Characterisation of fatigue crack extension by quantitative acoustic emission

A calibrated four-channel elastic wave recording system has been used to detect and characterise acoustic emission events associated with the growth of a fatigue crack in a compact tension specimen of 7010 aluminium alloy. The transducers sampled the elastic wave field in four independent directions in a plane perpendicular to the crack. The 2-D force dipole representation of each event, assumed to be a point source, was deduced by inversion of the Green's tensor. Each emission event was then characterised in terms of source type (e.g. microfracture, slip), orientation relative to fatigue crack and size (e.g. crack volume).All of the events (other than clearly distinguishable signals due to fretting at the loading pins) were located within 0.5 mm of the crack tip. 80% of the recorded events had the character of fracture, and were mostly oriented approximately parallel to the fatigue crack. The typical event size was 2,000 m³. It is believed that the principal source of emission was brittle inclusion fracture at, or close to, the crack tip. The largest inclusions ( 10 m) were much smaller than the deduced emission sources, implying that the fatigue crack modifies the elastic wave radiation from the inclusions and amplifies the apparent source strengths. Nevertheless the point-source model fitted most of the data reasonably well, with residual errors < 10%.

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Résumé On a utilisé un système étalonné d'enregistrement à quatre canaux d'ondes élastiques pour détecter et caractériser les zones d'émission acoustiques associées à la croissance d'une fissure de fatigue dans une éprouvette compacte de traction d'un alliage d'aluminium 7010. Les transducteurs captent le champ d'onde élastique dans quatre directions indépendantes d'un plan perpendiculaire à la fissure. Par une inversion du tenseur de Green, on déduit une représentation de chaque émission supposée être ponctuelle par un dipole de forces à deux dimensions. On caractérise ensuite chaque émission par le type de source d'émission (micro-rupture, glissement, etc.), par son orientation par rapport à la fissure de fatigue, et par son importance (par ex. volume de la fissure).On a pu établir que toutes les émissions-à l'exception de celles dont il était clair qu'elles provenaient des efforts dus aux tenons de mise en charge-étaient localisées dans une zone de 0,5 mm en avant de la pointe de la fissure. On a constaté également que 80% des zones d'émission détectées étaient caractéristiques d'une rupture, et se trouvaient le plus souvent orientées parallèlement à la fissure de fatigue. La taille typique de ces zones est de 2000· microns cubes. On pense que la principale émission d'onde est la rupture fragile d'une inclusion à l'extrémité d'une fissure ou en son voisinnage immédiat.Let inclusions les plus grandes ( 10 microns) se sont révélées beaucoup plus petites que la taille des sources d'émission telle que déduite des mesures, ce qui implique que la fissure de fatigue modifie l'irradiation d'onde élastique en provenance des inclusions, et accroit la puissance apparente d'émission.Toutefois, le modèle de source ponctuelle s'est révélé le mieux correspondre à la plupart des résultats, avec des erreurs résiduelles de moins de 10%.

     

Acoustic emission monitoring of corrosion fatigue crack growth in offshore steel

A research and development programme has been carried out to establish relationships between corrosion fatigue crack growth in offshore steel qualities and acoustic emission. Laboratory experiments on small-scale specimens and wide plates have shown that when a certain combination of crack size and crack surface corrosion deposit thickness has been reached, high acoustic emission event rates, in the range of 10–40 events per fatigue cycle, are recorded. The main activity is recorded on rising load, generated from crack surface activity, eg secondary emission. On the basis of the results from this programme general AE equipment can now be modified for offshore applications, and a research and development strategy to implement these modifications is presented,     

Statistical analysis of fatigue crack growth behavior for grade B cast steel

Tests for fatigue crack growth rate (FCGR) and crack-tip opening displacement (CTOD) were performed to clarify the fatigue crack growth behavior of a railway grade B cast steel. The threshold values of this steel with specific survival probabilities are evaluated, in which the mean value is 8.3516 MPa m^1/2, very similar to the experimental value, about 8.7279 MPa m^1/2. Under the conditions of plane strain and small-scale yielding, the values of fracture toughness for this steel with specific survival probabilities are converted from the corresponding critical CTOD values, in which the mean value is about 138.4256 MPa m^1/2. In consideration of the inherent variability of crack growth rates, six statistical models are proposed to represent the probabilistic FCGR curves of this steel in entire crack propagation region from the viewpoints of statistical evaluation on the number of cycles at a given crack size and the crack growth rate at a given stress intensity factor range, stochastic characteristic of crack growth as well as statistical analysis of coefficient and exponent in FCGR power law equation. Based on the model adequacy checking, result shows that all models are basically in good agreement with test data. Although the probabilistic damage-tolerant design based on some models may involve a certain amount of risk in stable crack propagation region, they just accord with the fact that the dispersion degree of test data in this region is relatively smaller.     

