On the relation between micro- and macroscopic fatigue crack growth rates in aluminum alloy AMS 7475-T7351

This paper discusses the relationship between striation spacing, i.e., the microscopic crack propagation rate, as measured in postmortem fractographic inspection of fatigue fractured surfaces, and the macroscopic crack propagation rate, i.e., da/dN, as monitored during fatigue crack growth tests. Compact tensile specimens C(T) in prevalent plane-strain conditions were extracted in LT orientation from the center of a 2-in. thick rolled plate of a SAE-AMS 7475-T7351 Al alloy. Testpieces were fatigue tested according to ASTM-E647 standard, at room temperature in a servo-hydraulic closed-loop MTS testing machine operating with the unloading elastic compliance technique. da/dN-ΔK data points were collected in the Paris’ law validity region, with crack growth rates typically ranging from 0.18 to 2.02 μm/cycle. Topographical survey was conducted on the test specimen fracture surfaces in a scanning electronic microscope in order to determine striation spacing created during the fatigue test. Macro- and micro-crack growth rates were compared and good correlation have been obtained for the data within the range of ΔK assessed in the study. Results of crack growth rates have been quantitatively evaluated in terms of fatigue life estimation.     

Multiple fatigue crack growth in pre-corroded 2024-T3 aluminum

Previous studies of fatigue crack growth in corroded aluminum have revealed that multiple crack-nucleating corrosion features often lead to the failure of individual test specimens. In the present work, this phenomenon was explored by performing quantitative fractography on forty 2024-T3 sheet aluminum fatigue specimens. Slightly over half of the specimens were found to have two or more crack-nucleating pits. The number of nucleating pits per specimen was found to be positively correlated with stress level, and an interactive effect with corrosion exposure duration was observed. A fracture mechanics-based model was developed to simulate the observed multiple crack growth process. Flaw interaction effects were investigated and the importance of modeling multiple crack growth at high stress levels was seen.     

A three-dimensional (3D) numerical study of fatigue crack growth using remeshing techniques

Numerical analyses based on the finite element (FE) method and remeshing techniques have been employed in order to develop a damage tolerance approach to be used for the design of aeroengines shaft components. Preliminary experimental tests have permitted the calculation of fatigue crack growth parameters for the high strength alloy steel adopted in this research. Then, a robust numerical study have been carried out to understand the influence of various factors (such as: crack shape, crack closure) on non-planar crack evolution in solid and hollow shafts under mixed-mode loading. The FE analyses have displayed a satisfactory agreement compared to experimental data on compact specimens (CT) and solid shafts.     

Modelling fatigue crack growth in shot-peened components of Al 2024-T351

Microstructural fracture mechanics concepts are used to develop a model to incorporate shot-peening effects into crack propagation laws and fatigue life predictions. Shot peening produces a residual stress which resists opening of the crack and also produces a work-hardened layer within which the flow stress is raised. The model takes account of these effects to give an accurate prediction of the increase in fatigue life. The model was also used to derive the conditions for crack arrest, and the results are presented in the form of a fatigue damage map (FDM). The FDM can be used for the determination of safe loads in durability and maintainability analyses.     

Studies on fatigue crack growth for airframe structural integrity applications

A review is made of efforts at the National Aerospace Laboratories in the development of fatigue crack growth prediction technology for airframe applications. The research was focused on extension of rainflow techniques for crack growth analysis and development of accelerated crack growth calculation methods for spectrum loading. Fatigue crack closure forms a crucial element of modelling and fractographic techniques were developed for its study. These, combined with binary coded event registration enabled crack growth and closure mapping for part-through cracks in metallic materials. Experimental research on short cracks at notches led to discovery of the hysteretic nature of crack closure, which explains well-known history-sensitive local mean stress effects in notch root fatigue. Optical fractography of failures obtained under simulated service conditions revealed that short cracks do not exhibit any more scatter than long cracks at comparable growth rates. The nature of multi-site crack initiation and growth of small cracks at notches was investigated and the effort extended to lug joints that are widely used in airframe applications. Results from this work suggest the possibility of modelling crack growth from a size smaller than 50 microns through to failure, thereby accounting for a major fraction of total life. The work described in this paper enjoyed the strong support of Dr S R Valluri, Prof R Narasimha and Dr K N Raju. Financial support for the effort was provided by Aeronautical Research & Development Board, Aeronautical Development Agency and Department of Science and Technology.     

