Fatigue Loading Conditions Research Articles

Fracture mechanics analysis method for cracked components under very low cycle fatigue loading

In this study, fracture mechanics analysis was performed to calculate crack growth under very low cycle fatigue loading conditions. The calculation results are compared with 11 experimental datasets concerning through-wall cracked pipes made from three different materials under fully reversed large-amplitude cyclic loading. In the analysis, it was assumed that the crack growth rate under large-amplitude cyclic loading is the linear summation of the fatigue crack growth rate and ductile tearing rate. Relevant fracture mechanics parameters were determined using the reference stress approach. It was found that the calculated failure cycles are around half of those using the experimental data regardless of the material and load amplitude. The most important factors affecting the calculation results were the accuracy of estimating J and ΔJ, and the crack closure coefficient when calculating ΔJ. Their effects on the analysis results are discussed herein.

Automatic edge detection of ply cracks in glass fiber composite laminates under quasi-static and fatigue loading using multi-scale Digital Image Correlation

Digital image correlation is utilized to detect automatically crack density in glass/epoxy laminates with different lay-ups under quasi-static and fatigue loading conditions. The DIC setups used in the present work are 2D-DIC and 3D-DIC which are considered to be at meso-scale and macro-scale, respectively. The main objective of this work is to study whether the DIC technique is reliable in automatic crack detection and functional in calculation of crack density. Therefore, an optimization analysis is implemented to evaluate the resolution and the standard uncertainty in strain and displacement measurements. An optimized compromise is attained between the resolution and the uncertainty of both measurements to decide about the final configuration of the DIC setup. The setup is executed by acquiring in-situ images under quasi-static and fatigue experiments so that the images are synchronized with the data acquired from the testing machine. The crack detection analysis is attained by DIC analysis using strain and displacement fields in order to obtain optimum results. Then a semi-automatic program is created to evaluate the crack density and weighted crack density from the edge and top of the specimen, respectively. Moreover, a comparison is established between both strain and displacement fields in crack characterization. Consequently, the displacement field has proved to deliver better and more accurate results than strain fields in crack density calculation and individual crack characterization. Accordingly, the results obtained from DIC analysis are compared and validated by microscopic images which are acquired after performing each test.

The influence of mode mixity and adhesive system on the fatigue life of adhesive joints

AbstractIn real applications, adhesive joints are commonly subjected to fatigue and mixed mode loading conditions. The aim of the current work is to experimentally analyse the influence of mode mixity on the fatigue strength of joints with an epoxy‐based adhesive. Different adhesive systems (acrylic and epoxies) were considered for pure mode I fatigue loading conditions. To achieve this, Arcan joints with an epoxy adhesive were manufactured and tested at different mode mixities. Based on stiffness degradation, monitored during the tests, damage evolution was calculated for different loading conditions and for all the tested adhesives. Finally, fatigue envelopes were constructed for different fatigue life regimes. Results show that a shear loading component reduces both the static strength and fatigue life of the joints. A small reduction rate of the stiffness was found throughout the most part of the life until a sudden drop was observed, indicating a smooth damage evolution.

Failure Analysis of Cooling Tower Fan-Arm

This work presents failure investigation of cooling tower fan-arms commissioned in a chemical processing plant. The analysis aims at understanding the mechanism and root-cause of the failure. The investigation involves site visits, microstructural analysis, fracture surface analysis, hardness measurements, numerical stress analysis and experimental simulation. Work concludes that the fan-arms failed due to the lack of post-weld heat treatment, which caused localized stress-corrosion and pitting at critical locations that served as crack initiation sites. Fatigue loading condition and presence of residual stresses at the weld enabled easy propagation of cracks that led to recurring premature failure. Based on the root-cause and the mechanism identified in this analysis, proper pre-heating and post-weld heat treatment is recommended to relieve the residual stresses at the critical locations and thus to avoid/minimize such recurring failures in future.