Micromechanisms of fatigue crack growth in cast aluminium piston alloys

The fatigue crack growth behaviour in as-cast and hot isostatically pressed (HIP) model cast aluminium piston alloys with hypoeutectic Si compositions of 6.9 wt% and 0.67 wt% has been investigated. The HIP alloys showed slightly improved fatigue crack growth resistance. Analysis of the crack path profiles and fracture surfaces showed that the crack tends to avoid Si and intermetallic particles at low ΔK levels up to a mid-ΔK of ∼7 MPa√m. However, some particles do fail ahead of the crack tip to facilitate crack advance due to the interconnected microstructure of these alloys. At higher levels of ΔK, the crack increasingly seeks out Si and intermetallic particles up to a ΔK of ∼9 MPa√m after which the crack preferentially propagates through intermetallic particles in the 0.67 wt%Si alloy or Si and intermetallics in the 6.9 wt%Si alloys. It was also observed that crack interaction with intermetallics caused crack deflections that led to roughness-induced crack closure and possibly oxide-induced crack closure at low to mid-ΔK. However, crack closure appears unimportant at high ΔK due to the large crack openings and evidenced by the fast crack growth rates observed.     

Random hydrogen-assisted fatigue crack growth in steel plates

A stochastic analysis of hydrogen-assisted fatigue crack growth in steel plates is presented. First, a simplified deterministic model of the process is proposed. It captures the basic empirical property that the influence of hydrogen diminishes, as the crack growth rate increases. However, it only applies to cases, when diffusion is rate limiting. Next, the model parameters are randomized to reflect the uncertainty inherent in the physical situation. On the basis of the obtained stochastic equation, probabilistic moments of the time, in which the crack reaches a critical length, are computed. Theoretical results are illustrated by a numerical example.     

Numerical computation of thermal fatigue crack growth of cast iron

Thermal fatigue experiments have been carried out on a single‐edge wedge specimen of the SiMo cast iron to reproduce the conditions experienced by exhaust manifolds during operation. The leading edge temperature was cycled between 20 and 750 °C and the temperature distribution on the specimen surface was measured by thermocouples throughout the thermal cycle. Due to the complexity of the loading and interaction effects between cracks, numerical simulation of crack propagation and shielding effects in multicracked structures appear a useful way to analyze this problem. Therefore, 3D thermo‐mechanical computation was performed with the finite element code ABAQUS of both un‐cracked and multicracked specimen. This computation allowed us to assess the temperature, stresses and strains distribution over a thermal fatigue specimen and the estimation of the crack growth rate using the energy criteria based on the calculation of the J‐integral crack tip.     

Fracture toughness and fatigue crack growth of grey cast irons

The fracture behaviour of alloys with a pearlitic matrix and lamellar and spherulitic graphite has been compared. Fatigue crack growth functions were measured and various critical stress intensities obtained from load-displacement curves. An analysis of microstructure in the uncracked and cracked state served as the base for a discussion of quantitative models for the relation between microstructure and bulk fracture mechanical properties. In addition to volume fraction and shape of graphite, crack branching (and eventually transformation of residual austenite) are required to explain the resistance of grey cast irons against stable and unstable crack growth. It is concluded that the standard methods for the evaluation of fracture mechanical properties are not satisfactory especially for cast irons with lamellar graphite.     

Hydrogen embrittlement contributions to fatigue crack growth in a structural steel

The detrimental effects of a hydrogen atmosphere on the fatigue resistance of BS 4360 steel have been assessed by a comparison of crack growth rates in air and hydrogen at a low cycling frequency (0.1Hz), and at a number of temperature (25, 50 and 80 °C). The crack propagation rates in air are almost independent of temperature over this range, but those measured in hydrogen differ by more than an order of magnitude between 25 and 80 °C. The greatest enhancement is seen at 25 °C and at high values of ΔK, the maximum occurring between 40–45 MPa √m at each temperature. There is little hydrogen contribution to crack growth at values of ΔK below 20 MPa √m for R = 0.1.

The enhancement of crack growth rates is reflected by the presence of ‘quasi-cleavage’ facets on the fatigue fracture surfaces of specimens tested in hydrogen. These are most apparent where the greatest increases in growth rate are recorded. The facets show linear markings, which run both parallel and perpendicular to the direction of crack growth. The former are analogous to the ‘river’ lines noted on brittle cleavage facets, and reflect the propagation direction. The latter are more unusual, and indicate that facet formation by hydrogen embrittlement during fatigue is a step-wise process.     

