PSI - Issue 23

Petra Ohnišťová et al. / Procedia Structural Integrity 23 (2019) 469 –474

470

Petra Ohnišťová et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Although the current trend in the aerospace industry is to produce either composite components or metal components by additive technologies, the most significant aircraft structures are made of specially threated aluminum, titanium and steel alloys. Aluminum alloys, also known as “ Duralumins ” , present an ideal combination of low density, specific high strength and good corrosion resistance. The specially developed aluminum alloys are used for aircraft wing components and also for other so-called primary structures. Significant portion of these components are manufactured by special machining strategies in very high material removal rates, Campbell (2006). High fatigue life of these components is expected, because any failure would importantly affect air traffic safety. It is known that an initiation of the fatigue cracks is closely related to the surface quality of the component. Fatigue crack nucleation is mainly initiated at the free machined surfaces which carry the most of the cyclic loading during operational mode. These fatigues cracks are initiated at the vicinity of the material inclusions and the incisions caused by a machining process or in any place of stress risers where the local stress concentration is above yield strength limit, Broek (1986). Ojolo et al. (2014) performed four-point bending fatigue testing of the end milled flat specimens from aluminum alloy 2024. According to his results the fatigue life increased as cutting speed increased. The decrease of the surface roughness was observed with increase of the cutting speed which may be the result of the thermal softening effect. The influence of the feed speed on the fatigue life has been described there as relevant. Furthermore, the increase of the feed speed resulted in decrease in fatigue life and the increase of the feed speed significantly affected the surface roughness. Ojolo (2014) supposes that at higher feed speeds, the tooth of the cutter doesn´t perform perfect swiping on the entire surface of the machined zone to make a perfectly smooth surface. It was as well observed that an increase of the rake angle of the cutter lead to a better surface quality, but with a shortening effect on the fatigue life of the component. Gómez et al. (2012) studied an influence of cutting parameters of the turning process on the fatigue performance of the aluminum alloy A92024-T351. Some fatigue tests using rotating samples and bending loading were carried out also. As the reference roughness parameter, the average roughness (Ra) was chosen. The increase of the feed speed led to an increase of the roughness. The influence of the cutting speed on the average surface roughness was important only for higher feed speeds. In this case the effect of the cutting speed on the fatigue performance was probably not related to the surface integrity but was given to the residual stresses after machining, the role of inter metallic particles or hardness of the machined surface. Many studies devoted to the effect of the machining strategies on the fatigue life. Some of them showed important influence of the surface topography, imposed by machining strategy. However, in case of the presence of the material inclusions or secondary phases in size larger than standard topography parameters the effect of the surface topography is usually suppressed - Piska et al. (2018). So, the fatigue cracks are usually initiated at the area of the material inclusions or secondary phases. It is therefore necessary to analyze the effect of the surface quality, including material structure and surface topography together with residual stresses, imposed by the machining process before releasing components into operation and safe use. The aluminum alloy 7475 is a material developed for aerospace applications with the so-called controlled toughness. This material is specific by its combination of high strength, good fracture toughness and high resistance to the fatigue crack propagation. The studied alloy 7475 is a technological refinement of the alloy 7075. Its fracture toughness for rolled plates is nearly 40% greater than for the previous version of 7075 alloy. The prevalence in some properties is a result of a reduction in Fe, Si and Mg contents and is also enhanced by thermo-mechanical and heat treatment procedures. The material 7475 is recommended when a high fracture toughness of a component (typically the aircraft wings or wing spars) is required. Chemical composition of aluminum alloy 7475 is given in Table 1 and its basic mechanical properties are shown in Table 2. The heat treatment T7351 can be defined by the following steps: solution heat-treatment, stress relief by controlled stretching, artificially over-aging to achieve a better corrosion resistance, mechanical properties. 2. Aluminum alloy 7475-T7351