Influence of hydrogen and frequency on fatigue crack growth behavior of Cr-Mo steel

Fatigue tests on Cr-Mo steel quenched at 860°C and tempered at 580°C were carried out under the frequencies of 0.2, 2, and 20 Hz with specimens containing a small artificial hole. An additional test in which the test frequency was alternately switched between 0.02 and 2 Hz was carried out. Hydrogen charging to the specimens was carried out by an immersion method. The fatigue life of the hydrogen-charged specimens remarkably decreased in comparison with that of the uncharged specimens. The fatigue crack growth rate da/dN increased with decreasing the test frequency f. The acceleration of da/dN saturated at \({\Delta K\,<\,17\,{\rm MPa}\sqrt{m}}\) for f ≤ 2 Hz. The presence of the upper bound for the fatigue crack growth acceleration was found with respect to the effects of hydrogen and test frequency in a hydrogen environment. The test switching the frequency between 0.02 and 2 Hz resulted the difference in fatigue crack growth morphology which is presumed to be caused by the difference in hydrogen concentration in the vicinity of crack tip. The particular crack morphology under the low test frequency with hydrogen was the localization of the slip around the crack tip and the linearization of the crack growth path. The hydrogen-enhanced striation formation model which was proposed to explain the effect of hydrogen on the fatigue crack growth for an austenitic stainless steel and low carbon steel can be applied also to the quenched and tempered Cr-Mo steel in this study.     

Effect of hydrogen charging on the torsional fatigue behaviour of 2024 aluminium

Torsional fatigue tests have been carried out on overaged and hydrogen charged specimens of 2024 aluminium in gaseous hydrogen and humid air. Hydrogen charging was found to significantly increase the number of fatigue crack initiation sites compared with uncharged specimens tested in argon, resulting in an overall reduction in fatigue life. Fatigue testing in gaseous hydrogen and humid air influenced both initiation and propagation of cracks. The fracture sites of both charged and uncharged specimens were similar, and the fracture mode was predominantly tensile in all specimens. However, specimens tested in humid air showed small amounts of longitudinal and transverse fracture, with ≈5% shear at low humidity and 10% at high humidity.     

Face/core debond fatigue crack growth characterization using the sandwich mixed mode bending specimen

Face/core fatigue crack growth in foam-cored sandwich composites is examined using the mixed mode bending (MMB) test method. The mixed mode loading at the debond crack tip is controlled by changing the load application point in the MMB test fixture. Sandwich specimens were manufactured using H45 and H100 PVC foam cores and E-glass/polyester face sheets. All specimens were pre-cracked in order to define a sharp crack front. The static debond fracture toughness for each material configuration was measured at different mode-mixity phase angles. Fatigue tests were performed at 80% of the static critical load, at load ratios of R = 0.1 and 0.2. The crack length was determined during fatigue testing using the analytical compliance expression and verified by visual measurements. Fatigue crack growth results revealed higher crack growth rates for mode I dominated loading. For specimens with H45 core, the crack grew just below the face/core interface on the core side for all mode-mixities, whereas for specimens with H100 core, the crack propagated in the core or in the face laminate depending on the mode-mixity at the debond crack tip.     

Effect of frequency on fatigue crack growth behavior of magnesium alloy AZ61 under immersed 3.5 mass% NaCl environment

Fatigue crack growth test of AZ61 magnesium alloy was carried out under immersed NaCl environment at frequencies of 15, 5 and 0.5 Hz under a stress ratio of 0.1. In order to investigate the effect of frequency on fatigue crack growth behavior in detail, additional tests at frequencies ranged from 15 to 0.01 Hz were conducted under a constant ΔK of 3.25 MPa m^1/2. Effect of frequency was clearly observed in low ΔK region, where fatigue crack growth rate decreased with decreasing frequency. Crack closure would be a dominant factor for the frequency effect observed under immersed NaCl environment at frequencies ranged from 15 to 0.5 Hz. However, fatigue crack growth rates at frequencies lower than 0.05 Hz were higher than those at frequencies higher than 0.5 Hz. The accelerated fatigue crack growth rates at frequencies lower than 0.05 Hz would be attributed to the corrosion attack at the crack tip.     

