THE ROLE OF DYNAMIC STRAIN-AGEING IN THE ENVIRONMENT ASSISTED CRACKING OBSERVED IN PRESSURE VESSEL STEELS

Abstract— The dynamic strain ageing (DSA) behaviour of A533B Class 1 reactor pressure vessel (RPV) steel, containing 5 ppm of free nitrogen, has been investigated over a wide range of strain rates from 3 × 10⁻⁷ to 3−10⁻³s⁻¹ and temperatures from 20 to 350°C. The DSA was observed within certain temperature ranges at all strain rates tested and its hardening effect in terms of the maximum strengthening stress (̀_p) decreased linearly with the increase of log strain rate. The temperature to stimulate DSA was observed to increase with increasing strain rate. The apparent activation energies of the characteristic strain rates for the onset, and peak of DSA have been determined, and their implications have been discussed. Compared with the available data, a positive effect of free nitrogen content on ̀_p has been evaluated. It has been found that the occurrence of susceptible environment assisted cracking (EAC) of A533B steel in high temperature water is co-related to the DSA behaviour. The results suggest that DSA reduces the ductility of RPV steel and its role in enhancing the EAC of RPV steels should not be neglected, in view of the coincidence with susceptibility zones for DSA and EAC in terms of strain rate and temperature.     

CYCLIC INDUCED CREEP OF A PLAIN CARBON STEEL AT ROOM TEMPERATURE

Abstract— Plain carbon steel specimens subjected to cyclic and superimposed mean stresses show significant plastic strain at intermediate life due to cyclic creep deformation at room temperature. Cyclic creep strains can also be observed in the high cycle region with N _f > 2. 10⁶. An extended Haigh-diagram provides information about the resulting mean strain at predetermined cycles.     

应变速率对一种无铼单晶高温合金低周疲劳性能的影响

研究了一种无铼镍基单晶高温合金在1223 K、不同应变速率(5×10^-4s^-1、1×10^-3s^-1、5×10^-3s^-1、1×10^-2s^-1)条件下的低周疲劳行为。结果表明:在四种应变速率条件下,合金均表现出循环稳定。随着应变速率的增加,合金的疲劳寿命逐渐增加,且其半寿命稳定滞后回线环内面积逐渐减少,表明低应变速率合金更容易积累蠕变塑性变形。疲劳裂纹源均萌生于试样表面,随着应变速率的增加,疲劳过程中产生的塑性变形越来越少,疲劳裂纹扩展区的面积逐渐增大。低应变速率时,较大的塑性变形导致合金取向发生明显的偏转,诱发多滑移系开动进而形成位错网;反之,高应变速率时,合金没有产生明显的塑性变形,只有单一方向的位错塞积形成位错束。     

THE APPLICABILITY OF NEURAL NETWORKS IN MODELLING THE GROWTH OF SHORT FATIGUE CRACKS

This paper examines the use of a neural network to model the chaotic behaviour of the growth of short fatigue cracks which are characterized by a decreasing crack growth rate with increasing crack length. Fatigue crack growth is modelled in terms of the Hobson short fatigue crack growth law. The neural network is exclusively trained and tested on Hobson's experimental data of short fatigue cracks propagating in a 0.4% carbon steel. The empirical constants d, α and C of Hobson's growth law are determined from the neural network predictions and are found to be within the following approximate ranges 63 < d < 400 (μm), −0.27 < α < 0.08 and 1 × 10⁻⁴ < C < 509 × 10⁻⁴ with no proportional relationship observed between the constant C and the applied cyclic stress. It is shown that neural networks are a viable computational tool for modelling the chaotic behaviour of short fatigue crack growth.     

