FATIGUE DAMAGE IN 1045 STEEL UNDER VARIABLE AMPLITUDE BIAXIAL LOADING

Abstract— During constant amplitude loading, two different types of crack systems have been reported In the high cycle fatigue (HCF) region, cracks nucleate on a small number of maxium shear strain amplitude planes One of these cracks becomes a dominant crack and leads to failure of the specimen In the low cycle fatigue (LCF) region, equally developed microcracks are observed over the entire gage section and grow during the majority of the life. The failure is due to a linking in which the microcracks join up during the last few cycles of the fatigue life.
To investigate the interaction of these two types of crack systems in biaxial fatigue, experiments were performed on thin-wall tubular specimens in tension, torsion and combined tension-torsion loading The test program included step loading and block loading in which two equivalent strain amplitudes were employed. One of the equivalent strain amplitudes is in the HCF region and the other was in the LCF region
Fatigue lives were predicted from constant amplitude damage curves when a single crack system dominated the fatigue process Two competitive crack systems were sometimes developed on the maximum shear strain amplitude planes in a single specimen under block loading This resulted in a conservative prediction of the fatigue life.     

Fracture mechanisms near threshold conditions of nickel alloyed PM steel

The paper examined fractographically four nickel alloyed powder metallurgy (PM) steels with total porosity between 3 and 9%. Fracture surfaces were inspected on smooth rectangular specimens from constant stress amplitude tests under axial loading with zero mean stress, at 30 Hz frequency.

The area containing the fatigue crack origin was observed in a region located invariably at a specimen surface. In all cases, the fracture surface was composed of three different morphological appearances (regions) associated with changed proportions of particular fracture mechanisms: macrocrack initiation region where cracks propagated preferentially through particles and there was no influence of pores on the propagation paths; in the other regions (macrocrack growth and unstable crack growth) cracks propagated mainly through the sintering necks by ductile rupture from microvoid coalescence and transparticle cleavage fracture.     

FATIGUE RESISTANCE OF A MEDIUM CARBON STEEL WITH A WEAR RESISTANT THERMAL SPRAY COATING

Abstract— The fatigue behaviour of a Ni-Cr-base powder flame-spray coating on a 0.4% C steel is investigated. Fatigue tests were carried out using mild hour-glass profile specimens. Cracks were detected and measured using plastic replicas and an image analysis system. Coated specimens showed a slightly lower fatigue endurance than plain specimens under torsion loading, while the opposite was observed for push-pull loading. Microcracks in coated specimens invariably form at pores.
Contrary to the usual case of stage I shear growth for a plain 0.4% C steel in tension or torsion loading, the coated specimens show initial crack growth from pores along directions perpendicular to the maximum tensile stress. The crucial behaviour of short cracks, and their growth rates, relative to the thickness of the coating, are discussed in some detail.     

Observation of fatigue damage in structural steel by scanning atomic force microscopy

Fatigue slip bands and plastic deformation around fatigue microcracks were observed by scanning atomic force microscopy. In fatigue slip bands, extrusions were observed but intrusions were not detected. Large extrusions were found in slip bands whose traces at specimen surface were almost perpendicular to the loading axis. Microcracks propagated under mixed mode condition of Mode I, Mode II, and Mode III.     

FATIGUE DAMAGE IN 1045 STEEL UNDER CONSTANT AMPLITUDE BIAXIAL LOADING

The progressive nature of fatigue damage under multiaxial stress states has been investigated. Experiments were performed on thin-wall tubular specimens of 1045 steel in tension, torsion and combined tension-torsion loading. Two equivalent strain amplitudes, one in the high cycle fatigue (HCF) region and one in low cycle fatigue (LCF) region were employed in this study. Four recently proposed damage theories were evaluated. Crack depth was used as a damage parameter in comparing damage curves under different loading modes.
Different types of crack systems were observed in the HCF and LCF regions. The damage curve obtained in tension loading can be used to evaluate the damage behavior under combined tension—torsion loading. The results of torsion loading show that torsional damage behavior is different from the above two loading modes.     

