Composites Engineering最新文献

Fracture mechanics analyses of composite skin-stiffener debond configurations using shell elements are presented. Two types of debond configurations are studied: a flange skin strip debond configuration and a skin-stiffener debond configuration. The flange-skin strip configuration examines debond growth perpendicular to the stiffener while the skin-stiffener configuration examines debond growth parallel to the stiffener. Four-node and 9-node shell elements are used to model both debond configurations. The stiffener flange and skin are modeled as two different layers of elements whose translational degrees-of-freedom, in the bonded portion, of the corresponding flange and skin nodes are constrained to be identical. Strain energy release rate formulae are presented for both 4-node and 9-node element models based on the virtual crack closure technique (VCCT). In addition, average values of the strain energy release rates are calculated using a gradient method. The VCCT formulae and the gradient method are used to compute the strain energy release rates (G-values) for both debond configurations. The G-values predicted by these methods are compared with those predicted by plane-strain and 3D finite element analyses. Excellent correlation is obtained among all the analysis results, thus helping to validate the VCCT formulae derived for the 4- and 9-node shell elements.

This study demonstrates that an in-situ nondestructive, ultrasonic surface wave technique can successfully detect the onset and extent of matrix cracking fatigue damage in a titanium metal matrix composite (MMC). A quasi-isotropic [0/±45/90]_sSCS-6/Timetal® 21S MMC material was used for room temperature fatigue tests and the resultant matrix cracking damage was ultrasonically monitored in situ as a function of cycle count. Damage accumulation in the material was successfully correlated with decreases in ultrasonic pitch catch amplitude and verified through the use of immersion ultrasonic C-scans and metallographic techniques. Damage initiation and progression was tracked through the use of complementary nondestructive and destructive techniques. The in-situ surface wave data show that the higher the fatigue stress level, the more quickly damage occurs; conversely, the lower the stress level, the slower the damage initiation. The in-situ surface wave technique proved to be more sensitive to the accumulating damage than standard load-displacement modulus measurements. The surface wave technique also indicated a change in material properties after only one fatigue cycle. The data acquired show that a better understanding of damage initiation and accumulation can be gained using the in-situ surface wave technique in comparison to current load-displacement modulus measurements.

The low-velocity impact of adhesive-bonded single-lap composite joints has been studied using a spring-mass model. In this quasi-static model, the impact response is represented by a time-dependent force, and the target joint is represented by an equivalent mass with equivalent stiffness. An analytical model has been developed to determine the equivalent mass and stiffness of the joint. The laminated anisotropic plate theory was used in the derivation of the governing equations of the two bonded laminates. The entire coupled system, as well as the assumed peel stress, were solved using both the joint kinematics and suitable boundary conditions. With the combination of a spring-mass equilibrium system and the developed joint model, a relationship between the impact force and the duration has been established. Adhesive stresses, which are believed to be the cause of failure, were predicted from the impact force. Impact tests of single-lap composite joints with different sample thicknesses and overlay lengths have been conducted to verify the proposed model.

The transient response of a fibre composite laminated plate due to a variety of impulsive line loads is examined. These loads act either on the outer surfaces or at an interface within the plate. The resulting straight crested waves travel through the plate in a direction at right angles to the load line. The analysis is based on taking a double transform of the displacement and stress components and solving the equations of motion subject to the appropriate boundary and interface conditions. An approximate numerical inversion technique is used to recover the solution to the problem. The results are in the form of graphs displaying the time histories of the normal surface displacement at epicentre and at distances varying from one to ten plate thicknesses.

This paper describes a method of calculating the water content of a composite material as a function of ageing and the degree of damage. The material studied is a glass fibre/epoxy laminate. Tubular specimens were manufactured by filament winding. The mechanical damage was created by tensile fatigue tests. Once damaged, the specimens were aged in demineralized water at 60°C. The paper describes the results of ageing tests on virgin and damaged material. The parameters of the Langmuir model and the diffusion law are determined. The water content in the damaged material can be calculated from the increased rate of absorption. The paper also proposes a model of the phenomena based on this principle.

In a recent series of papers, the authors have shown that the currently employed micromechanics approach applied to functionally graded materials, based on the concept of a representative volume element (RVE) assumed to exist at every point within the material, may produce erroneous results in the presence of macroscopically nonuniform material properties and large field variable gradients. As a result, a new higher-order theory for functionally graded materials has been developed that explicitly couples the microstructural and macrostructural effects, thereby providing both a rational methodology for analyzing the response of this new class of materials and a means for evaluating the uncoupled RVE-based micromechanics approach. Herein, the new theory is further generalized by combining it with a partial homogenization scheme applied along the nonfunctionally graded directions, while preserving the elements of micro-macrostructural coupling along the graded direction. As a practical consequence, composite plates functionally graded in the through-thickness direction and subjected to a thermal gradient along the same direction can now be analyzed in the presence of imposed average normal stresses in the nongraded inplane directions. Examples dealing with such composite plates are presented that illustrate the effect of partial homogenization on the internal stress fields and inplane moment resultants.

The boundary element method is utilized in this study to conduct thermal analyses of functionally graded composites, materials in which the internal microstructure of properties are explicitly tailored in order to obtain an optimal response, on the micromechanical (constituent) scale. A unique feature of the boundary element formulations used here is the use of circular shape functions to convert the two-dimensional integrations of the composite fibers to one-dimensional integrations. Using the computer code BEST-CMS, the through the thickness temperature profiles are computed for a representative material with varying numbers of fibers and fiber spacing in the thickness direction. The computed temperature profiles are compared to those obtained using an alternative analytical theory which explicitly couples the heterogeneous microstructure to the global analysis. The boundary element results compared favorably to the analytical calculations, with discrepancies that are explainable based on the boundary element formulation. The results serve both to demonstrate the ability of the boundary element method to analyze these types of materials, and to verify the accuracy of the analytical theory.