Application: Measurement of thickness of tear seams in automotive air bag covers. The procedure described here can also be applied to any other measurement of thickness inside thin grooves, notches, or channels in non-magnetic materials.

Background: In recent years the use of inflatable air bags as a safety device in automobiles and trucks has become nearly universal around the world. The air bags are stored in compartments on the steering wheel, dashboard, and doors that are enclosed by molded plastic covers. These covers have tear seams that are designed to break open instantly when the air bag inflates after an impact. The thickness of these seams is critical to proper air bag performance. If they are too thick the air bag may not fully deploy in an emergency, and if they are too thin they may separate if the cover is bumped or pressed during normal driving. Thus, measurement of seam thickness is an important part of manufacturing quality control. While seam thickness can be measured through destructive techniques, the Panametrics-NDT Magna-Mike 8500 offers a quick and nondestructive alternative to cutting and mechanically measuring air bag covers.

Equipment: Magna-Mike 8500 with 802PR-105 Chisel Tip Probe and target disks 80TD1 and 80TD2.

Procedure: The Magna-Mike 8500 is a Hall Effect thickness gage that utilizes electromagnetic principles to measure the distance between the tip of a probe and a magnetic target. The probe tip is held on one side of the test piece and the target is placed on the other, so that the distance between them represents the part's thickness. In tear seam applications, the Magna-Mike is used with the special 802PR-105 probe.
Typical tear seams in automobile air bags are approximately 1 to 2 mm (0.040 in. to 0.080 in.) wide, with a center thickness of approximately 0.5 mm to 1.5 mm (0.020 in. to 0.060 in.). The cross-sectional profile may be either flat on both sides, v-shaped on both sides, or one side flat and one side v-shaped.
The 802PR-105 probe has a narrow, pointed tip that is designed to fit into tear seams of either geometry. See Figure 1. It is normally used with one of two special target disks. The 80TD1 sharp-edged target disk (3/16" or 4.8 mm diameter) is designed to fit into v-shaped seams, and the 80TD2 square-edged target disk (1/2 in. or 12.7 mm) is designed for flat seams. The 802PR-105 probe may also be used with the standard 1/16th in. (1.6 mm) diameter target ball, although ball targets are normally not recommended for tear seam measurements.

Figure 1 -- cross-sectional drawing of 802PR-105 probe tip

Instrument calibration is performed in the same manner as for any other combination of probe and target, as described in detail in Chapter 2 of the Magna-Mike 8500 operating manual. When performing calibration with disk targets, it is important to insure that the target is lined up straight across the probe tip. If the target is misaligned during calibration, measurements will be inaccurate.

When making measurements, the probe will usually be placed on the outside of the air bag cover and the target disk on the inside, to permit free movement of the probe. See figures 2 and 3. The operator should monitor the position of the target disk to insure that it sits securely in the bottom of the tear seam groove. For accurate measurement, it is very important that the disk remains aligned straight with respect to the probe tip. If the probe/disk alignment is twisted, or if the disk is tilted within the seam, readings will be inaccurate. Remember that as with any other Magna-Mike application, the instrument is measuring the distance between the probe tip and the target, so if either the probe tip or the target is not securely in contact with the surfaces of the tear seam, the displayed thickness will be erroneouslyhigh. Similarly, if the target disk is misaligned with respect to the probe tip, it will appear to the instrument to be farther away than it actually is and readings will be too high. With proper calibration and target disk orientation, measurement accuracy of +/- 3% or better may be expected.

Figure 2 -- probe placement on outside of air bag cover

Figure 3 -- target disk placement inside tear seam