Friday, June 15, 2012

What is Tensile Strength and Yield Strength in Externally Threaded Building Fasteners?

Tensile strength and yield strength are two of the mechanical properties of externally threaded Building Fasteners such as Hex Cap Screws, Heavy Hex Bolts or Heavy Hex Screws. These factors help determine how strong the fastener will be and what situations and projects they can best be used for.
Lets look at some numbers of standard off the shelf 316 and off the shelf BUMAX
BUMAX vs. Stainless Steel Fastener 316

Tensile Strength
The maximum load that can be supported by a fastener before it fractures or breaks is tensile strength.  Tensile strength is expressed in pounds per square inch (psi) to describe the stress that a fastener can handle.  This is calculated by multiplying the stress area of the thread (in square inches) by psi.  Guidelines can be found for these calculations in the ASME (American Society of Mechanical Engineers).
Externally threaded products with less than 100,000 lbs. of tensile strength are usually tested in full size, whereas stronger fasteners use reduced size pieces that have been cut from the actual fastener and are machine tested.  Fasteners can also be wedge tested, which means that a hardened washer with a beveled surface will be placed beneath the test fastener; a wedging action provides extreme bending stress with a concentration on the part where the head and shank meet.  This is most common with hex flange screws, studs, hex socket screws, and hex bolts and screws.  This test shows that the fastener has structural integrity at the head-to-shank position, which is critical.
Yield Strength
A specific amount of deformation is experienced by a fastener when a tension-applied load is set to test yield strength.  It forces the fastener to be stressed beyond the elastic limit and enter the plastic zone.  It is easy to use machine testing for yield strengths because of the uniform cross sectional area that exists throughout the stressed length.  This kind of testing is not always an accurate reflection of the full sized product though.  When the specimen is machined from the parent product, the beneficial effects of the cold working can be lost.  Because of the different strain rates in the threaded section, the unthreaded shank, and the thread runout, it can be very difficult to conduct an accurate yield strength test.

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