An investigation into the damage development and residual strengths of open-hole specimens in fatigue

An extensive experimental program was carried out to investigate and understand the sequence of damage development throughout the life of open-hole composite laminates loaded in tension–tension fatigue. Quasi-isotropic carbon/epoxy laminates, with stacking sequence [45₂/90₂/−45₂/0₂]_S, [45/90/−45/0]_2S and [45/90/−45/0]_4S were examined. These were selected on the basis that under quasi-static loading the [45₂/90₂/−45₂/0₂]_S configuration exhibited a delamination dominated mode of failure whilst the [45/90/−45/0]_2S and [45/90/−45/0]_4S configurations showed a fibre dominated failure mode, previously described as “pull-out” and “brittle” respectively. Specimens were fatigue loaded to 1 × 10⁶ cycles or catastrophic failure, which ever occurred first. A number of tests were interrupted at various points as the stiffness dropped with increasing cycles, which were inspected using X-ray computed tomography (CT) scanning. A static residual strength program was carried out for run-out specimens of each configuration.     

EIFS-based crack growth fatigue life prediction of pitting-corroded test specimens

Corrosive environment causes corrosion pits at material surface and reduces the fatigue strength significantly. Fatigue crack usually initiates at and propagates from these locations. In this paper, a general methodology for fatigue life prediction for corroded specimens is proposed. The proposed methodology combines an asymptotic stress intensity factor solution and a power law corrosion pit growth function for fatigue life prediction of corroded specimens. First, a previously developed asymptotic interpolation method is proposed to calculate the stress intensity factor (SIF) for the crack at notch roots. Next, a growing semi-circular notch is assumed to exist on the specimen’s surface under corrosive environments. The notch growth rate is different under different corrosion conditions and is assumed to be a power function. Fatigue life can be predicted using the crack growth analysis assuming a crack propagating from the notch root. Plasticity correction is included into the proposed methodology for medium-to-low cycle fatigue analysis. The proposed methodology is validated using experimental fatigue life testing data of aluminum alloys and steels. Very good agreement is observed between experimental observations and model predictions.     

Measurement of fatigue crack growth in large compact tension specimens using an AC magnetic field method

Fatigue crack growth rate data are required in order to carry out a numerical analysis of the fatigue performance of complex structural components. These data are obtained by measuring crack growth in standard fracture mechanics specimens. A new method for measuring fatigue crack growth in compact tension specimens has been developed. The technique is based on the measurement of the surface magnetic fields produced when passing a high-frequency alternating current through the specimen. Fatigue crack growth data recorded using this method indicated an accuracy of ±0.02 mm when compared with optical measurements. The technique is suitable for computer-controlled operation and could easily be applied to other standard specimen geometries.     

Numerical predictions and experimental measurements of residual stresses in fatigue crack growth specimens

Controlling macro residual stress fields in a material while preserving a desired microstructure is often a challenging proposition. Processing techniques which induce or reduce residual stresses often also alter microstructural characteristics of the material through thermo-mechanical processes. A novel mechanical technique able to generate controlled residual stresses was developed. The method is based on a pin compression approach, and was used to produce well-controlled magnitudes and distributions of residual stresses in rectangular coupons and compact tension specimens typically used in fatigue crack growth testing. Residual stresses created through this method were first computationally modeled with finite element analysis, and then experimentally reproduced with various levels of pin compression. The magnitudes and distributions of residual stresses in experimental specimens were independently assessed with fracture mechanics methods and good correspondence was found between residual stresses produced using the pin compression and processing techniques. Fatigue crack growth data generated from specimens with low residual stresses, high residual stresses resulting from processing, and high residual stresses introduced through the new pin compression technique were compared and validated. The developed method is proposed to facilitate the acquisition and analysis of fatigue crack growth data generated in residual stresses, validate residual stress corrective models, and verify fatigue crack growth simulations and life predictions in the presence of residual stresses.     

