Key Engineering Materials Vols. 569-570

Abstract: This paper describes the development of a stress / strain based in-situ damage inspection strategy focused around, but not exclusively, using thermoelastic stress analysis (TSA). The underlying philosophy is that defects and damage in a component or structure only constitute a cause for concern if these influence the stress field, i.e. the defect or damage acts as a stress raiser that reduces the service load limit. To assess this, it is necessary for the inspection method to map the distribution of stresses in the component, rather than the location and extent of an irregularity in the material. Imaging based techniques, such as TSA, digital image correlation (DIC) or digital speckle pattern interferometry (DSPI) provide non-contact maps of the surface stresses, deformations and/or strains. The full field data enables the engineer to evaluate if stress concentrations are present within the structure and, if data from a previous inspection is available, to assess if the distribution of stresses within the structure has changed from a previous 'undamaged' state. One of the key issues addressed in the current work has been the transition from a standard test setup, as typically used in laboratory work, to a more flexible (portable) setup relevant to industry requirements, e.g. site inspections. An approach that enables similar resolution (by comparison to current laboratory standard setups) stress and strain data to be captured using natural frequency excitation of a structure has been demonstrated on various full scale components.

Abstract: Damage to many structural systems (including bridges, offshore platforms, airplanes and aerospace systems) can occur during normal service due to fatigue loading, a corrosive environment, collisions with extraneous objects, etc. For such structures, in order to guarantee safety, periodic inspections and maintenance are essential: Since undetected damage may result in catastrophic structural failure, the realisation of an accurate and trustworthy damage detection technique is fundamental.Vibration-based inspection offers the potential for detecting faults by monitoring the dynamic response of a structure, exploiting the fundamental principle that structural damage affects the stiffness distribution and hence the presence of the fault will change the dynamic properties of the structure under investigation.Many vibration-based inspection techniques have been developed over recent years which require knowledge of the baseline modal responses of the structure in the original undamaged state.However, for the vast majority of existing structures in operation, such data are simply not available. This keynote presentation reviews past and present research studies in which the author has been involved that aim to detect the presence of structural damage and identify its approximate location, using only post-damage vibration measurements. The techniques presented analyse either the mode shapes, operating deflection shapes or principal orthogonal modes and their corresponding derivatives. These features have been found to be good indicators of damage due to the spatial information that can be provided with respect to location of damage. The methodologies proposed are applied to isotropic/orthotropic uni/bi-dimensional structures and corresponding numerical and experimental results are presented.

Abstract: The paper describes the application of a 3D finite element model for prediction of impact induced damage in sandwich composites consisting of laminated skins bonded to a closed cell foam core. The major damage and fracture mechanisms typically developing in transversally loaded sandwich composites were simulated in the model. The model was implemented in the FE package ABAQUS/Explicit and used to predict the impact damage resistance of sandwich panels with different core densities, core thicknesses, and skins layups. Numerical results obtained by FE simulations were compared with experimental data and observations collected through impact tests carried out at various impact energies.

Abstract: Damage assessment of composite materials is crucial for health monitoring of engineering structures. It is particularly important to detect damage invisible to the human eye caused by low speed impact. Optical non-contact sensing techniques enable full-field measurements from structural responses. However, damage is generally associated with its local effect on deformation and strain patterns while full-field measured data is highly information redundant. It is possible to apply image processing techniques [1, 2] to extract succinct and efficient features, or attributes, from full-field data. Iterative reanalysis of large and detailed numerical models is generally very expensive and may not be feasible. Meta modeling is one of the practical ways to overcome the problem of high computational cost. In this paper, a case study of composite delamination assessment based on simulation is discussed. The damage mode in the composite plate is assumed to be a delamination of elliptical shape. Surface strain of a specimen under tensile loading is considered to be the measured structural output. The Krawtchouk moment descriptor is applied to extract a small number of features from the strain map. A meta model in the form of a Kriging predictor [3] is constructed to map the damage parameters to Krawtchouk shape features of the strain distribution. The delamination region is quantified using an inverse procedure based on the trained Meta model. Furthermore, a biomimicry algorithm, particle swarm optimisation, is applied to detect the location of the delamination.

