Quality Waterjet Newsletter 02/2005

Waterjet Component Life and Fatigue

Waterjet industry has been around for almost three decades. Today’s waterjet machines are much more reliable than those in the 70s’ and 80s’. As an example, operating pressure has increased from under 40,000 psi to over 60,000 psi and service life of high-pressure seal has increased from under 100 hours to over 1000 hours.  The improvement over the years came from steady progress in the understanding of the failure mechanisms, better design, better materials, and better manufacturing technology. High-pressure components usually fail in one of (or a combination of) these four modes: fatigue, wear, corrosion, and extrusion.

Fatigue is the major cause of failure of high-pressure wetted components --- those directly subjected to the load of high-pressure water. It is a function of stress level and number of cycles of loading. Every engineering material has its own fatigue characteristics, represented by a fatigue curve (S-N curve, please refer to www.qualjet.com/Fatigue.htm for example). This curve indicates, for a certain stress level (S), how many cycles (N) the material can take before it fails. By reducing the stress, a value can be found which will not cause failure, regardless of the number of applied cycles. This value is called Stress Endurance Limit. Ideally, high pressure wetted components should be designed with stress below the Endurance Limit. However, fatigue curves for materials used for high-pressure wetted components (such as 15-5 stainless steel) are usually not available. A typical practice in this industry is to test the real part under a cyclic pressure loading until it fails or survives a certain number of cycles. You may ask why these parts still fail in the field if they have already been tested in the lab. Reality is that not all the parts are made the same. First of all, raw material and heat treatment may not be consistent from batch to batch. Workmanship adds another variable. A single careless scratch can cause local stress level go beyond the Stress Endurance Limit and the part will fatigue before it reaches its expected life. Corrosion from unqualified water can change the characteristic of the fatigue curve and causes premature failure. This is a battle that is never over (at least not forever). When one component surpasses the expected number of cycles, it will raise the standard (and the pressure) and another part may become the weakest link. A later article will disclose how this industry has been fighting this battle.


  • New to waterjet cutting or abrasive waterjet cutting? www.waterjets.org is a good place to start.
  • Care about the safety information in dealing with waterjet injury? Check out WJTA’s article on “Water Jetting Safety” on p. 50, Feb. 2005, CleanerTimes.
  • Interested in retrofitting waterjet cutting machines? Take a look at Pikotek’s article on this topic in WJTA Jet News, Dec. 2005.

Sites of Interest Nomination

  • If your firm or someone you know is using waterjet for an innovative application and is willing to share with the industry, email us the information at info@qualjet.com. We will add a hyperlink to your site in the next newsletter.


QualJet LLC

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Q1 Promotion

·  10% discount for any retail purchase by March 31, 2005

·  25% discount for any stocking order in Q1


Intensifier Pump on Sale

Quality intensifier pump (50HP, 55,000psi, 1gpm) is available at 20% cheaper than equivalent pumps on the market, and an ideal replacement for worn-out pump or a cost-effective part for new waterjet system. Contact info@qualjet.com for details.


QualJet LLC

12819 SE 38th Street, #240

Bellevue, WA 98006

1-866-QUALJET (782-5538)

Fax: 425-378-7776

Email: info@qualjet.com

Website: www.qualjet.com


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