Short Crack Behavior Research Articles

SHORT CRACKS AND LIFETIME OF A NORMALIZED CARBON STEEL UNDER CYCLIC BLOCK LOADING

Abstract— The behaviour of short cracks and the lifetime of unnotched specimens of a normalized medium carbon steel is observed under constant cyclic loading and under cyclic block loading with changing mean stresses. The greatest part of the life is covered by the growth of short fatigue cracks. During cyclic block loading, the length of the loading blocks has a marked influence on the crack density and the lifetime of the specimens. Short blocks lead to high crack densities and low lifetimes. This effect can be explained by a short crack growth model which takes into account the crack arrest before microstructural barriers and crack linking in the case of high crack densities. The lifetimes for cyclic block loading are calculated on the base of the constant level data and found to be in good agreement with the measured lifetimes.

Behavior of short fatigue cracks initiated from a notch in 8090 Al-Li alloy

The rates of growth of short fatigue cracks initiated from a notch are much greater than the rates of growth of long fatigue cracks for the same values of Delta K. A decrease in the strength of materials caused by aging affects the behavior of long cracks. The geometric form of the notch strongly affects the behavior of short cracks. The growth rate of a short crack initiated from a sharp notch decreases and attains a minimum value at a length of 0.45 mm, which is far beyond the region of its influence. However, short cracks initiated from blunt notches exhibit slower growth in the region of stress concentration than outside this region. Strain fields induced by deformation of the tip of the notch are not the only factor inhibiting the propagation of short cracks from notches. To explain the behavior of a short crack initiated at a notch, one must take into account some other factors, in particular, crack closure.

A comparison of short and long fatigue crack growth in steel

In this study, the growth behaviour of short fatigue cracks in En7A steel with a high content of elongated MnS inclusions has been investigated by generating and evaluating short crack growth data for the six principal specimen orientations. The anomalous initiation and growth behaviour of short cracks was also compared with that of long crack data for the same material and loading conditions determined before. This comparison revealed the sub-threshold initiation and faster growth rate of short cracks compared with long cracks under the same stress intensity factor range. In order to simulate this discrepancy between short and long cracks, a crack closure model for long crack data was used.

Prediction of Notch Fatigue Threshold Based on R-Curve Method.

Single-edge-notched specimens of a structural carbon steel and a low-alloy steel were fatigued under axial loading. The initiation and propagation behavior of short fatigue cracks emanating from a sharp notch was investigated. The resistance-curve (R-curve) method was used to predict the fatigue thresholds of notched members. The R curve was constructed in terms of the experimentally determined threshold value of the maximum stress intensity factor which was the sum of the threshold effective stress intensity range ΔKeffth and the opening stress intensity factor Kopth. The value of ΔKeffth was constant and the R curve was independent of crack length and notch geometry. The predicted values of the fatigue limit for crack initiation, the fatigue limit for fracture, and the nonpropagating crack length agreed well with the experimental results. The effect of the material property or Vickers hardness on the fatigue notch sensitivity was discussed on the basis of the R-curve method. A simplified method to determine the R curve using Vickers hardness or the tensile strength was proposed for design applications.

Concepts in preventing the early stages of aqueous stress-corrosion cracking

Environmently induced crack growth generally progresses through several stages prior to component failure, including crack initiation, short-crack growth, and stage 1 growth. The importance of understanding early stages derives from the extended time that cracks exist in the early stages because crack velocity is slow. This slow crack growth provides an opportunity for preventing the transition into long cracks. Several important factors about short-crack behavior can be used to lessen the impact of stress-corrosion cracking: life prediction requires a knowledge of the total life cycle of the crack, including the early stages; greater reliability is possible if the transition between short-and long-crack behavior is known, because component life after this transition is short; and preventative actions are more effective for short cracks rather than long cracks.