Experimental investigation and numerical prediction of fatigue crack growth of 2024-T4 aluminum alloy

Compact specimens were employed to study fatigue crack growth of 2024-T4 aluminum alloy under constant/variable amplitude loading. Apparent R-ratio effect under constant amplitude loading was identified with the nominal stress intensity factor range. Fatigue crack growth rates predicted by a unified model agreed with the experimental data well. Single tensile overload resulted in significant retardation of crack growth which was fully recovered after propagating out of overload-affected zone. Retarded crack growth induced by three-step sequence loading was heavily dependent on two sequence loading parameters. The influence of variable amplitude loading on crack growth was reasonably characterized by Wheeler’s model.     

A study of fatigue crack growth of 7075-T651 aluminum alloy

Both standard and non-standard compact specimens were employed to experimentally study the crack growth behavior of 7075-T651 aluminum alloy in ambient air. The effects of the stress ratio (R), overloading, underloading, and high–low sequence loading on fatigue crack growth rate were investigated. Significant R-ratio effect was identified. At the same R-ratio, the influence of specimen geometry on the relationship between crack growth rate and stress intensity factor range was insignificant. A single overload retarded the crack growth rate significantly. A slight acceleration of crack growth rate was identified after a single underload. The crack growth rate resumed after the crack propagated out of the influencing plastic zone created by the overload or underload. A parameter combining the stress intensity factor range and the maximum stress intensity factor can correlate the crack growth at different stress ratios well when the R-ratio ranged from −2 to 0.5. The parameter multiplied by a correction factor can be used to predict the crack growth with the influence of the R-ratio, overloading, underloading, and high–low sequence loading. Wheeler’s model cannot describe the variation of fatigue crack growth with the crack length being in the overload influencing zone. A modified Wheeler’s model based on the evolution of the remaining affected plastic zone was found to predict well the influence of the overload and sequence loading on the crack growth.     

AFM and SEM observation on mechanism of fatigue crack growth in an Fe-Si single crystal

Fatigue crack growth tests are carried out on sheets of an Fe-3.2% Si single crystal with a crystallographic orientation appropriate for striation formation. The behaviour of slip near a crack tip during the loading and unloading parts of a fatigue cycle is observed using an Atomic Force Microscope and a Scanning Electron Microscope. The fracture surfaces are also analysed with an AFM and an SEM. The mechanism of fatigue crack growth is discussed based on the observations, and a fundamental kinematic model for fatigue crack growth is proposed. The model gives a reasonable explanation for both the crack growth and striation formation.     

Statistical modeling of fatigue crack growth rate in pre-strained 7475-T7351 aluminum alloy

In this study, 7475-T7351 aluminum strips were subjected to two tensile pre-strain levels of 3% and 5%. Using compact tension C(T) specimens, fatigue crack growth tests were conducted under constant amplitude loading at stress ratios of 0.1 and 0.5 in air and at room temperature. Three fatigue crack growth rate (FCGR) models, namely, Collipriest, Priddle, and NASGRO were examined. To handle the effect of stress ratio on FCGR, Walker equivalent stress intensity factor model was used. Consequently, generalized Collipriest (GC), generalized Priddle (GP), and generalized NASGRO (GN) models were developed and fitted to the FCGR data. It was shown that both GC and GP models fit the FCGR data in a similar fashion. However, the GP model provided a better fit than the GC model. The GN model was found to be the most appropriate model for the data. Therefore, this model may be suggested for use in critical applications, such as aeronautical structural design.     

The effect of sheet thickness on fatigue crack retardation in 2024-T3 aluminum alloy

Variable-amplitude fatigue studies of 2024-T3 aluminum alloy were performed to examine the effect of sheet thickness on fatigue crack growth rate retardation. Results indicated that the amount of retardation increased with decreasing specimen thickness. This phenomenon was attributed to enhanced plastic strains under plane stress conditions (i.e. in a thin sheet) which formed ahead of the advancing crack tip as a result of a high load excursion. These strains are believed to produce both crack closure and a favorable compressive residual stress field around the crack tip. Evidence of increased crack surface interference under plane stress situations was verified with electron fractographic observations.     

