Subcritical crack growth of trip steels in air under static loads

Pre-fatigued compact tension fracture toughness specimens of TRIP steel were held at constant loads at 25°C in 40 per cent r.h. air. Testing was done using an MTS closed loop universal test machine in the load control mode and the displacement was monitored as a function of time using a clip-on gauge and a strip chart recorder. Subcritical crack growth was observed and the experimental data was used to obtain a correlation between stress intensity and the rate of crack growth. The curves usually exhibited three distinct regions, including a plateau of stress intensity insensitive constant crack growth rates which have been observed by other investigators for titanium alloys and high strength steels. Based on comparisons with other investigators, the mechanism of subcritical flaw growth was tentatively identified as being due to hydrogen embrittlement. Fracture was observed to follow austenite grain boundaries and it was hypothesized that the austenite → martensite transformation sensitizes them to hydrogen by causing a large strain accumulation to be accommodated at the boundaries resulting in a large dilatation. Metallography revealed that the crack growth rate decreased as the strain-induced martensite increased and this was attributed to crack tip blunting by plastic deformation due to the invariant shear of the transformation. There are thus two apparently competing processes in the subcritical flaw growth of TRIP steels. Fractography showed that numerous fracture micromechanisms were operative. The subcritical crack growth characteristics compared favorably with other high strength steels tested under almost equivalent conditions and an apparent threshold for subcritical growth in air was determined to occur at about 56 per cent of K_IC.     

Sub-critical crack extension and crack resistance in polycrystalline alumina

Sub-critical crack extension can readily be observed in controlled fracture tests in fourpoint bending. A natural crack of any desired lengthc which exceeds the notch depthc ₀ by the amount c =c –c ₀ can be introduced into bend specimens by stable crack propagation. The stress intensity factor to achieve c increases considerably with increasing c. In pre-cracked specimens the stress intensity factorK _I0 to start the crack and the critical valueK _IC strongly depend on the natural crack length c whereasK _I0 andK _IC are independent ofc ₀ in solely notched specimens. From a quasi-continuous evaluation of the load-deflection curve recorded during controlled fracture, the differential work of fracture can be obtained as a function of the achieved crack length. It may be regarded as the crack extension resistanceR of the material because the balance between the energy release rateg ₁ andR is maintained throughout the experiment. By that, a formal analogy to theR-curve concept of fracture mechanics is given. The steady increase ofR is explained by multiple crack formation and by the interference of the fracture surfaces due to the angular development of the crack front.     

Mixed-mode crack surface interference under cyclic shear loads

A previous modelling analysis predicted that crack surface interference under cyclic shear loads is a combination of cyclic shear attenuation and cyclic wedge-opening. In the present study, experimental evidence is provided on notched thin-walled tubular specimens to evaluate the modelling predictions. Tests were carried out with varied static tensions superimposed on fully reversed ( R _τ = −1) or pulsating ( R _τ = 0) cyclic shear loads. The crack surface interference was measured by near-tip strain gauge methods. Based on the single and dual strain gauge readings, the strains induced by the mode I and II interference are separately identified so that the cyclic wedge-opening behaviour was noted as a companion of the cyclic shear attenuation. The crack surface interference under cyclic shear loads is compared with the influence from varied static tensions and shear stress ratios. A comparison of the mode I crack surface interference is also made between the conditions with cyclic shear loads and cyclic tensile loads. Finally, the characteristics of crack surface displacements are discussed, and the experimental results of effective mode I and II stress intensity ranges are compared with the modelling predictions.     

Modelling of crack surface interference under cyclic shear loads

Crack surface interference under cyclic shear loads is studied by an analytical method. The proposed model simulates the effects arising from both the residual stresses and the asperity interactions. A closed-form and a discrete approach are presented in obtaining the crack surface interference solutions. Backlashes of shear displacements, peeling or group sliding behaviours and induced cyclic mode I stress intensities are predicted under three configurations of residual stress distributions. The effects of a static mode I load, the facet angle and the frictional angle are also analysed. The predicted relationship between the effective shear mode stress intensity range and the R -ratio is discussed together with the experimentally observed 'contrasting' R effects.     

