Tham khảo tài liệu 'asm metals handbook - desk edition (asm_ 1998) ww part 14', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Constant displacement -decreasing tests do not have the problems of the -increasing tests. The plastic zone ahead of the crack tip does not increase with increasing crack size so that the stress condition always remains in the plane-strain mode. Also the constant displacement tests can be self-loaded and thus external testing equipment is not needed. Because in these tests the stress-intensity factor KISCC can be easily determined by exposing a number of specimens loaded to different initial K values. This can even be accomplished by crack arrest in one specimen. A major problem with this test method occurs when corrosion products form in the crack blocking the crack mouth and interfering with the environment at this crack tip. Moreover the oxide can wedge open the crack and change the originally applied displacement and load. Measurement of Crack Growth. In order to quantify the crack growth behavior in precracked stress-corosion specimens the crack length needs to be monitored so that the crack velocity da dt can be calculated and the relationship between the increasing K and the crack velocity can be determined. There are basically three methods to monitor the growth of stress-corrosion cracks visual optical measurements measurement of the crack-opening displacement using clip gages and the potential drop measurement which monitors the increase in resistance across two on either side of the propagating crack. Tests for Hydrogen Embrittlement HYDROGEN EMBRITTLEMENT is a time-dependent fracture process caused by the absorption and diffusion of atomic hydrogen into a metal which results in a loss in ductility and tensile strength. Hydrogen embrittlement is distinguished from stress-corrosion cracking generally by the interactions of the specimens with applied currents. Cases where the applied current makes the specimen more anodic and accelerates cracking are considered to be stress-corrosion cracking with the anodic-dissolution process contributing to the progress
