Internal Plies Research Articles

Underwater Implosion and Energy Mitigation of Hybrid Glass-Carbon Composite Shells

Experiments were conducted to investigate the dynamic buckling behavior of underwater hybrid composite tubes. The study focused on roll-wrapped hybrid layered glass-carbon fiber epoxy composite shells with a six-layer quasi-isotropic layup configuration. In addition to control specimens consisting of fully glass fiber-reinforced polymer and carbon fiber-reinforced polymer, four different hybrid layup patterns were examined. These specimens fitted with custom endcaps were placed inside a 7-kiloliter pressure vessel and subjected to increasing hydrostatic pressure until dynamic implosion occurred. High-speed cameras captured the failure event, and the resulting images were analyzed using Digital Image Correlation (DIC) techniques to obtain full-field displacement data. Additionally, tourmaline pressure transducers positioned around the specimens recorded local dynamic pressure histories. The results revealed that the contribution of each ply location varied in the overall failure behavior of the structures. The thickness of the internal plies played a dominant role in enhancing the structural performance, while the stiffness of the outer plies greatly influenced the bending stiffness. The energy released during the collapse was highly dependent on the failure mechanism of the internal plies. Specifically, for the considered geometries, tubes with glass fiber internal plies exhibited significantly lower energy emissions compared to carbon fiber inner plies.

Numerical and Experimental Studies of Multi-ply Woven Carbon Fibre Prepreg Forming Process

Woven carbon fibre prepreg is being increasingly used in high-performance aerospace and automotive applications, primarily because of its superior mechanical properties and formability. A wide range of forming simulation options are available for predicting material deformation during the prepreg forming process, particularly change in fibre orientation. Development of a robust validated simulation model requires comprehensive material characterisation and reliable experimental validation techniques.This paper presents experimental and numerical methods for studying the fibre orientation in multi-ply woven carbon fibre prepreg forming process, using a double-dome geometry. The numerical study is performed using the commercial forming simulation software PAM-FORM and the material input data are generated from a comprehensive experimental material characterisation. Two experimental validation methods are adopted for fibre shear angle measurement: an optical method for measuring only the surface plies, and a novel CT scan method for measuring both the surface plies and the internal plies. The simulation results are compared against the experimental results in terms of fibre shear angle and the formation of wrinkles to assess the validity of the model.

Forming Analysis of Internal Plies of Multi-Layer Unidirectional Textile Preforms using Projectional Radiography

The mechanical properties of fiber reinforced composites strongly depend on the fiber orientations that are created during preforming. Fiber orientations of inner plies of a multi-ply composite structure cannot be analyzed easily, especially for uniaxial unidirectional non-crimp fabric preforms. A novel approach is presented that uses photogrammetry software for optical forming analysis and projectional radiography (x-ray) for image generation. The process of applying a grid of dots with a contrast enhancing color onto the rovings of the fabric and the use of a computer tomograph for creating the images is described. As a result, full-field strain data, shear angles and fiber orientations of an inner ply of an 8-ply unidirectional preform were acquired. Fiber orientations of the analyzed inner ply are compared with the outer plies.

Fluid diffusion in cracked composite laminates – Analytical, numerical and experimental study

Composite structural components are often subjected to service loads that cause cracks in off-axis layers, without compromising the load bearing capability of the structure. In marine or humid environments fluid ingress in the material can be accelerated by the presence of such cracks, leading to further strength degradation.Several studies have been dealing with the effect of cracks on fluid diffusion in composites, sometimes with contradictory results. In this work the difference between cracks on the external and internal plies of the laminate has been addressed both experimentally and analytically, showing that the first ones have a strong influence on diffusivity, while the second have a negligible effect.

Ultrasonic Analytic-Signal Responses From Polymer-Matrix Composite Laminates.

