Applied Composite Materials (v.16, #6)

Flow Properties of Tailored Net-Shape Thermoplastic Composite Preforms by S. T. Jespersen; F. Baudry; M. D. Wakeman; V. Michaud; P. Blanchard; R. Norris; J-A. E. Månson (331-344).
A novel thermoplastic programmable preforming process, TP-P4, has been used to manufacture preforms for non-isothermal compression molding. Commingled glass and polypropylene yarns are deposited by robot onto a vacuum screen, followed by a heat-setting operation to stabilize the as-placed yarns for subsequent handling. After an optional additional preconsolidation stage, the preforms are molded by preheating and subsequent press forming in a shear edge tool. The in- and out-of-plane flow capabilities of the material were investigated, and compared to those of 40 wt% Glass Mat Thermoplastics (GMTs). Although the TP-P4 material has a fiber fraction of 60 wt%, the material could be processed to fill 77 mm deep ribs with a thickness of 3 mm, indicative of complex part production. The pressure requirements for out-of-plane flow were shown to depend on the fiber length and fiber alignment. Segregation phenomena were found to be less severe with TP-P4 than with GMT material.
Keywords: Preforming; Consolidation; Thermoplastic; Composites; Commingled yarns

Braiding Simulation and Prediction of Mechanical Properties by Anthony K. Pickett; Justas Sirtautas; Andreas Erber (345-364).
Rotary braiding is a cost effective method to manufacture near net shaped preforms that generally have a closed section and may have an arbitrary shape if braiding is performed over a shaped mandrel. The reinforcement architecture can be varied by the number and spacing of active bobbins, and by the speeds used to ‘take-up’ the braid and move the circumferential bobbins. Analytical methods are available that can reliably predict yarn paths and the final braid meso-structure for simple regular sections, and further analytical methods have been proposed to estimate composite braid elastic mechanical properties. A full simulation chain using the explicit Finite Element (FE) technique is presented for composite braid manufacture and mechanical stiffness prediction of the final composite. First simulation of the braiding process provides detailed information on yarns paths and braid meso-structure, from which Representative Volume Elements (RVE) of the braid may be constructed for analysis of stiffness properties. The techniques are general and can be applied to any braid geometry. A specific problem of meshing the yarn structure and interspersed resin volumes is overcome using conventional solid elements for the yarns and Smooth Particle Hydrodynamics for the resin, with link element to join the two constituents. Details of the background theory, braid simulation methods, meso- model analysis and validation again analytical and test measurements are presented.
Keywords: Braiding; Simulation; Stiffness; Mechanical properties

Scarf Joints of Composite Materials: Testing and Analysis by Y. W. Kwon; A. Marrón (365-378).
The objective of this study is to develop a reliable computational model in order to investigate joint strengths of scarf joint configurations constructed from carbon-fiber and glass-fiber woven fabric laminates with different material combinations like glass/glass, glass/carbon, carbon/glass, and carbon/carbon under various loading conditions such as axial, bending moment and shear loading. Both experimental and computational studies are conducted. For the experimental study, specimens made of hybrid scarf joints using carbon-fiber and glass-fiber woven fabrics are tested under compressive loadings to determine their joint failure strengths. Computational models are then developed using the discrete resin layer model along with fracture mechanics and virtual crack closure techniques. The numerical models are validated against the experimental data. The validate models are used to predict the joint strengths under different loading conditions such as axial, shear, and bending moment loadings.
Keywords: Scarf joints; Interface strength; Hybrid composite; Mixed mode fracture

Effects of Steam Environment on Creep Behavior of Nextel™610/Monazite/Alumina Composite at 1,100°C by Marina B. Ruggles-Wrenn; Tufan Yeleser; Geoff E. Fair; Janet B. Davis (379-392).
The tensile creep behavior of a N610™/LaPO4/Al2O3 composite was investigated at 1,100°C in laboratory air and in steam. The composite consists of a porous alumina matrix reinforced with Nextel 610 fibers woven in an eight-harness satin weave fabric and coated with monazite. The tensile stress-strain behavior was investigated and the tensile properties measured at 1,100°C. The addition of monazite coating resulted in ~33% improvement in ultimate tensile strength (UTS) at 1,100°C. Tensile creep behavior was examined for creep stresses in the 32–72 MPa range. Primary and secondary creep regimes were observed in all tests. Minimum creep rate was reached in all tests. In air, creep strains remained below 0.8% and creep strain rates approached 2 × 10−8 s−1. Creep run-out defined as 100 h at creep stress was achieved in all tests conducted in air. The presence of steam accelerated creep rates and significantly reduced creep lifetimes. In steam, creep strain reached 2.25%, and creep strain rate approached 2.6 × 10−6 s−1. In steam, creep run-out was not achieved. The retained strength and modulus of all specimens that achieved run-out were characterized. Comparison with results obtained for N610™/Al2O3 (control) specimens revealed that the use of the monazite coating resulted in considerable improvement in creep resistance at 1,100°C both in air and in steam. Composite microstructure, as well as damage and failure mechanisms were investigated.
Keywords: Ceramic-matrix composites (CMCs); Fibers; Coatings; Creep; High-temperature properties