Perfect spot weld: rapidly decreasing echo sequence, the echo distances correspond to the total thickness of the plates. Weld nugget too small: alternate large and small echoes, echo distances correspond to the individual plate thicknesses. Cold weld: long echo sequence, the echo distances are the same as a perfect weld No weld: long echo sequence, the distances are the same as a nugget which is too small.

In echo 35, page 36, we already gave you a brief account of spot weld testing used in the car body manufacture within the automotive industry as a complement to the classic testing method using a hammer and a chisel. What about the current state of the art in this testing technique?

The basic principle of ultrasonic evaluation hasn't changed: after the probe's orientation toward the welding spot has been optimized, the echo sequence appearing on the display screen of the ultrasonic instrument is evaluated. The criteria applied to this are:

	the length of the echo sequence from the total thickness,
	the occurrence of so-called intermediate echoes,
	the amplitude and position of these intermediate echoes within the echo sequence.

This makes it possible to differentiate welding spots that show an adequate weld nugget diameter and a high penetration from

	welding spots showing an adequate weld nugget diameter, but low penetration,
	welding spots showing an insufficient weld nugget diameter,
	cold welds,
	loose weld spots and
	burnt weld spots.

The intensive field application of this method and the experiences made in this connection, especially the increased use of surface-treated, e.g. galvanized, sheets within the car body production, led to further development of probes. As a result of this, the 20MHz probe series G 20 MN... is nowadays used for spot weld testing without exception.

The USIP 12 is still the preferred flaw detector due to its excellent detail resolution of the echo displays. The USIP 20 is its equal in every way, and what's more, with the option UP 17 this instrument includes the advantage of digital display documentation. The USD15 S is today's answer to the long-standing demand for display storage and display processing in a PC in connection with spot weld testing. This is a modified version of the well-known and proven flaw detector USD 15, with a high-resolution, extra large screen display (195 x 97 mm²!). It enables the inspector to easily see the display with all the details even from a normal distance during tests on the car body. In addition, the considerably increased pixel resolution leaves no details in intermediate echo sequences unrevealed.

As the economic advantages are easily calculable with consistent application of this test method, various branches of the automotive industry are already practicing this method today. Among these are eg:

	Testing of individual component parts. This replaces the hammer-and-chisel testing method and, at the same time, it eliminates the "test scrap".
	Experimental construction. The welding parameters can be optimized much more quickly and reliably.
	The car body manufacture. The echo display enables to already recognize at an early stage whether the welding parameters go beyond the tolerance range. Countermeasures can be taken in good time before any reworking becomes necessary or any scrap is produced.

Anybody who wants to know more details about the secrets of spot weld testing will find a wealth of information in our new special publication written by an expert who is known far beyond his place of work. Please send us your requests for this special documentation SD 272.

A Summary by R. Diederichs

The feasibility of using a modifed ultrasonic test system was examined for nondestructive testing of spot welds on steel plate in the automobile industry, and results showed that stick welds as well as cold welds could be detected. This method of testing is suitable for quality assurance of spot welds in the series production line.

A modified ultrasonic testing method clearly detects stick welds and cold welds, this is not possible with normal ultrasonic methods. When testing under production conditions, 98% of the welds tested are correctly evaluated, i.e. values obtained from the ultrasonic test satisfactorily corresponded to the results from the teardown test.

This method of testing can be applied in sectors where the classical teardown test produces unsatisfactory results, e.g. with thicker plates, inaccessible positions, body shells etc. This is a nondestructive and manual method of testing which can firstly be applied, in addition to the teardown test, within selected areas of quality assurance and then gradually replace the destructive method.

In the case of aluminum, this new method does not give clear results due to the porous welds, however, with spot welds on thin steel plate this form of testing has proved to be an effective method in the field of quality assurance.

Fig. 2 - Stick or cold weld on steel plale. Upper plate: St 14 electroplated with zinc, 1.0 mm thick; lower plate: ST 14, smooth, 0.88 mm thick; magnification 15: 1 Fig. 3 - Ultrasonic probe for spot weld testing (Krautkramer G 15 MN 3.6) Fig. 4 - Probe section. 1: Housing, 2: Socket; 3: Cable feed; 4: Damping element; 5: Crystal; 6: wafer delay path; 7: Membrane; & Plates

The disadvantages of previous test methods
In the modern automobile production line, resistance spot welding is the most important method of joining. In this case production quality is determined by a number of processing conditions. Systematic testing of spot welds are required in order to guarantee a level of quality. Normally this is done by a random teardown test. This is a destructive test and forceably separates the weld using a hammer and chisel. The quality of the spot weld is then possible by evaluating the break or shear and by determination of the shear diameter. For a long time, a nondestructive method was looked for which did not destroy the object to be tested. In the past, the different nondestructive methods were not convincing because the stick welds or cold welds could not be identified. The reproducibility of the measurement values and the correlation to the teardown test were not sufficient for quality assurance in the automobile industry.

With the support of an equipment manufacturer, an examination was made in 1989 regarding a new, modified ultrasonic (US) method for the nondestructive testing of spot welds, with the main emphasis on detection capabilities and application possibilities in the area of body production. The method as well as the submitted findings and observations during the examination are contained in this article.

The basic physical principle
The method is based on the different intensities of sound wave energy travelling through a test object. This is influenced by structural changes caused by heating during the welding process. The body sheet used in automobile production has a stretched grain in the rolling direction. The socalled "pancake" structure occurs which contains ferrite and globule cementite. This very fine grained structure only produces a low resistance to the sound energy. In the area of the nugget on a good spot weld there is a very rough dendritic fusion structure which strongly attenuates any sound pulses. With stick welds or cold welds a nugget does not exist. The heat developed during the welding process is not sufficient to melt the material and therefore only causes "normal annealing" (grain refinement). The sound attenuation is low in this case and can be compared to the parent material which is unaffected by heat (fig. 2).

The probe
Basically, for this test, special single crystal probes are used which are highly damped and operate according to the pulse echo method (fig. 3). The crystal has an alternate transmitter and receiver function (T/R probe). In order to determine the described differences in structure, a frequency of 15 MHz is required as opposed to the normal ultrasonic testing frequency of 0.5 MHz to 10 MHz (frequency is dependent on the material). The surface in the area of the spot weld has, more or less, a strongly formed or deformed indentation. Dispite the unfavourable topography, a good coupling is still achieved by using a water delay path. A curved membrane seals the water path which is in front of the crystal (fig. 4). This flexible arrangement matches itself to the surface conditions. The diameter of the weld nugget is determined by using probes having different crystal diameters.

Preparing the probe
As opposed to conventional probes, the special probe in this case requires preparation. Accurate results depend on the condition of the membrane and therefore the water delay path. To achieve optimum coupling it must be stretched tight so that about 4 mm to 7 mm protrudes. If this cannot be done then the membrane must be replaced and the probe refilled with water. In doing this, special attention must be given to avoiding air bubbles which cause interference echoes.

Coupling the probe
Normal couplant is used between the probe and the surface of the test object. The probe is positioned, using slight pressure, vertically and centrically onto the surface of the spot weld. By slightly moving the probe, an optimum position can be achieved. This requires a certain amount of