Fatigue Crack Growth Mechanisms Research Articles

Mechanisms of fatigue crack growth – a critical digest of theoretical developments

ABSTRACTRecent advances in the processing technology are permitting the manufacture of novel metallic materials with superior fatigue properties via microstructure tailoring. In the light of these promising developments, there is a rising need for establishing a synergy between state‐of‐the‐art experimental characterizations and physically based theoretical underpinnings. A revisit to the existing predictive literature is thus a timely requirement prior to furthering new design guidelines against cyclic damage. To that end, this paper recounts an overview of the key mechanistic and analytical theories on the fatigue crack growth mechanisms. Emphasis is placed on categorizing the proposed modelling endeavours based on their fundamental principles. In doing so, contributions and limitations thereof are carefully examined on the basis of most updated experimental revelations. The objective is to provide a perspective to the current generation of engineers and researchers alike. This concise yet critical narrative would essentially assist in formulating even more advanced microstructure–damage relationships in the modern context. A commentary is added at the end outlining the promising avenues for future research.

Evaluation of Fatigue Crack Growth Performance in different Hardmetal Grades based on Finite Element Simulation

Hardmetals (WC-Co) are a group of composite materials exhibiting outstanding combinations of hardness and toughness. Therefore, they are extensively used for highly demanding applications, such as cutting and drilling tools, where cyclic loading is one of the most critical service conditions.The micromechanics of fracture in hardmetals under static loads is well investigated and understood. Studies regarding failure by fatigue on the other hand, is mainly limited to experimental investigations conducted at a component scale and seldom refer to the influence of microstructure on the failure mechanism. Moreover, numerical studies evaluating the mechanisms of fatigue crack growth in hardmetals are also scarce.Experimental observations indicate that, the overall fatigue performance of hardmetals can be predicted from the early stages of the microcrack evolution. Taking this into consideration, a numerical methodology for evaluating the fatigue crack propagation in hardmetals was developed. In this respect, previously a model based on a continuum damage mechanics approach together with an element elimination method was implemented in a commercial finite element software for simulating the crack propagation in hardmetals. In the current study, the model is further extended to artificially generated hardmetal structures in order to simulate and evaluate the overall fatigue crack growth performance of different hardmetal grades.Fatigue crack growth rate diagrams based on the simulations were plotted for different hardmetal grades and the results showed good agreement in comparison to experimental observations. Such an approach is helpful for designing hardmetals at a microstructural scale without going through extensive experimental work.

Fatigue crack initiation and growth on an extruded titanium alloy in gigacycle regime: comparison between tension and torsion loadings

This paper is focused on the analysis of fatigue crack initiation and growth mechanisms in defect free VT3-1 titanium alloy (similar to Ti6Al4V) in VHCF regime under tensile and torsion loadings. Fully reversed fatigue tests were carried out between 107 and 109 cycles at 20 kHz under constant amplitude loadings (no pulse-pause). SEM observations of the specimens fracture surfaces were carried out in order to compare the crack initiation mechanisms and the different crack growth stages under different loadings. It has been shown that subsurface crack initiation may appear under tension (as usual in gigacycle regime) but in our experiments no inclusion was observed in the “fish-eye”. Furthermore, subsurface crack initiation was observed under torsion loading too, despite the maximum shear stress location at the specimen surface. Under this loading, a “fish-eye” was observed too, but again without any inclusion in its center. The suspected microstructural reasons responsible for subsurface crack initiations under torsion loading could not be observed due to significant destruction of the fracture patterns (crack lips friction under mixed mode). The results of the fatigue tests show a principal difference in crack initiation and early crack growth stage between tensile and torsion loadings. Torsion crack initiates on a plane of maximum shear stress (like in HCF regime) while tensile crack is on a plane of maximum normal stress. Sequences of changes in fracture surface roughness is the same for tensile and torsion loadings.

