Delamination Size Research Articles

Effect of delamination on the fatigue life of GFRP: A thermographic and numerical study

Delamination is the major failure mechanism in composite laminates and eventually leads to material failure. An early-detection and a better understanding of this phenomenon, through non-destructive assessment, can provide a proper in situ repair and allow a better evaluation of its effects on residual strength of lightweight structural components. Here we adopt a joint numerical-experimental approach to study the effect of delamination on the fatigue life of glass/epoxy composites. To identify and monitor the evolution of the delamination during loading, we carried out stepwise cyclic tests coupled with IR-thermography on both undamaged and artificially-damaged samples. The outcome of the tests shows that IR-thermography is able to identify a threshold stress, named damage stress σD, which is correlated to the damage initiation and the fatigue performance of the composite. Additionally, we performed FE-simulations, implementing the delamination by cohesive elements. Such models, calibrated on the basis of the experimental fatigue results, can provide a tool to assess the effect of parameters, such as the delamination size and location and composite stacking sequence, on the residual strength and fatigue life of the composite material.

Analysis of Open-Hole Compressive CFRP Laminates at Various Temperatures Based on a Multiscale Strategy

In this paper, a multiscale analysis strategy was proposed to analyze the failure behaviors of open-hole compressive (OHC) CFRP laminates. Micro-level intralaminar failure was defined in the constituents (fiber and matrix) with a modified micromechanics failure theory. In the multiscale stress transformation, the effect of thermal residual stress was considered using constant thermal amplification factor. Meanwhile, macro-level interlaminar failure was defined with cohesive elements. Based on the simulated and experimental results, the sub-laminate scaled OHC laminates of the stacking sequence [45/0/−45/90]4s were studied at different temperatures. The established multiscale model showed good precision in the strength and failure mode predictions. Transverse throughout damage at the hole section led to the final failure. As the temperature increased, the damage process began at a lower load level and the strength of the laminates decreased significantly. Stiffness reductions and small load drops were more likely to occur before final failure. The differences in the delamination size among all interfaces tended to be smaller. Besides, matrix failure lagged under shear loading conditions if the thermal residual stress was neglected in the multiscale analysis.

Experimental and numerical analysis of heat convection in cylindrical composite structures with internal defects

The present article discusses the influence of delamination and external loads on the heat transfer in flat and curved laminated panels. Three cases are considered in details: (1) the unloaded structure (length is a constant), (2) the static loading, and (3) the fatigue loading. Here, the comparison between experimental and numerical (finite element method) results is presented. The numerical model allows us to adopt implementations of various boundary conditions as well as the size of delamination. The influence of the shallowness parameter and the thermal resistance is demonstrated. The experimental detection of temperature contours during the cooling process has been conducted at different points in the area surrounding the delamination.

Diagnosis of Delaminated Composites Using Post-processed Strain Measurements under Impact Loading

Potentially having a destructive influence on the mechanical properties of composite laminates, the invisible phenomenon of delamination frequently occurs under impact loading. In the present study, simulating the performance of long-gauge fiber Bragg grating sensors, impact-induced average strains within laminated composites are utilized to develop a delamination identification technique. First, strain-based modal parameters are extracted from the estimator of frequency response functions using two different approaches. Next, these parameters are employed in various damage detection indicators. Then, the presence, location, and severity of delamination introduced at locations with the least effect on modal changes are detected. The results of different numerical examples with various delamination sizes and positions elucidate the considerable efficiency of average-strain measurements to diagnose delaminated composites.

A Comparative Study of the Behavior of CFRP and GFRP Laminates in a Plate Specimen Using Modified Virtual Crack Closure Technique (MVCCT)

