Fatigue crack growth from sharp notches

Notch-like stress raisers occur widely in engineering components. They are preferred sites for crack initiation when the components are subjected to cyclic loadings. Thus the growth of cracks initiated from notches is very relevant to design against fatigue failures. Schematic models proposed to explain the departure of notch crack growth from linear elastic fracture mechanics predictions are briefly reviewed. Different methods of measuring crack closure are compared. It is found that the commonly employed notch-mouth clip-gauge method is not sensitive enough to detect the closure of short cracks in regions of notch plasticity. Various mechanics parameters have been claimed to be able to bring the notch crack and long crack growth rate data to a single base. In the present work on double-edge notched AISI 316 stainless steel specimens, it is found that none of them is able to correlate satisfactory all the experimental data.     

SIF and threshold for small cracks at small notches under torsion

The basic approach to the problem of torsional fatigue strength of pieces containing defects is based on the stress concentration factor concept. However, experiments have shown that the torsional fatigue limit of specimens containing small holes is controlled by the threshold condition for small cracks emanating from small notches. Therefore, the ratio of torsional to bending fatigue limit ( τ _w / σ _w ) for specimens containing small defects must be studied from the viewpoint of fracture mechanics.
The scope of this paper is to address the calculation of the stress intensity factor for a small crack emanating from a three-dimensional hole under a biaxial state of stress by using the weight function method and to apply it to the fatigue limit prediction. The results obtained are in good agreement with experimental results on specimens with defects.     

A micromechanical approach to fatigue in small notches*

The present work shows the application to small notches of a micromechanical model which describes the growth of a short crack across the steep stress gradient generated at the root of a notch. The model, based on the theory of distributed dislocations, takes into account the interaction between short cracks and material barriers such as grain boundaries. The term ‘small notches’ refers in this paper to stress raisers the size of which is of the same order as the characteristic microstructural unit of the material. Typical examples are superficial scratches, corrosion pits, inclusions or pores. Comparisons between predicted fatigue limits and experimental results reported in the literature for different materials containing small artificial defects are shown and discussed.     

Prediction of threshold and ultra-low fatigue crack growth rates

A model was derived to predict the true threshold value for fatigue crack growth in the absence of crack closure. The model, based only on the tensile and cyclic properties of the material, was successfully verified against a set of experimental data on medium and high strength steels and one aluminium alloy. Good agreement with experimental results was also obtained for Region I of the $\text{d}\text{a/}\text{d}\text{N vs ΔK}$ curve using a fatigue crack growth rate equation based on the same model.Fatigue crack growth data obtained from the medium strength steel CK45 in the normalized state and two heat-treated conditions were analysed. Good data correlation was shown using a previously developed normalizing parameter, $\text{φ = (ΔK}{}^2\text{?ΔK}{}^2{}_{\mathrm{<mtext>th</mtext>}}\text{)/(K}{}^2{}_{\mathrm{<mtext>c</mtext>}}\text{?K}{}^2{}_{\mathrm{<mtext>max</mtext>}}\text{)}$, in the entire range of fatigue crack growth rates and for stress ratios ranging from 0.1 to 0.8.     

Small-crack growth and fatigue life predictions for high-strength aluminium alloys. Part II: crack closure and fatigue analyses