Controlling fatigue crack paths for crack surface marking and growth investigations

While it is well known that fatigue crack growth in metals that display confined slip, such as high strength aluminium alloys, develop crack paths that are responsive to the loading direction and the local microstructural orientation, it is less well known that such paths are also responsive to the loading history. In these materials, certain loading sequences can produce highly directional slip bands ahead of the crack tip and by adjusting the sequence of loads, distinct fracture surface features or progression marks, even at very small crack depths can result. Investigating the path a crack selects in fatigue testing when particular combinations of constant and variable amplitude load sequences are applied is providing insight into crack growth. Further, it is possible to design load sequences that allow very small amounts of crack growth to be measured, at very small crack sizes, well below the conventional crack growth threshold in the aluminium alloy discussed here. This paper reports on observations of the crack path phenomenon and a novel test loading method for measuring crack growth rates for very small crack depths in aluminium alloy 7050-T7451 (an important aircraft primary structural material). The aim of this work was to firstly generate short- crack constant amplitude growth data and secondly, through the careful manipulation of the applied loading, to achieve a greater understanding of the mechanisms of fatigue crack growth in the material being investigated. A particular focus of this work is the identification of the possible sources of crack growth retardation and closure in these small cracks. Interpreting these results suggests a possible mechanism for why small fatigue crack growth through this material under variable amplitude loading is faster than predicted from models based on constant amplitude data alone.

Fatigue crack growth behavior of Inconel 718 produced by selective laser melting

Additive layer manufacturing has recently gained a lot of interest due to the feasibility of producing metallic components directly from a computer-aided design file of the part. Selective laser melting, one of the main additive layer manufacturing technologies, is currently capable of producing nearly ready-to-use parts made of metallic materials. Their microstructure, however, differs substantially from that produced by conventional manufacturing. That is why a detailed study and knowledge of the relation of specific microstructure, parameters of the selective laser melting process and mechanical properties is of utmost significance. This study reports on the investigation of the fatigue crack growth behavior in Inconel 718 superalloy produced by selective laser melting. The fatigue crack growth curve and the threshold values of the stress intensity factor for propagation of long cracks were experimentally determined on compact-tension specimens fabricated using a RENISHAW A250 system and the recommended processing parameters. The fatigue crack growth rates and the fatigue crack paths both in the threshold and in the Paris region were investigated. The crack propagation curves and the crack propagation threshold were compared with literature data describing the behavior of conventionally manufactured material. The mechanism of fatigue crack growth was discussed in terms of the specific microstructure produced by selective laser melting.

Small fatigue crack initiation and growth mechanisms of nickel-based superalloy GH4169 at 650 °C in air

The aim of this paper is to identify the fatigue crack initiation and small crack growth mechanisms of GH4169 at 650°C in air. Results show that fatigue cracks tend to initiate at oxidized inclusions. The crack incubation period takes up more than 90% of the fatigue lifetime. The δ phase can improve the crack initiation life and the total fatigue lifetime. The similarity between small crack and long crack growth data at relatively high ΔK can be attributed to the similarity of propagation mode. Two three-stage models are proposed to describe the cracks initiation and propagation processes.

Effect of thermal aging on the fatigue crack growth behavior of cast duplex stainless steels

The effect of thermal aging on the fatigue crack growth (FCG) behavior of Z3CN20?09M cast duplex stainless steel with low ferrite content was investigated in this study. The crack surfaces and crack growth paths were analyzed to clarify the FCG mechanisms. The microstructure and micromechanical properties before and after thermal aging were also studied. Spinodal decomposition in the aged ferrite phase led to an increase in the hardness and a decrease in the plastic deformation capacity, whereas the hardness and plastic deformation capacity of the austenite phase were almost unchanged after thermal aging. The aged material exhibited a better FCG resistance than the unaged material in the near-threshold regime because of the increased roughness-induced crack closure associated with the tortuous crack path and rougher fracture surface; however, the tendency was reversed in the Paris regime because of the cleavage fracture in the aged ferrite phases.

Experimental investigation of fatigue crack growth behavior of GH2036 under combined high and low cycle fatigue

Fatigue crack growth rates have been experimentally determined for the superalloy GH2036 (in Chinese series) at an elevated temperature of 550°C under pure low cycle fatigue (LCF) and combined high and low cycle fatigue (CCF) loading conditions by establishing a CCF test rig and using corner-notched specimens. These studies reveal decelerated crack growth rates under CCF loading compared to pure LCF loading, and crack propagation accelerates as the dwell time prolongs. Then the mechanism of fatigue crack growth at different loadings has been discussed by using scanning electron microscope (SEM) analyses of the fracture surface.

