Permalink : http://hdl.handle.net/2115/58295

KEYWORDS : 跳ね返り危険度;弾性ひずみエネルギー;応力緩和;風倒木;無欠点小試験体;spring-back risk;elastic strain energy;stress relaxation;wind-thrown tree;small clear specimen

本研究は風倒木の切断処理中における樹幹の跳ね返り危険度を弾性ひずみエネルギーで評価して，相対樹種間差をみることを目的に6樹種の丸太の曲げ破壊試験を行った。また，風倒被害発生後の応力緩和による危険度減少を応力緩和試験によって評価した。さらに，樹幹の跳ね返り危険度の樹種間差および応力緩和による危険度減少を無欠点小試験体によって推定可能か検討した。丸太の樹種特性値MOR２/Ebを用いた跳ね返り危険度評価では，アカエゾマツ，トドマツに比べ，ミズナラ，シラカンバ，スギ，カラマツの危険度が有意に大きかった。丸太の野外曲げ応力緩和試験では，直径差，樹種間差による影響は見られず，概ね全試験体に共通して試験開始2日目以降は緩和速度が減少し，7日目以降の緩和速度はほぼ一定となった。無欠点小試験体によって樹幹の跳ね返り危険度および曲げ応力緩和の収束値を予測できることが示唆された。
Spring-back risk at manual cutting operations was evaluated to discuss the differences among 6 wood species by using elastic strain energy as a predictor, which was obtained from bending rupture test of logs. Furthermore, decrease of the risk considering stress relaxation in wind-thrown tree stems was evaluated by bending-stress relaxation test. We also discussed the validity of evaluating the differences in spring-back risks among species by using small clear specimens. Considering characteristic value of MOR2/Eb for logs, the risks of mizunara (Quercus crispula), shirakamba (Betula platyphylla), sugi (Cryptomeria japonica) and karamatsu (Larix kaempferi) were significantly greater than those of akaezomatsu (Picea glehnii) and todomatsu (Abies sachalinensis). In the stress relaxation test for logs, the relaxation speed decreased within 2 days after the test started and kept it nearly constant after the seventh day. No effect of diameters and species on the stress relaxation curves was found for almost all the specimens. It was suggested that spring-back risks could be estimated using mechanical properties obtained by small clear specimen tests.