Constant Amplitude Fatigue Research Articles

Cyclic deformation of aluminium alloys after the preliminary combined loading

Abstract This work explores sensitivity aluminium alloys under the influence of preliminary combined loading. There were examined 2024-T351 and D16ChATV aluminium alloys. The research was carried out under the conditions of tensile static and constant-amplitude fatigue tests on samples made of virgin material and after pre-combined loading. As a result of the research, it was concluded that the pre-combined loading leads to the modification of the mechanical properties of the tested materials. During the study reached similar results in increased strength for two aluminium alloys after the pre-combined loading, despite some differences in the structure of the output.

Fatigue crack growth behavior under multiaxial variable amplitude loading

This study compares both uniaxial and multiaxial variable amplitude experimental crack growth data for naturally initiated fatigue cracks in tubular specimens of 2024-T3 aluminum alloy to predictions based on two state-of-the-art analysis codes: UniGrow and FASTRAN. For variable amplitude fatigue tests performed under pure axial nominal loading conditions, both UniGrow and FASTRAN analyses were found to produce mostly conservative growth life predictions, despite good agreement with constant amplitude crack growth data. For variable amplitude torsion and combined axial-torsion crack growth analyses, however, the conservatism in growth life predictions was found to reduce. This was attributed to multiaxial nominal stress state effects, such as T-stress and mixed-mode crack growth, which are not accounted for in either UniGrow or FASTRAN, but were found in constant amplitude fatigue tests to increase experimental crack growth rates. Since cracks in this study were initiated naturally, different initial crack geometry assumptions were also investigated in the analyses

Fatigue strength prediction in composite materials of wind turbine blades under dry–wet conditions: An artificial neural network approach

In this article, the fatigue strength in composite materials of wind turbine blades under dry–wet conditions is predicted using artificial neural networks. Compression–compression constant amplitude fatigue tests were performed on thermoset polymer resins including polyesters and vinyl esters. Coupons were tested under an air temperature of 20°C and 50°C in both “dry” and “wet” conditions. The results show that artificial neural network can provide accurate fatigue strength prediction for different resin matrices under different values of temperature.

Fatigue properties of 6061-T6 aluminum alloy butt joints processed by vacuum brazing and tungsten inert gas welding

Tungsten inert gas welding and vacuum brazing butt joints of Al–Mg–Si alloy 6061 in the artificially aged condition T6 were studied. Constant amplitude and variable amplitude fatigue loading tests were performed. The experimental S-N curves were compared with the fatigue design curves recommended by the International Institute of Welding, British Standard, and Eurocode 9. Two mean stress correction methods, Goodman and Gerber, were evaluated. In terms of the size effect on the fatigue life, this article proposed an innovational thickness correction method based on the ratio of the ultimate tensile strengths of specimens with different thickness. For vacuum brazing components, the tensile strength–based thickness correction method was better than the thickness correction methods recommended by the International Institute of Welding and Eurocode 9.

Mechanisms and modeling of low cycle fatigue crack propagation in a pressure vessel steel Q345

The fatigue process near crack is governed by highly concentrated strain and stress in the crack tip region. Based on the theory of elastic–plastic fracture mechanics, we explore the cyclic J-integral as breakthrough point, an analytical model is presented in this paper to determine the CTOD for cracked component subjected to cyclic axial in-plane loading. A simple fracture mechanism based model for fatigue crack growth assumes a linear correlation between the cyclic crack tip opening displacement (ΔCTOD) and the crack growth rate (da/dN). In order to validate the model and to calibrate the model parameters, the low cycle fatigue crack propagation experiment was carried out for CT specimen made of Q345 steel. The effects of stress ratio and crack closure on fatigue crack growth were investigated by elastic–plastic finite element stress–strain analysis of a cracked component. A good comparison has been found between predictions and experimental results, which shows that the crack opening displacement is able to characterize the crack tip state at large scale yielding constant amplitude fatigue crack growth.

Fatigue Life Prediction of a Unidirectional Carbon Fiber Composite Under Off-Axis Spectrum Loads Using 3D Constant Life Diagram

Abstract In this study, a novel concept of 3D constant life diagram (CLD) for fatigue life prediction of a unidirectional (UD) polymer composite under spectrum load is proposed. Further, it is constructed and used to predict the fatigue life of a UD carbon fiber composite (CFC) subjected to a standard spectrum load sequence at an arbitrary off-axis angle. First, UD IMA/M21 CFC laminates were fabricated by an autoclave process. Static mechanical tests were conducted to determine the tensile and compressive strength at various off-axis angles ranging from 0° to 90°. Then the constant amplitude (CA) fatigue tests at three different stress ratios, R = σmin/σmax of 0.1 (tension-tension), −1.0 (tension-compression), and 10.0 (compression-compression) and at various off-axis angles between 0° and 90° at each of these stress ratios were performed to determine stress-life curves. Using the static and CA fatigue data generated, 3D CLD for UD CFC was constructed. Further, fatigue life of UD CFC subjected to a standard mini-FALSTAFF spectrum load sequence at an arbitrary off-axis angle of 20° was predicted following an empirical method using 3D CLD. Fatigue tests under the mini-FALSTAFF spectrum load sequence at an off-axis angle of 20° were also conducted at various reference stresses and compared with the predictions. A reasonably good correlation was observed between the predicted and experimental fatigue lives under off-axis spectrum loads.