Effect of cyclic frequency on fracture mode transitions during corrosion fatigue cracking of an Al-Zn-Mg-Cu alloy

Abstract There are complex issues remaining to be resolved before environment-assisted cracking models can be included in structural mechanics programs that are currently used to analyze mechanical fatigue crack growth. Considered here is the effect of cyclic frequency on fracture mode transitions that occur during corrosion fatigue of high-strength Al-Zn-Mg-Cu alloys. These alloys are used in civilian and military aircraft applications where they are exposed to detrimental aqueous saline environments. A previously developed “critical hydrogen at a critical distance” crack growth model is used to rationalize the observed transitions in crack-path fracture-modes, from intergranular (IG), to brittle transgranular (BTG), to ductile transgranular (DTG), as the alternating stress intensity factor and cyclic frequency increase beyond critical values. Corrosion fatigue crack growth rate data obtained on Al-Zn-Mg-Cu alloy 7017-T651 base metal and heat-affected “white zone” metal tested in aqueous NaCl solutions over a frequency range of 0.01–70 Hz are analyzed. For the white zone metal, dependence of the critical crack velocity on the critical frequency, at which the IG-BTG transition occurs, undergoes an abrupt reversal as the critical frequency increases above about 0.1 Hz. Mechanisms potentially responsible for this change in frequency dependency are discussed in the context of the critical hydrogen model. The transition from low to intermediate frequency behavior is speculated to be due to a change in the critical distance from microstructural control, for frequencies at or below 0.1 Hz, to control by the critical hydrogen criterion at higher frequencies. The low frequency behavior is discussed relative to the transition from static load stress corrosion cracking to low frequency corrosion fatigue, which occurs as cyclic frequency increases above zero.

Grain size effect on multi-scale fatigue crack growth mechanism of Nickel-based alloy GH4169

The mechanism of multi-scale fatigue crack propagation of Nickel-bases alloy GH4169 with different grain sizes was identified in this paper by using the specimens with micro-notches. Results showed that specimen with coarser grain exhibited longer fatigue life. Grain size effect on the growth rate of small crack cannot be distinguished due to the fluctuation of crack growth rate. The small fatigue crack stage accounted for a larger fraction of the total fatigue life for the specimen with finer grain size. An empirical formula was proposed to predict the relationship between the growth rate of small crack and crack length.

A physically based universal functional to characterize the mechanism of fatigue crack growth in materials

A unique relationship between the normalized remaining section size and the normalized remaining fatigue life has been discovered to exist during fatigue crack growth in a wide spectrum of materials. A crack growth functional that relates the two variables, and which is in excellent agreement with experimental data, is proposed. This functional suggests that all fatigue cracks propagate uniquely to sever the material cross-section and is promising as a new physical basis for characterizing fatigue crack growth mechanisms in materials.

Low Cycle Fatigue Behavior of Several Typical Powder Metallurgy Superalloys

Powder metallurgy superalloy has become the preferred material of aircraft engine turbine disc and turbine damper due to advantages of uniform structure, fine grain size, high yield strength, high fatigue strength, etc. The low cycle fatigue behavior of several typical powder metallurgy superalloys at different holding time was studied in this work. The analysis of the microstructure, curves of Lga-LgNi/Nf, da/dN-a and da/dN-N indicated the ratio of crack preparation, initiation, propagation and final fracture in whole process fatigue failure. The mechanism of fatigue crack growth was investigated by comparing the chemical composition, microstructure, fracture morphology of different alloys.

Low-cycle fatigue of a VZh175 high-temperature alloy under elastoplastic deformation conditions

The low-cycle fatigue of a VZh175 nickel superalloy is studied under conditions of complete deformation per loading cycle at an initial cycle asymmetry R = 0, a deformation amplitude ɛa = 0.4–0.6%, and a temperature of 20 and 650°C. The specific features of cyclic hardening/softening of the alloy under these conditions are detected. The mechanisms of fatigue crack nucleation and growth are analyzed as functions of the deformation amplitude and the test temperature.

