Issue 38

D. Carrella-Payan et alii, Frattura ed Integrità Strutturale, 38 (2016) 184-190; DOI: 10.3221/IGF-ESIS.38.25 188 Figure 2 : Variable Amplitude example in automotive application. For variable amplitude, traditional fatigue approaches for metallic material use SN curves, linear Miner-Palmgren damage accumulation and cycle (rainflow) based damage evaluations [3-5]. SN curves are test based on curve fitting techniques which does not take into account the loading history of the material. In 1945, Miner developed a linear damage accumulation method, based on the work of Palmgren and added the contribution of various stress amplitude loading to the damage. However, as for SN curves, the loading history of the material is not accounted for. In Rainflow counting methods the damage level depends on full closing hysteresis loops of load cycles (Fig. 3). Figure 3 : Illustration of rainflow method based on stresses/strains with nested cycles. In the case of composite materials, the fatigue behavior is changing over time due to changes in the matrix damage state. When applying variable amplitude loading, the largest load cycles – that contribute to the larger amount of damage – commonly take a very long time to complete, due to the many nested cycles (Fig. 3). In this case the approach to only consider cycles when they are completed can no longer be justified. The damage hysteresis operator approach based on Brokate works [6] is able to calculate damage at defined time increment instead of ‘closed load increment’. This operator allows to up-date the damage status depending on the pre- damage and any other external factors (i.e. temperature, humidity…). These operators are therefore suited to follow the progressive damage curves and also to include damage history of the material. This approach gives good prediction when applied to temperature dependent fatigue analysis with non-linear damage accumulation [7-8-9] and for full car structures with full load histories [10]. P ARAMETER IDENTIFICATION PROCEDURE Tests protocol n this study, two different tests protocols are proposed. The first one is a traditional approach with tensile tests on five layups and five load levels per layup capturing a representative span of fatigue life (based on test method [11]). This results in twenty five configurations. The second one is an application of a more innovative approach with one- I