INITIATION AND GROWTH OF CRACKS IN BIAXIAL FATIGUE

Abstract— Observations of fatigue crack growth in smooth specimens under biaxial loading are reviewed, with particular reference to the Stage I to Stage II and Stage II to Stage I transitions. Further results are presented for 1% Cr-Mo-V steel and AISI 316 stainless steel at various temperatures, showing that all cracks may be classed as either Stage I or Stage II. Predictive criteria are suggested for the typè of crack obtained, and the mechanisms for elevated temperature crack initiation are discussed.     

FORMATION AND GROWTH OF FATIGUE CRACKS IN REINFORCING STEEL FOR CONCRETE

Abstract— –The da/d Nvs Δ Krelation for Torbar Steel is obtained by testing specimens cut from 32 mm bar. This relation is compared with available data for Unisteel 410 and with medium strength steels. Also, the pattern of fatigue cracking in Torbar is studied by inspecting bars retrieved from reinforced concrete beams which were subjected to repeated loading. It has been observed that fatigue crack locations in the bars coincided with the locations of concrete cracks traversing the bars. Tension tests on the fatigue cracked bars show that the cracks considerably reduce the ductility of the bars.     

A REVIEW OF FATIGUE CRACK GROWTH IN STEELS UNDER MIXED MODE I AND II LOADING

The majority of methods for predicting the direction of propagation of mixed mode cracks have assumed that they branch to grow as mode I cracks. However, under some circumstances mode II crack growth occurs. Rolling contact fatigue cracks are one example of an industrial problem where cracks appear to grow under predominantly mode II loading without branching. This paper reviews the available models and the experimental studies in the literature on mixed mode I and II loading, and discusses the parameters that affect the mode of crack growth.     

INITIATION AND GROWTH OF FRETTING FATIGUE CRACKS IN THE PARTIAL SLIP REGIME

Abstract— —Recent work addressing the problems of fretting fatigue crack initiation and propagation under a carefully controlled axi-symmetric Hertzian contact is described. Both experimental work, enabling the fretting damage, sites of initiation, and crack trajectory to be viewed, and theoretical work, permitting a prediction of those processes are presented. Good correlation between the two strands of work is found. In particular, the initiation criterion proposed by Ruiz, Boddington and Chen for a very different geometry is found to work well, and would seem to indicate its potential as a design tool.     

CYCLIC FATIGUE CRACK GROWTH AND CLOSURE BEHAVIOUR IN ALUMINA CERAMICS

Cyclic fatigue crack growth behaviour in alumina ceramics is investigated and the effect of grain size discussed. Special attention is given to crack closure effects. Cyclic fatigue tests were carried out using four-point bend specimens, and the load–strain and load–differential strain curves were monitored. These curves show hysteretic behaviour probably related to frictional sliding of bridging grains, and also include non-linearities due to crack closure. The crack opening load is determined from the load–differential strain curve by using a method introduced in this study. Growth rates can be successfully described by the relationship da/dN = C[ΔK_{eff /(1 ? Kmax /KIC )]m} which is proposed in this study to account for the effects of crack closure and the maximum stress intensity factor. Irrespective of grain size, growth rates can be well represented by the above relationship, implying that the grain size exerts an influence on growth rates not only because of crack closure behaviour but also the material fracture toughness. The growth rate curve based on the proposed relationship shows a sigmoidal form for ceramic materials, which is similar to metals.     

FATIGUE GROWTH OF SURFACE CRACKS IN THE ELASTIC-PLASTIC REGION

Surface crack growth has been studied in centre notched and smooth panels of low carbon steel under symmetric axial elastic-plastic cyclic straining. The crack shape, followed by heat tinting, has been found to be approximately semi-circular for crack radii 0.3–5 mm. Both smooth and notched specimens have been fatigued until fracture. The smallest notches of 0.1 mm in radius and depth approximately represent defects of a critical size under present loading conditions. Crack propagation rates have been measured and plotted vs ΔJ. Both present part through-crack data and previous long through-crack data in the elastic and the elastic-plastic regions are compatible. A single dependence of dl/dN vs ΔJ is obtained.     

FINITE ELEMENT ANALYSIS OF CLOSURE BEHAVIOUR OF FATIGUE CRACKS IN RESIDUAL STRESS FIELDS

An elastic-plastic finite element analysis with high order elements is performed to examine closure behaviour of fatigue. cracks in residua1 stress fieids and the numerical results are then compared with experimental results. The finite element analysis, performed under plane stress using 8-node isoparametric elements, can predict fatigue crack closure behaviour through residual stress fields very well. The crack opening and closing behaviour through a compressive residual stress field is found to be complicated and influenced by the applied load magnitude and the location of the crack tip. Three different types of crack opening behaviour, namely, normal, unsymmetric partial and symmetric partial crack opening behaviour are observed through a compressive residual stress field. The partial crack opening stress intensity factor including the partial crack opening effect is recommended for the prediction of fatigue crack growth through a compressive residual stress field.     

