８０ｋｇ／ｍｍ?Q２?R高張力鋼すみ肉溶接継手の疲れ強さ ＩＩ 側面すみ肉溶接継手の疲れ強さ

Abstract

A pulsating tension fatigue test has been made of 80 kg/mm2 high strength steel double-strapped side fillet welded joints (weld length 1=20-80 mm, fillet weld design ratio r=Aw/Ap=0.22-0.8l) with the following results.(1) Fatigue failure characteristic of side fillet welded joint is widely different from its static failure characteristic ; the side fillet weld or the plate fails, depending on the number of cycles (N) which in turn depends on r or 1 and cyclic stress (fillet weld : τ ; plate : σ). Putting the critical fillet weld throat stress as τc, the critical plate stress as σc, and the critical number of cycles as Nc, when τ≥τc the fillet weld failure can occur (fillet weld throat stress being relatively high) and N≤Nc; when σ≤σc, the plate failure does (plate stress being relatively low) and N≥Nc; and both the fillet weld failure and the plate failure are likely to happen when τc≥(τ, σ)≥σc. Meanwhile, Nc, is the larger, the smaller r or 1 (Table 4); and using r and Nc, it is possible to express the fatigue characteristic and the static characteristic denfinitely (Fig. 8).(2) From the S-N diagrams (Fig. 5, Fig. 6), the fatigue strength of welded joint does not seem to improve very much even if the value of r exceeds 0.81. Thus, 1.0≥r≥0.8 would be desirable for side fillet welded joints in the high cycle range. From the standpoint of static performance, r≥ 1.4-1.7 is found desirable. After all, allowance for statics would assure sufficient fatigue performance.(3) For the side fillet weld, the fatigue strength at 2.106 cycles would be about 15 kg/mm2; and for the side fillet welded joint with r=0.4-0.8, the fatigue strength at 2.106 cycles (plate failure, plate stress) would be 5.5-9 kg/mm2.(4) The first fatigue crack in a side fillet welded joint is likely to initiate at the root of the side fillet weld end whee the maximum shear stress is concentrated. For this reason τc, has to be increased to improve the fatigue strength of the side fillet welded joint and the effect will be the greater, the larger the value of r.(5) The fatigue failures of side fillet welded joints have been broadly classified into four modes, which can be regionally indicated on the S-N diagram (Fig. 9).