PSI - Issue 19

Shinji Hashimura et al. / Procedia Structural Integrity 19 (2019) 204–213 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction In recent years, transport equipment such as automobiles and trains are required to reduce weight to lessen environmental load (Ureshino, 2018; Ufferman et al., 2018; Hana et al. 2010; Meschuta et al., 2014; Westphal et al., 2005). In order to achieve such weight reduction, it is necessary to replace steel with nonferrous materials not only for structural parts but also for joining elements such as bolts and nuts. It is well-known that the clamp force which acts as the mean stress on the bolt rarely have an influence on fatigue strength for high-strength steel bolts. Yoshimoto had suggested Haigh diagram for bolts based on Ishibashi’s hypotheses and explained that the influence of the clamp force on the fatigue limit were small (1983). Ishibashi’s hypotheses is an idea that the fatigue limit of notch specimen such as bolts is determined with the stress at slight inside of the root of notch (1969). In the bolted joint fatigue, Nakamura et al. revealed that this is because shakedown is caused by local plastic deformation at the root of the first thread (2016). In the bolted joints, the high stress concentration occurs at the thread roots. The stress concentration factor at the root of the first thread reaches approximately five. Therefore the plastic deformation generates locally at the thread roots although the bolt elastically deforms mainly. Therefore the mean stress is released only at the thread root. Consequently In general, the clamp force of high-strength steel of the bolted joint is thought to be as high as possible is better. Htoo et al. reported that the influence of the stress ratio on the fatigue limit significantly decreased for notched specimen made of aluminum alloy A2024-T4 if the stress concentration factor is high (2016). Recent years, the use of nonferrous bolts such as aluminum alloy bolts and magnesium alloy bolt are considering in many company. There are the several kinds of aluminum alloy bolts in the market at present and aluminum alloy A5056 bolts are widely distributed. Recently aluminum alloy A6056 bolts have also been developed for car manufactures and so on. Schwerdt et al. reported the fracture mechanism of A6056 bolts based on very high cycle fatigue tests (2010; 2011). Berger et al. also conducted the fatigue tests of A6056 bolts until very high cycle region (2006). Gröber et al. also reported hard anodizing coating decrease fatigue strength of self-tapping screw made of A6056 (2015). However an influence of the mean stress on the fatigue strength for the nonferrous bolts has not investigated yet. Aluminum alloy bolts are often tightened presuming that those has the same characteristics as steel bolts. However, it is difficult to consider that the characteristics of aluminum alloy bolts and steel bolts are the same. We have revealed that the tightening characteristic is significantly different from the steel bolts because of friction coefficient (Hashimura et al., 2014). Hence it is also considered that the fatigue characteristic of the aluminum alloy bolts is difference from that of the steel bolts. In this study, fatigue tests and FE analysis for aluminum alloy A5056 and A6056 bolts have been conducted to reveal the influence of clamp force on fatigue strength. Hexagon head bolts M8 with full threads were used in the experiments. The mean load as the clamp force was set to a few conditions considering the proof stress of the bolts. 2. Haigh diagram of high strength steel bolts proposed based on Yoshimoto’ hypotheses As mentioned above, the fatigue strength of high strength steel bolts has rarely been influenced by clamp force because the local plastic deformation at the root of the first thread occurs. Fig. 1 shows Haigh diagram of bolted joints proposed by Yoshimoto (1983). In Fig. 1, the ordinate is the stress amplitude  a and the abscissa is the mean stress  m . Haigh diagram of bolted joints is expressed dividing  w 0 -  T diagram by the fatigue notch factor. In the bolted joint, even if the compressive force applies to that, the compressive force does not act to the bolt. Hence left side than point A in Fig. 1 is neglected. From point A to point B, the fatigue limit decrease depending on the mean stress similar to the smooth specimen. Then the first thread root portion yields at point B. When the plastic deformation locally occurred at the root of the first thread, the effect of clamp force on the fatigue limit gets disappeared because the mean stress is partly released only at the thread root. But the clamp force is able to enough acted on the bolt because the bolt elastically deforms mainly. Although Yoshimoto just made a hypothesis with regard to this phenomenon, Nakamura et al confirmed the phenomenon (2016). In Yoshimoto’s Hypotheses, the fatigue limit  w of the bolt in the right side from point B is expressed a following equation.