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. Tips
Sites
of Interest Nomination
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QualJet LLC
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2005 ·
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QualJet LLC
12819 SE 38th Street, #240 Bellevue, WA 98006 1-866-QUALJET (782-5538) Fax:
425-378-7776 Email:
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