Fatigue Loading Conditions Research Articles

Crack growth behavior, fracture mechanism, and microstructural evolution of G115 steel under creep–fatigue loading conditions

Creep–fatigue crack growth (CFCG) behavior, fracture mechanism, and microstructural evolution of G115 steel under different hold times and initial stress intensity factor ranges (ΔKin) were investigated. The crack propagation durations increased significantly with decreasing ΔKin and increasing dwell time. In da/dN vs. ΔK plots, a “hook” was formed, and it entered the fast crack growth zone at approximately ΔK=36MPam, which corresponds to σref = σ0.2 under plane stress. In (da/dt)avg vs. (Ct)avg plots, data for a specific hold time in a wide load range exhibited a unique relationship. It appears that the parameter D0 in this relationship is affected by hold time and constraint, while φ is independent of these factors. Scanning electron microscopy analyses revealed that with increasing hold time (60–600 s), the fracture mode changed from transgranular-dominated to intergranular-dominated. Meanwhile, the fracture mode changed from intergranular-dominated to predominantly transgranular with high ductility when ΔKin increased (17–29 MPam). According to electron backscatter diffraction observations, many substructured and recrystallized grains formed after CFCG and the deformed grains further decreased as ΔKin increased. The increased crack resistance with decreased deformation grains may combine with the ductility fracture effect to decrease the CFCG rates under high load levels. Additionally, the local misorientation angle reduced significantly after CFCG, and the differences among various load levels were limited.

Notch Effect on Structural Strength of Components at Elevated Temperature Under Creep, Fatigue, and Creep-Fatigue Loading Conditions: Phenomenon and Mechanism

Structural discontinuities (e.g., nozzle, hole, and groove) widely occur in many high temperature components of nuclear and fossil power plants. In general, the notched component is used for simplified tests and analyses due to the complexity of the introduction of a practical component. In the previous work, the effects of the notch on failure life of the components have been reported experimentally, including the strengthening and weakening effects; however, the internal mechanisms have not been clearly demonstrated. This work reviews the notch effects on the structural strength of the notched components at elevated temperatures under creep, fatigue, and creep-fatigue loading conditions. Experimental phenomena (i.e., strengthening or weakening effects) for typical notched specimens subjected to the above three loading conditions are summarized, and the related factors for notch effects on creep rupture life or cycle to failure of the components are discussed. The mechanisms for the strengthening or weakening effects induced by a notch are described. Evaluation procedures for notch effect analysis under complex loading conditions are also included, and the primary challenges concerning the notch effect are provided for further investigations.

Static and fatigue behaviors of short glass fiber–reinforced polypropylene composites aged in a wet environment

In this paper, a new experimental study of the bending static and fatigue behaviors of a composite material reinforced with 40% by mass of short glass fibers (type E) and polypropylene matrix is presented. The composite material is obtained in the form of plates by an injection process, which inevitably affects the distribution of the fibers and therefore the behavior of the material studied. To do this, several techniques are implemented on specimens by cutting them in transverse and longitudinal directions. The effect of aging in distilled water at 40℃ on the mechanical characteristics is studied under static and fatigue loading conditions. The static tests, three-point flexure up to failure, allow us to choose the levels of stress for the fatigue tests. The endurance curves as a function of the number of cycles are plotted by adapting the end-of-test criteria N5, N10, and N20, which represent a rigidity drop of 5%, 10%, and 20%, respectively. An interpretation of the Wöhler curve equations defined for the end-of-test criteria allows defining the kinetics of material damage. The results highlighted the influence of distilled water on the mechanical behavior and the lifetime of the material. We also perform macroscopic observations of fracture and microscopic facies in order to identify the damage mechanisms of the composite material.

Micromechanical Fatigue Modelling of the Size Effect in Micro-Scale 316L Stainless Steel Specimens

For many years, computational modelling and simulation studies have been used by developers to advance device design and have been reported in regulatory medical device submissions. However, cardiovascular stent materials in such computational models are typically assumed to behave as a continuum. This approach assumes that bulk material properties apply to the micro-sized structure, i.e. material behavior is scale independent. However, as size is reduced, mechanical size effects arise as the grain size to specimen width ratio drops below a critical value. These size effects cause material behavior to deviate significantly from bulk material behavior. If such a deviation in material behavior is to be captured within computational models, it is necessary to represent the crystalline structure of a metal and to capture the anisotropic behavior of individual grains within these models. This paper describes the development of such a modelling methodology to investigate the phenomenon of strain localization within grains of a 316L stainless steel specimen under fatigue loading conditions.