Ultrasound has been used to inspect composite laminates since their invention but only recently has the response from the internal plies themselves been considered of interest. This paper uses modeling techniques to make sense of the fluctuating and interfering reflections from the resin layers between plies, providing clues to the underlying inhomogeneities in the structure. It shows how the analytic signal, analyzed in terms of instantaneous amplitude, phase, and frequency, allows 3-D characterization of the microstructure. It is found that, under certain conditions, the phase becomes locked to the interfaces between plies and that the first and last plies have characteristically different instantaneous frequencies. This allows the thin resin layers between plies to be tracked through various features and anomalies found in real composite components (ply drops, tape gaps, tape overlaps, and out-of-plane wrinkles), giving crucial information about conformance to design of as-manufactured components. Other types of defects such as delaminations are also considered. Supporting evidence is provided from experimental ultrasonic data acquired from real composite specimens and compared with X-ray computed tomography images and microsections.

Ply-orientation measurements in composites using structure-tensor analysis of volumetric ultrasonic data

Ply wrinkling in carbon fibre reinforced polymer (CFRP) laminates is often geometrically complex and difficult to quantify using non-destructive techniques. In this paper, an ultrasonic technique for mapping ply wrinkling is presented. The instantaneous-phase three-dimensional dataset obtained from a pulse-echo ultrasonic inspection is processed using the structure-tensor image processing technique to quantify the orientations of the internal plies of a CFRP laminate. It is shown that consideration must be given to the wrapped nature of the phase dataset during processing to obtain accurate orientation maps. Three dimensional ply orientation and ply-location maps obtained from a test coupon are compared with true ply angles and locations by overlaying the ultrasonically-derived results on X-ray CT image slices, showing that accurate orientation maps can be obtained using the proposed technique.

Assessment of Liquid Resin Infusion Impregnation Quality Using Scanning Electron Microscopy

Fibre reinforced composite materials are increasingly becoming the popular choice of materials for designers in many fields such as aircraft manufacturing. As well as the accompanying high costs, one of their main limitations is the difficulty of making shaped parts such as those of aircraft and UAVs. To overcome this problem, various composite manufacturing techniques have been developed including the vacuum resin infusion (VRI) process and its many derivatives. In this work, a cross ply glass fibre reinforced plastic (GFRP) composite laminate was manufactured using liquid resin infusion (LRI) process which is a derivative of VRI. Specimens cut from the manufactured laminate are tested under static tensile loading and then investigated for resin impregnation quality using two types of state of the art Field Emission Scanning Electron Microscopes (FESEM). Images taken showed some plies with well intralaminar (inter fibre within single ply) impregnation whereas in some locations - within mainly the internal plies - there was very little impregnation. The implication is that unidirectional (UD) laminated when stacked in a cross ply form present another LRI process factor that is not widely investigated. Consequently, infusing UD cross ply composite laminates may present more challenges compared to infusing woven fabrics which are helped by the gaps between the fibres weave and weft.

On evaluation of CFRP composite laminates using ultrasonic with polarization direction

Abstract This study has been intended for a precise evaluation of the polarity of ultrasonic transducer by a theoretical analysis of the phenomenon found in each internal plies of CFRP laminate when ultrasonic wave from the transducer passes through them. An ultrasonic experimental setup was made with 1 MHz ultrasonic transducer referring to the result of Dokun's experiment. Evaluation of A-scan images by ultrasonic transducer at angles of 0°, 30°, 90° and 150°, respectively, was made using CFRP quasi-isotropic laminate specimen. In addition, where a visual observation is difficult, C-scanning evaluation method was utilized for precise evaluation of polarity of the ultrasonic transducer. Further, based on the results of theoretical analysis, performed a simulation in the form of polar coordinates with increment of 1° from 0° to 360° of peak-to-peak amplitudes. With the transmitter fixed at 0° direction and the receiver changing the direction from 0° to 90°, the polarity of receiving transducer was evaluated by performing scan display three times at each angle. The results were compared with A-scan images. Through these processes, the reliability of experiment for evaluating the polarization direction of ultrasonic transducer could be upgraded.

