INFLUENCE OF SHOT-PEENING ON SHORT CRACK BEHAVIOUR IN A MEDIUM CARBON STEEL

The results of research into short fatigue crack behaviour in shot peened, medium carbon steel specimens under reversed torsion are presented. Characteristic features of short crack growth were established on the basis of optical and electron-optical observations of the replicas and micro-sections of the samples. Surface crack growth analysis, supported by plots of crack development, crack growth rate and crack length distributions against cycle ratio show that significantly slower crack growth and lower crack densities occur in shot peened specimens than in non-treated samples. That results can be linked to the effect of grain distortion, the packing of laminar grains, a high dislocation density in the plastically deformed surface layer and the introduction of compressive residual stresses. A fractographic analysis of the fracture surfaces has assisted an understanding of the mechanisms of fracture in shot peened specimens.     

SHORT FATIGUE CRACK GROWTH BEHAVIOUR OF A LOW CARBON STEEL UNDER CORROSION FATIGUE CONDITIONS

The effect of an aqueous chloride environment upon the development and growth of short fatigue cracks from smooth specimen surfaces has been studied under fully reversed torsional fatigue loading conditions. Crack initiation and growth has been monitored using a plastic replication technique enabling a full history of cracking characteristics to be recorded. Corrosion fatigue conditions were achieved by complete immersion in a 0.6 M NaCl solution, of nominal pH value 6.0, with specimens corroding at the free corrosion potential. Variations to these conditions were obtained by the addition of concentrated hydrochloric acid enabling test solution pH values to be altered, typically pH values of 3.5 and 2.0 were obtained. Further information regarding the effects of the environment on the early stages of crack development were obtained by conducting two stage alternate immersion type testing conditions. Evaluation of these effects through previously established Elastic-Plastic Fracture Mechanics models shows that the environment plays a major role during the early stages of microstructure-dominated crack growth particularly as cracks approach major barriers to propagation and at decreasing levels of applied shear stress.     

HIGH TEMPERATURE SHORT FATIGUE CRACK BEHAVIOUR IN A STAINLESS STEEL

Abstract— Continuous low cycle fatigue (LCF) tests with-and without-hold time in push-pull and torsion loading modes and sequential LCF tests in push-pull mode were carried out at 650°C in air on thin tubular specimens of 316 stainless steel; the sequential tests involving pure fatigue (PF) and creep-fatigue (CF) loadings. The growth of short fatigue cracks was studied by taking several replicas from the specimen surface which were subsequently observed under a scanning electron microscope. An analysis was done with respect to both crack density and the orientation of microcracks and macrocrack(s) which led to failure.
Crack density was higher on the surface of a CF tested specimen than that of a PF tested specimen. Mainly short cracks oriented at 45° to the specimen axis were observed on a torsion fatigue tested specimen surface. For push-pull specimens the microcracks propagated perpendicular to the specimen axis to form macrocracks that propagated in the same direction. On the other hand, for torsion specimens the microcracks which initially propagated at 45° to the specimen axis linked to form macrocracks oriented parallel and perpendicular to the specimen axis. However, the macrocrack responsible for the final fracture was always oriented parallel to the specimen axis.
Cumulative damage was dependent on the type of loading (PF or CF) in the first part of sequential tests. In particular microcracks initiated during an initial damage phase observed under sequential LCF tests in PF were found to be healed by oxide formation during the hold times applied in the subsequent CF loading and this produced a total damage summation significantly larger than one.     

A STUDY OF SHORT CRACK GROWTH IN TORSIONAL LOW CYCLE FATIGUE FOR A MEDIUM CARBON STEEL

A study of short crack growth for a medium carbon steel in low cycle torsional fatigue has been carried out in order to measure crack length and derive growth laws. Comparison with a previous analysis of uniaxial tests in low cycle fatigue for the same material provides a tentative basis for formulating an equivalent stress approach to describe uniquely the propagation of short cracks under multiaxial stress.     

CRACK INITIATION AND PROPAGATION BEHAVIOUR OF A HEAT-TREATED CARBON STEEL IN CORROSION FATIGUE

Abstract— When estimating fatigue damage quantitatively it is important to clarify its physical basis. In this study, rotating bending fatigue tests of a heat-treated 0.45% carbon steel were carried out in 3% NaCl solution, in order to clarify the physical basis of corrosion fatigue damage from successive observations of plastic replicas. The results show that corrosion pits are generated during the early stages of cycling, then most of them grow with further cycling until a crack is initiated from each corrosion pit. The initiation of corrosion pits from slip bands is observed only in the case when the stress range is relatively large, and in the range of stress for which slip bands are produced in air. After initiation of a crack, the crack propagates by frequent interactions and coalescence with other cracks. The growth rate of an especially small crack in NaCl solution is larger than that in air. However, the growth rate of a comparatively large crack is smaller in NaCl solution than in air.     

