Abstract: An apparatus and method for measuring interface properties of a composite test specimen includes a first jaw for gripping the specimen. The specimen has a fiber extending through a matrix forming a fiber-matrix interface with a portion of the fiber extending from the matrix. A second jaw grips the fiber extending from the matrix. A motor drive coupled to the first jaw rotates the fiber relative to the matrix. The torque transmitted through the fiber-matrix interface to the fiber is measured with a stationary torque transducer coupled to the second jaw with respect to the angular rotation of the fiber relative to the matrix. From the resulting torque-twist angle plot, composite interface properties such as interfacial fracture energy and frictional traction can be obtained.