MODE-I FATIGUE DCB TESTING ON COMPOSITE MATERIAL
Michel Boes - NLR (December 2022)
SUMMARY
Fibre-reinforced polymer composite (FRPC) material is predominately used in critical weight structures, and this includes structures with significant vibrations and cyclic loads such as aerostructures. Due to the nature of these loads, fatigue failure is a possibility and is, therefore, an important failure mode to understand. Two essential parameters in characterising a fatigue failure are the cyclic energy release rate (ERR) and the crack propagation. These parameters are commonly used in a formulation to relate applied loading to the crack propagation rate. Currently, there is no standard test method available to obtain data on the ERR and crack propagation rate of FRPC material in mode I loading.
For composite material with a unidirectional (UD) fibre orientation, exists a standard (ASTM D 6115) prescribing the measurement procedure and test setup for the measurement of the onset of delamination growth for mode I loading. Even though this standard is limited to the onset of delamination growth, many aspects of it can be re-used to create a test for analysing crack propagation beyond this point. This includes the test setup and double cantilever beam (DCB) specimen configuration. Previously this was explored with the use a dynamic testbench and a travelling microscope for the measurement of the crack length.
The method of the travelling microscope for the crack length measurement was found to have several downsides. Most significantly, the frequency of the measurements is relatively low at just several dozen measurements with as a consequence that the crack growth behaviour is not being captured with the satisfactory resolution over the complete test. In particular during relatively fast crack growth the used interval between the cycles does not provide a clear picture.
The goal was to create a method to improve the quality of the data, which has been achieved by using cameras to capture the crack tip and a computer algorithm to measure the crack propagation. This resulted in a great increase in the frequency of the measurements and the crack growth behaviour was obtained in greater detail.