Damage And Failure Behavior Of Composite Laminates Under Low-velocity Impact

Fiber Reinforced Polymer(FRP)has been widely used in the fields of traffic,aerospace,civil engineering and many other industries due to its advantages of high strength and modulus/weight ratio,superior corrosion resistance and durability,and good designability.However,composite products often need to bear the low-velocity impact load in practical application,and because of the poor impact resistance of composite materials,the composite structures subjected to low-velocity impact are easy to generate the internal damage,resulting in great safety risks,or being destroyed completely.Therefore,the damage and failure behavior of composite laminated structures under low-velocity impact have attracted wide attention and research interest.By summarizing the existing damage and failure theory of composite materials,this paper proposed a constitutive model to describe the damage and failure of composite materials under low-velocity impact.Then,based on experiments and simulation,the reliability of the proposed damage and failure model was verified,and the law of damage and failure of composite laminates under low-velocity impact was studied and revealed.The main research contents of this paper are as follows:(1)The damage and failure model of FRP composite materials under low-velocity impact was established and used in the simulation of the low-velocity impact damage of laminates.In the most existing damage and failure models of composite materials under low-velocity impact,only four modes of failuer are considered:the damage caused by longitudinal fiber tension,longitudinal fiber compression,the damage caused by transverse matrix tension and compression.However,when the composite laminated structures are subjected to the impact load in the out-of-plane direction,the compression stress in the thickness direction may exceed the corresponding compression strength,thus resulting in compression failure in the direction of thickness.Therefore,this paper established and compared two kinds of damage and failure models of composite materials with and without considering the compression failure mode in the thickness direction.The influence of the compression failure mode in the thickness direction on the accuracy of the low-velocity impact response was analyzed.The comparison of both experimental and simulation results shows that the damage and failure model with considering the compression failure mode in the thickness direction has higher accuracy in the prediction of the impact force-time/displacement curves,the energy absorption and impact damage of laminates than that without considering the compression failure mode in the direction of thickness.Thus the former is more suitable for the response analysis of low-velocity impact of composite laminated structures.In addition,the low-velocity impact damage of laminates was studied by simulation in this paper.The analysis results of the damage and failure model with considering the compression failure in the thickness direction show that the compression damage in the thickness direction occurred in the middle of the laminates during the impact process,and the damage was mainly concentrated in the lay-ups near the impact face of laminates.For the cross-ply composite laminates,in the lay-ups near the impact face,the damage caused by longitudinal fiber tension and transverse matrix tension mainly generate and develop in the annular region around the center of the laminates.(2)By studying the low-velocity impact response of frequency of composite laminates,the relationship between structural frequency and impact damage of composite laminates was revealed,and a new method to evaluate the impact resistance of composite laminates was proposed.In this paper,the modal tests of laminates before and after low-velocity impact were carried out to obtain the response law of frequency to evaluate the damage on the structures caused by impact.The results show that,similar to the case that there is knee point in the curve of dent depth-impact energy,the curve of residual frequency-impact energy also has knee point,and the position of knee point is basically the same as that of the former.Therefore,this paper proposes a new method to evaluate the capability of composite laminates to wtihstand impact(including impact resistance and damage tolerance),that is to use residual frequency to determine the position of knee point in the impact energy curve and then use the capability at the position of knee point to evaluate the impact resistance and damage tolerance of composite laminates.(3)The low-velocity impact damage and failure behavior of the FRP bolster protection suspender were investigated.The bolster protection suspender is a component installed on the bogie of train.Its main function is to bear the impact force of the falling joist and bolster,so as to prevent the falling joist and bolster from touching the rails and to avoid the derailment and rollover of trains.Therefore,the protection suspender is an important component to ensure the safety of the train and it mainly bears the random impact load of the falling joist and bolster.In this paper,a finite element modelling of the FRP protection suspender subjected to low-velocity impact was established,and the low-velocity impact damage and failure behavior of the FRP protection suspender with two lay-ups([0]₄₀ and[0₄/90₄/0₄/0₄]_S)were investigated with considering different bolt preloads(0 kN,5 kN and 20 kN)in simulation.The results show that,increasing the bolt preloads in an appropriate range can help to prevent the occurrence of crack damage around the installation holes,thus can improve the structural safety when the protection suspender is subjected to low-velocity impact.The dangerous position of the FRP protection suspender is located near the arc area of the protection suspender and the contact position between the protection suspender and the impactor.In addition,The FRP protection suspender with the lay-up of[0]₄₀ has better impact resistance than that with the lay-up of[0₄/90₄/0₄/0₄/0₄]_S.