Post- Impact Fatigue Damage Analysis of Quasi Isotropic CFRP Laminates through Infrared Thermography

Carbon fiber reinforced plastic (CFRP) materials used for lightweight aircraft structures are susceptible to low velocity accidental impacts during manufacture and while in service. In this study the CFRP laminates subjected to low velocity impact conditions of three different energy levels were tested under various fatigue loading conditions till it reached the failure state. The infrared thermography NDT technique was used for the damage analysis of these post-impact fatigue loaded CFRP specimens to understand the damage distribution across the specimen. The cooling response curves obtained from the active thermography of the post impact fatigue specimens under four different combinations (transmission or reflection mode with impacted or un-impacted surface facing camera) for each of the specimens were analyzed. The rate of cooling in the temperature response curves is different for each specimens subjected with various post impact-fatigue loading conditions and it seems to be correlated with the extent of post-impact fatigue damage present in each specimen quantified through X-ray CT image processing. The temperature rise observed in the passive thermography at the fiber/ply breakage during static strength tests were proportional to corresponding load drops and the non-homogeneous spread of damage at the onset of tensile failure was seen from the thermographs.

A modified constitutive model for tensile deformation of 9%Cr steel under prior fatigue loading

Reliable constitutive models are necessary for the precise design and manufacture of complicated components. This study is devoted to developing a modified constitutive model to capture the effects of prior fatigue loading on subsequent tensile deformation of 9%Cr steel. In the proposed model, a strain hardening rule combined with a defined fatigue damage parameter was introduced to represent prior fatigue damage. The defined fatigue damage parameter based on the inelastic strain range of each cycle is capable of describing the evolution of tensile strength, recovery of martensite laths and decline of dislocation density, regardless of the variation in fatigue loading conditions. To validate the predictive capacity of the proposed model, experimental tensile results at different strain amplitudes, lifetime fractions and hold times of prior fatigue loading were compared with the predicted results. Good agreement between experimental and predicted results indicates that the proposed model is robust in describing the tensile behaviour under prior fatigue loading. Moreover, few determined material parameters are required, which makes the proposed model convenient for practical applications.

Fatigue Strength Assessment of Trimaran Cross-Deck Structure Based on Spectral and Simplified Fatigue Method

In order to investigate the fatigue behaviour of trimaran cross-deck structural details, the spectral and simplified fatigue analysis approaches are proposed. In spectral fatigue approach, three-dimensional (3D) linear potential flow theory and global FE analysis are used for wave loads and stress transfer functions calculation; the stochastic spectral fatigue analysis is carried out considering the weighted wave headings factors. In simplified fatigue approach, based on the direct calculation procedure of LR rules, the evaluation of simplified fatigue loads and loading conditions are presented, and the stress ranges are obtained by global finite element (FE) analysis. Then the fatigue lives of a few hot spots are computed to demonstrate the application of the proposed method. The result shows that the method given in this paper has a good applicability. This study offers methodology for the fatigue analysis of trimaran cross-deck structure, which may be regarded as helpful references for structural design of these types of ships.

ПОШКОДЖЕННЯ ВІД ЩІЛИННОЇ КОРОЗІЇ В КОНСТРУКЦІЯХ ЛІТАКІВ ТА ЇХ ВИЯВЛЕННЯ

Presented examples of destruction of aircraft designs due to corrosion of metals under conditions of fatigue loading. It is shown that slit corrosion, which is an increase in corrosion in crevice and gaps between two metals, as well as in places of untight contact of metal with a nonmetallic material resistant to corrosion, leads to the appearance of corrosion products in the joints of the skin with the power suite, which supports it , which can lead to the swelling of some elements of the joint relative to other elements and provoke the tearing off of the heads of rivets with the further development of fatigue cracks and the destruction of aircraft structures. Shown, that visual inspection is not always effective for the detection of corrosion damage, and sometimes impossible, for example, in closed internal structures. New developments in the field of sensors and equipment for the detection of corrosive substances and corrosion damage are presented. Among them is information on the sensor (organic-ceramic composite) containing the conducting complex. When the composite is exposed to water liquids, its conductivity is lost. When the composite dries, the sensor reaches its initial values of resistance. Information is provided on the optical sensor for detecting corrosion in the construction of the airframe. This sensor is based on the remote detection of aluminum ions formed during corrosion. The development of a multi-parameter integrated sensor for assessing the structural integrity of aluminum alloys, the recording of the concentration of chloride ions, the release of hydrogen, changes in humidity and degradation of the material is presented. Information is provided on fluorescence-based optical sensors used to detect specific ions such as aluminum, indicating the beginning of corrosion of an aluminum alloy. Information is provided on the development of advanced digital X-ray methods for the detection of corrosion in the design of aircraft. The conclusion is made on the necessity of combining visual control and control with the use of means and methods for detecting corrosive substances and corrosion damage.

