Fiber composites with polymer matrices are established structural materials for various lightweight construction concepts, in fields such as lightweight automotive construction and the aerospace industry. A knowledge of the behavior of the material under loads representative of actual use is essential for future design of structural components. A key aspect of the performance of these composites under mechanical loading is the load transfer between the fibers and the surrounding polymer matrix. An indication of interfacial quality is provided by the debonding shear strength. This is determined by means of the push-out test, in which a flat punch indenter pushes a single fiber out of the specimen (a thin polished section of the composite material) and the force-indentation curve is recorded. For this test the indenter must be positioned exactly in the center of the fiber which is to be pushed out.
In line with the planned research ZwickRoell supplied a hardness testing system for instrumented indentation tests to ISO 14577 and classical hardness test methods (Vickers, Brinell and Rockwell). The testing system was expanded to cater for push-out tests. A cylindrical indenter is used to push a single fiber out of the surrounding matrix, allowing characterization of the interfacial shear strength of the fibers. The fibers typically have a diameter in the order of 10 µm.
The testing system delivers an impressively high level of reproducibility, with customer-specific positioning correction which allows exact alignment of the indenter with an accuracy of around 1 µm. The force-indentation depth curve used for additional material assessment is recorded regardless of the test method in use. Evaluation of the force-indentation depth curve supplies the elastic and plastic material characteristic values.
The core of the testing system is a hardness measuring head containing the measuring systems for test load and indentation depth, together with an indenter plus holding-down clamp; the system has been equipped with a motorized add-on unit with measuring microscope and motor-driven X-Y table to allow optical hardness testing and exact indenter positioning.