We shall concentrate on steel fasteners, which, because of their strength and cheapness, constitute over 90% of all fasteners used. Steels for commercial fasteners are graded into property classes as in Table 2.

Each class number consists of two figures

• the first figure is   S_u /100 where   S_u is the steel's nominal tensile ultimate (MPa)
• the second figure is the ratio   S_y /S_u where   S_y is the nominal 0.2% offset yield strength.

Minimum material strengths are not less than nominal values.

It is difficult to determine the yield of a full size threaded connector ( as opposed to a cylindrical test piece ) because of the different strain rates of shank, thread and runout. For this reason the proof stress rather than the yield stress is used as a criterion for failure assessment - the   proof stress S_p is the largest stress which does not lead to any permanent set. The proof load of a screw made from a particular material is the maximum load the screw can withstand without permanent deformation, and is given by ( 1) as the product of stress area ( Table 1 ) and proof stress ( Table 2 ). Selection of class 12.9 should not be undertaken lightly - its high strength begets other potential problems.

We have noted that the elastic behaviour of connected members has a significant effect upon the distribution of load between their connectors. In an elastic bolt and nut connected by a number of thread turns, the first 'thread' takes a disproportionate fraction of the total tensile load - Craddock op cit reports the load distributed among the threads as sketched here. It is clear that there is little benefit from an engagement length exceeding half-a- dozen threads. Special nuts which alleviate non-uniform load distribution are available. The variation of normal stress   σ over a cross- section adjacent to the nut face ( as sketched ) is dominated by stress concentration induced by the thread root, so it is hardly surprising that bolt fractures usually occur in the exposed threads close to the nut face.

Another failure mechanism is stripping of the nut threads, which is essentially shear failure of the nut material on the cylindrical surface at the thread major diameter. Stripping of the bolt threads is a similar shear of the bolt material at the minor diameter - but this is rare. Other possible failure mechanisms, such as crushing of the nut bearing surface and dilation of a thin nut due to its riding up the thread flanks, are not critical in themselves, but contribute to other modes.

If any failure is to occur, then bolt fracture is the preferred mode. Bolt fracture is clearly discernable and often occurs whilst tightening, when torsional shear stresses ( which dissipate quickly after tightening ) are superimposed upon tensile stresses. So the operator replaces the bolt and learns to exercise care in subsequent tightening. Thread stripping on the other hand is insidious and progressive - the first thread fails putting more load onto the remaining threads, hence the second succumbs . . . and so on. When threads strip it is often difficult to separate components for fastener replacement, whereas a broken bolt requires no further separation. The consequences of a tapped hole in a car engine block being stripped by over- zealous spanner wielding is a case in point.

For a given load, bolt fracture tendency will clearly be reduced by a larger stress area ( ie. a larger bolt size ) and/or a higher class of material. Given these parameters ( load, size and bolt class ), nut stripping will be reduced by a longer nut and hence an increased cylindrical shear area, by a thicker walled nut to decrease dilation, and/or by a superior nut (property) class.

The first stage of fastener design is bolt selection - size, class and other geometric attributes. ISO Metric nuts have been dimensioned to bias any failure towards bolt fracture rather than nut stripping, on the tacit presumption that the nut class is equal to or greater than the bolt class - ie. the material of the chosen nut is at least as strong as the preselected bolt's material. It is for such reasons that regular nut lengths are about 80% of bolt size, and of the hexagonal dimensions shown in Table 1 - though there are various styles of nut which differ from these figures.