Why Do High-Strength Bolts Break?

We subconsciously think that the higher the strength of a bolt, the less likely it is to break. However, this is not the case-on the contrary, high-strength bolts break more frequently than ordinary bolts, and there is a core logic behind this phenomenon.

First, we need to clarify a key principle: the higher the strength of a bolt, the higher its hardness (they are positively correlated); while the higher the hardness, the poorer the toughness (they are negatively correlated). This means high-strength bolts have low elongation. If the stress exceeds the limit, they will directly undergo brittle fracture, instead of first deforming significantly like ordinary bolts before failing. More importantly, high-strength bolts are inherently used in high-load scenarios and are designed to match their mechanical property range. If the actual stress exceeds the limit due to improper operation or abnormal working conditions, fracture is likely to occur. For low-load environments, ordinary bolts can be used to control costs, so there is no need for high-strength bolts-which is the core reason why high-strength bolts break more commonly.

The specific causes of high-strength bolt fracture mainly include the following categories:

1. Assembly Overload Fracture

The core of fastening high-strength bolts is to make the bolt tensile by tightening the nut to generate the specified preload (locking force), rather than "rotating and pressing the thread at the tail end of the bolt". Its tightening torque has clear standard parameters, usually controlled at around 75% of the bolt material's yield strength-this torque can make the bolt produce slight elastic deformation, and the reverse tension generated by the deformation is the preload. If the tightening torque exceeds the standard range, the bolt will bear excessive tensile load, directly causing overload fracture.

Controlling the tightening torque requires three conditions: reasonable on-site installation process design, precise installation tools (such as torque wrenches, torque multipliers), and operators who have received formal training before going on duty (they must be able to accurately read and set tool parameters). It should be noted that torque wrenches of different accuracy levels have different tolerances, usually ±4%~±10% (not 20%). Only when conditions such as power supply and air pressure are stable and the tool is within the calibration validity period will the tolerance not cause fracture risks; if the tolerance exceeds the range, improper torque is likely to occur.

2. Fracture Caused by Fluctuations in Friction Coefficient

When the bolt and nut threads engage, the friction coefficient will affect the actual preload-even if the same torque is set, fluctuations in the friction coefficient will cause preload scatter. If the friction coefficient is not fully considered and only torque parameters are relied on, insufficient preload or overload is likely to occur: when the friction coefficient is too large, the preload is too small under the same torque (which may lead to loosening); when the friction coefficient is too small, the preload is too large under the same torque (which may cause fracture).

In industrial scenarios, a common cause of reduced friction coefficient is unauthorized lubrication: some factories apply talcum powder, ordinary lubricating oil, etc. to the bolt threads for convenient assembly. Although this can reduce friction and facilitate screwing in, it will significantly reduce the friction coefficient, resulting in preload far exceeding the standard under the same torque, and ultimately leading to fracture. The correct approach is to use specialized anti-seize compounds (which need to match the bolt material) instead of random lubricating media.

3. Fatigue Fracture

Fatigue fracture is the most hidden failure mode of high-strength bolts-there are no obvious signs before fracture, and it may occur suddenly during static or working conditions. Moreover, the fracture location is mostly concentrated in stress concentration areas such as the transition fillet between the head and the shank, and the root of the thread.

The core cause of this type of fracture is "use beyond the fatigue limit": although high-strength bolts have high added value, some enterprises will reuse them indefinitely to save costs. When the number of uses or the alternating load borne exceeds their fatigue limit, microcracks will gradually form inside the bolt, eventually leading to fatigue fracture. Therefore, it is very necessary to conduct comprehensive regular inspections of high-strength bolts (such as magnetic particle inspection, ultrasonic testing), not "rarely necessary".

4. Fracture Due to Insufficient Tightening

It seems that bolts that are "not fully tightened" will not bear stress, but in fact, fractures can be caused by the clearance generated by loosening. For example: when two drill pipes are connected with high-strength bolts for drilling downward on the ground, if the bolts are not fully tightened, there will be a large clearance. When the high torque of drilling is transmitted through the drill pipes, the clearance will cause the bolts to bear additional shear force and alternating impact force-these forces far exceed the designed bearing range of the bolts, eventually leading to fracture. In essence, an insufficiently tightened bolt will change from a "tension member" to a "shear and impact member", failing because it exceeds its load-bearing type.

5. Fracture Caused by Quality Issues

Substandard materials or heat treatment processes are acquired quality problems and direct causes of fracture:

Substandard materials: Using steel grades that do not meet requirements (such as replacing alloy structural steel with ordinary carbon steel), or the materials have inherent defects such as impurities and cracks;

Substandard heat treatment processes: Deviations in parameters such as quenching temperature and tempering time will result in unqualified mechanical properties of the bolts (such as high hardness but extremely poor toughness).

Such problems can be completely solved by strictly controlling material procurement (verifying material certificates), production processes (monitoring heat treatment processes), and factory inspections (mechanical property testing).