In Situ Tensile-fatigue Testing Theory And Experimental Research Of Bulk Materials

Developping the in situ testing instruments based on various loads types to researchon various materials and accordingly obtaining testing data of material performance wouldbe the important foundation to materials development, the designing of structures andproducts, life prediction and reliability evaluation. Therefore, the developping of in situtesting instruments has important scientific significance, prominent social and economicbenefits.In situ tensile and fatigue testing are considered as one of most typical static anddynamic technologies to visually supervise the micro deformation, damage and structureschanges of materials. Currently, the primary method of in situ tensile testing of bulkmaterials mostly depends on commercial in situ tensile testers. However, these instrumentspresent specific function and function expansion is confined. The existing in situ fatiguemechanical testing mostly relies on the piezoelectric driving or hydraulic driving methodsto carry out the relevant tests on microscale specimen or bulk materials, however, thedriving methods respectively present problems of limited testing stroke and frequency.Furthermore, the current in situ testing methods have not involved combined loads testingbased on multiple loading modes and relevant decoupling methods. Therefore, accordingto the micro-mechanical properties testing and characterization of bulk materials,corresponding in situ testing devices and control system were developed in this paper. Andthree algorithms for correcting the mechanical parametres were proposed. On this basis,the in situ tensile-fatigue testing device was also systematically and deeply tested. Thetheories and experimental researches on the combined loads testing of bulk materials werealso carried out. The main research works of this paper could be summarized as follows:(1) Design and analysis of in situ tensile device and control systemThe ultra-low speed quasi-static loading mode was proposed by adopting thereducing mechanism with large reduction ratio, the loading mode could not only achievethe functions of reducing speed and increasing torque, but also effectively meet therequirement of high resolution imaging of materials’ deformation process by SEM.Addtionally, the loading mode could also achieve the compacting and lightweight oftesting devices. A self-positioning gripping method was also proposed based onequi-width constraint of the gripper and specimen’s gripping section. Furthermore, basedon the calibration of displacement sensor and load sensor, the author constructed aclosed-loop control system based on analog signal feedback of the load/displacement sensors and digital signal feedback of the encoder. The compatibility testing with SEMwas also conducted. On the basis, to achieve imaging depth requirements of SEM withsmall vacuum chamber or optical imaging conditions, a non-standard layout type deviceby layouting the specimen on the top horizontal plane was also developed to achieve thebroad compatibility with various types of imaging systems. The transmission error andnatural frequency of the device were also tested.(2) Correction methods of tensile testing dataAs for the important parameters for the characterization of material’s mechanicalproperties, such as the testing accuracy of elastic modulus, yield strength and elongation,systematically theoretical and experimental investigations on miniaturized in situ tensiletesting devices were carried out. Firstly, for the influences of various gripping positions onelastic modulus calculations, this paper proposed a theoretical model for calculating theactual strain of the specimen’s gripping section, and theoretical model between measureddisplacement and actual strain was also established. In addition, in situ observation imageswas adopted to verify the feasibility of the proposed model. Secondly, for the non-standardlayout device by layouting the specimen on the top horizontal plane, this papersystematically investigated the weakening of system stiffness caused by the transmissionchain gap and the overturn of load sensor, and proposed an algorithm which could correctthe stress and strain of full scale range. Finally, for the dimensional misaligment issuebetween the specimen’s axis and tensile direction, this paper proposed an analyticalmethod based on horizontal shear deformation component and vertical bendingdeformation component. Combined with the calculation of deflection curve, thespecimen’s actual stress-strain curve could be accurately calculated.(3) Tests and analysis on in situ tensile-fatigue deviceFor many components, the materials’ failure is often attributed to the coupled effectsof static and dynamic loading. By using the coupling driving method integrating the DCservo and piezoelectric actuator, fatigue testing with variable frequencies and wide stressratio could be carried out. By investigating the temperature rise and charge releasebehaviors of piezoelectric stacks, the rational number of cycles and interval time wereobtained. This paper also studied the dynamic output performance of piezoelectric actuatorbased on different initial static loads, and the effective stroke of piezoelectric actuatorbased on various testing frequency. The aggravation effects of alternating load on thetensile stress relaxation was also investigated.(4) In situ tensile testing of typical materials and prediction of cyclic stress-straincurveWith the aid of developed devices, the fracture mechanism of Cu-Al compositematerial was investigated, the necking process was observed by SEM and the testingresults showed: the plate layer (C11000copper) fractured antecedent to substrate layer (1060aluminum), the main fracture morphology of plate layer and substate layer wererespective tear toughening nest and pure shear typies. The research on the effect of alloycomposition Zn on the low cycle fatigue properties of copper-zinc alloy showed that basedon the identical strain amplitude, as the content of Zn increased, the copper-zinc alloyconsumed more irreversible plastic work during each cycle and presented shortertransition life. In addtion, for the bulk materials with typical cyclic hardening behaviors,an accurate prediction method of cyclic stress-strain curves at the materials’ strengthenstrain range was also proposed and investiagted based on uniaxial tensile stress-straincurve.(5) Combined loading and decoupling method of tensile-shearing mode and effects ofpre-loads on mechanical propertiesBy using a modified Arcan fixture, the uniaxial tensile, pure shear andtensile-shearing combined loading tests based on an offset angle of45°of AZ31Bmagnesium alloy were carried out. The results showed that the total consumed plasticwork did no significantly change, this indicated that the plastic work consumptionrequired for material fracturing without pre-stress was not sensitive with the loading typies.Furthermore, the decoupled model of the tensile stress-strain and shear stress-straincurves under tensile-shearing combined loads was proposed. Besides, the effects of tensilepre-strain and bending pre-deflection on the bending and tensile properties of AZ31Bextruded magnesium alloy were investigated. Specifically, in the elastic stage, the effectsof plastic prestrain on specimen’s elastic behavior were not obvious, in the transitionalhardening stage, as the prestrain increased, the specimen presented more obvioushardening behavior, in the large strain softening stage or necking stage, as the prestrainincreased, the softening behavior of specimen presented more obvious trend.In conclusion, this study is not only the expansion of the existing in situ tensiletesting technology, but also the further understand of the accurate correction of mechanicalproperties and combined loads coupling and decoupling theories and technologies. Thedevelopment of serialized in situ mechanical testing devices and the theoretical analysiswill provide a theoretical foundation and open up new avenues for developing in situmechanical testing technologies and equipments.