I'm welding a .125 plate to the rocker panel that is much thinner. Do you weld all the way around the plate or leave some space between your welds. My thinking is that IF your weld all the way around you weakened the area being weld. Making the thin metal brittle and prone to cracking?

What type of welding ??? and what kind of weld ? Lap, butt and why would you think it would crack ??? will it be exposed to stress or load ??

You will want to weld it all the way around, just make sure you weld in small 1/2"-1" lengths then switch to a different side of the plate to help keep the heat down. When i weld plates to the floor i always start the arc on the thicker material and swing it down to the thinner floor material.

Hopes this helps.

The thinner metal is about 1/2 the thickness of your plate so you want 2/3 of your weld focused on the thicker plate. I assume you're migging this so instead of aiming at a 45* into the joint you want to tilt the gun more towards the thicker metal. This allows you to use the heat setting for the thicker metal. Weld all the way around. Space your welds.

Also, be sure to clip off the ball at the end of the wire before EVERY weld AND chip off that shinny piece of silicon at the end of every bead before you continue that bead.

Weld all the way around,tack it down good weld top then bottom then each side.I prefer to make solid welds(like all of one side) if at all possible.Starting & stopping(short beads) weakens the weld,also a solid bead looks better.

WOW thanks for the input
I figure that you sometimes see factory welds that are not completely welded.
I will weld it all around and concentrate the heat to the thicker side.
I am mounting my main hoop to the floor. Instaed of welding it to the .125 plate that would be welded to the floor I decided( because the floor pan is not 100%) to weld the hoop to the .125 square tubing that attaches to the rocker and the subframe connections. I figure that would be stronger and safer

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Starting & stopping (short beads) weakens the weld

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Wow, I know a lot of metalurgists and welders who'll argue that statement !!

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also a solid bead looks better

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if I do a start and stop you'd never know it.

Round the corners of your plate, no pointed corners.

Sharp corners corners will burn away like RobX4406 said, and like P body said, put the heat into the thick piece, and persuade it to the thinner piece.......

Just make sure you practice on some scrap, before you get going to heavy.......

.....and as far as the tie ins, if you grind/blend in the starts/stops, the weld would theoretically be as strong all around.

Had to get re-certified for a particular contractor's job a couple months ago(316 stainless, 6" sch. 40, TIG root/hot pass, Flux core cap)100% x-ray, and had two starts in it on the cap(slipped )

Trust me, it does not make a difference whatsoever if you are thorough in the prep/tie in/heat control.......