Cracked Chuck Failure Analysis

Problem
Determine the root cause of a tool chuck that was cracking during product assembly.

Description
About 30% of the tool chucks were cracking. The chuck was made of a stainless steel alloy that was similar to PH 13-8 Mo.  The component was fabricated from wire stock that had been 70% cold drawn.  The following fabrication processes were used to make the chuck:

1. Machine hole through wire center
2. Machine other features inside the hole
3. Machine a slot into the side of the chuck
4. Heat treat at 900 °F to precipitation age the steel to a final hardness of >53 Rockwell C, which was the peak hardness of the steel.

The cracks initiated at the root of the slot when a mating part was inserted into the chuck.

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Analysis
To start the failure analysis, cracked samples were split open completely to allow the fracture surface to be examined with a scanning electron microscope. The analysis indicated that the fracture mode was dimple rupture, indicating that the metal was exposed to a stress that exceeded its tensile strength.  However, there was no plastic deformation of the material around the crack, which indicated that the metal had very little ductility.

The heat treatment that was used resulted in a steel with high yield and tensile strength and poor fracture toughness.  Using a higher heat treating temperature of 1000 or 1050 °F was recommended.  This heat treatment resulted in overaging, with a slight reduction in strength, but almost double the fracture toughness.  The reduction in strength was acceptable, so the recommendation was implemented, and the cracking no longer occurred.

Conclusions
The root cause of the cracking was poor design.  The heat treatment that was specified resulted in a material that had poor fracture toughness.

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Could the problem have been prevented?
Yes.  Completing a failure modes and effects analysis (FMEA) when the product was being designed would have probably identified cracking as a potential failure mode.  An FMEA involves reviewing components, assemblies, and subsystems to identify failure modes, and their causes and effects. For each component, the failure modes and their resulting effects on the rest of the system are recorded in an FMEA worksheet.  There are both design and process (Manufacturing and Assembly) FMEA analyses.

A successful FMEA activity helps to identify potential failure modes based on experience with similar products and processes - or based on common physics of failure logic. It is widely used in development and manufacturing industries in various phases of the product life cycle.

While performing an FMEA can be laborious, the output is often very powerful for giving design and manufacturing teams clear direction on engineering issues to resolve during product design and manufacturing process development.  Ultimately, the impact is improved product reliability, more capable manufacturing and assembly process reliability, and fewer problems and surprises.

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Had the design team for the product considered here performed a design FMEA, they may have identified the susceptibility of the chuck to cracking, and considered an analysis of the effects of different aging heat treatments on the cracking.  With this information, the design team could have optimized the mechanical design and metal properties requirements.

Information about how to perform an FMEAs is available in the publication "Potential Failure Mode & Effects Analysis", which is available from AIAG at www.aiag.org.