Fatigue Crack Initiation Model Research Articles

Microcracks of the alumina of the thin-film head: study and simulation

A study of the microcracks found in the alumina of a magnetic thin film head is presented. A model of the alumina fatigue crack initiation and growth is proposed. Thermal expansion of the energized wires leads to high stress at the pole tip area. Crack initiation usually occurs at the outer edge of the alumina, due to the mechanical damage caused by the rotating disk surface. Further crack growth is caused by fatigue in the alumina. The crack is attracted to the pole tip area, affecting the magnetic head performance. Magnetic Force Microscopy is used to study the effects of the crack on the magnetic fringe field of the head.

Atomic force microscopy and modeling of fatigue crack initiation in metals

We have obtained the first atomic force microscope (AFM) images of cell like boundaries and slip band emergence of fatigued metals surfaces (titanium and high strength-low alloy steel). The AFM has been used to obtain quantitative information about slip spacings and slip height displacements. Using these measurements we have determined the fraction of plastic strain that contributes to surface displacement upset. Values of 1.6% for Ti and 0.9% for HSLA resulted for completely reversed plastic strain cycling. We introduce a model for fatigue crack initiation based on slip band spacings, slip height displacement, and cumulative plastic strain. To first order these compare well with experimental initiation data obtained by other investigators. The relative ease of sample preparation for AFM, along with its outstanding height resolution, offers a higher degree of simplicity and precision in fatigue crack initiation studies compared to more conventional techniques.

A damage mechanics model of fatigue crack initiation in notched plates: Chow, C.L. and Wei, Y. Theor. Appl. Fract. Mech.16 2 (Nov 1991) 123–133

A damage mechanics model of fatigue crack initiation in notched plates: Chow, C.L. and Wei, Y. Theor. Appl. Fract. Mech.16 2 (Nov 1991) 123–133

FATIGUE LIFE ESTIMATION IN RESISTANCE SPOT WELDS: INITIATION AND EARLY GROWTH PHASE

Abstract— The main objective of this research is to develop a model of fatigue crack initiation and early crack growth in resistance spot welds that is specimen independent. This objective is achieved by examining the stress state around a resistance spot weld. A general expression for the structural stress around the weld is formulated that is dependent only on the loading immediately surrounding the weld. As such, it is specimen independent.An additional objective is to explore the feasibility of applying this fatigue crack initiation model of life estimation using structural response data from finite element analysis (FEA). This numerical technique is often used for evaluating structural integrity of assemblies. Limited verification examples show that the structural stress range as calculated from FEA reaction load data is capable of describing fatigue crack initiation and early crack growth in cyclically loaded resistance spot welds.

Fatigue properties and fatigue fracture mechanisms of SiC whiskers or SiC particulate-reinforced aluminium composites

Fatigue properties and fracture mechanisms were examined for three commercially fabricated aluminium matrix composites containing SiC whiskers (SiCw) and SiC particles (SiCp) using a rotating bending test. The fatigue strengths were over 60% higher for SiCw/A2024 composites than that for the unreinforced rolled material, while for the SiCp/A357 composites, fatigue strengths were also higher than that for the unreinforced reference material. For the SiCp/A356 composites at a volume fraction of 20%, the fatigue strength was slightly higher than that of the unreinforced material. Fractography revealed that the Mode I fatigue crack was initiated by the Stage I mechanism for the SiCw/A2024 and SiCp/A357 composites, while for the SiCp/A356 composite, the fatigue crack initiated at the voids situated beneath the specimen surfaces. On the other hand, the fatigue crack propagated to the whisker/matrix interface following the formation of dimple patterns or the formation of striation patterns for SiCw/A2024 composites, while for the SiCp/A356 and SiCp/A357 composites the fatigue crack propagated in the matrix near the crack origin and striation patterns were found. Near final failure, dimple patterns, initiated at silicon carbide particles, were frequently observed. Mode I fatigue crack initiation and propagation models were proposed for discontinuous fibre-reinforced aluminium composites. It is suggested that the silicon carbide whiskers or particles would have a very significant effect on the fatigue crack initiation and crack propagation near the fatigue limit.

A damage mechanics model of fatigue crack initiation in notched plates

A damage model is proposed to characterize fatigue crack initiation in notched plates. This is accomplished by assuming a damage fracture criterion and effective instantaneous tangent moduli matrix which accounts for damage accumulation. A finite element package is used to perform the numerical analyses for an edge-notched plate specimen made of Al alloy 2024-T3 under cyclic loading of varying constant amplitudes. The estimated fatigue crack initiation life agrees satisfactorily with those determined experimentally. An important observation from the results of the proposed damage model is its ability to identify the phenomenon of stress redistribution due to material degradation in two-dimension specimens during fatigue loading.

