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.

Minor damages occurring in composite materials during operation often necessitate repair over discard. This study investigates the critical factors influencing such repairs: patch size and the adhesive bond at the patch-parent interface under pure shear, both in static and fatigue loading conditions. Initially, the specimens were subjected to static loading, and upon repairing a damaged composite laminate with 30 mm and 50 mm square patches, the ultimate shear strength was enhanced by 1% and 28%, respectively, compared to the original damaged sample. Further, modifying the patch parent interface with varying graphene nanoparticle (GNP) concentrations (0.3, 0.5, 0.7, and 0.9 wt%) resulted in 1% to 7% change in the ultimate shear strength. In case of fatigue loading, among the GNP-reinforced patch-repaired samples, all concentrations demonstrated notable improvements in fatigue life, with the 0.5 wt% GNP addition yielding the highest improvement of 285% compared to the unmodified samples. The damage events captured by the acoustic emission (AE) sensors are investigated to classify the different types of damage that occurred in the specimen during testing. Finally, various damage and failure patterns in the specimens were analyzed and reported using Digital Image Correlation (DIC) and infrared imaging.

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.

In this work, the post-repair behavior of a quasi-isotropic laminate was studied in a high-velocity impact environment. The composite laminates were repaired using an external bonded patch repair procedure. Impact tests were performed to understand the effect of patch size and the result of filling the damaged region with a neat epoxy and hardener mixture. Two different size square patches, such as 30 and 50 mm, were used to analyze the effect of patch size. A continuum damage mechanics-based model was developed and incorporated into a finite element model interface to estimate the optimum patch size and patch stacking sequence for the impact environment. In the finite element model, the intralaminar failure criteria were implemented using the ABAQUS-VUMAT user subroutine, and the interlaminar failure criteria were implemented using the inbuilt cohesive contact properties. The 30 mm patch with resin filled in the damaged region performed better than the other combination patches. Also, placing 45 degrees plies toward the outer layer of the patches showed lower damage compared to the patches in which 45 degrees plies were placed at the inner surfaces.Highlights Post-repair impact behavior of repaired laminates was examined. Impact of patch dimension and resin filling in the damaged area was analyzed. A 3-D finite element model was developed to optimize patch parameters. Continuum damage mechanics model has been incorporated with VUMAT subroutine. Interface between lamina has been modeled using cohesive contact. Understanding the effect of patch size, stacking sequence, and resin infill in the impact loading environment. image

This paper examines the propagation characteristics of non-linear Lamb waves through delamination surfaces in the stiffened composite plate structures. Finite Element based simulations were performed by introducing contact non-linearity in the structure. The contact interface modeled at a delaminated surface shows the generation of higher harmonics when the A $$_{0}$$ mode interacts with it. The effect of presence and absence of spar was analyzed through amplitude ratio. It was observed that the amplitude ratio decreases with the presence of spar. Due to which there is a decrease in fundamental peak amplitude with the presence of a defect in the structure and energy is distributed in the form of subharmonic or higher-order harmonics. It is identified that as delamination length increases the amplitude ratio also increases. Delamination length greater than the wavelength of excitation frequencies introduces subharmonic in the signal whereas the amplitude ratio decreases when delamination length approaches wavelength in the structure. Based on this, we can infer relative size of the delamination length.

To measure strain in underwater applications, it is required to carry out waterproofing of strain gauges to protect them from the water ingress. In this work a novel approach using Polyurethane (PU) sealant has been adopted for the waterproofing. The approach was demonstrated on a composite laminate. Strain gauges were bonded on the laminate and PU sealant was applied over the gauges. Strains of PU sealant coated gauge and bare gauge is compared to see the effect of coating the gauge with PU sealant. Experiments were performed to measure strains in three cases - before application of the sealant, during curing of the sealant and presence of water over the sealant. In all three cases, the strains were found to be in accordance with each other. Strain value of PU sealant coated gauge varies less than 2 % of the bare gauge.

Lap joints of glass/epoxy (GFRP) laminates were made with an adhesive modified with nanofillers and studied in comparison with the control one. An ambient temperature curing epoxy used for making structural composites was also modified with nanofillers and its bond strength (lap shear) characteristics were evaluated in comparison to the virgin resin. Amine functionalized carbon nanofibers and silane modified nanosilica were used at concentrations of 0.1, 0.3 and 0.5% for altering the adhesive properties. Well dispersed nanofillers in the adhesive are found to have no significant effect on the lap shear strength of the GFRP composites.

This paper deals with experimental measurements on increase in attenuation of longitudinal bulk wave propagating in thickness direction in pristine and electrospun nylon 6, 6 nanofiber interleaved glass epoxy composite. Pristine and interleaved composites were manufactured using RFI process. Experiments to measure the difference in attenuation were performed employing air-coupled transducers of frequencies 100 kHz, 200 kHz and 500 kHz. Ratio of amplitude of the bulk wave in the pristine and the interleaved composite was calculated from experimental data and used to determine difference in attenuation of the wave due to the interleaved nylon nanofiber. It was found that the difference increases with increase in frequency of the bulk wave.

