Magnetic Field

T.O. 33B-1-1 3-29 must be taken with parts having low retentivity to minimize the risk of washing away an indication. On larger parts where the entire area of interest cannot all be flooded simultaneously, additional "shots" of current SHALL be applied immediately after the suspension application hose is moved away from each point of application. In addition to the minimum required current applications one or two current applications just before stopping the suspension application, if the equipment duty cycle permits, will help small indications to form. 3.3.8.2.3 It should be noted that the continuous technique requires more attention and alertness on the part of the inspector than does the residual. Careless handling of the suspension-current application sequence can interfere seriously with the results. Normally the duration of the magnetizing shots will vary from one-half second to 1 or 2 seconds, depending on the difficulty involved in showing the condition of interest. In some instances, when large forgings or steel castings are to be inspected with manual suspension application, the magnetizing current may be left on from 5 to 10 seconds during which time the part may be repeatedly swept with the suspension spray. The magnetizing field is maintained for a second or two after the final spray has ceased or been diverted. 3.3.9 Magnetic Field. 3.3.9.1 Direction. The proper orientation of the magnetic field in the part, in relation to the direction of the defect, is a more important factor than the value or amount of the magnetizing current. For reliable inspection, the magnetic lines of force should be at right angles to the defect to be detected. If the magnetic lines of force are parallel to the defect there will be little magnetic leakage at the defect, and therefore, if any indication is formed it is likely to be extremely small. 3.3.9.2 Amplitude. 3.3.9.2.1 Rule Of Thumb. ASTM E 1444, as did its predecessor MIL-STD-1949, suggests that sufficient magnetic field is present when an applied peak tangential field strength of 30 to 60 Gauss (Oersted) can be measured on the surface of the part where indications are expected to form. A recent study using DC magnetizing current confirmed that this field strength can produce good indications from small defects, with field strengths at less than this range, detectable indications can be produced. Other studies have suggested that while good to excellent indications of defects may be produced with a tangential field in the range of 30 to 60 Gauss (Oersted) and higher, the background produced from acceptable surface roughness may reduce the visibility of such indications. All studies agree that the “rule of thumb” formulae for estimating magnetizing currents, contained in ASTM E 1444 and reproduced in this section, will usually produce field strengths well in excess of 30 to 60 Gauss (Oersted) with the concurrent risk of producing a background that can hide defect indications. 3.3.9.2.2 Recommended. The most direct way of determining the magnetic field strength required would be to use a specimen representative of the parts to be inspected with a defect representative of those to be found. The specimen would be magnetized at sequentially higher field strengths until a good indication of the defect was formed without an excess of background from surface conditions. This magnetic field strength could then be measured and used for parts that are similar to the specimen utilized. Since suitable specimens are seldom available, an alternative is to use the techniques discussed in the following paragraphs to simulate a defect and measure the necessary magnetic field strengths. 3.3.9.3 Measurement. The measurement of magnetic flux or field strength, either within a part or at the part's surface, is extremely difficult. There are several practical methods or devices. These methods or devices have limitations. They do serve a purpose in technique development if their limitations are understood. 3.3.9.3.1 Hall Effect Gauss / Tesla Meter. This is a portable, hand-held digital instrument that can be used to measure magnetic-field strength. It applies a current to a Hall-effect probe or sensor and amplifies the output voltage that is proportional to the magnetic flux density that is present at the sensor and is at right angles to the applied current. It can be used in establishing MT testing