Proposal of Inspection Method for Shrinkage Cavities in Spheroidal Graphite Cast Iron by Electromagnetic Force Vibration Using Differential Vibration Measurement

Spheroidal graphite cast iron is often used for machine parts that require high strength. A shrinkage cavity may be generated inside the cast iron during the manufacturing process. If there is a shrinkage cavity in the cast iron, the strength of the cast iron is significantly reduced. In this article, a non-destructive testing method for detection of shrinkage cavity inside the cast iron by constant frequency vibration measurement using electromagnetic force excitation at low frequency is proposed. In this method, the electromagnetic force vibration is generated inside the cast iron from the static magnetic field by a permanent magnet and the eddy current by an alternating exciting coil. Then, the vibration is measured by an accelerometer that is in contact with the surface of the cast iron material. If there is a shrinkage cavity inside the cast iron, since the vibration intensity is changed, the shrinkage cavity is estimated by the vibration measurement. In this method, the shrinkage cavities inside cast iron can be inspected at high speed with an inexpensive device. In this article, the electromagnetic force vibration is calculated using the 3-D nonlinear electromagnetic finite element method (FEM) analysis. The displacement due to vibration in the cast iron is calculated by the 3-D displacement FEM analysis. The phenomenon of electromagnetic force excitation is clarified using these FEM analysis methods. Then, the effectiveness of the proposed inspection method is investigated by comparing the results of the verification experiment and analysis.