Have you ever broken a bolt? How do you determine why the bolt failed?

Many different things can cause bolts to break. It comes down to many factors that revolve around the material of the bolt or you might think you’re over-torqueing the screw. So what causes bolts to break? Many important reasons are the foundation of screw breakage which are including material quality, material strength, and environmental factors that compromise the materials. For this reason, you’ll always want to ensure you are choosing the right material for the job and also to make sure you use quality parts. Before knowing the reasons why breaking of the bolt, we have to understand to three types of stress on the bolts/screws as follow:-

Preload, or the internal stress keeping the bolted joints intact. The common cause of a bolt to break is related to pre-tension. When a bolt connects two materials securely and snugly, the bolt takes a much smaller amount of pressure when weight is added to it. If weight is added to the structure before the bolt is tightened, that placed the majority (if not all of the pressure) on the bolt which causes failure in many cases. So, to make sure your bolts are tightened down before extra weight is added to an assembly.

·      Service load, which is the external force of the joint material to exert separate stresses.

·      Tensile load, the most important stress, is static stress that stretches both sides of the bolt separately. If the bolt bears this load in over, it will be the cause of a shift in the load-bearing capabilities and ultimately, eventual cause bolt breakage. To prevent the additional force and breakage, you should refrain from tightening and overturning the bolts often. The tensile force exerted should never exceed the bolt’s tensile strength.

Now, we’re trying to conclude the main cause of breakage as below:-

1. Overloading / Overstressing: The most common reason for bolts failing caused by the pressure exuded on the bolt. When a bolt is working, multiple forces are working on that bolt at all times. Overloading or overstressing in one area which is the most common reason why of failure. All Screws/bolts have a point of maximum limit load that they can bear before they start to yield and this load is applied in the form of torque. It is very important to ensure that your bolts are adequately stressed and you are working with a material that is strong enough to withstand those pressures which help you can rest assured that your bolts’ longevity drastically improves.

Here is just for your information to see the step before the screw’s breaking.

– In case the pressure load is lower than expected, the bolts may yield before reaching the prescribed torque and at this point, the screws/bolts are able to reinstate to its original condition by simply reducing the load.

Stretched-Bolt

– Once the screws/bolts reach the yields point, exactly it will stretch and facing a “necking down” in the threaded area of the clamping, actually when reaching this point will be the permanent deformation and loosen of tightening holding.

– If you let the load continues or continues to increase slightly in this area, it will reach the point of screws/bolts breaking, and once the bolt breaks, you will see an obvious deform in surface area at the break where the bolt has necked down.

2. Fatigue: the main cause of fatigue failure which pertains to shear bolts which do not come with a lifetime value.

Fatigue-Screw

In simple terms, throughout the bolt’s lifetime is faced amount of fatigue that causes their efficiency degrades as the overstressing, fatigue can also be tied to preload and service load.

Then, that there may be doubts concern to. How does fatigue failure occur? Fatigue failure may cause from un-proper tighten the bolt and have loosened up during its lifetime.

Worse case, if the load is added to that point and acting on the loosened joint, it will result in extreme bending stress, which will weaken the clamp. So that it will eventually fail or fastener is broken.

If enough force is acting on the loosened joint during the use of fasteners, bending stresses can weaken the fastener, eventually causing it to break.

Best of all, to ensure that your equipment does not accede to fatigue, you should replace bolts every 1-2 years, especially if your equipment is faced with stress factors such as in stamping machines.

3. Vibration: the large amounts of vibration is the cause of fastener’s breakage as an uneven amount of stress on the bolts such as in the stamping machine or in the shift fork. The vibrations cause either the loosening of bolts or even complete breakage, which can result in an actual loss of control.

During the machine in an operation with high vibration, the fasteners/bolt should lock with washer, nuts, and pin to assure the property of locking bolt will not be degraded. Here are the factors which you have to keep in mind while designing, building, and maintaining the equipment can drastically improve bolt life and decrease the possibility of failures.

Keeping these factors in mind while designing, building, and maintaining your equipment can drastically improve bolt life and decrease the possibility of failures.

4. Corrosion: simply refers to the cause of the bolt’s material work with incompatible substances like citric acid, salty, oxygen, naturally occurring chemicals, or even other metal products.

Corrosion-Screw

Each material has its benefits and deterrents differently. For example, zinc plated steel is cheaper to produce but it unable to corrosion-resistant as stainless steel.

For incompatible metal corrosion or galvanic corrosion is comparatively rare but can happen in the mechanical or construction applications.

Such as in auto part application is more susceptible to chemical corrosion where bolts can be exposed to engine oil leaks or other fluid leaks, they become to the reasons why deteriorate over time.