Damage Characterization And Opening Analysis Of Composite Laminates Subjected To Low-velocity Impact

The application of composite materials in engineering structures is becoming more and more extensive and the materials have been used as the load-bearing part in some structures.However,the low-velocity impact seriously harms the stability and reliability of the composite structures with its characteristics of high incidence,high concealment and high hazard.In the background of these engineering applications,this paper studies the damage characterization of the composite laminate after a low-velocity impact and the influence of openings on the impact response.In this paper,a low-velocity impact test platform is built on a drop hammer impact test machine according to the American Society for Materials and Testing Standards.Moreover,a user-defined material subroutine,based on stiffness progressive reduction and the three-dimensional Hashin damage failure criteria,is developed using FORTRAN language.Using the subroutine and the cohesive element theory,a three-dimensional finite element model,which can simulate the low-velocity impact event,is established.By carrying out the drop hammer impact test to composite laminates,the damage mode and the damage situation are better understood,and the correction of the three-dimensional finite element model is verified.In addition,by modifying the damage coefficient,it makes the simulation analysis consistent with the experimental results.Through the comparative analysis of the simulation and experimental results,this paper has the following conclusions:(1)For the laminates without openings,the depth of the pit on the upper surface and the internal delamination,which are subjected to the low-velocity impact,are developed with the increase of the impact energy and they have the same growth trend.Therefore,the depth can be used as a measure of the delamination degree.The matrix and fiber damage of the laminates also extends with the increase of the impact energy,but the growth rate is becoming smaller and smaller,which is different with the above ones.(2)The openings will affect the impact response of the laminate,in which the influence of the distance between the opening and the impact point is larger than the influence of the opening's shape.With the increase of the distance between the opening position and the impact point,the duration of the low-velocity impact will become shorter and the peak value of the impact force will become larger.Moreover,the cohesive layer with the largest delamination will change and the extend direction of the delamination will change to the opposite direction.When it comes to the shape of the opening,the laminate with a circular opening have the lowest impact force and the smallest delamination around the opening's edge in the laminates with different shape openings.Besides,the other damages of the laminates with different shape openings are roughly the same.