Far-field Cyclic Compression Research Articles

Development of Fatigue Pre-Cracking Method into Micro-Sized Specimens for Measuring Fracture Toughness

ABSTRACTIn our previous investigations [1, 2], we have demonstrated that the introduction of fatigue pre-crack ahead of a notch is required to measure reliable fracture toughness values even for micro-sized specimens. However, it is rather difficult to introduce a fatigue pre-crack into a micro-sized specimen as once a fatigue crack starts to grow then the fatigue fracture occurs within one thousand cycles and this makes it extremely difficult to control fatigue crack length. Therefore, a new fatigue pre-cracking method is required for measuring fracture toughness. In this investigation, a new fatigue pre-cracking method has been proposed for micro-sized specimens and fracture toughness tests were carried out for the micro-sized specimens with fatigue pre-crack. Micro-cantilever beam type specimens with dimensions of 10 × 10 × 50 μm3 were prepared from an electroless deposited Ni-P amorphous alloy thin film and notches were introduced by focused ion beam machining. Fatigue pre-cracks were introduced ahead of the notches by far-field cyclic compression method using a mechanical testing machine for micro-sized specimens (MFT2000). Fracture tests were also carried out using the testing machine. Fatigue pre-cracks with length of 0.2 μm were confirmed on the fracture surfaces ahead of the notches in the far-field cyclically compressed specimens. This indicates that the fatigue pre-cracking method developed in this investigation is promising for measuring accurate fracture toughness for micro-sized specimens for MEMS applications.

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, ΔK, 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.

Crack growth and closure effects under far-field cyclic compression in a SiC-whisker-reinforced 2124 aluminium composite: Kwon, D., Kim, Y.-H. and Lee, S.

Crack growth and closure effects under far-field cyclic compression in a SiC-whisker-reinforced 2124 aluminium composite: Kwon, D., Kim, Y.-H. and Lee, S.

Effect of orientation on crystallographic cracking in notched nickel-base superalloy single crystal subjected to far-field cyclic compression

The effects of crystallographic orientation on the fatigue crack initiation and growth under far-field cyclic compression are discussed in single crystals of nickel-base superalloy MAR-M200. Results indicate that cracking occurs primarily due to planar slip on the {111}-type planes. Crystallographic cracking can occur on two or more slip planes simultaneously, but through-thickness cracks are not observed. In addition, it has been shown that the threshold stress intensity for crack initiation shows a strong dependence on orientation. The threshold stress intensity for crack growth in cyclic compression is 5 to 10 times the threshold for crack growth in cyclic tension.

Effect of orientation on crystallographic cracking in notched nickel-base superalloy single crystal subjected to a far-field cyclic compression: Aswath, P.B. Metall. Trans. A (Feb. 1994) 25A(2), 287–297

Effect of orientation on crystallographic cracking in notched nickel-base superalloy single crystal subjected to a far-field cyclic compression: Aswath, P.B. Metall. Trans. A (Feb. 1994) 25A(2), 287–297

Cyclic stress fields for fatigue cracks in amorphous solids Experimental measurements and their implications

The evolution cyclic residual stress fields within the damage zone ahead of fatigue cracks is known to have a significant influence in the fatigue fracture response of materials during constant-amplitude and variable-amplitude cyclic loading. In this paper, we present direct and in situ measurements of cyclic stress fields ahead of fatigue flaws in a model amorphous solid subjected to far-field cyclic compression loading. Photoelasticity and laser interferometry experiments conducted on single-edged-notched plates of a photoelastic resin are used to determine the residual stresses within the near-tip damage zone during the inception and subcritical growth of the fatigue flaw from the stress concentration. Quantitative analyses of the relevant stress components are used to establish a link between the evolution of cyclic near-tip fields and the conditions for the onset and advance of fatigue flaws. Transmission electron microscopy observations are presented for a rubber-toughened polystyrene to illustrate how the residual tensile stresses developing within the cyclic damage zone cause crazes to form along the plane of the fatigue crack, in a direction normal to the far-field compression axis. A series of systematic experiments on the effects of mean stress on fatigue fracture is reported, and the results of the experiments are rationalized with the aid of the near tip cyclic stress measurements. The applicability of the results of this study to a broad range of materials, including crystalline metals and ceramics, semicrystalline and amorphous polymers, and composites, is demonstrated. Implications of this work for a variety of fatigue phenomena involving constant- and variable-amplitude fatigue are also addressed.

Crack growth and closure effects under far-field cyclic compression in a SiC-whisker-reinforced 2124 aluminum composite

Crack growth and closure effects under far-field cyclic compression in a SiC-whisker-reinforced 2124 aluminum composite

Propagation of small corner cracks initiated in cyclic compression

The growth of physically small corner cracks under tension-tension fatigue ( R = 0.05) is investigated in a carbon low-alloy steel. A preliminary procedure involving crack initiation under far-field cyclic compression is used. This pre-cracking method is adopted to minimize the effect of residual damage at the tip of the crack grown (and arrested) under cyclic compression on its subsequent tensile fatigue crack growth. Experiments indicate that the growth rate of corner cracks is initially high, then decreases towards a minimum and finally increases, approaching the long-crack growth values. The crack extension at which the growth rate reaches a minimum depends on the initial crack size and shape. The transition to long-crack growth values does not occur at a unique crack length but takes place over a small range of crack lengths.