2013 Bower Award and Prize for Achievement in Science presented to Kenichi Iga, Tokyo Institute of Technology, Tokyo, Japan

The mechanical behavior of materials can be modeled and evaluated across multiple scales that encompass: atomic and molecular rearrangements to create incipient defect nuclei; combination or growth of such nuclei to form micro-scale and subsequently macro-scale defects; and finally development of critical flaw sizes that result in final failure of engineering structures. Through an understanding of the mechanisms governing these processes, which depend on the loading conditions and the specific materials, new or modified materials systems designs can be integrated with advanced structural concepts to advance our energy, civil, transportation and healthcare infrastructures, among others.Subra Suresh׳s research includes development of novel experimental techniques and the discovery of new mechanistic processes that are broadly applicable to essentially every major class of natural and synthetic materials, including metals, ceramics, polymers, composites, biomolecules and biological cells. Beginning in the 1970s, he elucidated mechanisms of near-threshold fatigue crack growth in steels and aluminum alloys, contributing to improved alloy design for nuclear pressure vessels, fossil plants, offshore structures and aircraft. Subsequent work identified mechanisms for toughening brittle ceramics against fatigue crack growth under cyclic compressive loads. Turning to the microscale and the study of thin films, he developed new methods for extracting elastoplastic and functional properties of materials from small-volume contact probing. This led to new theories, computation and experiments on material and surface design through controlled gradients in composition and properties, and applications to coatings. At the nanoscale, he experimentally discovered nanocrystallization of bulk amorphous alloys during nanoindentation, and also showed how nanoscale twins could optimize the strength and ductility of fine grained metals. Most recently, he is noted for demonstrating the effects of disease-induced changes to human red blood cell deformability and biorheology, and for linking these changes to human diseases such as malaria.

2013 Benjamin Franklin Medal in Mechanical Engineering presented to Subra Suresh

The mechanical behavior of materials can be modeled and evaluated across multiple scales that encompass: atomic and molecular rearrangements to create incipient defect nuclei; combination or growth of such nuclei to form micro-scale and subsequently macro-scale defects; and finally development of critical flaw sizes that result in final failure of engineering structures. Through an understanding of the mechanisms governing these processes, which depend on the loading conditions and the specific materials, new or modified materials systems designs can be integrated with advanced structural concepts to advance our energy, civil, transportation and healthcare infrastructures, among others.Subra Suresh׳s research includes development of novel experimental techniques and the discovery of new mechanistic processes that are broadly applicable to essentially every major class of natural and synthetic materials, including metals, ceramics, polymers, composites, biomolecules and biological cells. Beginning in the 1970s, he elucidated mechanisms of near-threshold fatigue crack growth in steels and aluminum alloys, contributing to improved alloy design for nuclear pressure vessels, fossil plants, offshore structures and aircraft. Subsequent work identified mechanisms for toughening brittle ceramics against fatigue crack growth under cyclic compressive loads. Turning to the microscale and the study of thin films, he developed new methods for extracting elastoplastic and functional properties of materials from small-volume contact probing. This led to new theories, computation and experiments on material and surface design through controlled gradients in composition and properties, and applications to coatings. At the nanoscale, he experimentally discovered nanocrystallization of bulk amorphous alloys during nanoindentation, and also showed how nanoscale twins could optimize the strength and ductility of fine grained metals. Most recently, he is noted for demonstrating the effects of disease-induced changes to human red blood cell deformability and biorheology, and for linking these changes to human diseases such as malaria.

Creep fatigue crack growth and fracture mechanisms of T/P91 power plant steel

High temperature assessment of crack like defects in power plant components is one of the most important task to ensure high energy conversion efficiency. Creep fatigue crack growth tests for P91 power plant steel, conducted at 600°C on standard C(T) specimens with 0·1, 1 and 10 h hold times, are presented. Microscopic features of the creep fatigue damage were observed along the crack path, on polished and etched cross sections of the tested specimens. The dominant damage mechanism is the formation of microvoids and microcracks associated with the crack tip, due to the creep effect during the hold time. In order to predict the experimentally measured crack growth rate a simple summation rule is used. The superposition of fatigue crack growth rate and creep crack growth rate, experimentally obtained on the same batch of material used for creep fatigue crack growth tests, well describes the behaviour under creep fatigue loading.

