3 Here, the symbol h, b, a and s are respectively represent the height, thickness, crack length and loading span. The fracture toughness of many specimens with different thickness or different prefabricated crack length, are shown in Figure 2 (a, b). Among them, the solid symbol indicates the fracture toughness of specimen without seawater immersion. And, the hollow symbols or vertical line is for the toughness value of specimen soaked in seawater for short-term that were unloading to zero after loaded firstly into the plastic stage. Figure 2(a) shows the variation of specimen fracture toughness with crack length. And Fig 2 (b) express the variation of toughness with specimen thickness. It can be seen that the toughness of seawater immersion specimen after plastic deformation is smaller than that of specimen without seawater immersion. The fracture toughness increases of specimen with the increasing of its thickness, and its toughness decreases with the increase of prefabricated crack length. 5 101520253035404550 70 75 80 85 90 95 100 105 110 h65b30a6 TJh65b30a6 h65b30a12 TJh65b30a12 h60b40a18 TJh60b40a18 h60b40a35 TJh60b40a35 h60b50a35 TJh60b50a35 h60b70a25 TJh60b70a25 Toughness K /MPa.m1/2 Pre-crack length a /mm a） 30 35 40 45 50 55 60 65 70 75 80 85 90 95 70 75 80 85 90 95 100 105 110 h65b30a6 TJh65b30a6 h65b30a12 TJh65b30a12 h60b40a18 TJh60b40a18 h60b40a35 TJh60b40a35 h60b50a35 TJh60b50a35 h60b70a25 TJh60b70a25 Toughness K/MPa.m 1/2 Specimen thickness b /mm b）
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