| With the development of new materials and high-end manufacturing and other industrial clusters,the demand for the academia and industry in the mechanical service performance measuring and testing techniques is enhanced.The traditional test machine with single load mode has been unable to meet the market demand.However multi-load and miniaturization in situ test instrument will be the future direction of material testing.The majority of engineering materials inevitably subjected to complicated loads in service conditions,not only single load.The mechanism of macro and microscopic failure of materials under different stress states are significantly different.Based on the microscopic mechanical behavior and deformation damage mechanism,considering with the current testing background of mechanical properties of materials,the research status at home and abroad on composite load in situ test technology was summarized.In this paper,the modified Arcan fixture was design to obtain different combined loading mode,Moreover,the design and experimental research of in situ combined loading testing instrument based on Arcan fixture are carried out.On this basis,independently completed the structural design,equipment installed and performance testing of entire in situ test device,and the fracture mechanism of Q235 steel under combined loads was analyzed by the self-made in situ instrument.The main research works of this paper could be summarized as follows:1、The design and analysis of in-situ combined loading testing Instrument based on Arcan Fixture.The modified Arcan fixture was design to obtain different combined loading mode,the main body of the fixture is designed as a ring structure.The geometric center of the positioning groove of the fixture is fully coincident with the geometric center of the fixture to ensure centration.In order to facilitate the in situ testing device compatibility with SEM,CCD and various types of imaging systems,the test device with a size of 206 × 183 × 53 mm,the design displacement is 0-8mm,displacement resolution is 1μm,load range is 0-1500 N,load resolution is 10 m N.Finally,the reasonability of the instrument design was proved by theoretical calculation and dynamic and static simulation.2、The performance testing and calibration of in-situ combined loading testing Instrument based on Arcan Fixture.Integrated electronic control system to test device and achieve the device signal with the host computer communication.Repetitive testing were carried out with 6061 aviation aluminum.In order to calibrate the miniaturized instrument,the error analysis of the test device was carried out,and two kinds of error correction mechanisms were put forward.The combination of systematically theoretical and theoretical model was carried out to analysis the cause of the error.In addition,three-dimensional digital speckle technology was adopted to verify the feasibility of the correction algorithm.Finally,the in situ test device was integrated with the OLYMPUS DSX500 high depth of field microscope.3、Experimental research based on self-made in situ deviceIn this paper,the modified Arcan fixture was designed and integrated with a self-made in situ instrument to investigate the fracture mechanism and the process of crack initiation and propagation of Q235 steel under combined loads.It is found that the distribution of the slip zone has a great influence on the initiation and growth of the crack.Moreover,the density of slip lines on the lattice surface increased with the increase of loading angle,the microcracks generally initiated at the grain boundaries and the transgranular behavior was conspicuous.With the increase of loading angle,the mechanism of crack initiation and propagation was transformed from the normal stress to shear stress domination,the failure mechanism of Q235 steel gradually transformed from microvoid accumulation fracture to ductile shear fracture.Under the combined action of 15 °,the micro defects stress concentration and expand into a strip crack.With the load increased,the metallographic surface was slowly stripped and the surface was like river.The main crack appeared at the root of the gauge part.It is found that the fracture mechanism of the material under different plane stress conditions is obviously different.Therefore,the self-made in situ device will provide a new application way to explore the micro mechanism of the material. |
