This study investigates the synergistic effect of hybrid matrix toughening on the fatigue behaviour of bidirectional glass fibre-reinforced polymer (GFRP) laminates. A novel hybrid system combining carboxylfunctionalized multi-walled carbon nanotubes (MWCNTs) and carboxyl-terminated butadiene acrylonitrile (CTBN) liquid rubber is employed to enhance the matrix fracture resistance so as to delay fatigue damage progression. MWCNTs contribute through crack bridging, bifurcation and deflection, while CTBN improves fracture energy via cavitation and plastic void growth under cyclic loading. CTBN forms bonds with functionalized MWCNTs and epoxy to achieve a uniform dispersion. Fatigue tests are conducted at a stress ratio (R) of 0.1. Infrared thermography is employed to correlate mechanical degradation with thermal signature. The hybrid filler comprising 0.25 wt% MWCNT and 5 wt% CTBN resulted in 17 % increase in tensile strength and 88-118 % improvement in the fatigue life compared to the neat specimen. In contrast, the MWCNTs alone under similar loading conditions exhibits a detrimental effect due to nanoparticle agglomeration. Fatigue stiffness degradation, hysteresis energy dissipation and temperature evolution studies demonstrate improved fatigue performance in case of hybrid composites. Fractographic analysis reveals enhanced fibre-matrix interaction and delayed fibre fracture for hybrid composite systems.

This study investigates the high cycle fatigue (HCF) performance of hybrid 316 L stainless steel (SS316L) at room temperature (RT) and elevated temperature (500 degrees C). The hybrid SS316L sample is composed of an additively manufactured (AM) zone deposited via wire-fed laser metal deposition (w-LMD) on a conventionally manufactured (CM) SS316L substrate. Computed tomography (mu CT) and X-Ray digital radiography are utilized to examine the hybrid SS316L samples to identify the defects such as porosity, lack of fusion, voids and cracks. Comprehensive mechanical characterisation (tensile, nano-hardness) and fatigue at stress ratio R = 0.1, are performed to assess the mechanical behaviour and fatigue performance of the hybrid SS316L samples. During tensile testing at RT and 500 degrees C, the hybrid SS316L samples consistently exhibits failure in the CM zone. In contrast, high cycle fatigue testing shows failure in the CM at RT and in the AM zones at 500 degrees C. Post failure electron back scattering diffraction (EBSD) analysis reveals a significant variation in the presence of Sigma 3 twin boundaries, geometrically necessary dislocations (GNDs), residual stresses and grain aspect ratio. These observations highlight the potential of wire-fed laser metal deposition (w-LMD) in the structural repair of SS316L components, which are designed for service exclusively at room temperature, as opposed to 500 degrees C.

This study investigates the fatigue response of bi-directional composites under block loading conditions. A new progressive damage model is employed for the fatigue life evaluation of the bi-directional composites. The proposed model addresses two key aspects essential for accurate fatigue life prediction of composites: (a) stress-ratio-dependent stiffness degradation, and (b) the influence of load sequence and cycle mixing on the cumulative damage evolution. Two separate damage evolution laws are given for tensile and compressive stiffness degradation. This enables a precise representation of damage progression under tension-tension and tension-compression fatigue loading. The model's predictive capability is evaluated across multiple loading scenarios, encompassing a range of load levels and stress ratios. Special emphasis is placed on assessing the role of load sequence and the nonlinear effects arising from prior compressive damage on subsequent tensile behaviour and accelerated damage induced by load reversals. The numerical predictions demonstrate an excellent agreement with the in-house experimental fatigue test results conducted on bi-directional GFRP to accurately capturing critical phenomena such as sequence-induced life reduction and stress-interaction effects. Overall, the study establishes a robust and simplified framework for the modelling of fatigue damage in composites under variable amplitude loading, offering significant advancements over conventional life prediction methods.