Fatigue crack growth in polymers subjected to fully compressive cyclic loads

It is experimentally demonstrated in this work that the application of cyclic compression loads to polymeric materials, specifically high-density polyethylene and polystyrene, results in the nucleation and propagation of stable fatigue cracks. The cracks grow at a progressively slower rate along the plane of the notch in a direction perpendicular to the far-field cyclic compression axis. The overall characteristics of this compression fatigue fracture are macroscopically similar to those seen in metals, ceramics, as well as discontinuously reinforced inorganic composites. It is reasoned that the origin of this Mode I compression fatigue effect is the generation of a zone of residual tensile stress locally in the vicinity of the notch-tip upon unloading from the maximum far-field compressive stress. The residual tensile field is generated by permanent damage arising from crazing and/or shear deformation ahead of the notch-tip. Evidence for the inducement of residual tensile stresses on the crack plane is provided with the aid of micrographs of near-tip region where crazes are observed along the plane of the crack, i.e. normal to the compression loading axis. Compression fatigue crack growth in polystyrene is also highly discontinuous in the sense that the crack remains dormant during thousands of fatigue cycles following which there is a burst of crack extension, possibly in association with fracture within the craze. This intermittent growth process in cyclic compression is analogous to the formation of discontinuous growth bands during the tension fatigue of many crazeable polymers. The exhaustion of the near-tip residual tensile field and the increase in the level of crack closure with increasing crack length cause the fatigue crack to arrest. The universal features of this phenomenon are discussed in the context of ductile and brittle, non-crystalline and crystalline, as well as monolithic and composite materials.     

Analysis of static and dynamic fatigue of polycrystalline alumina

The fatigue failure of polycrystalline alumina in a moist air environment at 30° C has been analysed in terms of a modified Weibull distribution function using fracture mechanics theory. The good correlation obtained between the fatigue test data and fracture mechanics theory indicates that fatigue is controlled by the slow crack growth of pre-existing flaws. The distribution of these pre-existing flaws can be represented by the modified Weibull distribution which provides an upper and a lower limit strength and thus is more realistic for the physical phenomena it represents. Comparison of proof-test predictions with experiment indicate that the proof test can be effective in eliminating weak samples from the population.     

Crack growth in transforming ceramics under cyclic tensile loads

The mechanisms of stable growth of short fatigue cracks (crack length up to 1 mm) at room temperature in magnesia-partially stabilized zirconia subjected to cyclic tensile loads were investigated. Single edge-notched specimens were fractured in the four-point bend configuration under cyclic and quasi-static tensile loads. At a load ratio of 0.1, the threshold stress intensity factor range, K, for fracture initiation in cyclic tension is as low as 3.4 M Pam^1/2, and catastrophic failure occurs at K=6.6 M Pam^1/2. For crack length less than 1 mm and for plane strain conditions, growth rates are highly discontinuous, and periodic crack arrest is observed after growth over distances of the order of tens of micrometres. Crack advance could only be resumed with an increase in the far-field stress intensity range. The mechanisms of short crack advance in cyclic tension are similar to those observed under quasi-static loads, and the tensile fatigue effect appears to be a manifestation of static failure modes. A model is presented to provide an overall framework for the tensile fatigue crack growth characteristics of partially stabilized zirconia. Experimental results are also described to demonstrate the possibility of stable room temperature crack growth under cyclic tension in fine-grained tetragonal zirconia polycrystals, partially stabilized with Y₂O₃. The growth of cracks in transformation-toughened ceramics is found to be strongly influenced by the crack size and shape, stress state and specimen geometry.     

Studies of stable crack growth under two independent loads

SEN specimens of two different materials were tested under different combinations of independently applied bending and tension. J _R-curves were determined and showed to be independent of the load combinations. Stability analysis using the tearing modulus concept showed to be possible but difficult.

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Résumé Des éprouvettes à entaille latérale simple en deux matériaux différents ont été soumises à essai sous diverses combinaisons de charges de flexion et de traction appliquées de manière indépendante. On a déterminé les courbes J _R et on a montré qu'elles étaient indépendantes de la combination de charge adoptée. Une analyse de stabilité recourant au concept de module d'arrachement s'est révélée possible encore que difficile à mettre en veuvre.

     

Crack growth under static and cyclic loading in hot-stretched PMMA

Crack growth behaviour under static and cyclic loading was investigated using anisotropic plates of PMMA oriented by hot-stretching. Both tests were performed at room temperature for samples with different degrees of orthotropy. A slight increase in the degree of orthotropy considerably improves the resistance to both static and cyclic crack growth in the case where the crack propagates perpendicularly to the hot-stretched direction. A power law relationship between crack growth rate and stress intensity factor may hold for both types of crack growth in the ranges of orthotropy tested. The experimental data for static crack growth were compared with a viscoelastic criterion based on the crack opening displacement theory for fracture. The criterion discussed here explains comparatively not only the beginning of cracking from a pre-introduced crack, but also the crack growth rate in oriented PMMA.     

Prediction of part-through crack growth under cyclic loading

A comparative analysis is performed of the applicability of the local, averaged and effective stress intensity factor ranges as part-through crack growth criteria under cyclic loading. The effective stress intensity factor range is found to be preferable for the purpose of part-through crack growth prediction. The surface and corner crack shape variations under cyclic loading are predicted and fatigue lives of cracked specimens are estimated. Part-through crack growth prediction results are compared with experimental data.