Response of Short Cracks in High-Strength Steels to Fatigue Loadings—Part II: Behavior in Seawater Under Cathodic Polarization

Experiments have been performed to determine the fatigue crack growth rate (FCGR) of short cracks (length from 0.1 to several mm) of five steels with yield stress in the range 370–570 MPa while cathodically polarized in natural seawater. Attention was focused on regions I and II of the classical FCGR-stress intensity range curve with particular consideration being given to the near-threshold behavior of short cracks. Single-edge notched, three-point bend specimens and a direct current potential drop crack monitoring system were employed; and test parameters were selected to simulate conditions experienced by deepwater offshore structures. These included a stress ratio of 0.5, a frequency of 0.3 Hz, and three levels of cathodic polarization (−800,−950, and −1100 mV, SCE). Crack growth rates were evaluated in terms of environment (air versus seawater), potential, material, and crack length. It was determined that cathodic polarization was generally beneficial with regard to FCGR compared to the freely corroding case, even at the most negative potential considered (−1100 mV, SCE), in contrast to what occurs for macro-cracks. The results are discussed within the context of design of offshore structures for resistance to fatigue.

Subcritical fatigue crack growth in alumina—I. Effects of grain size, specimen size and loading mode

The effect of both grain size and specimen size upon the slow fatigue crack growth characteristics in alumina is investigated using notched double cantilever beam type specimens. The fatigue crack growth rate can be described by the usual power law, d a/d N = A ( K max) n , with a value of n between 13 and 20. Both threshold stress intensity factor and crack growth rate, for a given value of the maximum applied stress intensity factor, are influenced by the load ratio R. An anomalous “short crack behaviour” was observed and is discussed. From the recorded load-notch opening displacement curves, it is shown that the fatigue cracks are bridged by both frictional ligaments, which give rise to a hysteresis loop, and elastic ligaments which decrease the specimen compliance. The equivalent bridging load (the load measured at loading pins) which is required to overcome the bridging resistance is measured as a function of crack length. It is shown that the elastic ligaments make the largest contribution to the bridging forces, which are much larger for the large grain size alumina. An effect of specimen size upon the crack growth resistance was also observed.

The double slip plane model for the study of short cracks

Apart from crack closure which has been employed extensively to explain apparent differences in the behaviour of short and long cracks, dislocation crack tip shielding is investigated as an additional contributing factor. The double slip plane (DSP) crack model is employed and a numerical approach is followed for the calculation of the dislocation density distributions on the crack and the slip planes and the plastic zone size. By accounting for dislocation crack tip shielding, the critical stress σcr and applied stress intensity factor KA,cr which define the onset of crack advance, exhibit a crack length dependence which is a manifestation of the anomalous short crack behaviour. It is shown, that below a critical crack length KA,cr is not constant, but rather decreases with decreasing crack length. Furthermore, a critical stress smaller than that predicted by linear elastic fracture mechanics is found. A method to calculate crack propagation rates based on dislocation density distributions is also presented. It is shown that under cyclic loading, a short crack propagates below the threshold stress intensity of a long crack and exhibits faster crack growth rates for the same nominal crack driving force. For the mild crack growth regime, Paris power-law exponents of 2 and 2.7 are predicted, depending on the value of an adjustable parameter in the configuration of the DSP model.

PROPAGATION BEHAVIOUR OF SHORT FATIGUE CRACKS IN Q2N STEEL

Abstract— The work described in this paper characterizes short fatigue crack growth behaviour of Q2N steel having a complex microstructure and designated for pressure vessel and offshore structure applications. Short and long fatigue crack growth tests for this steel were conducted under three point bend loading conditions. It was found that, in the initial stages of growth, short cracks propagate much faster than those of long cracks when correlated with the linear elastic fracture mechanics (LEFM) parameter ΔK. A period of crack growth retardation was observed at crack lengths of approx 50 μm. The theory of the interaction between short cracks and grain boundaries fails to predict the occurrence of this deceleration minima. A new short crack deceleration mechanism is proposed based on experimental observation. Observation of the characteristic behaviour of short cracks allowed the development of a short crack growth model based on microstructural fracture mechanics analyses.