Effect of acetic acids on the corrosion crack growth of 3.5NiCrMoV steels in high temperature water: synergistic interaction between stress corrosion and corrosion fatigue

Stress Corrosion Cracking (SCC) tests (pH: 3 ~ 5) and Corrosion Fatigue (CF) tests (R = 0.2, 0.1 Hz) were conducted to evaluate the effect of acetic acid on the corrosion crack growth behavior in high temperature water at 150°C. Acetic acid significantly influenced the corrosion fatigue cracking behavior of turbine disc steels in high temperature water. The CF crack growth rates of turbine disc steels increase until the organic acid concentration reaches a critical saturation value (between pH 4 and pH 3) because of the crack tip sharpening. Below the critical value of pH, the CF crack growth rates decreases because of the crack tip blunting. The corrosion fatigue crack growth rate is accelerated by the interaction of the fatigue and the stress corrosion in the test environment. The synergistic interaction should be accounted for in the realistic prediction of the corrosion fatigue life of turbine steel (3.5NiCrMoV steels) in high temperature water of acetic acid solution. With the high temperature corrosion fatigue data obtained in this study, it is possible to assess the life of turbine components in high temperature and high pressure.     

FATIGUE CRACK GROWTH FROM SMALL SEMI-ELLIPTIC HYDROGEN-INDUCED CRACKS IN QIN STEEL

Abstract— Small semi-elliptic hydrogen-induced cracks were produced in QIN (HY80) steel. Fatigue crack growth rate behaviour and threshold values for these cracks were investigated at several positive stress ratios ( R = 0.2 to 0.7) and compared with results from long through-cracks. At low R values the hydrogen-induced cracks gave higher thresholds, and lower crack growth rates at the same nominal Δ K value in the near threshold region. At high R values the growth rates of both crack types were almost identical. The results are explained by a combination of crack tip blunting and roughness induced closure of the intergranular hydrogen crack.     

Pre-strain effect on fatigue strength characteristics of SUH660 plain specimens

Precipitation particles in precipitation-strengthened materials are considered to be cut by pre-strain treatment, which affects the fatigue strength. In this study, fatigue tests were performed on precipitation-strengthened stainless steel SUH660 to investigate the effect of pre-strain on fatigue crack initiation and propagation characteristics. Fatigue test results showed that pre-strained specimens have a shorter fatigue crack propagation life compared to non-strained specimens; this is the opposite of results observed in carbon steel. The accelerated fatigue crack growth observed in pre-strained specimens was first assumed to be caused by precipitate cutting. A dislocation accumulation model for the fatigue crack tip in the precipitation-strengthened material was then suggested. Buff-polished specimens were also used for the fatigue tests, and the results showed that the work-hardened layer had a significant impact on the fatigue strength and fatigue life.     

CORROSION FATIGUE OF AN HSLA STEEL

Abstract— Single-pitted specimens of an HSLA steel, were tested in laboratory air and in 1 M NaCl solution to study the influence of a corrosive environment on its fatigue life.
The growth of fatigue cracks and the partitioning of the fatigue life into fatigue crack initiation and fatigue crack propagation were studied by photographing the pit and the cracks developing on it periodically during testing. Non-propagating or dormant surface cracks were not observed in this study. Fractography using SEM showed the locations of fatigue crack initiation. The mechanisms of corrosion fatigue were studied by performing tests in 1 M NaCl at different test frequencies. Corrosion pits proved to be crack initiation sites. Hydrogen embrittlement was found to be unimportant in the corrosion fatigue of HSLA steel in this study. The 1 M NaCl corrosive environment appeared to reduce the fatigue life of this material by a dissolution mechanism. The effect of pit depth was studied by testing specimens having various pit depths. An effect of pit size was apparent. Fatigue life decreased with increasing pit depth. Pit depth, rather than the ratio of pit depth to pit diameter, influenced fatigue behaviour. A non-damaging pit depth was found.     

