| 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 |
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| The major factor in determining the load a material can carry is its tensile strength, which is related to its hardness. |
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| 1. |
Tensile Strength - is an expression of the maximum capacity of a particular material to stretch under tension load, prior to failure. |
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It is normally expressed in: |
pounds/tons - imperial terms.
kilo newtons (kn) - metric terms. |
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| 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). |
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Stress is load ÷ area  the term will include a unit of area. |
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It is expressed as: |
lbf/in2 (PSI) - imperial terms.
N/mm2 (Mpa) - metric terms. |
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| 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. |
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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. |
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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). |
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| 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. |
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| 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). |
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| Proof loads and breaking loads are expressed as:- |
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imperial.............pounds force (lbf). |
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metric................kilonewton (Kn). |
| and are the units used by engineers in designing the elements of a joint. |
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| The strength properties of an individual fastener are achieved by a combination of:- |
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- Appropriate base material selection
- Manufacturing processes. |
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| Low carbon grades of steel are improved in hardness (strength) by cold working. |
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| Medium carbon grades of steel are improved in hardness (strength), after cold working, by controlled heat treatment and quenching. |
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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. |
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| Medium carbon grades of steel are improved in case hardness, after cold working, by heat treatment and quenching. |
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| Fasteners carry loads in one of two ways:- |
| Tensile Load. |
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| 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. |
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| Shear Load |
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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:- |
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in the imperial system as grades
in the metric system as product class. |
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| The approximate tensile strength comparison of steel grades and classes:- |
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| Points to Note |
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Product markings are not uniform over all stainless fasteners. |
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Where A2 and A4 are used without property class, assume it is lowest strength grade unless supplied with a certificate. |
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A2 and A4 may be replaced with 304 or 316. |
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'M' used in Australia on non-stainless product to indicate metric is not consistently used on stainless. |
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| Continued on next page |