Characterization of Failure Surfaces of Epoxy Adhesive Under Various Fatigue Loading Conditions

Characterization of Failure Surfaces of Epoxy Adhesive Under Various Fatigue Loading Conditions

Numerical simulation of internal stress in pavement concrete under rolling fatigue load

ABSTRACT This study explores the development law of concrete internal stress and strain under rolling wheel loads. To do this, strain gauges are embedded in different positions inside concrete, and the stress and strain values at different positions inside the concrete are measured using the rolling fatigue test. A numerical model is established using COMSOL Multiphysics software, and the concrete internal stress and strain values are simulated. A comparison between the calculated and measured values shows that the numerical model can simulate the actual stress state inside the concrete. The numerical model can predict the internal stress and strain variation of concrete specimens under more complicated load conditions. The prediction results show that under the same fatigue load conditions, the concrete stress peak is hardly affected by the fatigue loading times, but there is a marked increase in the strain value inside the specimen. When the stress peak of concrete does not reach its design strength, an excessive increase of the internal strain will cause hidden damage to the concrete. By investigating the internal stress and strain of the concrete, a theoretical basis can be determined for the deterioration mechanism of concrete microstructure and the durability design of concrete.

Effects of elevated temperature, hydraulic pressure and fatigue loading on the property evolution of a carbon/glass fiber hybrid rod

A carbon fiber core (CFC) and glass fiber shell (GFS) hybrid rod (HFRP) with the diameter of 22 mm was developed to replace the steel rod in the application of underground oil extraction. In the present study, the variation in the properties of the hybrid rod served for 553 days was investigated subjected to the coupling conditions of elevated temperature (T), hydraulic pressure (P) and fatigue load (L)/stress level peak (S), one exposure condition, closed to the ground (T = 25 °C, P = 0.5 MPa, Lmax/S = 70 kN/11.1%) and another in the underground oil-well up to 2600 m (T = 90 °C, P = 18 MPa, Lmax/S = 50 kN/7.9%). Exposed in the deep oil-well condition led to an extra degradation of interface shear strength (ISS) compared to the ground exposure. The degradation mechanism of ISS was attributed to the hydrolysis and plasticization of resin owing to formed multiple hydrogen bonds between the water molecule and chain structures of resin. The removal of free water from the hybrid rod brought about an average increase percentage of ISS, ~10.0% for the upper rod and ~7.4% for the bottom rod. Furthermore, the maximum recovery of ISS was observed for the removal of bound water for the drying at 100 °C for 14 days. The fatigue performance evaluation of hybrid rod exposed in underground oil-well condition showed the optimal rod diameter for the present fatigue load condition (Lmax/S = 70 kN/11.1%) was 19 mm with the service life of ~50 years. Furthermore, the applied maximum fatigue load was obtained to be 114.3 kN with the fatigue life of 22.6 years for the present hybrid rod.

Mechanical behaviours of granite containing two flaws under uniaxial increasing‐amplitude fatigue loading conditions: An insight into fracture evolution analyses

AbstractThis work aims to investigate the fracture evolution of granite containing two pre‐existing flaws under uniaxial increasing‐amplitude fatigue conditions using macroscopic stress strain descriptions and posttest three‐dimensional (3D) computed tomography (CT) technique. The impacts of flaw arrangement (i.e., approach angle of 20°, 50° and 70°) on the stress strain responses, hysteresis loop shape, damage evolution and crack coalescence pattern at rock bridge segment were investigated. Results show that rock structure has an obvious impact on macroscopic stress strain responses, volumetric strain, dynamic elastic modulus and damping ratio. The sparse‐dense pattern of hysteresis loop is different at each loading stage caused by the differential accumulative damage. The damping ratio increases and dynamic elastic modulus decreases with the increasing fatigue loading stage. Posttest 3D CT visualization reveals a most striking finding that crack coalescence is easy for rock having low approach angle, and complex crack network forms for rock having high approach angle.

Research on Microstructure and Fatigue Properties of Vibration-Assisted 5052 Aluminum Alloy Laser Welded Joints

Vibration-assisted laser welding experiment of 5052 aluminum alloys was carried out to focus on the influences of vibration parameters on microstructure and fatigue fracture. The experiment found that the vibration process homogenized the microstructure and promoted the formation of fine equiaxed grains significantly. In addition, vibration made the hardness uniform by reducing the area of the fusion zone. Experimental results of double welds showed that residual stress reduced by 58 to 77% under the action of micro-vibration. Vibration frequency and vibration acceleration had significant effects on the longitudinal and transverse residual stress, respectively. Under the same fatigue load condition, the average fatigue limits of the base metal (BM) and the welded joints were 160 and 120 MPa, respectively. Under the conditions of 107 cycles, the maximum fatigue strength of the welded joints has reached 74.95% fatigue strength of the BM. Vibration-assisted laser welding could obviously improve the fatigue performance and increase the fatigue life of the welded joints, which was a potential way to improve the quality of welded joints.

