If you’ve ever shopped for bolts, screws, or structural fasteners, you’ve likely encountered this term — often listed alongside grade designations like Grade 2, Grade 5, or Grade 8. For example, aluminum foil has a very low tensile strength and tears very easily, whereas something like a steel cable on a suspension bridge has a high tensile strength, engineered to withstand enormous tension loads without stretching or breaking.
Proof load, yield strength, and tensile strength are frequently confused, and it’s important to understand the distinction.
So what does this term actually mean, and why does it matter when selecting the right fastener for your application?
Tensile Strength Defined
This means the maximum amount of pulling force — or tension — a material can withstand before it permanently deforms or fractures. In fastener applications, it describes the maximum load a bolt, screw, or stud can handle along its length before it fails.
This is typically measured in pounds per square inch (PSI) or kilopounds per square inch (KSI) in the United States, and in megapascals (MPa) in metric standards. When a fastener’s strength is listed at 120,000 PSI, for example, that means the fastener can withstand up to 120,000 pounds of pulling force per square inch of cross-sectional area before breaking.
Tensile Strength vs. Proof Load vs. Yield Strength
Tensile strength is one of three related but distinct strength measurements you’ll encounter with fasteners:
- Proof load — The maximum load a fastener can sustain without any permanent deformation. This is the practical working limit for a fastener in most applications. Exceeding the proof load means the fastener has been stretched and will not return to its original dimensions.
- Yield strength — The point at which a fastener begins to permanently deform under load, even if it hasn’t yet broken. Once a fastener is loaded past its yield strength, it is compromised and should be replaced.
- Tensile strength — The absolute maximum load before fracture. A fastener loaded to its tensile strength will break.
In practice, properly engineered assemblies are designed so that fasteners never approach their strength capacity in normal use. Proof load is the more operationally relevant limit for most applications.
Why This Matters for Fastener Selection
This quality is the primary basis for fastener grade classifications. Higher grades mean more strength — and a meaningfully different performance profile:
- Grade 2 — Minimum tensile strength of 60,000–74,000 PSI. Suitable for light-duty, non-critical applications.
- Grade 5 — Minimum tensile strength of 120,000 PSI. The standard for automotive and general structural applications.
- Grade 8 — Minimum tensile strength of 150,000 PSI. Required for heavy machinery, high-stress structural assemblies, and demanding industrial applications.
Selecting a fastener with insufficient strength for the load it will carry is one of the most common — and most preventable — causes of joint failure. Conversely, over-specifying fastener strength adds cost without benefit in low-load applications.
Choosing the Right Strength for Your Application
When selecting fasteners, tensile strength should be matched to the joint’s actual load requirements, with an appropriate safety factor applied. For engineered or safety-critical connections, always consult the project specifications or a structural engineer to confirm the correct fastener grade.
At Tri City Bolt & Screw, we stock hex cap screws and structural fasteners across the full range of grade specifications — from Grade 2 for light-duty applications to Grade 8 for maximum-strength requirements. Our team can help you identify the right fastener grade and specification for your project.
Browse our full fastener selection or contact us today to request a quote.