CRACK GROWTH AND CLOSURE BEHAVIOUR OF SURFACE CRACKS UNDER AXIAL LOADING

Abstract— Crack growth and closure behaviour of surface cracks in 7075-T6 aluminium alloy are investigated under axial loading, noting the difference in fatigue growth behaviour at the maximum crack depth point and at the surface intersection point and also with through-thickness crack growth behaviour. The plane strain closure response at the point of maximum depth of a surface crack is monitored using an extensometer spanning the surface crack at the midpoint of its length. The plane stress closure at the surface intersection point is observed by multiple strain gauges placed at appropriate intervals ahead of the crack tip and continuously monitored without interrupting the fatigue test. The crack opening ratio is found to be about 10% greater at the maximum depth point than at the surface intersection point. Under axial loading, the difference in plane strain crack closure behaviour between the surface crack and the through-thickness crack is relatively small. Growth rates of surface cracks can be well described by the effective stress intensity factor range based on the closure measurements made in this study. The growth rates in terms of the effective stress intensity factor range seem to be slightly slower in surface cracks than in through-thickness cracks.     

DEVELOPMENT OF SEMI-ELLIPTIC FATIGUE CRACKS IN AK6 ALUMINIUM ALLOY UNDER BIAXIAL LOADING

Fatigue mechanisms are investigated in AK6 aluminum alloy under biaxial loading. The development of a semi-elliptic crack under conditions of biaxial compression-tension is connected with the emergence of spherical particles. A mechanism of rotational instability is suggested, connecting the processes of particle formation with a specific mechanism of material failure. A similarity is revealed in the formation of fatigue striations within the studied ratio λ of biaxial loads from –0.9 to +0.9. As λ increases, the crack-growth rate also increases. It is suggested that this be allowed for when introducing a correction to the mode I stress-intensity factor.     

THE GROWTH OF SMALL CORROSION FATIGUE CRACKS IN ALLOY 2024

Abstract— The corrosion fatigue crack growth characteristics of small surface and corner cracks in aluminium alloy 2024 is established. The damaging effect of salt water on the early stages of small crack growth is characterized by: (1) crack initiation at constituent particle pits, (2) intergranular microcracking for a≤100μm, and (3) transgranular small crack growth for a≥100μm. In aqueous 1% NaCl and at a constant anodic potential of −700 mV_SCE, small cracks exhibit a factor of three increase in fatigue crack growth rates compared to laboratory air. Small cracks exhibit accelerated corrosion fatigue crack growth rates at low levels of Δ K (< 1 MPa√m) below the long crack Δ K _th value. When exposed to Paris regime levels of crack tip stress intensity, small corrosion fatigue cracks exhibit growth rates similar to that observed for long cracks. Similar small and long crack growth behavior at various levels of R suggest that crack closure effects influence the corrosion fatigue crack growth rates of small cracks for a≥100 μm. Contrary to the corrosion fatigue characteristics of small cracks in high strength steels, no pronounced chemical crack length effect is observed for alloy 2024 exposed to salt water.     

PROPAGATION AND CLOSURE OF SHORT FATIGUE CRACKS AT NOTCHES UNDER COMPRESSIVE MEAN STRESSES

Abstract— Single-edge-notched specimens of a low-carbon steel were fatigued under cyclic in-plane bending with compressive mean stresses. The development of crack closure with crack growth was studied both experimentally and theoretically. The relation between the crack opening stress and the crack length was a function of the minimum (compressive) applied stress, irrespective of the maximum stress. The effective stress intensity range was a unique parameter in correlating the crack growth rate, even if the crack was embedded in the compressive plastic zone. Under a constant minimum stress, the length of nonpropagating cracks became longer with increasing maximum applied stress. A theoretical model was proposed for predicting the crack opening stress on the basis of the compressive stress distribution at the minimum applied stress. The predicted value agreed fairly well with the experimental result. The model gave upper bounds of the crack growth rate and the length of nonpropagating fatigue cracks within the plastic zone.     

A METHOD OF ESTIMATING BIAXIAL FATIGUE GROWTH RATES

Abstract —Conventionally, fatigue growth rate data are correlated in terms of the range Δ Kof the applied stress intensity factor. It is argued that a correlation involving both Δ Kand the constant stress T close to the tip of the equivalent elastic crack should be adequate for moderate stress, biaxial fatigue data. Practical application of these ideas would involve the preparation of " T -term" compendia similar to those already available for K -calibrations. " T -term" data are presented for the case of an edge-cracked strip in tension.     

FATIGUE GROWTH OF PHYSICALLY SMALL INCLINED CRACKS

The growth of physically small, self initiated, inclined corner and through-the-width cracks is investigated in a carbon steel under tension fatigue (R= 0.05). A preliminary procedure involving crack initiation under far-field cyclic compression is used. This precracking method is adopted to minimize the effect of residual damage at the tip of the crack grown (and arrested) under cyclic compression. Thus, the subsequent tensile fatigue crack propagates through a region with no (or very small) residual stress or damage. Experiments indicate that the early growth rate of the inclined corner cracks is discontinuous with a few decelerations. During the intermittent propagation period the corner cracks only extend into the thickness (depth) of the specimen and do not propagate along the width direction. However, after reaching a certain aspect ratio, the inclined flaws grow in a steady (continuous) manner in both the thickness and width directions. Through-the-width cracks, both inclined and perpendicular to loading direction, do not show the discontinuous growth pattern typical of the inclined corner cracks, but exhibit only one minimum in the crack growth rate behaviour.     