Corrosion fatigue short crack growth behaviour in a high strength steel

The influence of an aggressive environment (0.6 M, aerated NaCl solution) on short fatigue crack initiation and growth behaviour has been studied. The study involved three major test series, namely: air fatigue, corrosion fatigue, and intermittent air fatigue/corrosion fatigue. The above tests carried out under fully reversed torsional loading conditions at a frequency of 5 Hz, showed that it was the non-metallic inclusions which took part in crack initiation resulting from debonding at metal matrix/inclusion interface and pitting of inclusions in both air and corrosove environments, respectively. Short fatigue crack growth results in these two environments obtained by using plastic replication technique, indicated a large effect of microstructure i.e. prior austenite grain boundaries. The stage/stages at which the environmental contribution was dominant has been discussed by considering the results achieved from intermittent tests. However, the mechanisms involved in corrosion fatigue short crack growth have also been described in the light of results obtained from futher investigations carried out by conducting corrosion fatigue tests under applied cathodic potential conditions and tests on hydrogen pre-charged specimens under air fatigue and uniaxial tension conditions.     

Acoustic emission monitoring of a fatigue crack in 50D steel in a sea-water environment

Four specimens of 4360-50D steel plates were fatigued under a range of simulated marine environments and loads. AE events were detected using four broadband point-contact transducers and located in 3D (< 1 mm accuracy) relative to the crack front. This information was used together with the load at which each emission occurred to distinguish between ‘primary’ and ‘secondary’ source events.Little or no emission was observed from a dry fatigue crack, indicating that ductile crack growth by mode I loading in 50D steel plate is relatively quiet. In the presence of sea water, however, secondary emissions were observed at discrete points behind the crack tip where subsequent chemical analysis indicated a high concentration of calcium carbonate deposit. A third, cathodically protected, specimen gave much more AE, consistent with an increase in calcium carbonate between the crack faces. The emissions occurred at loads between zero and maximum tension. The characteristics of the AE signals did not change on reducing the maximum load, confirming that crack advance was not responsible. Crack face debonding, arising from fracture of calcareous deposits present, is believed to be the source of AE. A fourth specimen, also cathodically protected, was fatigued entirely in tension and gave less AE, consistent with much weaker crack face bonding.Measurements of the compressional wave arrival strengths at each probe were compared with calculated radiation patterns for different types of AE source. The experimental radiation patterns were always consistent with a double monopole combined with a microcrack source in varying proportions.Under these testing conditions, secondary events associated with corrosion product fracture dominated the emission. Whilst little or no primary emission was detected during fatigue crack growth in parent plate, this may not be the case in welds, regions of low toughness or where the stress fields are more complex.     

Relationships governing the growth of creep and fatigue cracks in a cast heat-resisting steel

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 27, No. 5, pp. 78–84, September–October, 1991.     

Influence of nonmetallic inclusions on fatigue crack growth in a structural steel

A study of the fatigue behaviour of a hardened and tempered steel, at two inclusion levels, has been carried out according to the linear elastic fracture mechanics criteria. The influence of inclusions on the fatigue crack growth rate has turned out to be a function of the local stress intensity factor range,K _I, at which fracture propagates. At lowK _I values, to which are related crack growth rates less than 10^–5 mm cycle^–1, the crack growth rate in the steel with higher inclusion content is lower than in the steel with lower inclusion content. AsK _I increases, an inversion in the difference between the two rates occurs. In the dirtier steel, the higherK _I, the higher the growth rate than in the other steel. The difference between the two rates becomes nil just below the fast propagationK _Ic level. By fractographic analysis, it has been possible to find out how inclusions affect fatigue behaviour.     

Acoustic emission and fatigue crack growth in rigid polyurethane foam

Compact tension specimens of a rigid polyurethane foam have been tested in fatigue and crack growth has been monitored visually and by means of acoustic emission (AE). During the load cycle it has been found possible to resolve the AE activity into four regions: the crack faces un-sticking, fracture events at or close to peak load, a period of zero AE just after peak load, and AE associated with crack closure lower down the unloading part of the cycle. The fracture AE has been found to increase rapidly with crack length — consistent with a seventh power dependence on K — and to occur during every cycle at high K values, but to be absent in an increasingly greater proportion of cycles as K is decreased below about 40 kPa m^1/2. AE data obtained on samples in which crack growth occurred across the layers of foam, through the high density inter-layer skins, show that the technique is very sensitive to the crack retardation effect associated with these skins well before this retardation is detectable visually.