Abstract: Experimental and numerical studies wereconducted to understand the effect of plate curvature on the blast response ofcarbon/epoxy composite panels. A shock-tube system was utilized to impartcontrolled shock loading to quasi-isotropic composite panels with varying radiiof curvature. A 3D digital image correlation (DIC) technique coupled withhigh-speed photography was used to assess the out-of-plane deflection ofcomposite panels. A finite element (FE) model integrating fluid-structureinteraction to represent coupling between the air surrounding composite panels,shock wave and panels, was developed using a general-purpose FE softwareABAQUS/Explicit. The numerical results were compared to the experimental dataand showed a good correlation.

Abstract: Conventional-drilling (CD) methods often initiate discrete damage phenomena such as micro-cracking, matrix burning; delamination and fibre pull-out in difficult-to-machine heterogeneous materials such as carbon fibre-reinforced polymer (CFRP) composites. Ultrasonically assisted drilling (UAD) is a promising machining technique suitable for drilling holes in CFRP composites. UAD has been shown to possess several advantages over CD, including reduction in a thrust force and torque, diminished burr formation at drill exit in ductile materials and an overall improvement in roundness and surface finish of the drilled hole. Recently, our in-house experiments of UAD in CFRP composites demonstrated remarkable reductions in levels of thrust force and torque (average force reductions in excess of 60%) when compared to CD with the same machining parameters. 3D Finite Element (FE) models of CD and UAD techniques for a CFRP laminate were developed using a general-purpose FE software ABAQUS/Explicit and validated using experimental results. The magnitudes of thrust force and torque obtained with FE analysis of UAD are compared with those for CD. The numerical results obtained with the developed FE model were found to be in a good agreement with the experimental data.

Abstract: This paper deals with effects of damage on Carbon FRP (CFRP) elements subjected both to defects and micro-cracking due to static tensile loading through an investigation of CFRP elements under natural vibration tests. The correlation between the response and frequency decrease due to damage for cross section reduction of CFRP cantilever beam elements has been analysed. Successively, the response of CFRP subjected to different values of tensile force has been investigated; experimental frequency values are compared with theoretical values and discussed.

Abstract: The unique potential to integrate functional elements into fibre-reinforced components combined with the recent progress in the simulation models of composite materials provides new perspectives for reliability improvement of the next generation components. Such combination is presented on the example of a carbon-fibre reinforced composite plate with integrated vibration measurement and excitation systems. The investigated structure was consolidated in an adapted resin transfer moulding process using additional layers for positioning, contacting and isolating of the active elements. The integrated elements enable an online estimation of the structural dynamic behaviour and its damage-dependent changes.The article considers the identification problem of diagnostic models enabling a precise interpretation of the measured vibration responses. An approach based on the generation of classifiers by means of inductive machine learning algorithms is applied. At the baseline phase, modal properties are measured that correspond to the undamaged state of the structure. Using these experimental data, a simulation model of the structure was fitted by means of a mixed numerical experimental technique and used for the generation of multiple vibration patterns resulting from different mass distributions. The unique combination of experimental and numerical results enables a generation of high resolved learning datasets for machine learning algorithms using a minimum amount of experimental data. The verification of the estimated classifiers by means of the achievable diagnostic performance is firstly conducted theoretically using standardised validation techniques and a high performance is identified. Then, at the inspection phase, the performance of the whole diagnostic system is additionally experimentally confirmed based on the dynamic response resulting from different unseen structural disturbances.

Abstract: Composite materials are increasingly being used in a wide range of structural applications in place of metallic materials. This presents a new range of challenges when considering the monitoring of damage and failure in complex components. This paper explores these challenges and presents a potential monitoring method using airborne acoustics which is both non-contact and easily implemented. A carbon composite panel was manufactured and statically loaded in tension until failure. During the test, Digital Image Correlation (DIC) was used to measure full field surface strain in the panel. An array of microphones, placed adjacent to the panel, was used to capture airborne acoustic signals between 400Hz and 20kHz during the test. The captured sound waves potentially contain signals originating from a range of sources, such as fibre failures and matrix cracking, but also contain background noise. A range of techniques have been used to examine the signals and determine the onset of failure, including Short-Time Fourier Transforms (STFT). The detection of failure using the airborne acoustic system has been validated using the strain data from the DIC measurements. The results presented demonstrate the applicability of the airborne system to monitoring of composite components.