Reversed Loading Conditions Research Articles

Fatigue analysis on flax-epoxy composites: insights and implications

This study investigates the performance of flax fiber and epoxy resin composites, focusing on fatigue testing, which is scarcely reported in the literature. Fatigue behavior is evaluated under full reverse bending loading conditions: R = −1. Apart of the definition of the classic S-N curve, the analysis deeply investigated the evolution of the hysteresis leaf generated during each cycle over the entire fatigue life. This approach allows evaluating interesting parameters such as the evolution of the loss factor and the apparent modulus over life, being these informations fundamental for practical application of natural-fiber composites in engineering applications.

A survey of the high-cycle fatigue performance of additively manufactured alloy 718

This paper presents a survey of the published high-cycle fatigue (hcf) data for the nickel–iron superalloy alloy 718 fabricated by additive manufacture. Approximately 680 fatigue data points were collected from the published literature and the reported data are presented in the form of stress versus cycles to failure curves for load ratios of R = −1, 0 and 0.1. Following this, curves showing estimated survival probabilities of 0.5, 0.95 and 0.99 are constructed using a statistical analysis based on the Weibull probability density function. Finally, a comparison between the fatigue performance of additively manufactured and wrought alloy 718 under fully reversed loading conditions ( R = −1) is provided.

Extended Gurson-Tvergaard-Needleman model considering damage behaviors under reverse loading

Void nucleation, growth, deformation and coalescence are the main microscopic damage mechanisms for plastic deformation of sheet metals. To describe the effect of reverse loading on the microscopic damage accumulation and ductile fracture, the damage evolution equations in regard of void nucleation, void growth, shear void nucleation and shear deformation damage were modified. Through the further introduction of the combined hardening model, an extended Gurson-Tvergaard-Needleman (GTN) model was constructed in this study, to improve the predictive ability of ductile fracture under reverse loading conditions. Two typical reverse loading experiments including compression-tension and shear-reverse shear were performed. The number, size and shape of microscopic voids were qualitatively analyzed and compared through the microscopic observations, it is indicated that the preloading and loading sequence strongly affect the damage accumulation and the final ductile fracture, which is the microscopic damage mechanisms under reverse loading. The proposed GTN model was verified by the load responses and displacements at fracture (DAFs) of the conducted reverse loading experiments. The good agreement between numerical simulations and experimental results proves the rationality of the proposed GTN model and its accuracy to the prediction of ductile fracture under reverse loading conditions.

Numerical analysis of non-proportional biaxial reverse experiments with a two-surface anisotropic cyclic plasticity-damage approach

This paper deals with the numerical analysis of ductile damage and fracture behavior under non-proportional biaxial reverse loading conditions. A two-surface anisotropic cyclic elastic–plastic-damage continuum model is adequately presented, which takes into account the Bauschinger effect, the stress-differential effect, and the change of hardening rate after reverse loading. An efficient Euler explicit numerical integration algorithm, based on the inelastic (plastic or plastic-damage) predictor-elastic corrector approach, is utilized to analyze the stress and finite strain loading histories. Detailed discussions are provided on different numerical integration-related consistent tangent operators that achieve convergence within the global Newton–Raphson scheme. The proposed continuum model is implemented into the commercial software Ansys as a user-defined subroutine (UMAT). Furthermore, the novel non-proportional biaxial tensile reverse experiments are performed to validate the proposed continuum model. The associated numerical simulations investigate the stability and accuracy of the proposed algorithm and material model.

Damage and fracture behavior under non-proportional biaxial reverse loading in ductile metals: Experiments and material modeling

This paper deals with a modified anisotropic stress-state-dependent plastic-damage continuum model incorporating combined hardening and softening rules to predict the plastic, damage, and fracture behavior of ductile metals under monotonic and reverse loading conditions. In the experimental part, a newly designed biaxially loaded cruciform flat HC-specimen of aluminum alloy EN AW 6082-T6 was subjected to biaxial non-proportional loading to generate a wide range of stress triaxialities during experiments. Thus, it is enabled to perform shear monotonic and reverse loading superimposed by various preloads without unloading processes. The experimental data reveal the Bauschinger effect, the strength-differential (SD) effect, and the change in the hardening ratio after shear reverse loading. This is captured by a Drucker–Prager-type yield criterion with an extended Voce isotropic hardening and modified Chaboche’s kinematic hardening rule. In addition, the damage surface is assumed to be translated in the direction of the rate of the damage strain. The numerically predicted global force–displacement curves and local strain fields are verified in terms of the digital image correlation (DIC) technique. Moreover, the scanning electron microscopy (SEM) is used to examine the fracture surfaces and to validate the proposed damage constitutive model.