A NOTE ON MODELLING SHORT FATIGUE CRACK BEHAVIOUR

Based upon experimental short fatigue crack growth data and adopting the Brown–Hobson model, new crack growth equations have been derived in an attempt to describe more precisely short fatigue crack growth behaviour that separates the physically small crack regime from the long crack regime. An empirical model for physically small crack growth was developed by employing elastic–plastic fracture mechanics parameters.
By considering the proposed approach to short fatigue crack modelling, a new second 'microstructural' threshold condition has been established using only short fatigue crack growth data. In the case of fatigue in an aggressive environment it is suggested that three transition (threshold) conditions can be identified representing: (i) a stress-assisted pitting/pit-to-crack transition; (ii) a microstructurally short shear crack/physically small tensile crack transition; and (iii) a physically small crack/long crack transition.
A comparison of this approach with that of existing models has been made, and predictions of total fatigue lifetime using the model have been presented. A reasonable agreement has been observed between predicted and experimental crack growth rates.     

SCATTER IN SMALL CRACK PROPAGATION AND FATIGUE BEHAVIOUR IN CARBON STEELS

In order to investigate the physical basis of scatter in fatigue behaviour, rotatory bending of smooth fatigue specimens was carried out using two steels with different microstructures (ferrite/pearlite and tempered martensite). Fifteen or sixteen specimens were fatigued at each stress, and the initiation and propagation behaviour of a crack which led to the final fracture were examined for all the specimens. The emphasis was to investigate the scatter characteristics of fatigue behaviour based on the scatter of small crack propagation data. The data were analysed by assuming the Weibull distribution, and a statistical investigation of the physical basis of scatter was performed, i.e. the distributions for fatigue life, crack propagation life and growth rate of small cracks were established and the relation between the scatter in those distributions was studied.     

CRACK GROWTH BEHAVIOUR OF FATIGUE MICROCRACKS IN LOW CARBON STEELS

The behaviour of low carbon steels has been studied, in particular the initiation and growth of the major crack which led to final fracture of smooth specimens, via surface replica and photomicrograph techniques. From this study, the fracture process and fatigue life prediction characterized by the growth of surface microcracks have been analysed by a new approach unifying the conventional approaches based on the final fracture of materials with the fracture mechanics approach. Knowledge of fracture parameters is critical for non-destructive inspection during service life and the application of fracture mechanics to life prediction and assessment.     

A STUDY OF SHORT FATIGUE CRACK GROWTH BEHAVIOUR IN TITANIUM ALLOY IMI 685

Abstract— The fatigue growth of short cracks in coarse grained IMI 685, having an aligned a microstructure, has been monitored using a two stage replication technique. Crack growth rates are presented in terms of the failure mechanism and compared with standard data obtained from through cracked, compact tension specimens. The maximum difference, of up to four orders of magnitude, between long and short crack growth rates is associated with separation along specific crystallographic planes resulting in a cleavage-like fracture appearance. The rate of short crack growth is also shown to be dependent on applied stress and a dwell at maximum load during the fatigue cycle.     

SHORT FATIGUE CRACK GROWTH BEHAVIOUR IN WASPALOY AT ROOM AND ELEVATED TEMPERATURES

Short fatigue crack growth tests have been carried out at room and elevated temperatures using the nickel-based superalloy known as Waspaloy. A fully automated computer controlled system has been developed and employed for controlling the testing and monitoring of the growth of freely initiated surface short cracks on smooth specimens. Surface cracks as small as 10 um in length have been detected and recorded at temperatures up to 700°C. Anomalous short crack propagation behaviour was observed when comparisons were made with the corresponding long crack behaviour. Some aspects of mechanical, microstructural and environmental effects on the short fatigue crack growth behaviour of the material are discussed.     

