Shear Strength and Welding: What You Need to Know
There’s no such thing as a perfect weld. Different kinds of welding and joints have different material properties.
Take, for example, a situation where you have two square tubes of steel, and you need to join them at a right angle. Do you:
- Put one flat end against the perpendicular surface of the other in a T joint.
- Put one next to the other and weld to the side?
- Adhere studs, drill holes, and use stud welding?
- Grind the ends into a 45-degree miter.
- Do you bevel the edges into a groove weld, or do you use a fillet weld on the surface?
Different means of attaching the materials to one another will have different properties and different strengths. Some will be better against twisting tension; others will be better against horizontal shear.
Shear strength is one of the most important kinds of strength for a weld to possess. In fact, since the shear force is the one most likely to break a weld joint – it’s the kind of stress welds are weakest – it’s an incredibly important consideration.
So, what exactly is shear strength in terms of welding, how can you calculate it, and how can you account for it in your welding?
What are Shear Stress and Strength?
Shear strength is defined as the ability of a material to resist forces that cause the material to slide against itself. When we’re talking about joining two materials, the shear force is the force that would slide the two materials along the axis of one of them.
Imagine a stack of books, one on top of another. If you press on one from the side, pushing it such that it slides cover across the cover to fall, this is a shear force. Shear force can be resisted in many ways; putting weight (or compressive force) downwards on the stack increases friction, making it require more shear force to “break” the stack. An adhesive like glue would join the books together and further inhibit shear forces.
Note: shear strength and shear stress are different. Shear stress is the force that applies to the material and is calculated based on the loads placed on the materials and the joint in question. Shear strength, meanwhile, is an inherent property of the material. Two identical pieces of steel will have the same shear strength but different resistance to shear stress depending on the joints they’re used in.
The same basic idea applies to welding. When you join two materials, various forces can apply to them, including to the joint. Some of those forces apply tension or torsion, some of them apply compression, some are ending, and some are shear.
The difference is that in metals that are fused together, it’s not shear forces applying between discrete objects like books (or metals); it’s applying between molecules.
Shear forces are important to resist primarily because the welding joint is a potential point of failure. It’s a fault line where metals have been melted, which can change their chemical and molecular properties. This is also why knowing whether or not a metal needs heat treating, choosing the right filler to melt and mix together, and ensuring proper depth of weld penetration are all critical.
This is also why, for example, porous welds full of impurities are bad. Those pores or impurities are weak areas that break the crystalline structure of the metal and, thus, can make the joint more susceptible to shear stress.
Sometimes, a joint is stronger than the material around it. This is common in woodworking, where wood glue forms a solid enough bond that the wood around it will break before the glue joint itself breaks. This can also be a concern when you’re welding certain metals.
How is Shear Strength Calculated?
Calculating the shear strength of a joint requires knowledge of a bunch of different factors. These include the size and shape of the weld, the type of joint, the materials, and even details about the tensile strength of the materials. Different parameters, including how thick the filler is, how thick the materials are, and whether or not the weld is sound, will all affect the end result.
In other words, this isn’t simple napkin math but a complex element of materials science and engineering. Fortunately, most welders aren’t going to be making these calculations on the fly; they’ll have a combination of ingrained knowledge and assurances from the engineers who designed the weld that they’ll be structurally sound.
Factors that Matter to Shear Strength Calculations
When you consider calculating the shear strength of a weld, numerous factors matter.
- The size of the weld. The larger the weld is, the stronger it generally will be in proportion to that increase in size.
- The geometry of the weld. A weld with improper geometry will be weaker than it might seem to be when other factors are the same. This is why consistency across the length of a weld is important and why picking the right type of joint is critical for strong welds.
- The materials involved. The base materials have their own known shear strength based on what kind of metal they are. The filler material will also have a known shear strength when used in welding, and the combination of the two (and how they interact in a weld pool) is critical. It does you no good to make a weld using the wrong filler, failing to adhere the materials, and leaving a weak weld behind.
- Welding technique. The skill and technique used by the welder are important, as an improper weld can be inconsistent in geometry and penetration, as well as deposition and size. All of these can weaken a weld.
- Joint preparation. In particular, when a material is coated, rusty, or otherwise layered with something that can adulterate a weld, it matters. Welding, while hot, doesn’t just burn away these impurities; it forms pockets or impurities in the core material, can cause oxidation, and leave behind a poor-quality weld.
- Welding process. Certain processes can be considered stronger than others. Most people consider TIG welding to be stronger than MIG or stick, predominantly because the process itself allows for better deposition, better penetration, less chance of inconsistent welding, and more.