Small-crack effects were investigated in two high-strength aluminium alloys: 7075-T6 bare and LC9cs clad aluminium alloys. Both experimental and analytical investigations were conducted to study crack initiation and growth of small cracks. In the experimental program, fatigue and small-crack tests were conducted on single-edge-notch tension (SENT) specimens and large-crack tests were conducted on middle-crack tension specimens under constant-amplitude and Mini-TWIST spectrum loading. A pronounced small-crack effect was observed in both materials, especially for the negative stress ratios. For all loading conditions, most of the fatigue life of the SENT specimens was shown to be crack propagation from initial material defects or from the cladding layer. In the analysis program, three-dimensional finite-element and weight-function methods were used to determine stress intensity factors, and to develop equations for surface and corner cracks at the notch in the SENT specimen. (Part I was on the experimental and fracture mechanics analyses and was published in Fatigue Fract. Engng Mater. Struct. 21 , 1289–1306, 1998.) This part focuses on a crack closure and fatigue analysis of the data presented in Part I. A plasticity-induced crack-closure model was used to correlate large-crack growth rate data to develop the baseline effective stress intensity factor range (Δ K _eff ) against rate relations for each material, ignoring the large-crack threshold. The model was then used with the Δ K _eff rate relation and the stress intensity factors for surface or corner cracks to make fatigue life predictions. The initial defect sizes chosen in the fatigue analyses were similar to those that initiated failure in the specimens. Predicted small-crack growth rates and fatigue lives agreed well with experiments.     

Fatigue crack growth under variable amplitude loading Part I: experimental investigations

During use, a component or a structure is exposed to variable amplitude loading, which influences the lifetime. Within the scope of this work, systematic investigations of different loading situations are carried out by means of experimental studies (part I) as well as analytical and numerical studies (part II). The experimental investigations show that overloads lead to retardation effects, which are influenced by several factors, e.g. the overload ratio, baseline‐level loading, number of overloads or the fraction of mixed mode. In a high–low–high block loading, both retarded and accelerated crack growth can be obtained, which is also influenced, e.g. by the block loading ratio and the length of the block. Moreover, experimental studies have been performed with load spectra, like FELIX/28, CARLOS vertical and WISPER. They have been applied in original form as well as in counted and reconstructed sequences.     

A threshold fatigue crack closure model: Part II – experimental verification

Predictions from an analytical model that considers contributions and interactions between plasticity, roughness, and oxide induced crack closure are presented and compared with experimental data. The analytical model is shown to correctly predict the combined influences of crack roughness, oxide debris, and plasticity in the near‐threshold regime. Furthermore, analytical results indicate closure mechanisms interact in a non‐linear manner such that the total amount of closure is not the sum of closure contributions for each mechanism.     

Fatigue crack initiation and growth in riveted specimens: an optical and acoustic microscopic study

While optical microscopy on riveted specimens reveals only surface cracks, the acoustic C-scan images reveal subsurface and buried cracks emerging to the surface. This is of particular interest for fatigue cracks that initiate below the outer surface such as with chamfered riveted panels. Fatigue crack initiation and growth in riveted panels of Alclad 2024-T3 were characterized using optical microscopy and scanning acoustic microscopy to obtain C-scan images. The C-scan images were obtained using a focused transducer with a center frequency of ca 50 MHz, and the peak value of the back surface echo of the plate with countersunk rivet holes was recorded in the C-scan images. Data on the initiation and development of fatigue cracks at rivets in riveted Alclad 2024-T3 are given.     

Fatigue crack growth and damage tolerance

The problems arising as a result of aging aircraft, rail and civil infrastructure have focused attention on tools for predicting the growth of cracks from small naturally occurring material discontinuities. To this end, the present paper discusses on the difference between the analysis tools needed for ab initio design and sustainment, modelling of cracks that grow from small naturally occurring material discontinuities and ways to determine the short crack da/dN versus ΔK data from long crack American Society for Testing and Materials (ASTM) tests. It also discusses how existing equations can be used to predict short crack growth and how to account for the variations seen in crack growth histories. Attention is also focused on the recent Federal Aviation Administration limit of validity ruling and the effect of the environment on widespread fatigue damage in civil transport aircraft.     

Fatigue crack growth of multiple interacting cracks: Analytical models and experimental validation

This article deals with the fatigue propagation of multiple cracks in finite width holed panels, which are typical of aircraft structural components. Theoretical studies in the literature have been considered and critically analyzed. Some of them have been translated into analytical models and implemented in a computer code. To check the effectiveness of the used models, a fatigue testing campaign has been conducted on six different configurations of notches and cracks. The comparison between experimental results and those obtained from the implemented models has shown a good agreement.     