Integrated Experimental, Analytical, and Computational Design for Fatigue Crack Growth Resistance in Cast Aluminum Alloys

Long and small fatigue crack growth (FCG) studies at various stress ratios have been performed on solution-strengthened A535-F and precipitation-strengthened A356-T6 cast aluminum alloys. Microstructures were altered through processing and chemistry in order to systematically investigate the individual and combined effects of materials’ characteristic microstructural features on FCG. Mechanisms of FCG at the microstructural scale of the studied alloys were identified at various growth stages, and load-microstructure-damage mechanisms design maps were created. Complementary to this work, a fracture mechanics methodology for data treatment and reduction has been developed for long-to-physically small crack growth corrections, and an original material science-based model that further compensates for the microstructurally small crack growth behavior was created and validated. A computational toolset has been constructed for microstructure-specific FCG simulations. FCG response is represented by the superposition of material's matrix and secondary phase behavior, with phase property data generated using a novel microhardness indentation technique. Actual microstructural images are used as the basis for meso-scale simulations to make numerical evaluations of the FCG response and to optimize the materials for FCG resistance. Examples on the application of these methods and tools will be given as they relate to design for FCG resistance, life predictions, and material optimization.

Grain size effect on the small fatigue crack initiation and growth mechanisms of nickel-based superalloy GH4169

The aim of this paper was to identify the grain size effect on the small fatigue crack initiation and growth mechanisms of nickel-based superalloy GH4169. Results showed that there was a transition of fatigue crack initiation mechanism between fine-grained material and coarse-grained material. Small fatigue crack growth rates were almost constant across a wide range of crack lengths. Once the surface crack length reached the critical size of 200μm, the crack would propagate fairly quickly. At a given experimental condition, the scattering of fatigue lives of the parallel specimens was dependent on the crack initiation mechanisms.

Electron back scattered diffraction characterization of thermomechanical fatigue crack propagation of a near α titanium alloy Timetal 834

Fatigue crack growth (FCG) mechanisms have been studied in light of the interaction of a propagating crack with local crystallographic orientations of primary alpha (αP) and secondary alpha (αS) colonies in a near α Timetal 834 Ti-alloy under thermomechanical fatigue (TMF) loading using electron backscattered diffraction (EBSD). FCG testing at in-phase (IP) and out-of-phase (OP) TMF loading have been carried out at two temperature intervals 300°C↔450°C and 450°C↔600°C. EBSD analysis and microhardness measurements have confirmed that larger cyclic plastic zone size at the crack tip leads to lower crack propagation under IP-TMFCG loading as compared to OP-TMFCG loading. EBSD analysis has also confirmed that crack dissipates more energy to propagate when it passes from a soft grain oriented with its c-axis normal to the loading direction and encounters a hard grain with its c-axis parallel with the loading direction.

Fatigue limit investigation of 6061-T6 aluminum alloy in giga-cycle regime

In order to investigate the fatigue limit micro-mechanism of a precipitation-hardened Al–Mg–Si alloy (6061-T6), the alloy was subjected to very-high-cycle fatigue (VHCF) of over 109 cycles by an ultrasonic fatigue method. Two kinds of specimens, one with smooth surface and the other with a small artificial hole on the surface, were compared. The smooth specimens showed no distinct fatigue limit. Conversely, the holed specimens showed clear fatigue limit which had been generally deemed to be absent in non-ferrous alloys. In addition to the conventional fatigue crack growth (FCG) observation by replica technique, metallographically critical analyses by electron backscattered diffraction (EBSD) and cross-sectional focused ion beam (FIB) were conducted to reveal the micro-plasticity associated with FCG. It was found that the fatigue life of smooth specimens at low stress amplitude was controlled by an unstoppable FCG mechanism mediated by persistent slip bands (PSBs). On the other hand, the emergence of distinct fatigue limit in holed specimens was attributed to a non-propagating crack having mode I characteristics in essence. No coaxing effect was, however, confirmed for such non-propagating cracks. The above results, which were somewhat different from previous ones obtained by rotating bending under normal frequency, were discussed in terms of both metallurgical and mechanical points of view.