Probabilistic Properties of Fatigue Damage and an Equivalent Damage Simplification Model for Multi-Level Load Spectra

An equivalent fatigue damage simplification model for multi-level load spectrum is proposed. On the basis of the probabilistic fatigue damage model which takes load sequence effect into account and the constant amplitude fatigue tests, probabilistic properties of damage accumulated in one loading block are achieved quantitatively by statistical methods. The new simplification model can ensure that, for the original spectrum and simplified spectrum, the probabilistic properties of fatigue damage are almost the same. Fatigue tests for Al-alloy straight plates are performed to test the validity of the model. The test results are in good agreement with predicted results.

Fatigue Performance of a Lightweight Composite Bridge Deck with Open Ribs

An innovative lightweight composite deck (LWCD) is proposed for steel bridges to avoid premature fatigue cracking. The composite deck is composed of an open-ribbed orthotropic steel deck (OSD) and a thin ultrahigh-performance concrete (UHPC) layer. This study is based on a suspension steel bridge in China, namely, the Second Dongting Lake Bridge. The following investigations were performed: (1) preliminary finite-element analysis (FEA) was carried out to evaluate the vehicle-induced stress ranges (i.e., Δσ = σmax − σmin) of six typical fatigue-prone details; (2) parameter analyses were performed to investigate the effects of the shape of cutouts and the thickness of the floor beams; and (3) two fatigue tests, one that used a full-scale LWCD panel and another that used a LWCD beam specimen, were conducted to reveal fatigue performance of the OSD and the stud shear connectors, respectively. Results of the preliminary FEA show that, with the contribution of the UHPC layer, the vehicle-induced stress ranges at some fatigue details of the LWCD, such as the rib–deck plate welded joints and the splice welds of the longitudinal ribs, were reduced to be less than their constant-amplitude fatigue limits, which indicates theoretically infinite fatigue lives of these details. The parameter analyses reveal that the apple-shaped cutout had relative good fatigue properties among the four cutout schemes and that the thickness of the floor beams is recommended to be 14–18 mm. According to the fatigue tests on the composite panel specimen and on the composite beam specimen, both the open-ribbed OSD and the stud shear connectors exhibited satisfactory fatigue endurances, which were much greater than 2 million cycles. The current theoretical and experimental investigations reveal that the proposed open-ribbed LWCD has favorable fatigue performances.

Fatigue damage of closed-cell aluminum alloy foam: Modeling and mechanisms

The objective of this work is to experimentally investigate the damage evolution and damage mechanism in closed-cell aluminum alloy foam under tension–tension fatigue loading. Constant amplitude fatigue tests are performed for the aluminum alloy foam, and experimental results indicate the large scatter of the fatigue damage in the aluminum alloy foam. To describe the fatigue damage with large scatter, a statistical fatigue damage model is developed on the basis of continuum damage mechanics. It is seen that the statistical damage model can describe the fatigue damage of the foam. Scanning electron microscopy (SEM) observation on the fracture surface of the tested specimen is carried out to understand the damage mechanisms of the foam. Four major categories of fatigue damage mechanism are concluded, i.e. damage initiates from the material surface, damage initiates on the cell wall, damage initiates at the intersection of several cell walls and damage initiates from the edge of cell. The high-resolution SEM images reveal that the fatigue mechanisms of the foam are mainly governed by the cell structure.

Fatigue life assessment of adhesive joints based on ratchetting strain evolution

ABSTRACTIn this article, experimental tests have been performed in order to examine the behavior of double-lap adhesive joints under constant amplitude fatigue loading and then a fatigue damage model was proposed based on ratchetting strain as a damage parameter. This parameter was employed in this study since it is feasible to be measured during the operational life of structures without interruptions and in a nondestructive manner. The predictions produced by the proposed model revealed acceptable agreements with the experimental data. Afterward, an alternative form of the F–N curve was obtained, which can be used to establish the design allowables.

