Visual simulation of fatigue crack growth

An attempt has been made to visually simulate fatigue crack propagation from a precrack. An integrated program was developed for this purpose. The crack-tip shape was determined at four load positions in the first load cycle. The final shape was a blunt front with an “ear” profile at the precrack tip. A more general model, schematically illustrating the mechanism of fatigue crack growth and striation formation in a ductile material, was proposed based on this simulation. According to the present model, fatigue crack growth is an intermittent process; cyclic plastic shear strain is the driving force applied to both state I and II crack growth. No fracture mode transition occurs between the two stages in the present study. The crack growth direction alternates, moving up and down successively, producing fatigue striations. A brief examination has been made of the crack growth path in a ductile two-phase material.     

The thermal fatigue behavior of the combustor alloys In 617 and HAYNES 230 before and after welding

The nickel-base alloys IN 617 and HAYNES 230 for welded high-temperature components have been subjected to thermal fatigue (TF) loading. In a series of TF tests in air, single wedge specimens were induction heated and compressed-air cooled at the leading edge for various temperature cycles between 200 °C and either 850 °C, 950 °C, or 1050 °C. The test rigs permitted simultaneous measurements of temperature and total strain along the edge of specimen during TF cycling. Both materials have been tested in conditions relevant for hot path components in the gas turbines, e.g., “as delivered,” “welded,” and “welded + notched”. Under identical temperature cycles and thermal gradients, HAYNES 230 showed a higher TF strength than IN 617 in the as-delivered condition. It is suggested that this advantage of HAYNES 230 is primarily related to its lower value of the relevant combination of properties of this alloy: coefficient of thermal expansion, thermal conductivity, elastic modulus, ultimate tensile strength, taken at maximal operating temperature. In addition, the advantage of the HAYNES 230 is described by a lower plastic strain, which is induced at the wedge region during TF loading. Moreover, microstructural details of crack initiation, crack propagation, and reactions with the gaseous environment play an important role. Both alloys investigated in the present work showed plastic deformation with a maximum in the central zone of the wedge tip. In this zone, slip bands and grain distortion occurred, whereas both ends of the wedge tip free of visible plastic deformation. The TF cycles led to multiple transgranular crack initiation and propagation. In welded specimens of IN 617 and HAYNES 230, cracks appeared first in the center of the weld. The susceptibility of welds to TF cracking depends considerably on the weld filler and the surface quality. It was shown for HAYNES 230 that a mismatched weld could reduce the TF life to less than 50 pct of non-welded specimens. The lower TF-fatigue strength of the welded specimens can be explained by the difficulty of the cast alloy in the welded zone to accommodate the repeated thermal shocks by plastic deformation. Notches introduced in the heat-affected zone (depth about 0.1 mm) reduced the TF life of both alloys by a factor as high as 4. The thermal fatigue strength of the welded material can almost reach the values of the base alloy provided the use of matching electrodes, post-weld heat treatment, and grinding off the weld beads is carefully executed.     

An Evaluation of the summation of cyclic plastic strain damage in two high strength aluminum alloys

A damage equation based upon the integration of low cycle fatigue plastic strain ranges was verified experimentally for two high strength aluminum alloys 2024-T4 and 7075-T651. The damage equation which has been used extensively for many fatigue crack propagation theories assumes cyclic damage under increasing plastic strain ranges. In order to verify the damage equation, low cycle fatigue specimens were subjected to a fully reversed strain cycle in which the total strain-range was increased linearly by a constant amount Δ[Δε_d] per cycle. An excellent agreement was obtained between the predicted and observed fatigue lifetimes. The stress-strain response of these alloys was also measured. The experimental results showed that these two alloys cyclically harden substantially and that the single strain increment stress-strain curve is a fair lower bound approximation of the cyclic stress-strain curve.     

