June 19, 2015

Bolt Fracture

Case Study: Bolt Fracture 

Hardware such as bolts, nuts and washers are generally used for connections and fixtures.  They form an integral part of the component structural integrity.  Bolts are governed by standards such as ASTM and are typically stamped with an identifying marking on the head during the manufacturing process.  The bolts selected for any application typically take into account the manner in which it is applied while taking into account the most prevalent mode of failure.  It follows that it would be in the designer’s best interest to understand how bolts fail and to avoid those situations.

Consider a simple bolt modeled in SolidWorks:

Roundhead Bolt 12mm DIA x 152MM LONG ASTM A307

bolt in solidworks

It is worth noting that the tensile strength for that grade of bolt is approximately 60,000 PSI.  Therefore, using plain carbon steel as a template, the tensile strength was adjusted accordingly.

tensile strength of bolt

Next, we can set up a test scenario using SolidWorks Simulation where the bolt is fixed at the top with a base tensile load of 10,000 PSI applied at the bottom

solidworks simulation of bolt

We can now mesh the bolt

meshing the bolt in solidworks simulation

After running the Static analysis we can do a section plot to understand how the stress a distributed in the cross section of the bolt.  We find that the highest point of stress concentration occurs at the neck as expected with a value of 28,067 PSI.  

static analysis of bolt in solidworks simulation

Note that this value is fairly close to yield strength of the material 31,994 PSI.  If a stress value approaches the yield strength, the general assumption of linear behavior of the material begins to breakdown.  Therefore it is in our best interest to run a non-linear analysis as well.

stress anlaysis of bolt

Creating a new non-linear study and running a similar analysis we find that the actual stress is approximately 27,197 PSI. This value is less than the linear analysis but it is still close to yield. 
Given these results it would be in the designer’s best interest to either reduce the load, increase the bolt diameter or chose a higher grade bolt.
Sometimes bolt failure can occur due to other circumstances as well.  Specifications such as how much a bolt must be tensioned at installation can make a difference.  In other cases large batches can yield bolts with manufacturing defects which may fail below the yield stress.  

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