Prediction of 3D small fatigue crack propagation in shot-peened specimens

Shot peening has been widely applied in industrial design to improve fatigue durability of high loaded machine components. The compressive residual stress induced by shot peening is in general assumed to be responsible for the improvement of material fatigue strength. In the present work a cyclic cohesive zone model is extended to analyze three-dimensional fatigue crack growth in shot-peened specimens. Fatigue crack growth behaviors in both unpeened and peened specimens are investigated using 3D finite element analysis. The parameters of the cohesive zone model have been identified in 2D unpeened specimens and are applied to predict peened specimens directly. The results indicate that shot peening strongly affects crack initiation time and crack profiles, but has little effect on crack propagation rate. It implies that the shot peening will hardly change Paris’ law used for the damage tolerant design.     

Fatigue crack paths in AA2024-T3 when loaded with constant amplitude and simple underload spectra

It is well known that variable amplitude fatigue loading produces progression marks on fatigue crack surfaces that are related to the loading sequence. These marks are generally a local change in the crack path. The same pattern of loading can produce a pattern of progression marks that have differences from material-to-material or from heat treatment-to-heat treatment, yet the crack path changes that produce these markings are not considered in the prediction of the crack growth behaviour. These path changes can be used to: investigate how the crack grows, aid crack growth measurement and shed light on the mechanism that forms striations. Consequently, an understanding of these path changes and their fundamental cause in structurally significant alloys is important to the explanation of how fatigue cracks grow and how their life can be predicted.In this paper, a number of simple loading sequences were used to investigate the influence that underloads have on the crack path, to develop a better understanding of the formation of fatigue striations and the coarser crack path changes associated with loading changes. The material chosen was aluminium alloy (AA)2024-T3. The results from the tests reported here were compared to previously investigated AA7050-T7451 specimens that were loaded in a similar manner. It is shown that the fatigue crack surfaces that were produced here were the direct consequence of the applied loading interacting with the crystal structure of the material. By changing the loading, via the addition of underloads it was possible to produce fatigue crack surfaces that where composed of not only striations but ridges, depressions and fissures. These features give an indication of the crack growth mechanism. Although, AA2024-T3 and AA7050-T7451 have different chemical compositions, mechanical properties and micro-structures, it was shown that both materials share essentially similar fracture features corresponding to crack propagation at the cycle-by-cycle level. It also appears that despite the above noted differences, similar failure mechanisms may take place.     

Interfacial fatigue crack propagation in microscopic model composite using bifiber shear specimens

Interfacial fatigue crack growth behavior in GF/epoxy model composites was investigated using bifiber shear (BFS) specimens in a scanning electron microscope. The specimen is composed of two E-glass filaments with diameters of 23 and 40 μm, and bisphenol A type epoxy is impregnated between the filaments. The crack growth behavior under different stress ratios was investigated to clarify the fatigue crack growth mechanism. The change in the crack growth rate, da/dN, was not monotonic with crack length, suggesting a variation in the resistance to fatigue crack growth along a single filament. The resistance to fatigue crack growth of the interface is much smaller than that of composite laminates. The fatigue crack growth mechanism of the glass fiber/epoxy interface under different stress ratios is controlled by the maximum energy release rate, G_max, which is completely different from that of composite laminates.     

Improved structural reliability assessment using non-linear regression techniques to process raw fatigue crack growth test data

A new method is proposed to process raw fatigue crack growth (FCG) test data for estimating the parameters of any crack propagation (FCG) model. This method is based on iterative minimization techniques for non-linear regression, extensively available in the literature. The method presented here permits a significantly reduced contribution of the variability inherent in the usual raw FCG data processing techniques to the predictions of the crack length distributions derived by any analytical FCG model and, consequently, improved structural reliability assessment is achieved.     

Strain profiling of fatigue crack overload effects using energy dispersive X-ray diffraction

Synchrotron based energy dispersive X-ray diffraction has been used to profile the strains around fatigue cracks in 4140 steel test specimens. In particular strain field comparisons were made on specimens prepared: with initial constant stress intensity fatigue; with this initial fatigue followed by a single overload cycle; and with this fatigue-overload sequence followed by an additional constant stress intensity fatigue. The strain profiles behind, at and in-front-of the crack tip are discussed in detail. Selected strain profiles measurements under in situ applied tensile stress are also presented. The technique of optical surface height profiling reveals surface depression effects which can be correlated with the interior strain profiles.