Retardation effects due to overloads in aluminium‐alloy aeronautical components

Fatigue data are generally derived under constant‐amplitude loading conditions, but aircraft components are subjected to variable‐amplitude loading. Without interaction effects, caused by overloads and underloads intermingled in a loading sequence, it could be relatively easy to establish a crack growth curve by means of a cycle‐by‐cycle integration. However, load‐spectrum effects largely complicate a crack growth under variable‐amplitude cycling. In this paper, fatigue crack growth behaviour of aeronautical aluminium alloy 2024‐T3 was studied. Effects of various loading conditions such as stress ratio and amplitude loadings were investigated. In particular, the effect of different overloads on the fatigue crack growth was simulated using Zencrack code. Preliminary analyses on Compact Tension (CT) specimens proved that the numerical results generated were in agreement with the results provided by an afgrow code for the same conditions. A case study was carried out on a helicopter component, undergoing repeated overloads, to compare numerical results obtained implementing yield zone models in Zencrack.     

FATIGUE CRACK GROWTH IN TWO ADVANCED TITANIUM ALLOYS UNDER CONSTANT AND VARIABLE AMPLITUDE LOADING AT 25 AND 175 °C

Fatigue crack growth of β-21S and Ti-62222 in sheet form was investigated under constant and miniTWIST flight spectra loading conditions at 25 and 175 °C. Variable amplitude results were compared with life calculations performed using NASA/FLAGRO software and constant amplitude fatigue crack growth results. Single tensile overloads under constant Δ K were performed to evaluate load interaction effects. Constant amplitude results showed that fatigue crack growth resistance was slightly better for Ti-62222 than β-21S at 25 and 175 °C. The presence of crack closure under various conditions caused moderate shifts in the fatigue crack growth data. Under miniTWIST flight spectra loading, Ti-62222 exhibited a greater extension in life in comparison to the β-21S at elevated temperature, consistent with the NASA/FLAGRO calculations. This was also consistent with the single tensile overloads where 25 °C tests were comparable for both materials, while at 175 °C, delay cycles were greater by a factor of almost three for Ti-62222. Extensive secondary cracking in Ti-62222 at elevated temperature accounted for the extended fatigue lives.     

Fatigue crack growth with overloads/underloads: Interaction effects and surface roughness

The generalization of damage tolerance to variable amplitude fatigue is of prime importance in order to maintain the reliability of structures and mechanical components subjected to severe loading conditions. Engineering spectra usually contain overloads and underloads which distribution may not be random. However for predicting the life of a structure, a simplified spectrum is usually determined from the real one, in order to reduce testing periods on prototypes. Therefore it is thus important to know which cycles can contribute to crack growth and which can be neglected. This paper presents an analysis of fatigue crack growth on M (T) specimens made of a medium carbon steel DIN Ck45. The specimens are subjected to repeated blocks of cycles made up of one or several (1, 2, 6 or 10) overloads (or underloads) separated by a variable number (10, 1000 or 10 000) of baseline cycles. The main objective of this study is to better understand the mechanisms at the origin of interactions effects due to the presence of overloads (or underloads) at different locations of each block loading. Under constant amplitude loading, single variables ΔK and K_max are required in crack growth relationships. The transferability of fatigue laws, obtained under constant amplitude loading to variable amplitude fatigue, requires at least an additional variable, whose evolution with crack length accounts for the interactions effects between cycles of different types. Results have shown that the interaction effects in fatigue crack growth are closely related to the mechanisms of crack growth: cyclic plastic behaviour of the material and fracture surface roughness. Measurements of roughness of the surface fracture were carried out in both constant amplitude and variable amplitude tests. The roughness characterization helped to determine the importance of the mechanisms on variable amplitude fatigue crack growth and determine the influence of overloads/underloads on fatigue crack growth.     

Fatigue crack growth in a structural steel under single and multiple periodic overload cycles

Preliminary results of a research program on fatigue crack growth in a low-carbon steel under a variable amplitude loading are presented. First, test results are reported on crack growth under simple loading sequences containing single and multiple tensile overloads applied periodically between smaller, constant amplitude cycles. Next, the observed crack growth behaviour is compared to predictions from a theoretical model developed by the authors.     

Effective strain–fatigue life data for variable amplitude fatigue

The usual analysis procedure for variable amplitude fatigue calculates fatigue damage based on constant amplitude strain controlled fatigue tests of smooth specimens. The resulting predictions are typically nonconservative due to a load interaction effect in variable amplitude fatigue. This paper reviews recent work which shows that large loads in a service load history decrease the crack opening stress and as a result increase the effective strain range for subsequent small cycles. A new strain–life fatigue test is introduced in which periodic large strain cycles reduce the crack opening stress for subsequent smaller cycles. The overloads are applied frequently enough that closure free fully open crack growth is achieved for the small cycles in the long life regime. An effective strain–life curve is derived and a crack opening stress equation calibrated by comparison of constant amplitude and effective strain ranges at given fatigue lives. The use of the effective strain–life curve in predicting fatigue lives is illustrated for service strain histories and for a variable amplitude load sequence applied to notched specimens. The predictions are good but somewhat conservative.     

