Influence of carbon content and tempering temperature on fatigue threshold characteristics of a low alloy steel

This paper attempts to describe the effect of carbon content on the fatigue threshold characteristics ΔK_th in various heat treated conditions. Essentially it has been shown that a tempering treatment increased ΔK_th while increasing the carbon content of steels from 0·13% to 0·8% significantly decreased the ΔK_th value by over 100%. At intermediate fatigue crack growth rates all the data show a linear relationship with ΔK level.

In terms of yield strength σ_y, the threshold stress intensity level could be given by the expression: ΔK_th = 8·74 − 3·42 × 10⁻³(σ_y).

At near threshold fatigue crack growth levels significant amounts of isolated intergranular failure were observed in the 400°C tempered condition. In the other heat treated microstructures only a flat trans-granular ductile striated failure mode was evident. A maxima in the amount of intergranular facets occurred at ΔK values approaching 15 to 20 MPa√m. It has been shown the existence of intergranular failure resulted from environmentally induced fracture (through the diffusion of hydrogen) that occurred within the crack tip enclave.     

Fatigue life response of ASME SA 106-B steel in pressurized water reactor environments

Fatigue strain-life tests were conducted on ASME SA 106-B piping steel base metal and weld metal specimens in 288°C (550°F) pressurized water reactor (PWR) environments as a function of strain amplitude, strain ratio, notch acuity, and cyclic frequency. Notched base metal specimens tested at 0·017 Hz in 0·001 part per million (ppm) dissolved oxygen environments nearly completely used up the margins of safety of 2 on stress and 20 on cycles incorporated into the ASME Section III design curve for carbon steels. Tests conducted with smooth base metal and weld metal specimens at 1·0 Hz showed virtually no degradation in cycles to failure when compared to 288°C air test results. In all cases, however, the effect of temperature alone reduced the margin of safety offered by the design curve in the low cycle regime for the test specimens. Comparison between the fatigue life results of smooth and notched specimens suggests that fatigue crack initiation is not significantly affected by 0·001 ppm dissolved oxygen, and that most of the observed degradation may be attributed to crack growth acceleration. These results suggest that the ASME Section III methodology should be reviewed, taking into account the PWR environment variables which degrade fatigue life of pressure-retaining components.     

The effect of frequency and environment on the fatigue crack growth behaviour of SA508 Cl. III RPV steel

This paper describes the effect of frequency and environment on the fatigue crack growth behaviour of SA508 Cl. III RPV steel. The study has shown that the effect of the Pressurised Water Reactor (PWR) environment is directly related to the frequency and the level of applied stress intensity of the test; these results further showed that the lower the frequency the greater the environmental effect, especially at low ΔK levels. No such frequency effect was observed in either the laboratory air or ultra-high purity argon environments. At a frequency of 0·1 Hz the PWR water test exhibited characteristic EAC growth, i.e. plateau growth behaviour. Fractographical examination of the fracture surface revealed that the fracture mode during plateau growth was intergranular failure. The experimental results are described and discussed in terms of the hydrogen assisted cracking mechanism.     

Effects of temperature on the cyclic deformation behaviour and microstructural changes of 12Cr-1MoVW martensitic stainless steel

Strain-controlled uniaxial push-pull fatigue tests were carried out with cylindrical specimens of 12Cr-1MoVW steel from room temperature up to 600°C. Specimens were tested at a total strain range of 2% at room temperature, 300°C, 400°C, 500°C and 600°C, respectively, and were also tested at 500°C for a total strain range of 1·6%, 1% and 0·6%. The fatigue test results showed that the cyclic deformation behaviour of this steel was strongly dependent on the test temperatures. TEM studies revealed that cellular dislocation structure comprised the major microstructure and the subgrain size increased with increasing test temperatures. Carbides tended to redistribute along the subgrain boundaries in the specimens subjected to low cycle fatigue at high temperatures. The undeformed section of each specimen exhibited a completely different microstructure from that in the gauge section in all the temperatures tested. The lath martensite structures were not changed obviously when HT-9 steel cyclic deformed at lower strain range.     

Corrosion fatigue behaviour of A508 class III steel in a simulated PWR water environment at 325°C

Fatigue crack growth data in a simulated PWR environment for A508 Class III steel at 325°C generally exhibited good agreement with crack growth data recorded for A533B steel at 288°C. All data were positioned below the ASME XI high R ‘wet’ line.