Prediction of fatigue crack formation in 304 stainless steel

Strain-controlled fatigue tests have been conducted on center-holed 304 stainless steel specimens. The fraction of total fatigue life spent until formation of an “engineering” crack ranged from about 15 to 85 pct, indicating the potential importance of being able to predict the fatigue crack formation life. A “just formed engineering crack,” as defined here, is a through crack long in the thickness direction, which has just emerged from the center hole. An energy based parameter, ΔσrΔε,, has been shown to correlate with the appearance of fatigue cracks in the center-holed 304 stainless steel specimens. This parameter is suggested to be more useful in predicting fatigue crack formation life than Δσ or Δε, alone. A good correlation was found over the limited range of data for two types of 304 stainless steel, a powder metallurgy (PM) stainless steel with higher than normal strength prop-erties and an ingot metallurgy (IM) stainless steel with normal strength properties. A better correlation was found for strain-controlled fatigue tests which did not go into compressive strain than for com-pletely reversed fatigue. Formerly a graduate student with the Materials Science and Engineering Department, Northwestern University, is     

Static and Cyclic Crack Growth Behavior of Ultrafine-Grained Al Produced by Different Severe Plastic Deformation Methods

Fracture-mechanics experiments were carried out on ultrafine-grained (UFG) samples of aluminum and two Al alloys to obtain the fracture behavior under static and cyclic loading. The UFG materials investigated show crack resistance behavior under static loading, which was confirmed by ductile fracture surfaces. Under cyclic load, the crack growth rate was described well by the ESACRACK model. The crack propagation results show no influence of the type of the severe plastic deformation method in the Paris region but more effect in the threshold region. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application,” which occurred September 25–27, 2006 in Kloster Irsee, Germany.     

High-temperature low-cycle fatigue behavior of K40S cobalt-base superalloy

An investigation was made on the strain-controlled low-cycle fatigue (LCF) of K40S cobalt-base superalloy at 900 °C in ambient atmosphere. The results show that K40S alloy possesses high LCF resistance in comparison with X-40 alloy. Under the testing conditions in this study, K40S alloy exhibits a cyclic stress response of initial hardening followed by softening. The cyclic stress response behavior has been attributed to dislocation-dislocation interactions and dislocation-precipitate interactions. The high response stress can lead to a large stress concentration at locations where inelastic strains of high amplitude accumulate, which account for the decreasing fatigue life with increasing strain rate. The well-distributed carbide particles are the “secondary” crack initiation sites. The secondary crack initiation relaxes the stress concentration at the crack tip, reducing the driving force of crack propagation. High-temperature LCF failure of K40S alloy results from the interaction of the mechanical fatigue and environmental oxidation.     

Additional evidence for the plastic blunting process of fatigue crack propagation

Current workers who deal with fatigue crack propagation mechanisms express the view that fracture surface striations are formed at the start of a tensile stroke instead of the end of a compressive stroke, as required by the older mechanism of the “plastic blunting process” (PBP). The current views are criticized and new fractographic evidence is presented in support of the PBP mechanism. The effects of a nonzero mean stress and of environment are also discussed. Variable amplitude tests are shown to be useful in dealing with questions about crack propagation kinetics.     

The effect of matrix microstructure on cyclic response and fatigue behavior of particle-reinforced 2219 aluminum: Part II. Behavior at 150 °C

The 150 °C cyclic response of peak-aged and overaged 2219/TiC/15p and 2219 Al was examined using fully reversed plastic strain-controlled testing. The cyclic response of peak-aged and overaged particle-reinforced materials showed extensive cyclic softening. This softening began at the commencement of cycling and continued until failure. At a plastic strain below 5 × 10³, the unreinforced materials did not show evidence of cyclic softening until approximately 30 pct of the life was consumed. In addition, the degree of cyclic softening (†σ) was significantly lower in the unreinforced microstructures. The cyclic softening in both reinforced and unreinforced materials was attributed to the decomposition of the θ′ strengthening precipitates. The extent of the precipitate decomposition was much greater in the composite materials due to the increased levels of local plastic strain in the matrix caused by constrained deformation near the TiC particles. formerly Research Assistant with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109 This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994 in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

