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Investigation of the PH model as a prediction tool in fatigue bending tests with rest periods
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ABSTRACT: The PH model (Partial Healing) is an excellent tool for describing the evolution of the complex stiffness modulus (modulus and phase lag) in continuous Four Point Bending tests (4PB). Parameters of the model obtained from 4PB tests can even be used in the prediction of the evolution of the complex stiffness modulus in uni-axial push-pull tests (Pronk 2005). Based on the parameters obtained from continuous fatigue tests in controlled deflection mode, predictions are made for the evolution of the complex stiffness modulus in bending tests with rest periods. The load-rest period was 40,000 (39,280)–400,000 (392,800) cycles at a frequency of 5 Hz. The strain amplitude was around 160 micro strains. After each load period and before the next load period started, the (beam) stiffness was also measured acoustically with an ultrasonic device. Although the ratio of rest and load period is rather high and after each load period the stiffness modulus recovered a bit, hardly any elongation in the life time was noticed. This was in accordance with the prediction using the PH model of which the parameters were based on fitting the first two load periods. The predictions based on a fit of the continuous fatigue tests for the same mix were less good.