Effect of loading speed on fracture of timbers with a crack

Effects of loading speed on fracture of a single-edge-notched tangential-lo ngitudinal system (SENT-TL) with specimen subjected to tensile force in per pendicular to fiber direction were studied. The crosshead speeds used were 0.01, 0.1, 1, 10, and 100 mm/min. The initial slope in load-crack opening d isplacement (COD) diagram increased, and the fracture became brittle with i ncrease of crosshead speed. The acoustic emissions (AE) occurred at small C OD with increase of crosshead speed. Load-GOD diagrams were reproduced base d on stress relaxation tests at several stress levels by using a generalize d Maxwell model. The total stress was shared by equilibrium stress by about 85% and the remains were by three Maxwell elements by almost equal rate. T he elastic constants showed no stress level dependence. However. the relaxa tion times of the Maxwell elements showed remarkable stress level dependenc e above about 80% of maximum load. The reproduced diagrams agreed very well with experimental results below the stress level of two thirds of maximum load. The critical value P-Q according to ASTM E399 was almost independent of crosshead speed and constant. However, at high loading speed, P-Q value located after frequent generation of AE due to initiation of catastrophic p ropagation of a crack. This result contradicts physical meaning of fracture . The proposed elastic COD criterion. namely a crack propagates when the el astic part of apparent COD arrives at a critical value, was almost independ ent of loading speed.