This next series of Connect articles will look at welding design.
Best practice in design is not simply a matter of deciding on the appropriate weld size or component thickness capable of carrying the service loads; there are many aspects of designing a welded component that need to be considered in addition to calculating permissible stresses. Weldability and mechanical properties such as tensile strength, toughness and fatigue resistance, all of which the designer must be familiar with, have been dealt with in a number of other Job Knowledge articles and will not be covered in this series on design.
In addition to selecting the material and specifying weld sizes, the designer must bear in mind that the decisions that he/she makes will directly affect the cost, safety and serviceability of the structure or component.
It is therefore necessary for the designer to:
- select the most appropriate material
- select the most cost effective design of welded joint
- design the component to be welded by the most cost effective process
- specify the smallest weld acceptable for both service and fabrication
- use the smallest number of welds
- ensure that there is adequate access for both welding and inspection
- ensure that realistic dimensional tolerances are specified and can be achieved
The topics mentioned above involve a range of specialised technologies and it is therefore essential for the designer to seek advice from other professions such as metallurgists and welding engineers and not to rely solely upon their own judgement. This must be done before the design process has proceeded beyond the point of no return; sadly this is often not the case!
To begin let us look at some definitions. Firstly, the joint type or configuration of which there are five fundamental forms as shown in Fig.1. Note that there are no welds associated with these joint types.
Fig.1. Joint types (a) - (e)
These various joint types may be joined by only two weld types. Firstly, the butt weld where the weld is within the plane of the components being joined and secondly, the fillet weld where the weld is completely or mostly outside the plane of the components ( Fig.2). Plug and edge welds are somewhat special cases and will be discussed later.
Fig.2. Weld types
A butt weld may be combined with a fillet weld to form a compound weld as illustrated in Fig.3:
Fig.3. Compound welds
Fillet welds are probably the most common type of weld, particularly in structural steelwork applications, so this first section will look at some of the design considerations of fillet welds. They may be used to make T, lap and corner joints ( Fig.4).
Fig.4. Single-sided fillet welded joint types
A fillet weld is approximately triangular in shape, the size being defined by the weld throat or leg length as shown in Fig.5.
Fig.5. Terms used to describe features of a fillet weld
Fillet welds sizes should be specified preferably by referring to the throat thickness 'a' although the leg length 'z' is often used and can be easier to measure during weld inspection. Conventionally, the leg lengths are regarded as being of equal dimensions, the weld forming an isosceles triangle in cross section.
The convex fillet is generally undesirable for two main reasons. a)The junction of the weld metal with the parent metal at the weld toe can form a significant stress raiser and will adversely affect both fatigue life and brittle fracture resistance; b) the excess weld metal in the cap costs both time and money to deposit without contributing to joint strength. The concave fillet weld can be beneficial with respect to fatigue strength and, if required, the minimum throat thickness MUST be specified.
Fillet welds are less expensive to make than butt welds as there is no requirement to cut or machine a weld preparation. Although they are capable of carrying substantial loads they should not be used where the applied loads put the root of the weld in tension, particularly where the loading is dynamic - fatigue life in particular is drastically reduced. Where such loading is a possibility then a double sided T-joint should be made using two fillet welds ( Fig.6).
Fig.6. Preferred fillet welded joint type under bending loads
It is commonly thought that the fillet weld is an easier weld for the welder to make than a butt weld as the weld is deposited on solid metal. However, this is not necessarily the case when full fusion into the root of the weld is required. It is not unknown for highly skilled welders to fail a fillet weld qualification test where this is a design requirement. This is an important point and needs to be considered firstly by the designer asking if it is an essential requirement and secondly by the fabricator when pricing a contract.
This also raises the point that the fillet weld is extremely difficult to volumetrically examine using non-destructive testing techniques to confirm its internal soundness. This applies particularly to the root region where it is not possible to measure, with any degree of precision, any lack of fusion, slag entrapment etc. Therefore the same reliance on joint integrity, and hence service performance, should not be placed on a fillet weld as may be placed on a fully inspected butt weld.
The next article (Part 2) will discuss the topic of fillet weld design before moving on to butt joints.
This article was written by Gene Mathers.