The growth of delaminations in polymer-matrix fibre composites under cyclic-fatigue loading in operational aircraft structures has always been a very important factor which has the potential to ...significantly affect the service-life of such structures. The recent introduction by the Federal Aviation Administration (FAA) of a ‘slow growth’ approach to the certification of composites has further focused attention on the experimental data and the analytical tools needed to assess the growth of delaminations under fatigue loads. Specific attention is given to the test and data-reduction procedures required to determine a ‘valid’ rate of fatigue crack growth (FCG), da/dN, versus the range of the energy release-rate, ΔG, (or the maximum energy release-rate, Gmax, in a cycle) relationship (a) to characterise and compare different types of composites, and (b) for designing and lifing in-service composite structures. Now, fibre-bridging may occur behind the tip of the advancing delamination and may cause very significant retardation of the FCG rate. Such retardation effects cannot usually be avoided when using the Mode I double-cantilever beam test to ascertain experimentally the fatigue behaviour of composites, so that a means of estimating a valid (i.e. ideally a ‘retardation-free’ or, at least, a very low-retardation) relationship is needed. The present paper presents a novel methodology, that is based on a variant of the Hartman-Schijve equation, to ascertain a valid, ‘retardation-free’, upper-bound FCG rate curves.
Strain energy release rate calculations for various cases of delaminations emanating from matrix cracks are developed and used to predict the onset of delaminations and their growth size as a ...function of applied tension and shear loads in composite laminates. The method determines the matrix crack spacing, the delamination onset load, the delamination size at onset and, through the use of a newly proposed delamination resistance curve, the size of delaminations as they grow under load. The method can be applied to any symmetric laminate. Comparisons to test results in the literature for a variety of layups and materials shows very good agreement with the exception of cases where significant edge delaminations appear before delaminations caused by matrix cracks.
A layerwise theory within the framework of first-order shear deformation theory is introduced to investigate the postbuckling behavior and the delamination growth of geometrically imperfect composite ...plates. The Ritz method is adopted and the displacement fields are assumed in a form of polynomial series leading to a reduced computational cost. The proposed method is capable of predicting both the local buckling of the delaminated sublaminates and the global buckling of the plate. Two types of delaminations are considered: through-the-width and edge delaminations. To inhibit the penetration of delaminated domain, contact constraints are applied. The debonding of adjacent plies is modeled to illustrate the effect of delamination growth on the postbuckling response of the delaminated composite plate. A parametric study is carried out to investigate the influence of size and location of delamination as well as the amplitude of imperfection on the postbuckling behavior with concurrent propagation of delamination. A three dimensional finite element analysis is also implemented. A comparison study is conducted for numerical predictions of the developed theoretical model and implemented finite element simulation. It is found that the snap-back instability can occur during the delamination growth. Moreover, the results show that predictions based upon an analysis without consideration for the delamination growth could not be reliable.
•The instability of laminates with single and multiple off-center delaminations is investigated.•A three dimensional finite element analysis is conducted using cohesive zone method.•The snap-back instability may occur during delamination growth.•The proposed method is able to significantly reduce the computational costs.
The compressive strength of impact damaged composite laminates may be reduced by ply damage and delaminations, which may grow under load. At coupon level, the residual strength of impacted laminates ...is assessed through compression after impact (CAI) tests; as a complement to the experiments, numerical tools are sought that predict the behavior of delaminated composites.
In this context, the paper investigates the influence of the cohesive elements parameters – the inter-facial strengths and the cohesive mesh size – in CAI test simulations. The study is carried out using a recently developed initialization technique that allows CAI simulations be run independently from the impact simulations (used to calculate induced delaminations).
The first part of the study uses a conventional sequential approach (CAI analyses follows the impact simulations which share the same parameters) to asses the sensitivity of force time histories and in-plane delaminations growths to parameters variation. Then, for a selected initial damage scenario, the inter-facial strengths sensitivity is performed also evaluating the computational efficiency.
The study shows how the cohesive parameters affect both the accuracy and the computational cost of the analyses and contributes to establish guidelines for an effective use of cohesive elements in CAI tests simulations.
Composite materials are extensively used in various applications like space, aircraft, and automobile sector because of superior physical and mechanical properties even though they are costly. In ...recent technological innovations, using Montmorillonite (Nanoclay) to reinforce polymer-based composites has raised attention to academic and industrial sectors since small addition could enhance Mechanical properties thereby decreasing failures like delaminations. In present research work, the effect of AWJ machining parameters on delaminations of glass fibre reinforced epoxy composite is investigated. The Main objective is determining delamination factor and reduce delaminations which is major failure in laminates. This paper investigates on effects of impregnated Nanoclay epoxy in Bi-directional GFRP where previous research has been made only in changing the parameters for reduction of delaminations. The samples were machined using AWJ, delamination factors are measured using image-J software and SEM analysis for comparing micrographs. Finally with increase in nano clay weight fraction, delaminations are checked.
