Toughness Crack Propagation Research Articles

Contact damage initiation in silicon nitride in Hertzian indentation: role of microstructure

A bearing-grade silicon nitride with fine microstructure and a turbine-grade silicon nitride with coarse microstructure were studied with respect to the influence of their microstructures on (a) crack-growth-resistance behavior, (b) strength degradation due to Vickers indentation, and (c) crack initiation in quasi-static indentation with WC spheres. The turbine grade exhibited strong rising crack-growth resistance and less strength degradation due to Vickers indentation as compared to the bearing grade. Partial-ring or C cracks initiated in Hertzian indentation and the critical loads exhibited linear (Auerbach) variation with indenter radius above a critical value. For smaller radius, indentation plasticity preceded C-crack initiation. The bearing grade exhibited higher critical loads for C-crack initiation, but showed greater extension toward a ring crack than the turbine grade. These differences in crack initiation and growth were consistent with the differences in crack initiation and propagation toughness of the two grades. A ball-on-ball impact analysis was used to predict the critical velocities for initiating C cracks in the impact of silicon nitride surfaces with WC spheres.

Crack Propagation Toughness of Rock for the Range of Low to Very High Crack Speeds

Crack Propagation Toughness of Rock for the Range of Low to Very High Crack Speeds

Numerical Simulation of One-Point-Bend Impact Test.

A numerical method to analyze a one-point-bend specimen with a fast propagating crack was developed using the Timoshenko beam theory. The displacement and the dynamic stress intensity factor can be obtained by this method if the time histories of the impact force and crack velocity are given. A numerical simulation of the one-point-bend impact test was carried out using the above method and the Hertz theory of contact. The time dependence of impact force (and hence the dynamic stress intensity factor) can be calculated if the impact velocity and the time history of crack velocity are given. The possibility of measuring the fast crack propagation toughness using a one-point-bend impact test machine was discussed.

Influence of Ultrafine Ceramic Reinforcement on the Microstructure and Properties of Al-Li-Mg Alloys

The microstructure and mechanical properties of Al-0.3Li-1Mg alloys reinforced with volume fractions of up to 12.5% TiO 2 additions (20 nm in size) have been evaluated. After extrusion, the alloy microstructure was fine grained (< 1μm) and remarkably stable at 400°C. XRD and TEM studies gave evidence for extensive particle/matrix reactions during hot processing of materials, leading to the formation of Al 3 Ti particles and, to a lesser extent, of Al 2 0 3 /TiO/MgO phases. The alloy strength increased with volume fraction of reinforcement, although the ductility tended to decrease. With a volume fraction of 7.5% Ti0 2 , a proof stress of 300MPa and 180MPa, at room and at 300°C, respectively, could be achieved. After extrusion, the hardness and tensile properties increased during heat treatment in the temperature range 500°C to 600°C and was accompanied by an increase in the amount of Al 3 Ti. The creep, fatigue crack propagation and fracture toughness behaviour of these materials have also been studied.

Experimental Determination of Dynamic Crack Initiation and Propagation Fracture Toughness in Thin Aluminum Sheets

An experimental investigation was undertaken to characterize the dynamic fracture characteristics of 2024-T3 aluminum thin sheets ranging in thickness from 1.63–2.54 mm. Specifically, the critical dynamic stress intensity factor Kdc was determined over a wide range of loading rates ( expressed as the time rate of change of the stress intensity factor KdI ) using both a servo- hydraulic loading frame and a split Hopkinson bar in tension. In addition, the dynamic crack propagation toughness, KD, was measured as a function of crack tip speed using high sensitivity strain gages. A dramatic increase in both Kdc and KD was observed with increasing loading rate and crack tip speed, respectively. These relations were found to be independent of specimen thickness over the range of 1.5 to 2.5 mm.

