Biaxial Fatigue Properties of Thin-Walled Composite Tubes

A testing method for obtaining fatigue and fracture data under biaxial loading and for negative R-ratios is described. Thin-walled tubes of one-inch (25.4 mm) outer nominal diameter with a tapered grip section were tested in an MTS Axial-Torsion Servohydraulic Testing Machine with an VTS 463 Data Control Processor at frequencies below 10 IIz up to the one-milli.on cycle range. Room tempera.ture, uniaxia.l constant amplitude fatigue data for a Ifercules high temperature cure Gr/E material are presented at R = -0.4 for a [±5] fiber orientation, The fatigue curve is almost horizontal. Tests on [± 5] 3M Co. SP-328 resulted in failures which predominantly originated in the grip section, Presently a new specimen design is being tested intended to eliminate cracking in the grip section. Biaxial in-phase loading of [0/± 45] tubes of Fiberite Gr/ Epoxy resulted in much steeper fatigue curves than observed under uniaxial loading,