Damage Characterization in Glass/Epoxy Composite Laminates under Normal and Oblique Planes of Cyclic Indentation Loading with AE Monitoring

This work focuses on the experimental investigation of indentation damage resistance in different stacking sequences of glass/epoxy composite laminates under cyclic loading on normal (0°) and oblique (20°) planes. The stacking sequence, such as unidirectional [0]12, angle ply [±45]6S, and cross ply [0/90]6S, were subjected to cyclic indentation loading and monitoring by acoustic emission testing (AE). The laminates were loaded at the center using a hemispherical steel indenter with a 12.7 mm diameter. The cyclic indentation loading was performed at displacements from 0.5 to 3 mm with an increment of 0.5 mm in each cycle. Subsequently, the residual compressive strength of the post-indented laminates was estimated by testing them under in-plane loading, once again with AE monitoring. Mechanical responses such as peak load, absorbed energy, stiffness, residual dent, and damage area were used for the quantification of the indentation-induced damage. The normalized AE cumulative counts, AE energy, and Felicity ratio were used for monitoring the damage initiation and propagation. Moreover, the discrete wavelet analysis of acoustic emission signals and fast Fourier transform enabled the calculation of the peak frequency content of each damage mechanism. The results showed that the cross-ply laminates had superior indentation damage resistance over angle ply and unidirectional (UD) laminates under normal and oblique planes of cyclic loading. However, the conclusion from the results was that UD laminates showed a better reduction in residual compressive strength than the other laminate configurations.

Damage Characterization in Glass/Epoxy Composite Laminates under Normal and Oblique Planes of Cyclic Indentation Loading with AE Monitoring

This work focuses on the experimental investigation of indentation damage resistance in different stacking sequences of glass/epoxy composite laminates under cyclic loading on normal (0°) and oblique (20°) planes. The stacking sequence, such as unidirectional [0]12, angle ply [±45]6S, and cross ply [0/90]6S, were subjected to cyclic indentation loading and monitoring by acoustic emission testing (AE). The laminates were loaded at the center using a hemispherical steel indenter with a 12.7 mm diameter. The cyclic indentation loading was performed at displacements from 0.5 to 3 mm with an increment of 0.5 mm in each cycle. Subsequently, the residual compressive strength of the post-indented laminates was estimated by testing them under in-plane loading, once again with AE monitoring. Mechanical responses such as peak load, absorbed energy, stiffness, residual dent, and damage area were used for the quantification of the indentation-induced damage. The normalized AE cumulative counts, AE energy, and Felicity ratio were used for monitoring the damage initiation and propagation. Moreover, the discrete wavelet analysis of acoustic emission signals and fast Fourier transform enabled the calculation of the peak frequency content of each damage mechanism. The results showed that the cross-ply laminates had superior indentation damage resistance over angle ply and unidirectional (UD) laminates under normal and oblique planes of cyclic loading. However, the conclusion from the results was that UD laminates showed a better reduction in residual compressive strength than the other laminate configurations.

Investigation of indentation damage resistance on normal and inclined plane of glass/epoxy composite laminates using acoustic emission monitoring

This work relates to the investigation of indentation induced damage resistance in glass fiber reinforced polymer composite laminates under normal and inclined planes. Uni-directional [0] and cross-ply [0/90] glass fiber reinforced polymer composite specimens were subjected to 0° and 20° indentation loading with acoustic emission monitoring. The specimens were loaded at the centre using a hemispherical steel indenter with 12.7 mm diameter. Mechanical responses such as force–displacement behavior, absorbed energy and resulting damage area were used for the quantification of the indentation damage. Acoustic responses such as normalized cumulative counts, energy, duration and peak frequency were considered for monitoring damage progression during 0° and 20° indentation loading. The residual compressive strength of the glass fiber reinforced polymer specimens following indentation was measured by testing them under in-plane loading, once again with acoustic emission monitoring. The correlation between mechanical and acoustic strain energy was used for the evaluation of the damage severity of the laminates. The result revealed indentation damage resistance in cross-ply laminates as 65.08% and 68.01% higher than uni-directional laminates for 0°, whereas for 20°, there was a reduction in the damage resistance in cross-ply laminates to 43.27% and 57.39%. These were higher than uni-directional laminates under the displacements of 2 mm and 3 mm, respectively. The conclusion from these results was that transverse shear load at 20° inclination of laminate leads to reduction in residual compressive strength. Moreover, uni-directional glass/epoxy laminates have better residual compressive strength than cross-ply laminates for both 2 mm and 3 mm indentation displacements.

Effect of Healing Agent Filled Microcapsules on Tensile Strength of Glass Fibre Reinforced Epoxy Composite Laminate

This work focuses on the study of variation in behaviour of Glass Fiber Reinforced Epoxy Composite laminate in the presence of healing agent filled microcapsules. Healing capacity was tested for the newly developed laminate with capsules. Two types of microcapsules were made and included in the resin system. One type consists of healing agent and other type consists of curing agent. These capsules were blended in the ratio of 10:3 in the resin system and laminate was fabricated by hand lay-up technique. Initially, the effect of microcapsules on the tensile properties of glass fiber epoxy composites was studied. No conspicuous variation was discerned in the tensile nature. The healing ability of the laminate was checked by producing minor indentation damage on the surface of the laminate with a 0.5 mm ball indenter. It was noticed that 80% of the indentation was recovered within the time span of 48 hours and 93% of healing efficiency was attained. FE analysis was performed with MIDAS NFX software to confirm the results. Both experimental and FE analysis results were observed to be identical.

Robust Localization and Classification of Barely Visible Indentations in Composite Structures by Fusion of Ultrasonic Damage Indices

This study aims to detect, localize, and assess the severity of barely visible indentation damage in a composite sandwich structure using ultrasonic guided waves. A quasistatic loading was gradually applied on a specimen of carbon fiber reinforced epoxy resulting in dents on the surface. Lamb-wave measurements, from a sensor network mounted on the panel's surface, were taken for the intact condition and three damage cases (0.2, 0.5, and 2.7-mm dents). Three approaches were adopted to define the damage indices (DIs) toward anomaly detection, namely, amplitude variation, symbolic dynamics, and root mean square deviation. Data fusion was performed between measurements from multiple excitation frequencies for single and multiple DIs, where the anomaly combination between all the frequencies and the DIs was called a total anomaly. An imaging algorithm was implemented for damage localization in conjunction with single and combined DIs. It was shown that combining the effects of different frequencies and/or different DIs increases the robustness and consistency of the damage detection and localization process. Moreover, a distance-based classification technique was applied using features from single DIs and the combined anomaly measure. Accuracies higher than 91% were attained for the majority of the cases tested.