FABRICATION AND TESTING OF COMPOSITE RING SPECIMENS.

Composite tubes are useful as structural members, but tubes and rings cut from them are also uniquely suited for certain types of tests for determination of material properties of composite laminates. The rings are tables specimens and as such present advantages over tabbed flat coupons for the determination of off-axis and shear properties of unidirectional composites and high rate properties of laminates. Special techniques and tooling for fabrication of composite tubes are described. Tubes can be fabricated in any desired ply orientation and stacking sequence. The tooling consists mainly of a perforated hollow stainless steel mandrel covered with a silicon rubber bladder and two outer halves forming the cylindrical mold cavity. After fabrication, tube quality is assessed in three different ways. The laminate thickness is measured directly at various points on the tube. The hoop strength is measured directly by cutting rings from the end of the tube and testing them under internal pressure. The tube is inspected ultrasonically. The technique for cutting ring specimens from the tubes and testing them under internal pressure is described. A special loading fixture is used for this purpose. Elastic and ultimate properties obtained from instrumented rings indicate that composite tubes can be fabricated with the same properties as flat laminates.