Crack propagation in ceramics under cyclic loads

Stable crack growth is observed in notched plates of polycrystalline alumina subject to fully compressive far-field cyclic loads at room temperature in a moist air environment andin vacuo. The fatigue cracks propagate at a progressively decreasing velocity along the plane of the notch and in a direction macroscopically normal to the compression axis. The principal failure events leading to this effect are analysed in terms of notch-tip damage under the far-field compressive stress, microcracking, frictional sliding and opening of microcracks, and crack closure. An important contribution to such Mode I crack growth arises from the residualtensile stresses induced locally at the notch-tip when the deformation within the notch-tip process zone leaves permanent strains upon unloading from the maximum nominal compressive stress. It is shown that the phenomenon of crack growth under cyclic compressive stresses exhibits a macroscopically similar behaviour in a wide range of materials spanning the very ductile metals to extremely brittle solids, although the micromechanics of this effect are very different among the various classes of materials. The mechanisms of fatigue in ceramics are compared and contrasted with the more familiar examples of crack propagation under far-field cyclic compression in metallic systems and the implications for fracture in ceramic-metal composites and transformation toughened ceramic composites are highlighted. Strategies for some important applications of this phenomenon are recommended for the study of fracture mechanisms and for the measurement of fracture toughness in brittle solids.     

Crack nucleation and growth periods in 30CrMnSiNiA steel under repeated-impact tensile loads

The article describes the results of a study of the effect of material structure (tempering temperature) and test temperature on the duration of the crack nucleation and growth stages during impact fatigue tests on 30CrMnSiNiA. It was shown that the period of nucleation of microcracks increases with an increasing UTS of the steel. The nucleation of microcracks is not sensitive to temper brittleness of steel. Lowering the test temperature leads to an increase in the crack nucleation period and shortens its growth period.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 5, No. 5, pp. 539–541, September–October, 1969.     

Crack growth retardation under aircraft spectrum loads

In the past, the prediction of the fatigue life of a part was often compared with Miner's rule (Σn/N = 1) wherein damage was computed by a linear accumulation of growth under individual loads. The difference between the predicted and observed damage for variable amplitude tests showed up as either retarded or accelerated fatigue life depending on test conditions. In this program, fatigue crack growth tests were performed with the compact tension specimen under aircraft spectrum loading to develop techniques for predicting fatigue life. Two models were investigated for their application to retardation behavior—linear cumulative growth and Wheeler. Constant amplitude crack growth rate data represented by the Forman equation was used as source data for the computer models. These models appear suitable for comparison with actual test results for diffusion-bonded Ti-6A1-4V and HP-9Ni-4Co steel at 0.3 and 0.2 carbon contents for the aircraft spectrums investigated. The 2219-T851 aluminum alloy, on the other hand, exhibited accelelerated behavior in relation to the linear cumulative growth scheme. An explanation is proposed by relating recent crack closure and spike overload studies in aluminum with the format of the applied spectrum. Also, the Wheeler model was found dependent on material and spectrum variables, and the Forman equation did not accurately represent crack growth rate below a rates of 10⁻⁵ in/cycle.     

Crack growth in ferroelectric ceramics under electric loading

Summary A crack with growth in ferroelectric ceramics under purely electric loading is analyzed. The crack tip stress intensity factor for the growing crack under small scale conditions is evaluated by employing the model of nonlinear domain switching. The electrical fracture toughness is obtained from the result of the stress intensity factor. It is shown that the ferroelectric material can be either toughened or weakened as the crack grows. Fatigue crack growth in a ferroelectric material under cyclic electric loading is also examined. The incremental fatigue crack growth under cyclic electric loading is obtained numerically. The fatigue crack growth rate is affected strongly by the electrical nonlinear behavior. It is found that the curve of fatigue crack growth rate versus electric field intensity factor is linear on the log-log plot at intermediate values of the electric field intensity factor.     

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.     

