Showing papers on "Paris' law published in 1995"

Fracture normal to a bimaterial interface: Effects of plasticity on crack-tip shielding and amplification

Abstract: The problem of a crack approaching a bimaterial interface is considered in this paper. Attention is focused on an interface between two elastoplastic solids whose elastic properties are identical and whose plastic properties are different. For the case of a crack approaching a bimaterial interface perpendicularly, it is shown by recourse to detailed finite element analyses that the near-tip “driving force” for fracture is strongly influenced by whether the crack approaches the interfaces from the lower strength or the higher strength materials. Specifically, it is demonstrated that the crack-tip is “shielded” from the remote loads when it approaches the interface from the weaker material, and that the effective J-integral at the crack tip is greater than the remote J when it approaches the interface from the stronger material. This plasticity effect determines whether a crack approaching the bimaterial interface continues to advance through the interface or is arrested before penetrating the interface. These theoretical findings are substantiated using controlled experiments of fatigue crack growth perpendicular to a ferrite—austenite bimaterial interface. The effect of the non-singular T-stress, acting parallel to the crack plane, on shielding and amplification of the stress fields is also discussed.

Classification of fatigue crack growth behavior

Abstract: A self-consistent theory has been developed to account for the variation in fatigue crack growth rates with load ratio,R, without reference to crack closure concepts. The theory states that (a) for an unambiguous description of cyclic damage, two loading parameters are required; (b) consequently, there are two thresholds corresponding to each parameter that must be satisfied for a crack to grow; (c) these two thresholds are intrinsic and are independent of specimen geometry; (d) a fundamental threshold curve can be developed that is independent of test methods defining these two thresholds from the asymptotic values, and last; (e) the two thresholds vary with the degree of slip planarity, microstructure, and environment. Based on these new concepts, we have classified the entire fatigue crack growth behavior into five different classes using the experimental ΔKth-R data. The characteristic feature of each class is discussed, and the supporting examples of materials behavior are provided. This classification could provide a basis for understanding the synergistic effects of mechanical and chemical driving forces and microstructure contributing to fatigue crack growth.

Multiple-cracked fatigue crack growth by BEM

Abstract: The dual boundary element method is applied for the two-dimensional linear elastic analysis of fatigue problem of multiple-cracked body. The traction integral equation is applied on ones of surfaces of cracks while the usual displacement integral equation simultaneously on the others. General multiple crack growth problem is solved in a single-region formulation. All crack surfaces are discretized with discontinuous quadratic boundary elements. J-integral technique is used to evaluate stress intensity factors. The real extension path of cracks is simulated by a linear incremental crack extension, based on the maximum principal stress criterion. For each increment analysis of the cracks, crack extension is conveniently modelled with new boundary elements. Remeshing is no longer necessary. Fatigue life analysis is carried out with Paris' formulae. Several numerical examples show high efficiency of present method.

Effect of silicon particles on the fatigue crack growth characteristics of Al-12 Wt Pct Si-0.35 Wt Pct Mg-(0 to 0.02) Wt Pct Sr casting alloys

Abstract: Fatigue crack growth (FCG) characteristics and mechanisms in Al-Si-Mg eutectic casting alloys containing 0.35 wt pct Mg and 0 to 0.02 wt pct Sr were investigated as a function of stress ratio,R, stress-intensity-factor range, ΔK, and silicon (Si) particle size. The fatigue crack propagation behavior was compared with that observed in commercial casting alloy A356. At the same applied ΔK level, the crack growth rate was found to increase with increasing stress ratio and Si particle size. Modified (fine Si morphology) and A356 alloys showed better FCG resistance than the unmodified (coarse Si morphology) ones, for a constant applied ΔK, due to increased closure. The effects of roughness-induced and plasticity-induced crack closures, crack branching, and crack meandering on the fatigue crack propagation observed in these alloys have been discussed. The fatigue crack propagation path is found to be dependent on the Si particle characteristics. The mechanisms of silicon particle decohesion and cracking are also discussed.

Effect of aging on fatigue crack growth at sn-pb/cu interfaces

Abstract: The effect of isothermal aging on fatigue crack growth behavior at the Sn-Pb solder/Cu interface was examined, with emphasis on the role of interfacial microstructure. Flexural peel interface-crack specimens were made from the joints of eutectic Sn-Pb solder and Cu and were further aged at 443 K for 7 and 30 days. Kinetics of fatigue crack growth along the solder/Cu interfaces were measured from flexural peel specimens as a function of strain energy release rate. Aging was found to induce not only microstructural changes in the solder and at the interface, but also degradation in fatigue crack growth resistance of the interface from the fatigue threshold to the fast fracture. The fatigue threshold decreased from 25 to 20 J/m2 after aging for 7 days and to 10 J/m2 following aging for 30 days. The degradation in the fatigue crack growth resistance is related to the formation of a Pbrich layer at the interface.