Low-Cycle Fatigue Behavior of an As-Extruded AM50 Magnesium Alloy

The low-cycle fatigue behavior of an as-extruded AM50 magnesium alloy has been investigated. The cyclic stress response of the alloy strongly depends on the imposed strain amplitude. It is also noted that at the higher total strain amplitudes, the alloy exhibits a pronounced anisotropic deformation behavior in the direction of tension and compression, where the width of the σ-ε hysteresis loop in the compressive direction is greater than that in the tensile direction. At the total strain amplitude of 1.5 pct, a serrated flow can be observed in both tensile and compressive directions of the σ-ε hysteresis loop. This means that dynamic strain aging takes place during fatigue deformation. The relation between elastic and plastic strain amplitudes with reversals to failure shows a monotonic linear behavior and can be well described by the Basquin and Coffin–Manson equations, respectively. In addition, crack initiation and propagation modes are suggested, based on scanning electron microscopy observations on the fracture surfaces of fatigued specimens. This article is based on a presentation given in the symposium entitled “Deformation and Fracture from Nano to Macro: A Symposium Honoring W.W. Gerberich’s 70th Birthday,” which occurred during the TMS Annual Meeting, March 12–16, 2006, in San Antonio, Texas, and was sponsored by the Mechanical Behavior of Materials and Nanomechanical Behavior Committees of TMS.     

Effect of dislocation substructures on fatigue fracture

The effect of dislocation substructures on fatigue is explored within the following categories: a) the dislocation substructures produced by cycling in annealed material and the effect of cycling on substructures introduced extraneously before cycling; b) hight and low strain fatigue fracture, expressed in terms of Coffin-manson or stress-life plots; and c) the effect of substructure on the mechanisms of crack nucleation and propagation. Usually, cycling is very effective in rearranging substructures introduced extraneously, especially at high strains. Consequently, there is little effect of the substructures on fatigue fracture except under some circumstances at low strains, and also if the agent of introducing the initial substructures has had an effect on the fracture process, for example, by opening cracks at inclusions. Dislocation substructure, however formed, is not significant,per se, to fatigue fracture except in acting as a source of plastic deformation. This paper is based on a presentation made at a symposium on “Mechanical-Thermal Processing and Dislocation Substructure Strengthening,” held at the Annual Meeting in Las Vegas, Nevada, on February 23, 1976, under the sponsorship of the TMS/IMD Heat Treating Committee.     

The Effect of ITMT’s and P/M Processing on the Microstructure and Mechanical Properties of the X7091 Alloy

The influence of microstructure and texture on the monotonic and cyclic properties of X7091-T651 was investigated. The various structures were developed from conventional ingot metallurgy (I/M), powder metallurgy (P/M) and intermediate thermal mechanical treatments (ITMT). Powder metallurgy produced a finer grain structure and particle distribution than I/M. Intermediate thermomechanical treatment produced a recrystallized, coarse grain structure with a weak texture, compared to the unrecrystallized grain structure and sharp texture obtained with conventional processing (CP). All materials had comparable monotonic properties. The resistance to fatigue crack initiation (FCI) increased with both a reduction in grain size and a finer particle distribution. Smaller grain sizes and finer particle distributions reduced the degree of cyclic strain localization. The CP-P/M alloy had the poorest ductility and FCI resistance of all the materials, although the slip was fairly homogeneous. This may be due to the presence of oxides at the grain boundaries and a sharp texture. The threshold stress intensity, ΔK_th, and the fatigue crack growth rate (FCGR) roughly follow a grain size dependence with the resistance of fatigue crack propagation (FCP) increasing with increasing grain size. It appears that large grains allow more reversible slip and reduce the amount of accumulated plastic strain within the reverse plastic zone. It is also believed that a greater degree of fatigue crack closure, which may be associated with large grains and a rough FCP surface, results in a lower FCGR in the lowΔK region. The intermediate thermomechanical treatment of P/M X7091 produced the optimum microstructure giving the best combination of mechanical properties. The important features include a small recrystallized grain structure, a fine particle distribution, a weak texture, and a low concentration of oxides at grain boundaries. Formerly Director, Fracture and Fatigue Research Laboratory, Georgia Institute of Technology, Atlanta, GA.     