A point defect model for fatigue crack initiation in Ni 3Al + B single crystals : Hsiung, L.M. and Stoloff, N.S.Acta Metall. Mater. June 1990 38, (6), 1191–1200

A point defect model for fatigue crack initiation in Ni 3Al + B single crystals : Hsiung, L.M. and Stoloff, N.S.Acta Metall. Mater. June 1990 38, (6), 1191–1200

Fatigue crack initiation and stage-I propagation in polycrystalline materials. II: Modelling

Based on the discussion of the micromechanisms involved with fatigue crack initiation presented in part I, an analytic model of stage-I fatigue crack initiation and the subsequent stage-II crack propagation based on a plastic strain intensity factor, ΔK p, is proposed. In this model, the stage-I crack initiation process is simplified as a straight line on a log-log plot of d a/d N against ΔK p. An inflection point that divides the crack growth into the initiation and propagation categories is introduced as the plastic crack propagation threshold, ΔK pth, whose value is determined by [(ΔJ)E] pth 1 2 , where ΔJ is the range of the J-integral and E is the Young's modulus. Following this, the integration of the crack growth rate as a function of the plastic strain intensity factor yields a fatigue crack initiation life estimate whose error is less than 5%. For comparison, the experimental data presented in this paper are also discussed on the baskis of the parameters ΔK and ΔK eff that have been proposed by other researchers.

A point defect model for fatigue crack initiation in Ni 3Al+B single crystals

Transmission electron microscopy of boron-doped Ni 3Al single crystals, oriented for single slip and cyclically deformed at room temperature, revealed a high density of dislocation dipoles and point defect clusters. Observations of circular perfect dislocation loops, Frank loops, vacancy tetrahedra and spherical voids provide evidence of vacancy condensation during fatigue cycling at room temperature. It is suggested that lattice misfit develops between persistent slip bands (PSB) and matrix as a result of the generation and coalescence of excess vacancies in PSBs. The misfit strain at PSB/matrix interfaces is considered to increase with increasing cumulative plastic strain. Together with SEM observations of surface topography, it is suggested that fatigue damage in Ni 3Al single crystals is initiated by the formation of microvoids (microcracks) at PSB/matrix interfaces. The microvoids (microcracks) break down the coherency of the PSB/matrix interfaces and thereby relieve the accumulated misfit strain at the interfaces. A model of fatigue crack initiation based upon a surface energy criterion is proposed.

Fatigue processes in metals—role of aqueous environments

The response of a wide range of metals under the action of an applied stress and in environments spanning an entire range of aggressiveness, from the very inert to aggressive has been the subject of several investigations. Most of these studies have focused on mechanisms governing fatigue crack propagation and little attention has been paid to the effect of environment on fatigue crack initiation. A comprehensive understanding of the influence of aqueous environments has been hampered by the complexity of the problem, the difficulties in understanding the various micromechanisms governing crack initiation and crack propagation and by an absence of a truly interdisciplinary attack of the problem. In this paper, several of the fatigue crack initiation models are presented and the micromechanisms governing fatigue crack initiation are examined. The various processes that control the phenomenon of cracking including aqueous environment enhanced fatigue crack growth are also highlighted in this review.

A unified model for fatigue crack initiation and growth, with emphasis on short-crack behavior, crack closure effects, variable-temperature fatigue and creep-fatigue interaction

A new unified phenomenological model for elevated temperature fatigue has been developed. In this model the incremental nature of failure is represented explicitly in terms of the initiation and growth of cracks occurring by a succession of local fracture events. This local fracture can take place by any one of several physical processes, whose rates are driven by the local mechanical conditions (i.e. the histories of stress and plastic strain) at a critical distance ahead of the crack tip. The individual local fracture processes which are represented within the computer model include (1) stage I transgranular fatigue (driven by the local plastic strain range), (2) stage II transgranular fatigue (driven by a combination of the local plastic strain range and the local peak tensile stress), (3) “r-type” intergranular cavitation (driven by the histories of local tensile stress and temperature), (4) “wedge-type” intergranular cavitation (driven by the local rate of grain boundary sliding and the extent of “r-type” cavitation), (5) ductile rupture (driven by the local tensile strain) and (6) cleavage (driven by the local tensile stress).The local mechanical conditions driving the above local fracture processes are predicted by a new “ligament” model which calculates the full triaxial stress and strain distribution histories around cracks of any length (including short cracks) for deformation behavior which encompasses creep, plasticity, cycling, variable temperature, crack tip blunting, crack closure, plastic flow concentration in near-surface grains, and blockage of slip by grain boundaries.Some of the important predictive capabilities of the new unified fatigue model include (1) the usual da/dN vs. ΔK behavior for initially cracked specimens, including a threshold ΔK, (2) the usual S vs. Nf behavior for initially smooth specimens, including a fatigue limit, (3) the anomalously high growth rates of short fatigue cracks, (4) plasticity-induced crack closure and its effects on crack growth, (5) mean stress and load sequence effects, (6) variable-temperature (thermomechanical) fatigue, (7) stress redistribution by creep and relaxation around crack tips, (8) creep crack growth by cavitation, (9) creep-fatigue interaction due to cavitation and (10) ductile-to-brittle transitions.

