Experimental Study
The sizes of the specimen are shown in Figure 1.Particularly,the width of the specimen is 10 mm.There were totally 5 specimens and No.1 graded Canadian spruce-pine-fir lumber was used to make the specimens.
Figure 1 Dimension of the specimens(unit:mm)
Reversed cyclic loading was applied with a stress-displacement mixed controlled cyclic protocol.When the specimen was under tension,the loading was controlled by the stress rate to make sure that the maximum tensile stress was 25 MPa.When the specimen was under compression,the loading was controlled by displacement to make sure that the maximum compressive strain was larger than the one in the previous loading cycle.The load speed was 0.8 mm/min.Typical test result is shown in Figure 2.
Figure 2 Stress-strain curves from cyclic loading tests
The test and data measurement device is shown in Figure 3(a).When the specimen was in large compressive damage,there was an obvious kinking band[8]on the middle part of the specimen,as shown in Figure 3(b).This indicated that the wood fiber was kinked.When the stress turned in tension(positive)from compression(negative),there was a low stiffness segment on the stress-strain curves and the kinking bands disappeared gradually.When the strain turned in positive,the wood fiber was straightened totally and the stiffness recovered.
Figure 3 Experiment device and specimen