This subject raises its ugly head in a lot of seemingly unrelated topics but is very important for DIY rodders to understand. In some cases it means the difference between life and death.
In designing ANY mechanical element that will see even moderate stress and especially vibration of any magnitude, be sure there are no sharp INNER corners in the design. The sketch below show a couple cartoon examples of what I am talking about. Note the shaft and weld joint at the left with the sharp inside corners and the cracks drawn in. These are very bad designs since the sharp inner corner is in essence a crack already initiated. It takes little added stress and/or fatigue from vibration to extend the crack into the body of the part, leading to total failure.
To understand why, just think of what stress is; it is a force borne by an area thus the term 'pounds per square inch' or 'psi'. The equation is simple,
force, #/area, sqin
Using this equation to analyze a sharp V-intersection that by definition has zero area, the equation is
X#/0.0sqin = infinite stress!
Thus it take very little force to propagate a crack. However when you design in a fillet or radius in a machined element or perfect your welding technique or at least grind proud weld beads to a fillet, you spread that force over a much greater area and eliminate the stress riser and inevitable crack.
The failure of the Heidt's MII X-members using strut rod eliminators that has been discussed many times on this board is due to a poor design that incorporates the 90deg welded intersection of two steel plates at a highly stressed point of the design. See the picture below. By simply continuing the bottom plate of the X-member up the side of the riser, those sharp stress-riser intersections are eliminated and the danger of cracks and catastrophic failure goes away.
In designing ANY mechanical element that will see even moderate stress and especially vibration of any magnitude, be sure there are no sharp INNER corners in the design. The sketch below show a couple cartoon examples of what I am talking about. Note the shaft and weld joint at the left with the sharp inside corners and the cracks drawn in. These are very bad designs since the sharp inner corner is in essence a crack already initiated. It takes little added stress and/or fatigue from vibration to extend the crack into the body of the part, leading to total failure.
To understand why, just think of what stress is; it is a force borne by an area thus the term 'pounds per square inch' or 'psi'. The equation is simple,
force, #/area, sqin
Using this equation to analyze a sharp V-intersection that by definition has zero area, the equation is
X#/0.0sqin = infinite stress!
Thus it take very little force to propagate a crack. However when you design in a fillet or radius in a machined element or perfect your welding technique or at least grind proud weld beads to a fillet, you spread that force over a much greater area and eliminate the stress riser and inevitable crack.
The failure of the Heidt's MII X-members using strut rod eliminators that has been discussed many times on this board is due to a poor design that incorporates the 90deg welded intersection of two steel plates at a highly stressed point of the design. See the picture below. By simply continuing the bottom plate of the X-member up the side of the riser, those sharp stress-riser intersections are eliminated and the danger of cracks and catastrophic failure goes away.