13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- 0.1 pz pz cr . Clearly the COD formula captures the beak-type opening at the tip of the crack. Figure 3. A COD in the crack – load configuration shown in Figure 2 left. Dashed line is COD due to ̅ = 0.4; the negative COD is the closure due to drawing stress ℓ̅ =0.1. 3. Single Edge Notch Specimen The PENT is a standard test widely used for evaluation of the slow crack growth (SCG) resistance of polyethylene (PE) pipes. In PENT the failure time is recorded and used to rank various PE with respect to SCG resistance. 3.1. SIF Formula A single edge notch (SEN) specimen is used in PENT. The specimen is subjected to a constant load ( 2.4MPa ) and undergoes creep until failure. A closing load dr is applied on the PZ that is in front of actual crack. The SIF due to is determined in the following way [14]: 1( / ) I K K L F L W . (14) where, L is CL length ( cr pz L ) in mm, W the width of the specimen in mm. The correction factor 1F LW with better than 0.5% accuracy can be found in [14]. Similarly the SIF Green’s function for SEN specimen is ( xis counted from the edge) is: 2 2 , SIF SEN G F x L LW L , (15) with the correction factor 2F LW given in [14]. Thus, 2 2 , cr L dr dr K F x L LW dx a and by superposition one readily finds the total SIF expression in terms of known correction factors: 1 2 2 ( / ) , cr L dr tot K L F L W F x L LW dx L . (16)
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