Bolt Strength & Failure. How Strong Are Nuts and Bolts?

Fasteners are often taken for granted; however, they are more than a piece of metal, and their working principle is fascinating and more complex than it seems.

In this article, I want to share some basic concepts that will help you better understand how bolts, nuts, and screws work, why they break, how to read their marks and other tips.
Bolt Strength explained

Bolts and Screws Basics: How Fasteners Work?

When you tighten a bolt or a screw, the torque (rotational force) you apply is converted into clamping force, making it hold the pieces tightly. Fasteners are designed to stretch when tightened, creating clamping force.

Besides torque, other factors like size and thread pitch help fasteners hold the parts together.

Let’s quickly go over two types of deformation that make the difference between a healthy and a broken fastener.

  • Elastic Deformation: it’s when an object returns to its original shape after being subjected to an external force. Imagine a ping pong ball; it gets deformed as you hit it or as it bounces but recovers its shape almost immediately.
  • Plastic Deformation: it’s when an object doesn’t return to its original shape after being subjected to an external force. Imagine a car runs over the same ping pong ball of the previous example and crushes it; it will stay permanently crushed.

Fasteners work thanks to elastic deformation.

The Spring Analogy

Fasteners are similar to springs. When a spring is in its natural or resting form (no forces are acting on it), it has a certain length and keeps that way indefinitely.

If you stretch spring, it will elongate, (as fasteners do when you tighten them), generating potential energy, also called load. When you release the load, the springs tend to go back to their original state and shape by nature. In a fastener, that load and tendency to recover the original shape are what generate clamping force.

As you stretch the spring more and more, it will store more potential energy, and a fastener will have a bigger clamping force. Every time you stretch a spring, and it goes back to its original shape, it undergoes elastic deformation, as happens when you tighten a fastener properly.

If you keep stretching the spring further, at some point it won’t be able to return to its original shape, and it will stay deformed. This state is called plastic deformation. In the case of a screw or bolt, it will get permanently damaged or break at the weakest point.

Phases of Tightening

Suppose you have two pieces held together by a bolt; the set of components formed by the two pieces and the bolt is called a bolted joint.

The fastening process has four stages, and I will use the two pieces and the bolt as an example.

  • Stage 1 – Rundown: There is no contact between the bolt and the two pieces about to be clamped.
  • Stage 2 – Draw-Down: The joint is seated, and there is contact along the components being clamped.
  • Stage 3 – Elastic: Torque is applied to the fastener, which is converted into clamp force, and holds the parts together. As torque is applied, the bolt suffers an elastic deformation.
  • Stage 4 – Plastic: This phase is the one we must avoid. Over-torquing or excessive stress to the bolted joint deforms, damages, or breaks the fastener, causing a plastic (irreversible) deformation.

Fastener Strength: Key Terms and Concepts

How strong is a bolt?
Below I will show you the most important factors that define the maximum force a fastener can withstand before failing. These terms are also helpful for reading nuts and bolt marks and charts.

Tensile Load: Using the spring analogy, when you tighten a bolt, it’s as if you were pulling the two ends of a spring. The load springs or fasteners endure when they are stretched or tightened, respectively. It’s called tensile load.

Tensile Strength: The tension (tensile load) a bolt can withstand without suffering permanent deformation is called tensile strength.

Ultimate Tensile Strength: It’s the maximum tensile load a fastener can withstand before breaking or snapping.

Yield Point: It’s the point at which a fastener’s material begins to deform permanently and won’t return to its original shape.

Yield Strength: It’s the maximum torque a fastener can withstand before suffering plastic deformation (damage).

Depending on the application, it’s recommended to torque fasteners up to 70 to 90 percent of the yield point, ensuring they only suffer an elastic deformation and recover their original shape. If you apply force (torque) beyond the yield point, the fastener will suffer a plastic deformation, become unable to recover its original shape, and will be permanently damaged. If you keep applying torque, you will end up breaking or snapping it.
The graph showing the effect of stress on bolts caused by torque
Shear Strength: It’s the maximum lateral stress a fastener can withstand before being damaged or broken. Imagine a suspension bushing’s bolt; as the control arm moves, the bolt endures several forces; the forces perpendicular to it would be the shear forces.

Material Ductility: It’s the material’s ability to deform before breaking. A bolt’s material ductility is highly related to its tensile strength.

Material Strength: A bolt’s material strength is defined by its elasticity or resistance to tension, shear, and bending resistance, which are related to the material and the process used to manufacture it.

Causes for Failure of Bolts and Screws

The most common causes for bolts and screw failure are related to their tensile, shear, and overall material strength.

  • Over-stress: It’s caused by over-torquing or under-torquing. (LINK to the over-torquing article?)
  • Material Fatigue: It can be caused by several factors, such as vibration, excessive shear forces, improper installation (i.e., misalignment), and installing and uninstalling the fastener multiple times. Some applications require replacing the fasteners every time you disassemble them.
  • Corrosion: Rust can damage fasteners, weakening their material and creating pits or cracks on their surface, which can make them lose clamping force and even break.
  • Temperature Fluctuation: Large temperature variations can make the fasteners’ and bolted joints loose due to the sudden material’s shrinking and expansions. Cast iron and other hard metals are less sensitive to this problem.
  • Wrong Grade: A fastener’s grade indicates its material quality, hardness, and strength. The higher the grade, the stronger the fastener. Whether you are replacing a damaged fastener or working on a new project, make sure that you are using the right grade for the intended application to prevent premature tear, wear, and other problems.

FAQ (Frequently Asked Questions)

What is Tensile Fracture?

Tensile fracture is when a bolt endures more torque than it can handle and suffers permanent deformation or snaps.

What is Elongation?

Elongation is the stretching and warping of a fastener caused by excessive torque or pressure.

What Does Torque to Yield (TTY) Mean?

Torque to yield means applying torque to a fastener up to its yielding point (permanent deformation). However, this is a common practice in many applications like cylinder heads, which are designed to be fastened like this. There are special bolts called TTY which are specifically made for this purpose.

What is Breakaway Torque?

Breakaway torque Is the minimum amount of torque needed to start rotating a bolt clockwise or counterclockwise.

What is a Fastener’s Tolerance?

Tolerance is the range of variation in size or dimension among fasteners that is still considered acceptable for a particular application or use.