An experimental investigation into damage modes and scale effects in CFRP open hole tension coupons

An experimental investigation was conducted to study the effect of notch size and length scale on the damage of carbon fibre-reinforced composite specimens. Open hole tension specimens in a range of configurations were tested quasi-statically to ultimate failure. The load response, damage modes and strain field development were experimentally recorded. The results demonstrated that changing the ply thickness and specimen dimensions markedly affected the damage modes and specimen behaviour. This output provides key insights into the nature of composite behaviour, and is also critical for the development and validation of analysis methodologies capturing damage initiation and progression

Intelligence in structural health monitoring of composite structures using a robust signal processing protocol

This paper reports the development of a Structural Health Monitoring (SHM) system for a 2-D polymeric composite T-joint, used in maritime structures. The system developed relies on the examination of the strain distribution of the structure under operational loading and passing this data through a series of in-house developed pre-processing algorithms and eventually onto an Artificial Neural Network (ANN)-based inference engine. This system prompted the development of sophisticated pre-processing algorithms for the strain data. Improvements of 82% or more in detection accuracy were observed when these algorithms were invoked. Finite Element Analysis (FEA) was also conducted with delaminations of variable sizes at various locations in two structures, a composite beam and a T-joint. This paper focuses on a few normalization procedures that were developed to reduce the dependency of the algorithm on variables such as loading vectors. The work here also demonstrates the capability of the algorithm to detect and quantify instances when multiple damage zones are present

Review of methodologies for composite material modelling incorporating failure

Advanced composite materials are finding increasing application in aerospace, marine and many other industries due to the advantages in performance, structural efficiency and cost they provide. However, despite years of extensive research around the world, a complete and validated methodology for predicting the behaviour of composite structures including the effects of damage has not yet been fully achieved. The Cooperative Research Centre for Advanced Composite Structures (CRC-ACS) is leading a currently running collaborative project to develop a methodology for determining mechanical behaviour and failure in composite structures. Key drivers of the project are the use of multi-axial testing machines for material characterisation and an appreciation of the issues involved due to the different length scales of any analysis. As part of the project, a critical review was performed to assess the state of the art in material constitutive modelling and composite failure theories. This paper summarises the results of the review, which includes a discussion of the various theories and approaches within the context of the dissipated energy density framework. The results of the review will be applied within the project to select appropriate constitutive modelling and failure approaches for implementation within a data-driven material characterisation methodology

Damage prediction models for composite T-Joints in marine applications

For a monocoque hull structure, the bulkhead is used to separate the hull into many compartments. A typical joint between the hull and bulkhead used in such structure is known as a T-joint. It consists of composite overlaminates over a shaped fillet to allow the transmission of direct and membrane shear stresses. This paper describes the numerical solutions of the analytical approach in studying of the effect of disbond for a composite ship T-Joint using two methods, the VCCT (Virtual Crack Closure Technique) and CTE (Crack Tip Element) method. The analysis was conducted for T-Joints with various disbond sizes. It was subjected to a straight pull-off load to simulate normal operational conditions. An experimental investigation was conducted to validate the FE (Finite Element) models. The results of the numerical and experimental studies are discussed to corroborate the effectiveness of using fracture mechanics for this analysing this structure. A modified CTE model for the T-joint is proposed to enable a like-for-like comparison to that evaluated by the VCCT method

An analysis methodology for failure in postbuckling skin-stiffener interfaces

Blade-stiffened structures have the potential to produce highly efficient structures, particularly when the large strength reserves available after structural buckling, in the postbuckling range, are exploited. In experimental tests of postbuckling stiffened structures made from fibre-reinforced composites, failure typically initiates at the interface of the skin and stiffener and leads to rapid and even explosive failure. A methodology has been developed for analysing collapse in postbuckling composite structures that involves predicting the initiation of interlaminar damage in the skin-stiffener interface. A strength-based criterion is monitored in each ply using a local model of the skin-stiffener interface cross-section. For the analysis of large structures, a global analysis is first run to obtain the complete postbuckling deformation field, which is then input onto a local model using a global-local analysis technique. The coordinates of the local model can easily be moved to rapidly assess failure initiation at numerous skin-stiffener interface locations throughout the global structure. The analysis methodology is compared to experimental results for two-dimensional T-section specimens and large, fuselage-representative stiffened panels and is shown to give accurate predictions of the failure load and failure mechanisms. The use of the approach for the analysis of postbuckling composite structures has application for the design and certification of the next generation of aircraft

