What is Loading and How is it Defined?
The point in the stress-strain curve at which the curve levels off and plastic deformation begins to occur. When something is stressed and stretches and after the load is taken off it goes back to the original position - this is plastic deformation, it has not yet ‘Yielded’. The Yield Load is the load which is great enough so it goes past this point, becomes inelastic and doesn’t return/go back to its original shape/position.
Offset yield point (proof stress)
When a yield point is not easily defined based on the shape of the stress-strain curve an offset yield point is arbitrarily defined. The value for this is commonly set at 0.1 or 0.2% of the strain. The offset value is given as a subscript, e.g., Rp0.2=310 MPa. High strength steel and aluminum alloys do not exhibit a yield point, so this offset yield point is used on these materials.
Tensile load or ultimate strength is the maximum stress that a material can withstand while being stretched or pulled before failing or breaking. This is higher than the Yield Load.
Shear strength is the strength of a material or component against the type of yield or structural failure where the material or component fails in shear. A shear load is a force that tends to produce a sliding failure on a material along a plane that is parallel to the direction of the force. When a paper is cut with scissors, the paper fails in shear. Shear strength is the strength a component or fastener has from a force at 90 degrees to the direction of the bolt or component. It would be calculated by an Engineer through testing or using the fastener/components Yield and Tensile Strengths. Unlike tensile and yield strengths, there are no published shear strength values or requirements for ASTM fastener specifications. The Industrial Fastener Institute (Inch Fastener Standards, 7th ed. 2003. B-8) states that shear strength for fasteners is approximately 60% of the minimum tensile strength.
Safe Working Load
This is the user’s engineer’s recommended maximum weight load for a line, rope, crane or any other lifting device or component of a lifting device or fitting for safe use. The SWL is determined by dividing the minimum breaking strength (MBS) of a component by a safety factor assigned to that type and use of equipment. The safety factor generally ranges from 4 to 6 unless a failure of the equipment could pose a risk to life; in that instance the safety factor would be a 10. For example, if a line has an MBS of 1,000 pounds and a safety factor of 5, then the SWL would be 200 pounds. 1000 / 5 = 200. Also called working load limit (WLL). Different applications have different safety factors used to determine Safe Working Load. For example, generally tension structures will use up to 60% of yield load as the Safe Working Load. But for Lifting purposes the safety factor will use a much higher, generally 24% of Tensile/breaking load.