Mechanical Properties
Mechanical properties, the forces acting on a weld, and why joint design determines what the electrode rating never can.
You place a 1-inch weld of 7018. What is that weld rated at?
Most people have a number ready. But the real answer is more complicated than you think.
Tensile strength only applies when a load is being pulled directly apart. A tensile test is performed in a lab under ideal conditions — a machine applying force in a single controlled direction, That is not what most welds encounter in service.
Real joints see shear forces, torsional forces, bending, fatigue, and often a combination of all of them at once. Each of those loading conditions affects the actual strength of the weld differently than a straight tensile pull would. PSI describes force distributed over an area. Without knowing the actual weld cross-sectional area and loading conditions, you cannot convert 70 ksi directly into weld capacity. Accepting the tensile rating as the full picture of weld strength is not only inaccurate — it can be dangerous.
The ‘70’ in 7018 means the deposited weld metal has a minimum tensile strength of 70 ksi (70,000 psi), but that is a stress rating, not a load rating in pounds. A 1-inch bead does not equal 70,000 pounds of capacity. Actual weld strength depends on joint geometry, weld size and throat, base metal, and the type of load the joint sees — tension, shear, torsion, fatigue, or a combination.
To understand what that actually means, you need to understand what mechanical properties describe.
1. Mechanical properties
Tensile strength, yield strength, ductility, hardness, and toughness are not the same thing. Each one describes something different about how a material behaves under stress. Treating them as interchangeable is where most of the confusion about weld strength comes from.
Green = safe elastic range · Orange = yield point (permanent change begins) · Red = fracture
“Below yield, the metal forgets the load. Past yield, it remembers it forever.”
A note on fatigue
A professor once explained it this way: think about fatigue in your own life. One hard day doesn’t break you. But the commute, the long hours, the constant grind — that accumulates. Over time it catches up. Metal works the same way. The load that did nothing on day one may be the load that causes fracture on cycle ten thousand.
Fatigue crack progression under repeated loading
“The load that didn’t break it the first time may be the exact load that breaks it on cycle ten thousand.”
2. The forces acting on a weld
Electrode tensile classifications are based on a straight pull under controlled test conditions. Real joints are rarely loaded that cleanly.
“Most welds aren’t pulled straight apart. They’re twisted, loaded sideways, and cycled repeatedly. That’s where the electrode rating stops mattering and joint design takes over.”
3. What the 7018 rating actually means
Back to the original question. You place a 1-inch weld of 7018. What is it rated at?
The ‘70’ is the minimum tensile strength of the deposited weld metal — 70 ksi, 70,000 psi. That is a stress rating measured under controlled test conditions. It is a property of the weld metal, not a load capacity for a specific joint in service.
| What determines weld capacity | Why it matters |
|---|---|
| Weld size & throat dimension | Directly sets the cross-sectional area resisting load |
| Joint type & geometry | Determines how load is transferred through the connection |
| Base metal classification | Joint is only as strong as its weakest component |
| Load direction & type | Tension, shear, torsion, and fatigue all behave differently |
| Weld quality | Discontinuities and defects reduce actual capacity below the rating |
4. Questions we hear on this
Does a bigger weld always mean a stronger connection?
Not necessarily. A larger weld on a poorly designed joint can still fail if the loading works against the geometry or if fatigue is the failure mode. Weld size matters, but joint design and service conditions matter more.
What does the 70 in 7018 actually mean?
It is the minimum tensile strength of the deposited weld metal — 70 ksi or 70,000 psi. That is a stress rating measured under controlled test conditions, not a load capacity for a specific bead or joint in service.
What is the difference between yield strength and tensile strength?
Yield strength is the point where permanent deformation begins. Tensile strength is the point where the material fractures. A material can go past yield and still carry load, but it will not return to its original shape. In structural work, yield strength is usually the more critical limit.
Does base metal strength affect weld joint capacity?
Yes. The joint is only as strong as its weakest component. If the base metal has lower tensile or yield strength than the weld metal, failure will typically occur in the base metal or heat-affected zone, not the weld deposit. Electrode selection should match or overmatch the base metal.
5. TLDR — Key Facts
The E7018 electrode has a minimum tensile strength classification of 70 ksi (70,000 psi) for the deposited weld metal. This is a stress rating measured under controlled laboratory test conditions. It is not a load capacity in pounds for a specific weld bead or joint configuration in service.
Tensile strength is the maximum stress a material can withstand under direct tension before fracture. Yield strength is the stress at which permanent deformation begins. These are two separate points on the stress-strain curve. A material can exceed its yield strength and continue to carry load, but it will not return to its original dimensions.
Plastic deformation is permanent change in a material that occurs when stress exceeds the yield strength. A36 structural steel has a minimum yield strength of 36,000 psi. Once A36 is loaded past that point, the deformation is permanent.
Elasticity is a material’s ability to recover its original shape after loading within the elastic range. Ductility is a material’s ability to deform plastically after yielding before fracture. These are distinct properties. A material can have high tensile strength and low ductility at the same time.
Hardness is resistance to indentation or penetration, measured by methods including Rockwell and Brinell. Hardness is not equivalent to toughness. A hard material may also be brittle and fail suddenly under impact or cyclic loading.
Toughness is a material’s ability to absorb energy before fracturing. It reflects a combination of strength and ductility. High tensile strength with low ductility typically produces low toughness. Toughness is a critical property in cold temperature service, impact loading, and cyclic applications.
Fatigue is progressive damage caused by repeated loading over time. Small cracks initiate and grow under cyclic stress until fracture occurs. Some steels exhibit a stress threshold below which fatigue damage does not appear to accumulate, but this behavior is not universal and is significantly affected by weld geometry, residual stress, and discontinuities. Welds are common fatigue crack initiation sites due to geometry changes at the weld toe and residual stresses from the welding process.
Welded joints in service are typically subjected to combinations of tension, shear, and torsion simultaneously, not tension alone. Actual weld load capacity depends on weld size and throat dimension, joint type and geometry, base metal classification, load direction and type, and weld quality. The electrode tensile classification describes the weld metal under test conditions. Joint design determines what the weld delivers in service.
Questions on this or topics you want covered next — drop them in the comments or reach out directly. More coming.
DJR Welding LLC
Mobile Welding & Custom Fabrication · Metro Detroit
· Wayne · Oakland · Macomb Counties
(586)569-9311 · djrwelding.com