4 Fig 2 The toughness of steel soaked in seawater or not for different pre-crack (a) or thickness (b) 4.The SEM morphology of specimen in seawater immersion As shown in Figure 3 (i,ii)，they are the SEM photos of the location of carbon steel specimen in uniaxial tension, and its appearance size is about long 20mm, wide 4mm, thickness 0.6mm. Fig3. (i) is about the specimen soaked in seawater for 90 days, and Figure (ii) is the photo of specimen without seawater. The size and crack length of both specimen is the same about 1.5mm. The specimen shown in Fig 3. (i) is loaded for tension 1240N, and that in Fig 3(ii) is for 1656N. As to the specimen immersed in seawater, it has the phenomena of cracking and slip twinning in disorder. The images show that the steel of soaked in seawater becoming deterioration in the mechanical properties of material, and its performance of resistance damage ability is also reducing obviously. （i） （ii） The SEM photos of specimen on local corrosion part in seawater are shown in Fig 4. Here, figure (1) is for conditions without load, and figure (2) for loading 1850N. The corrosion defects and corrosion surface shedding can be seen after loading on specimen in the right picture. 5. Conclusion 1) For the steel specimen of three point bending beam with crack, its seawater immersion of short time is not significant influence on the curve of loading and displacement of experiment. But, the specimen fracture toughness slightly reduced, deformation curve become shorter or becoming brittle, when it were reloaded after loaded to plastic stage Fig 3 SEM Photos of Steel specimen （i）after soaked in seawater at load 1240 N； （ii） not in seawater at load 1656N
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