IRRADIATION EMBRITTLEMENT OF A LOW ALLOY STEEL INTERPRETED IN TERMS OF A LOCAL APPROACH OF CLEAVAGE FRACTURE

Abstract— One heat of A 508 steel is investigated in both the unirradiated and the irradiated condition to determine the variation of the fracture toughness with temperature and specimen thickness. CT type specimens with three thicknesses B (12.5, 25 and 50 mm) are used. Two fluence levels (3·10¹⁹ and 8·¹⁹ n/cm²)( E < 1 MeV) are investigated. It is shown that the fracture toughness is a decreasing function of both specimen thickness and temperature. A model developed previously by Beremin is used to interpret the results. Axisymmetric notched specimens are tested to determine the factors used in the statistical approach of cleavage fracture. It is confirmed that the Beremin model is able to account for the large scatter in fracture toughness observed at a given temperature on the unirradiated material. The specimen thickness effect is also reasonably well interpreted by the model. The irradiation embrittlement can be explained by assuming that the cleavage fracture resistance is not modified by irradiation and by taking into account only the variations of yield strength with irradiation and test temperature.     

CYCLIC LOADING AND FRACTURE TOUGHNESS OF STEELS

Abstract— The paper considers the effect of cyclic loading and loading rate upon fracture toughness characteristics of steels at room and low temperatures. It is shown that fracture toughness of a low-alloy ferrite-pearlite steel with 0·1% C (steel 1) and for 15G2AFDps steel of the same class (steel 2) are 2 to 2·5 times lower under cyclic loading (50 and 0·5 Hz) and dynamic loading (= 1·5 × 10⁶MPa √m s⁻¹) than under static loading (= 0·6 to 9 MPa √m s⁻¹). For quenched and low-tempered 45 steel at 293 K and for armco-iron at 77 K fracture toughness characteristics do not depend on the loading condition. Macro- and micro-fractographic investigations revealed a correlation between the plastic zone size and the length of brittle fracture areas which are formed in steels 1 and 2, and in armco-iron during unstable propagation of the fatigue crack. Dependence of the decrease of the critical stress intensity factor under cyclic loading on the number of load cycles are obtained for repeating ( R = 0) and alternating bending ( R =−1) of specimens with a crack. A model for the transition from stable to unstable crack propagation is proposed involving crack velocity in the zone ahead of the crack tip damaged by cyclic plastic deformation. A new approach is suggested to the classification of materials on the basis of the sensitivity of fracture toughness characteristics to cyclic conditions of loading.     

FATIGUE MACROCRACK GROWTH IN TEMPERED HY80, HY130, AND 4140 STEELS:THRESHOLD AND MID-ΔK RANGE

Abstract— –The rate of propagation of macrofatigue cracks down to near threshold was measured in air in three tempered martensitic steels; HY80, HY130 and 4140 (650°C temper). The value of Δ K _th was determined by the load-shedding technique in center notched panel specimens. Of the three steels, 4140 tempered at 650°C had the lowest Δ K _th, 3–5 MN/m^3/2, while HY80 had the highest, 4.2 MN/m^3/2. The 4140 (650°C temper) is intermediate in strength between HY80 and HY130. The results are discussed in terms of a recent theory of one of the authors.
The fatigue crack propagation rates in the mid-Δ Krange in HY80 and HY130 in argon were also studied by measuring, with foil strain gages, the cyclic plastic work to propagate a fatigue crack by a unit area, U.HY 80 has a lower crack propagation rate and correspondingly higher U .This was attributed in part to the higher yield strength of HY130 but the dislocation structure and carbide composition and morphology also play roles. Microstructural changes due to cyclic plastic deformation inside the plastic zone in HY80 and HY130 were observed by TEM of thin foils. SEM studies of the fracture surfaces at Δ K = 20 MN/m^3/2 indicate a more ductile fracture mode for HY80 than for HY130. The fatigue crack propagation rate of HY130 is substantially higher in laboratory air (47% relative humidity) than in dry argon. This is not the case for HY80.     

