Development Of An In-situ Four-point Bending Instrument And Corresponding Expeirmental Research

In-situ mechanical testing technology combines mechanical loading andmicro/nano observation on the surface of materials together which can conduct onlineobservation of microstructure’s deformation and damage process under loading.In-situ mechanical test can provide mechanism explanation for improving mechanicalproperties of materials directly and it has a profound historical significance inpromoting the development of material testing technology.Firstly this paper introduces the research progress of in-situ testing technologyespecially bending test technology at home and abroad. Four-point bending test playsan important role in the test of semiconductor materials, thin-film materials, biologicalmaterials, coating materials and so on. In-situ four-point bending test just emerges inthe field of mechanical testing technology. Currently in-situ four-point bending testsmostly rely on the foreign commercial test instruments which are very expensive andhave confined function expansion. According to this situation, a small sized in-situfour-point bending test instrument was developed which could be compatible withmicroscopes. The outer dimension of proposed instrument is200mm105mm66mm.The instrument provides a maximal bending load of500N with resolution ratio of0.1N. The range of displacement is more than10mm and resolution ratio is1m. Thispaper presents structural composition and functional theory of the developedinstrument in details. By giving check calculation of instrument’s key components bymeans of theoretical calculation and finite element simulation, the four-point bendingtest instrument was verified to have the ability of working properly.When the instrument’s assembly work was completed, the transducers ininstrument were calibrated to fulfill the operating requirements. There is a problem ofstiffness shortage due to miniaturization of mechanical structure, so it is verynecessary to analyze the frame flexibility of the developed in-situ four-point bending instrument. The frame flexibility coefficient of the instrument was measured to be2.824610-4mm/N by means of strain testing. This paper proposes a four-pointbending automatic test method aiming at bulk materials’ elastic modulus. The elasticmodulus of6061Al alloy, c11000Cu alloy, machinable glass ceramic and K9opticalglass were tested by the calibrated four-point bending instrument and the test resultscoincide with standard elastic modulus of the materials which verifies the feasibilityof proposed method and developed instrument.Finite element simulation can find many phenomena which cannot be capturedby experiments and it is playing an increasingly important role in the field ofmechanical testing in the past few decades. The four-point bending process of6061Alalloy was simulated by Abaqus finite element software and simulationload-displacement curve was compared to experiment results to ensure rationality ofthe simulation. This paper makes a research of many influence factors on four-pointbending test, such as position error of four supports, friction between supports andtested specimen and asymmetrically placed specimen. Meanwhile, the stressdistribution rule under four-point bending load at different ratios between innersupports and outer supports was studied, and the strain was discovered to be the keyfactor to effect the uniformity of stress distribution.The in-situ four-point bending experiments of7075Al alloy V-notched specimenwere carried out by the developed instrument under super depth of field microscope.It was found that notch at compressive zone could bear more stress compared withwhich at tension zone and the final fracture of such ductile materials is likely to comefrom tension zone. In addition, the micro deformation of prefabricated indentation ofc11000Cu alloy and machinable glass ceramic was discovered by four-point bendingexperiments and the difference between plastic material and brittle material wascompared.