Short crack effects in fracture and fatigue

The mechanics and micromechanisms in fracture and fatigue have been reviewed in order to identify the different behaviours associated with short and long cracks. In particular, it is shown that the brittle-ductile transition is shifted to lower temperatures in specimens containing shallow cracks whereas the growth rate of short fatigue cracks is larger than that of long cracks. The loss of constraint in fracture and reduced crack closure in fatigue are confirmed as key factors which promote the anomalous behaviour of short cracks. The effects of these factors are analysed using recent advances in both global and local approaches to fracture.

FRACTURE BEHAVIOUR OF A HIGHLY ALLOYED HIGH SPEED STEEL

AbstractA high speed steel, processed by two powder metallurgy routes and heat treated to give a range of microstructures, was investigated in 4‐point bending at room temperature using smooth and precracked specimens. The finer microstructures were in the material from gas atomised powder which was hot isostatically‐pressed, commercial ASP60 alloy, while the coarser microstructures derived from laboratory vacuum sintering of water‐atomised powder. The resultant hardness values Hv50 were in the rage 780 to 1050, prior autenite grain sizes, 5 to 25 pm and maximum carbide sizes, 6 to 32 μm.Only some of the uncracked samples exhibited macroscopic yielding, at stresses in the range 1.64 to 2.59 GPa; the finer microstructures being asSociated with the higher strengths. Macroscopic plastic deformation never exceeded 0.33%; fracture strengths were in the range 1.46 to 2.75 GPa. Fracture toughness, Klc, varied from 12 to 17 MPa√m in the Hy50 range 920 to 800 for the directly sintered steel and only from 10 to 12 MPa√m in ASP60. The insensitivity of K1c to macroscopic hardness in ASP60 is asSociated with the plastic zone size of 1.5 μm which approximates to the average carbide spacing.Nucleation and growth of natural, i.e. stress‐induced, microcracks in un notched specimens was studied by surface replica microscopy. Crack nucleation took place at stresses between 0.5 and 1.5 GPa, i.e. below those for yielding and for fracture, σF, and was by debonding of inclusions (alumina and calcium‐alumino silicates) or cracking of carbides. In the coarsest microstructure monotonic stepwise subcritical crack growth was observed from stress levels of ∼1.3 GPa, i.e. ∼0.8 to 0.9σF. Similarities to the behaviour of short fatigue cracks in metallic materials and the R‐curve behaviour of ceramics are referred to.

The mechanics of fatigue crack growth in a medium with microdamage

Fatigue crack growth at a high stress level is considered using the model of a thin plastic zone taking microdamage accumulation in this zone into account. It is assumed that a crack grows when the stability condition of the “cracked body-loading” system is violated. This condition is treated in the framework of the principle of virtual work for systems with unilateral constraints. The damage accumulation process in the plastic zone and on its prolongation is assumed to depend on the opening stress range. The process of crack growth is studied by numerical simulation. Diagrams of the growth of fatigue cracks are obtained that describe all three stages of fatigue damage including the anomalous behaviour of short cracks and the accelerated growth near the final fracture. It is shown that the proposed theory predicts that the slope of the middle part of the diagram is close to two when the parameters of microdamage accumulation vary over a wide range.

