Tham khảo tài liệu 'asm metals handbook - desk edition (asm_ 1998) ww part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Eddy-current inspection is extremely versatile which is both an advantage and a disadvantage. The advantage is that the method can be applied to many inspection problems provided that the physical requirements of the material are compatible with the inspection method. However in many applications the sensitivity of the method to many inherent material properties and characteristics can be a disadvantage. Some variables in a material that are not important in terms of material or part serviceability can cause instrument signals that mask critical variables or are mistakenly interpreted to be caused by critical variables. Eddy-Current vs. Magnetic Inspection Methods. In eddy-current inspection eddy currents create their own electromagnetic field which is sensed either through the effects of the field on the primary exciting coil or by means of an independent sensor. In nonferromagnetic materials the secondary electromagnetic field is derived exclusively from eddy currents. However with ferromagnetic materials additional magnetic effects occur that usually are of sufficient magnitude to overshadow the basic eddy-current effects from electrical conductivity only. These magnetic effects result from the magnetic permeability of the material being inspected and can be virtually eliminated by magnetizing the material to saturation in a static direct-current magnetic field. When the permeability effect is not eliminated the inspection method is more correctly categorized as electromagnetic or magnetoinductive inspection. Principles of Operation Functions of a Basic System. The part to be inspected is placed within or adjacent to an electrical coil in which an alternating current is flowing. As shown in Fig. 1 the alternating current called the exciting current causes eddy currents to flow in the part as a result of electromagnetic induction. These currents flow within closed loops in the part and their magnitude and timing or phase depend on a the original or primary field