This study presents a robust, stress ratio-sensitive numerical framework for evaluating the fatigue behaviour of bi-directional fibre-reinforced polymer (FRP) laminated composites using continuum damage mechanics (CDM). The conventional damage evolution rate equations have been modified to facilitate the comprehensive assessment of the laminates' fatigue life, and a non-dimensional factor is introduced to accurately characterize the influence of stress ratios. The proposed numerical framework simulates progressive damage and stiffness degradation during cyclic fatigue loading. A specialized parameter identification strategy is developed to calibrate material parameters for bi-directional FRP composites. The model predictions closely align with the experimental fatigue test results conducted at different stress ratios on bi-directional glass fibre-reinforced (GFRP) composites with various stacking sequences. The study meticulously examines the effect of stress ratios on damage initiation, fatigue life, and failure mechanisms. Additionally, the research elucidates the influence of load levels on damage progression and fatigue life. The finite element simulations accurately capture the complete fatigue damage, from initial damage to final failure, capturing critical failure mechanisms such as matrix cracking, fibre-matrix debonding, and fibre breakage. This comprehensive modelling framework offers a powerful tool for predicting and enhancing the fatigue performance of bi-directional FRP composites.

Conventional modelling of leak tightness in pressure tube-end fitting (PT-EF) rolled joints is based on the standard theories of continuum solid mechanics, often implemented through macroscale finite element analysis. However, they are not capable of capturing micro or nano-sized flows of leakage, which are practically dominant in the PT-EF rolled joints. Therefore, for the first time, molecular mechanism of leak-tightness in rolled joints is formulated and analyzed in this work. A molecular dynamics (MD) based model is developed to investigate the helium leak tightness in the rolled joint. A hybrid interatomic potential based on embedded atom method (EAM) and Lennard-Jones (LJ) potential is used to define the interatomic interactions between the atoms. The leak-tightness of the rolled joint is investigated for various gap thicknesses (80 & Aring;, 100 & Aring;, 130 & Aring;, and 150 & Aring;), pressure differences (1 atm, 2 atm, 3 atm, and 4 atm), and temperatures (300 K, 400 K, 500 K, and 600 K). It is observed that change in gap thickness has greater influence on the leak-tightness than the temperature and pressure. The diffusion coefficients of helium gas are calculated at different values of temperatures and gap thickness. Subsequently, mathematical models are derived which describe the relationships of diffusion coefficient with temperature and gap thickness. The results reveal that during the diffusion of leaking atoms, the adsorption layers are formed near the zirconium-iron walls. The present model is validated with an analytical solution and available literature. It provides a microscopic understanding of leak-tightness in PT-EF rolled joints for effective engineering design.

This study presents an experimental investigation of thedamage-tolerant capability of Weldox-700 steel and its gas metal arc welding (GMAW) joints. Four different notch locations are considered for assessment, i.e., base material (BM), weld zone (WZ), and two notches in the heat-affected zone, namely HAZ1 and HAZ2. The fatigue crack growth rate (FCGR) tests are conducted at room temperature.The results indicate that BM exhibits the highest crack growth rate, whereas HAZ1 shows the minimum among all the sampling conditions. This unexpected result is primarily due to the crack closure effect. In the threshold region, HAZ1 has the highest crack growth resistance due to the roughness-induced crack closure effect, followed by HAZ2. On the other hand, after eliminating the crack closure effects, WZ shows the highest FCG resistance. Fracture surface examinations of BM revealed intergranular facets, which are rare in ductile alloys. Weld zone and HAZ1 exhibit transgranular fractures, whereas a mixture of intergranular and transgranular fracture is observed in the HAZ2 samples. These findings highlight the complex interplay among microstructure, mechanical, and FCGR behavior of the Weldox 700 steel. Furthermore, this study provides in-depth insights into the effect of microstructure, grain boundary misorientation, and crack path on the FCGR behavior of welded Weldox-700 steel.