Fatigue crack growth under variable amplitude loading Part II: analytical and numerical investigations

In part I, the effects of variable amplitude loadings on the fatigue crack growths were illustrated by means of experimental results. Within the scope of part II, systematic analytical and numerical investigations are presented. Using different analytical concepts it can be shown that the lifetime depends both on the concept used and on the loading sequence. Also, the influence of the parameters that must be fitted by experimental data for all analytical prediction models has been investigated. By means of detailed elastic–plastic finite element simulations it becomes obvious that not only the crack opening caused by large plastic deformations subsequent to overloads and block loadings, but also the stress field in the ligament is an indicator for the retardation effect. If the σ_y‐stresses both at maximum and minimum loading are identical with the σ_y‐stress distribution of an appropriate constant amplitude (CA) loading, one can assume that the interaction effect is annihilated.     

Fatigue crack initiation and crystallographic crack growth in an austenitic stainless steel

The lifetime of a specimen under cyclic loading is usually limited by the initiation and growth of microcracks. Experimental results for the austenitic stainless steel X6 CrNiNb 18-10 are given and a model for crack initiation and crack growth in the first grains of a polycrystalline aggregate is proposed.     

Fatigue crack growth testing at negative stress ratios: discussion on the comparability of testing results

It is an accepted fact in fatigue community that compressive loads contribute to fatigue crack growth. Evidences range from fatigue crack growth under fully compressive loads to effects of compressive underloads to negative stress ratio loading. Because the crack closes under compression and the crack flanks transmit compressive stresses, the loading situation is completely different to those of tensile loading. The present paper addresses the comparability of crack growth testing procedures at negative stress ratios. It reveals that compressive loading at the crack tip differs in different specimens for an equal maximum stress intensity factor K_max and negative stress ratio R. Furthermore, the crack length can significantly influence the loading conditions at the crack tip for tension–compression loading. Depending on the specimen type and crack length, a negative force ratio may lead to a change of algebraic sign of the stresses at the crack tip or not. As a consequence, the comparability of available literature results for R ≤ 0 tests is not ensured. Proposals to improve the comparability of tension–compression crack growth testing will be given.     

Fatigue crack growth, physical understanding and practical application

The paper presents a discussion on two problems associated with fatigue crack growth in aluminium alloys. First, the application of the similarity approach to crack growth prediction in specimens and structures of aluminium alloys is discussed. The significance of similarity conditions is emphasized and the K-dominated zone is briefly addressed. Secondly, the significance of water vapour for fatigue crack growth in aluminium alloy is reported with a case history of subsurface crack initiation and crack growth in vacuum. Some comments are presented on physical understanding and practical applications.     

Fatigue crack growth from stage I to stage II in a corrosive environment

How a corrosive environment such as a 3.5% NaCl aqueous solution would affect the transition of fatigue crack initiation mode from stage I (shear mode) cracking to stage II (normal mode) cracking was investigated, using notched specimens of HT80 and SNCM439 steels (corresponding to AISI 4340 steel).The experimental result that the transition of fatigue crack growth from stage I to stage II was accelerated in a hydrogen-related environment was discussed in terms of hydrogen induced dislocation multiplication near a crack tip. The microstructures immune to environmental damage were examined, using a precipitate-hardened steel.     

Local deformation at micro‐notches and crack initiation in an intermetallic γ‐TiAl‐alloy

The relation is studied between crack initiation from micro‐notches in a fully lamellar intermetallic γ‐TiAl alloy and the local strain field. These micro‐notches were introduced using femtosecond‐laser ablation and had dimensions below the average colony size. The specimen under investigation was then subjected to fatigue loading. Continuous monitoring using a travelling optical microscope allowed detecting microcracks at an early stage. Prior to fatigue loading, a sustained load was applied and the local strain field was determined using digital image correlation. This was supplemented by a Finite Element analysis of the notches and their neighbourhood. It was found that a crack was initiated from a notch causing high normal strains in lamella direction, whereas no crack was initiated from notches with high shear strains.