Study On Impact Damage Identification Of Fiber Reinforced Composites

FRP(Fibre reinforced polymer)has the advantages of high specific strength,large specific modulus,design flexibility,corrosion resistance,fatigue resistance,nsulation and thermal insulation,and the thermal expansion coefficient similar to concrete,etc.It is widely used in aerospace and civil construction,automobiles and ships,bridge structures,underground structures,etc.The layers of the FRP laminate structure are bonded by a resin matrix,and the strength of the interface of the laminate is relatively low compared to the fiber direction,and it is very sensitive to impact loads.FRP laminate structure is prone to damage in the case of low-speed impact during service,and delamination is one of the main forms of damage.Delamination damage reduces the strength and stiffness of FRP laminates,which can lead to structural instability and destruction,and thus cause great safety risk.Therefore,it is necessary to identify the impact damage of composite materials,especially the delamination damage that can cause a decrease in bearing capacity.The composite structure is damaged after being impacted,and the decrease of the structural rigidity will further cause the structural dynamic response such as vibration characteristics to change.Therefore,it is possible to reversely predict the existence of damage through the change of vibration characteristics before and after the damage,and even further identify the location,size and shape of the damage.In the service process,composite materials are often subjected to impact loads of different energies resulting in irregular damage of different degrees and types.Therefore,the identification of impact damage of composite materials has always been the focus and difficulty in the field of research.At present,for the damage in composite materials,especially the delamination damage that is easily formed under low-speed impacts,the general method of identification is to equivalent damage to rectangular,elliptical(circles)and other ideal regular shapes,and generally considered as single delamination.However,impact damage often appears as multiple overlapping damage at the impact location,and the damage types are diverse and the shape of the damage is irregular.Whether it is necessary to investigate multiple damages instead of single damage,whether different equivalent shapeshave different impact on the effect of damage identification,and whether there is good applicability of a certain equivalent shape to complex impact damage are all to be studied.Therefore,this topic will firstly study the impact damage of composite materials,and then the vibration characteristics of composite laminates before and after impact,and finally will identify the delamination formed after impact based on the change of vibration parameters before and after impact damage.The effect of equivalent shape on damage identification and the identification of multiple overlapping delamination at the impact location are studied.In this paper,the finite element software ANSYS was used to establish a damage-free FRP laminate model and a FRP laminate with layered damage.The delamination morphology was simulated as a rectangle and an ellipse,respectively.And a multi-layered damage finite element model was further established.Based on the finite element model,the influences of the shape,location,size,number and boundary constraints of FRP plates on the vibration characteristics of FRP laminates with damage were analyzed.Then,the FRP laminate test samples was designed and produced,then the drop weight impact test and modal test experiment were carried out.There are two kinds of damaged FRP laminates,including FRP laminates with irregular overlap and multiple layers formed by the drop weight impact test,and the other is regular elliptical and rectangular artificial delamination simulated by embedding Teflon thin film into FRP laminates.The morphology,location and size of damaged FRP specimens were verified by ultrasonic phased array test.Both the finite element simulation and the experimental results show that the vibration frequency of FRP laminates obviously decreases after the damage occurs,which provides and the possibility for identifying impact damage based on the change of vibration frequency.Afterwards,two inverse algorithms,support vector machine(SVM)and genetic algorithm(GA),were developed to predict the location and size of the damage through a series of frequency changes before and after the damage.The two inverse algorithms are tested by using the numerical test cases and the measured frequency change values of the experimental specimen.The results show that both algorithms have achieved good results.It is feasible to identify the delamination damage in the FRP laminates based on the frequency changes.Moreover,SVM has higher recognition accuracy than GA under the condition of small database samples while the genetic algorithm based on agent model has high efficiency and precision when the number of data samples is sufficient.It has been found through research that the irregular layering is approximately equivalent to a rectangle or ellipse according to its envelope shape,which has little effect on the accuracy of damage identification.In addition,the recognition of multiple overlapping layers based on frequency changes has also achieved good results.