EFFECTS OF NEUTRON IRRADIATION ON LOW-CYCLE FATIGUE AND TENSILE PROPERTIES OF AISI TYPE 304 STAINLESS STEEL AT 298 K

Abstract— Room temperature studies have been made of the effect of neutron damage on the mechanisms concerned with the low-cycle fatigue and tensile test behaviour of stainless steel AISI Type 304. Samples were irradiated in the HFR at Petten to a fast fluence of 5·10²⁴ n m⁻² ( E > 0·1 MeV) at 333 K followed by mechanical testing at room temperature. The low temperature irradiation caused irradiation hardening: the 0·2 yield stress increased from 230 MN m⁻² for the unirradiated material to a lower yield point value of 540 MN m⁻². Irradiation had no significant effect on fatigue life. The loop type damage was removed by glide dislocations resulting in cyclic softening. Dislocation substructures were observed after fatigue testing: cell structures were more pronounced after fatigue testing to failure the higher the applied strain ranges.
The formation of fatigue cracks at the surface of the specimens was observed in a series of specimens exposed to an increasing number of fatigue cycles.     

SLOW FATIGUE CRACK GROWTH AND THRESHOLD BEHAVIOUR IN IMI 685

Abstract— Fatigue thresholds and crack growth rates up to 10⁻⁴ mm cycle ⁻¹ have been measured in β processed IMI 685. The results obtained in laboratory air for material having an aligned α microstructure and a random basketweave microstructure displayed a pronounced load ratio dependence which increased with decreasing ΔK. This sensitivity to mean load was also apparent from the threshold results determined in a vacuum of 5 ± 10⁻⁶ torr.
Fractographic observations, compliance measurements, pd output and crack path replication have indicated that contacts can be made between the fracture faces at a number of points behind the crack tip during the load cycle. These contacts wedge the crack open, thus preventing the stress intensity from falling to the value associated with the minimum applied load. A critical stress intensity, K _op, has been determined which relates to the crack being fully "open" and the results are reanalysed and discussed in terms of an effective stress intensity range, Δ K _eff.     

INFLUENCE OF RETAINED AUSTENITE ON FATIGUE CRACK PROPAGATION IN HP 9-4-20 HIGH STRENGTH ALLOY STEEL

Abstract— Industrial multi-pass TIG weldments of HP 9-4-20 high strength alloy steel have been found to contain significant volume fractions (around 10%) of retained austenite which are not readily transformed after stress relieving and subsequent refrigeration procedures. To determine whether the presence of such retained austenite in tempered martensitic structures could be detrimental to fatigue resistance in HP 9-4-20 steel, fatigue crack propagation behavior was examined over six orders of magnitude in growth rate, in commercially heat-treated material (containing less than 3% austenite) and in intercritically heat-treated and tempered material (containing approx. 14% austenite) in an environment of moist, ambient temperature air. Whereas crack propagation rates were unchanged at growth rates exceeding 10⁻⁶ mm/cycle, structures containing 14% austenite showed somewhat superior resistance to near-threshold crack propagation at growth rates less than 10 ⁻⁶ mm/cycle, the threshold for crack growth (Δ K ₀) being over 20% higher than in commercially heat-treated material. The presence of retained austenite further appeared to inhibit the occurrence of intergranular fracture at near-threshold levels. It was concluded that significant proportions of retained austenite are not detrimental to fatigue crack propagation resistance in HP 9-4-20 steel, and may indeed have some beneficial effect at very low, near-threshold growth rates by increasing resistance to environmentally-assisted cracking.     

THE EFFECT OF STRAIN RATE ON THE TENSILE AND FRACTURE PROPERTIES OF BS 4360 'A' GRADE SHIP STEEL

Abstract— The results of high rate tensile and compact tension fracture toughness tests conducted on BS 4360 'A' grade ship steel are presented. Tensile results are reported for strain rates within the range 10–^2–10³/s and fracture toughness values at rates of increase of J integral within the range 10^3–10⁶ N/mm/s. The tensile properties of upper yield, lower yield and UTS are shown to be linearly dependent on the logarithm of strain rate whilst fracture toughness is shown to decrease with increasing loading rate prior to approaching a minimum value. The decrease in fracture toughness with increasing test rate is shown to be related to a change in the micro-mechanism of fracture.     

