Research On Strength And Failure Mode Of Sandwich L-joints Under Bending Load

L-joint has been widely used in composite ship structure and mainly been used in connection of deck and side plate.In practice,this area of the structure is weak due to the stress concentration.Consequently,the accurate prediction of its strength is important for the economic and safety benefits of composite materials.However,Due to the complexity of the structure and manufacturing process,and there are too many parameters that will influence the mechanical properties,the related research is rarely reported.Therefore,it is of great theoretical significance and practical value to carry out a study on the strength and failure modes of this joint type.Firstly,the constitutive relation of glass fiber reinforced panel and PVC foam were studied,and the applicability of sandwich panel theory was discussed,then the method of progressive failure was proposed to study the strength of L-joint.According to the applicability of different failure criterion and the characteristics of the test material,a Hashin failure criterion and corresponding stiffness degradation model were defined.The corresponding USDFLD subroutine was written in FORTRAN language to simulate the progressive failure analysis.The strain distribution,ultimate load and the displacement of the structure were obtained from the ultimate strength test.The phenomena observed in the test included the sound,the whitening of the brackets and the visible cracks,the locations of damage of four specimens occurred in the arc brackets and their surroundings.The failure modes observed in the dissected specimens include interface debonding,panel delamination,fiber breakage and sandwich deformation.The ultimate strength and failure mode of the L-joint under bending load were predicted by FEA.Comparison between the predicted values and the test results was made.From the comparison of calculation results and test results,the error of the ultimate load and ultimate displacement were 4.94% and 8.37%,respectively.And the failure mode and failure location were also consistent,which showed that the method was reliable and effective.In addition,the failure mechanism of L-joint was discussed and analyzed.It was considered that under the action of axial pressure and moment,the panel delamination occurred first,followed by fiber-based shear,fiber fracture,interface debonding and sandwich deformation.And then a variety of failure modes interact and aggravate each other,which made the carrying capacity of the structure reached the limit.Finally,in terms of the structure design,the influence factors of the strength and failure mode of the sandwich L-joint were analyzed.The results show that the triangular brackets cannot significantly improve the ultimate strength of the joints,however,they will reduce the failure modes.Meanwhile,the radius of arc bracket,L angle and load form have a significant effect on the ultimate load and failure mode of the L-joints.