Elevated-temperature crack growth in polycrystalline alumina under static and cyclic loads

An experimental investigation has been conducted to study the crack growth characteristics of a 90% pure aluminium oxide in 1050 °C air under static and cyclic loads. It is shown that the application of both sustained and fluctuating tensile loads to the ceramic, tested in a precracked four-point bend specimen configuration, results in appreciable subcritical crack growth. The crack velocities under cyclic loading conditions are up to two orders of magnitude slower than those measured in static loading under the same maximum stress intensity factor. Cyclic crack growth rates are markedly affected by the loading frequency, with a decrease in test frequency causing an increase in the rate of crack advance. Detailed optical and electron microscopy observations have been made in an attempt to study the mechanisms of stable crack growth and the mechanistic differences between static fatigue fracture. Under both static and cyclic loads, the predominant mode of fracture is intergranular separation. The presence of a glass phase along the grain boundaries appears to have a strong effect on the mechanisms of crack growth. Apparent differences in the crack velocities between static and cyclic fatigue in alumina arise from crack-wake contact effects as well as from the rate-sensitivity of deformation of the glass phase. Our results also indicate that the cyclic fatigue crack growth rates cannot be predicted solely on the basis of sustained load fracture data. White stable crack growth occurs in the 90% pure alumina over a range of stress intensity factor spanning 1.5 to 5 MPa m^1/2, such subcritical fracture is essentially suppressed in a 99.9% pure alumina, ostensibly due to the paucity of a critical amount of glass phase. Both static and cyclic fracture characteristics of the 90% pure alumina are qualitatively similar to those found in an Al₂O₃-SiC composite wherein situ formation of glass phases, due to the oxidation of SiC in high-temperature air, is known to be an important factor in the fracture process.     

Localized superplastic deformation of nanocrystalline 3Y-TZP ceramics under cyclic tensile fatigue at ambient temperature

Cyclic tensile fatigue tests were performed on 100±20 nm, and 0.35 m 3Y-TZP ceramic specimens at room temperature. Localized superplastic deformation of the grains in the 100 nm material at and near to the fracture surfaces was first identified by AFM imaging. Slip band-like microfeatures, similar to those reported on some metals, were also unexpectedly seen to develop on the side faces. In contrast, the 0.35 m specimens retained their equiaxed grain morphology after undergoing similar testing conditions. The micromechanisms underlining these phenomena were discussed. Grain boundary diffusion of the respective atomic species is reasoned to be the major governing process in operation. And the contribution of a dislocation slip mechanism is considered to play a possible or parallel role. © 1998 Chapman & Hall     

Crack growth from naturally occurring material discontinuities under constant amplitude and operational loads

This paper discusses how cracks that grow from small naturally occurring material discontinuities under operational load spectra behave. The growth of small cracks under a representative maritime aircraft flight load spectrum is discussed first. The results of this study, when taken in conjunction with the authors previous studies into cracks growing under combat aircraft load spectra, illustrate how for cracks that grow from small naturally occurring material discontinuities under operational load spectra crack growth can often be easily and accurately computed. It is also shown that the Hartman–Schijve variant of the NASGRO crack growth equation is able to accurately represent the growth of small cracks in two different rail steels. It is further shown that the growth of both small and long cracks can be described by a family of da/dN versus ΔK curves and that, for 7050-T7451, the experimental procedures commonly used to determine a closure free da/dN versus ΔK curve produce curves that are consistent with those obtained using the Hartman–Schijve equation and allowing for small variations in the term ΔK_thr.     

Crack growth in non-homogeneous transformable ceramics. Part II: Crack deflection

Crack growth in transformation toughened ceramics is studied using a micromechanics based continuum model which accounts for both dilatant and shear transformation strain components. In the computations, the transformable phase is taken to be distributed non-homogeneously in order to model Zirconia Toughened Aluminas that have not been optimally mixed, or Duplex Ceramics in which large zirconia inclusion are dispersed in an untransformable matrix. The small scale transformation problem is solved using a finite element approach. The influence of the transformation strains around the propagating crack on the stress intensity at the crack tip is computed using the transformation domain integral. The crack is modelled as a missing row of mesh elements and crack growth is simulated by nullifying the stiffness of a crack tip element. In contrast to Part I of this paper [1], this part is concerned with cases where the transformable phase is not distributed symmetrically with respect to the x ₁-axis, which causes the crack to deflect from its original crack path due to a local shear stress intensity factor at the crack tip. A computational method is developed which is capable of simulating this, assuming that the deflections from the original crack path are small. A parametric study is carried out of the effect of crack deflection and crack meandering on the overall crack growth resistance.     