短繊維強化ナイロン複合材料の疲労寿命とその応力比依存性

The static and fatigue behaviors of discontinuous carbon fiber-reinforced polyamide (DCF/PA) composite have been examined. Static tension and compression tests are first performed at different temperatures (RT, 70°C and 130°C) to quantify the temperature dependence of basic mechanical properties of the composite. Then, constant amplitude fatigue tests are carried out for different stress ratios (0.1, χ, -1, and 10) at different temperatures (RT, 70°C and 130°C), respectively. The fatigue test results demonstrate that the S-N relationship of the discontinuous fiber composite significantly depends on stress ratio, and the fatigue degradation occurs most rapidly at the critical stress ratio in line with the observations made thus far for continuous fiber composites. Finally, the anisomorphic constant fatigue life diagram approach developed for continuous carbon fiber composites is applied to the DCF/PA composite, and it is proved that this method can successfully be used to predict the fatigue life of the discontinuous carbon fiber composite at different stress ratios.

Delamination Propagation and Load Ratio Effects in DCB MD Woven Composite Specimens

Double cantilever beam (DCB) specimens fabricated from 15 plies of a plain woven prepreg (G0814/913) arranged in a multidirectional (MD) layup were tested by means of constant amplitude fatigue cycles under displacement control. Four different displacement cyclic ratios were used, namely 0.1, 0.33, 0.5 and 0.75. The delamination propagation rate da/dN was calculated from the experimental data and plotted with respect to different functions of the mode I energy release rate GI. When one of those functions, denoted here as ΔKI was used, all of the data obtained for the different displacement ratios collapsed into one master curve.

Fatigue behavior of AHSS lap shear and butt arc welds including the effect of periodic overloads and underloads

The effect of periodic overloads and underloads, which is a common occurrence in service load histories, on the fatigue behavior of AHSS (Advanced High Strength Steel) single lap shear and butt arc welds is investigated. A series of constant amplitude tests under tensile loading condition (R=Pmin/Pmax=0.1) were performed to obtain load amplitude-life curves for each type of arc welded joint. Variable amplitude load history tests, one with periodic overloads and another with periodic underloads were then conducted for each type of welded joint. Additional constant amplitude fatigue tests at a high load ratio (R≈0.6) representing the small cycles in the periodic underload loading were also performed. The validity of linear cumulative damage rule in the prediction of the fatigue life of these structures under variable amplitude loading is investigated. The results of the tests show that while periodic overloads did not affect the fatigue life of the welded joints, periodic underloads decreased the fatigue life. Finite element analyses were conducted to simulate the stress and strain states at the weld locations under overloads and underloads, the results of which were used to explain the observed experimental observations.

Multiaxial variable amplitude fatigue life analysis including notch effects

This paper evaluates the methodology for, and effectiveness of, common equivalent stress- and strain-based fatigue life analysis approaches when applied to variable amplitude multiaxial service loading conditions. Experimental results for both un-notched and notched specimens of 2024-T3 aluminum alloy, tested under axial, torsion, and combined axial–torsion loadings, are compared to predictions based on von Mises equivalent stress and strain. Mean stresses were considered using Smith–Watson–Topper, modified Goodman, and modified Morrow models. Variable amplitude life predictions are compared to constant amplitude predictions to highlight similarities and differences in life prediction trends. Overall, mixed results were obtained. Both constant and variable amplitude fatigue lives are predicted well for notched specimens, where a local uniaxial stress state always exists for the geometry considered. However, for un-notched specimens, where multiaxial loading effects come into play, no approach considered is able to predict more than 60% of fatigue data within a factor of ±3 of experimental life. A consistent trend of non-conservative life predictions for variable amplitude loading conditions is observed. The importance of considering stress gradient effects in notched specimen life prediction is also demonstrated. An extensive discussion on areas where improvements can be made with respect to fatigue damage quantification completes the paper.

Study of the fatigue behavior of a synthetic rubber undergoing cumulative damage tests

Fatigue damage tests were carried on polychloroprene rubber using a loading sequence with alternate blocks of two force amplitudes, one being damaging and the other not, and carried until the failure of the dumbbell-shaped samples. The duration of the blocks was investigated and chosen to be higher than 20% of the fatigue life of the damaging loading. The presence of cracks was assured by preliminary micro-tomography measurements on samples undergoing constant amplitude fatigue tests at the damaging solicitation. It was observed that the sum of cycles to failure at this damaging loading was more than 5 times the fatigue life at constant amplitude. The behavior of the hysteresis area, the maximal and the minimal strain hinted that the explanation of such an improvement of the fatigue characteristic was caused by the relationship between the crack growth, the strain-induced crystallized area at the tip of those cracks and the self-heating.

