Journal of Reinforced Plastics and Composites

A new injection molding process "DIF" (Direct Incorporation of Con tinuous Fibers) is proposed whereby roving strands are directly incorporated into the polymer melt by using a reciprocating-screw-plasticating unit. The DIF-technology offers the possibility to substitute the relatively expensive pultrusion process which is used to produce long fiber pellets. Furthermore it can be used as a fast and flexible R&D tool.

In order to improve fiber dispersion with minimum fiber breakage a new mixing non-return-valve" LFMR" (Long Fiber Mixing Ring) was developed based on the " Twente Mixing Ring." Its mixing capacity and influence on processing charactenstics, fiber breakage and property profile of the injection molded parts is examined and compared to a general purpose non-return-valve of the ring type. The results of fundamental mechanical and physical property investigations are presented including dispersion of fiber clusters and bundles, fiber length distribution and fiber orientation.

A molecular model, based on the concept of the activation rate theory, is developed for slip of a polymer as it flows through a die. The model correctly predicts the dependence of the apparent slip velocity of high molecular weight, linear polyethylene resins on wall shear stress, up to the onset of gross melt fracture. It also predicts the increased slip which occurs when die surfaces are coated with a fluoropolymer processing aid having a low surface tension.

Utilization of fiber reinforced plastics in the aerospace structures has become commonplace recently. However, considerable amount of research work is needed to understand the various coupling phenomena that arise from the use of FRP materials, and in efficiently modelling the coupling behavior in analytical formulations. The present effort describes the formulation and solution of FRP box beam type of structures through the use of the total potential energy functional in conjunction with the Ritz method. Results obtained from the present effort are compared to those obtained from a finite ele ment formulation and also to those from experimental results available in the literature, and it is shown that the present procedure is accurate for beams of symmetric layups.

A mathematical result is obtained for estimating the overall longitudinal Young's modulus of a unidirectional composite reinforced with (n - 1) different inclu sions, so as to have an in-depth study of the work [1], which was restricted to two phase composite body. The distribution of stresses and strains are discussed in accordance with the corresponding elastic energies and the mixture is assumed to possess the properties described in Reference [1].

Instrumented drop-weight impact testings were conducted in conjunction with the ultrasonic C-scan and scanning electron microscopy to characterize the impact re sponse at low velocities. Thermoplastic composite materials in the simply supported case showed dramatic improvements in impact damage resistance compared to the thermoset composite materials as reported. Scanning electron microscopy examination of the frac ture surface revealed fiber-matrix-debonding, fiber pull-out, and few deleminations. Matrix behavior can be characterized by a hackly appearance. On the other hand, the frac ture mechanisms of the transverse normal compression and shear punch were matrix cracking and grouped lamina delamination as opposed to individual lamina delamination, respectively.

In this article, the kinetics of crystallization for semi-crystalline polymers has been successfully incorporated into a cavity flow simulation. The flow resistance, in terms of filling pressure, can be significantly underestimated if crystallization is neglected during the calculation. In addition, the filling behavior for nucleated semi-crystalline materials is quite different from that of its virgin counterpart. With this simulation, the crystallinity distribution and degree of crystallinity in the part can be predicted at any instant of processing time, and hence part quality can be predicted accordingly.

Residual stresses are known to be detrimental to a number of mechanical properties in composite materials. The mechanisms by which they develop in the filament winding of composite cylinders and tubes is not fully understood. In this work a process model is presented to predict the residual stresses induced during the processing of two- layer composite cylinders. Chemical shrinkage, thermal expansion/contraction, and cure- dependent mechanical properties are accounted for in the model. The influence of mandrel stiffness and thermal expansion coefficient is examined and a case study of an aluminum and a steel mandrel is compared. The residual stress field is shown to be strongly affected by the mandrel thermal expansion coefficient.

A method for calculating the properties of advanced composite laminates, including their variations due to known variations in the properties of the individual plies and the laminate geometry, is presented. The method is useful for understanding scatter in the measured properties of composite laminates. This scatter is particularly important in the design of ultra-low coeflicient of thermal expansion (CTE) laminates. Such lami nates are designed with a theoretically zero CTE, but in practice have a distribution of non- zero CTEs. Information useful for designing ultra-low expansion laminates is discussed. A practical limit on how close to zero the CTE of a laminate can be assumed to be is found.

The need for effective and reliable joining methods continues to grow as the use of thermoplastic composites becomes widespread. It is now possible to join large- scale components with the development of an automated sequential resistance welding process. The thermal history generated by the heating element placed at the interface be tween adherends determines the quality and performance of the welded joint. This article presents a thermal analysis for the resistance welding of large-scale components that over comes the limitations of previous models. To simulate welding of the interface, a heat gen eration term was incorporated that accounts for the Joule heating of graphite fibers in the heating element. A parametric study was conducted to investigate the influence of welding parameters and assess the uniformity of interface temperatures. Components were joined by the resistance welding process to obtain experimental verification. Regions of localized overheating where potential current leakage may occur were identified as a function of process parameters. Insights on promoting more uniform heating for the resistance weld ing process are discussed.