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Application Notes

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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.

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 LTC, EPOCH 600, 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 backwall 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. For further details on transducer selection, contact us.

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 a good weld, such as the one below. It is helpful to draw a DAC curve marking the peak locations as a reference.

Spot Weld EPOCH 650 screen

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

No weld - EPOCH 650 Spot Weld

In the case of an undersized weld, 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:

EPOCH 650 weld screen

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, with larger peaks appearing on the screen. 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. If a DAC curve has been used to mark the peak heights (ringdown envelope) of a good weld, then the typical sign of a stick weld is a series of peaks that rise above that reference curve.

Stick Weld Screen EPOCH 650

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.


The EPOCH 600 is mid-level, handheld ultrasonic flaw detector. Weighing only 1.68 kg (3.72 lb.), its horizontal case is built to withstand the rigors of very harsh environments. EN12668-1 plus features such as 400 V PerfectSquare tunable square wave pulser, digital filtering for enhanced signal-to-noise ratio.


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.


The EPOCH LTC is a mid-level, handheld ultrasonic flaw detector in a compact 2.12 lbs (0.96 kg) vertical case. It is a full-featured instrument with EN12668-1 compliance and a wide variety of standard features as well as specialized options to meet your inspection needs.

Spot Weld Transducers

Spot weld transducers are specifically designed to provide optimal performance for nondestructive testing of numerous types of spotwelds, including those used in automotive and industrial applications.
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