Load sequence effects in fatigue crack growth of thick-walled welded C–Mn steel members

Many welded steel structures in marine, offshore, and infrastructural industries are subjected to variable amplitude (VA) fatigue loads. It is well known that the level and sequence of the load cycles can cause crack growth retardation or acceleration and thus influence the fatigue life. An important sequence effect is generated by a large stress cycle followed by smaller stress cycles. Whereas the effect of single large stress cycles in a further constant amplitude (CA) load on central through cracks in thin-walled aluminium sheet is well established, studies into the effects of practical VA loads on cracks in thick-walled welded steel structures are less common. This paper presents the results of CA tests with large stress peaks and VA tests on 70 mm C–Mn steel butt welded 4-point bending specimens with crack growth in thickness direction. It is demonstrated that loading by a sequence of accelerating and subsequent decelerating stress cycles cause significant retardation of the crack growth and that the same stress cycles but placed in random sequence hardly result in retarded crack growth. The obtained crack growth versus number of cycles for as-welded and stress relieved specimens have been simulated using two relatively simple crack rate retardation models, being the well-known Willenborg model and the Space-state model developed by Ray and Patankar. The latter model is also used to simulate crack growth of semi-elliptical surface cracks in welded steel structures tested by others. The Space-state model is able to predict experimental results with reasonable to good accuracy. A proposal is put forward for future improvement of the model.     

The role of microstructure in fatigue crack initiation of 9–12%Cr reduced activation ferritic–martensitic steel

Electron back-scattered diffraction together with scanning and transmission electron microscopy were used to study nucleation of microstructural fatigue cracks in reduced-activation ferritic–martensitic steel, EUROFER 97. Cylindrical specimens were cycled over different plastic-strain ranges in order to evaluate the evolution of the dislocation structure. Surface-damage evolution was studied in smooth notched specimens by an optical in situ system equipped with a high-resolution camera. In order to understand the crack-initiation mechanism, the dislocation microstructure that develops in the near-surface regions of the notch was compared with that of the bulk. The results demonstrate a strong influence of lath-martensite boundaries on fatigue-crack nucleation.     

Oxidation and fatigue crack propagation in the range of low stress intensity factor in relation to the microstructure in P122 Cr–Mo steel

Fatigue strength and life of weldment at high temperatures are important for the materials in power plants. The fatigue crack growth rate is accelerated by oxidation. Similarly, the high-temperature fatigue life is influenced by oxidation. The base metal, the weld metal and the heat-affected zone (HAZ) of the P122 (Cr–Mo steel) weldment were oxidized between 600 °C and 700 °C for up to 500 h in air, and their oxidation behavior was examined. The oxidation resistance increased in the order of HAZ, base metal and weld metal. The scales were mainly Fe₂O₃. Fatigue tests were performed to measure the fatigue crack growth rate in the range of low stress intensity factor, and the results are discussed from the viewpoint of different microstructures and oxidation.     

Data-driven prediction of the probability of creep–fatigue crack initiation in 316H stainless steel

Stainless steel components in advanced gas-cooled reactors (AGRs) are susceptible to creep–fatigue cracking at high temperatures. Quantifying the probability of creep–fatigue crack initiation requires probabilistic numerical simulations; these are complex and computationally intensive. Here, we present a data-driven approach to develop fast probabilistic surrogate models of creep–fatigue crack initiation in 316H stainless steel. We perform a set of Monte Carlo simulations based on the R5V2/3 high temperature assessment procedure and determine the sensitivity of the probability of crack initiation to loads and operating conditions. The data are used to train different supervised machine learning models considering Bayesian hyperparameter optimization. We discuss the relative performance of such models and show that a gradient tree boosting algorithm results in surrogate models with the highest accuracy.     

Effect of microstructure on the fatigue crack growth behaviour of a near-α Ti alloy

Fatigue crack growth behaviour of Ti–6Al–2Zr–1.5Mo–1.5V (VT-20 a near-α Ti alloy) was studied in lamellar, bimodal and acicular microstructural conditions. Fatigue crack growth tests at both increasing and decreasing stress intensity factor range values were performed at ambient temperature and a loading ratio of 0.3 using compact tension samples. Lamellar and acicular microstructures showed lower fatigue crack growth rates as compared to the bimodal microstructure due to the tortuous nature of cracks in the former and the cleavage of primary α in the latter. The threshold stress intensity factor range was highest for acicular microstructure.     

