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Spot Weld Testing


Spot Weld Testing

Application

Evaluating the integrity of spot weld joints in metal.

Background

Spot welding is a common technique for joining two pieces of metal, usually thin sheets or plates, by means of a series of small circular tack joints. The process is widely used in the automotive industry for body assembly and some chassis joints, and by other types of sheet metal fabricators as well. Spot welds are made by pressing electrodes against both sides of the parts to be joined and passing a high current through them, which momentarily creates a pool of molten metal that solidifies into a round joint known as a nugget.

Spot Weld Testing

If the weld is improperly formed, it is possible that the two parts may not fuse completely, or that the area of the nugget may be smaller than what is required for a strong joint. These problems are seldom visible to optical inspection, and destructive pull tests are inefficient and limited to a few samples. However, with proper equipment and technique, ultrasonic spot weld inspection can quickly and nondestructively provide valuable information about weld quality.

Equipment

The recommended instrument for spot weld inspection is a flaw detector such as the EPOCH 650 or EPOCH 1000. Spot weld testing requires coupling sound into a cup-shaped weld typically 3 to 6 mm (0.125 to 0.250 in.) in diameter and generating multiple back-wall echoes at high frequency. A wide variety of delay line and captive water column transducers in the frequency range from 10 to 20 MHz are available from Olympus for this purpose. Delay line transducers use a small plastic waveguide to couple sound energy from the transducer element to the test piece. Captive water column transducers incorporate a pliable rubber membrane enclosing a column of water that conforms to the topography of the spot weld to optimize coupling. In the case of delay line transducers, the delay line diameter and element diameter are normally matched to the nominal nugget diameter to within a few tenths of a millimeter. In the case of captive water column transducers, the element diameter is normally matched to the nominal nugget diameter. Please contact us for further details on transducer selection.

Spot Weld Testing

Theory of Operation

In a good weld, the spacing between echoes will be proportional to the thickness of the weld, and the decay rate (the rate at which successive echoes decrease in amplitude) will be related to the attenuation within the nugget. There will be a characteristic echo pattern associated with each weld condition; good, un-weld, undersized weld, and stick weld. It is encouraged to utilize the template storage software option which stores and recalls the outlines of the various echo patterns of each weld condition. Each echo pattern can be easily overlaid onto the live A-scan with a push of a button to facilitate comparison and enhance inspection. Multiple templates can be stored and an -A- denotes the active template selection.

Spot Weld Testing
Spot Weld Testing

If there is no fusion between the two pieces of metal (a "no weld" or "un-weld" condition), then the successive echoes will be much closer together and larger in amplitude.

Spot Weld Testing
Spot Weld Testing

In the case of an undersized weld, only a portion of the sound beam will reflect from the total thickness of the two metal sheets, while a portion reflects from a single thickness. This creates a pattern in which small peaks representing single-sheet thickness appear between the larger and more widely separated peaks representing the full weld thickness.

Spot Weld Testing
Spot Weld Testing

Finally, in a stick weld condition where the metal sheets are fused but because of insufficient heating the weld nugget is not fully formed, the echo decay rate will change, resulting in larger peaks appearing on screen with a longer ring-down time. This is because the grain scattering effect of a fully formed nugget is absent and the weld zone is more transmissive than in the case of a fully formed weld.

Spot Weld Testing
Spot Weld Testing

Olympus IMS

Products used for this application


EPOCH 1000 Series

The EPOCH 1000 is an advanced conventional ultrasonic flaw detector that can be upgraded with phased array imaging at an authorized Olympus service center. Key features include: EN12668-1 compliant, 37 digital receiver filter selections, and 6 kHz pulse repetition rate for high speed scanning.

EPOCH 650

The EPOCH 650 is a conventional ultrasonic flaw detector with excellent inspection performance and usability for a wide variety of applications. This intuitive, rugged instrument is a continuation of the popular EPOCH 600 flaw detector with additional capabilities.

OmniScan MX ECA/ECT

Performs eddy current array testing inspections. The test configuration supports 32 sensor coils (up to 64 with an external multiplexer) working in bridge or transmit-receive mode. The operating frequency ranges from 20 Hz to 6 MHz with the option of using multiple frequencies in the same acquisition.

Ultrasonic Transducers

Ultrasonic transducers are used for applications as diverse as flaw detection, thickness gaging, materials research and medical diagnostics. More than 5000 ultrasonic transducer types come in many styles, element diameters, frequencies, and connector styles.
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