Microcrack Initiation Mechanism of a Duplex Stainless Steel Under Very High Cycle Fatigue Loading Condition: The Significance of Load Partitioning and Micro Residual Stresses

Investigation on the cyclic response of metallic materials in very high cycle fatigue (VHCF) is a challenging problem, which hinders the development of fatigue theories. To overcome this difficulty, we initially applied several sensible techniques, e.g., High-energy X-ray diffraction (HEXRD), confocal laser scanning microscope (CLSM), nanoindentation and transmission electron microscope (TEM) to investigate the cyclic response of a duplex stainless steel (DSS) in the VHCF regime. In-situ XRD and in-situ digital image correlation (DIC) experiments were subsequently performed at observed changing stages, intending to explore the underlying mechanism of microcrack initiation. The first-hand results obtained revealed that the austenite phase exhibits cyclic softening-hardening-softening behavior during the VHCF process. The in-situ investigations performed at the cyclic softening and hardening stages showed a load partitioning and a load transfer between the two phases, implying the cyclic response can significantly affect the distribution of the applied load. Residual strain obtained by the DIC technique after unloading exhibited strong variations at phase boundaries, suggesting micro residual stresses have developed pronouncedly. Based on all the experimental findings, a unified crack initiation mechanism for the investigated DSS during VHCF loading was proposed.

Effect of build thickness and geometry on quasi-static and fatigue behavior of Ti-6Al-4V produced by Electron Beam Melting

The geometry and material properties of additively manufactured (AM) parts are closely related in a way that any alteration in geometry of the part will change the underlying manufacturing strategy. This in turn, affects the microstructure and consequently, the mechanical behavior of material. This paper aims to evaluate the effect of the AM part’s thickness and geometry on microstructure, surface roughness, and mechanical properties under quasi-static and fatigue loading conditions by performing experimental tests. A series of Ti-6Al-4V specimens with three different thicknesses and two different geometries were fabricated using electron beam melting (EBM). The results of microstructural analyses revealed that specimens with lower build thickness experience finer grain size, higher microhardness, and lower elongation at failure. Although the microstructure of the produced parts was strongly affected by the build thickness, different surface to volume ratios eliminated the effect of microstructural differences and governed the fatigue properties of the parts. The size effect on the microstructural features, geometrical appearance, mechanical properties of the AM parts should be considered for the design and failure analysis of complex structures.

Fracture and fatigue behavior of carbon/epoxy laminates modified by nanofibers

This research focused on fracture and fatigue response of carbon/epoxy laminates modified with electrospun nylon 66 nanofibers. For this purpose, AS4/8552 composite laminates interleaved by nanofibrous mat were considered under mode-I quasi-static and fatigue loading conditions. According to test outcomes, the fracture toughness increased about 133% under quasi-static tests. On the other hand, the plotted fatigue curves showed that the crack growth rate significantly decreased in modified samples and the threshold energy release rate enhanced about 128%. Finally, scanning electron microscopy (SEM) was also conducted to investigate the damage mechanisms.

Multiaxial fatigue behavior of additively manufactured Ti6Al4V alloy: Axial–torsional proportional loads

Additive manufacturing (AM) techniques are under constant development and selective laser melting (SLM) is among the most promising ones. However, widespread use of AM techniques in many industries is limited by the different/unusual mechanical properties of AM metallic parts, with respect to traditionally processed ones, especially when dealing with complex fatigue loading conditions. In fact, crack formation and propagation mechanisms are mainly affected by the development of internal defects, residual stresses, and microstructural changes. This is actually one of the major issues the materials engineering community is facing today. In many applications, AM components are subjected to multiaxial fatigue loads, arising from operating conditions and/or from complex geometries, that unavoidably generate crack initiation and propagation mechanisms. The aim of this study is to investigate the multiaxial fatigue behavior of additively manufactured Ti6Al4V samples, made by SLM. Fatigue tests, combining proportional axial and torsional loads, were performed on thin-walled tubular specimens. Full-field measurement techniques, such as the infrared thermography and digital image correlation, were also used to capture temperature and strain evolutions, at both local scales and global scales. Fatigue results highlighted damage mechanisms, and failure modes are strongly related to the applied stress level.