NEAR-THRESHOLD FATIGUE CRACK GROWTH AND CRACK CLOSURE IN A NODULAR CAST IRON

Abstract— Near-threshold fatigue crack growth and crack closure were investigated in a nodular cast iron. Fracture surface roughness was promoted by spheroidal graphites. The spheroidal graphites are partially crushed to form powder which accumulates within the crack and thus enhance crack closure. The marked influence of stress ratio on near-threshold crack growth is due to graphite-induced crack closure. When the contribution of graphite-induced crack closure is excluded, the crack growth characteristics are insensitive to stress ratio, and the threshold behaviour for crack growth tends to disappear. In this case, Young's modulus becomes a controlling material parameter for the power law relationship between crack growth rate and stress intensity range.     

CRACK CLOSURE AND FATIGUE CRACK GROWTH RATE IN THREE POINT BEND SPECIMENS OF DIFFERENT WIDTHS

Abstract— The crack closure stress intensity factor values and fatigue crack growth rates were determined in Three Point Single Edge Bend, SE(B), specimens prepared from rails manufactured using two different grades of rail steels. The width, and correspondingly the span, of the SE(B) specimens were varied eight fold; the thickness of all the specimens being the same. It is observed that the crack closure stress intensity factor values decrease with an increase in the width of SE(B) specimens. At a given value of Δ K _eff, the fatigue crack growth rate (FCGR) is independent of the width. However, at a given value of Δ K _eff, the FCGR is observed to decrease with increasing width. In view of the above results, the scope of application of the FCGR laws based on an effective stress intensity factor to the life prediction of components, requires careful examination.     

FATIGUE CRACK CLOSURE OF CORNER CRACKS LOCATED AT HOLES LOADED IN TENSION OR BENDING

Abstract— The objective of this paper is to report fatigue crack growth and closure behavior for corner cracked holes. An optical interference technique is employed with transparent polymer specimens to determine the three-dimensional crack surface displacement field. The local crack closure behavior is examined along the crack front under remote tension and pure bending conditions. It was observed that the crack opening loads along the hole and the free surface were significantly higher than the opening values at interior crack locations.     

THE GROWTH OF MICROSTRUCTURALLY SMALL FATIGUE CRACKS IN A FERRITIC-PEARLITIC STEEL

Abstract— The growth behaviour of microstructurally small fatigue cracks was investigated with smooth specimens of a ferritic-pearlitic steel, S45C. under rotating bending. The effects of microstructure, particularly the role of pearlite structure, on crack growth were evaluated based on detailed microscopic observations. In the region smaller than a certain crack length. small cracks tended to grow preferentially in the ferrite structure, and the crack growth rates decreased markedly at ferrite-pearlite boundaries when small cracks grew into the pearlite from the ferrite. The above region of crack length, i.e. the length of microstructurally small cracks, depended on stress level, increasing with decreasing stress level. The growth mechanism is also discussed in terms of the results obtained from fractogrdphy.     

FATIGUE CRACK GROWTH AND CLOSURE AT HIGH STRESS RATIOS

Abstract— Fatigue crack growth tests have been carried out on a medium carbon structural steel over a wide range of stress ratios, i.e. from 0 to 0.7. All tests were conducted under constant amplitude loading conditions corresponding to growth rates in the Paris regime. Crack closure behaviour was observed experimentally by a surface strain gauge technique, and numerically by a finite element analysis under plane stress condition. While the crack closure could not be detected by experimental measurements at stress ratios equal to or greater than 0.5, the numerical results showed that closure occurred even at high stress ratios up to 0.7. The differences between experimentally and numerically determined crack opening levels were found for each stress ratio. A cause for these differences is discussed. In addition, two new types of crack tip parameters which have been proposed recently are evaluated by finite element analysis and their relevance to fatigue crack growth are discussed. It is concluded that fatigue crack growth rates are substantially determined by the effective stress intensity factor range which is based on the crack closure concept.     

FATIGUE CRACK GROWTH UNDER BIAXIAL LOADING

Fatigue crack growth under biaxial loading for long cracks subjected to low cyclic stress levels was investigated. The biaxial stress ratio λ ranging from -0.5 to + 1.0 was considered. The strain energy density factor range was used as the criterion for predicting the crack growth rates and crack path. The agreement between prediction and experimental results was reasonable for crack growth rates and marginal for crack paths. The investigation highlighted the inherent difficulties for crack path prediction and indicated the increased sensitivity to initial crack angle and biaxial stress ratio when the biaxial stress ratio approaches unity.