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C have been studied to analyse defect assessment in a steam generator. Different grades of alloy 800 have been investigated to reproduce the in service conditions. Fatigue crack growth (FCG) tests were conducted on CT20 and tubular specimens, then on welded tubes. Furthermore the influence of hold times on fatigue crack growth behaviour was studied.

The results obtained on material simulating the weld heat affected zone are in agreement with the tests conducted on welded tubes. Fatigue crack growth characteristics of aged and cold-worked aged material seem to be slightly improved in comparison with base material. Finally a hold time of one minute increases strongly the FCG threshold value determined in pure fatigue but has a negligible influence on crack growth rates.     

The effect of bi-modal and lamellar microstructures of Ti-6Al-4V on the behaviour of fatigue cracks emanating from edge-notches

This paper is aimed at evaluating the influence of bi‐modal and lamellar microstructures on the behaviour of small cracks emanating from notches in α+β titanium Ti‐6Al‐4V alloy. Pulsating four point bending tests were performed at a nominal stress ratio of 0.1 and a frequency of 15 Hz on double‐edge‐notched specimens. The conditions of initiation and early propagation of fatigue cracks were investigated at two relatively high nominal stress levels corresponding to 88 and 58% of the 0.2% material yield stress. Crack closure effects were measured by an extensometric technique and discussed. Variations in crack aspect ratio were determined and considered in the ΔK calculation. Corresponding results were discussed by considering the effect of the yielded region at the notch tip calculated by elastic–plastic finite element modelling of the fatigue tests. The importance of the bi‐modal and lamellar microstructures on the material damage was highlighted and correlated to the observed oscillations in the crack growth rate. The crack growth rate data obtained were compared with those measured using standard C(T) specimens (long crack).     

Stages of fatigue fracture in nickel alloyed powder metallurgy steel

Abstract

Fatigue fracture of smooth rectangular specimens was investigated by testing at constant amplitude and zero mean stress, at a frequency of 30 Hz. Two nickel alloyed powder metallurgy steels with different contents of pores and MnS were examined. Fatigue fracture started with crack nucleation at pore interfaces by the formation of small steps. Growth of these nuclei was accomplished by stepwise crack tip blunting, which led to the formation of macrocracks. A dominant fatigue crack origin built up at the region of the largest number of macrocracks. Fatigue crack growth stages I, II, and III were found in this region.     

FATIGUE CRACK GROWTH BEHAVIOUR IN MOLTEN-METAL PROCESSED Sic PARTICLE-REINFORCED ALUMINIUM ALLOYS

Fatigue crack initiation and subsequent short crack growth behaviour of 2014-5wt%SiC aluminium alloy composites has been examined in 4-point bend loading using smooth bar specimens. The growth rates of long fatigue cracks have also been measured at different stress ratios using pre-cracked specimens. The distributions of Sic particles and of coarse constituent particles in the matrix (which arise as a result of the molten-metal processing and relatively slow cooling rate) have been investigated. Preferential crack initiation sites were found to be Sic-matrix interfaces, Sic particles associated with constituent particles and the coarse constituent particles themselves. For microstructurally short cracks the dispersed SiC particles also act as temporary crack arresters. In the long crack growth tests, higher fatigue crack growth rates were obtained than for monolithic alloys. This effect is attributed to the contribution of void formation, due to the decohesion of Sic particles, to the fatigue crack growth process in the composite. Above crack depths of about 200 μm “short” crack growth rates were in good agreement with the long crack data, showing a Paris exponent, m= 4 in both cases. For the long crack and short crack growth tests little effect of specimen orientation and grain size was observed on fatigue crack growth rates, but, specimen orientation affected the toughness. No effect of stress ratio in the range R=0.2-0.5 was seen for long crack data in the Paris region.     