PROPAGATION BEHAVIOUR OF SHORT FATIGUE CRACKS IN Q2N STEEL

Abstract— The work described in this paper characterizes short fatigue crack growth behaviour of Q2N steel having a complex microstructure and designated for pressure vessel and offshore structure applications. Short and long fatigue crack growth tests for this steel were conducted under three point bend loading conditions. It was found that, in the initial stages of growth, short cracks propagate much faster than those of long cracks when correlated with the linear elastic fracture mechanics (LEFM) parameter Δ K. A period of crack growth retardation was observed at crack lengths of approx 50 μm. The theory of the interaction between short cracks and grain boundaries fails to predict the occurrence of this deceleration minima. A new short crack deceleration mechanism is proposed based on experimental observation. Observation of the characteristic behaviour of short cracks allowed the development of a short crack growth model based on microstructural fracture mechanics analyses.     

THE INFLUENCE OF NOTCH PLASTICITY ON SHORT FATIGUE CRACK BEHAVIOUR

Abstract— A study has been made of fatigue crack formation and growth at the root of different notch profiles in a structural steel subjected to fully reversed tension-compression loading. The scale of stage I microstructural crack growth at notches decreased with increasing notch root strain and was comparable to the size of stage I cracks in shallow hourglass profile specimens at the same strain. Stage II crack growth rates were faster within the notch plastic field than in the elastic stress field of the bulk material.     

SHORT CRACK BEHAVIOUR IN NODULAR CAST IRON

Fatigue crack growth rates have been determined on standard specimens containing long cracks (∼5–10mm) and on specimens containing two-dimensional short cracks (∼0.10–0.50mm). Large differences have been observed indicating that at a given stress intensity factor short cracks propagate much faster than long cracks. Mouth opening displacement measurements for both specimen geometries have shown that the crack closure effect is largely responsible for the observed effect. These results are used to rationalize the behaviour of short cracks initiated from natural sites which were either graphite nodules or microshrinkage pores. The three-dimensional aspect of these natural small cracks is analysed and discussed in detail.     

TORSIONAL FATIGUE OF A MEDIUM CARBON STEEL CONTAINING AN INITIAL SMALL SURFACE CRACK INTRODUCED BY TENSION–COMPRESSION FATIGUE: CRACK BRANCHING,NON-PROPAGATION AND FATIGUE LIMIT

In order to examine the threshold condition for the fatigue limit of materials containing a small crack under cyclic torsion, reversed torsional fatigue tests were carried out on 0.47% C steel specimens containing an initial small crack. Initial small semi-elliptical cracks ranging from 200 to 1000 μm in length were introduced by the preliminary tension–compression fatigue tests using specimens containing holes of 40 μm diameter. The threshold condition for the fatigue limit of the specimens containing artificial small defects under rotating bending and cyclic torsion are also reviewed. Crack growth behaviour from an initial crack was investigated. The torsional fatigue limit for a semi-elliptical small crack is determined by the threshold condition for non-propagation of Mode I branched cracks. The torsional fatigue limit of specimens containing an initial small crack can be successfully predicted by the extended application of the √area parameter model in combination with the σ_θmax criterion.     

FATIGUE MICROCRACKS IN A LOW CARBON STEEL

Abstract— The behaviour of a low carbon steel has been studied, in particular the initiation, growth and coalescence of fatigue microcracks on the surfaces of smooth specimens via surface replicas and photomicrographs. From the study, quantitative information on the initiation period, growth and coalescence of cracks, statistical distributions of crack length and crack depth, density of cracks, distribution pattern and characteristics of the major crack, were obtained. Knowledge of these parameters is critical for non-destructive inspection during service life and the application of fracture mechanics to life assessment.     

ESTIMATION OF TORSIONAL FATIGUE STRENGTH OF MEDIUM CARBON STEEL BARS WITH A CIRCUMFERENTIAL CRACK BY THE CYCLIC RESISTANCE-CURVE METHOD

Near-threshold fatigue crack propagation tests were performed on circumferentially precracked round bars of a medium carbon steel under torsional loading. The crack propagation rate decreased with crack extension, because of the shear contact of crack faces. The crack propagation rate without the influence of crack-surface contact was determined by extrapolating to zero crack extension the relationship between the crack propagation rate and crack extension. The applied stress intensity factor range was divided into two parts: one was the effective value responsible for crack growth and the other was the value corresponding to crack-tip shielding. The resistance-curve method was used to predict the fatigue limit for crack initiation and fracture. The R -curve was constructed using the experimentally determined threshold value of the stress intensity range, which was the sum of the threshold effective stress intensity range and the threshold shielding stress intensity range. The threshold effective stress intensity range was constant. The R -curve was independent of the precrack length and specimen dimensions. The predicted values agreed well with the experimental results.