In this work, a new multiphysics phase field model is developed to simulate hydride-driven intermittent cracking in zirconium alloys. Unlike previous phenomenological models based on continuum-damage mechanics and cohesive-zone theories, this model conforms to the physics of energy-driven crack evolution. This is due to its inherent consistency with the principle of thermodynamic-irreversibility through phase field method. In this model, the hydrogen diffusion, mechanical equilibrium and phase-field damage are coupled and solved iteratively using a staggered approach. The plastic material behaviour is incorporated in the model by introducing a plastic degradation function which captures the relative contribution of plastic work in crack growth. Implicit gradient formulation of non-local strain is incorporated in the model to suppress the mesh sensitivity during strain-softening. The proposed model successfully captures the effect of hydride formation on the fracture behaviour of the zirconium alloy. Several numerical examples are solved using the proposed model to capture the multi-field behavior of hydride-driven intermittent cracking. The developed numerical model is validated by comparing the load-displacement response and critical stress intensity factors (KIC) with the experimental results. A structured description of the physics involved and its numerical implementation is presented for better understanding.

This study presents an experimental investigation of the fatigue performance and damage distribution mechanism of bi-directional GFRP composites. Uniaxial fatigue tests have been conducted under load-control, at stress ratios, R = 0.1, 0.5 and critical stress ratio (chi =-0.9). The influence of gauge length and surface roughness on fatigue life has been examined for R = 0.1. An infrared (IR) camera is employed to monitor temperature evolution and capture thermal images during the fatigue experiments. Fatigue stiffness degradation, energy dissipated per cycle, and severity of damage progression have been analyzed to elucidate the effects of stress levels and mean stress on fatigue performance. At higher stress levels, the damage is intense and localized, resulting in relatively shorter life due to fiber-breakage accompanied by rapid fatigue stiffness degradation. At lower stress levels, the damage is uniformly distributed and less severe, primarily involves stress concentration, resulting in longer fatigue lives. The study highlights the contrasting damage progression mechanisms for tension-tension and tension-compression fatigue. Under tension-tension fatigue, an oval-shaped damage zone forms perpendicular to the loading direction indicating transverse crack propagation, while under tension-compression fatigue, the damage zone aligns parallel to the loading direction indicating longitudinal crack propagation due to compressive loading.

This study presents a novel experimental approach to evaluate the fatigue performance of bi-directional glass fiber-reinforced polymer (GFRP) composites under variable amplitude sequential block loading conditions. The proposed method incorporates advanced loading protocols to simulate realistic service conditions, capturing the complex nonlinear stress interactions and damage progression unique to composite materials. The experimental setup ensures precise control over load sequences and stress blocks, facilitating a comprehensive understanding of fatigue performance and failure mechanisms based on fatigue life, stiffness degradation and thermal response. Results demonstrate a significant influence of load sequence, cycle mixing and stress level on fatigue behaviour, providing critical insights into material performance and damage evolution. Key findings highlight the pronounced effect of high-to-low (H-L) compared to low-to-high (L-H) loading regimes on fatigue performance, showing up to 38 % reduced fatigue life. The study also demonstrates the influence of crack initiation and propagation rates on fatigue performance under both loading regimes. A load sequence sensitive damage law has been proposed to capture the nonlinear damage accumulation under variable loading conditions. This work provides a robust framework for improving the design and reliability of GFRP composites subjected to variable amplitude loading in engineering applications.

The Al-Si cellular microstructure in additively manufactured AlSi10Mg plays a pivotal role in dictating its mechanical properties, such as strength and work-hardening. However, the micro-mechanism leading to the inter-relationship between the mechanical properties and microstructure is not yet well understood. Therefore, this investigation explores the processing-structure-property correlation in AlSi10Mg using a realistic 3D microstructure-based crystal plasticity (CP) approach. The work hardening in AlSi10Mg has been explored based on two mechanisms: (i) load bearing by the Si-phase and (ii) dislocation-driven hardening. The strain incompatibility introduced by the harder Si-phase generates geometrically necessary dislocations, which influence the hardening behavior. The role of the slip system interaction based on the slip activity on all active slip planes was also explored in understanding the work hardening behavior. The transformation in silicon morphology after the heat-treatment changes the interaction of dominant slip systems, thus leading to higher work hardening in the as-built condition during tensile deformation. The result of this study predicts that the tailoring of the Al-Si cellular structure can help to achieve the desired mechanical properties in the additively manufactured AlSi10Mg.

