Graphite Epoxy Composite Laminates Research Articles

Evaluating SIFs in finite orthotropic composites from experimentally determined stress coefficients

Unlike with isotropy materials, determining stress intensity factors in orthotropic composites is complicated as they can depend on constitutive properties and principal material directions. Acknowledging the above, this paper develops a novel technique to evaluate stress intensity factors in finite, plane-stressed, orthotropic composites. Upon substantiating the numerical reliability of the approach, KI is determined experimentally in a finite tensile loaded double-edge cracked [013/905/013] graphite-epoxy composite laminate from displacement-evaluated stress coefficients. Only a single DIC-measured displacement field is need. Advantages of the method include applicability beyond a laboratory environment and to other structural shapes, loads and crack configurations, wide acceptable range of stress coefficients and highly arbitrary source locations of the measured input data. Unlike purely numerical or theoretical approaches, the method does not require knowing the external loading. This is significant as loads are often unknown in practice. Results are verified independently.

Compressive failure behaviour of glass epoxy and graphite epoxy composite laminates due to the effects of different cut-out sizes and variations in fibre orientation

Composite laminates consisting epoxy are widely used in major industries. Hence, the mechanical property of these materials needs to be fully understood. Currently, there is limited studies on the effect of various square cut-out sizes related to glass epoxy and graphite epoxy composite laminates, therefore, the behaviour of these laminates is not fully understood. This paper aims to study the effect of the square cut-out sizes on the failure performance of glass epoxy and graphite epoxy composite laminates with various fibre orientations. Prior to the compressive failure analysis, convergence and numerical validation were conducted. The failure analysis is conducted by using the finite element method (FEM) using ANSYS with maximum stress theory as the failure criteria. The results show a substantial difference in the strength of the laminates without and with cut-outs. The main finding that can be deduced from the study is, the presence of cut-out reduces the strength of the composite laminates. Different fibre orientations effected graphite epoxy composite laminate at a considerable amount compared to glass epoxy laminates. From the results, it is proven that the present study is important in understanding the compressive failure behaviour of the composite plate in which can be useful in contributing the knowledge for further study.

Neural Network-Based Prediction Model to Investigate the Influence of Temperature and Moisture on Vibration Characteristics of Skew Laminated Composite Sandwich Plates.

The present study deals with the development of a prediction model to investigate the impact of temperature and moisture on the vibration response of a skew laminated composite sandwich (LCS) plate using the artificial neural network (ANN) technique. Firstly, a finite element model is generated to incorporate the hygro-elastic and thermo-elastic characteristics of the LCS plate using first-order shear deformation theory (FSDT). Graphite-epoxy composite laminates are used as the face sheets, and DYAD606 viscoelastic material is used as the core material. Non-linear strain-displacement relations are used to generate the initial stiffness matrix in order to represent the stiffness generated from the uniformly varying temperature and moisture concentrations. The mechanical stiffness matrix is derived using linear strain-displacement associations. Then the results obtained from the numerical model are used to train the ANN. About 11,520 data points were collected from the numerical analysis and were used to train the network using the Levenberg–Marquardt algorithm. The developed ANN model is used to study the influence of various process parameters on the frequency response of the system, and the outcomes are compared with the results obtained from the numerical model. Several numerical examples are presented and conferred to comprehend the influence of temperature and moisture on the LCS plates.