The applications of polymer composites in aircraft industry have exponentially increased in the recent years due to their high strength to weight ratio. Presence of delaminations in composites is inevitable which affects the structural stability due to reduction in structural stiffness and strength. The degradation of a structural component depends on the geometric characteristics of delamination, nature of loading and material characteristics. Damage tolerance study is thus essential to determine the extent of degradation of the structure due to the presence of delamination. The present paper brings about a comparision between the behaviour of a standard plate specimen made up of Carbon Fibre Reinforced Polymer (CFRP) and Glass Fibre Reinforced Polymer (GFRP) laminates with circular delaminations of varying diameters and subject to compressive load.A constant compressive load in terms of initial displacements was applied on a quasi-isotropic square plate specimen of dimensions 200 mm x 200 mm with a thickness of 2.88 mm of both CFRP and GFRP configurations. A circular delamination was introduced at the centre of the plate and its diameter and position along the thickness direction were varied and studied. Using ABAQUS codes of practice, Strain Energy Release Rate (SERR) was computed. The principles of Modified Virtual Crack Closure Technique (MVCCT) was used to compute SERR. Delamination propagates when the computed mixed mode energy release rate exceeds the critical value, GC. Depending on the external loading and material properties, Total strain energy release rate, GT (GT=GI+GII+GIII) was used to predict growth of delamination. The onset of delamination growth was determined by plotting the values of (GT/GC) across various delamination sizes along the thickness of the plate and reported.

Investigating the Effect of Delamination Size, Stacking Sequences and Boundary Conditions on The Vibration Properties of Carbon Fiber Reinforced Polymer Composite

Investigating the Effect of Delamination Size, Stacking Sequences and Boundary Conditions on The Vibration Properties of Carbon Fiber Reinforced Polymer Composite

Drilling Behavior of Flax/Poly(Lactic Acid) Bio-Composite Laminates: An Experimental Investigation

ABSTRACT This paper aims to investigate the effects of machining parameters such as spindle speed, feed rate and drill diameter on machinability of flax/poly(lactic acid) bio-composites, to analyze the relations among cutting forces, drilling-induced damages and crack propagation of the drilled samples. In particular, a set of drilling experiments were conducted using different drilling conditions and a new low-cost measurement set-up was developed to measure the cutting force during the drilling operation. In addition, the analysis of variance (ANOVA) was applied to identify the significance of each individual cutting parameter. The experimental results indicate the relation between the thrust force and the machinability parameters of flax fiber reinforced bio-composite. The increase in spindle speed reduces thrust force and delamination size of the drilled holes, whereas an increase in feed and drill diameter leads to a considerable increase in both thrust force and delamination factor. The effect of spindle speed on peripheral damage was not significant for the drills tested, though the feed rate was found to play the key role on the delamination damage area. The best hole quality was achieved with the samples drilled at spindle speed and feed rate of 3000 rpm and 0.11 mm/rev, respectively.

Post-buckling induced delamination propagation of composite laminates with bi-nonlinear properties and anti-penetrating interaction effects

This paper presents a unique theoretical analysis for the multiple failures of in-plane loaded composite laminates containing a through-width delamination. Under the action of the in-plane load, the delaminated laminates undergo first post buckling failure, then delamination propagation. The anti-penetrating contact effect occurred at the interfaces of the delamination is identified by analyzing the local transverse deformation and is incorporated into the multiple failure stages. Based on von Karman nonlinear geometric relationships, the post-buckling equilibrium path of the delaminated laminates characterized by the in-plane load vs deflection amplitude curves is solved. Expression of energy release rate is derived from variational principles as the summation of the products of shear forces and deflection slopes at the delamination fronts. With the available solutions from post-buckling analysis, threshold to initiate the delamination to propagate is computed according to Griffith fracture criterion and is calculated for different delamination conditions by developing Matlab program. It is found that the delaminated laminate undergoes a consistently macroscopic deflection with microscopic local buckles on the interfaces of the delamination. For all cases, Mode I delamination propagation can never occur due to the contact behavior. Instead, Mode II delamination propagation prevails regardless of the delamination size and position. The analysis presented in this paper accounts for nonlinear geometric deformation, nonlinear constitutive behavior and anti-penetrating interaction effects and identifies multiscale deformation modes involved in different failure mechanisms, thereby demonstrates the latest research idea for the current frontier problems.