FATIGUE BEHAVIOR OF A PLASMA-SPRAYED 8%Y2O3-ZrO2 THERMAL BARRIER COATING

Abstract— Thick thermal barrier coatings with thicknesses on the order of a few millimeters are being developed for use in diesel engines with operating temperatures of about 800°C. In this environment, a coating will experience thermomechanical cycling due to differences in elastic and thermal expansion properties between the coating and the substrate. The inelastic constitutive behavior of the coating material results in both compressive and tensile stresses. To observe the effects of such stresses, specimens of plasma-sprayed 8%Y₂O₃-ZrO₂ were fabricated to allow testing of the coating material independent of the substrate. Cyclic compression fatigue tests were conducted at room and high temperature (800°C) to simulate the loading environment to which the coating materials will be exposed during service. At high temperature, the compressive fatigue strength of the coating material increased by nearly 100%. Fatigue tests in tension and combined tension/compression were conducted at room temperature to evaluate the effect of mean stress. It was observed that a varying mean stress had no significant impact on the fatigue lives of the coating material and the fatigue life was controlled by the maximum tensile stress of the cycle. Results from fatigue tests and SEM observations indicated that the damage accumulated during the tensile and the compressive portions of the fatigue cycle were independent of each other.     

FATIGUE OF A PARTICULATE REINFORCED ALUMINIUM METAL MATRIX COMPOSITE SUBJECTED TO AXIAL, TORSIONAL AND COMBINED AXIAL/TORSIONAL LOADING CONDITIONS

Abstract— The goals in this research were to analytically and experimentally investigate the fatigue behavior of a particulate reinforced metal matrix composite subjected to axial, torsional and combined axial/torsional loadings. A series of fully-reversed uniaxial, torsional and combined axial/torsional fatigue tests were performed on a 6061/Al₂O₃/20p-T6 metal matrix composite material. This research investigated the ability of the Fatemi-Kurath and the Smith-Watson-Topper (SWT) damage parameters to correlate the experimentally obtained fatigue life data and also to represent the fatigue life using uniaxial strain-life constants. The Fatemi-Kurath damage parameter correlated the experimental fatigue data from all loading cases better than the SWT damage parameter. Using uniaxial strain-life constants, both damage parameters predicted fairly reasonable fatigue life calculations for the intermediate fatigue lives (10³ to 10⁴ cycles to failure), while producing non-conservative results for the shorter fatigue lives (< 10³ cycles to failure).     

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.     

A GENERALIZED CONCEPT OF A FATIGUE THRESHOLD

Abstract— The conditions for non-propagating LEFM type fatigue cracks were investigated on an Inconel, 617 Alloy in the range of K _max between 10 and 50 MPa m^1/2 under four different types of fatigue loading conditions. In all tests, K _max was held constant during the fatigue cycling prior to determining the non-propagation condition. It was found that with decreasing range of applied K the fatigue tolerance range Δ K _{eff, th} increases. Furthermore, there is slight increase of Δ K _eff,th with decreasing K _max of approximately 20–30% when K _max is decreased from 50 to 10 MPa m^1/2. The results of the four types of tests are considered in respect to the damage in the near-region of the crack front, i.e. increasing K _max increases the damage zone and therefore decreases the fatigue tolerance range Δ K _eff,th.     

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.     

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.     

BIAXIAL HIGH CYCLE FATIGUE TESTS ON A GAS TRANSMISSION PIPELINE STEEL

Abstract— Gas transmission pipes are sometimes subject to external damage due to interference by excavators. Optimized grinding of gouges may offer a solution in repairing pipes. Since pipelines may be subjected to internal pressure variations, the Dang Van criterion has been used to size the allowable grinding depth. The criterion's boundary has been determined from uniaxial fatigue testing and extrapolated to the higher hydrostatic pressure which occurs in practice. The aim of this paper is to check by means of biaxial fatigue tests on specimens, that this extrapolation ensures a lifetime of 10⁵ cycles for the ground pipes. The state of stresses in the central area of the biaxial specimen was calculated from an elastic finite element simulation. The test machine had independent motions of the two perpendicular axes of loading. The fatigue testing was limited to 10⁵ cycles and was carried out at a hydrostatic pressure of 200 MPa. The results validated the extrapolation technique for the Dang Van criterion.     