Stringer-sheets fatigue cracking of helicopter-tail-boom in welding joints of 01420 AL–Li alloy

Regularities of fatigue crack appearance and growth in-service cracked stringers of helicopter Mi-26 tail-boom area were demonstrated. Statistic analyses of cracks distribution in helicopter tail-boom has shown crack appearance not only in stringers manufactured from Al–Li alloy but in frames too. Fatigue tests of complicated sheet-stringer specimens reproducing spot-welding-bonding joint assembly experienced constant cyclic loads amplitude were performed. Result of tests have shown fatigue cracking by the regular change of inter- and transgranular crack propagation. This manner of material cracking is similar case for in-service cracked stringers. The discovered mechanism of Al–Li alloy fatigue cracking is considered based on the results of fractographic investigations in scanning electron microscope. The discussed mechanism of the crack propagation reflects Li diffusion in grain boundary under stressing ahead of a crack tip during fatigue crack propagation that weakened grain boundary and accelerate crack propagation. Crack growth periods for tested specimens were estimated and compared with in-service registered crack growth duration based on regular non-destructive inspections. In-service fatigue cracking period of stringers was established and tolerance in-service duration for helicopter between two inspections has been recommended.     

Hydrogen-enhanced cracking of 2205 duplex stainless steel

Tensile and fatigue crack growth tests of 2205 duplex stainless steel (DSS) were performed in laboratory air, gaseous hydrogen at 0.2 MPa and saturated H₂S solution. The longitudinal specimen showed a lesser degradation of tensile properties than the transverse ones in saturated H₂S solution. The orientation of specimens with respect to rolling direction had little influence on the fatigue crack growth rate (FCGR) of the alloy in air. Furthermore, 2205 duplex stainless steel was susceptible to hydrogen‐enhanced fatigue crack growth. Transmission electron micrographs, in addition to X‐ray diffraction, revealed that the strain‐induced austenite to martensite transformation occurred near the crack surface within a rather narrow depth. Fatigue fractography of the specimens tested in air showed mainly transgranular fatigue fracture with a small amount of flat facet fracture. Furthermore, extensive quasi‐cleavage fracture of 2205 duplex stainless steel was associated with the hydrogen‐enhanced crack growth.     

Four-point bending fatigue behaviour of an iron-based laser sintered material

New material processing methods such as laser sintering of metal powder necessitates new knowledge and characterization of the material to support its implementation in technical applications. Fatigue behaviour of a laser sintered FeNiCu-alloy was studied with emphasis on crack path, initiation and propagation. Fatigue crack growth was investigated by surface replication in four-point bending fatigue tests. The fatigue behaviour was controlled by the complex layered structure. Pores on or under the surface were preferable places for crack initiation. Crack linkage and deflection occurred due to crack tip interaction with microstructure and sinter layers where microcracks initiated at pores adjacent to the advancing crack tip. Crack growth rate and stress intensity factor were calculated from surface replicas and showed an oscillating behaviour.     

MICROSTRUCTURAL AND ENVIRONMENTAL EFFECTS ON FATIGUE CRACK PROPAGATION IN DUPLEX STAINLESS STEELS

Abstract— Fatigue crack initiation and propagation in duplex stainless steels are strongly affected by microstructure in both inert and aggressive environments. Fatigue crack growth rates in wrought Zeron 100 duplex stainless steel in air were found to vary with orientation depending on the frequency of crack tip retardation at ferrite/austenite grain boundaries. Fatigue crack propagation rates in 3.5% NaCl solution and high purity water are increased by hydrogen assisted transgranular cyclic cleavage of the ferrite. The corrosion fatigue results are interpreted using a model for the cyclic cleavage mechanism.     

Effects of different indentation methods on fatigue life extension of cracked specimens

In this paper, 3 different indentation methods have been investigated for crack arresting and fatigue life enhancement of cracked components. The influence of residual stresses induced by indentation on fatigue crack growth (FCG) rate was explored by experiments and numerical simulations. Fatigue tests were conducted on a group of specimens which were indented on the crack tip by various indentation load magnitudes. For another group of specimens, the double indentation and triple indentation methods were applied on the cracked specimens with the aim of obtaining proper residual stress fields that contribute to higher crack growth retardations. Both the numerical and experimental results revealed that the higher indentation loads led to larger domain of compressive residual stress around the crack tip and consequently to higher fatigue life extension. In addition, the triple indentation method resulted in more FCG retardation compared with single and double indentation methods. Furthermore, for the specimens repaired by double and triple indentation methods, indenting ahead of the crack tip led to retardation in more crack growth compared with the other horizontal positions of indentation.     