Damage evolution in high density polyethylene under tensile, compressive, creep and fatigue loading conditions

Influence of various loading histories on mechanical properties and damage evolution of polyethylene (PE) is quantified through a two-stage test method. The first-stage tests are to introduce damage by subjecting the specimens to different prestrain levels under tensile, compressive, creep and fatigue loading condition. Two months later, the second-stage tests apply monotonic tensile loading at a crosshead speed of 0.01 mm/min to characterize the mechanical properties for specimens that have had damage generated in the first-stage tests. Experimental results suggest that yield stress and loading stiffness decrease and residual plastic strain increases with the increase of prestrain introduced in the first-stage tests. Damage evolution law, based on the degradation of loading stiffness has been established as a function of prestrain and residual plastic strain. Results show that damage variable expressed as a function of prestrain strongly depends on loading conditions applied in the first-stage tests. However, damage evolution described by the relationship between damage variable and plastic strain is found to be independent on the loading condition, suggesting that a unified, plastic strain controlled damage evolution law can be established for PE materials.

Fatigue and Static Crack Growth Rate Study of Carbon Steel for Corrosion Prevention of Natural Gas Transmission Pipelines

Internal metallic and non-metallic coatings are being considered as parts of answers to allow current natural gas transmission pipelines to safely batch transport a variety of products, including natural gas, carbon dioxide and hydrogen. Sacrificial anode coatings, that contain Zn, are being considered for internal corrosion protection of standard carbon steel pipeline as they are known to galvanically protect steel. However, this can lead to the creation and ingress of hydrogen into the line pipe material, resulting in increased susceptibility to cracking under high static and cyclic loads associated with both steady state as well as startup and shut down conditions. The current study simulates the conditions expected to be encountered in field by performing crack growth rate measurements under static and fatigue loading conditions on fracture mechanics-based specimen extracted from API 5L X-65 steel. The effect of cyclic loading frequency on fatigue crack growth rate (FCGR) and the effect of applied cathodic polarization on static crack growth rates (SCGR) was investigated in a 3.5% NaCl solution in contact with CO2 at 25°C. The partial pressure of CO2 (pCO2) was 101.325 kPa. The applied cathodic potentials were those associated with mixed potential of the Zn- based coatings.

Experimental and FEM numerical assessment of multiaxial fatigue failure criteria for a rolling Stock's seats structure

This paper deals with the experimental and numerical investigations of the structural behaviour of a rolling stock's seats system under static and fatigue loading conditions, aimed to assess the reliability of some multiaxial fatigue failure criteria implemented in the FE model. According to the NF F31–119 standard, a specific full-scale multiaxial testing machine has been used for the fatigue test. During the test, a visual inspection approach with a detection interval of 3·105 cycles has been adopted. After 1.2·106 cycles the test has been stopped and a Liquid Penetrant Inspection (LPI) has been carried out in order to detect accurately cracks nucleation and propagation. Moreover, numerical analyses, based on the Finite Elements (FE) Method, have been performed on the EN-AW 6060 T5 aluminium alloy structure of a rolling stock's seats to predict the most affected zones where the cracks nucleation may occur under multiaxial fatigue loads. Since the in-service applied loads belong to Multiaxial High-Cycle Fatigue (MHCF) load class, Sines, Crossland and Dang Van criteria have been used to post-process the predicted results achieved by the FE analyses. For validation purpose, numerical and experimental results have been compared. According to the numerical-experimental results comparison, Dang Van criterion provided the best level of accuracy.

Effect of fiber orientation distribution on constant fatigue life diagram of chopped carbon fiber chip-reinforced Sheet Molding Compound (SMC) composite

The material behavior of chopped carbon fiber chip-reinforced Sheet Molding Compound (SMC) composite is investigated under both quasi-static and cyclic fatigue loading conditions in the present study. Quasi-static tensile and compressive strength data is collected, followed by a series of fatigue tests under three different cyclic loading conditions, i.e., Tension-Tension (T-T), Compression-Compression (C-C), and Tension-Compression (T-C). The experimental results show a considerable amount of variation for each loading condition, which is found to be induced by the spatial distribution of fiber orientation within the tested specimens. An analytical approach is developed to correlate the fatigue performance of the SMC material with the local modulus, which is determined by the fiber orientation distribution. Additionally, the similar failure modes are observed under different loading conditions. Based on these findings, a new form of Constant Fatigue Life (CFL) diagram is established in which the impact of fiber orientation tensor to the fatigue behavior of SMC is considered. This developed CFL diagram is examined by comparing the measured fatigue performance with prediction where local fiber orientation distribution is quantified from microscopic images.