Characterization of ductile damage and fracture behavior under shear reverse loading conditions

AbstractThis paper deals with experimental and numerical analysis of the ductile damage and fracture behavior undergoing biaxial non‐proportional shear reverse loading conditions. In the numerical analysis, an anisotropic cyclic plastic‐damage constitutive model is considered to characterize the damage and fracture behavior. Additionally, the change in the hardening rate is observed after shear reverse loading. Thus, a newly modified non‐hardening strain region method is introduced to capture the change in the hardening ratio between isotropic and kinematic hardening. Furthermore, the damage surface is assumed to translate in the direction of the damage strain increment. Hence, a softening law based on the damage strain is proposed. The modified constitutive model predicts macroscopic force‐displacement and microscopic evolution of plastic and damage behaviors under shear reverse loading with a tensile preload. Through digital image correlation technique and scanning electron microscopy, numerical results can be validated by experimental ones. Furthermore, scanning electron microscopy pictures also reveal different damage and fracture mechanisms on the micro‐level.

Numerical modeling of cyclic softening/hardening behavior of carbon steels from low- to high-cycle fatigue regime

The aim of this study is to characterize the stress–strain behavior of three construction steels (SM490, SM570, and F18B) through both experimental and numerical investigations. The material performance was evaluated by conducting tests on round bar specimens subjected to monotonic, fatigue, and incremental step fully reversed loading conditions. The experimental campaign was conducted to provide valuable information on the mechanical performances of the steels and data for calibrating the material constants required for numerical analyses. The numerical simulations aimed to demonstrate the effectiveness of the proposed unconventional plasticity model, the Fatigue SS model (FSS), in describing the non-linear behavior of the materials under a broad range of loading conditions, including stress states below and beyond the macroscopic yield condition. This aspect is a significant advantage of the FSS model, as conventional elastoplastic theories fail to provide a phenomenological description of inelastic material deformation under stress states within the yield condition. The good agreement between the experimental and numerical results confirms the validity of the calibration of the material constants and the reliability of the computational approach.

Effect of Defect Variability in Aluminum Alloys on Ultrasonic Fatigue Performance across Additive Manufacturing Platforms

Abstract Optimization of process parameters is a critical step toward realization of component builds across multiple additive manufacturing (AM) platforms and broadening the deployment of AM processes. The main focus of the present work is the quantification of defect variability and fatigue performance of builds across multiple laser powder bed fusion (L-PBF) equipment. This study of variability is predicated upon the hypothesis that, given process variables yielding a similar percentage/type of porosity based on process maps for multiple AM platforms, the defect size distribution across them should be similar. Aluminum alloy (AlSi10Mg) specimens of cylindrical geometry built on three different L-PBF machines are characterized using computed tomography. The distributions of defect size are compared using Kolmogorov-Smirnov tests. Ultrasonic fatigue testing is used to quantify fatigue properties of the builds from the different L-PBF machines under fully reversed loading conditions. Finally, fatigue performance is correlated to the defect distribution in builds across the three platforms.

Analysis of ductile damage evolution and failure mechanisms due to reverse loading conditions for the aluminum alloy EN‐AW 6082‐T6

AbstractThis paper deals with experiments and numerical simulations for reverse uniaxial tension‐compression and shear tests to investigate the ductile damage and fracture behavior of the aluminum alloy EN‐AW 6082‐T6. For this purpose, experiments with different loading sequences and number of loading cycles with uniaxial tensile and new shear specimens have been performed. Furthermore, the digital image correlation (DIC) technique captures the deformations and strain fields during the experimental processes. A modified anisotropic stress‐state‐dependent elastic‐plastic‐damage continuum model is proposed and is validated by experimental strain fields measured by DIC. Moreover, numerically predicted distributions of the principal damage strains are compared with fracture images taken by scanning electron microscopy (SEM).