Fatigue crack growth and crack closure in an AlMgSi alloy

This study reports an experimental investigation of fatigue crack propagation in AlMgSi1-T6 aluminium alloy using both constant and variable load amplitudes. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. For the constant amplitude tests four different stress ratios were analysed. The crack closure parameter U was calculated and related with Δ K and the stress ratio, R . The threshold of the stress intensity factor range, Δ K _th , was also obtained. Fatigue crack propagation tests with single tensile peak overloads have been performed at constant load amplitude conditions. The observed transient post overload behaviour is discussed in terms of the overload ratio, Δ K baseline level and R . The crack closure parameter U trends are compared with the crack growth transients. Experimental support is given for the hypothesis that crack closure is the main factor determining the transient crack growth behaviour following overloads on AlMgSi1-T6 alloy for plane stress conditions.     

INTERACTION OF OVERLOADS ON FATIGUE CRACK GROWTH FOR 40CrNi STEEL

This work was designed to improve the general understanding of loading sequence effects on fatigue crack growth and lead to the development of improved methods for predicting crack propagation behaviour. Two loading histories were selected (1) a baseline amplitude with periodic overloads or underloads, and (2) several overloads without interactive effects. The specimen used was of a Wedge Opening Loading type and the material was a low alloy high strength steel, i.e. 4OCrNi. It was found that the Linear Summation of Damage (LSD) assumption could be applied in predicting fatigue crack growth rate (FCGR) under periodic overloads or underloads within the scatterband of the constant amplitude data obtained using an Alternating Current Potential Drop technique for measuring crack length for multiple specimens with several load amplitudes. A discrepancy existed between the FCGR predicted from LSD and the one actually measured during several hundreds of loading cycles immediately following every non-interactive overload of the latter loading history although the overload ratio was the same as that of periodic overloading. The causes of this phenomenon are discussed.     

Fatigue crack growth in fibre metal laminates under selective variable-amplitude loading

To extend the predictive capability of existing crack growth models for fibre metal laminates under constant amplitude fatigue loading to variable-amplitude loading, further research on variable-amplitude fatigue mechanisms in fibre metal laminates is necessary. In response to this need, an experimental study into the effects of multiple overloads, underloads and various block-loading sequences on crack growth in the fibre metal laminate Glare was investigated. Crack growth retardation effects were observed in the tests; however, the magnitude of these effects was lower than seen in monolithic aluminium because of fibre bridging. As a result, predictions of the observed behaviour were attempted using an existing constant-amplitude fatigue crack growth model for Glare in combination with a linear damage accumulation law.     

Elastic–plastic fatigue crack growth analysis under variable amplitude loading spectra

Most fatigue loaded components or structures experience a variety of stress histories under typical operating loading conditions. In the case of constant amplitude loading the fatigue crack growth depends only on the component geometry, applied loading and material properties. In the case of variable amplitude loading the fatigue crack growth depends also on the preceding cyclic loading history. Various load sequences may induce different load-interaction effects which can cause either acceleration or deceleration of fatigue crack growth. The recently modified two-parameter fatigue crack growth model based on the local stress–strain material behaviour at the crack tip [1,2] was used to account for the variable amplitude loading effects. The experimental verification of the proposed model was performed using 7075-T6 aluminum alloy, Ti-17 titanium alloy, and 350WT steel. The good agreement between theoretical and experimental data shows the ability of the model to predict the fatigue life under different types of variable amplitude loading spectra.     

Fatigue crack propagation assessment based on residual stresses obtained through cut-compliance technique

Crack growth rate versus crack length curves of heavily overloaded parent material specimens and fatigue crack propagation curves of friction‐stir‐welded aluminium samples are presented. It is shown that in both cases the residual stresses have a strong effect on the crack propagation behaviour under constant and variable amplitude loading. As a simplified engineering approach, it is assumed in this paper, that in both cases residual stresses are the main and only factor influencing crack growth. Therefore fatigue crack propagation predictions are performed by adding the residual stresses to the applied loading and by neglecting the possible effects of overloading and friction stir welding on the parent material properties. For a quantitative assessment of the residual stress effects, the stress intensity factor due to residual stresses K_res is determined directly with the so‐called cut‐compliance method (incremental slitting). These measurements are particularly suited as input parameters for the software packages AFGROW and NASGRO 3.0, which are widely used for fatigue crack growth predictions under constant and variable amplitude loading. The prediction made in terms of crack propagation rates versus crack length and crack length versus cycles generally shows a good agreement with the measured values.     