Comparison and analysis of Acoustography with other NDE techniques for foreign object inclusion detection in graphite epoxy composites

This paper presents the use of a novel through-transmission ultrasonic (TTU) Acoustography non-destructive evaluation (NDE) method to detect foreign object inclusion (FOI) defects in graphite epoxy composite laminates. The study employed three different composite test standards with varied size FOI defects embedded at varying depth within the composite laminates. For validation, Acoustography results were directly compared with conventional immersion TTU testing and infrared thermography (IRT) methods. From results obtained, it was demonstrated that the Signal-to-Noise Ratio (SNR) measurements for Acoustography were more than 6:1 and were in good correlation with immersion TTU and IRT results. The defect sizing ability of TTU Acoustography for FOI defects in graphite epoxy composite laminates were also in strong correlation with immersion TTU and IRT techniques. Finally, for the three laboratory systems employed in this study, typical panel TTU Acoustography inspection time was just about three minutes to scan a 300 mm × 300 mm (11.8″ × 11.8″) area, which was more than three times faster compared to IRT and sixty times faster to conventional immersion TTU C-Scan techniques. This is a very significant finding for the reason that Acoustography is being developed as a faster, more efficient, and affordable alternative to traditional ultrasonic inspection systems for composite manufacturing quality control and quality assurance (QC/QA) and field maintenance of composite structure applications.

Effect of oblique angle on low velocity impact response of delaminated composite conical shells

This paper presents the effect of oblique impact angle on low velocity transient dynamic responses of delaminated composite pretwisted shallow conical shells. An eight-noded isoparametric quadratic plate bending element is employed in the finite element formulation incorporating rotary inertia and effects of transverse shear deformation based on Mindlin’s theory. The modified Hertzian contact law which accounts for permanent indentation is utilized to compute the contact force, and the time-dependent equations are solved by Newmark’s time integration scheme. A comparative study is carried out on torsion stiff, cross-ply, and bending stiff laminates to investigate the effects of triggering parameters like angle of twist, plate displacement, striker’s velocity, and displacement for graphite-epoxy composite laminate subjected to low velocity oblique impact at the center.

Application of the point stress criterion to the failure of composite pinned joints

An analysis was performed to evaluate the bearing strength of pin-loaded composite joints using a two parameter characteristic curve model. This model involves determination of characteristic dimensions in tension and compression and based on this model, a two-dimensional stress analysis was used to determine the stress distribution around the fastener hole. In this analysis, characteristic dimensions in tension and compression were evaluated using the point stress failure criterion and joint bearing failure evaluated using the Yamada–Sun failure criterion. Results were compared with available experimental data for joints made from AS4/3501-6 graphite epoxy composite laminates and good correlation observed when evaluated as function of edge distance to hole diameter. However, the analysis yields conservative results when joint strength is evaluated as a function of plate width to hole diameter.

Elastic waves from localized sources in composite laminates

This paper is concerned with the analysis of elastic waves generated by localized dynamic sources in structural composites. The source can be external, involving acoustic wave loading as in the so called leaky Lamb wave experiment, and low-velocity foreign object impact on the surface of the structure, or internal, as in sudden crack initiation and its rapid growth from existing internal flaws. All three problems are of critical importance in the safe operation of composite structures, due to their vulnerability to hidden delaminations that can occur in composite materials when they are subjected to this type of loads. It is well known that both the dynamic surface loading associated with impact, and the sudden “opening” of an internal crack associated with the extension of a preexisting flaw act as sources of elastic waves in the material of the structure. The research reported here consists of model-based analysis of the guided waves generated by surface loading and microcrack initiation in graphite epoxy composite laminates commonly used in aircraft and aerospace structures. The objective of this study is to develop a mechanics based understanding of the causal relationship between the properties of the source and the characteristics of the waves generated by its initiation and propagation. The results of this research are expected to be useful in developing effective health monitoring systems for new as well as aging aircraft and aerospace structures.

