Numerical investigation of fatigue crack-growth in a composite specimen featuring a cut-ply under vibration and four-point bending load
Topic(s) : Special Sessions
Co-authors :
Jordy SCHÖNTHALER (NETHERLANDS), Matias LASEN (NETHERLANDS), Mohammed Iqbal ABDUL RASHEED , Dario DI MAIO (NETHERLANDS)Abstract :
Currently, mechanical fatigue tests of composite materials are conducted at low loading rates up to 10 Hz to avoid self-heating, and, therefore, takes a considerable amount of time for high-cycle fatigue. Furthermore, each different lay-up must be tested to evaluate its fatigue life and hence study the variations. It is paramount to reduce the testing time for fatigue evaluation to allow for more long-term testing to be done. This allows for faster design iterations, aiding in designs that guarantee long-term structural integrity of
composite components. One of the testing techniques capable of speeding up the fatigue process is a vibration test [1, 2].
This novel method proved to be successful in identifying a failure criterion during fatigue tests of components excited at a constant drive frequency, near the resonance, and under a constant strain load [1]. Previous researches have shown that fatigue tests can be accelerated and that self-heating is an understandable behaviour which can be modelled. However, to replace the current mechanical fatigue tests, the results of the vibration test must be validated against existing standards. Therefore, the aim of this study is to compare the mode-mix ratio of a composite specimen between the vibration test and a four-point bending test. The specimen will feature a cut ply in its stacking sequence to ensure the
initiation point of the delamination.
To compare the mode-mixity of the vibration test to the traditional methods, the Strain Energy Release Rate (SERR) is simulated for a test carried out by (i) vibrating the component at its first bending mode, and (ii) deflecting the component using a four-point bending test [3]. In both simulations a stress ratio of R = -1 is used knowing that, in practice, four-point bending tests are not carried out with such a stress ratio. Further, both the simulated tests are configured to use the same strain severity and deflection
shape. The Virtual Crack Closure Technique (VCCT) is used to evaluate the SERR over a prescribed delamination path. The ambition is to evaluate a metric which allows reading the SERR over a prescribed delamination path for the positive and negative deflections between the two tests, thus establishing a correlation. This study allows to create a translation between the fatigue results produced by vibration tests and mechanical fatigue tests.
composite components. One of the testing techniques capable of speeding up the fatigue process is a vibration test [1, 2].
This novel method proved to be successful in identifying a failure criterion during fatigue tests of components excited at a constant drive frequency, near the resonance, and under a constant strain load [1]. Previous researches have shown that fatigue tests can be accelerated and that self-heating is an understandable behaviour which can be modelled. However, to replace the current mechanical fatigue tests, the results of the vibration test must be validated against existing standards. Therefore, the aim of this study is to compare the mode-mix ratio of a composite specimen between the vibration test and a four-point bending test. The specimen will feature a cut ply in its stacking sequence to ensure the
initiation point of the delamination.
To compare the mode-mixity of the vibration test to the traditional methods, the Strain Energy Release Rate (SERR) is simulated for a test carried out by (i) vibrating the component at its first bending mode, and (ii) deflecting the component using a four-point bending test [3]. In both simulations a stress ratio of R = -1 is used knowing that, in practice, four-point bending tests are not carried out with such a stress ratio. Further, both the simulated tests are configured to use the same strain severity and deflection
shape. The Virtual Crack Closure Technique (VCCT) is used to evaluate the SERR over a prescribed delamination path. The ambition is to evaluate a metric which allows reading the SERR over a prescribed delamination path for the positive and negative deflections between the two tests, thus establishing a correlation. This study allows to create a translation between the fatigue results produced by vibration tests and mechanical fatigue tests.