Numerical Evaluation of Fatigue Crack Growth Rate of Hot-Rolled 16Mn Steel Based on Compliance Method

Reliable evaluation of the fatigue crack growth rates in different kinds of specimen is necessary for the design of engineering components working in fatigue-load conditions. In this study, a new numerical model for evaluation of the fatigue crack growth rate in a three-point-bend specimen was proposed on the basis of compliance method. A three-point-bending fatigue test machine with a displacement-controlled loading system was developed to measure decrease of the fatigue load due to increase of the compliance of the specimen caused by crack growth. Evaluation of the fatigue crack growth rates of hot-rolled 16Mn steel was carried out using the test machine and the numerical model. The analysis results of the crack growth rates revealed different tendencies of slope for the former and latter processes of the subcritical propagation. Paris’s constants of hot-rolled 16Mn steel for the former process were obtained as c=1.17×10−9 and m=4.47, whereas the same parameters for the latter process were obtained as c′=3.09×10−5 and m′=0.84. The threshold value of the stress-intensity factor range for the material was determined as ΔKth=10.8 MPa√m.

Welding-repair effect on F357-T6 aluminum castings: analysis of fatigue life

Demands for quality, complexity, and product integrity are crucial in the growing market of castings. Usually, the more complex is the manufacture of the casting, and the more difficult is to preserve the quality requirements. Additionally, casting manufacturing remains economically acceptable when the process scraps are limited in number. Welding repair can be adopted to eliminate some defectiveness due to the casting process although the effect of repairing on the mechanical performance of structural components has to be accounted. In this paper, a welding-repair procedure for cast aluminum samples, specifically designed to reproduce typical refurbishing of a complex full-scale cast frame, was implemented. A set of 14 weld-repaired aluminum samples was tested under fatigue loading condition, and the comparison with the unrepaired casts’ behavior is presented. The statistical analysis was performed by Maximum Likelihood Estimation (MLE) method. The repaired casts belonging the Class 1—Grade C assignment of the SAE Aerospace material specification (AMS 2175) show a slight lowering of the high cycle fatigue strength. However, in the higher stress regime, a reduction of the median fatigue life of the weld-repaired set was observed. Failure mechanisms of repaired cast samples were investigated and discussed.

Improved density peak clustering-based adaptive Gaussian mixture model for damage monitoring in aircraft structures under time-varying conditions

Abstract For the reliable damage monitoring of aircraft structures under Time-Varying Conditions (TVCs), Gaussian Mixture Model (GMM)-based damage monitoring methods combined with Guided Wave (GW) have been studied increasingly frequently in recent years. In this paper, to enhance the performance of the GMM-based damage monitoring method, an Improved Density Peaks Clustering (IDPC)-based Expectation Maximization (EM) algorithm is proposed to improve the constructing algorithm of the GMM. In the algorithm, the unique initial value of the GMM parameters is obtained based on an adaptive searching strategy of the probability density peaks of GW Damage Features (DFs) first. Next, the GMM is constructed simply and efficiently by the EM algorithm based on the initial parameters. The IDPC-EM algorithm can stably construct the GMM with fewer experience-dependent initialization parameters while promising the probability modelling accuracy and maintaining high computational efficiency. Based on the IDPC-EM algorithm, a new adaptive GMM-based damage monitoring method combined with the GW is established. The method is simple, stable, highly computationally efficient and less experience-dependent due to the adaptive GMM. In this method, two GMMs of the DFs obtained in the healthy state and the damage monitoring state of an aircraft structure are constructed, respectively. By measuring the similarity of the two GMMs, the variation trend of the probability distributions of the DFs between the two states is obtained to detect the damage. Finally, the method is validated in a full-scale aircraft fatigue test which is close to an in-flight load test on the ground. The cracks of the right landing gear spar and the left wing panel are monitored reliably and stably under random fatigue load conditions.

Fractal analysis of the bending-torsion fatigue fracture of aluminium alloy

This work presents the relationship between fracture surface morphology, fatigue loading conditions and notch radius. Some specimens have been destroyed during fatigue tests under bending with torsion. Then, the fracturing surfaces of the rectangular cross-section specimens AW-2017A-T4 with notches have been analysed.This study presents fractal analysis for quantification of the geometry of the fracture surface. Surface geometry analysis is carried out on the whole fracture surface, using fractal dimensions and, surface isotropy. Fracture surfaces are observed using a focus variation microscope (FVM), which allows the acquisition of data sets with large depth of focus.A correlation between fractal dimension and stress concentrator, fatigue loading history, and total fatigue life is examined. These systematic dependencies may serve as a basis for extracting fatigue loading information from fracture surfaces of destroyed materials and structures.