Hygroscopically induced residual stresses in composite laminates

In this paper the exact moisture concentration function m(z, t) is used to compute the hygroscopic residual stresses in laminated composites. The governing equations were established by using a full higher-order displacement theory. Hygroscopically induced residual stresses in thick graphite-epoxy composite laminates: Cross-ply [0/90] 8 based on this approach were compared with those obtained by assumption: the moisture concentration m is a constant in all plies through the laminates thickness

Experimental investigation of the dynamic response of graphite-epoxy composite laminates under compression

In this study, the dynamic stress–strain response of graphite-epoxy composite laminates is investigated. The laminates are interposed in a section of a split Hopkinson apparatus. A quasi-rectangular wave is generated at one end of the incident bar by striking it with another bar of known length. This bar is accelerated using a compressed air gun. Approximate average stresses and strains can be obtained by measuring the incident, reflected and transmitted waves in the split bar. The dynamic behavior is evaluated for a range of impact velocities. The dependence of the response on impact velocity is analyzed and discussed. Three different specimen thicknesses have been used. These are obtained by increasing the repetition factor of a base stacking sequence: (+45°, −45°, 0°, 90°). This process is called sublaminate scaling; it is preferred to ply scaling since it has been shown that the accumulation of layers of the same orientation decreases the failure load to such an extent that residual stresses may crack the specimen before any external load is applied. The laminates considered are: (+45°, −45°, 0°, 90°) ns , n=2,3,4. The scale effects observed in the experimental response are analyzed and discussed.

Prediction of the location of delamination in the drilling of composite laminates

An analytical approach for the determination of the position of the onset of delamination during the drilling of composite laminates based on linear elastic fracture mechanics is presented in this paper. The critical thrust force calculated from the predicted position of the onset of delamination has a reasonable agreement with experimental results obtained in the drilling of T300/5208 graphite epoxy composite laminates reported in the literature. Thus, the analytical method developed in the present study may be applied to the drilling of composite laminates drilling with the avoidance of delamination.

Postbuckling analysis of composite laminates with a delamination

A finite element modeling is presented to study the postbuckling behavior of composite laminates with an embedded delamination. The postbuckling analysis of graphite-epoxy composite laminates with a delamination is studied for a through-the-width delamination, and an embedded delamination. Three different possible modes of instability are identified at the critical load, which is global, mixed and local buckling modes. It is found that there exists three types of the postbuckling behavior, which depend on the delamination buckling mode. The postbuckling behavior of composite laminates with an embedded delamination shows the same pattern as that of composite laminates with a through-the-width delamination.

光ファイバによるＣＦＲＰ積層板の内部ひずみの測定およびクラックの検出

Optical fibers were integrated into graphite-epoxy composites in their manufacturing process. Two types of embedded fiber optic sensors were tested. One was an embedded fiber optic strain sensor, and the other was an embedded sensor for detection of cracking, utilizing optical fiber breaks. First, in order to evaluate the bonding property between optical fiber and epoxy resin, the optical fibers were integrated into epoxy resins, and the tensile test was conducted. As a result, by removing an acrylate coating of the optical fiber with H2SO4, the optical fiber showed good adhesion to the surrounding epoxy resin. The ultimate strain of the uncoated optical fiber was about 1.4 to 1.5%. Second, the embedded fiber optic strain sensor was evaluated. The fiber optic strain sensor consisted of Michelson interferometer using a single-mode coupler. The strains in graphite-epoxy composite laminates were measured by the embedded fiber optic strain sensor, and their strain sensitivities agreed well with the theoretical values. Third, the crack detection experiment using the embedded optical fiber was conducted. When the crack was induced by the tensile load, the embedded optical fiber was broken and the light through the optical fiber went out immediately. Such a relation between cracking and optical fiber break was confirmed by tension test with ultrasonic C-scan images. The cracking induced by the impact load in the composite laminates was also monitored. Internal cracking was induced at the early stage of impact load. As the results of these evaluations, it was proved that the embedded fiber optic sensors were effective for the measurement of strain and the detection of cracking in graphite-epoxy composite laminates.

Mixed-Mode Fracture in Unidirectional Graphite Epoxy Composite Laminates with Central Notch

Mixed-mode matrix fracture in central notched off-axis unidirectional composite laminates was investigated. A limited number of unidirectional tensile type specimens with a central, horizontal, notch were tested. Crack initiation and propagation were examined under various local stress fields that were controlled by fiber orientations. The tested specimens were simulated using a two dimensional finite element method with constant strain loading. The strain energy release rates along the crack were evaluated via crack closure technique. The variation of critical strain energy rates with off-axis angle was studied. The results from single (one-sided) and double (two-sided) crack simulations were presented and compared.

