Ultrasonic Testing of Safety Parts in Automobile Manufacturing

The Principles of Ultrasonic Testing

It is a matter of choice whether the ultrasonic testing of safety parts takes place on-line or off-line. Longitudinal and transverse flaws, internal and external flaws, such as shrink-holes, cracks, and inclusions etc., must be nondestructively tested according to the terms of delivery arranged between supplier and customer.

In ultrasonic testing high-frequency sound waves produced by a piezo-crystal element are transferred into the test piece through a liquid coupling medium, usually water. If there are flaws in the test piece -- they are called discontinuities -- and if these are encountered by the sonic beam, the sound waves are reflected by these discontinuities.

The reflected sound waves are picked up by the crystal element ("received") and transformed back into electric signals. The ultrasonic instrument processes the electric signals and represents the flaws as echos on the monitor. The time of flight, the amplitude and sometimes the echo shape are called on to evaluate the detected discontinuities.

Carrying Out the Test in the Factory

For a statistical test or random sampling off-line it is usually enough to make a manual test with a portable ultrasonic testing device and one probe. The critical zones are sonically beamed and tested for discontinuities. This method is called direct contact mode and the probe is coupled directly to the surface of the material through a coupling medium.

Automatic ultrasonic test machines designed for on-line tests in series usually employ the immersion technique. The safety part which is to be tested is automatically inserted into a water tank where several probes are mounted corresponding to the number of desired testing positions.

For the automatic evaluation of the test results the electronics of such a machine include the so-called "monitor gates". These are placed in such a way that they cover the critical zones of the test piece. If echos enter these gates and if they exceed in amplitude a pre-adjustable threshold, a "bad" signal is produced. In this way pieces which are found to be flawed can be identified optically or accoustically, marked with color or appropriately sorted.

In order to simplify communication with other units in the production line it is possible to use a computerized control system to coordinate the functions and the processes. Depending on the desired degree of automation and the geometrical forms of the test pieces, various solutions are possible for transporting the test pieces. All kinds of custom made equipment can be supplied from the simplest manual insertion to loading and unloading with the help of chutes, conveyor belts, transference with pick-up arms or intake and outtake with chain conveyors.

Test Examples for Various Safety Parts

Testing ball bearings. Ball bearings are usually tested for longitudinal and transverse flaws by turning the piece 360°. As adjusting aids original pieces with artificially introduced flaws are used: e.g. a notch 0.3 mm deep and 0.5 mm wide. The immersion technique is used and the coupling medium is water. As a rule, test frequencies between 5 MHz and 10 MHz are employed. In the test cycle a function check is carried out by means of an additional through transmission cycle. This assures that no test piece leaves the testing machine untested. In practice, cycle times of about 3600 pieces per hour are reached with an automatic piece testing machine.

Testing steering columns. Steering columns are among the safety parts which are especially subject to cracks because of the special processing by cold forming. The parts are tested for external and internal longitudinal cracks. In the transition zone to the pivot an additional quasitransverse flaw test is carried out. The testing is performed using the immersion technique. The coupling medium is thin-bodied oil. As a rule longitudinally focusing probes are used with a test frequency of 4 MHz. With one probe transverse flaws in the transition zone can be detected and with two more probes longitudinal flaws in the cylindrical zone.

Testing of cardan shafts. In front-drive automobiles the cardan shafts are among the safety parts. Here it is necessary to detect internal and external longitudinal flaws in the cylindrical housing and tranverse flaws in the internal transition zones from the housing to the pivot. This automatic test is also carried out with the immersion technique using water as a coupling medium. The test frequencies are 4-5 MHz and focusing probes are used. All probes are fixed but they can be moved with an additional transverse drive in order to increase the testing density. The piece is turned in a water tank at least 360°. A function check of the transverse flaws takes place by means of the initial pulse and of the longitudinal flaws by means of a through-transmission cycle. This ultrasonic test is performed with automatic testing machines which are fully integrated into a production line and reach cycle times of about 8 seconds.

Testing of pre-chamber housings. Especially in welding technology such welding techniques as electron-beam welding and laser welding have become established. But even today the technique must be controlled at every point, because not even the smallest welding defects can be permitted in safety parts. Like most automatic ultrasonic tests the testing of electron beam welding on the pre-chamber housings of diesel engines is carried out with the immersion technique. These parts are scanned spirally with special probes with a water delay line of about 20 mm corresponding to the admitted zone which is not welded.

Testing pistons and piston pins. When testing bonding of piston ring supports one of the best known testing methods in automobile manufacture is employed. With the aid of special probes pistons are tested with the immersion technique using water or thin-bodied oil as a coupling medium. The special probes are adapted to the geometrical forms of the pistons and can usually be applied to particular geometrical zones.

Testing valves. Valves of various dimensions should be tested for welding defects between the stellit welding and the valve head and/or the friction welding on the shaft of the valve. Bonding and welding defects should be detected. With the aid of probes of the types H5M with a water delay line these parts are tested using the immersion technique. The coupling medium is normally water. The water delay line is about 20 mm. The probes can be arranged in a variety of ways: the bonding tests as in the sketch or in the through-transmission mode; for the friction welding test a "tandem arrangement" is possible.

Testing of drive shafts, external lamellar supports, sun gears and synchronizing rings. Because of the need to save weight and to use rational production methods and also with regard to the development of improved welding procedures gear parts today are often welded with laser or electron beams. Ultrasonics is used to test so called weld drifts applying the immersion technique; in the case of complex geometrical forms the sound beam is directed into the test piece with deflecting mirrors at 90°. The machine is calibrated on a master piece and in special cases on natural flaws. Usually, testing mechanics which include turntables are employed. For automatic testing a handling robot can insert the parts and sort them out.

Testing of pins and wheel bearings. Forging defects and internal material flaws are not permissible in safety parts. The flaws that occur demand a probe arrangement appropriate to each case. This ultrasonic test is usually supplemented with an x-ray test (random sampling) and a magnetoscopic test. Depending on the location, size and position of the flaw, various probes with water delay lines are used. A wide range of automatic testing machines and even simple immersion technique testing machines with turntable or roller block can be employed. A selection of these testing machines depends on the parts to be produced and the desired cycle time.

Testing of cup valve tappets. In high-powered motors and low-pollution engine units, cup valve tappets welded with laser beams are used, making the adjustment of the tappet clearance unnecessary. Welding defects and weld drifts should be detected. In the immersion technique these parts are usually tested with an immersion technique probe. A master part with an artificially introduced test flaw serves as a calibration aid in ultrasonic testing. In these cases a single-channel ultrasonic flaw detector is usually employed since one probe is normally enough.

Testing oftrock axles. In heavy-transport trucks with their extreme requirements the testing of safety parts is just as necessary as in the case of cars. Truck axles are usually welded from profiles. Here, the problem is to detect welding defects in the complete weld area. The immersion technique is used with the probes placed at calculated distances and at defined incident angles of acoustic waves in order to include the complete area in accordance with the law of refraction. Calibration is performed on test flaws (drill holes and/or notches). A function check is carried out by means of a separate through-transmission cycle. For the evaluation of the test results, multi-channel test electronic instruments or packages are used.