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Design Part 2

   

The article in the last issue of Connect introduced the fillet weld, the least costly weld type to make since the components to be joined do not require flame cutting or machining of a weld preparation, the pieces can be propped against each other and the welder can then deposit a single pass of weld metal against a solid metal backing.

Whilst this sounds simple there are some aspects of making a fillet weld that must be taken into account (in addition to those already mentioned in the previous article Design part 1).

Cooling rates in a fillet weld are greater than in a similar thickness butt joint. There are three paths by which heat will be lost from the weld. This fact means that lack of fusion/cold start defects are more likely, particularly in high thermal conductivity metals such as aluminium and the risks of cold cracking are increased in carbon and low alloy steels. What may be acceptable in terms of heat input and/or preheat temperature for a butt weld may therefore not be acceptable with a fillet weld configuration. This point has sometimes been overlooked, particularly when welding on temporary attachments such as strongbacks, where quality control may be somewhat lax. This has led to major cracking problems for some fabricators.

Unlike a butt weld where the required weld throat is generally the thickness of the parent metal, the size of a fillet weld is determined by the loads that it is expected to carry. It can therefore be of any size that the designer specifies although there are practical limitations with respect to both minimum and maximum throat thickness.

With the conventional arc welding processes it is difficult to deposit a fillet weld with a throat less than some 2mm. This is in addition to the possibility of the lack of fusion/cold cracking mentioned above due to the rapid cooling rates experienced by small fillet welds. The maximum size of fillet weld is generally that of the thickness of the thinner of the two items being joined but very large fillet welds may cause unacceptable distortion and/or extremely high residual stresses. In addition, above a certain size it may be more economical to make a T-butt, rather than a fillet weld.

Although the throat thickness is regarded as being the most important dimension for design purposes it is a fact that mechanical failure of fillet welds is often along the fusion line or through the parent material itself. One reason for this in carbon or low alloy steels is that the weld metal is mostly substantially stronger than the parent metal.

As mentioned in Connect article No. 90 there are a variety of fillet weld shapes that make the accurate measurement of the throat thickness a little more difficult than may be first thought.

The throat is the shortest distance from the root to the face of the weld. To measure this dimension in a regular mitre or flat faced fillet weld is relatively simple. The shape is that of an isosceles triangle, the throat being 0.7 of the leg length. Convex, concave and deep penetration welds, however have throat thicknesses as illustrated in Fig.1.

a) mitre fillet weld
a) mitre fillet weld
b) convex fillet weld
b) convex fillet weld
c) concave fillet weld
c) concave fillet weld
d) deep penetration weld
d) deep penetration weld

Fig.1.Throat dimensions in fillet welds

It is apparent then, that measurement of either leg length or actual throat thickness alone is not reliable in determining the design throat thickness of a weld but that the weld shape must be taken into account. The excess weld metal of the convex weld gives no benefit with respect to design strength and, from a cost point of view, the fillet weld face should be as flat as possible.

The deep penetration weld is a very cost effective way of increasing the joint strength as only a proportion of the weld metal is from deposited filler metal. However, it is not possible to measure the throat of a deep penetration weld. To guarantee that the minimum design throat has been achieved it is necessary to control welding parameters and fit-up within very tight tolerances. This type of weld is therefore generally made using an automated or mechanised welding process (submerged arc or spray transfer MIG/MAG) in order to achieve sufficient and consistent control of the welding parameters.

When deciding on the size of a fillet weld it should be remembered that a small increase in throat thickness will result in a large increase in deposited weld metal as the cross sectional area of a fillet weld is a function of the square of the leg length (area = z2/2). Increasing the throat from, say, 5 to 6mm results in an increase in area and therfore weld metal of around 45%. This equates to almost 0.1kg extra weld metal per 1 metre length of weld. There are thus substantial cost and weight penalties to be paid if the joint is either over-specified by the designer or over-welded by the welder. There are no hard and fast rules about the point at which it is more economical to change from a fillet weld to either a double sided fillet weld or a partial penetration butt weld. Areas quoted in Fig.2 are worth bearing in mind when deciding on fillet weld sizes.

Fig.2. Relative cross sectional areas

Fig.2. Relative cross sectional areas

For a fillet weld loaded in shear (the load parallel to the weld) the calculation of stress on the weld is simple; it is the load divided by the area of the weld throat.

Fig.3. Calculation of fillet weld throat

Fig.3. Calculation of fillet weld throat

It is assumed for design purposes that fillet welds fail through the throat and it is therefore a simple matter to calculate the cross sectional area capable of carrying this applied load when the strength of the weld metal is known.

Note that the shear strength of a metal is generally around 70% to 80% of the tensile strength. This figure is often factored to give an acceptable margin of safety. In the UK for plain carbon steels a shear strength of 115N/mm2 is frequently used, enabling the throat thickness to be calculated from the simple formula throat:- throat 'a' = P/(L x 115).

The throat dimensions of a double fillet weld T-joint loaded in tension can be determined using the same approach. Note, however, that this is a very simplistic calculation and does not take into account any other stresses (bending, torsion etc) that the weld may experience. It is however beyond the scope of these brief articles to cover in any depth the stress analysis of welds.

Part 3
Part 4
Part 5

This article was written by Gene Mathers.

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