Effect on fatigue crack growth of interactions between overloads

ABSTRACT Various types of interactions between overloads were studied in a 0.38% C low carbon steel. The retarding effect due to consecutive overloads is found to increase with the number of overloads, until it reaches a maximum. Similarly, it is found that a critical distance between overloads ensures the highest retarding effect, while shorter or longer spacing are less efficient for retarding crack growth. These effects are successfully explained using FEM calculations of the effective stress intensity factor. The kinematic hardening of the alloy, which is very efficient in ferritic–pearlitic steels, is shown to be mostly responsible for those effects. Taking into account the amplitude of kinematic hardening allows qualitative explanation of the observed effects. The order of application of the cycles during variable amplitude fatigue is thus important and should be taken into account for predicting fatigue lives.     

FATIGUE CRACK PROPAGATION BEHAVIOUR UNDER VARIABLE AMPLITUDE LOADING IN THE NEAR-THRESHOLD REGION OF A HIGH-STRENGTH LOW-ALLOY STEEL

The near-threshold behaviour of a high-strength low-alloy steel used in helicopter rotors under variable amplitude loading is investigated in ambient air and in vacuum. Strong sequence effects occur, in particular a high decrease in crack growth rates after overloads of 40% on maximum load while keeping the amplitude constant. A detailed study of the retardation phenomenon due to the overloads provides an explanation for the different behaviour observed in air and in vacuum and has laid the basis for a method of predicting crack propagation under helicopter loading spectra.     

Fatigue behaviour and life prediction of fibre reinforced metal laminates under constant and variable amplitude loading

ABSTRACT Fatigue crack growth of fibre reinforced metal laminates (FRMLs) under constant and variable amplitude loading was studied through analysis and experiments. The distribution of the bridging stress along the crackline in centre‐cracked tension (CCT) specimen of FRMLs was modelled numerically, and the main factors affecting the bridging stress were identified. A test method for determining the delamination growth rates in a modified double cracked lap shear (DCLS) specimen was presented. Two models, one being fatigue‐mechanism‐based and the other phenomenological, were developed for predicting the fatigue life under constant amplitude loading. The fatigue behaviour, including crack growth and delamination growth, of glass fibre reinforced aluminium laminates (GLARE) under constant amplitude loading following a single overload was investigated experimentally, and the mechanisms for the effect of a single overload on the crack growth rates and the delamination growth rates were identified. An equivalent closure model for predicting crack‐growth in FRMLs under variable amplitude loading and spectrum loading was presented. All the models presented in this paper were verified by applying to GLARE under constant amplitude loading and Mini‐transport aircraft wing structures (TWIST) load sequence. The predicted crack growth rates are in good agreement with test results.     

Block-by-block approaches for spectrum fatigue crack growth prediction

Two block-by-block approaches for improving spectrum fatigue crack growth prediction were proposed and developed in this paper from the observations and analyses of fatigue crack growth behaviours in either representative specimens or real aircraft structures under flight spectrum loading by using the quantitative fractography method. The first approach is the flight-by-flight approach that can be used to predict crack growth history curves for a tested spectrum crack growth data at different stress level for a critical location. The second approach called the effective block approach can be used to predict crack growth histories for un-tested spectra based on some previously tested spectrum crack growth data. In order to demonstrate the robustness of the block-by-block approaches for aircraft damage tolerance analysis, verification and consistency studies were conducted and presented using fatigue test results for different aircraft structures under several flight spectra. It was found that the block-by-block approaches are able to provide significant advantages over conventional fatigue lifing approaches for aircraft damage tolerance analysis.     

Bonded aircraft repairs under variable amplitude fatigue loading and at low temperatures

