What Causes The Failure Of Bolted Joints?

The failure of bolted joints can be a significant concern for engineers. They frequently assess whether a joint can withstand the pressure applied to it or not. However, it is vital to note that the safety and strength of bolted connections largely depend on the functioning of the joint.

Although the capacity of the joint connections can satisfactorily be determined by experts in lab-based analysis, common failure modes can be recognized by visual examinations with little training.

Some common causes of bolted joint failures can result from various mechanical issues, such as:

1. Inappropriate Joint Clamp Force
2. Bolt Fatigue Failure
3. Bolt Overload
4. Excessive Bearing Stress
5. Material Brittleness

Let's take a closer look at all of the causes of bolted joint failures.

Inappropriate Joint Clamp Force

The functionality and longevity of bolted joint connections at full capacity primarily depend on the clamping force generated by the tensioning or torquing of bolts and the stress they are meant to handle. The primary forms of stress of bolted joints result from:

• Internal stress occurs when the joints are not kept in their right place.
• External stress (also called service load) of the jointed materials
• Static stress (tensile load) pulls on both ends of the bolted joint. Excessive static stress on the bolts can cause bolt breakage. To prevent such failures and safeguard your project, ensure the bolts are not overtightened or overturned beyond the recommended levels. The tensile force should never exceed the bolt's tensile strength.

What if a gasket is included in a bolted joint to prevent leakage? In this case, you have to apply sufficient clamp force to allow effective sealing. The extra force applied to the bolt is transmitted by friction to the joint, leading to a secure connection.

Bolt Fatigue Failure

The efficiency of bolted connections tends to diminish over time. Due to the high-stress gradient, bolt fatigue failures mostly start from the threaded area, particularly the point where the bolt engages in the internal thread.

Fatigue failures are very hazardous because they mainly occur without visible warning signs. The failure of bolted joints can be decreased by using gaskets or replacing the bolts after a couple of years, especially if your project involves regular high-stress levels.

Bolt Overload

"Bolt overload" can occur from:

• Over-torquing during installation
• Overloading the joint during service leads to shear or tensile failure.
• Using severely understrength bolts

With a bit of training or with the help of professionals, you can do visual examinations regularly to determine bolt qualities such as strength, hardness, steel type, etc., to avoid related mechanical problems and other losses.

Excessive Bearing Stress

Bearing is one of the ways bolts transfer force to materials in a joined connection. This method assumes that a comprehensive force develops in contact between the sides of the hole and the bolt used. The magnitude of this force is limited to the strength of the materials used.

Typically, bolts are made from highly strong steel that, when fully tightened, can exert a very high clamp force on the bolted joint. The region directly below the nut face or the bolt head sustains the bearing stress produced. If the stress exceeds the maximum limit, deformation starts occurring.

Material Brittleness

Poor service conditions and improper heat treatments can compromise steel quality, leaving bolts and screws susceptible to failure even under slight bending or strain. This failure may occur during installation or later after continuous degradation.

Brittle fractures mostly form recognizable fractures. They can be controlled by doing lab-based analysis to confirm the degree of potential brittle fractures, fatigue, and ductile failures.

Decrease Your Chances of Bolt Failure

Fasteners frequently fail for different reasons, requiring independent solutions to avoid repeated occurrences. Whenever you experience bolt and bolted joint failures, you should not guess the cause because you may do it wrong, especially if you don't have any training.

It is prudent to let a reliable professional or engineers identify the root causes and advice on the efforts to be applied to prevent similar future failures. The professional should also help you choose top-quality fasteners and other joint connection tools, such as hydraulic bolt tensioners and hydraulic torque wrenches.

At Big Bolt, we have a team of qualified professionals with profound experience in ensuring the effectiveness of bolted joint connections. We offer different types of fasteners and customizable services such as CNC machining, forging, and threading, among others. Contact us today.