Stability and failure analyses of delaminated composite plates subjected to localized heating

A layerwise B-spline finite element formulation is used to study the stability and first-ply failure of composite plates with embedded delaminations and subjected to localized thermal heating over the plate surface with uniform temperature rise through its thickness. The step functions are used to represent jump discontinuities in displacement fields at the zone of delamination between layers of composite plates. As the applied thermal loading is nonuniform, in the first step the prebuckling stress distribution within the plate is evaluated by solving the thermoelasticity problem. Subsequently, thermal buckling temperature is computed using the plate prebuckling stress distribution. Postbuckling response of the plate is obtained considering the von Kármán type geometric nonlinearity. In the delaminated region, virtual springs are added to prevent interpenetration of lower and upper sublaminates. The localized thermal load at which the first-ply failure of the lamina occurs has been detected by Tsai-Wu quadratic interaction criterion. The effects of the area of heating region, delamination size and its position in the thickness direction and plate boundary condition on the composite plate critical buckling temperature and on first-ply failure load are reported. The present model can capture global, local and combined global–local buckling mode shapes.

The role of n-3 polyunsaturated fatty acids in the formation of the psychophysiological status of ski racers

A layerwise B-spline finite element formulation is used to study the stability and first-ply failure of composite plates with embedded delaminations and subjected to localized thermal heating over the plate surface with uniform temperature rise through its thickness. The step functions are used to represent jump discontinuities in displacement fields at the zone of delamination between layers of composite plates. As the applied thermal loading is nonuniform, in the first step the prebuckling stress distribution within the plate is evaluated by solving the thermoelasticity problem. Subsequently, thermal buckling temperature is computed using the plate prebuckling stress distribution. Postbuckling response of the plate is obtained considering the von Kármán type geometric nonlinearity. In the delaminated region, virtual springs are added to prevent interpenetration of lower and upper sublaminates. The localized thermal load at which the first-ply failure of the lamina occurs has been detected by Tsai-Wu quadratic interaction criterion. The effects of the area of heating region, delamination size and its position in the thickness direction and plate boundary condition on the composite plate critical buckling temperature and on first-ply failure load are reported. The present model can capture global, local and combined global–local buckling mode shapes.

Influence of temperature and impact energy on low velocity impact damage severity in CFRP

This study focuses on the effect of temperature and impact energy on the damage in CFRP under low velocity impact. Quasi-isotropic laminates with a thickness of 4.1 mm were impacted with energies between 8 and 21J at temperatures ranging between 20 and 80°C. The resulting damage was assessed using ultrasonic C-scans, radiography and confocal microscopy. The residual strength was evaluated utilizing compressive tests at 20°C. It was found that delamination size is decreasing with increasing temperature. However, severe fibre failure on the impacted side occurs at elevated temperatures. This increases the visual damage detectability on the impacted side heavily while decreasing it on the opposite side. Nevertheless, the residual compressive strength is mainly dependent on the delamination area. Different impact energies can have the same effect on the residual strength, when impacted at different temperatures. As a result, visual damage severity does not correlate with the residual compressive strength.

Effects of rebars on the detectability of subsurface defects in concrete bridges using square pulse thermography

The ability to predict critical conditions on highway concrete bridges is important for the avoidance of unpredictable collapses during the operation period. Potential deteriorations can develop, such as terms of cracks, air voids, and subsurface delaminated areas that are usually detected through nondestructive testing (NDT) techniques together with conventional sounding methods, i.e. hammer sounding and chain dragging. Of these deteriorations, delamination is one of the major indicators of concrete bridge health, i.e., the measurement of the percentage of delaminated area. Due to vehicle loading, changes in weather and environment, and corrosion of reinforcing steel bars, delamination can occur above or even below the rebar layer, and it can be difficult to identify and determine the size and depth of delamination, especially for delamination below the rebars. In this study, two concrete specimens were analyzed in the laboratory as used with a wide range of different concrete covers that are commonly applied in bridge structures, from 2 to 8 cm at near surface. Square Pulse Thermography (SPT), one of the active infrared thermography (IRT) techniques, is used to analyze the specimens in terms of their various heating regimes and environmental conditions. This study aims to comprehensively investigate the effect of steel reinforcement on the detectability of delamination when the delaminations have a width-to-depth ratio equal to or smaller than 2.0. The results show that delamination above the rebars indicates a higher absolute contrast than that below the rebars. In addition, the observation time of delamination below the rebars decreases significantly. As a result, the predicted depth of a delamination will be less than the value of the real depth.