MEAN STRESS EFFECTS ON LOW CYCLE FATIGUE FOR A HIGH STRENGTH STEEL

Abstract— ASTM A723 Q & T steel with a yield strength and ultimate strength of 1170 and 1262 MPa respectively was evaluated for mean stress-strain effects under smooth specimen axial strain controlled low cycle fatigue conditions with strain ratios R of −2, −1, 0, 0.5 and 0.75. Cycles to failure ranged from 15 to 10⁵. Cyclic stress-strain response based upon half-life hysteresis loop peaks were similar for all R ratios. Mean stress relaxation occurred for R ≠−1 only when plastic strain amplitudes were present and this occurred above total strain amplitudes of 0.005. Thus, mean stress relaxation was completely dependent upon cyclic plasticity. Mean strains did not affect low cycle fatigue life unless accompanied by half-life mean stress. Tensile mean stress was detrimental and compressive mean stress was beneficial and these effects only occurred at strain ampltidues below 0.005. Three different mean stress models were used to evaluate the low cycle fatigue data and the SWT log-log linear model best represented the data. These results can be used with the local notch strain fatigue life prediction methodology.     

THE INFLUENCE OF LOADING RATE ON HYDROGEN INDUCED CRACKING OF MICRO ALLOYED STEELS

Abstract— Since the degradation effect due to environment on the cracking of materials depends on time, the loading rate has an important influence on the parameters that characterise its behaviour. This work analyses the effect of loading rate on the resistance to hydrogen induced cracking (HIC) of two microailoyed steels, E690 and E500. Monotonic loading tests were performed on precracked CT samples using a slow strain rate machine. Tests were done under constant displacement rate varying from 4.1 × 10–⁷ m/s to 8.2 × 10–¹⁰m/s on the two steels that were cathodically charged with hydrogen at different current densities (1, 5 and 10 mA/cm²) to obtain different hydrogen concentration levels inside the material.
Based on an analytical study, the initiation conditions for cracking as well as the crack propagation rates were determined in each case, and analysed as a function of K ₁. An extensive fractographic SEM study has been performed to help in the analysis of the different zones of behaviour obtained as an effect of loading rate, for each material and environmental condition used.     

Impact Tensile Stress–strain Characteristics of Wrought Magnesium Alloys

Abstract:  Impact tensile properties of three different wrought magnesium alloys (AZ31B-F, AZ61A-F and ZK60A-T5) are evaluated using the split Hopkinson bar. Reliable tensile stress–strain data up to fracture in the extrusion direction at strain rates of nearly 1000 s⁻¹ are presented and compared with those at quasi-static and medium rates of strain obtained on an Instron testing machine. The effect of strain rate on the tensile strength, elongation at fracture and absorbed energy is examined in detail. It is demonstrated that the tensile strength increases with increasing strain rate, and the strain-rate dependence of elongation at fracture and absorbed energy varies, depending on the magnesium alloys tested. The limitations of the test technique are discussed.     

LONG LIFE SPECTRUM FATIGUE OF CARBON AND STAINLESS STEEL WELDS

Abstract— Fatigue tests of non-load carrying carbon and stainless steel fillet welds have been performed using spectrum loading typical for rail vehicles. The proportion of spectrum cycles exceeding the constant amplitude fatigue limit ranged between 0.86% and 100% and cycles to failure ranged from 4.2 ± 10⁵ to 2.1 ± 10⁷. For the longest tests, the majority of fatigue damage was contributed by cycles with stress ranges less than the constant amplitude fatigue limit. For the carbon steel welds a significant portion of fatigue damage was produced by cycles with stress ranges less than 50% of the fatigue limit but only a small fraction of damage was produced by cycles of this size for the stainless steel welds. The carbon steel welds had slightly better fatigue strength at lives less than 10⁷ cycles but results suggest that stainless steels may have superior long-life variable amplitude fatigue strength when a greater portion of life is spent in the early stages of crack nucleation and growth.     