The quasi-static and cyclic fatigue fracture behavior of 2014 aluminum alloy metal-matrix composites

In this article, the quasi-static and cyclic fatigue fracture behavior of aluminum alloy 2014 discontinuously reinforced with fine particulates of aluminum oxide are presented and discussed. The discontinuous particulate-reinforced 2014 aluminum alloy was cyclically deformed under fully reversed, tension-compression loading over a range of strain amplitudes, well within the plastic domain of the engineering stress-strain curve, resulting in cyclic fatigue lives of less than 10⁴ cycles. The influence of both ambient and elevated temperatures on cyclic stress and cyclic stress-strain response is highlighted. The underlying mechanisms governing the fracture mode during quasi-static and cyclic fatigue are discussed and rationalized in light of the concurrent and mutually interactive influences of intrinsic composite microstructural features, deformation characteristics of the metal matrix and reinforcement particulate, cyclic strain amplitude and resultant fatigue life, and test temperature. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

The quasi-static and cyclic fatigue fracture behavior of 2014 aluminum alloy metal-matrix composites

In this article, the quasi-static and cyclic fatigue fracture behavior of aluminum alloy 2014 discontinuously reinforced with fine particulates of aluminum oxide are presented and discussed. The discontinuous particulate-reinforced 2014 aluminum alloy was cyclically deformed under fully reversed, tension-compression loading over a range of strain amplitudes, well within the plastic domain of the engineering stress-strain curve, resulting in cyclic fatigue lives of less than 10⁴ cycles. The influence of both ambient and elevated temperatures on cyclic stress and cyclic stress-strain response is highlighted. The underlying mechanisms governing the fracture mode during quasi-static and cyclic fatigue are discussed and rationalized in light of the concurrent and mutually interactive influences of intrinsic composite microstructural features, deformation characteristics of the metal matrix and reinforcement particulate, cyclic strain amplitude and resultant fatigue life, and test temperature. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Facture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

The effect of matrix microstructure on cyclic response and fatigue behavior of particle- reinforced 2219 aluminum: Part I. room temperature behavior

The low-cycle and high-cycle fatigue behavior and cyclic response of naturally aged and overaged 2219/TiC/15p and unreinforced 2219 Al were investigated using plastic strain-controlled and stress-controlled testing. In addition, the influence of grain size on the particle-reinforced materials was examined. In both reinforced and unreinforced materials, the naturally aged conditions were cyclically unstable, exhibiting an initial hardening behavior followed by an extended region of cyclic stability and ultimately a softening region. The overaged reinforced material was cyclically stable for the plastic strains examined, while the overaged unreinforced material exhibited cyclic hardening at plastic strains greater than 2.5 × 10⁻⁴. Decreasing grain size of particle-reinforced materials modestly increased the cyclic flow stress of both naturally aged and overaged materials. Reinforced and unreinforced materials exhibited similar fatigue life behaviors; however, the reinforced and unreinforced naturally aged materials had superior fatigue lives in comparison to the overaged materials. Grain size had no effect on the fatigue life behavior of the particle-reinforced materials. The fatigue lives were strongly influenced by the presence of clusters of TiC particles and exogenous Al₃Ti intermetallics. formerly Research Assistant with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109 This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994 in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

Hydrogen-involved tensile and cyclic deformation behavior of low-alloy pressure vessel steel

The temperature- and strain-rate-dependent tensile behavior of hydrogen-charged low-alloy pressure vessel steel ASTM A508 C1.3 has been investigated. The fatigue crack initiation and propagation behavior of the steel in high-temperature water environments has also been evaluated. It was found that hydrogen played significant roles in both tensile and cyclic deformation processes, especially in the temperature and strain-rate region of dynamic strain aging (DSA). The presence of hydrogen resulted in a distinct softening in tensile strength and a certain loss in tensile ductility in the DSA region. Remarkable degradation in fatigue crack initiation and propagation resistance in high-temperature water environments was observed in the DSA strain-rate region. Typical hydrogen-induced cracking features also appeared on the corresponding fatigue fracture surfaces. The interactions between hydrogen and DSA in tensile and cyclic deformation processes are discussed as well as their combined effects on the environmentally assisted cracking (EAC) mechanism of pressure vessel steels in high-temperature water environments.     