Investigation on fracture behaviors and damage evolution modeling of freeze-thawed marble subjected to increasing- amplitude cyclic loads

Rock is often subjected to fatigue loads in many mining and civil engineering, rock fatigue mechanical behaviors under constant stress amplitude have been widely investigated. Yet the increasing-amplitude fracture mechanical characteristics of rock and the associated damage evolution model are not well understood. In this work, uniaxial increasing-amplitude fatigue loading experiments were conducted using GCTS RTR 2000 rock mechanics system on marble obtained from an open-pit slope with 0, 20, 40 and 60 freeze-thaw (F-T) cycles. Results show that the previous F-T damage of rock affects the fatigue lifetime, secant modulus, Poisson’s ratio, deformation and damage evolution. The damage accumulation and volumetric deformation increase with the increase of F-T cycles. Damage growth rate becomes weaker with increasing fatigue cyclic levels and a two-stage damage accumulation pattern was found, namely, the initial damage and steady damage stage. The sudden increase moment of stress amplitude has obvious effect on rock damage. Based on the experimental data, a damage evolution model was proposed to describe the two-stage fatigue accumulation characteristics. The model is not only suitable for damage description of each fatigue loading level but also the whole loading process. It is found that the applicability of the damage evolution model is not influenced by the initial damage of rock, and will be applied to intact or jointed rock. The studies in this work are expected to improve the understanding the fatigue damage mechanism of rock subjected to multiple level fatigue loading conditions.

On the progressive fatigue failure of mechanical composite joints: Numerical simulation and experimental validation

In this study, a novel holistic progressive damage model for mechanical fiber-reinforced polymer (FRP) joints under static and fatigue loading is presented. The model is implemented as a user-defined subroutine in the finite element software ABAQUS/Implicit. First, the theoretical basis of both the static and the fatigue damage model are explained in detail. Here, special emphasis is put on the introduction of the FRP fatigue damage model (FDM), which is based on a physically-motivated hypothesis enabling the consistent coupling of static and fatigue damage properties. Furthermore, detailed insights concerning the experimental calibration of the FDM are presented for the first-time. Subsequently, the presented damage model is validated using first-hand experimental results for standard bolt bearing, as well as T-bolt bearing, test setups. The experimental bearing setups are tested and analyzed until ultimate failure, applying static and fatigue loading. It is shown that the model prognoses are in close accordance with experimental measurements and observations in terms of static joint strength, cyclic stiffness degradation and cycles to failure. The rich information provided by the damage model concerning the progressive damage evolution under static and fatigue loading conditions allows for its application within virtual test rigs for mechanical FRP joints. In that context, this work demonstrates the promising abilities and features of the energy hypothesis applied for the FDM for use-cases of significant relevance in the composite industry for the first time in literature.

Nonlinear Span Assessment by Amplitude-Dependent Linearization

Abstract Although it has long been recognized that vortex-induced vibrations of subsea pipeline spans involve nonlinear and inelastic behavior, the current practice to assess such spans for fatigue and ultimate loading conditions is based on the modal analysis assuming linear behavior. Nevertheless, nonlinearity can be captured approximately by making the linearization amplitude dependent. The eigenvalue problem to be solved for the natural frequencies and mode shapes then involves a stiffness matrix that depends on the mode shape and amplitude of vibration. An important part of the nonlinearity comes from the soil, which is generally represented by springs. This paper presents a simple and particularly effective algorithm to solve this nonlinear eigenvalue problem by using the same algorithm that serves to track the bifurcated solution branches in quasi-static structural stability (buckling) analyses. This method is applied to an example in which the nonlinearity comes from the soil springs. The results demonstrate the importance of the nonlinearity, even at relatively low vortex-induced vibrations (VIV) amplitudes typical of the pure inline response. The inelasticity of the soil springs is also used to calculate the associated contribution to the modal damping ratio.

