measured and approximated by a mixed-condition relation proposed by us. This change and the gammer-letter relation between the velocity and a dynamic stress intensity factor KD (or a dynamic strain energy release rate, SD) gives the latter (KD or SD) as a function of the crack propagation distance. This provides a good theoretical explanation of the above variations of fracture marks.

Fatigue crack layer propagation in polystyrene?Part I experimental observations

Fatigue crack propagation experiments performed on thin, single edge notched, polystyrene specimens show that during slow crack propagation, an intensive zone of crazing surrounds and precedes the propagating crack. The system of the crack and the craze zone constitute a crack layer (CL). The part of the CL ahead of the crack tip, where crazing accumulates prior to CL growth, is called the active zone. The width of the active zone is found to increase by almost an order of magnitude during slow CL propagation. Fractographic analysis reveals two types of discontinuous crack growth bands: multicycle bands and single cycle bands. The point of transition from multicycle to single cycle bands is established and used to extract crack speed from fracture surface markings. The experimental results demonstrate that fracture propagates by the translation, isotropic expansion and distortion of the active zone. Analysis of craze distribution within the active zone reveals that the craze density decreases away from the crack. A considerable difference in the critical energy release rates is observed in specimens fatigued under different loading levels. This is attributed to the difference in the density of crazes at the critical crack tip.

Relationship between sharply notched fracture strength and fracture surface marks in a brittle plastic in tension. (In the case of an inside loading)

The fracture strength was varied by changing an inter-grip distance (which we call a gage length here) of a sharply center-notched tensile sheet specimen of unsaturated polyester resin, where the specimen width, thickness, crack length and loading method were kept constant. The loading method is such that the main load works on a central-line parallel to the axis of loading which crosses perpendicularly the central crack length. We call this an inside loading in contrast to an outside loading (1). The number, density, area ratio of fracture surface marks vary with the fracture strength of the material. The total number of marks increases with the fracture strength. The density and area ratio marks increase at each different rate with respect to fracture strength. These results are discussed with the analytical relations of the fracture strength, the dynamic stress-intensity factor and the dynamic strain-energy-release rate by employing the reported relations. The fracture surface marks are examined by the two-dimensional equation of fracture surface marks, and the analytical area ratio of the marks is also calculated.