Corrosion fatigue of a 2024-T3 aluminum alloy in the short crack domain

It is well accepted that the steady-state fatigue crack growth rates of long cracks depend uniquely on AK for a fixed load ratio R and test environment. Anomalous growth behavior of short cracks in either inert or deleterious environments that has been reported [1-7], however, calls into question the validity of using only long-crack results in evaluating the service life of a structural component and argues for the need for considering the effects of crack size. Crack-size effects have been extensively studied from the perspectives of mechanical, metallurgical and chemical principles. Microstructurally and mechanically short cracks [1-3] are associated with the influences of fine-scale microstructure, the limitation of continuum mechanics (or LEFM-linear elastic fracture mechanics), or crack closure. Chemically short cracks [4-7], on the other hand, normally extend to longer lengths and are attributed to the differences in local crack-tip chemistry (e.g., pH, [Oz], potential) between the long and short cracks and also, perhaps, from the bulk solution. The objective of this study is to explore the effect of crack size in various environments and to identify the key variables that affect crack growth behavior. The crack size investigated herein was chemically short but microstructurally and mechanically long. A 1.6 mm thick 2024-T3 (bare) aluminum alloy was used and its chemical composition (in wt%) is as follows: 4.24 Cu, 1.26 Mg, 0.65 Mn, 0.15 Fe, 0.08 Zn, 0.06 Si, 0.031 Ti, <0.01 Cr and balance A1. The elongation, 0.2% offset yield strength and tensile strength are 17.0%, 355 MPa (51 ksi) and 480 MPa (70 ksi), respectively. The specimens were cut in the L-T orientation as dog-bone shaped, center-pin-loaded, single-edge-cracked specimens with a 31.75 mm wide and 76.2 mm long (final dimension) mid-section. A 1.6 mm long electro-discharge machined (EDM) starter notch was introduced at the center of one edge of each specimen. The specimens were precracked by fatigue in air to a crack length of 3.42 mm (including the notch). Extra material (2.92 mm wide) was included on the notched side, and was removed by EDM to produce the final specimen, with a symmetrical test section and an approximaely 0.5 mm long edge crack. The following K calibration equation was used:

Effect of residual stress on fatigue behaviour of notches

The effects of surface-rolling-induced residual stresses on the endurance limit and the growth behaviour of short cracks were investigated for notched steel parts. Unlike the case of smooth specimens tested in laboratory conditions, the endurance limit of notches under axial loading was enhanced by surface rolling, but the improvement was limited owing to the detrimental effect of tensile residual stresses at a distance from the surface. The second fatigue origin, which was just ahead of the crack tip and destroyed the condition of slowed propagation rate of the short crack in the compressive residual stress field, would be produced at the position where the maximum tensile residual stress was acting. This effect could be effectively eliminated by tensile preloading in such a way that the tensile residual stresses were greatly released by plastic deformation and the short fatigue crack in the compressive residual stress field became non-propagating at the loading of endurance limit. The effect of compressive residual stress on growth behaviour of short fatigue cracks lies in two aspects: (a) increasing the crack closure effect; and (b) decreasing the maximum stress intensity factor. A predicted d a/dN versus ΔK eff curve based on this assumption agreed with the experimental results.

Short‐Crack Mechanical Properties and Failure Mechanisms of Si3N4‐Matrix/SiC‐Fiber Composites

SiC‐fiber‐reinforced Si3N4 composites were fabricated by hot pressing. The indentation‐strength technique was applied to study the mechanical properties of these composites. This enabled the investigation of short‐crack behavior of continuous‐fiber ceramic composites (CFCCs). The flaw tolerance of composite ultimate strength, matrix‐cracking stress, and work‐of‐fracture were investigated. Scanning electron microscopy was used to examine crack–fiber interactions. The ultimate strength was found to be independent of indentation load at a fiber volume fraction f= 0.29, while at f= 0.14 it exhibited a transition from flawsensitive to flaw‐independent. The work‐of‐fracture was found to be independent of indentation load at both fiber volume fractions. The matrix‐cracking stress was found to correspond to the first load‐drop on the load–displacement curve. It decreased with increasing flaw size and therefore is the steady‐state matrix‐cracking stress. A failure mechanism transition from catastrophic failure to non‐catastrophic failure, coupled with the transition from flawsensitive to flaw‐tolerant behavior, was observed by varying the preexisting flaw size and the fiber volume fraction. These transitions were explained by analyzing the relations between ultimate strength, matrix‐cracking stress, fiber volume fraction, and preexisting flaw size of the composite materials. Experimental results were compared with predictions from available models.