Boundary element study on the optical method of caustic for measuring fast crack propagation toughness K ID

The dynamic stress field near the tip of a crack tip which is accelerated and decelerated in an elastic plate with finite width under impact loading is analyzed by the boundary element method, and a simulation of measuring fast crack propagation toughness K ID by the caustic method is performed. The results of the simulation agree qualitatively with the experimental results by Arakawa and Takahashi, and indicate the dependence of the ‘measured’ K ID not only on crack acceleration but also plate width. The dependence of ‘measured’ K ID on crack acceleration may result from the fact that under the condition of high loading rate or abrupt change in crack velocity, the transient stress field near the initial curve of caustic can not be represented fully by the dynamic stress intensity factor K I(t, v), as suggested by Rosakis et al. The dependence of ‘measured’ K ID on plate width may be attributable to the fact that the transient stress field near the initial curve is affected directly by the reflected stess wave and also indirectly through crack acceleration which depends on the reflected stress wave. The possible dependence of the ‘measured’ K ID on loading rate, loading history, crack propagation history is also discussed.

B.E. Analysis on Caustic Method for Measuring Fast Crack Propagation Toughness.

The acceleration and deceleration behaviors of a semi-infinite crack in an elastic plate with a finite width, which is subjected to time-dependent loading, are analyzed and the caustic method for measuring the fast crack propagation toughness KID is simulated by using the boundary element method. It is shown that the "measured" KID depends on the shape of the specimen when the dynamic stress intensity factor KI ( t, v) and/or the crack speed v changes rapidly. The dependence is explained by the numerical results that the KI(t, v) -controlled field does not develop enough to cover the location of the initial curve of caustics, i. e., the displacement field near the initial curve is influenced not only by KI (t, v) but also by (1) the superposition of stress wave reflected at the boundary and (2) the change of the crack acceleration due to the reflected stress wave

A Strain Gage Analysis of Fracture in Wide Plate Tests of Reactor Grade Steel

A new method of fracture analysis is described. Strains recorded by gages in the immediate vicinity of a propagating crack are analyzed. From the analysis, crack tip position, propagation toughness, and crack velocity are determined. The analysis procedure Is demonstrated using data from the dynamic fracturing of a large-scale, wide plate test. The results are then used to describe the propagation toughness-crack velocity-temperature relation for the 2.25 Cr-1 Mo steel used in the test.

Crack Propagation in Nial and FeAl

The crack-propagation behavior and fracture toughness at room temperature of extruded and heat-treated NiAl and FeAl were examined by testing chevron-notched, three-point flexural specimens at constant crosshead speeds. In Ni-50 at. % Al, sudden load drops occurred repeatedly, indicating run-arrest crack propagation. The fracture resistance was not found to depend ci the crosshead speed. Iron additions of up to 1 at. % and boron additions of 0.01 at. % did generally not improve the fracture toughness. By contrast, crack propagation in Fe-40 at. % Al occurred in a stable manner. In agreement with the environmental sensitivity of this intermetallic alloy, fracture resistance did depend on the crack-propagation velocity, indicative of the kinetic nature of this process. While the crack-growth resistance of iron aluminides was reduced by changing the aluminum content from 40 to 45 at. %, it was increased significantly by small additions of boron.

Boundary Element Analysis of Caustic Method. Estmation of Fast Crack Propagation Toughness KID.

An elastodynamic boundary element analysis of a fast crack propagation test is performed. A propagating crack subject to a time-dependent uniform pressure on its surfaces is considered. The crack speed is determined in such a way that the dynamic stress intensity factor K1(t, v) is equal to the fast crack propagation toughness KID. Caustics in reflection are generated by making the assumption of plane stress. The time variation of the measured KID is obtained from the diameter of the generated caustics. The KID thus obtained is found to depend on crack propagation history due to the transient nature of the elastic stress field near a propagating crack tip. This may explain to some extent the physical meaning of the experimental data showing the dependence of KID on crack acceleration.

