The Effect Of Seawater Environment On Mechanical Properties Of PET Foam Sandwich Structure

Composite sandwich structure composed of polymer foam core and resin based fiber reinforced plate face sheet is a kind of advanced structure and provides excellent properties like high strength,lightweight,good stability,pronounced sound and heat insulation ability,good corrosion resistance,etc.Composite sandwich structures are broadly used in structure engineering,aerospace and ship engineering for its excellent engineering application,and its application in ships can help to achieve lightweight of the whole structure for its high strength-to-weight ratio and rigidity-to-weight ratio.While in service on ships,composite structures are frequently exposed to various marine climatic conditions,polymer foams are sensitive to humid and hot environment,and the absorption of water are bond to induce the degradation of the mechanical properties of the sandwich structure.To achieve a thorough understanding of the mechanism for water absorption and mechanical properties degradation for PET sandwich structures and thus provide efficient index for the design of composite materials,researches has been done on composite sandwich structure made of PET foam core and epoxy resin based plate face sheet under 40℃ constant temperature marine environment.(1)Water absorption experiments are performed on GFRP laminate,PET foam and PET foam sandwich to obtain the diffusion coefficients of the specimens,based on the second Fick’s law.A piecewise function form of diffusion coefficient is established for PET foam,and the experiment results are simulated in the multi-physics software COMSOL and the exported data are compared to the experimental ones,it turns out the two showed good agreement.(2)Compression tests are performed on dry PET foam specimens and those soaking in40℃ constant temperature marine environment for 20 days and 32 days to analyse the effect of water and soaking duration have on the compression property of PET foam.tensile tests and three points bending tests are performed on dry GFRP laminates and those soaking in water for 20 d and 32 d to analyse the effect water and soaking duration have on tensile property and bending property of GFRP laminate.Interlaminar cracking tests are performed on dry PET sandwich composites and those soaking in water for 20 d and 32 d to analyse the effect of water and soaking duration have on interlaminar cracking property of PET sandwich structures,the test process is simulated through COMSOL and the theoretical results arecompared with experimental data,the experiment results shows that seawater environment has a substantial impact on mechanical properties of GFRP laminate,PET foam and PET foam sandwich structure,the longer time for soaking,the more obvious for the impact.(3)Three points bending fatigue tests are performed on dry GFRP laminates and those soaking in water for 32 d to analyse the effect of water has on fatigue property of GFRP laminates,S-N curve is established by testing fatigue life of specimens under different load levels.four points bending fatigue tests are performed on dry PET sandwich structures and specimens soaking in water for 32 d to analyse the effect of water has on fatigue property of PET sandwich structures,S-N curve is established by testing fatigue life of specimens under different load levels,the results indicate that the fatigue life of dry specimens are longer than that of wet ones at the same load level,fitted curve for wet specimens lies on the lower left part of dry ones.