Fatigue-crack growth and fracture resistance of a two-phase (γ + α2) TiAl alloy in duplex and lamellar microstructures

Abstract: The fatigue-crack propagation and fracture-toughness behavior of a two-phase (γ + α2) Ti-47.3Al-2.3Nb-1.5Cr-0.4V (in at.%) alloy in fine-equiaxed (duplex) and fully-lamellar microstructural conditions were examined at room temperature. It is observed that the lamellar microstructure displays superior fracture toughness and fatigue-crack growth resistance compared with the duplex microstructure, although the extent of improvement is significantly greater during quasi-static fracture. Crack extension under monotonic loading is characterized by resistance-curve behavior with plateau (steady-state) toughnesses of about 30 MPa m 1 2 in the fully-lamellar condition compared to a crack-initiation toughness of about 10 MPa m 1 2 for the duplex. Corresponding thresholds for cyclic fatigue-crack propagation are of the order of 10 and about 6.5 MPa m 1 2 in the lamellar and duplex structures, respectively. Such improved properties of the lamellar alloy are attributed principally to extrinsic shielding effects from tortuous crack paths, microcracking and formation of shear ligaments resulting from the microlaminated, composite-like features of the alloy.

Analysis of Fatigue Crack Closure and Thresholds

Abstract: A critical examination of fatigue crack closure concepts indicates that (a) procedures commonly followed in estimating crack closure are in error, (b) complete premature crack closure occurs only when cracks are completely filled with oxides or corrosion products, (d) such closure manifests as an infinite slope in the load-displacement (crack mouth opening) curves; rarely observed in experiments, (e) asperities cause only local closure, (f) their contribution is less significant than is assumed and (g) plasticity does not contribute to closure. New concepts are developed to explain fatigue phenomena hitherto attributed to crack closure. Implications of these concepts to understand fatigue crack growth are discussed.

On the calculation of closure-free fatigue crack propagation data in monolithic metal alloys

Abstract: A computational method is described for the determination of ΔK b , corresponding to a fatigue crack growth rate of b/cyc, where b is the Burgers vector for a monolithic metal alloy. ΔK b is found to be numerically equal to E √ b for the case of closure-free crack growth behavior. Given that the closure-free FCP rate of many monolithic metals varies with ΔK 3 , the growth rate of metal alloys at ΔK ⩾ ΔK b is given by d a /d N = (Δ K/E ) 3 (1/√ b . Excellent agreement is found between experimental and computed FCP data for the case of monolithic metal alloys. The limits of these relations for metal-matrix composites and ceramics are discussed.

The effect of grain size on fatigue crack growth in an aluminium magnesium alloy

Abstract: Fully reversed uniaxial fatigue tests were performed on aluminium magnesium alloy Al 5754 with four different grain sizes in order that the effect of grain size on fatigue crack growth could be examined. Surface cracks were monitored by a plastic replication technique. Fatigue strength was shown to improve with a decrease in grain size. The endurance stress is a function of the inverse square root of the grain size and is described empiricdty by a Hall-Petch type relation. The effect of grain size on fatigue crack growth is most significant when the crack length is of the order of the microstructure. Fluctuations in the growth rate of microstructurally short cracks are most marked in a fine grained microstructure and may be related to the need to transfer slip to adjacent grains. Crack path deviation is greatest in the coarsest grained microstructure and SEM fractography reveals a more pronounced crack surface roughness in the coarser grained alloy than in the finer grained alloy.

Separating the effects of Ti3Al and silicide precipitates on the tensile and crack growth behavior at room temperature and 593 °C in a near-alpha titanium alloy

Abstract: Aging of silicon-bearing, near-alpha titanium alloys produces two types of precipitate, Ti3Al and (TiZr6Si3 (silicide). A postaging heat treatment that relies on the phenomenon of the critical ordering temperature is used to remove the Ti3Al precipitate while leaving the silicide intact. Three materials—unaged (precipitate free), overaged (Ti3Al plus silicide), and post aging heat treated (silicide only)— are compared. The objective is to identify the separate effects of the Ti3Al and silicide precipitates on tensile and fatigue crack growth behavior at room temperature and 593 °C. The Ti3Al precipitate is largely responsible for the increase in yield stress and for the decrease in ductility at both test temperatures. In contrast, the increase in room temperature fatigue crack growth rate associated with aging is attributed to the silicide, with Ti3Al playing only a minor role. Aging produces a slight improvement in fatigue crack resistance at 593 °C, which also appears to be due to the presence of the silicide precipitate.