ＳｉＣ粒子強化６０６１アルミニウム合金複合材料の破壊靭性

Metal matrix composite fractures in a complicated manner such as exfoliation between matrix and reinforcement or failure of reinforcements. Therefore, evaluation of fracture toughness becomes important for the application as structural materials especially from the view point of the guarantee of higher security. In this study, the effect of volume fraction of SiC particles on the fracture toughness and fracture mechanism of 6061 aluminum alloys reinforced with SiC particles (SiCp/6061-T6 composites) manufactured by powder metallurgy process were investigated by characterizing with fractography and microstructure. It was found that dynamic fracture toughness and crack propagation toughness were remarkably decreased with increasing the content of SiC particle. Furthermore, volume fraction of SiC particles in SiCp/6061-T6 composites showed a great influence on the crack propagation toughness. The micromechanism of the fracture process in the lower volume fraction of SiC particle was concluded as follows: 1, voids were formed at the ends of SiC particles, 2. voids grew and coalescenced, and then crack propagated. Stable voids growth was difficult with increasing SiC particles. Eventually the rapid unstable fracture occurred.

Fatigue crack propagation and cryogenic fracture toughness behavior in powder metallurgy aluminum-lithium alloys

Fatigue crack propagation and cryogenic fracture toughness properties of powder metallurgy (P/M) aluminum-lithium alloys have been examined by studying the behavior in mechanically alloyed (MA) Al−4.0Mg−1.5Li−1.1C−0.8O2 (IN-905XL) and rapid solidification processed (RSP) Al−2.6Li−1.0Cu−0.5Mg−0.5Zr (Allied 644-B) extrusions. Results are presented as a function of microstructure, mean stress, and specimen orientation and are compared with previous data on equivalent high-strength aluminum alloys fabricated by both ingot metallurgy (I/M) and P/M methods. It is found that the fatigue crack propagation resistance of the RSP Al−Li alloy is superior to traditional RSP aluminum alloys without lithium and even comparable to I/M Al−Li alloys, particularly at near-threshold and intermediate stress intensity levels. In contrast, crack growth rates in MA 905XL P/M extrusions are nearly three orders of magnitude faster and do not show benefits of alloying with lithium. Growth rate behavior in both alloys, however, is anisotropic; for example, crack growth rates in RSP 644-B alloy are up to three orders of magnitude faster in theT-L, compared toL-T, orientation. However, when characterized in terms of a closure-corrected near-tip “driving force,” ΔKff such differences are reduced. With respect to toughness, plane strainKIc values (L-T orientation) in the RSP alloy are observed to increase with decrease in temperature from 298 to 77 K; conversely, the MA alloy shows a small decrease inKIc at 77 K. Such results are interpreted in terms of the micromechanisms influencing fatigue and fracture behavior in Al−Li alloys, specifically involving the microstructural role of hardening mechanism, slip mode, grain structure, and texture on the development of crack tip shielding (crack path deflection and crack closure) and short-transverse delamination cracking.

Processing property characteristics for long glass fibre reinforced polyamide

The influence of microstructure on the stiffness and toughness of ‘long’ glass fibre reinforced nylon 66 mouldings was studied. Using the results of previous work which established the influence of processing conditions on moulding microstructure, the effects of processing on resulting mechanical properties are inferred. Whilst impact toughness is shown to be insensitive to processing conditions, slow crack propagation toughness is greatest in situations where the processing produces a pronounced core region of long fibres orthogonally aligned to the direction of crack propagation. Model studies of the influence of microstructure on stiffness suggests little sensitivity of modulus to likely variations in the processing parameters.

