V. Arumugam

A Global Method for the Identification of Failure Modes in Fiberglass Using Acoustic Emission

The various failure mechanisms in bidirectional glass/epoxy laminates loaded in tension are identified using acoustic emission (AE) analysis. AE data recorded during the tensile testing of a single layer specimen are used to identify matrix cracking and fiber failure, while delamination signals are characterized using a two-layer specimen with a pre-induced defect. Parametric studies using AE count rate and cumulative counts allowed damage discrimination at different levels of loading and Fuzzy C-means clustering associated with principal component analysis were used to discriminate between failure mechanisms. The two above methods led to AE waveform selection: On selected waveforms, Fast Fourier Transform (FFT) enabled calculating the frequency content of each damage mechanism. Continuous wavelet transform allowed identifying frequency range and time history for failure modes, whilst noise content associated with the different failure modes was calculated and removed by discrete wavelet transform. Short Time FFT finally highlighted the possible failure mechanism associated with each signal.

Failure modes characterization of impacted carbon fibre reinforced plastics laminates under compression loading using acoustic emission

Composite laminates have low resistance under dynamic loading, particularly impact loading. A low-velocity impact on laminated composites causes various types of damage, such as delamination, fibre breakage, matrix cracking and fibre matrix interfacial debonding. Post-impact compressive strength is one of the greatest weaknesses in carbon fibre reinforced plastics laminates. After impact, due to the delaminations present in the laminates, local instability is triggered, which ultimately reduces considerably their residual strength. In this work, symmetric cross ply carbon fibre reinforced plastics laminates [(0°/90°)2]12 were subjected to falling weight impact at two different velocities, 2.5 and 3.5 m/s. Compression after impact studies showed substantial differences in failure mode between the two cases, passing from end crushing to crack propagation with higher impact energy. Acoustic emission technique was able to confirm this result and characterize the different types of failure modes during compression after impact test, in particular by frequency distribution.

Ultimate Strength Prediction of Carbon/Epoxy Tensile Specimens from Acoustic Emission Data

The objective of this paper was to predict the residual strength of post impacted carbon/epoxy composite laminates using an online acoustic emission (AE) monitoring and artiflcial neural networks (ANN). The laminates were made from eight-layered carbon (in woven mat form) with epoxy as the binding medium by hand lay-up technique and cured at a pressure of 100 kg/cm 2 under room temperature using a 30 ton capacity compression molding machine for 24 h. 21 tensile specimens (ASTM D3039 standard) were cut from the cross ply laminates. 16 specimens were subjected to impact load from three difierent heights using a Fractovis Plus drop impact tester. Both impacted and non-impacted specimens were subjected to uniaxial tension under the acoustic emission monitoring using a 100 kN FIE servo hydraulic universal testing machine. The dominant AE parameters such as counts, energy, duration, rise time and amplitude are recorded during monitoring. Cumulative counts corresponding to the amplitude ranges obtained during the tensile testing are used to train the network. This network can be used to predict the failure load of a similar specimen subjected to uniaxial tension under acoustic emission monitoring for certain percentage of the average failure load.

A novel approach for classification of failure modes in single lap joints using acoustic emission data

New innovative basalt fiber/epoxy composite materials are used in engineering applications such as aerospace, automotive, and civil structures due to the potential low cost of this material together with its mechanical characteristics and its failure mechanisms. Acoustic emission is a passive nondestructive testing technique for real-time monitoring of damage developed in materials and structures, which have been used successfully for the identification of damage mechanisms in composite joints under tensile loading. The present study is focussed on acoustic emission characterization of failure modes in three prominent joining methods namely, bonded, riveted, and hybrid joints during tensile test. Parametric analysis is performed on the acoustic emission data obtained during the tensile testing of these types of joints to discriminate the failure modes. Fast Fourier transform analysis using acoustic emission waveform analysis is carried out to analyze the different failure events and associate them with their dominant frequency ranges. The predominance of failure modes in each signal is used as a key in the study to discriminate failure modes on single-lap joints in basalt/epoxy composite laminate, and the results are validated with fast Fourier Transform analysis.

Estimation of residual flexural strength of unidirectional glass fiber reinforced plastic composite laminates under repeated impact load

The aim of this study is to investigate the effect of low-velocity multiple impacts at ambient (35℃) and elevated temperature (65℃ and 85℃) on unidirectional glass fiber reinforced plastic (GFRP) composites. Low-velocity repeated impact tests were conducted using a falling weight tower at a constant velocity of 1.5 m/s. The dominant parameters such as energy, contact force, and deflection were recorded during multiple impacts. The residual strength of laminates following repeated impact was evaluated by conducting three-point bending tests with acoustic emission (AE) real time monitoring. The temperature was revealed to play a key role in the impact response of composite materials, especially due to the progressive softening of the epoxy matrix. The nature and extent of damage during multiple impacts at ambient and elevated temperatures was investigated using real time AE monitoring: this analysis indicated delamination as a predominant failure mode, whose extent and criticality depended on temperature and number of impact events.