Fatigue crack propagation in 99.99+ and 1100 aluminum at 298 and 77 k

The fatigue crack propagation rate,dc/dN, in cold-rolled and annealed 99.99+ Al is about 80 times slower at 77 K than at 298 K. In annealed 1100 Al which contains constituent particles,dc/dN decreases by a factor of 20 on cooling from 298 to 77 K. At 298 and 77 K, annealed 99.99⁺ Al and 1100 Al cyclically harden but the amount is greater at 77 K. Cold-rolled 99.99⁺ Al cyclically hardens at 77 K but cyclically softens at 298 K. The much slower fatigue crack propagation rate at 77 K in aluminum is attributed in part to the increase in cyclic yield stress, σ_y′, on cooling. At 77 K the high rate of work hardening at large strains is also thought to result in high plastic work per unit area of fatigue crack thereby reducing the fatigue crack propagation rate. Rice’s theory for a Mode I plane stress crack predicts the measured plastic zone size if the local stress corresponding to zero plastic strain in the cyclic stress-strain curve is employed in the formula.     

High-cycle fatigue of nickel-based superalloy ME3 at ambient and elevated temperatures: Role of grain-boundary engineering

High-cycle fatigue (HCF), involving the premature initiation and/or rapid propagation of cracks to failure due to high-frequency cyclic loading, remains a principal cause of failures in gas-turbine propulsion systems. In this work, we explore the feasibility of using “grain-boundary engineering” as a means to enhance the microstructural resistance to HCF. Specifically, sequential thermomechanical processing, involving alternate cycles of strain and annealing, was used to increase the fraction of “special” grain boundaries and to break up the interconnected network of “random” boundaries, in a commercial polycrystalline Ni-based superalloy (ME3). The effect of such grain-boundary engineering on the fatigue-crack-propagation behavior of large (∼8 to 20 mm), through-thickness cracks at 25 °C, 700 °C, and 800 °C was examined. Although there was little influence of an increased special boundary fraction at ambient temperatures, the resistance to near-threshold crack growth was definitively improved at elevated temperatures, with fatigue threshold stress intensities some 10 to 20 pct higher than at 25 °C, concomitant with a lower proportion (∼20 pct) of intergranular cracking.     

The effect of high vacuum on the low cycle fatigue law

Push-pull fatigue tests have been conducted on several materials at various frequencies and temperatures in air and high vacuum (10⁻⁸ torr) and the fatigue life determined in terms of the cyclic plastic strain. In contrast to a changing exponent of the Coffin-Manson law with increasing temperature in air, in high vacuum this exponent is found to remain nearly constant at a value of about 0.5. Further, the temperature sensitivity of this exponent and of life at a specific plastic strain range in high vacuum is slight. Pronounced plastic instability (specimen shortening and fattening) was observed for the ductile metals investigated and crack nucleation was retarded. In all cases crack propagation was transgranular in vacuum. It is concluded that for the materials, temperature, and frequencies investigated, the degradation of fatigue life at elevated temperature is due to environmental enhancement of intergranular fracture. Materials investigated include A286 at room temperature and 593°C, nickel A at 550°C, 304 stainless steel at 816°C and 7075T6 aluminum alloy.     

Cyclic deformation of ferritic steel—II. Stage II crack propagation

The crack propagation process is dominated by the formation of ductile fatigue striations and the classical “one load cycle per striation” concept is only valid in a narrow crack propagation rate interval. However, over 4 orders of magnitudes in crack propagation rates the striation rates is independent of the ΔK, which implies that the number of cycles necessary to form one striation probably is greater than one. The similarities between the fatigue damage processes in the cyclic plastic zone and in plastic strain controlled specimens are documented in detail. The micromechanisms of crack propagation is related to a local plastic collapse of the dislocation substructure at the crack tip. A model for fatigue crack propagation rate predictions has been developed. The model is a refinement of the existing accumulated damage, LCF-models and the most important parameters taken into account are: the constant striation spacing, a realistic dynamic cyclic yield stress, the Coffin-Manson constants, a threshold plastic strain and a constant which coincide with the average grain size.