A.C. generates an alternating field Alternating field generates - - PowerPoint PPT Presentation

• An alternating current is passed

through a coil

• A.C. generates an

alternating field

• Alternating field

generates eddy currents in conductors

• An alternating current is passed

through a coil

• A.C. generates an

alternating field

• Alternating field

generates eddy currents in conductors

• Eddy currents generate
• pposing field which

modifies current in coil

Defects will interrupt the eddy current Interruption in the coil current is displayed on the set

Equipme pment

Electrical currents induced in metals by alternating magnetic fields

The size of the current is affected by

• Electrical conductivity
• Stand off distance
• Flaws
• Permeability
• Specimen dimensions

• Sensitive to surface defects
• Can detect through several layers
• Can detect through surface coatings
• Accurate conductivity measurements
• Can be automated
• Little pre-cleaning required
• Portability

• Very susceptible to permeability changes
• Only on conductive materials
• Will not detect defects parallel to surface
• Not suitable for large areas and/or complex geometry's
• Signal interpretation required
• No permanent record (unless automated)
• Expensive equipment

• High frequency sound

sound waves are introduced into a material

• Reflected sound gives

information on the material under test and signals displayed on a CRT

Principle

The presence of a Defect in the material shows up on the screen of the flaw detector with a less distance than the bottom of the material The BWE signal Defect signal Defect

The depth of the defect can be read with reference to the marker on the screen 0 10 20 30 40 50 60 60 mm

A A B B C C The THINNER the material the less distance the sound travel The closer the reflector to the surface, the signal will be more to the left of the screen The thickness is read from the screen 68 46 30

May 1999: tmin = 29.6 mm

• Dec. 1999: tmin = 29.6 mm

June 2000: tmin = 29.4 mm April 2001: tmin = 29.2 mm