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.
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.
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.
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.
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.
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 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.
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.