It has been shown through sulphur printing and fractography that the morphology and distribution of non-metallic sulphide inclusions play an important part in initiating environmentally assisted crack (EAC) growth which is identifiable by its fan-shaped fracture mode.

In regions of EAC crack growth a significant area of the fatigue fracture surface can still exhibit ductile striated growth. Indeed it has been shown that significant EAC growth occurs when the amount of fan-shaped growth prevalent on the fatigue surface exceeds 20%. Also, fan-shaped growth tended to occur at the centre of the test specimen.     

Effect of temperature on the threshold fatigue crack growth behaviour of spheroidal graphite cast iron

The influence of temperature at various R ratio values on the fatigue crack propagation response of a ferritic spheroidal graphitic cast iron has been studied. It has been established that the influence of R ratio on ΔK_th is strongly dependent upon the test temperature. At elevated temperatures the influence of R ratio is significantly less than that at ambient temperature. At low to intermediate R ratio values temperature initially decreased, then, with increasing temperature, increased the ΔK_th levels, causing a minimum in ΔK_th to occur at 250–300°C. At high R ratio, however, ΔK_th exhibited a small but consistent increase with temperature. The influence of temperature on the ΔK_th at various R ratio values could be adequately explained in terms of crack closure. Much intergranular failure was observed on the fatigue fracture surfaces at ambient temperature, whereas at elevated temperatures there was little evidence of this particular failure.     

Defects in boiler evaporator tubes—A detailed case study concerning defect initiation and extension processes

The present report attempts to describe a detailed failure analysis that was conducted on a series of full and partial through-thickness tube wall defects. Essentially two different defects were observed. One was a full through-thickness defect, the shape of which strongly suggests that it was driven by an active corrosion-dominated process. The second type of defect was partial through-thickness in nature and exhibited a common development route inasmuch as it extended through the evaporator tube wall by at least four discrete events or defect extension processes, viz.

1. (i) semi-elliptical pit formation, 0·3 mm deep;

2. (ii) thick porous-like magnetite growth (up to 400 μm thick) at the base of the pit;

3. (iii) heavy or active corrosion tunnelling, 1.3 mm in diameter; and

4. (iv) an environmentally assisted crack (EAC) fatigue crack propagation region which varied between 0.5 and 1.4 mm in size.

In some instances, a fifth defect extension event, which was corrosion dominated, was observed at the end of the fatigue-dominated event (iv). This event was similar to event (iii). Detailed fractography demonstrated that pit formation was the result of a mixed transgranular and intergranular corrosion process while the corrosion tunnelling was caused by a predominantly intergranular corrosion activity. The fatigue-dominated (event (iv)) fracture surface region contained a mixture of flat, cleavage-like facets which are indicative of an EAC growth process, and a fissured, ductile striated, failure mode which is commonplace in the case of pure, mechanically driven, fatigue processes. In the case of the full through-thickness defect, only an intense intergranular corrosion process was evidenced. The likelihood of these discrete events being caused by on-load or off-load process is discussed, while the presence of localised concentrations of sulphur and chlorine were detected on the corrosion-tunnelled and EAC fatigue growth surface regions.     

Some formulae for the effective stress range ratio used in crack growth of 6063-T6 aluminium alloy

The influence of stress ratio R and stress intensity range (ΔK) on fatigue crack growth experiments were determined for 6063-T6 aluminium alloy and crack growth data were analysed with different formulae for the effective stress intensity range ratio U. The data covered R values from 0 to 0·3. A good correlation was obtained from da/dN and ΔK_eff using the equation for U as a function of R.     

Crack propagation rate as a function of crack tip characteristics

The dependence of crack growth rate on various crack tip parameters was studied. Experiments were performed on thin sheets of 6063-T6 Al-alloy having a central notch, to find crack tip opening displacement, total strain range, plastic strain range, crack opening stress and crack growth rate. Crack tip opening displacement and crack opening stress were measured, using a surface measurement technique, with small crack opening displacement gauges. The theoretical predictions of crack tip opening displacement compare fairly well with the experimental values. It is found that crack propagation rate vs total strain range-plastic strain range gives a straight-line fit on a log-log graph and, for positive stress ratios, the fatigue crack growth rates are found to be independent of R.

Experimental results show that the crack opening stress is not affected by the position of the gauge when it is mounted behind and near the crack tip.