Micromechanism of improvement in crack initiation and propagation toughness of a Ti–Al–Sn–Zr–Mo alloy by coarsening prior β-grains

Instrumented Charpy V impact tests and static and dynamic fracture toughness tests were carried out on Ti–6Al–2Sn–4Zr–6Mo alloys in which the prior β-grain size was varied by heat treatment. The effect of microstructure on the toughness was then examined. With increasing prior β-grain size, the elongation, crack initiation, and particularly propagation toughness increased and the strength decreased slightly. The increase in crack initiation toughness was caused mainly by the increase in Widmanstätten α-lath size or spacing, while the increase in crack propagation toughness was caused by the deflection of the crack propagation path, which was brought about by the decrease in intersubcolony spacing. The intersubcolony spacing decreased with increasing number of ‘diffusion controlled’ Widmanstätten α nucleating sites, which were introduced by the deformation strain.MST/786

Micromechanism of improvement in crack initiation and propagation toughness of a Ti–Al–Sn–Zr–Mo alloy by coarsening prior β-grains

AbstractInstrumented Charpy V impact tests and static and dynamic fracture toughness tests were carried out on Ti–6Al–2Sn–4Zr–6Mo alloys in which the prior β-grain size was varied by heat treatment. The effect of microstructure on the toughness was then examined. With increasing prior β-grain size, the elongation, crack initiation, and particularly propagation toughness increased and the strength decreased slightly. The increase in crack initiation toughness was caused mainly by the increase in Widmanstatten α-lath size or spacing, while the increase in crack propagation toughness was caused by the deflection of the crack propagation path, which was brought about by the decrease in intersubcolony spacing. The intersubcolony spacing decreased with increasing number of ‘diffusion controlled’ Widmanstatten α nucleating sites, which were introduced by the deformation strain.MST/786

Ti-6 Al-2 Sn-4 Zr-6 Mo合金のき裂進展特性におよぼす旧β粒内下部組織の影響

Ti-6 Al-2 Sn-4 Zr-6 Mo合金のき裂進展特性におよぼす旧β粒内下部組織の影響

On the Mechanics of Dynamic Fracture

Recent progress in the study of dynamic fracture mechanics is reviewed. Attension is forced on the mode-I (crack opening mode) fracture from a practical viewpoint. In contrast to static fracture mechanics, both mathematical and experimental difficulties are encountered in the efforts to understand the dynamic fracture phenomena and there still remain some unanswered questions. For example, an enormously sharp increase in the impact fracture toughness KId observed at very high loading rates can be explained by assuming the existence of a fracture incubation time, but its physical background remains unknown or uncertain. For another example, the experimental data indicating the geometrical or crack-acceleration dependence of the crack propagation toughness KId raise a serious question about the extent to which the basic assumption, K(t, v) = KID (K(t, v) denotes the dynamic stress intensity factor), can be validly used. Further study is expected to answer these questions.

Dynamic analysis of crack growth and arrest in a pressure vessel subjected to thermal and pressure loading

Predictions of crack arrest behaviour are performed for a cracked reactor pressure vessel under both thermal and pressure loading. The object is to compare static and dynamic calculations. The dynamic calculations are made using an explicit finite-element technique where crack growth is simulated by gradual nodal release. Three different load cases and the effect of different velocity dependence on the crack-propagation toughness are studied. It is found that for the analysed cases the static analysis is slightly conservative, thus justifying its use for these problems.

A FEM analysis of crack arrest experiments

Crack arrest experiments are performed on edge-notched specimens of a high strength steel. The crack velocity and the displacements at the boundaries are continuously measured during the experiments. This information is then used in a subsequent FEM analysis to evaluate the dynamic stress intensity factor before and after crack arrest. Dynamic effects are seen to influence the process a considerable time after the arrest. A discussion of the validity of the arrest condition in a linear theory is made based upon experimental results. The inverse problem is also considered, i.e. to predict the crack growth event, the arrest length and the corresponding stress intensity factors when the crack propagation toughness is known for the material. The predicted values are shown to exhibit good agreement with the experimental results.

Numerical evaluation by FEM of crack propagation experiments

A number of crack propagation experiments are performed on sheets of a high strength steel. Momentaneous forces, crack lengths and velocities are measured. With this as input the dynamic stress-intensity factor is calculated with aid of a FEM program especially modified for the purpose. Observations regarding the dependence of crack propagation toughness on crack tip velocity and acceleration are made. A simple statistical analysis of the results is performed.