The initiation and propagation behaviour of short fatigue cracks in Waspaloy subjected to bending: Yates, J.R. Zhang, W. and Miller, K.J. Fatigue Fract Eng Mater Struct (Mar. 1993) 16 (3), 351–362

Classical definitions of boundary layer mass and momentum flux deficiency thicknesses can lead to gross errors when applied to measurements near a trailing edge where the flow curvature in the free stream is appreciable. This paper presents a double vortex sheet model as a development from the single vortex sheet model of Helmholtz and others. Two bound vortex sheets define a potential function which can describe a flow with the same mass and momentum flux deficiencies as the viscous regions. The bound nature of these sheets allows the modelling of the integral properties of these regions while retaining the advantages of a potential flow. The application to the flow near the trailing edge of a lifting aerofoil is given

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.

Crack growth and closure behavior of short fatigue cracks

Crack growth and closure behavior of short cracks are investigated for various stress ratios, using in-plane bending specimens of 2024-T351 aluminum alloy. Artificially prepared two-dimensional, short through-thickness cracks are used. Crack length and closure of short cracks are measured continuously during the test by employing an unloading elastic compliance technique and a personal computer system. For most of all the stress ratios tested, short cracks grow faster than long cracks in the low stress intensity factor range region, and the growth rates of short cracks merge with the long crack growth curve with increases in the value of stress intensity factor range. The variation in growth rates between short and long cracks is reduced as the value of stress ratio is increased, and finally disappears at a stress ratio of R = 0.5. The growth rates of short cracks are well described by the effective stress intensity factor range. The growth rates of short cracks in terms of the effective stress intensity factor range coincide well with the long crack data. However, the closure behavior of short cracks is significantly different from that of long cracks. The crack opening ratio of a short crack decreases from an initial high value corresponding to a fully open crack linearly with the value of effective stress intensity factor range, and merges with the long crack results. Based on the test results obtained, a relatively simple procedure is proposed to predict closure and growth behavior of short cracks. The validity of the proposed procedure is examined using the test data of other workers. In addition, some methods proposed by other workers are also discussed.

Fatigue Crack Propagation from Notch in SiC-Particulate-Reinforced Aluminum Alloy.

The initiation and propagation behavior of short fatigue cracks emanating from a sharp notch in an aluminum alloy (2024-T6) reinforced with 20 volume percent of SiC particles was investigated. The development of crack closure with crack propagation was measured through the compliance method. The propagation rate, dc/dN, of short cracks was higher than that predicted using the dc/dV-ΔK relation for long cracks. For the case of a low stress amplitude, the propagation of short cracks decelerated and finally stopped. The relationship between dc/dN and the effective stress intensity range, ΔKeff, for short cracks at notches was identical to that obtained for long cracks. The relationship between dc/dN and ΔKeff/E(E=Young's modulus) was nearly identical for short and long cracks in reinforced and unreinforced alloys. The roughness of the fatigue fracture surface was determined using a computer image processing method. The roughness of the fracture surface of composites was larger than that of unreinforced aluminum alloy.

Application of Short and Long Fatigue Crack Growth Behaviour in Lifetime Prediction of Mine Auxiliary Ventilation Fans

Growth behaviour of short and physical small cracks in a C-Mn steel heat treated to two microstructural conditions has been investigated experimentally on hour-glass shape specimens subjected to push-pull loading at room temperature and using the replica technique for crack length measurements. A crack growth equation which considers the microstructural effect on fatigue crack growth was used to explain the experimental behaviour obtained from constant amplitude tests. Long crack growth behaviour was studied using three point bend and cruciform specimens. The experimental data was analysed using fracture mechanics and presented in the form of a Paris law. The theoretical and empirical derivations were applied to predict the inspection intervals and fatigue life time of mine auxiliary ventilation fans.