Evaluation of fatigue crack propagation by mode I and mixed mode in 1050 aluminium

The mechanism of mixed‐mode fatigue crack propagation was investigated in pure aluminum. Push‐pull fatigue tests were performed using two types of specimens. One was a round bar specimen having a blind hole, one was a plate specimen having a slit. The slit direction cut in the specimen was perpendicular or inclined 45 degrees relative to the centre of the specimen axis. In both cases, cracks propagated by mode I or by the mixed mode combining mode I and shear mode, depending on the testing conditions. In these cases the crack propagation rate was evaluated with a modified effective stress intensity factor range. Crack propagation retardation was observed in some specimens. However, it was found that the crack propagation rate could also be evaluated by the effective stress intensity factor range independent of the crack propagation mode.     

INITIATION AND GROWTH OF SMALL CRACKS IN DIRECTIONALLY SOLIDIFIED MAR-M247 UNDER CREEP-FATIGUE PART II: EFFECT OF ANGLE NETWEEN STRESS AXIS AND SOLIDIFICATION DIRECTION

This paper deals with the effect of anisotropy on fracture processes of a directionally solidified superalloy, Mar-M247, under a push–pull creep-fatigue condition at high-temperature. Three kinds of specimen were cut from a cast plate such that their axes possess angles of 0°, 45° and 90° with respect to the 〈001〉 orientation that is aligned parallel to the solidification direction (also to the grain boundaries and primary dendrite axis); these specimens being denoted the 0° specimen, the 45° specimen, and the 90° specimen, respectively. The tests were conducted at 1273  K (1000 °C) in air under equal magnitudes of the range of a Δ J -related parameter, Δ W _c , which represents the driving force for crack growth in creep-fatigue. Although the grain boundaries are macroscopically parallel to the solidification direction, they are wavy or serrated microscopically. Small cracks nucleate along parts of the grain boundaries perpendicular to the stress axis in all specimens. The 90° specimen has the shortest crack initiation life and the 0° specimen has the longest. In the 90° and 45° specimens, intergranular cracks continue to nucleate and a main crack is formed along the grain boundary due to the frequent coalescence of small cracks. In the 0° specimen, cracks grow into the grain, and transgranular cracks coalesce along the primary dendrite or grain boundary. The 0° specimen exhibits the slowest crack growth rate and the 90° specimen the fastest. These differences in the initiation and growth behaviour of small cracks cause the longest failure life in the 0° specimen and the shortest in the 90° specimen.     

Behaviour of short cracks in alloy 800HT under biaxial fatigue

Biaxial in phase fatigue tests were carried out on thin walled tube specimens of alloy 800HT at ambient temperature. The loading modes included tension, torsion, and combined tension—torsion with a tensile/shear plastic strain range ratio Δ?p/Δγp = 3^1/2. The influence of effective strain amplitudes and biaxiality on the initial growth of fatigue cracks was investigated using the replica technique. The results indicated that the loading conditions strongly affected the growth rates of short cracks. In torsion the cracks grew significantly more slowly than under axial or biaxial loading. A mean tensile stress perpendicular to the shear crack promoted its growth and reduced the fatigue life. The growth of the cracks could be described by the ΔJ integral for axial and biaxial loading; the integration predicted the fatigue life under axial and biaxial loading correctly. However, significantly conservative lifetime predictions were obtained for pure torsional loading since ΔJ does not include crack closure and crack surface rubbing.

MST/3234     

Possibility of Hydrogen-Assisted Propagation of Short Fatigue Cracks in WT3-1 Titanium Alloy

We make an attempt to explain brittle propagation of short fatigue cracks in specimens of WT3-1 titanium alloy tested by simultaneous bending and torsion. It is assumed that the fracture process can be affected by hydrogen absorbed from the atmosphere and penetrating to the tips of microcracks in the subsurface layer of the material. Higher concentrations of hydrogen observed near the crack lips located near the specimen surface as compared with the bulk of the specimen confirm the assumption that the effect of hydrogen is responsible for the brittle propagation of short fatigue cracks. __________ Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 3, pp. 25–28, May–June, 2005.     

A dilatancy model of tensile macrocracks in compressed rock

A model is developed for tensile fracture under compression for a brittle material with microcracks. The final stage of failure with the formation of macroscopic-splitting cracks is considered. Pre-existing microcracks act as a converter of compression into tension in one direction. This results in the nucleation of other tensile microcracks. Rupture of spacings between the microcracks generates a mode I macrocrack parallel to the direction of maximum compression. Crack propagation is due to sliding along planes that are inclined to the compression microcrack surfaces and is stimulated by the forces distributed along the interacting macrocrack surfaces. Equilibrium, stability and growth of cracks are studied on the basis of the theory of fracture mechanics under the assumption of the plane strain state. The behaviour of both short and long macrocracks are analysed. Parameters of the model are evaluated with the help of data from fracture experiments on some rocks.     

