Destructive weld tests: bend, tensile and Charpy

Why destructive tests exist

Nondestructive methods find flaws without harming the part, and the NDE methods guide covers them. They can't tell you the strength of the weld metal. Destructive tests can, which is why they're used to qualify procedures and welders rather than to check each production weld. The test piece is a coupon welded under controlled conditions. You sacrifice the coupon to learn something about every weld made the same way.

The tests and what each proves

Tensile testing

A machine pulls a specimen until it fails. You learn the load it carried, and from the specimen dimensions you can work out stress. After failure you can measure how much it stretched and how much the cross section shrank. Those give elongation and reduction of area, both signs of ductility. Know where the break happened too: a break in the base metal away from the weld says the weld was at least as strong as the plate.

For practice with the arithmetic, the CWI math calculator works tensile stress, elongation and reduction of area step by step. Try one with your own invented numbers until the formulas stick.

Guided bend testing

A strip cut from the weld is forced around a shaped former. This shows whether the weld can deform without cracking. Know the three orientations: face bend (the face of the weld is on the outside of the bend), root bend (the root is on the outside) and side bend (the strip is cut across the weld and bent so a side surface stretches, which suits thick material). Face and root bends are used on thinner material. A crack or open flaw on the stretched surface is the thing the inspector watches for, and the code sets the limit.

Charpy impact testing

Strength and toughness are different. A strong material can still shatter when hit, especially when cold. A notched bar is struck by a swinging pendulum, and the energy it absorbs measures toughness. The test is run at a stated temperature because many steels turn brittle as temperature drops. When a question mentions low temperature service, think toughness and Charpy.

Hardness testing

Hardness tests press a tool into the surface and measure the indent. Common scales are Rockwell, Brinell and Vickers. Hardness is a quick indicator of the heat-affected zone's condition and, in some steels, of cracking risk, since very hard microstructures are crack-prone. Know that the scales differ and that hardness roughly tracks strength.

Macro-etch

Cut a cross section, polish it and apply an etchant. The weld layers, fusion lines, penetration and heat-affected zone show up clearly to the eye or low magnification. It's the quick way to confirm fusion at the root and sidewalls and to check fillet size and profile on a section.

Fillet break and nick break

Fillet welds are often tested by breaking them open, as in a fillet break test. A nick-break specimen is cut with a small notch and snapped. You then examine the fracture surface for porosity, slag, lack of fusion and incomplete penetration inside the weld.

Match the question to the test

When an item names a property, translate it:

• "How much load can it carry?" is tensile.
• "Will it crack when formed or deformed?" is bend.
• "Will it survive an impact at low temperature?" is Charpy.
• "Is the heat-affected zone too hard?" is hardness.
• "Did the weld fuse at the root and sides?" is macro-etch or break test.

Where this fits in the exam and on the job

Destructive tests tie directly to procedure qualification. A coupon is welded, cut into specimens, tested, and the results go into the record, as covered in WPS, PQR and WPQ explained. On Part A you answer from memory with no code book, so the purpose of each test needs to be automatic. The Part A guide shows the full domain list.

Find out whether this is a gap

Close the notes and explain each test in one sentence, then say what the specimen looks like. Where you stall, drill. The free 10-question Part A check is scored by area and shows exactly which topics to study first, so you can take it here before you decide how long to spend on this topic.

FAQ

What is the difference between a tensile test and a bend test?

A tensile test pulls a specimen to failure to measure strength and ductility. A bend test forms a strip around a former to see whether the weld can deform without cracking.

What does a Charpy test measure?

Toughness, meaning how much energy the material absorbs before it fractures when struck. It's run at a stated temperature because toughness often drops in the cold.

What are face, root and side bends?

They describe which surface is stretched on the outside of the bend. Face puts the weld face outside, root puts the root outside and side bends across the thickness.

Are destructive tests done on production welds?

Normally not. They're run on test coupons to qualify procedures and welders. Production welds are checked with visual and nondestructive methods.