Need car parts? Select your vehicle
Reading Time: 4 minutes
Summary
  • Fasteners are responsible for connecting and securing parts of a vehicle. Despite being made from durable metals, fasteners can still fail for various reasons.
  • Fasteners can fail because of tensile overload, torsional wear, corrosion, hydrogen embrittlement, and cyclic fatigue.
  • If you want to avoid fastener failures, you have to understand a fastener’s weaknesses so that you can implement correct design and installation practices.

Studs, rivets, bolts, screws, clamps, and pins are examples of fasteners. Without them, many components won’t be able to be connected securely. Despite being made from durable metals, fasteners can still fail for various reasons. In this article, we’ll go through the reasons why fasteners fail.

Mechanical Failure or Tensile Overload

Tensile strength is a measure of the stress a bolt can withstand while being stretched before it is pulled in two. When a fastener such as a bolt has too much load or if it was overtightened, then the bolt can stretch. This is the simplest form of fastener failure, as the bolt or screw fails simply because it’s bearing too much weight.

It bears mention that on most engines built since the ‘80s, it has become increasingly common for cylinder head bolts to be designed to stretch for maximum clamping force. These head bolts are torqued to a given foot pound value and then torqued another 90 degrees to provide the required stretch. But they’re only supposed to be used once and replaced with new bolts once removed.

It bears mention that on most engines built since the ‘80s, it has become increasingly common for cylinder head bolts to be designed to stretch for maximum clamping force.

– Richard McCuistian, ASE Certified Master Automobile Technician

Tensile overload tends to occur between a bolt’s shank and threads. The shank is the portion below the head that doesn’t have threading. The telltale signs of tensile overload are cup and cone formations around the break point.

Avoiding mechanical failures because of tensile overload underscores the importance of correct design, as having undersized or misaligned bolts is typically the result of poor planning.

Torsional Wear

The shear failure threshold of a bolt is around half its tensile strength. This means that if it takes 100 pounds of force to pull a bolt apart, then it’ll take around half the amount of force to twist it until failure.

, Common Reasons Why Vehicle Fasteners Fail

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Bolts are all about clamping force. Lubricating the threads and the underside of the bolt head will tremendously increase clamping force even if the bolt is torqued the same.

Fasteners aren’t usually subjected to torsional stress or stress caused by twisting. However, torsional stress can occur in a vehicle driveshaft, input shaft, and output shaft. Misalignment between male and female threads or using the wrong lubricant can also lead to this type of failure.

, Common Reasons Why Vehicle Fasteners Fail

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Over-torquing with an impact wrench or a cheater pipe tends to twist bolts to the breaking point, but bolts have different grades, and a lower grade bolt will twist off easier than a higher grade bolt.

illustration showing the difference between imperial bolts and metric bolts
 Illustration showing the difference between imperial bolts and metric bolts | Image Source: Richard McCuistian

Embrittled Metals Because of Leftover Hydrogen

This is a type of failure that can happen to high-stretch steel alloys such as L-19®, H-11, 300M, and Aeromet. The weakened metal can fail within an hour of torquing the bolt. Fasteners will fail early because there’s an existing issue that stems from the electroplating process during production. As water is made up of hydrogen and oxygen, the electricity involved in electroplating can free hydrogen molecules and permeate the steel, which can weaken the bolt.

To prevent hydrogen embrittlement, the hydrogen molecules must be burnt off after electroplating by heating the metal to a certain temperature. The US passed the Fastener Act of 1990, which established manufacturing practices and forced traceability of fasteners to protect the public.

Stress Corrosion

When steel alloys are exposed to moist, corrosive environments, they can develop rust, which can cause cracks. High-strength steel alloys like L-19, H-11, 300M, and Aeromet are prone to stress corrosion, and they must be kept well-oiled and protected from moisture and electrolytes like salt.

This type of failure occurs similarly to hydrogen embrittlement because the fastener head can break off. Unlike hydrogen embrittlement, stress corrosion can take place in the first 24 hours to 2 months after fasteners are torqued.

If you want to avoid stress corrosion and hydrogen embrittlement, you should carefully consider the use of high-strength metals in fasteners.

Cyclic Fatigue

Cyclic fatigue occurs when a load is constantly applied, removed, and then reapplied on a fastener. While fasteners are built to withstand a set load, they can be weakened if smaller loads are constantly applied, removed, and reapplied. A case where this can occur is if the fastener is improperly installed, as vibration or play can cause cyclic fatigue.

If you want to avoid fastener failure, you have to understand a fastener’s weaknesses. So far, their weaknesses include excessive load, cyclic fatigue, corrosion, and torsional stress. Properly preloading bolts are also critical in preventing failures. Engineers and mechanics should always acknowledge the possibility of bolt failure so that they can design structures and parts carefully.

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

File Under : Maintenance , DIY Tagged With : , , , ,
headlights and components
Subscribe
Notify of
guest
0 Comments
Inline Feedbacks
View all comments

View all Questions & Answers

expand_more
CarParts.com Answers BE PART OF OUR COMMUNITY: Share your knowledge & help fellow drivers Join Now