By definition,the proof load is an applied tensile load the fastener can support without permanent deformation.The bolt will return to its original shape once the load is removed.The bolt may be reused provided you are absolutely certain the bolt never exceeded this point and began to yield:a simple enough definition,but one that requires and extensive explanation.
As the fastened joint is put into use, it will encounter all types of various loads, including tension loads, shear loads, cyclic loads, prying loads and loads which may be a combination of these and other possibilities. Things like pressure changes in a pipeline, vibration from an engine or an impact of a hydraulic ram, may produce these loads. These external loads add to or subtract from the initial load of the fastener and in extreme cases may yield the fastener.
Heat will lower the yield value of the fastener. The yield strengths are determined at room temperature. ASTM A193 B7 has a yield strength of 75-105 ksi(75 ksi for sizes over 4 inches in diameter and 105 ksi for material in diameters up to 2-1/2 inches) at 70 degrees, and drops to approximately 53-74 ksi at approximately 800 degrees. So, if we install a B7 fastener at room temperature expecting each fastener to support a tensile load of 85 ksi, which would not yield the fastener of diameters 2-1/2 inches and smaller. Now, if we were to intoduce heat into our fastener application, the fastener would begin to yield at much lower load.
Was the fastener installed properly? This is one of the most difficult questions to answer. Extreme caution should always be used when relying on torque from a formula to indicate the tension induced. Torque is only an indirect indication of tension. Improper installation may yield the fastener.
One important factor to remember; typically you will not know whether or not the fastener has been yielded. Especially in critical situations, you should never reuse a fastener unless you are certain the fastener has never been yielded.