Performances are a key concern in aerospace vehicles, requiring safer structures with as little consumption as possible. Composite materials replaced aluminum alloys even in primary aerospace ...structures to achieve higher performances with lighter components. However, random events such as low-velocity impacts may induce damages that are typically more dangerous and mostly not visible than metals. The damage tolerance (DT) approach is adopted for the fatigue design of aircraft, but fracture mechanisms and propagation of failure prediction in composite structures are much more challenging. Consequently, the DT approach is still costly for these types of structures. It can be achieved only through expensive experimental testing and a drastic reduction of allowable stress levels and maintenance intervals by applying scattering factors due to the uncertainties involved in their original estimations. Structural health monitoring (SHM) systems deal mainly with sensorised structures providing signals related to their “load and health status” to reduce maintenance and weights. At the same time, the use of Deep Neural Networks (DNNs) based on strategic engineering criteria, for instance, may represent an effective and efficient analysis tool to promote faster data analysis and classification. In the field of aircraft maintenance, this approach may lead, for example, to a faster awareness of an aircraft/fleet situation or predict failures. Deep learning-based networks provide automatic feature extraction at different levels of abstraction. With the universal function approximation property of neural networks, it learns the inverse mapping from input space (signals) to target space (damage classes). Starting from the well-established Structural Health Monitoring (SHM) technologies, a network of distributed sensors embedded throughout the structure could be used for real-time structural monitoring and data acquisition. Structural data will constitute an enormous amount of information that can be adequately filtered with the help of specific DNNs designed and trained for the structural context and aimed to classify and identify significant parameters. The authors have collaborated for some years to collect wave propagation signals through experimental tests and validated numerical models of healthy and damaged composite structures, and developed machine learning algorithms (mainly dense and convolutional neural networks) aimed at signal classification and analysis for damage detection and localization. This paper presents a brief review of relevant works about SHM employing Machine Learning methodologies and summarizes the most promising approaches developed during the last years jointly by the two research groups and presents a critical analysis of obtained results and subsequent future activities.
Delaminations are flat subsurface defects parallel to the sample surface. Recently we have demonstrated that lock-in infrared thermography, with optical excitation, allows sizing the geometrical ...parameters (length, depth and thickness) of a semi-infinite delamination. Here, we analyse the ability of this technique to resolve several parallel and semi-infinite delaminations. First, we develop an analytical method (based on the thermal quadrupoles) together with a numerical formulation to calculate the surface temperature of a sample containing several semi-infinite parallel delaminations. We verify that both methods provide the same temperature values, indicating their consistency. Then, we study the ability of lock-in infrared thermography to resolve two close delaminations. In particular we focus on two main configurations: two non-overshadowed delaminations and two superimposed delaminations. Next, after analysing the inverse problem in terms of residual function minimization, we develop a dedicated parametric estimation procedure able to retrieve the geometry of the studied defects. Finally, we test this procedure with synthetic temperature amplitude and phase data to retrieve the geometrical parameters of both delaminations.
Splits are delaminations that may appear perpendicular to the crack plane during fracture toughness tests of certain materials, such as hot-rolled metal alloys. X-ray computed tomography (CT) was ...used to conduct a 3D analysis of the geometrical and morphological characteristics of the splits in SE(B) specimens machined from a DH36 steel. Tomograms and 3D reconstructions of the CT results were compared with high-resolution images obtained through optical microscopy (OM) and scanning electron microscopy (SEM). Quantitative and qualitative comparisons revealed a good agreement between the results, validating the split characterization by CT. It was discussed whether characterizing the splits just by the routinary fracture surface observation conducted in fracture mechanics specimens can hide important phenomena such as plane changes, branching, and interactions between delaminations. On the other hand, CT enables an accurate and comprehensive characterization of the morphological and geometrical attributes of splits. Contrasts between the analysis and characteristics of deformed and undeformed splits were made. Finally, the limitations and challenges of the 3D split characterization by CT were also discussed, exploring experimental and image processing issues. These findings emphasize that a more thorough understanding of the internal structure of splits can be achieved by applying CT analysis, contrasting with traditional fracture surface examination. This study highlights the relevance of CT in revealing hidden complexities within the internal structure of specimens with splits.
This paper presents an approach for computing the growth of Mode I, II and Mixed Mode I/II delaminations in carbon fibre reinforced polymer composites (CFRP) using a modified Hartman–Schijve ...equation. Unlike other equations it does not involve splitting the energy release rate into its various components. One advantage of this formulation is that the exponent of the associated power law appears to be independent of the mode as is the constant of proportionality. This formulation is shown to accurately compute the delamination growth rates associated with a range of Mode I, II and Mixed Mode I/II data available in the open literature. The potential for this approach to be used to overcome the no growth philosophy associated with current composite designs is also discussed.
Delaminations are flat subsurface flaws parallel to the sample surface. Lock-in infrared thermography with optical stimulation is proposed to size the geometrical parameters of a delamination. Here ...we deal with semi-infinite delaminations, which are characterized by three geometrical parameters: length, depth and thickness. In a recent paper we calculated analytically and numerically the temperature oscillation of a sample containing a semi-infinite delamination when illuminated by a homogeneous and modulated laser beam. In this work, we present lock-in infrared thermography experiments performed on calibrated delaminations manufactured on AISI-304 stainless steel. We fit the numerical model to the experiments to retrieve the delamination length, depth and thickness. The agreement between nominal and retrieved values confirms the validity of the method and paves the way for characterization of finite delaminations.
PDF
