Applied Composite Materials (v.16, #5)

A Comparison of the Vibration Characteristics of Carbon Fiber Reinforced Plastic Plates with those of Magnesium Plates by Joel S. Hoksbergen; M. Ramulu; Per Reinhall; Timothy M. Briggs (263-283).
An experimental and numerical investigation is conducted to evaluate vibration characteristics of an advanced composite material system, namely carbon fiber reinforced plastic (CFRP), relative to a magnesium alloy currently used in the vibration testing industry. Experimental test specimens for both materials, with varying thicknesses, are tested with two boundary conditions — the free condition, to evaluate the natural frequencies and damping of the two materials and, secondly, a general constrained condition, typical of vibration testing. Experimental modal analysis techniques are used to measure the vibration characteristics including natural frequencies, damping and mode shapes. The results from these tests show that the natural frequency and mode shape for relatively thin CFRP plates were comparable with those of magnesium plate. Although the mode shapes also compare well for thick CFRP and magnesium specimens, the natural frequencies were found to have significant differences between the two material systems. The largest difference between the two material systems, present for all thicknesses, is found to be the damping values for the respective vibration modes. This unique characteristic of the CFRP material presents an opportunity for a performance increase in the vibration testing system’s community.
Keywords: CFRP; Magnesium; Vibrations characteristics; Damping; Frequency; Mode shapes

As the most frequent defect in composite structures during fabricated by autoclave process, delamination has evidently influence on compressive behavior of delaminated composite panels. For structure design purposes, delamination area is of great importance parameter to be considered. Based on considerable delamination information from non-destructive identification for composite components, a statistical study was in detail presented to investigate how delamination area in different composite components distributed. The distribution model of delamination area was hypothesized and tested by normal W test and normal D test. With consideration combined by the dispersibility and the qualified rate of delamination area, an effective method for estimating the manufacturing quality of composite components was also established. The results indicated that a lognormal distribution was found to represent the delamination area quite well and about 100 mm2 had the maximum probability. Moreover, the complexity and the thickness of composite structures had significantly influence on the qualified rate of delamination area. The more complex structure and the thinner or the thicker composites decreased the qualified rate of delamination area.
Keywords: Composites; Autoclave; Delamination; Manufacturing defects; Normal distribution

Bearing Behavior of Drilled and Molded-in Holes by Massimo Durante; Antonio Langella (297-306).
Glass/epoxy composites were manufactured using RIFT (Resin Infusion under Flexible Tool), a closed mould process capable of obtaining large and complex forms, by impregnating, under a vacuum, a dry preform placed on a flat rigid mould. At certain points of these composite laminates, molded-in holes were made in the dry perform before the resin infusion phase, using two different methods: displacing or cutting the yarn of the fibers. After the resin treatment, other holes were made in the same laminates by drilling. Single-point pin-loaded specimens, cut from laminates, were tested for different values of specimen width-to-hole diameter ratio (W/D) and edge distance-to-hole diameter ratio. In the results of the experiment, the specimens with molded in holes made by displacing the fiber yarn showed higher bearing strength values.
Keywords: Drilled hole; Molded-in hole; Bearing strength; Infusion process

This paper presents a process model for simulating the manufacturing process of prepreg filament wound composite tubes developed based on the finite element analysis. The model relates the process variables, such as degree of cure, viscosity, material property and temperature etc., to the parameters characterizing (residual stresses, warpage deformation) the composite tube and the mandrel. From the simulating results, several important trends in both the data and model are observed (1) Low temperature will go with low reaction rate and the reaction starts under low temperature will later compared with high temperature; (2) The results using CHILE model after demolding will smaller than the one using linear elasticity which assumes a stress-free prior to cool-down. After the mandrel (mold) is removed, some residual stresses, especially hoop stress will be released. (3) Remarkable stress concentration appeared in the transition zone between the boss and cylinder. In order to prevent the structural failure due to interlaminar shear or delamination, both the outer surface of the cylinder and the inner of the boss should have the same ply orientation angle.
Keywords: Filament wound; Viscosity and degree of cure; Residual stresses; Warpage deformation

This paper presents an algorithm for shape optimization of composite pressure vessels head. The shape factor which is defined as the ratio of internal volume to weight of the vessel is used as an objective function. Design constrains consist of the geometrical limitations, winding conditions, and Tsai-Wu failure criterion. The geometry of dome shape is defined by a B-spline rational curve. By altering the weights of control points, depth of dome, and winding angle, the dome shape is changed. The proposed algorithm uses genetic algorithm and finite element analysis to optimize the design parameters. The algorithm is applied on a CNG pressure vessel and the results show that the proposed algorithm can efficiently define the optimal dome shape. This algorithm is general and can be used for general shape optimization.
Keywords: Filament wound vessel; Shape optimization; Genetic algorithm; Tsai-Wu failure criterion; B-spline rational curve