Research On The Dynamic Testing Techniques And Properties Of Lightweight And Soft Materials

Lightweight and soft materials are one of the most intensively investigated materials,and they usually have the features of low modulus,low strength and low wave velocity.Cellular lightweight materials,like metal foam,have been widely used in aerospace,energy absorption,transportation and other fields.Soft materials,represented by hydrogel,have broad application prospects in biomedicine,structure bionic,agriculture and environment protection.Lightweight and soft materials and their composite structures are often subjected to impact loading in application,therefore it's necessary to test and characterize the dynamic mechanical properties of these materials.Split Hopkinson bar(SHPB)technique is the most important and effective method to test dynamic stress-strain relation of a material at high strain rates,however,great difficulties arise when SHPB is employed to lightweight and soft materials.For metal foam,reliability and usability of SHPB are being questioned because of the deformation localization of specimen during the test,but other test methods can hardly replace SHPB to characterize the dynamic stress-strain relation.For hydrogel,owing to its extremely low elastic modulus(usually lower than 1 MPa),there still exist arguments and problems in modification of experimental facility and design of specimen shape.Moreover,study on dynamic properties of polyvinyl alcohol(PVA)hydrogel which is a substitute material for rehabilitating articular cartilage is almost empty.Aiming at these problems,this thesis studied the usability of SHPB to test metal foam and presented a reliable correction method.A new measurement technique in SHPB experiments based on digital image correlation method(DIC)was put forward.A modified SHPB facility which applied to soft materials was designed and made up.Compression tests of hydrogels with different contents of PVA under varied strain rates were conducted,and a rate dependent constitutive model was proposed.The main contents researched in this thesis are as follows:(1)The usability of SHPB to characterize dynamic stress-strain relations of metal foam was investigated.Simulative SHPB tests of metal foam were carried out based on 3D Voronoi model.The effects of specimen thickness on the deformation nonuniformity and size effect have been studied quantitatively.Appropriate specimen thickness has been recommended and a concept of “effective specimen” has been put forward.A correction method has been presented to obtain reliable dynamic stress-strain relation.We proved that SHPB is still usable to test metal foam.(2)A DIC-based technique which can measure the specimen's strain and stresses at both ends synchronously during SHPB tests was proposed.Speckles were made on surfaces of incident and transmission bars which were close to the specimen,and high speed photography system was employed to record the speckle images during the SHPB test.DIC was used to directly obtain the strains of specimen as well as the stresses at both ends at each moment.This technique can avoid the errors caused by determining the start points of pulse signals artificially in conventional experimental data processing.The usability and robustness of the technique were verified by an SHPB experiment.The accuracy of identification of this technique was evaluated.(3)A modified SHPB facility for soft materials was developed.In this SHPB facility,the bars were made of polycarbonate,and a double-striker electromagnetic drive system was designed to ensure excellent repeatability of impact velocity.The dispersion of waveforms were analyzed and corrected.Both the modified SHPB facility and the conventional aluminum SHPB setup were employed to conduct dynamic tests on silicone specimens,and the experimental results were compared to validate the reliability of the modified SHPB facility.(4)Dynamic mechanical properties of PVA hydrogel were examined and a rate dependent constitutive model was presented.For the soft materials with elastic modulus of less than 1 MPa(like PVA hydrogel),the influences of specimen shape on experimental result were analyzed,and an optimal experimental scheme was suggested.Compression tests of hydrogels with different contents of PVA under varied strain rates were carried out,and the content threshold of PVA of hydrogels was predicted.The strain rate sensitivities of PVA were discussed and a rate dependent constitutive model was established.The model indicates that the effects of the PVA content,water's contribution and strain rate to the stress-strain response are relatively independent.