Mechanisms of fatigue crack initiation in metals: role of aqueous environments

Fatigue crack initiation in engineering materials has been the subject of considerable research. Most of these investigations focused on gaseous environment effects, and extensive review articles have appeared in recent times discussing the role of gaseous environments on crack initiation. Because of experimental difficulties, the effect of aqueous environments on mechanisms of fatigue crack initiation has received little attention, despite their unquestionable importance from an engineering standpoint. In this review, several of the fatigue crack initiation models are examined in detail and their anomalies discussed. The physics and micromechanisms of crack initiation during cyclic deformation in aqueous environments which are highly corrosive in nature are examined. The characteristics of the crack initiation process in aqueous environments are critically reviewed in the light of the specific role of several concurrent factors involving the nature of the aqueous medium, corrosion interactions, alloy chemistry, processing treatments, intrinsic microstructural effects and test variables.[/p]

On the role of point defects in fatigue crack initiation

Analysis of inhomogeneous cyclic straining, observation of surface relief at emerging persistent slip bands (PSBs) and data on point defect generation and migration in fatigued metal single cyrstals and polycrystals are the origin of a new model for fatigue crack initiation in PSBs. Because of the high plastic strain amplitude, non-equilibrium point defects are generated in PSBs. They are distributed inhomogeneously according to the inhomogeneous sink distribution. Point defect migration over short distances causes mass transport and results in the formation of extrusions and intrusions on the metal surface. The distribution of extrusions and intrusions is related to the dislocation substructure of the PSB. Cyclic straining under stress together with the strain concentration at sharp intrusions result in environment-assisted formation of new surfaces and therefore in crack initiation.

Fatigue crack initiation on slip bands: Theory and experiment

A model for fatigue crack initiation is presented based on accumulation of dislocations in slip bands modeled as elliptical domains with the domain size for tension being slightly different from that for compression. This takes account of the irreversibility. Using continuous dislocation theory, an accumulation of dislocation dipoles was calculated at the boundary between the slip domain and the matrix. Cracking was assumed to occur when the accumulated dipoles reach the number to give the critical displacement to cause fracture in a perfect metal. Rather good agreement between the theoretical predictions and measured values for the cycles to initiation was obtained. The theory also accounts for the very small size of fatigue crack just after initiation.

Measurement of fatigue-induced surface plasticity

A method is presented for measuring at a surface the localized plastic strains induced by fatigue within individual grains. The technique uses mica flakes distributed on a sample surface as reference gauges, relative to which strains in the surface can accurately be determined. An application of the method to the study of fatigue induced microplasticity in an Al 2219-T851 alloy is discussed. On an unfatigued specimen, subjected to applied stresses less than the yield strength, deformation is elastic over gauge lengths comparable with the grain size. After fully-reversed cyclic loading at a peak tensile stress of 75% of the yield strength for 20×103 cycles, the larger grains in the alloy exhibit a residual tensile strain after a tensile loading cycle. Neighbouring smaller grains are driven into elastic compression to accommodate this tensile plastic deformation. Peak localized tensile plastic strains may exceed 0.5% at the surface. This technique will be useful in evaluating models of fatigue crack initiation and surface damage accumulation.

A theory of fatigue crack initiation at inclusions

The dislocation dipole accumulation model for fatigue crack initiation previously proposed by the authors is extended to an analysis of the fatigue strength reduction due to inclusions in high strength alloys. The initiation of a fatigue crack is determined by an energy criterion under the assumption that the crack initiation takes place when the self strain energy of dislocation dipoles accumulated at the damaged part in the material reaches a critical value. Explicit formulae for the crack initiation criterion in several cases are derived as functions of the applied stress, the inclusion size, the slip band shape, and the shear moduli of the inclusion and matrix. The following three types of fatigue crack initiation at inclusions are considered: the slip-band crack emanating from a debonded inclusion, the inclusion cracking due to impinging of slip bands, and the slip-band crack emanating from an uncracked inclusion. The first mechanism was reported to be operative in high strength steels, while the last two mechanisms were reported in high strength aluminum alloys. The present theoretical results are in good agreement with the experimental data published for each case of fatigue crack initiation at inclusions.

A model for fatigue-crack initiation at stable slip bands

A model for fatigue-crack initiation at stable slip bands

The fatigue behavior of fine grained two-phase alloys

The fatigue properties of a number of ultrafine grained two-phase alloys have been examined. Compositions and processing treatments were altered to produce volume fractions of the individual phases ranging from 0 to 100 pct along with changes in the grain sizes of the individual phases. In all two-phase alloys in this investigation, the second phase was distributed at grain boundaries rather than within the grains of the primary phase. The fatigue strengths of the two-phase microduplex alloys were generally higher than those of the corresponding single phase alloys. Tension-compression fatigue tests showed that a Coffin law type of fatigue relationship was obeyed in the low cycle fatigue tests. Fatigue crack growth rate studies in tension-tension on IN-744 sheet gave results very comparable to those reported for other steels. Fatigue cracks propagated through both phases without following any obvious crack paths and no indication of delamination or crack blunting was detected. The high cycle fatigue performance of microduplex alloys can be rationalized in terms of a critical crack opening displacement model for fatigue crack initiation.