A numerical model for hydride embrittlement in Zirconium alloy (Zr–2.5Nb) is developed utilizing the extended finite element method (XFEM). Hydride embrittlement reduces the ductility and failure time of a metal/alloy. During hydride embrittlement, stress-directed hydrogen diffusion, metal-hydride phase transformation, mechanical deformation, and hydride precipitation occur simultaneously. The present model incorporates all these processes and is able to predict the hydrogen concentration and the hydride fraction distribution under any externally applied stress field. In this work, both the steady and transient hydrogen diffusion cases are evaluated. Further, the XFEM is utilized to develop a model of hydride embrittlement in the presence of a crack. The first step of the hydride embrittlement process is the diffusion of hydrogen. According to Fick’s law of diffusion, hydrogen diffusion is directly dependent on hydrostatic stresses and hydrogen concentration gradient under external stresses. The next step is the hydride precipitation in hydride embrittlement, where the expansion of material takes place that changes the hydrostatic stress field. Thus, studying the effect of precipitation of hydride on hydrostatic stresses is essential. Moreover, the process of hydride embrittlement is highly influenced by residual stresses in the structure. Hence, the effect of residual stress present in the zirconium alloy pressure tube (PT) is also evaluated. The results indicate that the residual tensile stresses contribute to the growth of hydride, which will reduce the material failure time.

This study investigates the mechanisms for cyclic failure of base and welded weldox-700 steels under high cycle fatigue regimes. Strain-controlled tensile tests at a strain rate of 10-3/s and loadcontrolled axial fatigue tests at different stress ratios, i.e., -1, 0.1, and 0.5 have been conducted on base and weld materials. It has been observed that the fatigue strength of the welded weldox700 steel is found significantly lower than the base material, despite a higher tensile strength of the weld material. The effect of weld parameters and surface roughness on fatigue failure has been analyzed. The fracture surface is further investigated using a scanning electron microscope that reveals the micro-mechanisms of fatigue failures. Multiple crack initiation sites have been observed for both base and weld materials. The welded samples exhibit unusual fatigue failure where a crack is initiated from the heat-affected zone and weld toe/root. The increased stress concentration, surface irregularities, and microstructure inhomogeneity are the dominant factors for poor fatigue performance of the weld materials.

The conventional strategy for simulating thermo-mechanical failures using localizing gradient damage is based on an elastic material-model. It does not incorporate the physics of plastically-driven failures under combined thermal and mechanical loads. Moreover, the conventional formulation neglects the effect of damage on heat-capacity and avoids certain essential physics-based couplings among the deformations, damage and temperature. Therefore, in this work, a novel computational framework based on non-localizing and localizing gradient damage is developed for simulating thermo-elasto-plastic failure of materials, under the influences of both mechanical and thermal loads. The present strategy is derived from the law of thermodynamic power-balance and the free-energy density function. Unlike previous works, the present framework considers the effect of damage-based degradation on both thermal conductivity and heat-capacity. A new set of constitutive relations for thermo-elasto-plastic damage are developed in incremental form to incorporate the stress fields, local and non-local equivalent plastic strains, damage and temperature. Using these constitutive equations, new formulations of coupled-stiffness matrices and heat-capacity matrices are derived in the context of gradient damage. The cross-influences of damage, temperature and deformation on each other are incorporated through these matrices. The capability of the present framework is demonstrated by solving several examples on thermo-elasto-plastic ductile failures using finite element approach.

