Impulse Test - Pressure Fatigue Test


Impulse Tests | Pressure Fatigue Testing

Pressure Fatigue Testing, also known as Impulse Pressure Testing or impulse test, can help you determine pressure fatigue endurance of a component or review of design and/or calculation concepts (FEA simulations). The MAXIMATOR pressure pulsation tests, will provide the assurance that you need. Our Service Centers perform verification of both prototypes in the development phase and qualities of serial production components.

What is Pressure Fatigue Testing?

A leak can be defined as a change in pressure from high to low, typically a consequence of an unintended hole or crack. When an internal component or pressure vessel is above normal atmospheric pressure it is said to be in a state of positive pressure, if a seal or crack bridges the sealed positive pressure area with a lower pressure region or atmospheric pressure region the positive pressure will escape and decay or defuse to a lower pressure; the location of escaping pressure is the leakage location.

Benefits of Pressure Fatigue Testing

  • Determine failure mode and find the “weakest link” on a component or an assembly
  • Assist in the development of a new prototype or the redesign of existing component
  • Fast results
  • Test multiple parts simultaneously
  • Compare your parts with competitors
  • Confirm the benefits of life extending processes such as autofrettage

One of the prime benefits of pressure fatigue testing is determining the “weak link” in a product’s design. In some cases, it may come as a surprise as to the actual location of the weakest part of a component. Characteristics such as surface finish may have a much more profound impact on fatigue life than most designers suspect. High surface loading near gaskets and seals can hold significant mean stress levels, adversely effecting fatigue life.

Another prime objective of pressure fatigue testing is to reveal the actual number pressure cycles a component can withstand prior to failure. In some cases this can be quite complicated, especially when there are multiple pressure cycles at varying levels to be tolerated. Some of the things that can affect fatigue life include: surface finish, changes in geometry without smooth transitions, sharp corners, cross hole transitions, etc. Even today’s best analytical tools are challenged to accurately predict fatigue life, especially in light of complex duty cycles. In most cases, stress calculations are made (or provided by FEA models) and fatigue is assessed by conventional methods such as equations or software. Actual test data can be used in these cases to verify the accuracy of, or refine the models and methods used in the Engineering product life calculations. In some cases, it may be advantageous to determine the pressure fatigue resistance of a product at various pressure levels in order to produce a Wöhler curve (S/N curve) of stress versus fatigue cycles, as seen in the next illustration. This gives the Engineer a wealth of fatigue information by being able to then interpolate between test points and evaluate the life of the product at multiple pressure levels.

Additional Uses of Pressure Fatigue Testing

Additional uses of pressure fatigue testing involve evaluating alternative designs, such as material selection or design changes regarding cross section, weight reduction and sealing locations. All of the above can play a vital role in driving down costs in today’s economically sensitive commercial environment, but could have a significant impact on fatigue life.

Another important aspect of pressure fatigue testing is to evaluate the trade-offs of various manufacturing processes such as machining methods and heat treating. Are the processes that you are considering impact pressure fatigue life? Does a process change potentially being considered affect the bore surface finish or features and thus have an effect on fatigue life?

Competitor Benchmarking

Some manufacturers are keen to take advantage of competitor benchmarking when it comes to product testing, and pressure fatigue is no exception. A lot of information can be gleaned from testing a competitor’s product, and as well it may bolster your competitive advantage in the marketplace if your product is clearly superior.

Fatigue Test Pass/Fail Criteria

Fatigue test pass/fail criteria will typically be dictated by customer usage of a particular product and its application(s). Some objectives may be simply to meet a pressure cycle loading without failure occurring for a given number of cycles. The number of cycles required can vary from one customer to another and one industry to another; for many high pressure diesel fuel systems manufacturers the required number of cycles can range from 5,000,000 to 10,000,000. Whereas a pressure gauge for a hydraulic application may only require 100,000 cycles. Other objectives may be more complicated, with different usage scenarios, or even multiple loading parameters. In some cases, the manufacturer may dictate the pressure fatigue life of the product, but in today’s demanding market the more technically adept customers will be defining the requirements. Still other pass/fail criteria may require that the pressure vessel manufacturer display a maximum failure rate at a given reliability level. The last mentioned criteria requires usually that many samples be tested to gain confidence in the results, however there are statistical and reliability analysis techniques like Weibul analysis and accelerated testing to counteract that need.

Testing Multiple Parts

It is typical that even the best techniques for analyzing the endurance of your products may lead to uncertainty regarding their life expectancy. Some of the most consistent products on the market when pressure fatigue tested could have as much as a 2:1 ratio of tested cycles in a small batch of 4 or 5. Such is the nature of fatigue that the results of testing even under the tightest repeatable controls involve significant scatter. Always make sure that you are testing a statistically significant number of samples to obtain accurate results. There are ways of saving money by testing multiple parts simultaneously.

Recreate Acurate Features

When pressure fatigue testing products, it is our policy at Maximator Test to as closely as possible recreate the exact mating features in pressure seals and joints as accurately as possible, and use specified torque values for all nuts and fittings. Careful attention to detail here will produce accurate test results and realistic failure locations. This can minimize the potential for accidentally removing a real failure mode, or introducing a false one.

Accelerated testing at elevated pressure levels is a very practical way to determine a lot of information regarding the pressure fatigue capability of a design in the least time possible. One caution should be followed here – do not test at abnormally high pressure levels which can induce alternate, unrealistic failure modes. Generally, testing between the maximum operating pressure and short of the proof pressure is a good guideline.

Maximator-Test has been providing solutions for clients throughout the transportation and energy industries.  This includes significant experience in High Pressure applications.  These solutions have included testing, analysis, and materials evaluation of equipment designed for harsh environments.  Let us know how we can help you with your project.

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