Fatigue Mechanism Analysis And Life Prediction Of Lattice-web Reinforced GFRP-balsa Sandwich Structure

Due to advantages such as high specific strength,favorable corrosion resistance,and low price,glass fiber reinforced polymers(GFRP)have become valuable materials in the field of infrastructure.Composite sandwich structure,as an efficient structural form,has good application prospects in civil engineering and marine engineering,such as road plate,bridge deck,rapid assembly houses and bridge collision avoidance devices.It is found that fatigue is one of the main factors of damage occurrence and accumulation by analyzing and testing the application of composite sandwich structure in civil engineering,especially in bridge structure.Under the constant action of loads,the FRP facesheet and core of the components will be damaged in different degrees,and the interfacial delamination will occur,which directly affects the bearing capacity and durability of the composite components,and restricts the reliability and safety of the structure.In this paper,the fatigue behavior and failure mechanism of GFRP-balsa sandwich structures with and without lattice-web reinforced were studied by integrating experimental investigation with theoretical analysis.The corresponding life prediction models were established,and the fatigue life of the sandwich structure was analyzed and predicted.The main contents and achievements are as follows.(1)The investigation on fatigue properties of component materials for the sandwich structure.The tensile fatigue tests of GFRP laminates were carried out with load level as variables,and the shear fatigue of balsa core in short beams were tested under three-points bending.The fatigue failure modes,failure mechanism and fatigue life influencing factors of the two compositions were obtained,and the corresponding tensile fatigue S-N curves and shear fatigue S-N curves were established.The results show that the tensile fatigue behavior of GFRP facesheet is related to the load level.The fatigue shear fracture of the balsa core material follows the grain of wood fiber,and the fatigue failure process presents obvious brittleness characteristics.The fatigue life distribution of GFRP facesheet shows three stages of "static strength failure,degradation and fatigue limit".The shear fatigue life of balsa core material shows great discreteness,but the corresponding relationship between life and load grade still has obvious regularity.The fatigue limit is about r=0.165 for the GFRP facesheet and about r=0.6 for the balsa core.(2)Experimental study on bending fatigue properties of light wood sandwich composite structures.Four-point bending fatigue tests were carried out on composite sandwich beams with different span-to-height ratios and lattice-web configurations.The fatigue failure process,failure modes and fatigue life of the sandwich beams were studied.The interface debonding behavior in the fatigue tests was analyzed.The results show that the failure modes of sandwich beams without lattice-web were core shearing and debonding,while the failure modes of lattice-web reinforced sandwich beams varied with the lattice-web setting and load levels,including the buckling or compression of the upper facesheet and the tensile fracture of the lower facesheet,etc.The fatigue life of sandwich beams is related to the lattice-web setting,load grade and span-to-height ratio of the specimens.The lattice-web reinforced specimens still can persevere in working for a long time after cracks appear,so that the failure modes of them show obvious ductility characteristics.The interfacial cracks are I-II mixed mode,which proves that the shear crack of core material caused the interfacial stripping between core material and panel.The lattice-webs of the sandwich beams can effectively slow down or even prevent the growth of interfacial cracks.(3)Fatigue life analysis of GFRP-balsa sandwich structures.Both the linear and nonlinear fatigue life curve models were established systematically,the fatigue life of GFRP-balsa sandwich structure was quantitatively analyzed and the fatigue limit of the beams were predicted.The results show that,the three-parameter S-N curve model used in GFRP laminates performs well in the prediction of fatigue life of GFRP-balsa sandwich structure.From the perspective of applicability,the double-parameter model is only used in the description or the prediction of fatigue life in medium to high cycle,but the three-parameter model can accurately predict the whole life-cycle of the sandwich beam.In addition,the parametric regression analysis of the three-parameter life model based on the equivalent section method and multivariate non-linear regression method was conducted to reveal the relationships between parameters of the three-parameter model and span-to-height ration,lattice arrangement,which expand the application range of the three-parameter life model.(4)Study on fatigue damage mechanism of GFRP-balsa sandwich structure and establishment of cumulative damage model.The deflection variation and the stiffness attenuation of sandwich beams are analyzed.It is concluded that the fatigue damage of sandwich beams without lattice-web is the sprouting and expanding process of micro-cracks and micro-holes in the core material,while the fatigue damage of lattice reinforced sandwich beams is mainly caused by tensile or compressive fatigue of the upper and lower GFRP facesheet.Based on the cumulative damage theory,the first-order shear deformation theory and the residual stiffness theory,the “exponential type” fatigue cumulative damage models of sandwich structure with and without lattice-web reinforced were established respectively,and the fatigue life of sandwich structure was predicted according to the models.The cumulative damage models reasonably explained the life prediction and damage assessment of sandwich structures with different lattice-web arrangement and different span-to-height ratios.The distributions of parameters in the cumulative damage model of sandwich beams were analyzed,and the damage degree and fatigue life of sandwich beams were predicted.(5)Fatigue life prediction of GFRP-balsa sandwich structure based on BP neural network(BPNN).The fatigue damage cumulative behavior of GFRP-balsa sandwich structure is considered as an unstable process.The BPNN was applied to predict the fatigue life of GFRP-balsa sandwich structure,where the loading level,equivalent area ratio of web,span-to-height ratio were selected as input items and the fatigue life was the single output item.The result of this method was compared to both the test data and cumulative damage model prediction value.The results show that,the nonlinear relationship between input and output data was obtained by BPNN and it reflects the inner characteristics of entire sample data;The BPNN has a better clustering performance and ensures the prediction effect of every data point;Meanwhile,the weights of each input parameter was quantization by the established BPNN model.