IN-REACTOR LOW-CYCLE FATIGUE OF TYPE 0Kh16N15M3B STAINLESS STEEL

An investigation has been carried out on austenitic stainless steel 0Kh16N15M3B under normal conditions and also to neutron irradiation of 6.8 × 10¹⁶nm^-2s^-1 ( E > 0.1 MeV) intensity. Thin-walled torsion cylindrical specimens were tested in strain-controlled fully reversed loading mode at 923 K. Various ranges of strain, pre-loading fluences and half-cycle hold times (1, 5 and 30min) were applied. Neutron irradiation was found to result in hardening of the steel, stimulating cyclic stress relaxation and a reduction in cyclic life. When acting together, neutron irradiation and static loads cause a more significant reduction in the number of cycles to failure than if summed up as independent factors. Application of a kinetic failure criterion based on a damage parameter enables an estimation to be made of the limiting state of the steel under high-temperature cyclic loading with hold periods.     

MIXED MODE SMALL CRACK GROWTH

Abstract— The fatigue crack growth behavior of small part-through cracks in 1045 steel and Inconel 718 subjected to biaxial loading has been investigated. Experiments were performed on thin-wall tubular specimens loaded in tension, torsion and combined tension torsion. Crack sizes analyzed ranged from 20 μm to 1 mm and growth rates ranged from 10^-7 to 10^-4 mm/cycle for 1045 steel and from 10^-5 to 10^-2 mm/cycle for Inconel. Nucleation and the early growth of cracks occurs on planes of maximum shear strain amplitude for both of these materials even in tensile loading. An equivalent strain based intensity factor was employed to correlate the crack growth rate under mixed mode loading conditions In loading conditions other than torsion, a transition from mode II to mode I was observed for 1045 steel. Principal strains were used to analyze mode I cracks. Cracks in Inconel 718 grow in mode II for the majority of the fatigue life. The maximum shear strain amplitude and the tensile strain normal to the maximum shear strain amplitude plane were used to calculate the strain based intensity factor for mixed mode loading.     

FATIGUE STRENGTH OF 7075-T6 ALUMINIUM ALLOY UNDER COMBINED AXIAL LOADING AND TORSION

The fatigue strength of 7075-T6 aluminium alloy under combined axial loading and torsion was examined. The S-N relations were correlated with the von Mises criterion for the high cycle region ( N _f≥ 10⁴ cycles) and with the Tresca criterion for the low cycle region ( N _f < 10⁴ cycles), where N _f is the cycles to failure. This transition at N _r= 10⁴ cycles was related to the occurrence of macroscopic plastic straining and a change in fracture modes. The results are discussed in comparison with those for a high strength steel (SNCM8) in a previous study. Particular attention is given to differences in cyclic deformation behaviour, fracture modes and fatigue crack growth rates between the two materials.     

Analysis of strain rate dependence of tensile elongation for a mechanical milling Al–1.1Mg–1.2Cu alloy tested at 748 K from a dislocation dynamics viewpoint

Tensile deformation was carried out for a mechanically milled and thermo-mechanically treated Al–1.1Mg–1.2Cu (at.%) alloy at 748 K and three nominal strain rates of 10⁻³, 10⁰, and 10² s⁻¹. Despite the prevailing belief that superplasticity occurs by grain boundary sliding which requires slow strain rates at high temperatures, the maximum elongation was observed at the intermediate strain rate of 10⁰ s⁻¹, neither at the lowest nor the highest strain rates. In order to explain this phenomenon, the true stress–true strain behaviors at these three nominal strain rates were analyzed from a viewpoint of dislocation dynamics by computer-simulation with four variables of the thermal stress component σ*, dislocation immobilization rate U, re-mobilization probability of unlocked, immobile dislocations Ω and dislocation density at yielding ρ₀. It can then be concluded that the large elongation (>400% in nominal strain) at the intermediate strain rate is produced by a combination of a very large Ω and a moderate U, resulting in a large strain rate sensitivity m value.     