Cracking simulation‐based fatigue life assessment

A fracture mechanics numerical model is developed to simulate the collective behavior of growing short fatigue cracks originating from the surface of unnotched round specimens made of a two‐phase alloy. The specimen surface roughness is considered resembling microcracks of different sizes and locations along the minimum specimen circumference. Material grains of different phases, sizes, and strengths are randomly distributed over that circumference. Variations in mechanical and microstructural features of grains are randomly distributed. Possible activities of surface cracks are predicted against loading cycles till either fracture occurs or all existing cracks become nonpropagating. The material's S‐N curve and fatigue limit can, thus, be assessed. Published experimental data on ferritic‐pearlitic steel specimens in push‐pull constant amplitude loading (CAL) were utilized. Different specimens were randomly configured and virtually tested. Comparison of experimental results and corresponding predictions validates the model, which, further, recognizes the effect of surface roughness, specimen size, and mean stress on lives.     

Low‐cycle fatigue/high‐cycle fatigue interactions in notched Ti‐6Al‐4V*

Combined low‐cycle fatigue/high‐cycle fatigue (LCF/HCF) loadings were investigated for smooth and circumferentially V‐notched cylindrical Ti–6Al–4V fatigue specimens. Smooth specimens were first cycled under LCF loading conditions for a fraction of the previously established fatigue life. The HCF 10⁷ cycle fatigue limit stress after LCF cycling was established using a step loading technique. Specimens with two notch sizes, both having elastic stress concentration factors of K_t = 2.7, were cycled under LCF loading conditions at a nominal stress ratio of R = 0.1. The subsequent 10⁶ cycle HCF fatigue limit stress at both R = 0.1 and 0.8 was determined. The combined loading LCF/HCF fatigue limit stresses for all specimens were compared to the baseline HCF fatigue limit stresses. After LCF cycling and prior to HCF cycling, the notched specimens were heat tinted, and final fracture surfaces examined for cracks formed during the initial LCF loading. Fatigue test results indicate that the LCF loading, applied for 75% of total LCF life for the smooth specimens and 25% for the notched specimens, resulted in only small reductions in the subsequent HCF fatigue limit stress. Under certain loading conditions, plasticity‐induced stress redistribution at the notch root during LCF cycling appears responsible for an observed increase in HCF fatigue limit stress, in terms of net section stress.     

Fretting fatigue behaviour of hard anodizing coated 2014‐T6 aluminium alloy with dissimilar mating materials under plane bending loading

The effect of hard anodizing coated 2014‐T6 aluminium alloy test samples with dissimilar mating materials on fretting fatigue was investigated. Fretting fatigue configuration involved bridge‐type pads on the flat specimen. Bridge‐type pads were made of AISI 4140 steel. All the fretting fatigue tests were conducted under plane bending loading with a stress ratio of R=?1. Coated and uncoated specimens were compared for microhardness, surface roughness, tangential force. The specimens were tested under both plain fatigue and fretting fatigue loading at ambient temperature. Micrographs obtained from scanning electron microscope showed that hard anodizing coating had tiny cracks through the thickness of the anodized layer. The hardness of hard anodized coating was higher than that of uncoated specimens and they also exhibited lower tangential force. However, the fretted region of the hard anodizing coated specimens was rougher than that of uncoated samples and despite lower tangential forces, fatigue lives of hard anodizing coated samples were inferior to those of uncoated samples. As the hard anodizing coating had pre‐existing tiny cracks and tension residual stress, cracks propagated from the hard anodizing coating through the interface into the substrate. We conclude that these may be the main reasons for inferior fretting fatigue lives compared with uncoated samples.     

