I'm a noob when it comes to fabrication, but I find it really interesting. How do you know that what you've built will be strong enough to winch off of? Is it bolted anywhere, or just welded, and is one stronger than the other?
This comes from experience, and/or having a mentor, and/or copying what you've seen others use successfully.
You can also look up the rated strength for specific parts and do some math.
Take the minimum yield in psi of the ASTM grade (y), multiplied by the stress area of the specific diameter (a). This formula will give you the ultimate yield strength of that size and grade of bolt (x).
y*a²=x
Example with 5/8"-11 grade 8 bolt: 130,000psi*0.226in²=29380lbs
So the 5/8"-11 grade 8 bolt will hold 29380lbs before yielding.
Take the minimum tensile strength in psi of the ASTM grade, multiplied by the stress area of the diameter. This formula will give you the ultimate tensile strength of that size and grade of bolt.
t*a²=x
Example with 5/8"-11 grade 8 bolt: 150,000psi*0.226in²=33900lbs until failure.
Now to find the shear strength you just multiply the ultimate tensile strength of the bolt by 60%. It is important to understand that this value is only an estimate. Unlike tensile and yield strengths, there are no published shear strength values or requirements for ASTM specifications. The Industrial Fastener Institute states that shear strength is approximately 60% of the minimum tensile strength.
So for the 5/8"-11 grade 8 bolt you have 33,900*.60= 20340lbs to shear the bolt.
Lets just say 20k lbs and lets imagine this is for a front bumper with a winch. Say you have 6 of these bolts holding the bumper to the frame/body. It should take 120k lbs of force to break the bolts. Before that would happen you would probably pull the bolts out of the thin metal on the frame/body.
Ok, to prevent that you plate the area it bolts to with 1/8 or 1/4" steel plate. This now makes the weakest point of your setup the winch cable which is usually rated for ~12k-20k lbs if steel and ~15-20k lbs if synthetic. You'll snap your cable before you rip off your winch/bumper.
For areas that are welded, if you're getting enough penetration into the metals your weld should be stronger than the surrounding metal. You can test this by cutting a test piece and inspecting the penetration or having it x-rayed if you can't destroy the prototype. If it's not a critical weld that will get someone killed if it fails you can just weld up a similar piece and put it in a press or vise and bend the joint and see what fails first.
