Interlaminar Fracture Of Laminated Composites Under Impact Loadings

Carbon fiber reinforced polymers(CFRPs)have been finding extensive applications in a wide range of global transport and energy industry structures due to their high specific stiffness,high specific strength and excellent fatigue resistance.However,up to now,the failure mechanism of this kind of materials under impact loads remains unclear and the application of PMCs has been refrained by the delamination damage when the composite structures are dynamically impacted.Delamination or interlaminar fracture is one of the predominant damage modes in laminated composites due to the relatively weak and brittle behavior of matrix.It poses a serious threat to the structure integrity,which significantly degrades the stiffness of composite structures and cannot be detected visually until the catastrophic failure of the whole structure.The speed of interlaminar crack can be several hundred meters per second(200-500 m/s).Thus,delamination damage spreads laterally to a considerably large area in a very short period of time.Therefore,the loading rate effect on both crack initiation and propagation of delamination should be considered for a wide range of engineering applications.Although the dynamic propagation of the cracks belongs to the subject of the fracture dynamics,the rate effect on initiation and propagation behavior has not been considered in engineering applications,mainly due to the immaturity of the dynamic interlaminar fracture tests.On the other hand,there is no exact criterion for predicting the equation of motion of the dynamic crack.Based on the requirement of engineering application and basic scientific problems,the emphasis of this research is to develop reliable dynamic interlaminar fracture tests and study the rate effect on the initiation and propagation toughness of interlaminar fracture.Both static and dynamic fracture experiments have been conducted to study the initiation and propagation behavior of unidirectional carbon/fiber composite T700/MTM28-1 under quasistatic and impact loadings.Under quasi-static conditions,the double cantilever beam(DCB),end-notched flexure(ENF),and mixed-mode bending(MMB)specimens have been loaded by Instron micro-tester to obtain mode I,mode II and mixed-mode I/II interlaminar fracture toughness.The maximum loading rate is 8 mm/s.Under dynamic loading conditions,the electromagnetic Hopkinson pressure bar has been employed.The maximum loading rate is 30 m/s.Pure mode I interlamianr fracture has been achieved in DCB specimens by symmstric loadings,and mixed-mode I/II interlamianr fracture has been achieved in MMB specimens by asymmetric loadings.Hybrid experimental-numerical method has been performed to determinate the dynamic initation and propagation toughness of the interlaminar crack propagation.Besides the imposed dynamic loading displacements,the crack initiation time and crack propagation history should also be provided by dynamic fracture experiments.To ensure the crack initiates at the same time along the width of specimen,three methods were used to monitor the crack initiation:strain gage,crack-propagtion gage,and high-speed photography.And the latter two methods were also employed to record the crack propagation history.A user-defined cohesive element integrating experimentally measured crack propagation history was developed to model the dynamic propagation of interlaminar crack,which was implemented into ABAQUS/Explicit through user defined element subroutine(VUEL).The parameters(initiation stiffness,length of fracture process zone(FPZ)and the damage distribution function)was chosen carefully after elaborate parametric analysis to reduce the numerical flucturation.Three methods were used to obtain continuous crack propagation history: linear fitting,linear interpolation and Hermite cubic interpolation.Concurrently,2D dynamic finite element simulations were performed using elastic material model for unidirectional laminated composites.The critical energy release rate was calculated by virtual crack close technique(VCCT)and dynamic J-integral.The main conclusions are drown and listed as follows:(1)Under quasi-static conditions,due to the effect of bridging fibers,the propagation fracture toughness increases with the crack length,which can be described by R-curve.And the Rcurve effect decreases with the degree of the mode mixture increasing.When the mode mixture is greater than 0.5,the R-curve effect disappears.Besides,the geometry of DCB specimen affects the shape of the R-curve,implying that the R-curve is not a material property.Under quasi-static conditions,the mixed-mode criteria could be described by B-K criteria,and the parameter η equals to 3.56 for the composites used in this research.While the crack velocity is lower than 0.06 m/s,the R-curve of mode I interlaminar fracture does not depend on the loading rate.(2)For mode I and mode II interlaminar fracture,the results indicate the presence of a critical loading rate for interlaminar fracture,below which the fracture toughness remains constant while the fracture toughness increases rapidly once the loading rate goes beyond this critical value.For mode I fracture,the fractography results suggest the transition of failure mechanism from fiber/matrix interface failure under quasi-static loadings to ductile matrix fracture under dynamic loadings.Although the mixed-mode fracture has been achieved by asymmetric loadings using the electromagnetic,extensive friction has been introduced which limited the reliability of the technique.Two novel methods have also been suggested for mixed-mode interlaminar fracture test and their reliability has been examined and validated by numerical analysis.(3)Pure mode I interlaminar propagation was ensured by the symmetric loadings.The crack velocities before crack arresting were in the range of 50 and 250 m/s which were obtained by changing the settings and the geometry of DCB specimens.Results from extensive case studies indicate that the dynamic propagation toughness of mode I interlaminar fracture is not a single value function of crack speed.The propagation of stress waves emanating from the crack tip was also studied.The results show that the fracture propagation behavior is not only affected by the imposed dynamic loads but also by the interaction between the stress waves and the moving crack tip.The traction-separation(TS)curves have been studied extensively,which show that the interlaminar dynamic propagation toughness is affected by two factors: crack velocity and loading rate which can be explicitly expressed by local opening rate of cohesive element.(4)The arrest fracture toughness of interlamianr crack is not a constant,which could vary in a large range around the quasi-static initiation fracture toughness.The arrest of a dynamic crack is a kinetic process,which could be affected by the distribution of bending moment and flexural velocity(the propagation of bending waves).