- Type of joint. As mentioned above, a simple tee joint with a fillet weld is going to have different material properties than a bevelled joint, a stud weld, a butt joint, and other kinds of joints. In fact, many of these alternative styles of welding were developed specifically to improve the shear strength of the resulting joints.
As you can see, there are many considerations in play.
One of the simplest possible weld joints to use as an example is a tee joint using a fillet weld with no beveling, no additional preparation, the same materials and thickness, and other details controlled for. A basic calculation is this:
Shear Strength = 0.707 x Weld Throat x Weld Length x (Tensile Strength/3). (source)
The throat is the depth of the fillet weld’s bead. This can be calculated by measuring the coverage depth of the bead and calculating the depth at the middle angle. To put it more generically:
Shear Strength = Weld Size x Weld Length x Allowable Shear Stress
The weld size is the distance between the joint’s root and the outer edge of the weld. This is close to, but not exactly, the throat length; however, it’s measurable on the fly and much easier to estimate and use in calculations than the throat size of the same weld. Weld size can, of course, be adjusted by using multiple passes for the weld and depositing more material.
The weld length is the total length of the weld along the joint. Longer welds are generally stronger welds because there’s more material and more “surface area” to prevent the material from breaking under shear forces.
Allowable shear stress is the force expressed in a measurement like PSI that depends on the material, design of the weld, and industry standards.
What Kinds of Welds are Strongest?
When considering how strong a weld is, there are many considerations that affect the answer. Unfortunately, there’s no clear “this is the best” weld or joint; it all depends.
The most important considerations are the materials, the purpose, and the design of the weld joint.
The materials have a significant impact on the strength of a joint. This is why making a building out of wood versus making one out of steel is a real consideration. Houses, which aren’t subject to extreme forces in general, can be made of wood; skyscrapers, which have much more force acting on them, use steel for structure. Each individual connection in a massive building may be a weld, a rivet, or a combination, and they are all designed for that strength and the forces acting on the building.
The purpose of the weld joint is also important because it defines what kinds of forces the joint will have to resist. Imagine a table. The surface is supported by legs attached to the bottom. There’s no real shear strength in the vertical direction, which is where the majority of the force on the table will come from. Shear strength would be horizontal forces, which are atypical for a table.
But, if you flip the table on its side and put vertical pressure on the legs, you have two kinds of stress acting on them: shear and bending. This is the same structure, physically, as a cantilever. These joints, to be supportive, need to resist these forces to a higher degree than in the other orientation.
The design of the joint is also critical. Butt joints, tee joints, corner joints, lap joints, edge joints; they all have different pros and cons when it comes to the forces they can resist and the forces that are more likely to break them.
All of this goes back to the very first question we asked at the start. How would you design such a joint? Answering that question has cascading effects on the rest of the process.
Ensuring the Strongest Possible Welds
Ensuring the strongest possible joint means making sure every factor that goes into the strength of the joint is skewed in your favor.
Designing the weld is the engineer’s task. Picking the right materials is the designer’s task. Understanding the overall loads and construction, again, falls on the engineer. At the ground level, as a welder, you have more specific concerns.
- Make sure you’re following any welding specifications precisely. Cutting corners weakens welds and makes them more susceptible to various forces, including shear.
- Use the right filler material. Fillers with high ductility are generally stronger against shear stress because they can deform slightly without snapping, whereas brittle materials will break, often suddenly.
- Use a fast, consistent process. Something like TIG allows for a minimal amount of heat-affected material, which avoids weakening parts of the material that aren’t then reinforced as part of the weld pool.
- Use consistent speeds, deposition rates, and travel rates. Consistency is critical because any area where there’s variability means a weakness or disparity in the overall strength of the joint.
Something that can help with almost all of these factors is the overall quality of the welding machine you’re using. A good, high-quality machine will input consistent current and allow you to use modern technologies like pulsed current, which can ensure high-quality welds while reducing the heat put into a workpiece. They can also have a variety of useful features to help ensure consistent welds, including automatic deposition adjustments, current adjustments, and even welding automation.
So, if you’re on the lookout for a high-quality tool to help you ensure that you produce high-quality welds, look no further. At Red-D-Arc, our welding equipment rentals can do all of that and more. You can find excellent machines for individual welders, all the way up to fully automated and robotic systems, alone or in bulk. Whatever your needs, we have something that can help. All you need to do is reach out to talk about your needs or browse our catalog to see what’s on offer.
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