Damage evolution in sinter-hardening powder-metallurgy steels during tensile and fatigue loading

The damage approach was used to compare the tensile and push-pull fatigue behaviour of two high-strength sinter-hardened powder metallurgy (PM) steels, with a density of 6.7 and 7.2 g/cm³. In both alloys, tensile damage was found to start when the ratio of the applied stress to UTS was greater than about 0.3. The tensile damage was due to localized yielding and, in a later stage, to the formation of several micro-cracks that joined to form more than one macrocrack. Fatigue damage was followed in the finite fatigue life regime and was found to develop through three stages. During the first one, damage increased with fatigue cycling up to the attainment of a plateau (second stage) that lasted to a fraction of about 0.9 of the fatigue life. The damage recorded during the second stage was very similar to that encountered in the tensile tests at the same stress to UTS ratio. The formation of microcracks was observed in the third stage only, when the fraction of fatigue life was greater than 0.9. During this stage damage increased very sharply and this was due to the growing and joining of the microcracks to form one long crack able to lead to final fracture.     

MICROSTRUCTURAL EFFECT ON LOW CYCLE FATIGUE BEHAVIOUR IN Ti-ALLOYS UNDER BIAXIAL LOADING

Abstract— Low cycle fatigue tests under axial, torsional and combined axial-torsional loading were conducted using thin-wall tubular specimens of Ti-6A1–4V titanium alloys. Two kinds of alloys with different microstructures, the (α+β) and β alloys, were investigated in fatigue tests at room temperature. When the failure life was correlated with the equivalent plastic strain, the life in axial loading shifted toward the lower life region compared with those in other loading modes in both alloys. Dominant surface cracks propagated in mode I under axial and combined loading in the two alloys. Although growth by the mode II type was predominant under torsional loading, the growth direction of the main crack coincided with the specimen axis in the (α+β) alloy, but the circumferential direction in the β alloy. The cracking morphology depended on the microstructure, especially under the torsional mode of loading, and was simulated successfully by using the proposed model for crack initiation.     

The application of microstructural fracture mechanics to various metal surface states

The fatigue crack initiation period, previously thought to be a necessary precursor to fatigue crack propagation and eventual failure, is considered here to be a negligible phase in the fatigue failure of polycrystalline metals. Rather this period is considered to be the propagation of a defect of microstructural dimensions by a variety of processes. The significance of this alternative view is examined in relation to corrosion fatigue, models of short crack growth, different loading modes, and the enhancement of fatigue resistance by surface shot-peening treatments. In both inert and aggressive environments, the fatigue lifetime of plain steel specimens of various strengths and treatments is predominantly determined by the early propagation of short cracks of microstructural dimensions. Microstructural fracture mechanics, rather than continuum mechanics, can quantify both pit growth and Stage I shear crack growth behavior before the defect reaches the dominant microstructural barrier which controls the fatigue behavior of the material. The important processes that determine lifetime are those that are strongly dependent on the synergism between the aggressive environment and cyclic stresses; these are the pitting, Stage I and the Stage I-to-Stage II crack propagation processes. A model has been produced to quantify these three important stages of lifetime named above. Under torsion loading, where Stage I cracks prefer to propagate along the surface, an intermittent series of deceleration/acceleration events of crack growth occur across the first few grain boundaries until the defect is blocked in its further development by a major microstructural barrier. When this barrier is breached, the environmentally-assisted Stage I crack rapidly becomes a Stage II crack. Under push-pull loading, the Stage I environmentally-assisted crack can propagate faster into the bulk material and, as a consequence, the transition to a Stage II environmentally-assisted crack is rapid thereby eliminating the need for the intermittent process observed under torsion loading. With no environmentally-assisted fatigue processes (i.e., testing in air) reversed torsion and push-pull loading test data can best be correlated by a von Mises criterion. Corrosion fatigue lifetimes can best be correlated by a Rankine (tensile stress) criterion. Shot-peening enhances the corrosion fatigue resistance of polycrystalline metals by inducing residual compressive stresses in the surface and creating numerous and more rapidly formed microcracks. This is probably caused by the presence of variously oriented plastically deformed bands within the surface microstructure and “short crack-short crack” interactions both of which delay the progress of the dominant crack toward its final Stage II phase. The present work is published according to the kind permission of the Royal Society in London. In 1997, Prof. K. J. Miller celebrated his 65th birthday. Congratulations of the Editorial Board on this occasion are presented at the end of this issue. Research Institute for the Integrity of Structures, Sheffield University (SIRIUS), England. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 1, pp. 9–32, January–February, 1997.     

