Journal of Reinforced Plastics and Composites

A numerical computation and experimental tests have been performed for the investigation of dynamic behavior of thick composite beams. The present finite element method can effectively evaluate transverse shear deformation by considering layerwise in-plane displacements. The natural frequencies and damping values were measured by fitting the frequency response function obtained from FFT analyzer. The frequencydependent damping properties of composite material were measured to utilize them for the finite element analysis of thick cross-ply cantilevered beams. The experimental results are in good agreement with those of finite element analysis.

Failure behaviour of composite materials presents several characteristics which depend on the materials selected, the manufacturing process used and the state of stress or strain. An important number of strength criteria have been applied to model failure in the last two decades. Each criterion is different and can be used to fit particular experimental results. Up to now, there is no general and systematic approach to describe the failure of composite materials. In this study, a general strength theory based on dual kriging is proposed to model the failure behaviour of composite materials. The theoretical background of the method is first presented, then its application to model failure of composite materials is demonstrated on two examples: a graphite-epoxy composite and paperboard. The mathematical expression of the criterion can be obtained either through a parametric or an implicit formulation. The parametric formulation is used to predict three-dimensional failure envelopes of graphite-epoxy fabrics and paperboard. The method shows an excellent agreement with available experimental data. Different types of interpolating functions, can be used such as polynomials or trigonometric functions. The results obtained are compared with the tensor quadratic, tensor cubic and parametric criteria. The model can be improved when a nugget effect is added in the kriging equations: the interpolating function does no longer fit the data points. This approach permits incorporating the effect of measurements errors in the interpolation procedure. The intensity of the nugget effect is usually chosen to be proportional to the variance error. The uncertainty of the measurements is thus reflected on the shape of the failure envelope. Note that general tensor polynomial criteria can be derived as a limit case of this approach, and additional experimental data incorporated in the model to refine its accuracy. The proposed methodology can be used as a general and systematic tool to model effectively a great variety of failure behaviours.

An analytical sublaminate model of intralaminar cracking, i.e., representative of a particular ply or sub-set of adjacent plies (layers) of equal orientation, is developed by an internal state variable (ISV) approach. Furthermore, a finite element based plylevel damage formulation is established to represent the variation of material properties with intralaminar cracks. Simple linear and parabolic "layer" material models are shown to be useful in providing for a smooth transition in stiffness characteristics as crack densities increase. The utility of the formulation is established in the comparison of predicted laminate stiffness variation and constitutive relations with experimental results. In addition, some results are presented based on implementation of the ply-level damage model in a finite element formualtion.

In the present investigation a higher-order shear deformation theory (HST) and the conventional first-order theory (FST) are used to develop a finite element method to analyze the impact behavior of laminated composite beams. The higher-order theory assumes all the displacement components u, v, w that contain variation up to cubic power of z. The effects of various parameters, such as span to thickness ratios, support conditions and stacking sequence on the impact behavior of laminated composite beams are studied. Both eight-noded and nine-noded isoparametric elements are employed to compare the computational efficiency. However, to save computational time, numerical results are generated using only eight-noded isoparametric elements. The present results compare well with those of W. Goldsmith. It is observed that stresses computed using HST and FST exhibit wide variations.

The sensitivity of the Acoustic Impact Technique (AIT) has been investigated in detecting delaminations, impact and moisture induced damage in laminated composites. The use of Acoustic Impact Technique has shown encouraging results by previous investigators in identifying defects like delamination and disbonds in honeycomb structures. Very limited work has been reported in the literature about the utility of AIT as a NDT tool for testing laminated structures. The present work investigates the sensitiveness of the AIT method in detecting delamination in laminated composites in terms of size, shape, position and degree of concentration. The technique has also been applied in identifying low velocity impact and moisture induced damage in laminated composites. The significant advantage of AIT is that this technique is attractive for field applications. The AIT method involves striking the structure with an instrumented impacter in the region of interest and recording the time history of the impulse. The response of the signals received from both good and defective zones of a specimen were anlayzed in the time domain. The experimental configuration used by previous investigators was very restrictive. In this study a different experimental set up for studying AIT is developed. The effectiveness of AIT was evaluated by making a comparative study with ultrasonic measurement in detecting similar types of defects.

The results indicate the sensitiveness of AIT in detecting delamination in laminated composites in terms of size, depth and degree of concentration. The low velocity impact damage can also be identified using the AIT method. Moisture induced damage, shape of delamination and position of delamination along the edges and the center appear to be less sensitive for detection by the AlT method.

Dielectric Analysis is routinely used for monitoring the cure of thermosetting resins. Implantable, disposable sensors allow measurements of the curing process to be made in various locations throughout a part. Permanent, reusable sensors flush mounted in the mold allow the measurements to be made automatically during each molding cycle. This paper discusses process development, QA/QC, and production control in the polyester SMC/BMC industry. The curing rate through the cross-section of SMC parts of thicknesses up to 3/4 of an inch is examined. The effect of aging temperature on the reactivity of BMC demonstrates the use of Dielectric Analysis as a QA/QC test. Finally, it is shown that overall cycle times can be reduced by real time identification of the part-to-part variability in cure times during production molding. This allows closed-loop feedback to the press controller as to the proper time to demold each part.