Recently our CEO was out driving along one of our local major interstates and as he came down the off ramp he noticed lying on the side of the road a broken wheel stud with its two piece wheel nut attached.  The assembly apparently fell off of a tractor trailer truck that had been driving along this major interstate.

As an owner of a fastener company and an inquisitive person, Kirk stopped to pick up the bolt.  He brought it into the Earnest Testing Lab to determine what happened to cause it to fail.

The interesting characteristics of this M22 x 1.5 x 120 wheel stud were that the head had fallen off, the serrations on the shoulder were worn all the way down to the shoulder diameter and the first 1/2” of thread below the shoulder had been worn and corroded down to the minor diameter of the thread, removing almost all trace of the thread that had been present, yet the front end of the stud and the wheel nut looked in good shape.

It appears that the last time that the truck that this wheel stud was installed onto was in for service no one gave the wheel stud a full inspection  If the mechanic just gave this wheel stud a visual examination they would not discovered any of the damage and wear that was going on behind the wheel.  All the mechanic would see on a visual examination are the exposed threads and top of the nut that showed no signs of excessive wear or rusting.

Just because an assembly looks good does not mean that it is good.  Critical fastener assemblies must be inspected at regular intervals to ensure reliability.

Failure Analysis

Earnest’s testing lab went to work to determine the cause of failure and developing recommendations to improve the wheel bolt assembly to prevent future failures.

Earnest’s test lab is capable of testing the physical, dimensional and chemical properties of all types of fasteners (whether we stock them or not).  Our lab has the industry standards for both inch and metric fasteners that permits us to verify if the fastener was made correctly and our personnel has the fastener experience to recommend where improvements in design can be incorporated to improve the safety and reliability.

Examination of the top of the wheel stud showed that the strength level was still visible.  The “10.9” marking on the head showed that this product was designed to meet a property class 10.9 strength level per the requirements of ISO 898-1.

Failure analysis starts with determining if the fastener was properly made to the standard that it was designed to meet.

Earnest test lab has the capability to perform the following tests:

Physical Properties

1. Tensile Test / Proof Load Test:  Determines the actual strength of the product. Tensile testing is performed on studs and bolts.  Proof load testing is performed on nuts. Earnest’s tensile machine can test up to 400,000 lbs making it one of the largest in the industry.
2. Rockwell Hardness:  The hardness of a material provides a correlation to the strength of the material and the uniformity of the heat treatment.  Our Rockwell hardness testers are capable of testing all of the Rockwell scales (C, B, A, N, etc) and all superficial hardness scales.
3. Micro Hardness:  Micro hardness testing determines if a product has been carburized or decarburize which can result in premature failures.
4. Plating / Coating Thickness:  Our X-Ray Fluorescence plating thickness testing is a non-destructive test that measures platings and coatings thickness and can identify the major chemical analysis of the plating material.
5. Salt Spray Cabinet:  Our salt spray cabinet meets the requirements of ASTM B117 salt spray testing and can measure the actual hours to rust for plated and coated fasteners

Dimensional Properties

1. Thread Gaging:  Earnest’s lab has a complete set of Go and No Go gages for not only inch coarse and fine threads (up to 5” in diameter), we also have a complete set of gages for metric coarse and fine threads.
2. Contour Tracer:  A contour tracer can measure and verify the critical features of any thread form.
3. Geometries:  Intricate geometries can be easily measured with Earnest’s Smart Scope. The Smart Scope is capable of measuring linear and radius dimensions to an accuracy of .0001” in three dimensions, along with the capability to measure angles in all three plains.
4. Dimensions:  Earnest’s labs micrometers and calipers are calibrated and certified to exacting industry standards for precise linear measurements.

Material Properties

1. Chemical Analysis: Determining the chemical analysis of a material establishes the grade of material used in the manufacture of the product.  Our mass spectrometer is capable of determining the makeup of steel, alloy and stainless materials.
2. Microscopic Examination:  Microscopic examinations determine if defects are present in the material from either the raw material used, the manufacturing process or from heat treatment.

Conclusions

Earnest’s testing of the failed wheel stud showed that it was manufactured to the hardness level specified for a property class 10.9 strength level stud.   The hardness range showed that the heat treatment was uniform across the cross section of the product.

Core Hardness

Rc 35.8 / 36.6

Since the outside of the head was broken off starting at the fillet radius through the center of the head, it could not be verified if damaged had occurred to the fillet radius because of insufficient clearance between the edge of the material being clamped and the fillet radius.

Microscopic examination of this failure zone indicated that stress corrosion was occurring at the fillet radius.  Areas of high stress concentration make high strength alloy steels prone to stress cracking at areas where corrosion is occurring.

The ratio of the head thickness (7mm) to the body diameter (22mm) also adds to the potential for high stresses to occur in this fillet radius area.  Since the shear strength of alloy steel is 60% of the tensile strength of the material, the shear area of the head needs to be 170% larger than the stress area of the threads.  In this case the shear area of the head is only 150% of the stress area of the threads.

Recommendations

Earnest’s testing of the failed wheel stud did show that it was manufactured to hardness requirements of a property class 10.9 strength level stud.

The type of head failure evident indicated that damage had occurred to the fillet radius that permitted excessive stress to develop under the head which resulted in the head breaking off.

In order to minimize the stresses from occurring under the head it is recommended that a larger fillet radius be incorporated into the stud.  This will require that the material the stud is being clamped down onto also has sufficient clearance at the edge of the hole to prevent damage from occurring to the fillet radius.  The wheel stud should also have a larger head thickness to minimum stresses at the fillet radius.

It is also recommended that a more corrosion resistant coating be used on the wheel stud.  From the condition of the stud it appears that it was coated with a standard phos and oil coating.  In order to prevent stress corrosion from occurring it is recommended that a more durable barrier coating is used (like a Dacromet or Magni coating) that will prevent corrosion from occurring.