<論文>鋼板くぎ打ち接合部の疲労特性

Abstract

ツーバイフォーサイズのスプルース材を突付けにし,厚さ1mm,幅40mm,長さ90mmの鉄板2枚をその両側面に普通釘(ZN 40)8本で打ちつけた接合試験体にくり返し(疲労)荷重,一定(クリープ)荷重及び定連増加荷重の3種の荷重形式の下における変形と強度を調べた。結果は以下の通りである。(1)くり返し荷重の下でぱ荷重と破壊までに要するくり返し数の関係は単一な疲労曲線を描き,疲労限度は静的強度のおよそ23%である。(2)定連増加荷重の下では接合は釘の引き抜けで破壊するが,くり返し荷重の下では荷重が大きい場合のみ釘の引き抜けで破壊し,荷重が低い場合は釘の頭から5～10mmの点が疲労破壊する。中間の荷重ではこれらが混在する。(3)くり返し荷重や一定荷重を与えた後の接合の静的財力ぱこの試験の範囲でぱ低下していない。(4)定連増加荷重では荷重速度と共に接合の財力が増す。(5)くり返し荷重または一定荷重の下でぱ接合の相対変位(開口量)は荷重のくり返し数の対数またぱ負荷時間の対数に比例的である。(6)接合の開口量はいずれの荷重形式の下でも静的財力の50%以上の荷重に対して著しく増大する。

Repetitive loading (fatigue), constant loading (creep) and ramp loading tests in tension were made on the butt-jointed 2×4 sized spruce specimen with two flat metal plates (1.2×40×90 mm) nailed on both surfaces of the specimen with four common nails (ZN 40) each. Performance of the joint under these three different types of loading was discussed on the relative displacement (opening of the butt-joint). The results are summarized as follows: (1) The relationship between applied repetitive load (Pr) and number of cycles to failure (N) is expressed with a single continuous curve, and the fatigue limit of the joint is estimated at 23% of the standard static tensile strength (Po). (2) The failure mode of the joint in ramp load tests is characterized by withdrawal of the nails. But in fatigue tests, it is classified into three categories same as already observed with metal plate connectors. Nails are pulled out of the wood block by repetition of loading at relatively high levels (P_r/P_o<55%), but they are sheared off in fatigue at location 5-10 mm distant from the head at relatively low loading levels (P_r/P_o<30%). At intermediate loading levels, some nails are pulled out and the rest are sheared off. (3) The residual strength of unbroken joints does not decrease after repetition or duration of loading in the extent of this experiment. (4) The ultimate strength of the joint increases much with increasing loading rate in ramp load tests. (5) The relative displacemant of the joint by repetitive or constant loading are roughly proportional to logarithm of number of cycles or loading time, respectively. (6) The relative displacement are remarkable when loads more than fifty percent level of the standard static strength are applied.