Tham khảo tài liệu 'volume 17 - nondestructive evaluation and quality control part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Ị Fig. 27 Variations of the magabsorption signal and stress with strain for an annealed nickel plated 3 mm in. diam aluminum rod These meager data show that with a calibration curve the stress in nonferromagnetic materials can be measured using the magabsorption signals from a thin nickel plating on the nonferromagnetic material. The magabsorption versus stress graph for the nickel plated aluminum seems to obey nearly the same equation as does the nickel wire. To date no measurements of the stress in bars of aluminum with small areas of nickel plating have been accomplished as yet but are planned for the future. Residual Stresses and Magnetism. Asymmetries in magabsorption signals may be indicative of residual stresses or magnetism. In the case of residual magnetism the magnetic domains are not entirely haphazard instead they do some ordering in a particular direction Fig. 4f . This will produce asymmetries in the magnitude of the magabsorption signal depending on the orientation of the bias field HB with respect to the orientation of the residual magnetism. Similarly residual stresses are indicated by the ratio of magabsorption signals from two orientations 0 90 of the bias coil. When the 0 and 90 amplitudes are equal the stress is zero whatever the amplitudes are. When the parallel perpendicular ratio is greater than 1 the stress value is positive and is tensile when the ratio is less than 1 the stress is negative or compressive. For example in one investigation magabsorption measurements were made on steel samples before and after turning cutting and shaping operations. For the turning operation the sample was reduced in diameter with a cutting tool for the cutting operation a sample was reduced in thickness by a ram shaper for the shaping operation the sample was reduced in thickness by an end mill. With the turning operation the ratio of the 0 to the 90 magnitude of the magabsorption decreased from to when a 130 . m 5 mil cut was made. When a 230 . -
