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Permissible stresses in welds

By Jeffrey Cook,2014-04-24 16:42
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Permissible stresses in welds

Weld poster Explanation

    Eric Sawyer

    4/24/2012

Permissible stresses in welds

    The permissible stress in welds is based on the American Institute of Steel Construction (AISC) building codes. AISC uses the yield strength to determine the max allowable stress. The comparable safety factors are also shown in the chart. These are minimum safety factors and maximum stress conditions. These permissible stresses could be used to determine the necessary size of the weld. If the fillet weld is loaded in torsion the following equations can then be back solved to find either the length or leg height of the weld.

     ;VMr;;J'''.707hJu JA

    The primary and secondary shear stress must be combined to obtain a total shear stress this will equal the max allowable shear stress. Then h can be found which is the leg height of the weld. Length of the weld is imbedded in Ju. Values of Ju can be found in table 9-2 (Shigley, page 392).

For fillet welds in bending:

     ;My;;I.707hIu I

    The stress is again known from the permissible shear stress calculation. The leg length (h) or other weld dimensions can then be solved for. Length of weld and/or distance between welds are again imbedded in Iu. Values of Iu can be found in table 9-3 (Shigley, page 396-397).

Type of loading Type of weld Permissible stresses Safety factor (n)

    Tension Butt 0.60Sy 1.67

    Bearing Butt 0.90Sy 1.11

    Bending Butt 0.60-0.66Sy 1.52-1.67

    Simple compression Butt 0.06Sy 1.67

    shear Butt or fillet 0.40Sy 1.44

Stress Concentration Factors in Welds

    Stress concentration factors should be use for parent metal as well as weld. There is often a trade off between stress concentration and over all size of weld. As the size of the weld grow so does the strength; unfortunately so dose the stress concentration, so the over all strength may be about the same. It is more important however, to focus design of weld process and components based on time and economic factors rather than machinability. For a reinforced butt weld the reinforcement on the top of the weld will increase the throat area thus decreasing stress in the weld. However, the stress concentration factor at the edge of the weld may negate any advantages gained from the reinforcement. Therefore it may be equal as strong to make a flat weld with out the

    Weld Poster 2

    Sawyer

    reinforcement and with out the stress concentration. The same idea applies to the T-butt

    (fillet) weld. If the fillet weld is done in such a way to have a very concave shape the

    transitions will be much more gradual and the stress concentration less. However as the

    weld becomes more concave in shape the throat area becomes less and the stress then

    increases.

    Post Weld Processes

    Welding always causes some distortion, shrinkage or warpage along with changes to the

    micro-structure of the parent material. Welding processes can be grouped into two levels;

    common welds and critical welds. Common welds are still very high quality welds.

    However for such situations other factors drive the design (such as deflection) so the

    stress safety factor is quite high by the time the deflection is with in limits. Such would

    include welds on vehicles frames, bridges, buildings and steam pipes. Most welds fall

    into this category. For such applications a full re-heat-treatment of part after welding is

    impractical to impossible. The best post weld procedure for this level of welds is stress

    relieving. Critical welds would be welds subjected to very high stresses or weld on very

    exotic material. After the weld process is done the entire part is re-heat-treated (heat,

    quench, temper) so the weld and heat affected zone is restored to the properties of the

    parent metal. Such processes are slow, expensive and performed in only the most crucial

    applications.

    Post weld processes temperatures will differ depending on the composition of the

    material. The specific processes/temperatures on the chart should be acceptable for

    common, low carbon, mild steels.

Process Procedure Effects

    Stress Relieving Heat to approximately 1125?F then

    (mild steel) slow cool (50?F per hour) back to

    room temperature

    Reduce or eliminate residual stresses caused by Stress Relieving Heat to a temperature well below the distortion and shirking from welding process (alloy steel) temper temperature for the particular steel No micro-structural changes Stress Relieving Heat to a temperature well below the (Al alloys) age temperature for the particular

    aluminum

    Annealing Heat to austenite, slow cool (50?F per Eliminate all effects of heat treatment or cold

    (most steels) work due to manufacturing or welding hour) back to room temperature

    Return material to softest weakest condition but

    increase % elongation and ductility

    Normalizing Heat to austenite, quench in room Increases tensile and yield strengths but

    (most steels) temperature air decreases ductility

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