A COMPREHENSIVE STUDY OF FATIGUE CRACK INITIATION AND GROWTH UNDER VERY HIGH CYCLE TORSIONAL FATIGUE LOADING

This paper investigates the fatigue behavior of smooth specimens made of VT3-1 titanium alloy under fully reversed loading conditions. Mathematical modeling results are presented for the fatigue fracture of smooth specimens under high cycle torsional fatigue loading. The fatigue quasi-crack initiation and growth are calculated using a multimode two-parameter model of fatigue damage accumulation. The surface and subsurface quasi-crack initiation under torsional loading is studied. The numerical results are in good agreement with experimental data and can be used to predict the change in crack growth mechanisms under complex multiaxial loadings such as torsion.

Evolving asymmetric and anisotropic hardening of CP-Ti sheets under monotonic and reverse loading: Characterization and modeling

Commercially pure titanium (CP-Ti) sheets are promising to manufacture high-performance and lightweight components in many fields such as aerospace, marine, energy, electrics and healthcare. However, due to its low symmetric hexagonal close-packed (HCP) structure and complex thermal-mechanical processing history, CP-Ti sheets may exhibit significant asymmetry and anisotropy under monotonic and reverse loading. Therefore, the coupling effect of loading-path dependent asymmetric and anisotropic hardening behaviors of CP-Ti sheets need to be systemically characterized and accurately described under monotonic and reverse loading paths. In this study, firstly, the rheological tests were performed to systemically explore the evolving asymmetric and anisotropic hardening behavior of CP-Ti sheet under monotonic and reverse loading modes. With the help of anti-buckling support fixture and DIC system in tension and compression tests, the coupling effect of asymmetric and anisotropic hardening under monotonic and reverse loading of CP-Ti sheet was identified. Secondly, a loading-path dependent constitutive modeling framework was constructed in which the stress invariants-based model (SIM) integrated with distorted hardening (DH) under reverse loading (RL) conditions (SIM+DH+RL) were considered. Within the proposed modeling framework, a judging criterion of stress reversal for explicit solution method and an activation criterion of different reverse loading modes were introduced. Finally, the proposed SIM+DH+RL model was implemented, calibrated and further applied to simulate typical V-/U-bending of CP-Ti sheets. The results show that the proposed SIM+DH+RL model can significantly improve the prediction accuracy of V-/U-bending springback with the relative errors less than 7.8% and 2.1%, compared with 15.2% and 12% relative errors of Mises+IH, 19.5% and 7.3% relative errors of SIM+DH, 15.1% and 8.4% relative errors of Yoon2014+IH, 14.5% and 5.2% relative errors of CPB06+IH, and 17.3% and 11.5% relative errors of Hill48+IH. It is noted that for the thickness distributions after V-/U-bending springback, the above used models have similar prediction accuracies with the relative errors less than 1.8%.

Slow fatigue crack growth in 2024-T3 and Ti-6Al-4V at low and ultrasonic frequency

Abstract Fatigue crack growth in 2024-T3 has been studied in ambient air and in vacuum at load ratios R = –1, R = 0.05 and R = 0.5 using ultrasonic equipment (cycling frequency 20 kHz) and servo-hydraulic equipment (20 Hz). In vacuum, no strain rate influences were found and similar growth rates and threshold stress intensities were measured at both frequencies. In ambient air, threshold stress intensities were similar at 20 Hz and 20 kHz and were 53 –62% of the respective values measured in vacuum. Above threshold, fatigue crack growth rates at ultrasonic frequency are slower (at R = –1) or similar (at R = 0.5) to growth rates at 20 Hz. Ultrasonic fracture mechanics tests in Ti-6Al-4V at load ratios R = 0.1, R = 0.5 and R = 0.8 in ambient air delivered threshold values similar to cycling frequency 50 Hz, whereas growth rates above threshold are approximately a factor of 3 higher at 20 kHz. The compressive part of a load cycle under fully reversed loading condition causes additional fatigue damage, and the maximum stress intensity factor at threshold is lower at R = –1 than at R = 0.05 or R = 0.1.

