Peel Tests and Spot welding AHSS Steels
Welding AHSS Steels On Automobiles and Peel Testing
Advanced High-Strength Steels (AHSS) have been resistance welded in automotive production lines in the last few years. However, the high strength and hardness can be expected to affect spot weld failure modes during the typical peel testing and chisel testing performed for weld quality evaluation.
A well-established industry-standard associated with peel testing of conventional steels is that an acceptable test is one in which the peel test “pulls a nugget” or a “full button,” as shown below.
However, with AHSS, full button pulls are less likely due to the high Carbon Equivalent that is likely to produce hard weld nuggets. This fact is compounded by the higher yield strengths of the material that will tend to produce greater stresses concentrating at the edge of the nugget during a peel or chisel tests. Therefore, the conventional modes of testing such as peel and chisel testing may be more likely to produce interfacial or partial interfacial failure modes such as those shown below.
With AHSS, these types of failure modes may occur even though the weld strengths may be acceptable for the intended application. Even full interfacial failures may exhibit high strength, although it may sometimes be challenging to differentiate between an interfacial failure and a “stuck” weld condition (which refers to an unfused bond of unacceptable strength).
The above statements provide a great insight into the plight of the vehicle repair technician in how do they ensure that a vehicle is welded correctly?
Many spot welding machines now have Auto selection modes that will assess the type, resistance, and thickness of the steel stack to be welded, this enables greater quality in theory but still requires the welder operator to perform a destructive( peel) test and observe visual weld size characteristics.
To carry our a peel test spotweld at the edge of two weld tags in the middle.
Place one of the lengths in a vice and gripping the top piece of steel with a pair of grips "peel" the steel towards you, hopefully tearing a button on the vice gripped plate.
In The UK BS EN 1140 gives instructions for destruction and testing of spot welds you can download it here.
Contributed by Menachem Kimchi, Ohio State University