Fastener Forensics:
How to Determine the Cause of an Accident Based on a Broken Bolt Automatic translate

Metal doesn’t lie. When a joint fails, the bolt fragments preserve a precise record of the events leading up to the disaster. For an experienced engineer or mechanic, the fracture surface is an open book, written in the language of solid-state physics. The ability to read these traces transforms repair from guesswork into an exact science, allowing you to address the root cause, not just the symptom.

Any accident begins long before the final impact. The failure process is often initiated by microscopic changes in the metal structure, which accumulate over hours, days, or even years. When the bolt head breaks off, we see only the final act of a long drama. The main goal of the investigation is to reconstruct the chronology and determine the nature of the loads acting on the component.

Anatomy of plastic fracture

The most obvious and common type of failure is related to simple overload. This occurs when the applied force exceeds the metal’s tensile strength. Engineers call this ductile fracture. The main symptom here is deformation. The metal "flows" before giving in.

Visually, such a fracture resembles a neck. If you’ve ever stretched chewing gum or a piece of play dough until it breaks, you’ve seen this process. At the point of greatest stress, the diameter of the bolt shank begins to decrease, forming a characteristic narrowing called a neck. The fracture surface itself appears dull, fibrous, and uneven.

Suppliers and managers see fasteners as just a line item on a cost sheet. In the rush to complete projects, they might search for queries like "M30 nut price in Onyx ," focusing on logistics and budget. However, a mechanic holding a broken stud understands that the quality of the steel, the strength grade, and the proper installation procedure mean more than the numbers on the invoice. A poor choice or a tightening error always leaves its mark on the broken part.

The classic ductile fracture pattern is called a "cup and cone." One part of the broken bolt has a depression in the center with raised edges (the cup), while the other has a corresponding protrusion (the cone). The edges are often beveled at an angle of approximately 45 degrees to the load axis. This is a clear sign that the fastener was made of high-quality ductile steel, but the load on it was excessive.

The culprit of ductile fracture is almost always found in the workshop. It’s either the use of an extension pipe on a wrench, or a design error by the designer, who selected a fastener that was too weak for a particular component. The bolt resisted faithfully until the very end, stretching and warning of a problem until it exhausted its ductility.

Brittle Fracture: Sudden Death

The complete opposite of ductile fracture is brittle fracture. There are no warning signs. The bolt does not stretch, and the neck diameter does not decrease. The rupture occurs instantly, often accompanied by a loud, gunshot-like sound. This is the most dangerous scenario, as it occurs without any visible warning.

The surface of a brittle fracture looks different. It’s flat, perpendicular to the bolt axis, and has a granular, crystalline structure. The metal shines, reflecting light with its many fine facets, reminiscent of a chip of refined sugar. Sometimes, "chevron patterns" — V-shaped marks whose sharp ends point to the site of the crack’s origin — are visible on the surface.

The causes of brittleness lie in the structure of the material or operating conditions. High-strength bolts (grades 10.9, 12.9) are more prone to this behavior than mild steel. A common cause is improper heat treatment, where the metal is overheated and hardened to excess.

Low temperatures also contribute to brittleness. Ordinary steel loses its impact strength in extreme cold. If equipment operates in the north, the impact load on a frozen bolt will cause glass-like fragmentation. If you see a grainy, shiny cut with no signs of tapering, look for a problem with the quality of the metal or the temperature, not with overtightening.

Fatigue failure: beach lines

Metal fatigue is the most insidious enemy of mechanics. It is responsible for the vast majority of accidents in mechanical engineering. The peculiarity of fatigue is that it destroys a bolt under loads significantly below its ultimate strength. Failure occurs not from the force of impact, but from cyclical failure.

Imagine a wire that you bend and straighten repeatedly. At some point, it breaks. The same thing happens with a bolt, but on a microscopic scale. The process always begins with a tiny defect on the surface: a scratch, a corrosion pit, or a thread gullet. This is a stress concentrator.

With each load cycle (engine vibration, wheel rotation), the crack advances by microns. This process leaves remarkably distinct marks on the fracture surface, known as "beach marks." They resemble tree rings or wave marks in the sand. These arcs radiate from the crack’s origin.

The fatigue growth zone is usually smooth, almost polished, because the crack edges constantly rub against each other during operation. But when the bolt’s cross-section becomes too small to support the load, final full fracture occurs. This full fracture zone looks like a typical brittle or ductile fracture — rough and rough.

The presence of beach lines is a death sentence for the quality of the installation. Bolt fatigue only occurs when the connection "breathes." If the bolt is tightened to the correct preload, it doesn’t feel variable external loads. The appearance of a fatigue crack indicates that the tightening was insufficient, and the components began to move relative to each other. The fault lies not with the metal, but with someone who didn’t use a torque wrench.

