Spot Welding - Weld Inspection
1. Weld Inspection

Both visual inspection and destructive testing are used for spot weld inspection. One of the drawbacks of spot welding is visual inspection difficulty because the steel layers are stacked on top of each other. Because of this, destructive testing is the main inspection method. However, visual inspection is still important to check for the finishing quality. Destructive testing often requires equipment not found in most shops, such as a tensile strength test machine. The chart below details the easiest and most practical spot weld inspections.

Visual Inspection:
Inspection Points
Judgment Criteria
Spot locations
Nugget is located in the middle of flange, no holes, and no nugget overflow into flange edge.
The repair spot welds avoid original spot weld locations where possible.
Number of spots and pitch
Number or spot welds is slightly increased over the factory number of welds where possible.
Welding pitch is set according to model specific body repair manual procedures.
Pinhole
No visible holes.
Spatter
No excessive spatter.

2. Destructive Inspection
Destructive Inspection (using test piece):
1.
Prepare test pieces made of the same material and thickness as the parts to be welded and weld together as illustrated.
2.
Apply force in the directions shown by the arrows to separate the spot weld. Then inspect the condition of the broken test parts.
When testing thicker test pieces, use a vise to hold one piece and hammer the other piece to rotate and separate the weld nugget area.
3.
There are three types of spot weld fractures:
Plug fracture (Nugget pullout).
Interfacial fracture.
Partial plug fracture.
4.
Spot welds pass inspection if the weld nugget diameter is equal to or greater than 4.5 times the square root of one plate thickness.
NOTE: In general, high-strength steel contains a higher amount of carbon and hardens the weld nugget more than mild steel. This means high strength steel has a lower workability (ductility) and fractures more easily than mild steel.
Plug Fracture (Nugget pullout):

The weld nugget pulls completely out of one of the test pieces, along the outer diameter of the spot welded area.

The weld diameter or nugget diameter is calculated as [d1 + d2]/2, to get a mean diameter. However, because the diameter of plug fracture is larger than the weld nugget diameter as illustrated, calculate using the weld nugget diameter.





Interfacial Fracture:

The weld nugget fails by cracking through the spot welded area of the test pieces.

The weld nugget diameter of interfacial fracture is calculated as [d1 + d2]/2 to get a mean diameter.