NASA CP Basic Principles Book
Indice ROCarneval


Operating and Maintenance Instructions

adaptaed from:

Hard spot inspection trolley • PLAMAT-M • 18201
Operating and Maintenance Instructions • V2.0
Fundamental principles

Usually materials with high mechanical hardness have also a high magnetic hardness, that means these materials need a high magnetic energy for magnetization and in reverse soft materials can be magnetized with few energy. Figure 2 shows the hysteresis curve of a soft and a hard material. The shape of the hysteresis curve of the soft material narrow and high and the one of hard materials is wide and low. A characteristic parameter of the hysteresis loop is the coercivity HC, which is smaller for soft materials than for hard material.

soft versus hard material
Figure 2: Hysteresis for magnetic hard (Martensitic) and soft (Ferrite) materials

The precise method for measuring a hysteresis curve makes high demands on the measurement set up and the sample geometry. So the shape of the sample must be a long rod which
must be magnetized very homogenously with very low frequencies. The measurement of the magnetization is taken with a coil enveloping the rod shaped sample. As this measurement set-up is not useful for practical applications, the 3MA-X8 has been developed to measure the hysteresis behavior indirectly by using sensors which can be put onto the surface of nearly any sample geometry.
The following three testing methodologies are used for the trolley:
  • Eddy current impedance analysis
  • Incremental permeability analysis
  • Upper harmonics analysis
Methodologies applied
Figure 3: Methodologies applied for material characterization

From these methods testing statistics are derived which correlate with material properties. They satisfy special requirements for recording magnetic hysteresis (e.g., stepwise increasing H, sample geometry, …) and offer a real-time characterization of heavy plates under harsh conditions in steel mills.

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