During fatigue testing of any material (especially for steels), why is strain-control mode preferred over stress control mode?
because metals, like steel, are ductile and will stretch before breaking. Straining the metal is stretching it, stressing the metal is applying a shearing force or bending until there is an actual metal failure. A sheet metal strap 1inch across and 24 gauge metal is commonly used to support metal ducting systems in construction. These have an average strain to failure weight load as high as 16,200 lbs. As reported by the smacna index, they will safely hold 6800 pounds. Of more interest is the screws used to hold the strap to the duct. While they are grade 3, fairly strong, they strain to 1400 pounds when properly installed, and are safely able to support 650 pounds, unless they are overtorqued (stripped) and this quickly drops to 400 pounds and 125 pounds...big difference.