Experimental study on mechanical behaviour of replaceable energy dissipation connectors for precast concrete frames

An innovative replaceable energy dissipation connector (REDC) is proposed in this paper that can be used as a connecting component as well as a metal-yield damper in the beam end of a precast concrete frame. In addition to the precast concrete frame for the REDC (REDC-PCF), a construction and post-earthquake repair method for it is proposed. An experimental study on the mechanical behaviour of REDCs under cyclic reverse loading was conducted to study their axial stiffness, hysteretic behaviour, energy dissipation capacity, and other properties. The results revealed that REDCs manufactured using high ductility steel exhibited a stable hysteretic property and high low-cycle fatigue performance under cyclic reverse loading conditions. On the basis of the test results, the REDC has a promising future in seismic design because it can be widely applied in precast concrete frames in high-seismic activity zones. A numerical analysis of the transverse displacement of the mechanical property of the REDC was also conducted. The results showed that the transverse displacement of the specimen had little effect on its mechanical property and low-cycle fatigue capacity. Thus, the uniaxial testing of REDC specimens would be effective in predicting the properties of beam–column joints.

Experimental Evidence of Specimen-Size Effects on EN-AW6082 Aluminum Alloy in VHCF Regime

The present paper investigates the influence of the specimen size of EN-AW6082 wrought aluminium alloy subjected to very high cycle fatigue (VHCF) tests. The hourglass specimens were tested under fully reversed loading condition, up to 109 cycles, by means of the ultrasonic fatigue testing machine developed by Italsigma® (Italy). Three specimens groups were considered, with a diameter in the middle cross-section ranging from 3 mm up to 12 mm. The stress field in the specimens was determined numerically and by strain gauge measurements in correspondence of the cross-section surface. The dispersion of experimental results has been accounted for, and data are reported in P-S-N diagrams. The decrease in fatigue resistance with increasing specimen size is evident. Theoretical explanation for the observed specimen-size effect is provided, based on Fractal Geometry concepts, allowing to obtain scale independent P-S*-N curves. The fatigue life expectation in the VHCF regime of the EN-AW6082 aluminium alloy full-scale components is rather overestimated if it is assessed only from standard small specimens of 3 mm in diameter. Experimental tests carried out on larger specimens, and a proper extrapolation, are required to assure safe structural design.

Mixed formulation for geometric and material nonlinearity of shear-critical reinforced concrete columns

This study presents a new beam element formulation following a Hellinger-Reissner variational principle for shear critical reinforced concrete members, and considering large displacement and rotation effects. The corotational formulation is used to describe the large displacement at the element nodal level and degenerated Green-Lagrange strain measures are used at the basic element level. A new displacement shape function has been developed considering section kinematics and compatibility conditions to satisfy stability criteria. New explicit equations for the internal geometric stiffness matrix to consider large deformation effect has been developed. A robust state determination algorithm for the large deformation mixed-based formulation is proposed. The Timoshenko beam theory has been adopted to consider shear deformations in deriving the section kinematics equations. The multi-axial stress state in reinforced concrete fibre has been simulated through the fixed crack smeared softened membrane model. The developed shear fibre beam element is capable of accurately reproducing the experimentally-observed large displacement effect on the post-peak shear strength degradation in the softening region. The accuracy and efficiency of the mixed-based formulation was evaluated by examining the responses at local and global levels for both monotonic and reverse cyclic loading conditions along with the process of reproducing experimentally-observed failure modes.

Variable Amplitude Fatigue Life Prediction of Rock Samples Under Completely Reversed Loading

Many rock structures may experience cyclic loading with variable amplitude. Therefore, considering the fatigue behavior of rocks subjected to different loading sequences is necessary. In this study, the accumulated fatigue damage for a Green onyx rock sample with two-step high to low sequences of loading levels was investigated under completely reversed loading condition. In order to apply completely reversed loading to the rock specimens, a new rotating fatigue test machine is employed. A comparison between the predicted behavior of Manson damage curve approach and experimental data was conducted. The results showed that the applicability of Manson damage curve approach depends on the fatigue life ratio of specimens subjected to high-low loading conditions. A new damage curve model was proposed based on the experimental observation and verified with a large number of laboratory tests. Also, the results manifested the higher loading level plays a significant role in the fatigue life of samples. Since the amplitude of the dynamic load conducted on the rock structures is not constant, it is necessary that variable amplitude loading to be taken into account. Otherwise, the prediction of fatigue life is overestimating.