Tension stiffening in textile-reinforced concrete under high speed tensile loads

Damage mechanism in glass textile-reinforced concrete (TRC) with and without the addition of Alkali resistant short glass fibers under high speed tensile loading was investigated. The high strain rates ranging from 25 to 100 s⁻¹ were achieved using a high speed servo-hydraulic testing machine. Image analysis by means of digital image correlation (DIC) method was used to obtain the evolution of crack width which was subsequently correlated with stress response. The non-uniform strain distribution was characterized as three distinct response zones of localization, shear lag, and uniform strain and quantitatively measured in each zone. Mechanism corresponding to the basic aspects of tension stiffening modeling were identified by computing the average stress in the matrix phase between two cracks. The width of crack localization zone as well as crack spacing were also obtained using DIC as indications of bonding properties. A finite difference method simulating tension stiffening behavior was employed to predict crack spacing and stress–strain responses of TRC systems. Improvements in bond properties and mitigation of cracking with the addition of short fibers were verified using multiple methods.     

Crack growth rate under cyclic bending in the explosively welded steel/titanium bimetals

The paper presents the results of the tests on fatigue crack growth in a steel/titanium composite under oscillatory bending. Two kinds of specimens of rectangular cross sections were tested. In the tested specimens, the ratio of heights of basic and overlaid materials was h₁:h₂ = 2.5:1 and 1:1. In the specimens, the fatigue crack growth was parallel to the applied loading and its direction changed at the interface line. Next, the crack growth along the interface line or the crack growth passing through the interface line were observed. When the crack growth passed along the interface line, decrease of the crack growth rate took place. The specimens have the uniform crack growth at both sides of lateral surfaces. At the composite fractures in the steel and titanium, transcrystalline cracks are dominating.     

Crack growth in polymethylmethacrylate under low frequency cyclic loading. Effect of cycle duration

Crack growth in polymethylmethacrylate under low frequency cyclic loading was studied by double torsion method. The torques were varied with time in the triangular wave form. The cycle duration, , was ranged from 1 to 2000 secs. It was found that there were two qualitatively different modes of crack growth under cyclic loading. They are determined by the value of G, the crack extension force. At high values of G the length increment of the crack per loading cycle is proportional to . At lower values of G it does not depend on . The transition from one mode to the other is determined by a certain value of G=G _th, the threshold value, characterising a given material and ensuring continuous crack propagation under static loads.

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Résumé La propagation des fissures de fatigue dans le polyméthacrylate de méthyl a été étudée par la méthode de la double torsion. Les forces de traction ont été variées avec le temps en forme de l'onde triangulaire, le temps du cycle de chargement a été varié de 1 à 2000 sec. Il a été etabli qu'il y a deux modes qualitativement différents de la propagation des fissures de fatigue. Ils sont determinés par la valeur de G, la force d'extension de la fissure. Aux hautes valeurs de G l'incrément de la longueur de la fissure de fatigue par cycle de chargement est proportionnel à . Lorsque les valeurs de G sont plus petites, il ne dépend pas de . La transition d'un mode à l'autre est déterminée par une certaine valeur de G=G _th, la valeur qui caracterise le matériau en question et assure la propagation continuelle de la fissure sous une charge statique.

     

Crack growth and residual stress in Al-Li metal matrix composites under far-field cyclic compression

The growth of cracks under far-field cyclic compressive loading in aluminium-lithium (Al-Li) alloys reinforced with SiC particulates is investigated in notched compact tension specimens (CT). When cracks were initiated from the root of the notch, progressive deceleration occurred with the initial crack growth being largest. After crack arrest, analysis indicated that the initial residual stress diminished as the crack became non-propagating and at arrest the crack faces appeared to be open. When the crack closure loads were determined, it was shown that not all the stress amplitude produced crack growth and opening. This effect of crack closure was enhanced for small stress fields when the effective stress intensity dropped to the fatigue threshold of the alloy. For large residual stress fields the effective stress intensity range was well above the threshold and the initial crack growth rates were largest in the alloy containing the reinforcement particles. A residual strain model was used to determine the residual stress introduced in the root of the notch from the first compressive preload. It is shown that the fatigue crack growth was confined to a region of tensile stress within the residual stress field and the initial crack propagation rates were enhanced by the presence of the reinforcement. A dependence of the stress magnitude on growth rates was also established — the greater the residual stress at the root of the notch the larger the growth rates. The reinforcement had an additional amplification effect in terms of tensile distance from the notch. The effective stress intensity range, K, was investigated using compliance measurements and a model is introduced which explains the underlying features and mechanism of accelerated growth in both alloys, taking into account the reinforcement phase, plastic zone-size dependence and the residual stress field of the MMC.