Joint efficiency is a measurable quality that helps to assess the safety and reliability of welded joints.
What is Joint Efficiency?
Joint efficiency is important because a strongly welded joint is one way of reducing the allowable stress before failure of a given material. Efficiency also means the strength of the welded joint, which is dependent and in relation to the material strength of the joint.
Ultimately joint efficiency should be an important design consideration, not something that is haphazardly calculated after the weld is created. Material failure can have critical safety implications if not properly taken into account. Efficiency will change depending on joint type, as well as the extent of the joint.
So how exactly is joint efficiency measured? It is measured from the strength of the joint relative to the strength of the material the joint is welded on. The ideal version of a joint has an efficiency of 1.00, which means the joint is judged to have the same strength of the base material, and is said to be a seamless weld.
When we say a joint is judged to have an efficiency of 1.00, or 0.75, it is another way of saying that the joint is 100% efficient, or 75% efficient. And judged is the term used here, because either the designer or a specific inspector is in charge of looking over the welded joint and determining its efficiency. These spot checks are either full or partial and extrapolated.
How Do You Calculate Joint Efficiency?
The rating of a joint slides on a scale aiming towards the perfection of a welded joint being a smooth extension of the base metal. It is a ratio of welded joint strength to strength of the base. Any defects of discontinuities will contribute to a lower joint efficiency rating.
Joint efficiency is just that: a rating. This rating emerges from an inspection of the joint known as a Non-Destructive Examination (NDE). As well as the results of an NDE, joint efficiency is based on the category and the type of joint, some of which will be limit how high the efficiency may be rated.
An NDE must be performed by someone who has been trained in conducting proper examinations. There are typically two ways someone can get certified to perform an NDE: either through their employer or a central certification authority that adheres to ISO standards. Additionally, there are three varying levels to being NDE certified, ranging from technician level capable to conducting a test, to engineer level capable of setting up a test, to a more director level in charge of understanding codes and establishing new NDE rules and tests.
NDEs do not necessarily need to be performed along the full length of the weld. For very large joints, this may be quite impractical. Sometimes an NDE can be performed and the results extrapolated to the rest of the joint, and this is known as a partial inspection.
A partial inspection, meaning not every weld has been looked at, will never result in a joint efficiency of 1.00. Even if the weld(s) inspected are in essence “perfect” and without defect, and it is possible or even probable the rest of the welds will be as well, a 1.00 designation is not allowed to be given. The ramifications of a less than perfect joint efficiency means that there will be less allowable stress in the material if standards are followed.
Total and partial inspections are also known as “Full” and “Spot” NDEs. There are no total requirements for Full inspections, rather it is up to the discretion of the engineers.
Most importantly, joint efficiency designations are based off of the equipment and techniques used, and the knowledge of the designer and inspector. Even before the inspection is performed, the joint efficiency will have a baseline rating due to these conditions. How the weld is tested, and even just knowing how it was made before inspection, will all have a large impact on the final rating.
While it may seem prudent to use the most qualified inspector and the highest rated joints, remember that there is typically a tradeoff between joints efficiencies, and time plus costs. Sometimes a weaker joint with a less rigorous NDE is acceptable for the type of joint being welded, and the higher costs are not justifiable.
Joint Efficiency of Various Weld Types
Different welds will inherently have different efficiencies, regardless of whether full or spot checks are performed (though the final ratings will obviously change depending on which NDE is done). There are two main types of welds covered by ASME standards commonly adhered to: Butt and Lap joints.
Butt Joint Efficiency
A butt weld is a very simple type of joint. Two pieces of metal are touched side by side in parallel on the same plane, and welded together in the small gap between. Occasionally a groove or gap can be made where the pieces touch, preparing the metal for the weld and allowing for more secure welding. These welds are easy to create and provide high strength when done correctly, but are not suited for more complicated joints.
Butt welds are easy to make full welds, completely fusing two metal pieces together. It’s easier to machine two pieces being joined by a butt weld too, and inspection is generally easier for this type of joint, as the final welds are easier to control against distortions while being completed.
There are disadvantages to using a butt weld, with the biggest drawback being that they can be unusable for complex pieces. Then can also be more costly or time consuming when backing pieces are used, and if there are any gaps in the welded joint, corrosion is more likely.
Here are the efficiencies of certain types of butt joint welds:
Double grooved and welded butt joint:
- Full inspection efficiency of 1.00
- Spot inspection efficiency of 0.85
- No inspection efficiency of 0.70
Single grooved and welded butt joint with backing strip:
- Full inspection efficiency of 0.90
- Spot inspection efficiency of 0.80
- No inspection efficiency of 0.65
Single grooved and welded butt joint, no backing strip:
- Limited to 0.60 efficiency, regardless of inspection type
Lap Joint Efficiency
A lap joint is similar to a butt joint, however instead of the two pieces of metal lying on the same plane, there is overlap. One portion will sit above the other, on a different plane. Typically lap weld joints are used when needing to joint together two metal pieces of varying thicknesses.
However, lap joint welds are not often used for thick pieces of metal, and typically reserved for things like stretches of sheet metal. Lap joint welds have the potential to corrode easier because of this overlap, and are also more susceptible to lamellar tearing.
These overlapping pieces of metals can be welded on both “faces” of the overlap, leading to better strength outcomes. These are known as double fillet lap welds. Additionally, lap joints can be supplemented by plug welds, where in a hole in one sheet of metal is overlapping with the second sheet underneath, and that hole is filled with a weld.
Here are the efficiencies of certain types of lap joint welds:
Double full fillet lap joint:
- Joint efficiency of 0.6
Single full fillet lap joint with plug welds:
- Joint efficiency of 0.5
Single full fillet lap joint, no plug welds:
- Joint efficiency of 0.45
It’s clear that butt joint welds are typically more efficient than lap joint welds, even without a full inspection done. However, that does not always mean butt joints are appropriate or even the better option for a weld.
What is Joint Efficiency in a Tank?
A tank, also known as a “pressure vessel”, is a solidly welded hollow metal structures used to store liquid or gas. It’s obvious why you wouldn’t want leaks in any container but in some instances, like storing toxic gases, it is absolutely critical that welded joints are secure!
One way to add extra security to joints for tanks is to make them rivet joints. A rivet joint essentially bolts two pieces of metal together with bolt-like pins known as rivets. Multiple rows of rivets provide more security, and higher joint efficiencies.
Rivet joints for tanks also follow the same joint efficiency scale, and the most common types of these joints are once again butt and lap joints. It is common to see lap rivet joints ranging from one to four rows of rivets, correlating to joint efficiencies ranging from 0.45 to 0.75. For the more secure butt rivet joints, rivet rows commonly range from two to six, which range from 0.75 to 0.92 efficiency, respectively.