Theoretical Approaches And Experimental Applications In The Mechanics Test Technology Of Materials

With rapid development of aviation, high-speed railway, chemical engineering and reactor engineering, the engineering demand for the fracture properties of materials has been evidently expanded in the new century. However, there still exist a series of problems of load separation method, unloading compliance method, and the effective application of fracture test standards in the traditional fracture mechanics testing. Some issues still have highlighted the value of theoretical research and engineering application. Portable measurements of uniaxial stress versus strain curves have significant impact on the inservice inspection of engineering components. Fortunately, the indentation technique on the basis of hardness testing presents good prospects in this field. However, some improvements are needed for this method because of less effective applications in the existing researches. This thesis systematically carried out researches on the load separation method, unloading compliance method, the determination of uniaxial stress-strain curves by spherical indentation technique. The main works are introduced as follows:1. Based on similarity theory, a nondimensional load separation principle is proposed, so the dimensionless asymmetry problem in the current load separation principle has been solved. Correspondingly, a modified load separation parameter Spb method is developed according to the nondimensional load separation principle. This new method diminishes the effect of the reference blunt crack specimen and the choice of calibration points on the crack length determination of sharp crack specimen. Experiments on the determination of JIc for Cr2Ni2MoV and A508-â…¢steel, the estimation of CTOD for welded joints of certain gas pipeline ball valve and the estimation of CTOA for A508-â…¢steel are completed by use of the modified load separation parameter Spb method, and the validity of this method is successfully certified. According to researches on load separation method, a draft of corresponding appendix for the new national and unified test standard for quasistatic fracture toughness is given.2. Detailed investigation on the problems of current rotation correction method for CT specimen is conducted, then new reasonable rotation correction methods for CT, SEB and SET specimens are developed on the basis of rigid hinge hypothesis and work equivalence principle. The results show that, the rotational radiuses of the three types of specimens obtained by elastic-plastic finite element analysis are more reasonable than the current results. The rigid rotation has little effect on the measurement of crack length, but the J-integral computation is seriously affected. For SEB specimen, the rotation effect can be ignored when the displacement is measured at the load point. When using unloading compliance method to determine JIC, a reasonable approach is to correct the measured load versus displacement records firstly, but not correct the measured compliance directly. Based on the new rotation correction method, elastic-plastic transforming formulas between crack mouth opening displacement and load point/load line displacement for SEB specimen and straight-notch CT specimen are proposed, respectively. These formulas are feasible for the JIC determination on the basis of crack mouth opening displacement measurements, and the defect of current fracture testing standards in this field has been made up.3. In unloading compliance method, unified and simplified formulas for the crack length measurement for both SEB and CT specimens are proposed when the displacements are measured at different location of specimens, and thus simplified formulas for effective elastic modulus are deduced correspondingly. An unreasonable expression for the incremental iteration of plastic J-integral is corrected. Based on finite element analysis, a compliance method for a CT specimen with layered structure is developed, which is benefit to the fracture test for some special materials such as welded joint. In order to expand the application of compliance method in high-temperature fracture testing, a point-touch high-temperature compliance method is developed. The differences for various versions of JIC and CTOD test standards are discussed in detail, and some typical metallic materials are employed to investigate the influence of different test standards on the evaluation of fracture toughness. According to researches on unloading compliance method, a draft of corresponding appendix for the new national and unified test standard for quasistatic fracture toughness is given.4. Based on spherical indentation technology, a direct method to estimate the uniaxial stress-strain curves from the load versus indentation depth records is developed. Consequently, the validity of this method is successfully certificated by some indentation tests for A533-B steel, C40 steel,361L stainless steel, annealed Cu, OCr18NilOTi stainless steel, and A105 steel.