The effect of mechanical properties and loading on crack growth were also studied. The specimens were fatigue cracked to a predetermined length and some specimens were annealed and again loaded cyclically. The application of cyclic loads to annealed specimens caused significant increase in crack propagation rates in comparison with the specimens having no heat-treatment. The load-displacement record was found to stabilize in about 10 cycles; the crack then extended slowly as a fatigue crack. Crack propagation rates for different values of R for annealed and work-hardened material were plotted against a crack tip parameter, ΔK*, based on notional crack lengths. Since the results of da/dN vs ΔK* for both states of material (as-received and annealed) seem to lie on the same straight line on a log-log graph, the study provides a hope that the results for a material tested in any state (annealed or work-hardened) for positive values of R (0·0≤R≤0·3) will lie on this line, thus eliminating fatigue tests on the same material under different work-hardening conditions for different values of R. Models for da/dN have been developed using various crack tip parameters.     

The effect of microstructure on fracture mechanisms of 2·25Cr1Mo low alloy steel, part A: the influence of non-metallic inclusions

The effect of variations in the size and distribution of non-metallic inclusions on the fracture behaviour of 2·25 Cr1Mo low alloy steel has been studied by undertaking a programme of disk bend testing at temperatures between 20°C and −196°C. Analysis of load/displacement data in combination with detailed fractographic evaluation indicates that inclusions greater than about 4 μm in diameter significantly increase the susceptibility for the formation of ductile voids. The ease with which ductile voids form then has a direct effect on the load and strain to failure. In contrast, inclusions do not appear to influence behaviour when fracture occurs by cleavage.     

Threshold fatigue crack extension characteristics of a low alloy steel: The influence of environment and microstructure

The present investigation is aimed at examining the influence of environment and microstructure on the fatigue crack extension behaviour of a low alloy steel. Significant environmental effects were recorded under low R-Ratio testing but not under high R-Ratio conditions, viz., the ΔK_th values recorded in salt solution were significantly larger than those for a dry air environment and a constant value of 10 MPa√m was recorded irrespective of microstructure (yield strength). Such effects were attributed to oxide induced closure effects.

Above threshold, environmental assisted cracking behaviour in the salt solutions was the result of the appearance of transgranular cleavage facets on the fatigue surfaces. The extent of this environmentally enhanced cracking was simply related to the extent of cleavage failure.     

Experimental observation of crack propagation in 6063-T6 al-alloy under constant amplitude loading

Crack propagation experiments were performed on 6063-T6 Al-alloy for various load ranges and stress ratios. Experimental results show that for a constant load range, the life of specimens decreased as stress ratio increased. At constant maximum load, the life of the specimen increased as the load ratio increased. The crack growth data were analysed in terms of ΔK_eff as a function of stress ratio R. The data covered R values from 0 to 0·5 and a good relationship was obtained for K_eff/K = 0·55 + 0·12 R². A crack growth rate equation was developed.     

Creep deformation of induction pressure welded 2·25Cr-1Mo steel

In this paper, experimental results involving the effect of stress and temperature on creep behaviour of induction pressure welded (IPW) 2·25Cr-1Mo steel are presented. Creep rupture tests were conducted at 550–700°C in steps of 50°C over a stress range of 112·5–180 MPa. Above 650°C failure of the specimen was enhanced due to the microstructural instability. Failure in the specimens occurred invariably in the heat affected zones (HAZ), and the fracture surfaces indicated ductile failure.     

Experimental observations on mixed mode fatigue crack propagation

Fatigue crack propagation experiments were conducted on D16AT Al alloy to study the effect of load ratio on mixed mode fatigue crack propagation. The experiments were conducted on a Compact Tension Shear specimen at different crack angles in both mode I and mixed mode conditions. At each angle tests were conducted at three different load ratios of 0·1, 0·25 and 0·5. It has been observed that the crack growth rate increases with decrease in load ratio. The fracture surfaces were analysed under the scanning electron microscope. The effect of load ratio was studied in terms of striation spacing.     

Electrical properties of CuInSe2 films prepared by evaporation of Cu2Se and In2Se3 compounds

We prepared CuInSe₂ films by evaporating In₂Se₃ and Cu₂Se compounds instead of elemental sources. The resulting CuInSe₂ film grown at 680°C had a smooth and dense microstructure with the grain size of 2 3 μm. But the CuInSe₂ films were Cu-rich, with a low resistivity of about 0.1 Ω cm. So we conducted H₂ post annealing to control the electrical resistivity and composition of CuInSe₂ films. In a H₂ atmosphere, the resistivity increased to about 100 Ω cm by annealing at 350°C for 1 h. The resistivity decreased again when the annealing temperature was above 350°C. This resistivity change might be related to the contents of Cu, In, Se atoms and the valency states of Cu and In ions in the films. We discussed the reason of resistivity change caused by H₂ post annealing in this paper.     