Creep‐fatigue deformation micromechanisms of a directionally solidified nickel‐base superalloy at 850°C

In the present exploration, it was attempted to understand the creep‐fatigue (CF) deformation micromechanisms of alloy CM 247 DS LC by conducting low‐cycle fatigue (LCF) and CF tests employing strain amplitude ranging from 0.6% to 1.0% at T = 850°C in the air and performing extensive electron microscopic examinations. The cyclic life of the alloy lessens for all CF tests conducted at 1 and 5 minute dwell time in comparison to LCF tests. Transmission electron microscopy (TEM) examinations confirmed that during CF tests substructure consists of dislocation loop, mixed dislocations, and γ' rafting, a typical creep deformation signature of nickel‐base superalloys, it also consists of features observed during fatigue deformation such as anti‐phase boundary (APB)‐coupled dislocations inside γ' precipitates and local tangles of dislocations. This confirms that the deformation of CF‐tested specimens is ascribed to the synergistic effect of both creep and fatigue. This fact was further verified by scanning electron microscopic (SEM) examinations.     

INITIAL FRACTURE MECHANISMS IN NICKEL ALLOYED PM STEEL

Abstract— Initial fatigue crack propagation mechanisms at near threshold conditions were studied for four nickel-alloyed, powder-metallurgy (PM) steels. Fatigue fracture surfaces were obtained by testing smooth rectangular specimens at 30 Hz and under constant amplitude and zero mean stress conditions. Materials based on Distaloy AE were used in two densities, namely 7.15 and 7.45 g/cm³.
All the fracture surfaces were composed of three morphological regions (i) a macrocrack initiation region Rl where cracks propagated preferentially through particles (ii) a macrocrack growth region R2 and (iii) an unstable crack growth region R3 where cracks propagated preferentially between particles. Initial fatigue crack growth, in region R1, was controlled by the propagation of short cracks whose dimensions were comparable to the material microstructure. The subsequent fatigue crack growth in regions R2 and R3 was controlled by ductile rupture between microvoids. Transparticle fracture in region R1 was independent of pore distribution, while interparticle fracture in regions R2 and R3 was dependent on pore distribution.     

Investigation on the Static Fatigue Mechanism and Effect of Specimen Thickness on the Static Fatigue Lifetime in WC–Co Cemented Carbides

The static fatigue mechanism and effect of specimen thickness on static fatigue lifetime for four WC–Co cemented carbides were studied with different binder contents and carbide grain sizes. Static fatigue tests under three-point bend loading were conducted on different sized specimens. The fracture surfaces of rupture specimens were examined by scanning electron microscopy to investigate the static fatigue micromechanisms. Experimental results show that microcracks nucleate from defects or inhomogeneities and the connection of microcracks produces a main crack. The main crack propagates rapidly, resulting in the fracture of specimens. The extension of static fatigue lifetime with the increase of specimen thickness is due to the decrease of plastic zone size near the crack tip and relevant energy change during the crack growth. The effect of specimen thickness on static fatigue lifetime is much greater for cemented carbides with larger WC grain size or higher cobalt content, which is attributed to operative toughening mechanisms.     

Weld arrangement effects on the fatigue behavior of multi friction stir spot welded joints

In this study, the effects of friction stir spot weld arrangements as multi type on fatigue behavior of friction stir spot welded joints is investigated. The joints that are considered with five different styles for friction stir spot welded joints: one-row four joints parallel to loading direction, two-row four-joint specimen, one-row four joints perpendicular to the loading axis, three-row as diamond shape with four joints in each edge and five friction stir spot welded specimen in three rows that middle row consist three joints. The correlation between micro hardness, cyclic material constants and mechanical strength of different zones around the friction stir spot welds are assumed to be proportional to base material hardness. A non-linear finite element analysis was carried out for simulating tensile shear multi friction stir spot welded joints with ANSYS software by considering gap effects. Using the local stress and strain calculated with finite element analysis, fatigue lives of specimens were predicted with Morrow, modified Morrow and Smith–Watson–Topper (SWT) damage equations. Experimental fatigue tests of welded specimens have been carried out using constant amplitude load control servo-hydraulic fatigue testing machine. The results reveal that there is relatively good agreement between fatigue life predictions and experimental data in reasonable fatigue life regime.