Hydrogen embrittlement: the invisible saboteur

There’s a type of failure that baffles even experienced mechanics. A bolt is tightened according to the instructions, the load is within specifications, but a day later, the head simply falls off while the mechanism is sitting in the garage. This phenomenon is called delayed failure, and its cause is hydrogen.

Atomic hydrogen penetrates the steel’s crystal lattice during acid etching or galvanic zinc plating. If the manufacturer violates the process and fails to perform dehydrogenation (baking the parts in an oven), the gas remains within the metal. Under mechanical stress, hydrogen atoms migrate to areas of stress concentration, creating colossal internal pressure.

A fracture caused by hydrogen embrittlement is always intergranular. Under a microscope, it’s clear that the crack extends not through the grains of the metal, but along their boundaries, as if unraveling the structure. Visually, this appears as a dirty, gray, rough fracture with no traces of plastic deformation.

This is the bane of high-strength fasteners. Regular construction bolts of class 5.8 or 8.8 rarely suffer from this, as their structure allows hydrogen to escape or does not create critical stresses. But for class 10.9 and 12.9 products, the presence of hydrogen is fatal. If you see a shiny galvanized bolt crack on its own while at rest, it’s almost certainly hydrogen embrittlement.

Thread stripping: when the nut is stronger than the bolt

Sometimes the bolt shank remains intact, but the connection fails because the threads completely disappear. Thread stripping occurs when the bolt and nut materials are improperly matched in hardness, or the tightening length is insufficient.

In an ideal connection, the bolt should break before the threads strip. A broken bolt is immediately obvious, while stripped threads may go unnoticed, creating the illusion of reliability. If you notice that the threads look like sheared shavings, or the threads are clogged with metal, then the threads have slipped.

Typically, threads are sheared off at the base, at the root. If the nut threads are sheared off, it means the nut was too soft. If the bolt is "bald," the problem is in the bolt. Standard practice requires the nut to be slightly softer than the bolt, but the height of a standard nut (approximately 0.8 times the thread diameter) should be sufficient to support the tensile load of the rod.

Thread shear often indicates overtightening with a small thread engagement. If the bolt has only entered the nut a couple of turns, the thread contact area will be insufficient. The metal simply won’t withstand the shear pressure and will flow, forming a cylinder.

Knee-based diagnostics

A laboratory with an electron microscope is not required for a preliminary analysis. Good lighting and a 10x magnifying glass are sufficient. The first thing to do is avoid trying to force the bolt halves together. This destroys the microscopic details on the fracture surface, crushing the peaks and valleys.

Inspect the crack’s origin. With fatigue, it’s often located in the root of the thread or under the bolt head — where the geometry changes abruptly. If you see radial lines converging at a single point, that point is the origin. The presence of rust on the fracture surface indicates that the crack was there a long time ago, and moisture has had time to do its work. A fresh fuller will have a shiny, clean metal sheen.

Check the geometry. Place a ruler against the remaining rod. Is there any distortion? A bent bolt indicates that the connection was subjected to a shear load, which bolts are not designed to withstand. A bolted connection operates due to the friction created by preload. If the bolt begins to act as a shear pin, the friction was insufficient — again, the issue is related to the tightening torque.

The role of stress corrosion

Another failure scenario involves a corrosive environment. Stress corrosion cracking (SCC) is the silent killer of stainless steels. A component may appear almost new on the outside, but branching cracks develop internally.

This type of fracture has a very characteristic structure. It is often multiple, with numerous secondary cracks extending deep into the material. The fracture surface may be covered with corrosion products, but not in a continuous layer, but in patches.

This occurs when three factors combine to create a fatal problem: tensile stress, a susceptible material (such as austenitic 304 or 316 stainless steel), and a specific environment (usually chlorides, even in the form of sea fog or road salt). If a stainless steel bolt cracks without visible deformation in a humid environment, suspect SCC. The solution here is not to increase the bolt diameter, but to change the material to duplex steels or special alloys.

Practical conclusions

Fastener fracture analysis is no longer the secret domain of metallurgists. It’s a mechanic’s go-to tool, preventing repeated mistakes.

• Is there a neck and stretched threads? You’ve got too long a lever. Buy a torque wrench.
• Is the fracture smooth, grainy, and shiny? The bolt was defective (overheated) or was frozen. Change the supplier or strength grade.
• Are there visible arcs (beach lines)? The tightening is weak. The unit was vibrating. Use thread locker or check the tightening torque.
• Did a bolt crack on its own on a shelf? Hydrogen embrittlement. Don’t galvanize high-strength fasteners in a garage.

Every broken bolt is evidence of unreliability. Ignoring this evidence and simply replacing the broken part with a new one is planning for the next accident. A careful look at the broken end of the bolt will save thousands of dollars and hundreds of hours of downtime, transforming the chaos of emergency repairs into predictable equipment operation.