Use of the Shadow Optical Method of Caustics to Predict Fatigue Crack Growth in Compression

The shadow optical method of caustics has been applied to the measurement of the bulging out of the surface of aluminum alloy 7075-T6 in the vicinity of a blunt notch in a single edge notch tensile sample after a compression overload and during compression fatigue. The size and shape of the caustic were found to be sensitive functions of the amount of elastic and plastic strain in the vicinity of the notch. Examination of the caustic during application and after release of the overload shows the elastic zone to be larger and more circular than the plastic zone, as predicted. A crack was propagated through the plastic zone using only far-field cyclic compression loading: the crack was self-arresting at the edge of the elasticplastic boundary created by the initial compression overload. The size of the plastic zone as measured by the caustic was found to agree with the maximum length of the crack. The actual surface profile in the vicinity of the notch was measured and its shape was used along with geometrical arguments to develop a model for interpretation of the caustics results. These results indicate that the caustics technique can be used to detect the presence of a plastic zone at the tip of a notch after a compressive overload, and that the size of this zone as measured by caustics is approximately equal to the size as determined by subsequent fatigue crack propagation.

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.

Mechanics and micromechanisms of fatigue crack growth in brittle solids

This paper is concerned with the mechanics and micromechanisms of stable mode I crack growth in brittle solids subjected to compression-compression fatigue and tension-tension fatigue loads. Constitutive models, results of finite element analyses, and experimental observations are described for monolithic ceramics and ceramic-matrix composites, plain concrete, and a transformation-toughened ceramic in an attempt to deduce a general theory on the origin of mode I fracture in notched plates under uniaxial cyclic compression at room temperature. An analysis of the residual stress field which develops at elevated temperatures in response to power law creep and far-field compressive cyclic loads is also presented. The principal “driving force” for mode I fracture in cyclic compression is the generation of a near-tip zone of residual tension, when the deformation at the notch-tip leaves permanent strains upon unloading from the far-field compressive stress. The results indicated that materials with very different microscopic deformation mechanisms, i.e., microcracking, dislocation plasticity, martensitic transformation, interfacial debonding/slip, or creep, exhibit a macroscopically similar, stable fracture under far-field cyclic compression because the zone of residual tension is embedded in material which is elastically strained in compression. It is shown that cyclic compression loading offers a unique method for fatigue precracking notched specimens of brittle solids prior to tensile fracture testing, whereby an unambiguous interpretation of the critical stress intensity factors for crack initiation and growth can be achieved. Fatigue crack growth characteristics of a transformation-toughened ceramic and a creeping ceramic composite under tension-tension fatigue loads are also discussed.

The failure of hard materials in cyclic compression: Theory, experiments and applications

In this paper a summary is presented of recent developments in the understanding of mode I crack growth at room temperature in hard materials subject to uniaxial cyclic compressive stresses. Experimental observations of fatigue crack growth in cemented carbides, single-phase ceramics, transforming ceramics, ceramic composites and concrete are briefly reviewed. A detailed discussion of the “driving force” for the growth of a stable crack in a direction macroscopicaly normal to the far-field compression axis is presented. The “universal features” of cyclic compression fracture in a wide range of brittle and ductile solids are described. Some important applications of the compression fatigue phenomenon to the fracture toughness testing of brittle materials are high-lighted. The overall framework for a general theory of fatigue failure under far-field cyclic compression is developed.

Load Interaction Effects on Compression Fatigue Crack Growth in Ductile Solids

The influence of the very first compression cycle on subsequent Mode I crack growth in notched plates of ductile solids subject to fully compressive cyclic loads has been examined in this work. Whereas stable crack growth occurs from the root of the notch over hundreds of thousands of compression cycles, the amplitude of the very first compression cycle is found to have a decisive effect on the total distance of crack advance. Specifically, a three-fold increase in the stress amplitude during the first compression cycle alone leads to more than a ten-fold increase in the total distance of crack growth during subsequent cycles. Finite element calculations of the crack closure stress levels during cyclic compression are in concurrence with the experimental results of closure stresses (measured from changes in compliance) for a lower strength steel. A brief discussion is also presented of the effects of periodic compressive overloads on crack growth behavior under far-field cyclic compression.

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 initiation under far-field cyclic compression and the study of short fatigue cracks

A procedure involving crack initiation under far-field cyclic compression is used to study the fatigue behavior of small flaws which are ~0.3–0.5 mm in length and are amenable to linear elastic fracture mechanics (LEFM) characterization. This technique enables the determination of the threshold stress intensity range at which crack growth begins for small flaws and provides insight into some closure characteristics. Cracks were propagated in notched specimens of a bainitic steel subjected to fully compressive remote cyclic loads, until complete crack arrest occurred after growth over a distance of only a fraction of a mm at a progressively decreasing velocity. Following this, physically small flaws were obtained by machining away the notch. For the loads examined, the results indicate that the extent of damage left at the tip of the crack grown (and arrested) under remote compression is not large enough to affect subsequent tensile fatigue crack growth, when closure effects are not significant (e.g. at high load ratios). At high load ratios, the growth of small linear elastic cracks is identical to that of corresponding long flaws subjected to the same stress intensity range, which corroborates the similitude concept implicit in the nominal use of LEFM. At low load ratios, however, short tensile cracks propagate substantially faster than the longer flaws and exhibit lower threshold stress intensity range levels. Such apparent differences in their growth rates seem to arise, to a large extent, from the differences in their closure behavior, as indicated clearly from various aspects of the compression method. Global measurements of closure, with their inherent uncertainties, however, cannot account completely for the anomalous behavior of short flaws and for the effect of load ratio on short crack growth. Closure of short flaws begins to develop after growth over a minimum distance of about 0.5 mm in this steel. The significance and limitations of the compression technique are discussed and possible mechanisms responsible for the differences between long and short fatigue cracks are outlined.