Nonlinear finite element solutions of thermoelastic deflection and stress responses of internally damaged curved panel structure

• A novel numerical model is developed to study the deflection and stress values of the pre-damaged composite structure. • The internal damage has been modeled via sub-laminate approach including two kinematic theories. • The nonlinear FEM solutions are obtained under the combined thermomechanical loading. • The numerical solutions (deflection and stress) are evaluated for different panel geometries via the derived models. Thermoelastic deflection and corresponding stresses of the pre-damaged layered panel structure are investigated numerically in this article including the large deformation kinematics under the linearly varying temperature field. The composite structural deformation kinematics is derived using two different polynomial type of kinematic theories including the through-thickness stretching effect. The inter-laminar separation between the adjacent layers is incurred via the sub-laminate approach and Green–Lagrange strain to count the total structural deformation. Also, the intermittent displacement continuity conditions are imposed in the current mathematical model to establish the displacement continuity between the separated layers. The variational principle is adopted for the evaluation of the nonlinear structural equilibrium equations and solved via total Lagrangian approach . The convergence and the corresponding validity of the currently derived nonlinear finite element solutions are checked by solving different sets of numerical examples. Additionally, the comprehensive inferences are drawn from various numerical examples for the well-defined important input parameter including the size, position, and location of delamination .

Lamb wave interaction with composite delamination

Elastic guided wave based evaluation is useful to detect delamination in composites. This paper reports a detailed account of guided wave interaction with delamination in laminated composites modeled using time domain spectral finite element (TSFE) method. Wave scattering due to different delamination positions is studied. Wave field near the delamination in different layer-wise positions is investigated, which is useful to further characterize and correlate the far field wave packet carrying the parametric signature of delamination. Off-axis delamination in composite laminate creates an asymmetry in the elastodynamic stress transfer. It leads to wave mode conversions. Sensitivity of the delamination to the wavelengths is studied. The outcome of this study indicates potential possibilities to use frequency-wavelength information to discriminate various sizes of delamination. Energy of the scattered waves and dissipation/conversion of the wave energy due to the defects depend on the resonance characteristics of the sub-laminates. Wave scattering effect due to length-wise multiple delaminations and edge delamination is also analyzed. The resonance patterns in the signals are analyzed with reference to the defect quantification problem.

On the physics of applying finite width and geometry correction factors in fatigue crack growth predictions of GLARE

In general, a finite width correction to stress intensity factor (SIF) is required in the fatigue crack growth. The finite width correction factor can be explained physically from the energy point of view. It is assumed that the finite width correction factor primarily constitutes an energy correction factor, i.e. it corrects the applied load for the work applied. To evaluate the finite width correction for FMLs, constant amplitude load fatigue crack growth tests were performed on monolithic aluminium 2024-T3 and the Fibre Metal Laminate GLARE containing 2024-T3 aluminium layers. The loads and displacements were recorded to quantify the total amount of work applied throughout each fatigue test. The crack length and delamination size were monitored by using digital image correlation technique to evaluate the dissipative energy. It appears that the Feddersen’s and all other standard finite width correction significantly overestimates the effect for FMLs. The finite width correction to SIF for FMLs is small but cannot be neglected, and it is also greatly related to the Glare grades, stress ratio and stress level.

Effect of Z-pinning on the impact resistance of composite laminates with different layups

The paper presents the results of an experimental investigation into the effect of Z-pinning on the impact resistance of composite laminates with different multi-directional layups. Unpinned and pinned [02/902]s, [02/±45]s and [0/±45/90]s carbon/epoxy samples were subjected to low-velocity impacts resulting in damage severities ranging from barely visible damage to full laminate penetration. The responses to impact of the laminates were characterized in terms of damage evolution, load-carrying capacity, and resistance to perforation and penetration. It was found that even though Z-pins are not capable of delaying the onset of delamination, they significantly reduce delamination size for high-energy impacts. Z-pinning improves the resistance to perforation and penetration of [02/902]s, laminates, while it does not significantly affect that of [02/±45]s and [0/±45/90]s samples. The dependence of the efficacy of Z–pins on the laminate layup is attributed to the different distribution of the delaminations through the thickness of the investigated laminates.