Uncertainty-quantified parametrically homogenized constitutive models (UQ-PHCMs) for dual-phase \u03b1/\u03b2 titanium alloys

This paper develops an uncertainty-quantified parametrically homogenized constitutive model (UQ-PHCM) for dual-phase α/β titanium alloys such as Ti6242S. Their microstructures are characterized by primary α-grains consisting of hcp crystals and transformed β-grains consisting of alternating laths of α (hcp) and β (bcc) phases. The PHCMs bridge length-scales through explicit microstructural representation in structure-scale constitutive models. The forms of equations are chosen to reflect fundamental deformation characteristics such as anisotropy, length-scale dependent flow stresses, tension-compression asymmetry, strain-rate dependency, and cyclic hardening under reversed loading conditions. Constitutive coefficients are functions of representative aggregated microstructural parameters or RAMPs that represent distributions of crystallographic orientation and morphology. The functional forms are determined by machine learning tools operating on a data-set generated by crystal plasticity FE analysis. For the dual phase alloys, an equivalent PHCM is developed from a weighted averaging rule to obtain the equivalent material response from individual PHCM responses of primary α and transformed β phases. The PHCMs are readily incorporated in FE codes like ABAQUS through user-defined material modeling windows such as UMAT. Significantly reduced number of solution variables in the PHCM simulations compared to micromechanical models, make them several orders of magnitude more efficient, but with comparable accuracy. Bayesian inference along with a Taylor-expansion based uncertainty propagation method is employed to quantify and propagate different uncertainties in PHCM such as model reduction error, data sparsity error and microstructural uncertainty. Numerical examples demonstrate the accuracy of PHCM and the relative importance of different sources of uncertainty.

Cyclic hardening and softening behavior of the low yield point steel: Implementation and validation

The low-yield-point (LYP) steel exhibits complicated cyclic hardening and softening response under reversed loading condition when applied in the seismic control and isolation system, thereby the evaluation of its hysteretic behavior demanding a sophisticated constitutive model with robust multi-dimensional implementation for reasonable finite element analysis. In this paper, the implementation and validation of an advanced constitutive model for LYP steel developed in a previous study is illustrated, which formulates the cyclic hardening and softening features by decomposed A-F hardening laws with strain memory effect and a novel concept of “transformation zone” with multi-stage evolution rule, respectively. To convert the theoretical model into numerical utilization, a modified radial return algorithm is proposed for the fully-implicit stress integration, and the corresponding consistent tangent moduli are derived. Moreover, the constitutive model is incorporated into a general finite-element software package based on the ABAQUS UMAT platform to facilitate its universal application. A number of experimental results, including the material coupons, the shear panels/links, and a frame-shear wall system made of LYP steel, are collected to validate the model ranging from material level to structural level. The close fit between simulations and experimental results indicates that the developed model is capable of accurately describing the nonlinear cyclic hardening and softening properties with memory effect for LYP steel, which proves the model suitable for the structural seismic simulation and analysis.

Life Prediction of Spur Gear Under Fully Reversed Loading Using Total Life Approach and Crack-Initiation Method in FEM

This paper focus on the comparative fatigue life prediction of spur gears based on finite element method under fully reversed load conditions. Gears being the vital components of any automobiles, power generation systems and in heavy machinery industries, need to have good fatigue properties such as fatigue life, endurance limit and fatigue strength for better life and performance of the equipment or machinery. Therefore, the main aim of this study is to simulate fully reversed loading conditions in the fatigue life prediction on general gear materials, SAE materials like ALSI4027, SAE1045-450-QT, SAED 5506 and SAE5160-825-QT. The finite element method (FEM) has been performed on the gear models to observe the distribution of stress and damage. A comparison was made on the fatigue life and the results were analyzed. Finally, conclusions were given.

A rotating gear test methodology for evaluation of high-cycle tooth bending fatigue lives under fully reversed and fully released loading conditions

Gear bending fatigue life is a primary consideration in gear design. Each gear tooth is subjected to cyclic root stresses that may cause fatigue fracture. A gear in a transmission is subjected to either fully released or fully reversed loading conditions, with different fatigue lives. This study develops an experimental methodology for evaluation of high-cycle tooth bending fatigue lives of gears under both loading conditions. Unique specimens for tooth bending fatigue and an adaptive diagnostics method for test termination are designed while root strains under high-speed conditions are quantified. Example fatigue tests are presented to demonstrate the effectiveness of the proposed methodology.