FATIGUE CRACK GROWTH FROM A CIRCULAR NOTCH UNDER HIGH LEVELS OF BIAXIAL STRESS

Abstract— A series of experiments have been conducted on cruciform specimens to investigate fatigue crack growth from circular notches under high levels of biaxial stress. Two stress levels (Δσ₁= 380 and 560 MPa) and five stress biaxialities (λ=+1.0, +0.5, 0, −0.5 and −1.0; where λ=σ₂/σ₁ were adopted in the fatigue tests in type 316 stainless steel having a monotonic yield strength of 243 MPa. The results reveal that fatigue crack growth rates are markedly influenced by both the stress amplitude and the stress biaxiality. A modified model has been developed to describe fatigue crack growth under high levels of biaxial stress.     

Measurement of residual stresses in a Noryl telecommunications co–ordinate selector switch

The moulding residual stress level in a miniature non–metallic (Noryl) telecommunications switch was measured using strain gauge and photoelastic techniques. Strain gauge results were obtained using a modified Waisman and Phillips¹ layer removal technique, which indicated maximum residual stresses of the order of 650 lbf/in² (4.5 MN/m²) and a considerable stress gradient in the cross section of the component. Photoelastic measurements were made on slices from components in polycarbonate which has the same mould shrinkage value as Noryl. The results showed a maximum surface residual stress of 240 lbf/in² (1.7 MN/m²). Interpretation of the exceptionally high fringe orders observed, was very difficult and the validity of this application of photoelasticity is questioned.     

Gigacycle fatigue of ferrous alloys

The objective of this paper is to determine the very long fatigue life of ferrous alloys up to 1 × 10¹⁰ cycles at an ultrasonic frequency of 20 kHz. A good agreement is found with the results from conventional tests at a frequency of 25 Hz by Renault between 10⁵ and 10⁷ cycles for a spheroidal graphite cast iron. The experimental results show that fatigue failure can occur over 10⁷ cycles, and the fatigue endurance stress S _max continues to decrease with increasing number of cycles to failure between 10⁶ and 10⁹ cycles. The evolution of the temperature of the specimen caused by the absorption of ultrasonic energy is studied. The temperature increases rapidly with increasing stress amplitudes. There is a maximum temperature between 10⁶ and 10⁷ cycles which may be related to the crack nucleation phase. Observations of fracture surfaces were also made by scanning electron microscopy (SEM). Subsurface cracking has been established as the initiation mechanism in ultra-high-cycle fatigue (>10⁷ cycles). A surface–subsurface transition in crack initiation location is described for the four low-alloy high-strength steels and a SG cast iron.     

THE EFFECT OF CYCLE WAVESHAPE ON THE LOW CYCLE FATIGUE BEHAVIOUR OF 20%Cr–25%Ni–Nb STAINLESS STEEL AT 650°C

Abstract— Low cycle fatigue tests at 650°C on 20% Cr–25% Ni–Nb stainless steel have been carried out under conditions of equal tension/compression ramp rates (10⁻³ s⁻¹) and slow tension–fast compression ramp rates (10⁻⁶ s⁻¹/10⁻³ s⁻¹). It was found that the latter cycle significantly reduced endurance. Detailed metallography revealed that life reduction is due to the accumulation of creep damage during the slow tensile ramp. It is proposed that at high strain ranges, failure is creep dominated and this changes to a creep-fatigue interaction failure mechanism at low strain ranges.
Predictive models have been developed based on cavity growth mechanisms and ductility exhaustion. Both techniques provide an accurate life prediction and only limited data are required to use them.     

THE COMBINED EFFECTS OF STRESS RATIO AND YIELD STRENGTH ON THE LEFM FATIGUE THRESHOLD CONDITION

Abstract— Long-crack (LEFM) fatigue threshold, Δ, K ₀ values are predicted which include the commonly observed effects of stress ratio, R , and yield strength, σ _y . It is assumed that the yield strength effect on threshold is indirectly related to grain size and so is not an independent variable. Two intrinsic thresholds of a material are invoked to explain the observations of higher Δ K ₀ values and a higher R -ratio sensitivity of Δ K ₀ in low strength materials compared to high strength materials. The paper shows that Δ K ₀ is almost independent of both yield strength and stress ratio at high values of the stress ratio. Quantitative relations are developed to estimate curves of (i) Δ K ₀ versus R and (ii) Δ K ₀ versus σ _y . These curves show good agreement with experimental data for steels and aluminium alloys. A method is presented that may be used as an alternative procedure for obtaining quick and conservative estimates of Δ K ₀ for design applications.     