Bonded repairs can replace mechanically fastened repairs for aircraft structures. Compared to mechanical fastening, adhesive bonding provides a more uniform and efficient load transfer into the patch, and can reduce the risk of high stress concentrations caused by additional fastener holes necessary for riveted repairs. Previous fatigue tests on bonded Glare (glass‐reinforced aluminium laminate) repairs were performed at room temperature and under constant amplitude fatigue loading. However, the realistic operating temperature of ?40 °C may degrade the material and will cause unfavourable thermal stresses. Bonded repair specimens were tested at ?40 °C and other specimens were tested at room temperature after subjecting them to temperature cycles. Also, tests were performed with a realistic C‐5A Galaxy fuselage fatigue spectrum at room temperature. The behaviour of Glare repair patches was compared with boron/epoxy ones with equal extensional stiffness. The thermal cycles before fatigue cycling did not degrade the repair. A constant temperature of ?40 °C during the mechanical fatigue load had a favourable effect on the fatigue crack growth rate. Glare repair patches showed lower crack growth rates than boron/epoxy repairs. Finite element analyses revealed that the higher crack growth rates for boron/epoxy repairs are caused by the higher thermal stresses induced by the curing of the adhesive. The fatigue crack growth rate under spectrum loading could be accurately predicted with stress intensity factors calculated by finite element modelling and cycle‐by‐cycle integration that neglected interaction effects of the different stress amplitudes, which is possible because stress intensities at the crack tip under the repair patch remain small. For an accurate prediction it was necessary to use an effective stress intensity factor that is a function of the stress ratio at the crack tip R_{crack tip} including the thermal stress under the bonded patch.     

Effect of salt-water fog on fatigue crack growth behaviour of 7050 aluminium alloy in different orientations

The fatigue crack growth behaviour of 7050 T73651 high strength aluminium alloy that was originally developed for the aircraft industry was investigated in this study. The tests were conducted by using C-T specimens machined in six orientations under the action of constant amplitude sinusoidal load cycles. The tests were first carried out in laboratory air and then repeated in salt-water fog of a 5% NaCl solution to observe the effect of the environment on the fatigue crack growth behaviour. The experimental results showed that the fatigue life, maximum stress intensity range and the fatigue crack growth rate of the specimens were seriously affected by the environment. The severity of the effect, on the other hand, was observed to be dependent on the orientation. The strongest orientation was determined to be L-S, while the weakest was S-L.     

Random fatigue crack growth with retardation

On basis of a study of the literature concerning empirical findings in fatigue crack growth in metal specimens under constant amplitude loading with occasional overloads, the paper summarizes the reported qualitative effects of the overloads. The great scatter of the observations and the difficulty of setting up a clear physical mechanism, which in deterministic terms explains the crack growth retarding effects of the overloads, motivates attempts to formulate stochastic process models of phenomenological type. The paper shows that birth processes have features that make them applicable in modelling fatigue crack growth processes. In fact, this process type allows a time transformation that reduces the case of variable amplitude loading to the case of constant amplitude loading. The mean growth curve defined as the mean time of growth to a given crack length as function of this crack length may in the constant load amplitude case be calibrated to the Paris-Erdogan law. For the case of occasional overloads it may be further calibrated to the empirical results reported in the form of the Wheeler model of crack retardation based on the concept of a strengthening plastic zone at the crack tip caused by the overload.     

Analysis of fatigue crack growth in an attachment lug based on the weight function technique and the UniGrow fatigue crack growth model

A generalised step-by-step procedure for fatigue crack growth analysis of structural components subjected to variable amplitude loading spectra has been presented. The method has been illustrated by analysing fatigue growth of planar corner crack in an attachment lug made of Al7050-T7451 alloy.Stress intensity factors required for the fatigue crack growth analysis were calculated using the weight function method. In addition, so-called “load-shedding” effect was accounted for in order to determine appropriate magnitudes of the applied stress intensity factors. The rate of the load shedding was determined with the help of the finite element (FE) method by finding the amount of the load transferred through the cracked ligament. The UniGrow fatigue crack growth model, based on the material stress–strain behaviour near the crack tip, has been used to simulate the fatigue crack growth under two variable amplitude loading spectra. The comparison between theoretical predictions and experimental data proved the ability of the UniGrow model to correctly predict fatigue crack growth behaviour of two-dimensional planar cracks under complex stress field and subjected to arbitrary variable amplitude loading.     

Fatigue crack propagation under variable amplitude loading in PMMA and bone cement

Fatigue failure of PMMA bone cement is an important factor in the failure of cemented joint replacements. Although these devices experience widely varying loads within the body, there has been little or no study of the effects of variable amplitude loading (VAL) on fatigue damage development. Fatigue crack propagation tests were undertaken using CT specimens made from pure PMMA and Palacos R bone cement. In PMMA, constant amplitude loading tests were carried out at R- ratios ranging from 0.1 to 0.9, and VAL tests at R = 0.1 with 30% overloads every 100 cycles. Palacos R specimens were tested with and without overloads every 100 cycles and with a simplified load spectrum representing daily activities. The R- ratio had a pronounced effect on crack propagation in PMMA consistent with the effects of slow crack growth under constant load. Single overloads caused pronounced crack retardation, especially at low da/dN. In Palacos R, similar overloads had little effect, whilst individual overloads at low da/dN caused pronounced acceleration and spectrum loading retarded crack growth relative to Paris Law predictions. These results demonstrate that VAL can have dramatic effects on crack growth, which should be considered when testing bone cements.