Main Takeaways
Screw breakage falls into five major root‑cause categories: static overload, fatigue fracture, material‑manufacturing defects, environmental‑induced brittle fracture, and improper installation‑assembly. Most fractures initiate at stress‑concentration zones such as thread roots or head‑to‑shank fillets. Correct diagnosis needs fracture‑surface observation, hardness testing and metallographic analysis rather than only visual inspection.
| Failure Mode | Primary Causes | Typical Features |
|---|---|---|
| Static Overload Fracture | Excessive tightening torque; insufficient‑grade specification; superimposed pre‑load plus service load | 8.8/10.9 shows necking; 12.9‑grade often has almost no necking |
| Fatigue Fracture | Long‑term alternating / vibration load; insufficient pre‑load leading to joint slip | Crack expands progressively; conchoidal beach marks on fracture face |
| Material‑Manufacturing Defect | Unqualified heat‑treatment; raw‑material inclusions; thread‑processing burrs; sharp fillet | Cracks originate from internal or surface defects |
| Environment‑related brittle fracture | Hydrogen embrittlement (high‑strength bolts after plating); stress‑corrosion cracking under chloride / corrosive medium | Brittle fracture without obvious plastic deformation, delayed breakage |
| Improper installation | Eccentric / bending load; mis‑alignment; thread galling; misuse of impact‑type tools | Additional bending stress accelerates crack growth |
Detailed explanation
- Overload fracture: Produced when actual axial/torsional load exceeds material bearing capacity. It may happen during over‑torquing assembly; or pre‑load already approaches yield, and superposed working load pushes total stress beyond strength limit. Improper specification selection in design also triggers overload. Note: necking on fracture face is not absolute proof of overload; poor heat‑treatment may also produce necking.
- Fatigue fracture: Occurs under cyclic vibration even below static strength limit. If pre‑load decays and the connected members slip, screws bear repeated alternating shear‑tension stress; cracks start at thread‑root stress‑concentration points and gradually expand until final snap. This accounts for a large proportion of field screw breakage.
- Material and production issues: Improper quenching‑and‑tempering, unqualified core hardness, material inclusions, burrs and sharp transitions at thread‑root / head fillet create stress‑raising origins for crack initiation.
- Environmental‑brittle failure: High‑strength fasteners are susceptible to hydrogen embrittlement after pickling / electroplating without dehydrogenation treatment, leading to delayed brittle fracture. In coastal / chemical sites, combined tensile‑stress plus corrosive medium causes stress‑corrosion cracking.
- Installation‑related factors: Eccentric clamping, forced mis‑alignment introduces bending moment onto the screw; reckless use of impact wrenches brings uncontrolled torque; thread galling damages thread surfaces, all accelerating breakage.
Engineering diagnosis & prevention tips
- Do not judge failure cause merely by appearance. Carry out hardness testing and metallographic analysis for critical broken samples.
- Match strength‑grade and dimension according to calculated load; control tightening torque with torque‑wrench / hydraulic tensioner.
- Eliminate eccentric‑bending stress during assembly; guarantee adequate pre‑load for vibration‑prone joints and deploy valid anti‑loosening measures.
- For plated high‑strength bolts, perform dehydrogenation baking; select corrosion‑resistant material or proper surface treatment for corrosive working conditions.
- Reject fasteners with visible burrs, cracks and defective thread profiles.
The main reasons for screw breakage are as follows:
01 Quality issues
The quality problems of screws include the quality of raw materials and the production process. If the raw materials are of poor quality, containing a lot of impurities and being impure, it will cause the screws to be insufficiently hard. Problems in the production process, such as improper heat treatment, may also lead to the breakage of screws.
02 Pre-tightening torque
If the pre-tightening torque of the screw is too large, it may cause the screw to break.
03 Strength and fatigue strength
The strength and fatigue strength of screws are also important factors affecting their fracture. If the strength and fatigue strength of screws are insufficient, they may break during use.
04 Corrosion
Physical or chemical reactions between screws and the service environment may lead to failure and fracture. 05
05 Loosening
Screws gradually loosen under conditions such as impact, vibration, variable loads, and large temperature changes, generating kinetic energy. For bolts subjected to axial force, the threads are damaged and the bolts are pulled apart. For fasteners subjected to radial force, the bolts are sheared off and the bolt holes are deformed into ellipses. 06
06 Usage issue
If a user applies too much force when using a screw, it may also cause the screw to break.
In fact, the vast majority of bolt breakages are due to loosening. They are broken because of loosening. The situation of bolt breakage due to loosening is largely the same as that of fatigue breakage.
We can always find the cause from the fatigue strength. In fact, the fatigue strength is so high that we cannot imagine it. The fatigue strength is never utilized in the process of bolt usage.
Take the M20×80 8.8 grade high-strength bolt as an example. It weighs only 0.2 kilograms, yet its minimum tensile load is 20 tons, which is a hundred thousand times its own weight. Generally, we only use it to fasten components weighing 20 kilograms, which is only one-thousandth of its maximum capacity.
Even if other forces act on the equipment, they cannot exceed a thousand times the weight of the component. Therefore, the tensile strength of the threaded fastener is sufficient, and it is impossible for the bolt to be damaged due to insufficient strength.
In the transverse vibration loosening test, the threaded fastener can become loose after just a hundred cycles, while in the fatigue strength test, it needs to be vibrated a million times. In other words, the threaded fastener becomes loose when only one ten-thousandth of its fatigue strength is utilized.
We have only used one ten-thousandth of its maximum capacity. Therefore, the loosening of the threaded fastener is not due to the fatigue strength of the bolt.
When the threaded fasteners become loose, they generate huge kinetic energy. This huge kinetic energy directly acts on the fasteners and the equipment, causing damage to the fasteners. Once the fasteners are damaged, the equipment cannot operate in a normal state, further leading to equipment damage.
Fasteners subjected to axial force have their threads damaged and the bolts are pulled apart. When fasteners are subjected to radial force, the bolts are sheared off and the bolt holes are deformed into ellipses.
When a bolt breaks, the most common conclusion people draw is that its strength is insufficient. Therefore, they usually adopt the method of increasing the bolt’s diameter and strength grade. This approach can enhance the bolt’s preload, and its frictional force is also increased.
The carbon steel screws break off due to hydrogen embrittlement. When hydrogen molecules penetrate into the carbon steel screws, even when the pressure applied is far below the tensile strength that the carbon steel material can withstand, the screws will break off.
Most hydrogen embrittlement in carbon steel screws is internal hydrogen embrittlement. Generally, atomic hydrogen may penetrate into carbon steel screws during pickling or electroplating, and if hydrogen remains in the screws, it will accumulate at the place with the highest pressure and form small voids. When the pressure increases, it will cause the sudden fracture of the screw head.
Environmental hydrogen embrittlement is one of the reasons for the breakage of carbon steel screws. It is a type of hydrogen embrittlement that occurs when hydrogen penetrates the screws under pressure in the surrounding environment.
Carbon steel screws with surface corrosion also increase the chance of hydrogen penetrating the screws. This is the least obvious type of hydrogen embrittlement and is also the most easily overlooked and difficult to solve.
If you are looking for quality screws, please visit us here https://hktl-fastener.com/screws/.