Cyclic crack growth in titanium aluminides

Abstract: Micromechanisms of fatigue crack growth have been studied in specific Ti 3 Al-based aluminides and TiAl-based aluminides. Effects of test temperature, environment and microstructure on fatigue crack growth resistance are considered. For both intermetallic systems brittle mechanisms of crack extension are observed at room temperature, and such mechanisms persist in the TiAl-based aluminides considered to a test temperature of 800 °C. The implications of such observations for the engineering application of these materials are addressed briefly.

Fatigue crack growth at arbitrary angles to bimaterial interfaces

Abstract: The fracture characteristics of monotonically loaded cracks located parallel to, in the vicinity of, and at arbitrary angles to the interface between two elastic solids have the subjects of considerable theoretical research in the literature. The present experimental work on steel-steel bimaterials clearly demonstrates that, for essentially the same elastic properties, the plastic mismatch between the constituent layers of bilayer composite can dictate whether a fatigue crack approaching the interface at various angles continues to advance through the interface or arrests prior to penetrating the interface. The former crack arrest phenomenon occurs when the fatigue crack approaches the interface from the plastically weaker material, whereas continued crack growth results when the crack approaches the interface from the plastically stronger material. The observed experimental trends are qualitatively consistent with the numerical predictions of crack-tip shielding or amplification from the interaction of the near-tip plastic zone with an interface oriented normal to the plane of the crack. When the fatigue crack is initiated parallel to the interface, it deviates from the strongly bonded interface to advance in one of the two steels. This trend appears to experimentally confirm predictions of the possible evolution of strong mode mixity ahead of interface cracks subjectedmore » to cyclic tensile loading.« less

Corrosion Fatigue of AZ91E-T6 Cast Magnesium Alloy in a 3.5 Percent NaCl Aqueous Environment

Abstract: The objective of this research was to obtain and compare constant and variable amplitude fatigue behavior of AZ91E-T6 cast magnesium alloy in both an air and 3.5% NaCl aqueous corrosive environment. An additional objective was to determine if commonly used models that describe fatigue behavior and fatigue life are applicable to this material and test environment. Fatigue tests included constant amplitude strain-controlled low cycle fatigue with strain ratio, R, equal to 0, {minus}1 and {minus}2, Region 2 constant amplitude fatigue crack growth with load ratio, R, equal to 0.05 and 0.5 and variable amplitude fatigue tests using keyhole notched specimens. In all fatigue tests, the corrosion environment was significantly detrimental relative to the air environment. Mean strains influenced fatigue life only if accompanied by significant mean stress. The Morrow and Smith, Watson, and Topper mean stress models provided both accurate and inaccurate fatigue life calculations. Likewise, variable amplitude fatigue life calculations using the local strain approach and based upon the formation of a 1 mm crack at the keyhole notch were both accurate and fairly inaccurate depending on the specific model used.

Effects of microstructure and temperature on fatigue crack growth in the TiAl alloy Ti-46.5Al-3Nb-2Cr-0.2W

Abstract: The fatigue crack growth behavior of forged Ti-46.5Al-3Nb-2Cr-0.2W (at.%) was investigated. Heat treatments were used to generate both a nearly fully lamellar microstructure (grains of α 2 - γ lamellae) and a duplex microstructure (equiaxed γ and lamellar grains) to span the wide range of microstructural conditions available through thermomechanical processing of these alloys. Fatigue crack growth tests using load-shedding threshold and constant-load-amplitude techniques were conducted at room temperature, 600 °C (below the ductile-to-brittle transition temperature (DBTT)) and 800 °C (above the DBTT). Results show that the fatigue crack growth resistance of the lamellar microstructure is superior to that of the duplex microstructure. The nature of fatigue crack advance depends strongly on microstructure, which explains, at least in part, the differences observed in crack growth rates for the lamellar and duplex microstructures. Fractography was conducted to identify the dominant crack growth mechanisms in both the lamellar and duplex microstructures.

