| Metals used in fastener
manufacture are elastic materials which will stretch (elongate)
under applied loads and return to their original shape when the
load is removed. |
| However, if sufficient load is
applied, the material will stretch beyond its yield point and
enter a plastic zone, losing its elasticity and becoming
permanently stretched. |
| Further increased load on the
material will stretch it to its ultimate tensile strength at
which point the material will fracture. |
| The materials of our particular
concern are: |
|
Steels |
- low tensile (mild steel)
- high tensile
- stainless steel |
|
| The major factor in determining the
load a material can carry is its tensile strength, which
is related to its hardness. |
 |
|
1. |
Tensile Strength - is an
expression of the maximum capacity of a particular material
to stretch under tension load, prior to failure. |
|
| |
It is normally expressed in:
|
pounds/tons - imperial terms.
kilo newtons (kn) - metric terms. |
|
|
|
2. |
Yield Stress (yield
point) - is an expression of the theoretical point of stress
(pressure) beyond which the material loses its elasticity
and becomes permanently stretched; (realistically, a range
rather than a single point). |
|
|
Stress
is load ÷ area
the term will include a unit of area. |
|
It is expressed as: |
lbf/in2 (PSI) - imperial
terms.
N/mm2 (Mpa) - metric terms. |
|
|
|
3. |
Proof Load Stress - is an
expression of the minimum stress a material must achieve,
prior to permanent elongation and, the stress which would be
applied to test and remeasure a specific fastener to prove
it had not permanently stretched and that it will carry the
required load. |
|
|
These terms will also include a
unit of area, are approximately between 80% and 90% of the
theoretical yield stress and are expressed in the same
terms. |
|
|
Proof load stresses also apply
to nuts and are the point at which the nut is deemed to have
failed; (= to the bolt UTS in a given diameter). |
|
|
|
4. |
Ultimate Tensile Stress -
is the theoretical minimum point at which the material will
fracture. It is expressed in the same terms as yield stress
and proof load stress. |
|
|
| These properties are used to
calculate the proof load and breaking load for
each diameter of each grade or class of product. (The calculated
figures for each of these properties are listed by diameter in
the relevant standards). |
|
| Proof loads and
breaking loads are expressed as:- |
|
imperial.............pounds
force (lbf). |
|
metric................kilonewton
(Kn). |
| and are the units
used by engineers in designing the elements of a joint. |
|
|
| The strength
properties of an individual fastener are achieved by a
combination of:- |
|
- Appropriate base material
selection
- Manufacturing processes. |
|
|
 |
| Low carbon grades of steel are
improved in hardness (strength) by cold working. |
|
 |
| Medium carbon grades of steel are
improved in hardness (strength), after cold working, by
controlled heat treatment and quenching. |
|
 |
Austenitic grades in various
strengths are improved in hardness by cold working.
Martensitic grades in various
strengths are improved, after cold working, by controlled heat
treatment and quenching. |
|
 |
| Medium carbon grades of steel are
improved in case hardness, after cold working, by heat treatment
and quenching. |
|
 |
| Fasteners carry loads in one of two
ways:- |
| Tensile Load. |
 |
| Where the load is acting to separate
the fastened components along the shank length, it is referred
to as a tensile load. Tensile loads try to elongate the
fastener. |
|
| Shear Load |
 |
|
Where the load is acting to separate
the fastened components across the shank diameter, it is
referred to as a shear load. Shear loads try to cut the
fastener in half.
The load carrying capability of a fastener is somewhat less in
shear than in tensile and will further vary if the shear plane
is across the threads rather than the plane shank.
Some applications could exert a
combination of tensile and shear loads.
The strengths of a
product group of fasteners are expressed:- |
|
in the imperial system as grades
in the metric system as product class. |
|
| The approximate tensile strength
comparison of steel grades and classes:- |
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| Points to Note |
 |
Product markings are not uniform
over all stainless fasteners. |
|
|
Where A2 and A4 are used without
property class, assume it is lowest strength grade unless
supplied with a certificate. |
|
|
A2 and A4 may be replaced with
304 or 316. |
|
|
'M' used in Australia on
non-stainless product to indicate metric is not consistently
used on stainless. |
|
| Continued on
next page |
