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What Causes Aerospace Fasteners To Fail?
Ever wonder what causes bolts to fail? Or do you have experience with gaskets that no longer have the clamping force required to hold them? Failed fasteners are an irritating nuisance in any application, but a disturbing potential loss in the case of aircraft.
For such simple components, fasteners like bolts require great attention to detail, including conducting a Root Cause Analysis (RCA) on the failed part. Although lab-based analysis is the most reliable, adding a visual examination helps determine the primary cause of failure. With the increasing demand for fasteners in the aerospace industry, design and aeronautical engineers must understand the leading reasons for their failures to determine the material suitable for the manufacture of fasteners and the force to be applied while securing them.
This article addresses the leading causes of failure in aerospace fasteners, which may occur either in isolation or concurrently. Let's get right into it!
5 Reasons Why Aerospace Fasteners Fail
- Gasket Creep
- Fatigue Failure
- Hydrogen Embrittlement
- Thermal Expansion
All fasteners have a maximum load they can withstand before starting to yield. It mainly draws from the torque of the aerospace fastener: where there is lower friction, the rivet may end up flexing before the required torque is reached. This flex causes a necking down in the clamping zones that are yet to mate with the threading areas. When a bolt or any other fastener stretches beyond its holding capacity, you will take more rotations with your wrench until, finally, the bolt yields and breaks, leaving loose the part it held.
2. Gasket Creep
Gasket creep results from the decrease in thickness of the gasket that has been under constant load. The gasket acts as a spring that pushes back against the force exerted by the joint face and the bolt. Once this springiness is lost, clamping strength decreases. When this occurs, the force wielded by the bolt is released. Gasket creep can also arise when the fasteners are not secured evenly on the joint face.
3. Fatigue Failure
Fatigue failure occurs when tightening of fasteners is loosely done or in cases where the pins have long been in service and have come loose during their years of service. The force acting on the loose joint follows bending stress that eventually causes the fastener to weaken and fail. A low-grade fastener may also cause fatigue failure. In cases of suspected fatigue failure, it’s essential to examine any fractures to rule out other potential reasons for failure.
4. Hydrogen Embrittlement
Embrittlement can happen after assembly if hydrogen introduced into the plating is not fully baked out. Damage is only evident days or weeks after being placed under tension when the fastener cracks and fails. Hydrogen can also be introduced due to corrosion. Some high-strength fasteners, such as zinc-plated bolts with a hardness valued at over 39 Hardness Rockwell C, are also victims of hydrogen embrittlement. Diagnosing hydrogen embrittlement can be done simply by examining the fracture surface.
5. Thermal Expansion
Rapid environmental changes may cause aerospace fasteners to expand and cool exponentially, leading to loosening clasps. This differential thermal expansion is mainly attributed to different materials between fasteners and joint faces. Once exposed to thermal expansion and contraction, the fasteners loosen, yield, and fail.
Big Bolt Aerospace Fasteners
Aerospace fasteners can fail for various reasons, but an essential factor when considering how to best mitigate failure is the quality of the fastener. However, do not trouble yourself seeking quality when working with Big Bolt. At Big Bolt, we take pride in providing quick turnarounds and excellent quality for our clients.
When it comes to quality aerospace fasteners, Big Bolt is one of the largest manufacturers of, custom, specialty fasteners in the United States. Here at Big Bolt, quality speaks, and quality is a must as a push distributor or a design engineer for aerospace applications.