CRACK GROWTH BEHAVIOUR IN A THERMAL FATIGUE TEST. EXPERIMENTS AND CALCULATIONS

Abstract— This study is concerned with the results of experiments in which thermal cycles have been repeatedly applied through the wall thickness of axisymmetrically cracked tubular specimens. The investigated material is a Cr–Mo steel used for the moulds when fabricating centrifugally cast iron pipes. Crack growth rates have been measured by using the interrupted tests technique. A methodology is proposed to model the crack growth rates under such thermal fatigue loadings. The elastic and plastic stress-strain fields are calculated on the uncracked specimen by means of a finite element code. Special attention was paid to reach a mechanical steady state regime. Fatigue crack growth rates data were obtained, both under isothermal and anisothermal conditions, on CT and SEN specimens. The latter specimens were tested under large-scale yielding in order to obtain the data appropriate to the cyclic stress-strain field calculated in the thermal fatigue specimens. An effective stress intensity factor, which takes into account both plastic strains and crack closure effect, was used to correlate the results of isothermal tests on CT and SEN specimens and to calculate the thermal fatigue crack growth rates in tubular specimens. It is shown that the use of the effective stress intensity factor gives a satisfactory agreement between the observed and the calculated crack growth rates.     

FATIGUE CRACK GROWTH BEHAVIOUR AND FRACTURE RESISTANCE IN CERAMICS

Fatigue crack growth and the fracture resistance curve (R-curve) were investigated in a polycrystalline alumina (AD90) and a silicon carbide whisker-reinforced alumina composite (Al₂O₃-SiC_w) at room temperature in air using a combined loading technique for stabilizing crack growth, and a surface film technique for monitoring crack length. Fatigue crack growth was evaluated successfully with those experimental techniques. Load shedding tests were performed until the crack became dormant, in order to determine the threshold stress intensity factor K_th. Subsequently, the specimens were used for quasi-static crack growth tests under a monotonic loading condition. The R-curves were determined in this experiment; however, fracture resistance did not increase markedly with crack growth. Detailed observations of the crack growth behaviour revealed that the flat R-curve was attributed to the shielding effect of the fatigue crack tip wake. Thus, the fatigue precrack introduced by the load shedding test was not regarded as an ideal crack for determining the R-curve. Fractographic observations were performed to investigate the mechanistic difference between fatigue and quasi-static crack growth. It was found that the cyclic loading produced fretting damage in the wake region and it reduced the shielding effect of the fatigue cracks. Based on the experimental results, the relationship between the fatigue crack growth and the R-curve is discussed as is the significance of K_th as a material parameter.     

The observation of compliance change in the transition from hydrogen to air environment during the fatigue test

A compliance change was observed during fatigue testing of ASTM A710 HSLA steel using constant “K” CDCB specimen. The compliance decreased from 1.296 × 10⁻⁵ mm/N to 1.235× 10⁻⁵ mm/N when the environment was changed from hydrogen to air under the fatigue test conditions of f = 0.2 Hz, R = 0.1 and Δ K = 10 MPa✓m. The compliance change was observed in all fatigue testing while changing the environment from hydrogen to air. This compliance change can be explained numerically using the differential method for the design factors of the CDCB specimen. It was found from the calculation that the compliance change corresponded to a 6.3% change in Young’s modulus. It is proposed that the increased compliance resulted from the decreased Young’s modulus, the reduced Young’s modulus resulted from the increased lattice dilation which in turn resulted from a significantly increased hydrogen concentration at the crack tip region. The increased hydrogen concentration at the crack tip resulted from stress-induced hydrogen diffusion at the crack tip region.This work was conducted at Illinois institute of Technology (IIT).