A review of analytical methods for aircraft structures subjected to high-intensity random acoustic loads

A review of the acoustic fatigue design process for aircraft structures is presented in this paper, together with the current design guides, which are used to predict the stresses that an acousticallly loaded aircraft structure may experience in service. These methods are based on linear theory and use the single-degree-of-freedom approximation method. A recent programme of research which uses this method together with the finite element method to predict the root mean square strains experienced by acoustically excited, doubly curved sandwich panels is briefly discussed. Recent developments in prediction methods based on the non-linear dynamic response of thermoacoustic loaded structures are reviewed, and suggestions are made as to possible future directions in the area of acoustic fatigue researc

The characterisation of notched 3D woven composites using Thermo-elastic Stress Analysis and Digital Image Correlation

Although adhesive bonding of fabricated composite structures would be the preferred option from a design for manufacture point of view, there will always be a need to use mechanical fastening for highly loaded regions. Fatigue behaviour and notch sensitivity are two of the main concerns for fabricated composite structures, despite the excellent performance of the material in the former regard. For 3D woven composites previous research has concentrated on the un-notched fatigue behaviour with less research focused on the notched properties. In the present work, the quasi-static and tension-tension fatigue behaviour of notched orthogonal and angle-interlock 3D woven carbon/epoxy composites are investigated. Digital image correlation was used to obtain the full-field strain distribution during the quasi-static tensile tests, and thermo-elastic stress analysis was used to characterise the damage progression during the fatigue tests. The DIC results revealed that the tensile strength of the 3D woven composites was not sensitive to notch size, with the notched tensile strength being less than 17% lower than the un-notched tensile strength. Limitations of the test machine capacity meant that the fatigue specimens were loaded at 60% of the ultimate failure stress. The specimens were cyclically loaded for 5,000,000 cycles without complete fracture, although the TSA results revealed progressive surface cracking around the notch. The orthogonal weave was found to have a larger surface damage area than the angle-interlock weave

Multi-scale damage modelling of 3D woven composites under uni-axial tension

This paper presents a detailed numerical analysis of two types of 3D woven composite architecture. a unit cell, full finite element meso model, and a mosaic macro model are developed to simulate the elastic and damage progression behaviour of the two weave types. Both models predicted the tensile modulus and strength within 20% of the experimentally measured values, and the predicted failure sequence was similar to the experimental observation

Numerical modelling of the compression-after-impact performance of a composite sandwich panel

A numerical model for the quasi-static indentation and compression-after-impact behaviour of a composite sandwich panel is presented, using cohesive surfaces for interlaminar damage prediction. Intra-laminar damage and core crushing is also included. The models show generally good agreement with experimental results for residual strength, performing best when two cohesive surfaces are used in the impacted skin, but tend to over-estimate the undamaged panel strength. Damage extent predictions from the indentation phase of the analysis are often quite poor, but do not necessarily correlate with the accuracy of the strength estimates. The model provides a promising basis for further development

Mixed-mode delamination in layered isotropic and laminated composite beam structures

Completely analytical theories are presented for calculating the total energy release rate (ERR) in a mixed-mode delamination in layered isotropic and laminated composite straight beam structures and for partitioning it into opening mode I and shearing mode II components. The theories are developed within the contexts of both the Euler and Timoshenko beam theories. The theories are extensively verified against numerical simulations using the finite element method. The developed theories provide a valuable means for the design of such beam structures against delamination

Experimental and numerical investigation of the effect of asymmetry on the residual strength of a composite sandwich panel

Asymmetric sandwich panels with skins of differing thickness are subjected to various degrees of damage via quasi-static indentation before compressive loading to failure. These are compared with panels with skins of equal thickness. The experiments show that the asymmetric panels experience an improvement in strength with small amounts of indentation compared with undamaged asymmetric panels, and for more severe damage, show greater residual strength than the symmetric panels. The two configurations are numerically modelled using Abaqus, including inter- and intra-laminar damage, and core crushing. The strength predictions from the models agree well with the experiments

Open hole quasi-static and fatigue characterisation of 3D woven composites

This paper presents a comprehensive study on the open-hole quasi-static tensile and tension-tension fatigue behaviour of an orthogonal and an angle-interlock 3D woven carbon/epoxy composite. The full-field strain distribution during quasi-static tests was characterised using digital image correlation (DIC), and the fatigue damage behaviour was monitored using an infra-red camera. The notched tensile strength was less than 17% lower than the un-notched tensile strength and not very sensitive to the notch size. The fatigue specimens were loaded with maximum stress of about 60% of the ultimate failure stress and no complete fracture occurred after 5,000,000 cycles. The residual fatigue strength was also found to be similar to the quasi-static tensile strength in both weaves. The surface crack initiation and progression during fatigue loading was identified using thermoelastic stress analysis which revealed that the orthogonal weave had larger surface damage area than the angle-interlock weave

Influence of fibre architecture on the tensile, compressive, and flexural behaviour of 3D woven composites

This paper presents a comprehensive study on the tensile, compressive, and flexural performance of six types of 3D woven carbon-fibre/epoxy composites which were manufactured using a traditional narrow fabric weaving loom and resin transfer moulding. Four orthogonal and two angle-interlock weaves were tested with the primary loading direction parallel to the warp direction. The mechanical performance was found to be affected by the distribution of resin rich regions and the waviness of the load-carrying fibres, which were determined by the fibre architectures. The binding points within the resin rich regions were found to be the damage initiation sites in all weave types under all loading conditions, which were confirmed with both visual observation and digital image correlation strain maps. Among all weave types, the angle interlock weave W-3 exhibited the highest properties under all loading conditions

Impact of mode shapes on experimental loss factor estimation in automotive joints

This paper presents the experimental work carried out on single-lap joints fastened together with bolts and nuts to investigate the contribution of mode shapes, and the effect that bolt sizes has in dissipating energy in built-up structures. Five different bolt sizes are chosen to assemble five single-bolted single-lap joints using aluminum plates. An analogous monolithic solid piece carved from the same aluminum material is used to determine the material damping and compare it against the damping from bolted joints. The dynamic response of all structures is captured under free-free boundary conditions, and the common modes are analyzed to understand the contribution and primary source of damping in the same range of the sampling frequency. This investigation has revealed that the source of damping in the joints is heavily linked to the mode shapes of the structure and structural damping (also referred to as material damping) contributes more during specific mode shapes compared to the joint damping itself. The findings allow a more accurate implementation of energy loss in automotive structures which contain bolted joints, allowing an implementation of both material and joint loss factor, respectively

Fatigue life prediction of 2524‐T3 and 7075‐T62 thin‐sheet aluminium alloy with an initial impact dent under block spectrum loading

This paper presents a fatigue life prediction model of post‐impacted sheets considering the effects of dent size and stress ratio. Low‐velocity impact tests at four different impact energies were performed on specimens cut from sheets of 2524‐T3 and 7075‐T62 aluminium alloy. Following the impact tests, static tensile and uni‐axial constant amplitude and block spectrum fatigue experiments were conducted. Numerical models were generated to determine the initial residual stress patterns, residual stress relaxation, and stress concentration factors around the impact dent. The S‐N curves and corresponding stress concentration factors and relaxed residual stresses of three of the post‐impacted specimens were used to determine the model parameters. Good agreement was achieved between the predictions and experimental results, and it has been demonstrated that the fatigue life prediction model can effectively simulate the effects of residual stress, stress concentration, and stress ratio on fatigue damage for post‐impacted thin sheet aluminium alloy materials