Stochastic materials design of composite materials. (Development of simulator for failure path of composite materials).

A basic formulation for evaluating the failure path of composite laminates is presented. The proposed formulation is based on an in-plane failure criteria and first-order reliability method (FORM). The sequential failure path of each layer of composite laminates is simulated by means of modulus degradation and safety indices under the off-axial load conditions. Numerical analyses of graphite epoxy composite laminates are performed corresponding to various lamination configurations with symmetric lamination angles subjected to off-axial in-plane strain conditions. From the simulation analysis, a typical failure path is evaluated corresponding to an in-plane strain vector. The mechanical behaviors such as decreasing modulus and increasing in-plane strain are also simulated in correspondence with each stage of sequential ply failure. The proposed simulator enables one to search the effective failure path in order to evaluate the last ply failure probability of laminates.

Determination of the properties of composite interfaces by an ultrasonic method

The feasibility of using a recently developed ultrasonic technique to determine certain macroscopic properties of the interface zones of composite laminates is studied. The strong influence of the elastic properties and the thickness of the interface zone on the phase velocity of guided waves is demonstrated by means of a simple model of a single fiber embedded in a layer of the matrix material. The overall dynamic elastic moduli of a unidirectional graphite-epoxy composite laminate are determined through inversion of guided wave dispersion data obtained by the leaky Lamb wave experiment. The thickness and elastic properties of the interlaminar interface zone in a cross-ply graphite-epoxy laminate are also estimated by the same approach.

Detection of Delamination Onset in a Composite Laminate Using Moiré Interferometry

Moire interferometry was used to detect and measure delaminations in a graphite-epoxy composite laminate. The laminate used was a [(±30)2/¯9¯0]s layup. A uniform strain was subtracted from the full-field moire pattern by changing the frequency of the reference grating. This technique accentuated the nonuniform strains allowing very high resolution measurements of strain concentrations. These strain concentrations include free-edge effects, resin rich regions and delaminations.

Ultrasonic and Mechanical Characterizations of Fatigue States of Graphite Epoxy Composite Laminates

Abstract : Fiber reinforced composites are inherently nonhomogeneous materials in which fabrication procedures can affect their service reliability without effect on their visual appearance. Williams and Doll observed in a unidirectional graphite fiber epoxy composite that a small change (14 deg c) in the precure temperature resulted in significant changes in trans fiber compressive fracture strength and the transfiber compression-compression fatigue life. Because (for some modes of) fatigue the failure of graphite fiber composites tends to be sudden and occurs without any visible evidence of damage any means of nondestructively monitoring fatigue damage or predicting fatigue behavior of graphite fiber composites is likely to enhance their effective use. In the investigation conducted by Williams and Doll they observed that the ultrasonic attenuation of the composite in the as fabricated state can be an indicator of the composite fatigue life. The purpose of the present study is to further explore the relationship between ultrasonic attenuation and fatigue survivability of graphite fiber epoxy composites fabricated under various processing temperatures and pressures.

Low Velocity Impact of a Viscoelastic Beam

A theoretical solution for the response of a viscoelastic beam to off-center low speed transverse impact is presented. The flexural model adopted for investigation consists of a uniform Bernoulli-Euler beam whose behavior has been generalized to include a linear viscoelastic constitutive relation for each element of the beam. Further, the beam and rigid impactor are assumed to remain in contact during the resulting motion and a consistent set of initial displacement and velocity distributions is adopted for the beam. The solution method utilizes two Laplace transforms, i.e., one with respect to space and the other with respect to time. Comparison of the numerical predictions of the theoretical model with central impact test results on graphite-epoxy composite laminates indicates a good agreement between theory and experiment.