Issue 37

C. M. Sonsino et alii, Frattura ed Integrità Strutturale, 37 (2016) 200-206; DOI: 10.3221/IGF-ESIS.37.26 200 Focussed on Multiaxial Fatigue and Fracture Multiaxial fatigue of cast aluminium EN AC-42000 T6 (G-AlSi7Mg0.3 T6) for automotive safety components under constant and variable amplitude loading C.M. Sonsino, R. Franz Fraunhofer Institute for Structural Durability and System Reliability LBF, Bartningstr. 47, D-6429 Darmstadt / Germany A BSTRACT . Regarding the fatigue behaviour of EN AC-42000 T6 (A 356 T6), which is the most frequently used cast aluminium alloy for automotive safety components, especially under non-proportional constant and variable normal and shear stress amplitudes with changing principal stress directions, a poor level of knowledge was available. The reported investigations show that, under non-proportional normal and shear stresses, fatigue life is increased in contrast to ductile steels where life is reduced due to changing principal stress directions. This behaviour caused by the low ductility of this alloy (e < 10%) compared to quenched and tempered steels suggests the application of the Normal (Principal) Stress Hypothesis (NSH). For all of the investigated stress states under multiaxial constant and variable (Gaussian spectrum) amplitudes without and with mean stresses, the NSH was able to depict the life increase by the non-proportionality and delivered, for most cases, conservative but non-exaggerated results. K EYWORDS . Cast aluminium; Multiaxial fatigue; Spectrum loading. I NTRODUCTION ince the 1970s, cast aluminum has found increasing access into automotive applications, especially for safety parts, such as wheels, steering knuckles, brake brackets, axles, and engine carriers [1-4]. Fig. 1 shows an example of prominent automotive safety components, i.e. the multiaxial loading of a car wheel and its appertaining steering knuckle with acting reaction forces, which cause a combined non-proportional bending and torque on the shaft. Because of missing necessary knowledge on the multiaxial fatigue behaviour of cast aluminium for the design of such safety parts a large investigation was initiated focusing on the mostly used cast aluminium alloy EN AC-42000 T6 [5]. The present paper will report the results and the applicability of the NSH. M ATERIAL PROPERTIES , SPECIMENS AND TESTING PROGRAMME Material Properties and specimens he precipitation hardened alloy EN AC-42000 T6 with the heat treatment T6 contains mainly 7 % Si and 0.38 % Mg. The porosity state, according to ASTM E 155 [6], is P ≈ 0, i.e. pore diameters d ≤ 0.3 mm. The investigated alloy is a cyclic hardening material, revealing monotonic and cyclic yield stresses R p0.2 = 253 MPa and R’ p0.2 = 307 S T