Locally resolved hysteresis measurement of advanced glass-mat thermoplastic composites

With hysteresis measurement stiffness, creep and damping can be recorded at cyclic loading. Using a laser extensometer a locally resolved hysteresis measurement was formerly developed. It allows to observe several hysteresis-loops simultaneously. Local hysteresis measurement is suitable for fatigue tests of inhomogeneous materials or materials which have inhomogeneous damage process. Locally resolved hysteresis measurement was demonstrated for textile-reinforced advanced glass-mat thermoplastic composites which are interesting for structural components in automotive applications. Cyclic tension tests show stiffness degradation, creep and increasing damping especially for the fracture zones. In cyclic compression loading, the change of the material properties is less pronounced. A component test at a U-profile shows a good agreement between hysteresis measurement and major strain in the fracture zone of the U-profile recorded with digital image correlation. For the interpretation of creep and stiffness degradation at cyclic tension loading, strain oriented considerations were made. It can be shown that for the longitudinal and transversal samples a similar location of the trend-line in a total strain Wöhler-diagram can be found, therefore a total strain dominated failure behavior seems obvious. For further analysis a relative consideration concerning the development of strain amplitude and mid-strain is made. For each single test the ratio between the development of the strain amplitude and the development of the strain middle is calculated. A significant tendency of the above mentioned quotient concerning the stress amplitude can be observed.