The Dynamic Mechanical Properties And Failure Mechanisms Of Lightweight Protective Materials

With the rapid development of science and technology,in real life,many structures or surfaces of objects are often subjected to various impact loads such as impacts or explosives.For example,the collision accident in driving,the impact on the bottom door of plane from gravel that rolled up by plane tire,the bird's impact on the windshield of an airplane and so on.Therefore,there is a great demand for the protective properties of materials and structures in these fields.For the rapid development of materials in various industrial fields,lightweight fiber reinforced composite materials and polymer materials with advantages of low density,high strength and excellent impact resistance have been widely concerned by researchers.The 3D angle interlock woven composites,layered structural functional materials and PU polyurea materials are three kinds of polymer materials with excellent protective performance in light protective materials.At present,the research on the former two types of materials is few and mainly focused on the simple ballistic impact test research and numerical simulations.The study of PU material lacks systematic dynamic experimental research.In addition,the proposed constitutive equations that can describe its mechanical properties is relatively complex,which is not conducive to the application of engineering technicians.Therefore,in this paper,glass fiber reinforced 3D angle interlock woven composites and Kevlar/Carbon fiber layered structural functional materials were selected for dynamic mechanical property experiment,microscopic damage mechanism analysis and constitutive model study.Two new PU materials were selected for dynamic mechanical property and constitutive model study.Specific research contents are as follows:(1)The warp direction(in-plane)compression experimental investigations at strain rates ranging from 0.001/s to 800/s,the thickness direction(out-plane)compression experimental investigations at strain rates ranging from 0.001/s to 1400/s,and the warp and weft direction tensile experimental investigations within the strain rate range of 0.001/s-1800/s were conducted for the 3D angle interlock woven composite.The mechanical properties of the woven structure materials at different strain rates and different loading directions were obtained through a series of dynamic experiments above.The experimental results indicated that this kind of composite material had significant strain rate effects,and its stress also increased with the increase of strain rates.The macroscopic and microscopic analysis of the failure morphology of the specimens under different test conditions showed that the failure mechanisms of the specimen in the compression test is mainly shear fracture of the fiber bundle,while the failure mechanisms of the specimen in the tensile test is mainly tensile fracture of the fiber bundle and matrix cracking.Due to the mechanics curves are nonlinear,so based on ZWT model,a parameter D introduced to built a strain rate related to the in-plane tensile and out-plane compression constitutive model,through comparing the constitutive model results are in good agreement with the test results.(2)The compression tests in strain rates range of 0.0001/s-1300/s and compression-shear tests in strain rates range of 700/s-1300/s were performed with three differnt Kevlar content(5%,20% and 30%)specimens.The experimental results showed that the strain rate effects of the mechanical properties of this material is not obvious in the compression test,while the mechanical properties of this material have obvious strain rate effects in the compression-shear hybrid loading test.The effect of the content of Kevlar on the overall properties of the material was obtained through the above experiments.The results showed that the strength of specimens would gradually decrease with the increase of the Kevlar content,when the content reached a critical value,the strength no longer decreases significantly as the Kevlar content increases further.Since the material has obvious effect of strain rates in compression-shear test,the dynamic compression-shear constitutive model of strain rate and Kevlar content is established based on the function term introduced by ZWT model,and the prediction results of the model are in good agreement with the experimental results.The reflection tensile tests were performed to 5% and 30% Kevlar fiber content specimens,the damage process of material was recorded by using high-speed photography.The changing law of strain at different positions on the surface of specimen during the deformation process were revealed.Meanwhile,the law of stress wave reflection and transmission in layered structure materials were revealed.In the final,through the microscopic failure mechanisms analysis,the strain rate effects is not obvious in the compression test and sensitive to strain rate in dynamic compressive-shear test were explained,the main cause of this phenomenon is the different failure mechanisms of Kevlar fiber layers.(3)A series of experimental research on two new polyurea(PU105,PU605)materials were conducted.The research content mainly include the compression tests with confine pressure and unconfined pressure load,temperature range of 233 k to 293 k,strain rates within the range of 0.001/s-15000/s.Based on the experimental results,the temperature effects,strain rate effects and pressure effects of the materials were analyzed.The results showed that the stress and strain characteristics of the two polyurea materials in uniaxial compressive are largely depend on temperature and strain rate.Young's modulus will increase with the decrease of temperature,but will increase with the increase of strain rate.Meanwhile,it increased slowly in low strain rate area and rapidly in high strain rate area.Through the analysis of the experimental results,it is also found that the strain hardening slope of the two polyurea materials are not sensitive to temperature,but have a strong sensitivity to the strain rate,especially in the high strain rate area.The strain hardening slope of PU105 in low strain rate area(0.001/s-10000/s)is lower than PU605,while in high strain rate area(15,000/s)is higher than PU605.Finally,a more succinct double linear constitutive model were proposed,the model contains the material strain rate effects,temperature effects and pressure effects,and can predict the stress-strain curves of PU materials well.(4)The mechanical properties of the three kinds of lightweight protective materials in this paper are compared and analyzed.