Considering the superior strength properties of polymer based composites over metallic materials, they are being used in primary structures of aircrafts. However, these polymeric materials are much more complex in behaviour due to their structural anisotropy along with existence of different materials unlike in metallic alloys. These pose challenge in flaw detection, residual strength determination and life of a structure with their high susceptibility to impact damage in the form of delaminations/disbonds or cracks. This reduces load-bearing capability and potentially leads to structural failure. With this background, this study presents a method to identify location of delamination interface along thickness of a laminate. Both numerical and experimental studies have been carried out with a view to identify the defect, on propagation, mode conversion and scattering characteristics of fundamental anti-symmetric Lamb mode (Ao) when it passed through a semi-infinite delamination. Further, the reflection and transmission scattering coefficients based on power and amplitude ratios of the scattered waves have been computed. The methodology was applied on numerically simulated delaminations to illustrate the efficacy of the method. Results showed that it could successfully identify delamination interface.

A numerical study was conducted to find the effect of wrinkle in a metal sheet on its stiffness under tensile, compressive and flexural loading conditions. To define wrinkle of different sizes in model 'wrinkle factor' - ratio of thickness of specimen at wrinkle location to thickness of the specimen at no wrinkle location, is defined. Stiffness of a sheet with wrinkle has been compared with that of a pristine sheet under various loading conditions. From numerical results it is seen that, the stiffness in both tension and compression loading reduces as the wrinkle factor increases. But the stiffness in flexural loading is not affected considerably; contrary it increases for some wrinkle factor. Considering various loading conditions, it is observed that, stiffness in compression is affected more than in any other case. Furthermore, an attempt has been made to investigate force picked up by wrinkled portion and compared with the total force applied on the sample.

During the interaction of the anti-symmetric funda mental Lamb mode (A o) with the front edge of a delamination, it undergoes mode conversio n. The mode converted A o mode (So) propagates as A oSo mode along with the incident A o mode, which propagates as A oAo mode in top and bottom sub-laminates. A detailed study on varia tion in transmission factors and transmission coefficients revealed that, transmission factor of AoAo mode is nearly independent of thickness ratio and excitation frequency. Whereas, the transmission factor of AoSo mode depends on the thickness ratio and excitation frequency. Moreover, power tra nsmission coefficient of A oAo mode is a strong function of the thickness ratio, but, the power ass ociated with the AoSo mode is very less as compared to its AoAo counterpart.

A new hybrid method based upon nonlinear Lamb wave response in time and frequency domains is introduced to locate a delamination in composite laminates. In Lamb wave based nonlinear method, the presence of damage is shown by the appearance of higher harmonics in the Lamb wave response. The proposed method not only uses this spectral information but also the corresponding temporal response data, for locating the delamination. Thus, the method is termed as a hybrid method. The paper includes formulation of the method and its application to locate a Barely Visible Impact Damage (BVID) induced delamination in a Carbon Fiber Reinforced Polymer (CFRP) laminate. The method gives the damage location fairly well. It is a baseline free method, as it does not need data from the pristine specimen.

Attenuation of a bulk wave, generated by a point source, propagating in an isotropic medium, is due to the geometry and nature of the material involved. In numerical simulations, if the complete domain of propagation is modeled, then it captures the attenuation of a wave caused due to its geometry. To model the attenuation of the wave caused due to the nature of the material, it is required to know the material’s attenuation coefficient. Since experimental measurement on attenuation of a wave involves both the effects of geometry and material, a method based on curve fitting to estimate the material’s attenuation coefficient from effective attenuation coefficient, is proposed. Using the material’s attenuation coefficient in the framework of Rayleigh damping model, numerical modeling on attenuation of both the bulk waves - longitudinal and shear excited by a point source was carried out. It was shown that the proposed method captures the attenuation of bulk waves caused on account of geometry as well as nature of the material.

•Numerical studies on effect of ply stacking on amplitude of So mode transmitted into sub-waveguides.•Estimation of transmission factors of the modes transmitted from main waveguide to sub-waveguide.•Correlation of the transmission factors with phase velocities of the modes.•Experimental validation of the observations drawn in numerical simulations.

This paper focuses on quantification of attenuation coefficient of fundamental anti-symmetric (Ao) Lamb wave when it propagates through hybrid structural composites with nanomaterials. Investigations were carried out on nanosilica and carbon nanofiber hybridized-glass and carbon fiber reinforced epoxy composites in comparison to their respective control composite laminates with no nanofillers. Composite specimens were fabricated through Resin Film Infusion (RFI), ensuring near zero void fractions. Effective and actual attenuation characteristics of Ao mode were evaluated at three different frequencies – 100kHz, 200kHz and 500kHz in hybrid glass and carbon based composites. It is observed that effect of the nanomaterials (nanofillers) on attenuation of Ao mode is more significant at high frequencies. Furthermore, attenuation behavior of Ao mode depends on the type of nanomaterial. It is also observed that Ao mode undergoes more attenuation in glass fiber reinforced composites compared to their carbon fiber based counterparts, irrespective of the nanomaterials.

The fundamental anti-symmetric Lamb mode (Ao) radiation characteristics of air-coupled transducers in isotropic (aluminium) plates were measured experimentally for the first time. To evaluate the radiation characteristics, experiments were performed by fixing the position of one of the transducers (either transmitter or receiver) and then moving other one (either receiver or transmitter) along the circumference of a circle, whose radius was equal to that of the distance of separation between the transducers. Experiments were conducted to evaluate the radiation pattern of 100 kHz, 200 kHz and 500 kHz air-coupled transducers in one mm and three mm thick aluminium plates. It is interesting to note that, Ao Lamb mode radiation pattern emitted by air-coupled transducers in isotropic plates is characterised by Gaussian curve.