Microcracking in Cross-Ply Laminates due to Biaxial and Thermal Loading

DOI: 10.2514/1.20798 This paper presents a methodology to predict microcracking and microcrack density in both surface and internal plies ofa symmetriccross-ply laminateunder biaxial mechanical andthermal loadingconditions. Thethermoelastic properties of the microcracked laminates at different crack densities were determined by finite element analysis of the unit cells bounded by the microcracks. Analytical expressions for the stiffness and coefficients of thermal expansion as functions of crack densities were obtained in the form of response surface approximations. These analytical expressions were then used to predict the formation of a new set of microcracks by equating the change in strainenergyintheunitcellbeforeandaftertheformationofthemicrocrackstothecriticalfractureenergyrequired for their formation. Analytical expressions obtained as response surface approximations were also used to predict progressive microcracking. Both displacement and load control cases were considered along with thermal loading. Results from the current methodology agree very well with published data. Nomenclature

Compressive strength of composite laminates with terminated internal plies

Composite laminates are commonly tapered by terminating internal prepreg plies, creating geometrical defects which can induce failure. In this study, experiments have been conducted on tapered laminates loaded in axial compression. Failures by microbuckling or delamination were observed to nucleate near the dropped plies. The results indicate that existing models of microbuckling give adequate predictions of strength in specimens with mild geometric defects, while a new model given here for delamination provides satisfactory predictions of strength for more severe defects.

A mixed variational formulation for interlaminar stresses in thickness-tapered composite laminates

Stress fields in laminated plates containing an abrupt thickness taper are determined following Pagano's methodology of using the Hellinger-Reissner functional with the stress components approximated within layers by expressions explicit in the thickness coordinate [Pagano, N. J. (1978). Stress fields in composite laminates. Int. J. Solids Structures14, 385–400; Pagano, N. J. (1983). Axisymmetric stress fields in involute bodies of revolution. In Advances in Aerospace Structures, Materials and Dynamics; A Symposium on Composites, AD-06, (eds U. Yuceoglu, R. L. Sierakowski and D. A. Glasgow) ASME, NY, pp. 57–64]. The Euler equations from the variational principle area set of variable coefficient, differential-algebraic equations (DAEs) in the longitudinal coordinate. Difficulties with the number of differential equations and boundary conditions are resolved. Solution of the system is by higher-order one-step finite difference scheme. Numerical ill-conditioning encountered when modeling layers that are thin relative to other layers in a model was remedied by choosing stress shape functions and displacement weighting functions that are different than those used by Pagano. The example problems discussed are dropped-ply laminates (laminates with terminated internal plies), that are subjected to in-plane compression or shear under the assumption that the response is adequately modeled by generalized plane deformation elasticity.

Delamination in Thickness Tapered Composite Laminates

The structural performance of thickness-tapered laminates has been investigated using an energy-based damage tolerance methodology. The geometry studied is a thin laminate with discontinuous internal plies and a through-width delamination embedded at the interface between continuous and discontinuous sublaminates. An analytic model, based on shear deformation plate theory and linear-elastic fracture mechanics is employed to determine the Mode I and Mode II components of strain energy release rate. A two-dimensional plane strain finite element analysis is conducted to confirm the accuracy of the analytic predictions. The resulting pure mode strain energy release rates are combined with a mixed-mode growth criterion to predict the axial load required to induce delamination growth. Finally, the analytic and numerical model were used to predict failure in a delamination critical test specimen. Reasonable agreement of the actual and predicted failure loads was observed.

Damage Tolerance of Composite Structures: The Role of Interlaminar Fracture Mechanics

Layered fiber-reinforced composite structures are susceptible to crack initiation and growth in the resin-rich layer between plies. Delamination represents one of the most prevalent life-limiting failure modes in laminated composite structures. Interlaminar fracture mechanics represents one approach to assess the damage tolerance of composite structures. This paper is organized into two major sections. The first sections introduces interlaminar fracture mechanics and test methods that have been developed to characterize the Mode I, II and III interlaminar fracture toughness of composite laminates. In the second section, the role of interlaminar fracture mechanics in assessing damage tolerance of composite structures is defined through the following case studies: residual compression after impact strength, instability related delamination growth in compressively loaded laminates and delamination growth in composite laminates with discontinuous internal plies.