Surgical Screw Failure Analysis, Metal Fatigue

 Read Consulting performed a metal failure analysis on two surgical screws that were used to hold wire for a surgical implanted back support system. These titanium screws failed after less than one year after the surgical implant.The failure analysis determined that both screws failed at the minor diameter due to  metal fatigue in bending. In one case there was a machining mark that was associated with the initiation of the fatigue failure. This machining mark acted as a stress riser that magnified the applied stress. In addition, the bending stresses were higher on both screws because these titanium screws were not  inserted completely.

Diamond Tool Design{ Glass Expert Discusses Glass Damage From Diamond Grinding

Chart Showing Damage Depth  of Diamond
 Grind  v. Diamond Particle Size

Checks in Glass Revealed by
Etching with Buffered HF

100 Grit Damage Remaining After
0.0075" Had Been Removed

 Certain types of glass must be ground to shape with a diamond tool, and they are then lapped and polished to make them transparent. If this glass piece must also exhibit a certain strength level, each subsequent finishing step must remove all the damage from the previous step. These considerations dictate tool design and diamond grit size selection. The chart above shows that damage depth from diamond grinding is directly related to the size of the diamond abrasive Note: As the grit size number increases, the abrasive size decreases). The nature of subsurface damage from a 100 grit diamond are shown in the other two 390 X photo micro-graphs. These pieces were polished to be optically clear; they were then etched with a buffered HF solution. Etching highlighted the damage remaining from the diamond grind. Even after 0.0075" had been removed, there was remaining subsurface damage. This remaining grind damage significantly weakens the glass. Thus, it must be completely removed for reliable mechanical properties.

Plastic Pipe Expert Performs Failure Analysis on Failed PVC Pipe Elbow

The PVC pipe expert at Read Consulting was asked to perform a failure analysis of a failed PVC "glue joint" elbow that had failed in service and that had caused a significant amount of water damage. The failed PVC pipe elbow was submitted for failure analysis. Upper left is the as received elbow with a visible crack. This crack went through the entire elbow and had caused the leak. This crack was "opened" up to reveal the fracture surface. This pipe failure initiated on the outside of the elbow. It is a progressive failure that is most likely fatigue. The plumbing expert was unable to view the elbow as it was installed. So, at this point all that can be said of this plastic failure is that it is a plastic fatigue failure.