On the mechanism of fatigue failure in the superlong life regime (N>107 cycles). Part II: influence of hydrogen trapped by inclusions

High cycle fatigue fracture surfaces of specimens in which failure was initiated at a subsurface inclusion were investigated by atomic force microscopy and by scanning electron microscopy. The surface roughness R _a increased with radial distance from the fracture origin (inclusion) under constant amplitude tension–compression fatigue, and the approximate relationship: R _a ≅ C Δ K ²_I holds. At the border of a fish-eye there is a stretched zone. Dimple patterns and intergranular fracture morphologies are present outside the border of the fish-eye. The height of the stretch zone is approximately a constant value around the periphery of the fish-eye. If we assume that a fatigue crack grows cycle-by-cycle from the edge of the optically dark area (ODA) outside the inclusion at the fracture origin to the border of the fish-eye, we can correlate the crack growth rate d a/ d N , stress intensity factor range Δ K _I and R _a for SCM435 steel by the equation
   
and by d a/ d N proportional to the parameter R _a .
Integrating the crack growth rate equation, the crack propagation period N _p2 consumed from the edge of the ODA to the border of the fish-eye can be estimated for the specimens which failed at N _f > 10⁷. Values of N _p2 were estimated to be ∼1.0 × 10⁶ for the specimens which failed at N _f ≅ 5 × 10⁸. It follows that the fatigue life in the regime of N _f >10⁷ is mostly spent in crack initiation and discrete crack growth inside the ODA.     

THE FATIGUE LIFE OF THICK-WALLED AUTOFRETTAGED CYLINDERS WITH CLOSED ENDS

Abstract— It is well known that the fatigue strength of a thick-walled cylinder is enhanced by autofrettage. However, this does not appear to have been explained from fracture mechanics. The present paper shows that two uncertainties arise when this is attempted. Firstly, the distribution of residual stress resulting from the autofrettage pressure must be estimated and secondly a realistic stress intensity factors for subsequent fatigue cracking must be defined. A number of available stress intensity solutions are modified with the author's predictions to the residual stress following an elastic-plastic autofrettage pressure in a closed cylinder of hardening material. A comparison with experiment has enabled the various approaches to be appraised. It is shown that a modified stress intensity factor of Bowie and Freese is most consistent with the propagation fatigue life observed in autofrettaged cylinders provided their solution is adapted to account for the propagation of a semi-elliptical crack front in the presence of residual stress. Other K ₁ estimates appear to lead to dangerously optimistic predictions particularly within the range of fluctuating pressure where failure occurs between 10⁵ and 10⁶ cycles. The contribution to fatigue failure from initiation cycles is expressed as a power function of the observed life for cyclic pressures in the region of the fatigue limit.     

A SHEAR STRESS BASED CRITICAL-PLANE CRITERION OF MULTIAXIAL FATIGUE FAILURE FOR DESIGN AND LIFE PREDICTION

Abstract— The use of a previously presented general criterion of failure for high cycle multiaxial fatigue, τ_a/ t _A,B+σ_n.max/2σ_T= 1, is extended to cases where the shear and normal stress on the critical plane are non-proportional and also to give life predictions in the range of 10⁴ to 10⁶ cycles. The criterion takes account of whether case A cracks, growing along the surface, or case B cracks, growing in from the surface, occur.     

Superplasticity and grain boundary sliding in rolled AZ91 magnesium alloy at high strain rates

The superplastic deformation characteristics and microstructure evolution of the rolled AZ91 magnesium alloys at temperatures ranging from 623 to 698 K (0.67–0.76 T_m) and at the high strain rates ranging from 10⁻³ to 1 s⁻¹ were investigated with the methods of OM, SEM and TEM. An excellent superplasticity with the maximum elongation to failure of 455% was obtained at 623 K and the strain rate of 10⁻³ s⁻¹ in the rolled AZ91 magnesium alloys and its strain rate sensitivity m is high, up to 0.64. The dominant deformation mechanism in high strain rate superplasticity is still grain boundary sliding (GBS), which was studied systematically in this study. The dislocation creep controlled by grain boundary diffusion was considered the main accommodation mechanism, which was observed in this study.