A cohesive zone model for fatigue and creep–fatigue crack growth in single crystal superalloys

A numerical analysis using cohesive zone model under cyclic loading is proposed to develop a coupled predictive approach of crack growth in single crystal. The process of material damage during fatigue crack growth is described using an irreversible cohesive zone model, which governs the separation of the crack flanks and eventually leads to the formation of free surfaces. The cohesive zone element is modeled to accumulate fatigue damage during loadings and no damage during unloadings. This paper presents the damage model and its application in the study of the crack growth for precracked specimens. The use of cohesive zone approach is validated through a convergence study. Then, a general procedure of parameters calibration is presented in pure fatigue crack growth. In the last section, an extension of the cohesive zone model is presented in the case of creep–fatigue regime at high temperature. The model showed its capability to predict with a good agreement the crack growth in the case of complex loading and complex specimen geometries.     

Relationships between microstructure and fatigue crack propagation paths in Al–Si–Mg cast alloys

Fatigue crack propagation of long and small cracks was investigated for hypoeutectic and eutectic Al–Si–Mg cast alloys. Crack propagation behavior in the near-threshold regime and Regions II and III was related to microstructural constituents namely primary α-Al dendrites and volume fraction and morphology of eutectic Si. Long crack thresholds reflect combined closure effects of global residual stress and microstructure/roughness. The small crack threshold behavior is explained through closure independent mechanisms, specifically through the barrier effects of characteristic microstructural features specific to each alloy. In Regions II and III changes in fracture surface roughness are associated with different crack propagation mechanisms at the microstructure scale. The extent of the plastic zone ahead of the crack tip was successfully used to explain the observed changes in crack propagation mechanisms.     

Environmental effect on fatigue strength of stainless steel in PWR primary water – Role of crack growth acceleration in fatigue life reduction

The influence of the pressurized water reactor (PWR) water environment on fatigue life and fatigue crack growth rate was discussed. The fatigue lives of Type 316 stainless steel in the PWR water environment were investigated using cylindrical hollow specimens. The acceleration in the crack growth due to the environment was quantified by investigating spacing of striations and crack growth tests using compact tension specimens. The growth rates obtained could be represented by the strain intensity factor. It was shown that the fatigue lives estimated by crack growth prediction agreed with those obtained by the tests. Then, it was concluded that the reduction in the fatigue life due to the PWR water environment was brought about not by enhancement of crack initiation but by the acceleration of the crack growth.     

Effect of microstructure on appearance of near-threshold fatigue fracture in Cr–Mo–V steel

Near-threshold fatigue crack growth behavior in 25Cr2NiMo1V steel with different microstructures was investigated by utilizing the load-shedding technique at ambient temperature. Crack surface morphology was observed by SEM with special emphases on the incidence of intergranular fracture and the influence on crack growth rates. Results show that the maximum intergranularity occurs at the ΔK corresponding to the cyclic plastic zone size being equivalent to the prior austenitic grain size. Two types of crack growth mode were observed in the near-threshold regime, i.e., the crystallographic mode of crack growth and the striation mode of crack advance. The incidence of faceted fracture was mainly rationalized by comparing the cyclic plastic zone size with the grain size. It is concluded that, in the crystallographic mode, lower crack growth rates in samples with higher heat treatment temperatures are caused by a greater degree of roughness-induced crack closure (RICC), faceted fracture induced crack closure (FFICC), and oxide-induced crack closure (OICC). The faceted fracture shows negligible influence on crack growth rates when cracks grow in a striation controlled mode.     

Fretting–fatigue behavior of steel wires in low cycle fatigue

The effect of strain amplitude on fretting–fatigue behavior of steel wires in low cycle fatigue was investigated using a fretting–fatigue test rig which was capable of applying a constant normal contact load. The fretting regime was identified based on the shape of the hysteresis loop of tangential force versus displacement amplitude. The variations of the normalized tangential force with increasing cycle numbers and fretting–fatigue lives at different strain amplitudes were explored. The morphologies of fretting contact scars after fretting–fatigue tests were observed by scanning electron microscopy and optical microscopy to examine the failure mechanisms of steel wires. The acoustic emission technique was used to characterize the fretting–fatigue damage in the fretting–fatigue test. The results show that the fretting regimes are all located in mixed fretting regimes at different strain amplitudes. The increase in strain amplitude increases the normalized tangential force and decreases the fretting fatigue life. The abrasive wear, adhesive wear and fatigue wear are main wear mechanisms for all fretting–fatigue tests at different strain amplitudes. The accumulative total acoustic emission events during fretting–fatigue until fracture of the tensile steel wire decrease with increasing strain amplitude. An increase of the strain amplitude results in the accelerated crack nucleation and propagation and thereby the decreased life.     