MODELING THE LONG-LIFE FATIGUE BEHAVIOR OF A CAST ALUMINUM ALLOY

As-cast specimens and smooth specimens of a AA 319 cast aluminum alloy containing casting porosity were fatigue tested with special attention given to the long-life region ( N 1.25 × 10⁸ cycles). Fatigue cracks were observed to initiate from the near-surface casting pores or from discontinuities resulting from the as-cast surface texture. The observed fatigue lives were strongly dependent on the size (√area) of these casting defects.
The effect of casting defects on the fatigue life was modeled assuming the fatigue life to be the sum of the crack nucleation and the crack propagation life (including both the growth of short and long cracks). The crack growth behavior of (mechanically) short cracks was considered in detail by a developed crack-closure-at-a-notch (CCN) model. The CCN model predicted the fatigue lives for both as-cast and machine-notched specimens. Extension of the CCN model to reliability-based design was attempted using the measured size distribution of the fatigue-initiating casting pores.     

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.     

Flexural fatigue resistance of ultra-high molecular weight polyethylene at ambient and low temperature

Specimens of ultra-high molecular weight polyethylene have been subjected to flexural fatigue tests at −40° and 23°C, and the temperature of some of the specimens recorded throughout the test. It is found that when the specimen life exceeds 10⁶ cycles, the temperature of the specimen stabilizes. However, if the temperature of the specimen does not reach equilibrium with the testing temperature, the specimen life is short (< 10⁴ cycles). The stabilization of the speciment temperature is related to a critical stress level, which is different for each testing temperature.     

On the mechanism of fatigue failure in the superlong life regime (N>107 cycles). Part 1: influence of hydrogen trapped by inclusions

The fracture surfaces of specimens of a heat-treated hard steel, namely Cr–Mo steel SCM435, which failed in the regime of N = 10⁵ to 5 × 10⁸ cycles, were investigated by optical microscopy and scanning electron microscopy (SEM). Specimens having a longer fatigue life had a particular morphology beside the inclusion at the fracture origin. The particular morphology looked optically dark when observed by an optical microscope and it was named the optically dark area (ODA). The ODA looks a rough area when observed by SEM and atomic force microscope (AFM). The relative size of the ODA to the size of the inclusion at the fracture origin increases with increase in fatigue life. Thus, the ODA is considered to have a crucial role in the mechanism of superlong fatigue failure. It has been assumed that the ODA is made by the cyclic fatigue stress and the synergetic effect of the hydrogen which is trapped by the inclusion at the fracture origin. To verify this hypothesis, in addition to conventionally heat-treated specimens (specimen QT, i.e. quenched and tempered), specimens annealed at 300 °C in a vacuum (specimen VA) and the specimens quenched in a vacuum (specimen VQ) were prepared to remove the hydrogen trapped by inclusions. The specimens VA and VQ, had a much smaller ODA than the specimen QT. Some other evidence of the influence of hydrogen on superlong fatigue failure are also presented. Thus, it is concluded that the hydrogen trapped by inclusions is a crucial factor which causes the superlong fatigue failure of high strength steels.     

DETERMINATION OF THE ENERGETIC PARAMETERS CONTROLLING CLEAVAGE FRACTURE INITIATION IN STEELS

Abstract— The process of brittle fracture in steels can be divided into three distinct steps: (1) initiation of a microcrack in a brittle particle, (2) propagation of the microcrack into the surrounding matrix and, finally, (3) crack progression through the matrix. Depending on microstructure, temperature and loading rate, the critical step which controls cleavage fracture is subject to change. In this work the behaviour of different microalloyed steels is considered and the energies γ_pm and γ_mm, which define the stress necessary for the microcrack to surmount steps 2 and 3 have been experimentally determined. While the γ_pm value remains constant around 7 J/m², it has been observed that γ_mm is dependent on temperature. At −196°C the value is lower than 50 J/m² and at room temperature it is higher than 200 J/m². This increase in the matrix-matrix energy with temperature increases the probability of microcracks, generated in particles, arresting at grain boundaries. This is the reason why refinement of grain size has an important effect in improving the fracture toughness at room temperature.