Mathematical modelling and simulation of delamination crack growth in glass fiber reinforced plastic (GFRP) composite laminates

Delamination crack growth is a major source of failure in composite laminates under static and fatigue loading conditions. In the present study, damage mechanics based failure models for both static and fatigue loadings are evaluated via UMAT subroutine to study the dela- mination crack growth phenomenon in Glass Fiber Reinforced Plastic (GFRP) composite laminates. A static local damage model proposed by Allix and Ladev`eze is modified to an non-local damage model in order to simulate the crack growth behavior due to static loading. Next, the same classical damage model is modified to simulate fatigue delamination crack growth. The finite element analysis results obtained by the proposed models are successfully compared with the available experimental data on the delamination crack growth for GFRP composite laminates.

Fatigue Testing of Large-Scale Steel Structures in Resonance with Directional Loading Control

Offshore structures such as jacket and monopile foundations for wind towers and platforms for oil and gas are large steel structures that are subjected to severe fatigue loading conditions. When developing constructions using novel welding techniques, high-strength steel grades or specific bolted assemblies there is a need for validation testing of large-scale components. Conventionally, such fatigue tests are carried out on servo-hydraulic test which are time-consuming.Alternatively, fatigue tests can be performed in resonant bending by a cyclic excitation force with frequency close to the first eigenfrequency of the test specimen. In this paper a new test setup is presented suitable for testing large-scale steel components in resonance with control of the loading direction by two counter-rotating excentric masses.A wide range of loading conditions can be applied with a frequency from 20 to 40 Hz. The test specimen, that can weigh up to 25 ton, is supported in the nodes of its natural wave-form, so that no dynamic forces are transmitted to the setup.The working principles of the test setup are illustrated based on test results of a 711 mm diameter x 25.4 mm wall thickness steel pipe, a welded X-node representative for a welding detail of an offshore jacket foundation structure and a large-scale HE-beam. Crack initiation is detected using acoustic emission while crack growth is monitored by local strain gauge measurements as well as the global stiffness reduction of the test specimen. Finally, the beach marking method is used to visualize crack fronts for post-mortem analysis.

High Cycle Fatigue strength evaluation of welded joints in handling equipment

This study falls within the scope of the fatigue design of handling machines manufactured by the Manitou Group, which are usually composed of a chassis and a boom. These welded assemblies are subjected to fatigue loading conditions and include highly stressed zones acting as crack initiation sites. In view of the geometrical complexity of the structures, the degree of conservatism incorporated in design methods based on the usual international standards is difficult to evaluate. Alternative design approaches based on the structural stress are being developed and used by other companies such as PSA and LOHR. The present work focuses on developing a numerical approach to take into account the local stiffness of welds. The proposed strategy is based on shell finite element modelling of the assembly and an equivalent stiffness matrix for welds. This makes it possible to describe the local mechanical behaviour of seam welds with the computational cost of a shell element based model. To validate the proposed approach, a comparison in terms of stiffness is undertaken for three welded structures using solid finite element models as a reference. The proposed approach is also compared to the Manitou, Fayard and Lohr approaches.

Crack initiation from a clinically relevant notch in a highly-crosslinked UHMWPE subjected to static and cyclic loading

Crosslinked Ultra High Molecular Weight Polyethylene (UHMWPE), which is used as a bearing material in total joint replacement components, is subjected to static and cyclic loads in vivo. Resistance to crack initiation from a notch as a function of static and cyclic loads is not well understood for crosslinked UHMWPE. This study estimated the resistance of crosslinked UHMWPE (crosslinked with 100 kGy gamma radiation and remelted to extinguish free radicals) to crack initiation for a clinically relevant notch under both static and cyclic loading conditions. For cyclic loading, four frequencies were applied with a sine waveform and two frequencies were applied with a square waveform to independently estimate the effect of frequency and rate of loading on crack initiation. Crack initiation time and cycles to crack initiation were determined. Crack initiation time for fatigue loading conditions was substantially lower compared to static loading conditions. Crack initiation time decreased with an increase in test frequency. A square wave resulted in shorter crack initiation time compared to a sine wave. The results suggest that crosslinked UHMWPE is more resistant to crack initiation from a notch under static loading conditions compared to fatigue loading conditions.