Analysis of fatigue crack growth in rotary bend specimens and railway axles

Abstract: Fatigue crack growth tests have been carried out on pre-cracked specimens of steel AFNOR XC 38 under rotary bending conditions. The semi-elliptical surface crack shape evolution was determined and the results were analyzed by da/dN = f(ΔK) using a stress intensity factor solution for semi-elliptical surface cracks in round bars subjected to a bending stress modified to take into account the rotatary bending conditions. Results show a very good correlation with fatigue crack growth data obtained in standard centre cracked tension specimens of the same steel. An application to the sensitivity of design parameters to an analysis of cracked railway axles is proposed, allowing an improvement in maintenance procedures.

Damage tolerance engineering property evaluations of aerospace aluminium alloys with emphasis on fatigue crack growth

Abstract: This report presents, discusses and evaluates the results of many NI& investigations into the damage tolerance engineering properties of aerospace aluminium alloys. The investigations cover a period of more than twenty years and represent a considerable source of data. The report is in four parts. Part I comprises chapters 1 and 2, which give an introduction to the significance of aluminium alloys for the aerospace industry and the scope of the report. Parts I1 and I11 contain chapters 3-21, which form the main body of the report. Part IV comprises chapters 22 and 23. These provide general conclusions based on the results in Parts I1 and 111, and also recommendations for further work. In Part 11, containing chapters 3-15, the engineering properties of new and conventional aluminium alloys are compared, discussed and evaluated. This work commenced in 1969 and ran through to 1990. In Part 111, containing chapters 16-21, several topics on fatigue crack growth are investigated: ” short cracks long crack growth at low stress intensities fatigue durability entirely in terms of crack growth. flight simulation fatigue crack growth and correlation of the data by characteristic stress intensity factors

Crack propagation studies to determine benign or catastrophic failure modes for aerospace thin-rim gears

Abstract: : Analytical and experimental studies were performed to investigate the effect of rim thickness on gear tooth crack propagation. The goal was to determine whether cracks grew through gear teeth (benign failure mode) or through gear rims (catastrophic failure mode) for various rim thicknesses. Gear tooth crack propagation was simulated using a finite element based computer program. Principles of linear elastic fracture mechanics were used. Quarter-point, triangular elements were used at the crack tip to represent the stress singularity. Crack tip stress intensity factors were estimated and used to determine crack propagation direction and fatigue crack growth rate. The computer program used had an automated crack propagation option in which cracks were grown numerically using an automated re-meshmg scheme. In addition, experimental studies were performed in the NASA Lewis Spur Gear Fatigue Rig. Gears with various backup ratios were tested to validate crack path predictions. Also, specialized crack propagation gages were installed on the test gears to measure gear tooth crack growth rate. From both predictions and tests, gears with backup ratios (rim thickness divided by tooth height) of 3.3 and 1.0 produced tooth fractures while a backup ratio of 0.3 produced rim fractures. For a backup ratio of 0.5, the experiments produced rim fractures and the predictions produced both rim and tooth fractures, depending on the initial crack conditions. Good correlation between the predicted number of crack propagation cycles and measured number of cycles was achieved using both the Paris fatigue crack growth method and the Collipriest crack growth equation when fatigue crack closure was considered.

Fatigue crack growth and closure at high stress ratios

Abstract: — Fatigue crack growth tests have been carried out on a medium carbon structural steel over a wide range of stress ratios, i.e. from 0 to 0.7. All tests were conducted under constant amplitude loading conditions corresponding to growth rates in the Paris regime. Crack closure behaviour was observed experimentally by a surface strain gauge technique, and numerically by a finite element analysis under plane stress condition. While the crack closure could not be detected by experimental measurements at stress ratios equal to or greater than 0.5, the numerical results showed that closure occurred even at high stress ratios up to 0.7. The differences between experimentally and numerically determined crack opening levels were found for each stress ratio. A cause for these differences is discussed. In addition, two new types of crack tip parameters which have been proposed recently are evaluated by finite element analysis and their relevance to fatigue crack growth are discussed. It is concluded that fatigue crack growth rates are substantially determined by the effective stress intensity factor range which is based on the crack closure concept.

Weight function for a single edge cracked geometry with clamped ends

Abstract: A single edge cracked geometry with clamped ends is well suited for fracture toughness and fatigue crack growth testing of composites and thin materials. Analysis of fiber bridging phenomenon in the composites and determination of stress intensity factors due to non-uniform stress distributions such as residual and thermal stresses generally require the use of a weight function. This paper describes the development and verification of a weight function for the single edge cracked geometry with clamped ends. Finite element analyses were conducted to determine the stress intensity factors (K) and crack opening displacements (COD) due to different types of stress distributions. The weight function was developed using the K and COD solution for a constant stress distribution. K and COD predicted using this weight function correlated well with the finite element results for non-uniform crack surface stress distributions.