Fatigue performance of damaged RC beams rehabilitated with GGBS based ultra high performance concrete

The study investigates the fatigue behaviour of damaged RC beams strengthened with GGBS based UHPC strips. Parent RC beams are pre-damaged under static and fatigue loading conditions. The damaged beams are rehabilitated with thin UHPC strips of various thicknesses (5, 10 and 15 mm) and tested under fatigue load until failure. The results illustrate that the fatigue damaged beams rehabilitated with thin UHPC strip (5 mm) can efficiently restore its fatigue life despite the severity of the damage. From experimental studies, the mechanism of the strain that exists in the rehabilitated RC beams subjected to static and fatigue damage is illustrated. Two-stage finite element analyses have been performed to predict the fatigue behaviour of damaged and rehabilitated beams. Various aspects considered in the numerical analysis include concrete damage model, failure criterion, stress crack-width relation and fracture energy. The deviation in the predicted numerical and the corresponding experiment results are lesser than 25%.

Effect of Electron-Beam Treatment on the Structure of Commercial-Purity Titanium Subjected to Fatigue Failure

The fracture surface, the phase composition and the state of the defect substructure of the titanium VT1-0 alloy subjected to preliminary irradiation by a high intensity pulsed electron beam and failed under fatigue loading conditions have been studied. A surface layer with a nanocrystalline multiphase structure is shown to form during fatigue tests of the samples preliminarily annealed in air. The surface layer structure of the samples after irradiation is found to differ substantially from the structure of the unirradiated samples: a 5-μm-thick surface layer, in which the grain volumes have a subgrain structure, forms.

Examining metrics for fatigue life predictions of additively manufactured IN718 via crystal plasticity modeling including the role of simulation volume and microstructural constraints

There is a growing demand for the development of precise, accurate, and industrially applicable predictive models for evaluating the fatigue performance of additively manufactured (AM) materials to accelerate their qualification. Six fatigue metrics based on microstructure-sensitive crystal plasticity finite element (CPFE) simulations are utilized to assess the fatigue performance of Inconel 718 produced via selective laser melting (an AM technique). Each metric is used to predict the probable locations of failure and the scatter in fatigue life under high cycle fatigue loading conditions. The predicted scatter from all the fatigue metrics was in good agreement with the experimental test data. The predictions locations of failure using two of the metrics, plastic strain accumulation, and plastic strain energy density, were found to correlate in a statistical sense with the post mortem fractography results. Moreover, an additional set of CPFE simulations were performed with varying volumes of the input microstructures, with the largest microstructure having a volume close to that of the test specimen's gauge section. This additional analysis was intended to provide informed guidelines for simulation volume that is both computationally tractable, and results in consistent scatter predictions, which was estimated to be a simulation volume consisting of ~200 grains. Lastly, fatigue life predictions obtained from traction-free and more realistic microstructure constraints were compared to understand the role of boundary conditions (BCs) in fatigue life predictions. This work is beneficial in determining the statistical minimum for safe-life analyses, thereby reducing the overall number of experimental tests and accelerating the qualification process.

Influence of microcork particles on the lap shear strength of an epoxy adhesive subjected to fatigue loading and different environmental conditions

The present study deals with the effects of temperature and cyclic loading on the strength of single lap adhesive joints enhanced with different amounts of microcork particles. To achieve this, different joints with various overlap lengths were manufactured and tested. The joints were tested at two different temperatures (−20 ℃ and 75 ℃) in quasi-static conditions. Results of high temperature (75 ℃) show that the joints tensile strength decreases with the amount of cork particles. For low temperature (−20 ℃), the strength of the joint with 1 vol.% of cork increased; however, higher amounts of cork led to a reduction of the joint strength. To investigate the influence of cork particles on the fatigue endurance, joints were tested at room temperature under sinusoidal cyclic loading. Based on the experimental data it was found that the cork particles can significantly improve the fatigue life of adhesive joints (up to 4.6 times). Microfailure analysis of the tested joints was also performed using the scanning electron microscope technique.