رفتار فرسودگی چرخه بالا
Abstract
1-INTRODUCTION
2-MATERIAL AND EXPERIMENTAL METHOD
3-Results and discussions
4-CONCLUSION
Acknowledgements
References
Abstract
The aim of this study is to identify the impact of High Speed Machining defects on the fatigue behaviour of the Al7050 aluminium alloy. A vast experimental campaign under fully reversed plane bending loads containing different surface states has been undertaken to characterize the effect of the surface topography on the fatigue behaviour. The results show that the fatigue strength decreases only when the surface roughness is significantly degraded. It is also pointed out that manual grinding eliminates the effect of the machining defects on the fatigue behaviour. In order to predict the influence of the surface condition on the fatigue behaviour, a numerical approach based on the real surface topology has been developed. It is shown that the numerically identified crack initiation sites are in agreement with the experimental results. A probabilistic approach based on the weakest link concept, associated with the definition of a stress based crack initiation threshold has been integrated in a FE model. This approach leads naturally to a probabilistic Kitagawa type diagram, which in this case explains the relationship between the size defect and the scale effect on the fatigue strength.
INTRODUCTION
This study is part of a French research project that deals with the control of the machining and grinding of large structural aeronautical components. The material under investigation is the 7050-T7451 aluminium alloy. During HSM (High Speed Machining) of aircraft parts, macro-geometrical defects can be created. Mismatch may appear due to the gap left between two consecutive passes of the machining tool. Chatter can also be created due to the tool/part vibrations which may occur during different types of machining. In order to respect the strict specifications imposed by the aeronautical industry, expensive manual grinding is often performed to remove the machining defects. By the local thermomechanical effects induced, machining can change the surface integrity of manufactured parts and therefore change their fatigue strength. Shahzad [1] and Guillemot [2] showed that changing the cutting conditions can generate a change in surface topology and therefore a change in the fatigue life. Furthermore, Shahzad pointed out that the fatigue resistance shows a significant decrease when surface roughness increases. Even if HSM is known to introduce residual stresses in the machined parts, Tang [3] and Rao [4] showed that these residual stresses are mainly located in the first 50 micrometers below the surface. A variation of the microstructure in the upper surface (hardness variation, grain recrystallization) can also be detected after HSM. Campbell [5] showed that for the Al7000, these changes are located in the first 30 micrometers below the surface.