Creep–fatigue crack development from short crack starters

Abstract

A series of isothermal strain controlled creep–fatigue tests on fully instrumented cylindrical specimens with shallow chordal crack starters has been conducted for an advanced 9%Cr turbine rotor steel at 600 and 625°C. Cyclic/hold wave shapes involving a dwell period at peak strain in tension or compression were also performed with crack development being monitored by means of electrical potential drop instrumentation. It is found that temperature, total strain range and hold period are the most influential factors on short creep–fatigue crack propagation rates and specimen life. In order to establish a reliable relationship to represent subcritical crack development for high temperature component integrity assessment, the effectiveness of candidate correlating parameters such as cyclic strain range, cyclic J integral and strain energy density factor have been evaluated. Their application to circumstances involving short crack development due to fatigue, and interacting and non-interacting creep loading are evaluated with reference to the evidence determined from post-test metallurgical examination.     

Effects of solidification structure on fatigue crack growth in rheocast and thixocast Al–Si–Mg alloys

Fatigue crack growth test was performed for rheocast and thixocast Al–Si–Mg aluminum alloys. At small stress intensity factor range (ΔK), fatigue crack growth (FCG) rate of sample with coarse acicular Si particles decreased slightly compared with specimen with small acicular Si particles. However, at large ΔK, fatigue crack growth rate of specimen with coarse acicular Si particles drastically increased. This is because large acicular Si particles induce high strain hardening at small ΔK, but such particles are easily cracked with the increase in ΔK. Morphology of the Si particles strongly affects striation formation.     

On fatigue crack growth in a Ti-4.5Al-5Mo-1.5Cr alloy with metastable β-phase

Fatigue crack growth behavior has been examined in a Ti-4.5Al-5Mo-1.5Cr alloy, for two different levels of β-phase metastability. The resistance to fatigue crack growth appears to be marginally enhanced with the presence of metastable β-phase in a microstructure also containing some primary α-phase (~30 pct) of high aspect ratio. This enhancement appears slightly greater for β-phase water quenched from 899°C, than as more slowly cooled in helium from this same solution treatment temperature, at approximately an air-cooling rate. In the case of the former, nearly full retention of solute in the β-phase is apparent, while in the latter, significant precipitation of secondary α-phase is evident in thin-foil transmission electron micrographs.     

Investigating environmental effects on small fatigue crack growth in Ti–6242S using combined ultrasonic fatigue and scanning electron microscopy

In situ ultrasonic fatigue with a cyclic frequency of 20 kHz was employed in an environmental scanning electron microscope (ESEM) to characterize fatigue crack formation and growth in the near alpha titanium alloy Ti–6242S. The role of environment on small fatigue crack initiation and growth was investigated in vacuum and in variable pressures of saturated water vapor, as well as in laboratory air. Small crack growth behavior from cracks initiated at FIB-produced micro-notches indicated a significant environmental dependence, with fatigue crack growth rates increasing with increasing partial pressures of water vapor. Environment also influenced crack initiation lifetime in that cracks initiated earlier in laboratory air than in vacuum or saturated water vapor environments. Transgranular, crystallographic crack growth was observed in each environment, with the crack path in primary α grains producing facets parallel to basal planes when crack size was small. Small crack growth resistance had a marked sensitivity to microstructural features, such as α/α grain boundaries with high misorientation and α/α + β boundaries. These initial investigations demonstrate the usefulness of in situ ultrasonic fatigue instrumentation (UF-SEM) as a new tool for the characterization of environmental and microstructural influences on very high cycle fatigue (VHCF) behavior.     

Post-impact fatigue damage growth in fiber–metal laminates

Fiber–metal laminates (FMLs) are a family of hybrid materials currently being considered for use in airframe structural applications. Post-impact fatigue strength tests were carried out on several varieties of GLAss REinforced (GLARE) aluminum laminates. The panels were impacted in a drop weight impact tower located at the Institute for Aerospace Research of the National Research Council of Canada. Observations made by other researchers that the internal impact damage in FMLs is confined to the immediate impact site were confirmed. The impacted specimens were cycled in tension–tension fatigue until failure. Cracks developed along side the dent and also at the edges of the gauge section of the specimen. Aluminum baseline specimens had significantly lower fatigue lives than the FML specimens. The stress-state surrounding the dent is complicated and contributed to unusual fatigue crack initiation behavior in some GLARE variants.     

Environmental embrittlement of Al–Sn alloys and its implication for fatigue crack growth

Abstract

The environmental embrittlement of a series of Al–Sn alloys has been studied using the technique of slow strain rate testing in laboratory air, in a packing of anhydrous magnesium perchlorate, and in a salt solution (2%NaCl–0·5%Na₂CrO₄ buffered to pH 3·5). The embrittlement occurring at slow strain rates is attributed to reversible hydrogen embrittlement. Only the alloy containing a continuous network of tin phase suffers a reduction in ductility. This is attributed to an increase in hydrogen diffusivity which is demonstrated through a series of permeation tests. These results are then used to interpret fatigue crack growth tests conducted as a function of environment in the absence of closure.

MST/744