有限要素法によるばねの疲労き裂発生開始点の推定

自動車の燃費向上のために、部品の一つであるばねの軽量化が求められている。軽量化は、材料の高強度化で対応可能であり、高強度化に伴う加工性、靭性、耐久性等の低下といった問題の解決をし、疲労特性を向上させることでばねの寿命を延ばす、つまり、き裂の発生を遅らせることが必要となる。き裂は、繰り返し負荷により転位が蓄積され、その結果、表面に降伏が発生することで発生する。近年、有限要素法を用いて、圧縮負荷を受けるコイルばねなどの複雑な寸法の部品の応力分布解析を行うことが可能となった。本研究では、降伏現象の重要な指標であるミーゼス応力の分布をもとに、コイルばねの疲労破壊の発生点の推定を試みた。その結果、ミーゼス応力はばねの内側で最大値を示し、これが疲労き裂の発生箇所を示唆していることが他の実験の結果の論文と比較してわかった。

Due to the need of increasing fuel efficiency, the weight of springs and many other components in automobiles need to decrease their weight. The weight reduction can be achieved by increasing the strength of material, however, it may cause the problems of reduced workability, toughness and durability. Even increasing the material strength, enhancing their fatigue properties will expand the service life of them, i.e., delaying the initiation of cracks. A crack initiation is caused by the accumulation of dislocations, which is resulted by yielding on the surface due to repeated loading. Finite element analysis was recently available to apply the stress distribution analysis of a complicated shape, e.g., a helical coil spring subjected to compression loading. In this research an attempt was made to presume the point of the fatigue fracture initiation in coil springs according to the distribution of Mises stress, that is the crucial index of yielding phenomenon. The point inside and surface of a coil spring wire at which the Mises stress has its maximum value is the same place as a crack nucleation during fatigue progress according to referring experimental papers.

Finite Element Method, Coil Spring, Mises Stress Distribution, Fatigue Crack, Yielding