Fatigue crack propagation under gaseous hydrogen in a precipitation-hardened martensitic stainless steel

A test device has been developed at P′ Institute in order to investigate the mechanical behaviour of structural materials under high pressure of gaseous hydrogen. In this paper, preliminary results on fatigue crack propagation in a martensitic stainless steel are presented. A tremendous fatigue crack growth enhancement is observed at high pressure (9 MPa). This enhancement is dependent of pressure. It is noticed that the maximum enhancement is associated with a brittle fracture mode. However no intergranular decohesion is noticed in this regime.     

Low temperature μc-Si film growth using a CaF2 seed layer

This paper describes low temperature thin film Si growth by remote plasma chemical vapor deposition system for photovoltaic device applications. Using CaF₂/glass substrate, we were able to achieve an improved μc-Si film at a low process temperature of 300°C. The μc-Si film on CaF₂/glass substrate shows that a crystalline volume fraction of 65% and dark conductivity of 1.65×10⁻⁸ S/cm with the growth conditions of 50 W, 300°C, 88 mTorr, and SiH₄/H₂=1.2%. XRD analysis on μc-Si/CaF₂/glass showed crystalline film growth in (1 1 1) and (2 2 0) planes. Grain size was enlarged as large as 700 Å for a μc-Si/CaF₂/glass structure. Activation energy of μc-Si film was given as 0.49 eV. The μc-Si films exhibited dark- and photo-conductivity ratio of 124.     

Measurement of crack propagation velocity in nuclear pressure vessel steel (SA516 gr. 70)

An attempt has been made to develop a simple, reliable and cost-effective device for measuring the dynamic crack propagation velocity in a nuclear pressure vessel steel (SA516 gr. 70). The experimental method is described and a simple digital approach is proposed. The experimentally determined dynamic crack velocity has been utilized to obtain elastic dynamic stress intensity factors by INSAMCR (a two-dimensional dynamic finite element code which is a modified version of SAMCR developed by Dr Schwartz at the University of Maryland). A relationship between instantaneous crack tip velocities and dynamic stress intensity factors for pressure vessel steels is estimated using dynamic crack propagation velocities determined by a proposed measuring device. The relationship between the dynamic stress intensity factor and time history and the dynamic arrest toughness for each test are obtained using the generation mode dynamic finite element analysis. A function ƒ(å) = 1·356 − 2·672å + 6·494å² − 4·539å³ + 1·461å⁴ is suggested which may be useful to predict the relationship between the dynamic fracture toughness (K(å)) and the dynamic crack arrest toughness (K_Ia) for SA516 gr. 70 steel (say K(å) = K_Ia ƒ(å) where å is the dynamic crack propagation velocity).     

Preliminary investigation of PWR feedwater line cracking

Circumferential cracks were found in large diameter carbon steel piping that carries feedwater on the secondary side in PWR nuclear power plants. The cracks were confined to a narrow band in the vicinity of the counterbore region of the piping near the weld joining the piping to the feedwater nozzle on the steam generator. In most cases, the cracks were numerous, but small; a typical penetration was less than 20% through the wall. However, in some cases, the cracks were much greater in depth and, in one instance, propagated through the pipe wall, approximately 1·6 cm (0·65 in.). The environment was high temperature water containing less than 1 ppm oxygen.

A metallurgical analysis, in which both light and electron microscopy were utilized, led to the conclusion that the cracks, particularly the large ones, were caused by corrosion fatigue. This conclusion was based upon crack morphology, crack location, the presence of bench marks and striations, fracture appearance, and the presence of corrosion products. The next step in the failure analysis was to determine the source(s) of the cyclic stresses that resulted in the corrosion fatigue.

Stress and fatigue analyses were made in an effort to determine the combinations of loads and cycles necessary to produce the observed phenomena. Environmental effects were taken into account in carrying out the analyses. The results showed that plant specific mechanisms must have been responsible for the rapid fatigue growth. An analytical and experimental programme involving the investigation of thermal fluctuations was planned to determine the precise source of the required combinations of loads and cycles.