The present work aims to study the influence of hot deformation on microstructural development and flow behavior in the Al-4.8Mg-0.3Sc alloy produced by the laser powder bed fusion (LPBF) technique. Uniaxial compression tests have been conducted within a temperature regime of 200-350 degrees C at a 0.01-1 s(-1) strain rate using a Gleeble-3800 (TM) thermomechanical simulator. The major results show that the flow behavior is governed mainly by strain hardening at 1 s(-1) strain rate and dynamic recrystallization (DRX) at 0.1 s(-1) within 250-350 degrees C. The constitutive equations have been developed by employing activation energy (Q) and other material constants to forecast the influence of deformation temperature and strain rates on flow stress. Compared to other Al alloys (114-227 kJ/mol), the mean Q for hot deformation is found to be significantly higher (similar to 340 kJ/mol at a 0.69 true strain), indicating more stress requirement for deformation, also confirmed by the flow curves. Moreover, the processing map developed with MDMM+Poletti instability criteria is found to be appropriate compared to DMM and MDMM models. The safe workable zone is obtained in the range of 250-350 degrees C/0.01-1 s(-1) with a maximum power dissipation efficiency of 45.8 %. Microstructural analysis shows that recrystallization starts primarily at melt pool boundaries which formed during the LPBF process. The highest recrystallization fraction is observed for the specimen deformed at 350 degrees C/0.01 s(-1) (59.3 %). SEM analysis of the samples deformed at 200 degrees C/0.01-1 s(-1) and 250 degrees C/1 s(-1) depict the formation of various defects, such as voids and micro-cracks, mainly governed by the non-uniform deformation at the particle/matrix interface and due to the presence of voids/pores. A detailed investigation of Q, stress exponent (n), flow stress behavior, and constitutive equations suggests that the hot deformation is mainly governed by both dislocation climb and cross-slip mechanisms.

The original computational framework of elasto-plastic localizing gradient damage, also called as the localizing gradient plasticity (LGP) model, considers that damage in a material causes reduction of yield strength alone. It does not account for the physical process of stiffnessdegradation due to damage of ductile materials. Therefore, in this work, a new constitutive model of elasto-plastic localizing gradient damage is developed which incorporates the effect of damage on stiffness as well as yield strength. A thermodynamic basis is provided for the model through the law of energy-balance. Constitutive relations are developed in variational form and incorporated into a finite element framework which has fully-coupling between deformations and non-local equivalent plastic strains. Damage is induced in the material by these non-local strains. Moreover, the original localizing gradient plasticity model suffers from oscillating stress fields in its computations. To counter this issue, an anisotropic form of non-local averaging is used in the present work. Accordingly, a smoothed stress-based gradient matrix is used to model the nonlocal interaction region. The capability of the present model is investigated through several problems of ductile fracture and compared with the conventional model and some experimental evidences. It is observed that the proposed model satisfactorily captures the material-softening induced by damage.

In the present work, gradient plasticity-based damage methodology has been employed to capture the plastic softening behaviour under monotonic loading at elevated temperatures. The conventional approach to capture the stress-strain curve of materials using classical elastoplasticity-based simulation often suffers from its incapability to accurately capture the softening curve. This drawback becomes more pronounced at elevated temperatures where an early onset of softening occurs. Therefore, the present work investigates the stress-strain softening behaviour of steels under tensile loads at elevated temperatures through gradient based elasto-plastic damage methodology. The framework, originally developed by Engelen et al. (2003), has been modified by using a temperature-dependent damage evolution law. The damage variable, which is incorporated in the yield function, captures the accumulation of damage through non-local strains. The gradual reduction of stiffness with increasing load captures the softening behaviour of the material. Two example problems are numerically investigated, firstly a S900 steel specimen and secondly a Q960 steel specimen. From experimental evidences, it is found that the present methodology satisfactorily captures the plastic softening upto 700˚C for both materials. The present results are also compared with classical von-Mises elastoplasticity through Ramberg-Osgood behaviour, and it is observed that the present methodology provides better agreement with the experimental data of S900 steel (Narimani et al., 2023) as well as Q960 steel (Wang et al., 2020).