The effect of delamination size and location to buckling behavior of composite materials

In this study, the size and location of delamination effects to layered composites' mechanical properties in buckling behavior are inspected experimentally. Composite plates were produced in accordance with the different dimensions of delaminations and the positions between the layers. These plates consist in delamination rates of a/L=0.3,0.5,0.7 (a: Delamination width and L: Buckling length). The specimens were subjected to buckling tests with placed on its two edges, while the others are free. In the experiments two results are considered. First, we determined load-displacement graphs. Critical buckling loads (Pcr) were determined from the obtained load-displacement graphs. Second, we acquire lateral displacements, which occurred with the effect of applied load in the delamination area in direction of plate thickness. By comparing Pcr with each other, an optimum delamination dimension was determined for composite materials with delamination zones. In the applications, the negative effect on the mechanical properties due to delaminations can be seen, proportionally according to the delamination location.

Studies on shape memory alloy-embedded GFRP composites for improved post-impact damage strength

ABSTRACT Embedding shape memory alloys (SMAs) in composites is a promising method resistance against impact loading.In the present paper, an attempt is made to quantify the improvement in damage mitigation properties of FRP (fibre reinforced plastic)composites by embedding SMA.Numerical modelling of low velocity impact was carried out topredict the delamination in composites and composites embedded with SMA and steel wires. Through modelling, effect of location and quantity of SMA material in decreasing the impact induced delamination size was also studied.Compression after impact (CAI) tests were carried out on pristine and SMA hybrid composite (SMAHC)specimens to quantify the change in damage resistance. Experimental results show an improvement in compressive load-carrying capacity after impact in SMAHC as compared to pristine composites.Experimental results match well with numerical findings in terms of; location of placement of SMA wire and optimum quantity of SMA wires.

Mixed mode embedded circular delamination propagation in spar wingskin joint made with curved FRP composite laminates

This paper presents the mixed mode propagation of circular delamination pre-embedded at the interface of 1st and 2nd plies of wingskin adherend of the Spar Wingskin Joint made with curved fibre reinforced plastic composite panels. Three dimensional finite element analyses of these joints under uniformly applied out of plane transverse load have been carried out using contact and multi point constraint elements. The growth of embedded delamination is simulated by sequential release of the constraints of these elements ahead of the delamination front. The inter-laminar stresses and strain energy release rate values being indicative parameters in delamination growth studies are computed in the vicinity of the curved delamination fronts. These values are evaluated using virtual crack closure technique. It has been observed that the circular delamination size significantly influences the magnitudes of inter-laminar stresses and the three components of SERR values in the vicinity of delamination front. Maximum values of peel stress and mode I SERR occur at orthogonal locations to the loading direction i.e. at θ = 90° and 270° measured counter clockwise from positive X-axis along the periphery of the delamination fronts. The variations of inter-laminar stresses and the values of SERR are non-uniform along the circular delamination front indicating variable rates of propagation. This simulation procedure will be used for the assessment of loss of structural integrity of the SWJs having circular delamination when the embedded delamination undergoes propagation upon increase of loading.

Experimental study on drilling of jute fiber reinforced polymer composites

The use of natural fiber composites is growing at a fast rate in terms of industrial applications due to their eco-friendly, recyclable, renewable nature, and low density/costs. Recently, jute fiber is being used as a reinforcement material in the development of natural fiber reinforced plastics for engineering applications. Drilling is an essential machining operation that is frequently performed for making of holes in composite structures to facilitate assembly of several components into a complex part. The literature reviews indicate that only a few of research articles investigated drilling of natural fiber reinforced composites. Hence, in the present research, drilling of woven jute fiber reinforced polymer composites has been examined. This study aimed at evaluation of cutting parameters and drill bit types effect on thrust force, delamination size, and surface roughness in the jute fiber reinforced polymer composites drilling using an experimental design based on full factorial technique. The percentage contribution of the cutting parameters and drill bit types were determined through the analysis of variance. The optimal setting of these parameters was found through observation. The experimental results indicated that the most significant effect was the drill bit type influencing both delamination factor and surface roughness.