EFFECT OF STRAIN HARDENING ON FATIGUE CRACK CLOSURE IN ALUMINUM ALLOY UNDER CONSTANT AMPLITUDE WITH SINGLE OVERLOAD

In this study effect of strain hardening on crack closure has been examined with the help of experiments and finite element method on the side edge notched specimen of five different Aluminum alloy (3003 Al, 5052 Al, 6061 T6, 6063 T6, 6351) in mode I under constant amplitude fatigue loading with single overload using Abaqus® 6.10 which is very well accepted FEM application in research. Extended Finite Element Method Module has been used to determine effective stress intensity factor at the crack tip while propagation takes place. FEM results have given good agreement with experimental results. Regression analysis has also been done with SPSS® 16 and dependency of strain hardening coefficient on crack closure has analyzed. A generalized empirical formula has been developed based on strain hardening to calculate effective stress intensity range ratio and a modified Paris law has also been formulated for these aluminum alloy.

Surface Irregularity Factor as a Parameter to Evaluate the Fatigue Damage State of CFRP.

This work presents an optical non-contact technique to evaluate the fatigue damage state of CFRP structures measuring the irregularity factor of the surface. This factor includes information about surface topology and can be measured easily on field, by techniques such as optical perfilometers. The surface irregularity factor has been correlated with stiffness degradation, which is a well-accepted parameter for the evaluation of the fatigue damage state of composite materials. Constant amplitude fatigue loads (CAL) and realistic variable amplitude loads (VAL), representative of real in- flight conditions, have been applied to “dog bone” shaped tensile specimens. It has been shown that the measurement of the surface irregularity parameters can be applied to evaluate the damage state of a structure, and that it is independent of the type of fatigue load that has caused the damage. As a result, this measurement technique is applicable for a wide range of inspections of composite material structures, from pressurized tanks with constant amplitude loads, to variable amplitude loaded aeronautical structures such as wings and empennages, up to automotive and other industrial applications.

Probabilistic anisomorphic constant fatigue life diagram approach for prediction of P–S–N curves for woven carbon/epoxy laminates at any stress ratio

A general engineering methodology to construct a family of anisomorphic constant fatigue life (CFL) diagrams with probability of failure as the parameter that allows efficiently predicting P–S–N curves at any stress ratios is developed and validated for a plain weave fabric carbon/epoxy laminate. Constant amplitude fatigue tests are first performed to obtain statistical samples of fatigue life at different stress levels and stress ratios, respectively. Static tensile and compressive strength data are also collected. The Kolmogorov–Smirnov and Anderson–Darling goodness-of-fit tests suggest that both two-parameter lognormal and Weibull distributions are acceptable as the distributions for the static strength and fatigue life data, respectively, at the significance level of 5%. Then, we attempt to develop a methodology for efficient construction of the anisomorphic CFL diagrams for different constant values of probability of failure. It requires the P–S–N curves for any percentile points of the distribution for the critical stress ratio. To come up with this requirement, a probabilistic scaling law is formulated. It takes account of the probability-of-failure dependence of the critical stress ratio and the stress-ratio dependence of the P–S–N curve for the critical stress ratio. Finally, the anisomorphic CFL diagrams for different constant values of probability of failure are predicted using the proposed methodology, and they are shown to be in good agreement with the experimental results. It is also demonstrated that the P–S–N curves can efficiently and accurately be predicted for the woven CFRP laminate at any stress ratios using the proposed probabilistic anisomorphic CFL diagram approach.

Crack growth and delamination behaviour in advanced Al–Li alloy laminate under constant amplitude fatigue loading

Al–Li alloy laminate, as an advanced fibre metal laminate, has not been researched in fatigue crack growth and delamination behaviour. This paper investigates the crack growth, delamination shape and size of Al–Li alloy laminate under constant amplitude fatigue loading. The results indicate that the crack growth rate of Al–Li alloy laminate is almost constant like other fibre metal laminates. More connected bridging fibres would result in slower crack growth and crack growth rate would accelerate at the high stress level. The delamination shape becomes larger and transforms from triangle to a semi-elliptical shape with crack length increasing.

Effect of ply-drop on fatigue life of a carbon fiber composite under a fighter aircraft spectrum load sequence

Carbon fiber composite laminate containing symmetric internal ply-drop simulating thickness variation was fabricated and tested to determine fatigue life under a standard mini-FALSTAFF spectrum load sequence. The fatigue life of ply-drop composite was significantly lower than that of plain composite due mainly to initiation and growth of delamination near the ply-drop location. The spectrum fatigue life was also predicted by empirical method. For this purpose, static and constant amplitude fatigue data was generated to construct constant life diagrams (CLDs). The spectrum fatigue life predicted using CLDs was in good agreement with experimental results for both plain and ply-drop composites.