Ultrasonic Guided Wave-based Evaluation Technique For Interlaminar Properties Of Modified Composites

The composites have been widely used in engineering field due to its comprehensive properties.As a typical layered structure,the composites are susceptible to delamination during their service process,which seriously affects the bearing capacity of structures.Under this background,the modified composite technology emerges as the times require and becomes one of the effective solutions.The accurate evaluation of interlaminar properties can provide an important reference value for the mid-term service-condition and later service-life assessment of the modified composite.However,the existing evaluation methods mainly focus on the macro properties test,or micro fracture surface analysis,physical and chemical analysis,etc.,lacking a convenient,efficient and low-cost nondestructive testing(NDT)technology.In this study,a new method for the preparation of interlaminar toughening composites was proposed by using the microcapsule as the modified material,and the interlaminar properties of virgin and toughened samples were characterized by ultrasonic guided wave technique.In addition,based on the existing research of domestic and overseas,the microcapsule self-healing composites were prepared,and the recovery effect of the interlaminar properties of the self-healing samples was evaluated by ultrasonic guided wave technique.The main research work is as follows:Firstly,following the principle of gradually deepening,the extending-delamination of virgin composites was detected using the ultrasonic guided wave technique.The results show that the peak value of the response signals decreases with the extending-delamination,and the difference between waveforms increases.Besides,the second harmonic appears in the frequency-spectrum of the response signals,which indicates that there is closed micro-crack in interlamination,and the occurrence time of second harmonic advances with the delamination growth.The local wavenumber method can be used to localize the delamination of composites.The two-dimensional model of composite with delamination was established using the finite element simulation software ABAQUS.The simulation calculation results show that the incident wave undergoes mode conversion at the crack-tip of the composite,and transmitted waves propagate in the sublayer at different speeds.Next,the interlaminar-toughening composite samples with clustered microcapsules were prepared using hot-compaction technology.The toughening effect was evaluated through Mode I interlaminar fracture tests,and the ultrasonic guided wave technique was used to detect the undamaged samples.The interlaminar fracture surfaces were observed by scanning electron microscope(SEM)and ultra-depth microscope.The results indicate that the clustered microcapsules were crushed during hot-pressing process,which triggers the polymerization of cores,and forms multiple viscosity enhancing regions in interlamination,thus effectively improving the interlaminar fracture toughness of the samples.Besides,the clustered microcapsules and the matrix resin forms a second phase material layer,which absorbs the fracture energy and inhibits the expansion of interlaminar cracks.What’s more,the filling of microcapsules changes the characteristics of the interlaminar matrix of the samples,which increases attenuation of the propagating guided waves.Meanwhile,the clustered microcapsules change the vibration response of the samples to the excitation of the five-peak wave with a center frequency of 125 kHz,resulting in the amplitude of the center frequency in the signal spectrum being lower than that of the virgin sample.Finally,the microcapsules were filled into the composite interlamination in a dispersed form,and two types of self-healing composite samples with different microcapsule concentration were prepared by hot-pressing technology.The interlaminar properties and self-healing efficiency of the samples were evaluated by Mode I interlaminar fracture tests.The ultrasonic guided wave technique was used to detect the samples in the undamaged,damaged and repaired state.The interlaminar fracture surfaces were observed by SEM.The results show that the microcapsules were punctured by the interlaminar cracks and the encapsulated core material was released.Then the cores polymerizes to repair the crack.The recovery efficiency of interlaminar properties of high-concentration samples is less than that of low-concentration samples.Low-concentration microcapsules reduce the original interlaminar fracture toughness of samples,while high-concentration microcapsules improve that.The response signals peak value decreases,and the waveform difference between the response signals and reference signal becomes larger when the samples were damaged.When the samples were healed by themselves,the response signals peak value rises,and this waveform difference becomes smaller.However,there is no linear change rule between the peak value,waveform similarity and self-healing efficiency.