In this work, numerical simulations of high-cycle fatigue and fatigue crack growth have been performed using localizing gradient damage methodology (LGDM) for base and welded weldox-700 steel. LGDM models crack growth phenomenon by avoiding any non-physical widening of damage bands. The effect of stress ratio is included in the strain-based damage evolution in the present work. Accordingly, the generalized expressions of damage parameters have been derived. Experimental fatigue tests are performed on the base material and the welded joints. In order to numerically investigate the welded joints, residual stresses developed during welding are obtained through sequentially coupled thermo-mechanical finite element analysis. Subsequently, the fatigue-life and crack-propagation are simulated by applying the residual stresses as a pre-existing field in the LGDM framework, which uses full coupling between deformations and non-local equivalent strains in the finite element analysis. The simulated results and experimental fatigue data are found to be in good agreement.

The present work describes the usefulness of artificial neural networks (ANN) for predicting residual stresses in a butt-welded plate joint. The motivation for the present work is driven by the recently emerging usage of machine learning techniques to reduce computational cost of finite element (FE) analysis. A sample specimen with unequal plate thickness, often used in aircraft panels and shipyards, is investigated here, in order to consider the effects of plate thickness variation in the prediction process. Datasets for building ANNs are generated from sequentially coupled thermal and elasto-plastic FE analysis of the welded specimens using varying inputs like length, width, thickness of plates, weld power, and welding speed. The longitudinal and transverse residual stresses in the domain are obtained as outputs for every set of inputs. ANN models comprising of different activation functions and hidden layers are trained and tested using inputs of three-dimensional coordinate points of the specimens along with the inputs–outputs of FE simulations. The ANN models are then used to predict residual stresses using new input data, and their validity is investigated through FE simulations. It was observed that predictions of ANN models agree well with the simulations, thereby establishing the ANNs as powerful alternatives to FE analysis of welding. For the welding process and material investigated in the present work, the ANN scheme can be used to predict the residual stresses at a considerably faster speed than the conventional FE computations for varying weld-process parameters and geometric parameters.

Accurate fatigue life prediction of polycrystalline materials is crucial for many engineering applications. In polycrystalline materials, a significant portion of life is spent in the crack nucleation phase at the microstructural scale. Hence, the total fatigue life shows high sensitivity to the local microstructure. To predict fatigue life accurately, the microstructure models of polycrystalline material i.e., titanium alloy are virtually generated with the help of the Voronoi tessellation technique. These models incorporate critical microstructural features such as grain size, grain shape, and the volume fraction of different phases within the material. To efficiently predict microstructure sensitive fatigue life, the smooth finite element method (SFEM) is coupled with continuum damage mechanics (CDM). The SFEM provides flexibility in the meshing of complex microstructure geometries as it alleviates the need to use only triangular and quadrilateral elements. Moreover, there is no need of isoparametric mapping and explicit form of shape function derivatives in SFEM, hence it requires less computation time. To obtain the fatigue life (in number of cycles), jump in cycles algorithm is implemented using SFEM-CDM. The numerical results of fatigue life data obtained from simulations are compared with experimental data, which reveals the validity of the present approach. This approach is useful to find out the scatter in fatigue life data of polycrystalline materials along with the source of scatter.

Standard non-local gradient damage methodology for fatigue analysis has an intrinsic drawback of unusual widening of the damage zone. This causes a rapid growth of crack in the simulations which often violate experimental evidences. In order to tackle this undesirable behaviour, the localizing gradient damage methodology has been formulated for high cycle fatigue crack growth simulations. The framework comprises of coupling damage and elasticity through continuum mechanics, a fatigue damage law and an interaction function which reduces the influence of damaged regions on the surrounding locality. The present scheme prevents the spurious widening of the damage-band around the critically damaged area and therefore the non-physical growth of fatigue crack in the simulations is successfully countered. The developed framework is tested on various standard specimens under mode-I and mixed-mode high cycle fatigue loads. Nonlinear finite element analysis is used for this purpose. The discretized form of solver equations for the localizing framework is mathematically derived. Numerical examples show that the simulated crack-growth curves using proposed localizing framework agree closely with the experimental data and has a higher accuracy than the standard non-local framework.