Because of the many factors affecting performance, the design and analysis of a multiple trap joint were performed using Finite Element Analysis (FEA) models. The CPR FEA model is shown in Fig. 3. The thickness and material properties of each layer in the filament-wound laminate are represented as accurately as possible, based on design data. X-rays and cross-sections of finished parts were also examined. End Fitting geometries were taken directly from the model and used to create production drawings.
The most useful results of the FEA modeling are the fiber stress results and stress contour plots (Fig. 4). When failure modes observed in testing were correlated with stress predictions, this data was used to evaluate the performance of a given configuration relative to material design allowables.Having established good correlation between test observations and analysis, the design can now be confidently evaluated for the effects of geometric and material changes.
Composite Material Testing. Based on many years of experience building and testing composite pressure vessels and shafts, the composite manufacturer has a high degree of confidence in the material allowables used to predict fiber dominated failures. The fiber stress levels achieved are dependent on the tensile or compressive strength of the reinforcing fiber and will be the same regardless of matrix material selection (assuming of course that service temperature is below the glass transition temperature of the matrix). However, for matrix dominated failures, such as are often observed in MCI failures, the off-axis or transverse strengths of the composite material become important.
An important effort in the NIST ATP CPR program has been the material characterization program conducted by personnel from the Composites Engineering and Applications Center (CEAC) at the University of Houston. Tubular specimens were made by Lincoln Composites using the CPR fiber reinforcements and epoxy resin. These specimens were tested under a variety of load combinations to determine material strengths. CEAC was also successful in characterizing the interaction between the various stress levels in bi-axial stress states for the CPR composite material systems.
CPR Manufacturing Development
Since composite performance is manufacturing process dependent, all tests were conducted on filament wound CPR specimens that are full-scale diameter/short length. The specimens were fabricated with the proposed laminate and metal-to-composite interface (MCI) with a composite tube body length of eight (8) feet between the steel end fittings for a total length of approximately 12 feet (see Fig. 5). Internal and external HNBR liners were installed on the specimens as required by the type of test to be performed. A stub Acme thread was used to mate with the test equipment instead of the specified Hydril MAC-II 12 3/4-inch, 94.20 lb/ft premium thread in order to reduce the cost of the end fittings. The manufacturing and inspection processes are shown schematically in Fig. 6.
End Fitting Fabrication and Preparation. The end fittings for the CPR specimens were machined from L-80, type 1 steel tubing in a multi-tool CNC machining center. Inspection of the finished fitting included dimensional verification, material certification and magnetic particle inspection. The fittings were then phosphate coated and packaged for shipment.
Upon receipt at the composite manufacturer, each fitting was vapor degreased or solvent wiped, and then all non- bonding surfaces and threads were masked. The bonding surfaces were grit blasted with aluminum oxide, and solvent wiped again. Chemlok 205 primer was applied and allowed to cure. Then a HNBR preform was laid up on the fitting MCI profile, and the assembly was installed into a mold and oven cured.
Tube Body Fabrication. Filament winding of the composite laminate was accomplished in a computer- controlled multi-axis winding machine. Prior to winding the composite, the end fittings are installed on the mandrel. The fittings are accurately located and held in place by specialized tooling details at each end of the mandrel. Next, uncured HNBR is wound onto the mandrel to form the inner liner. The low-angle helical layers, which give the CPR joint its axial strength and stiffness, are wound over the end fittings and reversed through low-profile domes located outboard. After the completion of a helical layer, it is secured between the trap geometry and dome, and then cut loose from the dome. The helical is then compacted into the traps or grooves in the fitting by winding structural fiber over it. Localized axial reinforcements are incorporated into the trap in similar fashion to increase joint performance.
To facilitate high-rate production, the fiberglass and carbon rovings are impregnated with resin during the winding process. The method of impregnation is proprietary, and consistently controls the resin percentage within ± 2% by weight. Tensioning of the fiber is another proprietary process that is controlled at the point of impregnation. Fiber tension is pre-set and controlled within ± 0.25 pounds during the wind process. The layer sequence and wind angles, as well as the resin content and fiber tension, are all controlled by the wind program.
Following winding, the mandrel/wound assembly was removed from the winding machine and placed into a rotating cure dolly. The parts were rotated throughout the entire cure cycle. The cart is then rolled to a cure oven with controls that are calibrated and automatically control and record the entire cure process. After cure, the composite was visually examined for fiber wrinkling and distortion and the cure process was verified. Then the assembly was placed back into the winding machine, where a thick layer of HNBR was spiral wrapped on the O.D. of the composite.