Porosity Effect on Residual Flexural Strength following Low Energy Impact of Carbon Fibre Composites

Studies of the combined effects of the presence of porosity (as it may result from partially effective cure cycles) and of low-energy impact damage on the residual properties of CFRP laminates have led so far to controversial results. In particular, it is not clear from the literature whether the presence of voids would blunt crack propagation following impact or rather would promote damage development. These effects would respectively either increase or reduce post-impact residual strength, relative to that of the laminate with virtually no voids, as the result of an optimal manufacturing procedure. With this in mind, different cure cycles have been applied to produce carbon fibre-reinforced polymer (CFRP) composites with various levels of void content, which were subjected to low energy impact damage (3, 4.5 and 6 J) and then to post-impact flexural strength measurement. Damage assessment using micro-focus computed tomography (μCT) was used to complement traditional ultrasonic C-scans, which proved ineffective on the high-porosity samples. Three cure-cycles were investigated: one which led to high porosity (average void content 4 vol%) and two conventional low-porosity cure cycles, only one of which included a post-cure cycle. This study has found that, despite a lower initial flexural strength, higher residual flexural strength was retained after impact in the high-porosity material than in the low-porosity one. This is explained by the lower extent of impact damage observed in the high porosity material, where voids had the effect of suppressing delamination propagation.

Acoustic emission characterization of local bending behavior for adhesively bonded hybrid external patch repaired glass/epoxy composite laminates

This article investigates the influence of homogeneous and hybrid external patches based on glass and Kevlar plain weave woven fabrics on local bending response of adhesively bonded external patch repairs in damaged glass/epoxy composite laminates. The intent of using hybrid external patches was to combine the excellent high displacement to failure property of Kevlar fiber as a ductile reinforcement with the superior mechanical property of glass fiber as a brittle reinforcement. The undamaged normal specimens were taken as the standard specimen for evaluation of residual mechanical properties. In all hybrid patches, the proportion of Kevlar and glass fibers was equal (i.e. 50% of Kevlar and 50% of glass by volume fraction), while lay-up configurations were different. This further allowed studying the associated effects of hybridization and lay-up configuration on local bending response of the repaired laminates. All the specimens were subjected to cyclic quasi-static indentation tests with a step loading method. The indentation tests have also been monitored in real time by acoustic emission system. The acoustic emission results illustrated various damage profiles and correlates with the mechanical test results to point out the load to a transformation in damage mechanisms during indentation loading with respect to the effect of each patch material on the performance of the repaired glass/epoxy specimens. Results showed that hybridization and lay-up configurations of the external patches played a significant role on local bending response (i.e. ultimate load, stiffness, residual deformation, displacement to failure, and damage pattern) of the repaired glass/epoxy specimens. Specimens repaired using intra-ply hybrid patches showed the best local bending response.

Safety and precautions

Reducing human error during maintenance operation can improve aviation safety. This chapter discusses present approaches to detecting, reporting, and controlling human error in maintenance operation. Potential directions for decreasing the occurrence and extenuating the influences of error in maintenance and, thus, enhancing safety and performance of operations are discussed.

Design, analysis, and durability of composite repairs

Initially, the parameters influencing the mechanical performance and environmental durability of the adhesively bonded repairs are discussed in this chapter. As bolted or hybrid bolted/bonded repairs are still largely employed as repair technique in primary composite structures, mechanics of mechanically fastened repairs are also discussed in this chapter. Afterward, the damage modes and damage prediction under static and dynamic loads are discussed. Then, the influences of environmental conditions (i.e., temperature and moisture) on residual strength and damage modes and nondestructive inspection methods for detecting the damage modes in the repairs are discussed. Lastly, the complex problems of the certification of repairs in composite structures are discussed. Subsequent to explaining processes presently employed to verify new structure, recommendations for satisfactory certification of repairs are discussed.

Key stages of adhesively bonded repairs

In this chapter, a comprehensive survey of the technology, advantages and drawbacks of various nondestructive evaluation techniques, machining procedures, adhesives, surface preparation methods, and curing processes employed for repairing fiber-reinforced polymer matrix composite materials is provided. Initially, a detail explanation of various nondestructive evaluation techniques most commonly employed for detecting flaws in composite materials is discussed. Subsequently, a review of the most important kinematic relationships for nonconventional machining processes (abrasive water-jet and laser-jet machining) that are often employed in machining fiber-reinforced polymer matrix composites is discussed. Next, the factors influencing the mechanical response and performance of the various adhesive material systems, bonding mechanisms, and surface preparations in polymer and composite adherends are reviewed in detail. Finally, a detailed review of the technology used in processing adhesively bonded repairs in composite materials, together with details of characteristic resin kinetics, curing cycle, and curing sources, is discussed.