Structural consequences of sensor cavities in scarf repairs

Structural health monitoring (SHM) may be applied to bonded composite repairs to enable continuous through-life assessment of structural integrity. Adhesively bonded joints are an ideal starting point to develop real-time, in-situ monitoring due to knowledge of modes and locations of failure. Similarly, the ability to accurately monitor the health of a joint has potential to aid acceptance of adhesive bonding and move away from the contemporary 'black aluminum' approach to design. Comparative Vacuum Monitoring (CVM) developed by Structural Monitoring Systems Pty Ltd (SMS) has potential for monitoring local 'hot spots' such as joints. The CVM concept is simple, by creating a cavity on the surface or within a structure and drawing and holding a light vacuum, any damage breaching the cavity will cause a measurable leak. To monitor composite structures sensor cavities must extend from the interrogation system to potential locations of damage. For an adhesively bonded scarf repair, sensors cavities must reach or be integrated into the bondline itself. Consequently, it is vital to understand the influence of sensor cavities on structural performance of scarf repair joints. This paper focuses on the static and cyclic tensile performance of soft repairs (where the new material is co-cured in-situ) with internal sensor cavities. Microstructure changes such as fibre distortion and localised resin richness are characterised and discussed. The reduction of joint strength was found to be as much as 34%. While overall fatigue performance was lowered, the sensitivity of the joint to accumulation of fatigue damage was reduced by the inclusion of galleries. Although this represents an important first step, considerable work remains for the implementation of a viable SHM system for scarf repairs or any form of adhesively bonded structure

Structural health monitoring of composite structures using artificial intelligence protocols

This study discusses a structural health monitoring (SHM) system developed to detect the presence of delamination, and predict its location and size in a composite structure. Two structures are considered in this study: a composite beam and a T-joint structure used in ships. Finite element (FE) models of these structures are created, embedded with delaminations, and the strain distribution along the bond-line and surface of the structures is used as a damage characteristic, to get information about the structures' condition. Experimental tests are then conducted to verify the FE model, an excellent corroboration is achieved between the two. Artificial neural networks is then used in tandem with a pre-processing program developed, called the damage relativity assessment technique (DRAT), to determine the presence of the damage and then predict its size and location. This SHM system developed is completely independent of the structures' loading condition and it detected the presence, and predicted the size and location of delaminations with an acceptable level of accuracy

Effects of biaxial deformation of the knitted glass preform on the in-plane mechanical properties of the composite

The effect of biaxially deforming a weft-knit Milano rib fabric on the overall composite tensile and compressive properties has been studied for a glass/vinyl-ester knitted composite. A range of combinations of wale and course stretch ratios was considered. It was found that the tensile modulus, strength and strain-to-failure were all affected to varying degrees by fabric deformation, whereas the compression properties of these structures, on the whole, appeared to be closer to isotropic and relatively insensitive to fabric deformation. This latter observation is believed to be due to the dominance of the matrix properties. Despite not detecting any gross changes in the knit structure, it is believed that some re-distribution and re-orientation of the fibres did occur during fabric deformation, which in turn altered the relative contents and/or directionality of the fibres in the composites. Consequently, the tensile properties were affected by the stretching of the knitted fabric. Post failure microscopy revealed that tensile fracture typically occurred in the vicinity of the highly stressed crossover points of the knit structure. A feature of the compressive failure was kinking of the highly bent yarns, particularly in resin rich regions having reduced lateral support for yarns

Effect of weaving on the tensile properties of fibre tows and woven composites

Tension tests were conducted on (consolidated) tow and woven carbon/epoxy composites in an attempt to quantify the effect of fibre damage induced during weaving on the mechanical performance of fibre tows and their corresponding composites. Two commercially-available carbon fibres were considered. The tension tests were carried out on composites with fibre tows sampled from different locations in the loom setup. These included samples from the tensioning, heddles and reed regions of loom setup. In addition, samples were taken from the external (or, surface) and internal warp layers of the woven preforms. The present work has shown that the tensile properties of fibre composites are detrimentally affected only if the fibres suffer sufficiently large amounts of damage during the weaving process. Compared with unidirectional, non-woven fibre tow composites, further deterioration in properties is expected in multilayer woven composites due to the inevitable presence of fibre crimping