Fastener Failure Analysis: What Causes Automotive Fastener Fatigue Fracture and Hydrogen Embrittlement Under Dynamic Loads?

Quick Answer:

Fasteners used in automobiles may be susceptible to failure due to fatigue, shearing, corrosion-related cracking, or loosening caused by vibrations. Dynamic loading makes these issues much more dangerous since engine vibration, impact from roads, braking, and suspension system vibrations will cause the forces on a joint to change continuously.

Fastener failure analysis must be conducted considering the fracture, joint condition, bolt loading, corrosion, and history of assembly. The position where the crack or break occurred may help to determine which type of problem was responsible for failure. CNRL produces various types of elementi di fissaggio per autoveicoli, including bolts, studs, screws, nuts, among others, for engines, chassis, transmissions, brakes, and safety systems.

Why can fastener failure become costly in automotive applications?

A simple bolt can have a great impact on a big vehicle assembly system. If a crucial fastener loses its clamping ability or breaks, there could be movement of parts beyond their intended limits.

Consequences would include noise production, leakage, misalignment, part damage, or failure of joint functionality. The engine, suspension, steering, transmission, and brake systems are particularly vulnerable since they have fasteners that are subjected to dynamic loading.

Dynamic loading also means failure may develop gradually. A bolt can survive thousands of cycles before a small crack grows enough to cause a final fracture. This is why fastener fatigue failure may occur even when the applied load never appears large enough to break the bolt in one event.

What are the common automotive bolt failure modes?

The main bolt failure modes do not all leave the same evidence. Understanding how each develops helps engineers trace the cause instead of simply replacing the broken part.

Fatigue failure

Fatigue arises when a fastener is exposed to repetitive stress variations. A small crack could start developing at the root of a thread or even on the surface of the fastener.

With every load cycle, the crack could grow until the bolt is reduced in size to such an extent that it can no longer support the load, causing sudden fracture.

The condition will be worsened by inadequate preload, which results in separation or slippage of the joint, resulting in increased cyclic loads in the bolt.

It is therefore vital for the fastener failure analysis to go beyond just examining the failed bolt.

Shear failure

Failure by shear is the result of forces applied transversely to the bolt rather than axially. This may be the case when there is any sliding action between the two connecting pieces.

Bolt shearing causes are overloading the joint, use of bolts of improper size, grade, design, or inadequate clamp load.

Clamp load is especially significant in such cases. In most cases, friction between the clamped surfaces in a joint serves to resist motion. However, in a case where there is a reduction in the clamp load, the members will start slipping until the bolt shaft comes under the stress of shear.

Corrosion-induced failure

Corrosion causes a reduction in the cross-sectional area of the fastener. Additionally, it causes pitting, which leads to stress concentrations. These are particularly important in locations with road salt, moisture, heat, and chemical contact around the areas of vehicles such as underneath vehicles and exhausts.

Another problem that might affect some high-strength steel fasteners is hydrogen embrittlement. Hydrogen can get into susceptible steel through manufacturing and coating processes or even the environment.

In turn, the fastener becomes prone to delayed cracking when subjected to tensile stresses. Initially, the bolt will look alright but will crack after some time.

Thus, the choice of corrosion protection needs to correspond to the type of material, strength class, coating procedure, and service environment. Just getting a better bolt is not an answer to all problems.

Loosening from vibration

Vibration does not always break a bolt directly. Instead, repeated transverse movement can gradually reduce clamp force.

Once preload falls, joint surfaces can begin slipping. The bolt may then experience larger cyclic stresses and movement than intended.

Effective fastener loosening prevention begins with achieving the specified preload. Thread condition, tightening method, surface finish, joint stiffness, and locking method can also influence resistance to loosening. This matters in suspension, engine, chassis, and powertrain joints where vibration continues throughout vehicle operation.

How can each type of fastener failure be prevented?

Prevention starts by matching the fastener and joint design to the actual load. Replacing a failed bolt with a higher strength class without identifying the cause can simply move the problem elsewhere.

Controlling fatigue and shear

Fatigue control depends on keeping cyclic bolt stress within the intended range. Correct preload helps the clamped parts stay in contact and reduces movement at the joint.

For shear-related problems, the joint should also have enough clamp force and suitable fastener dimensions for the expected transverse load. Understanding the actual bolt shearing causes helps determine whether the problem comes from load, joint movement, size, or material selection.

Managing corrosion and hydrogen risk

Material and coating selection should reflect exposure to water, road salt, chemicals, and temperature. For susceptible high-strength steel, manufacturing controls are also important because hydrogen introduced during processing can contribute to delayed cracking. Coating processes and required post-treatment should therefore match the fastener specification.

Keeping vibration from reducing preload

Proper tightening remains central to fastener loosening prevention. Too little preload allows movement, while excessive tightening can stretch or damage the fastener.

Locking features may also be used where the joint requires them. These can include prevailing-torque nuts or other mechanical locking designs. The chosen method should support the joint rather than compensate for an incorrect fastener or tightening specification.

High Tensile 12.9 Grade Socket Shoulder Bolts

Maggiori dettagli

How does CNRL test fasteners before automotive use?

Testing provides a way to find dimensional, mechanical, surface, and material problems before fasteners enter an automotive assembly.

CNRL lists tensile testing machines, hardness testers, salt spray equipment, coating thickness tools, dimensional inspection equipment, and 3D measurement systems among its testing resources. The company also uses magnetic particle and eddy current inspection for defect detection in applicable production processes.

Mechanical and dimensional checks

Tensile and hardness testing help confirm whether mechanical properties meet the specified requirements. Dimensional inspection checks features such as diameter, thread geometry, and other drawing requirements.

These checks are important because incorrect dimensions or mechanical properties can contribute to several bolt failure modes.

Surface and defect inspection

Salt spray and coating thickness testing can evaluate specified surface treatments. Magnetic particle or eddy current inspection can also help detect certain defects that may not be obvious through visual inspection alone.

Together, these controls support a more complete fastener failure analysis approach. Preventing failure starts before installation by checking whether the fastener matches its drawing, material, mechanical, and surface requirements.

Learn about CNRL‘s testing center capabilities and discuss the inspection requirements for automotive fastener projects.