Autofrettage

What is Autofrettage?

Autofrettage is a technique used on tubular metal components in which enormous amounts of internal pressure causes portions of the part to yield plastically, resulting in internal compressive residual stresses once the pressure is released.

The goal of autofrettage is to increase the durability of the final product by inducing residual compressive stresses into the material.

The autofrettage process is used to change the inner surface of a container/tube by inducing permanent compression to its inner layers.

With our specialized equipment and high pressure experience, Maximator Test can assist you with your autofrettage project large or small; whether you have 5 parts or 5,000 we can help determine how best to seal your components and develop the process to help increase their fatigue life.

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Autofrettage Applications

The autofrettage process is widely used in various industries for increasing operating pressures for advanced industrial, automotive, aerospace and defense systems; the application of autofrettage process is required to improve component fatigue life. The technique is commonly used in the manufacture of high-pressure pump cylinders, fuel injection systems for diesel engines and warship and tank gun barrels. Similarly, the process is also used in the expansion of tubular components in oil and gas wells.

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How Does Autofrettage Work?

Autofrettage works by yielding a portion of the wall thickness of the material of a high pressure carrying element to produce a beneficial compressive stress on the product’s ID surface when the applied pressure is removed. Essentially, autofrettage works as if the pressure carrying element has a band clamp on its outer circumference, minimizing or eliminating the pressure vessel ID from experiencing the negative effects of pressure pulsations. Generally, cracks will tend to initiate from the largest stress riser or the largest defect on the ID surface. The only time this is not the case is when large stress risers with residual tensile stresses are positioned on the outer perimeter of the part. The following graphic illustrates the process in two steps. In the first step, a very high pressure is applied to the part. The inner most bore will generally experience a small plastic deformation; while the lesser stressed OD will expand elastically. When the pressure is released in step two, the elastic stresses in the OD of the part overcomes the lesser material in the bore and instills a compressive hoop stress which is largest at the inner surface. It is even possible to plastically deform the entire wall of the pressure carrying part and still impart large beneficial compressive stresses at the bore. There is very little measurable dimensional difference between a part that has undergone autofrettage and a part that has not.

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How Autofrettage Improves Pressure Fatigue Life

There are two major beneficial mechanisms at work due to autofrettage.

  1. First, there is a hindered or slowed crack initiation phase. In most cases, the crack does not develop due to the lack of sufficient tensile hoop stresses at the bore during peak pressure. In some cases, an already developed crack (or defect acting as a crack) may become arrested and grow no further with repeated cyclic pressure loading.
  2. Second, with autofrettage the bore of the pressure vessel experiences alternating stresses with a superimposed mean compressive stress. This means that the oscillating pressure allowed can approach its maximum potential. In the case where there is no autofrettage and the part is exposed to a cyclic pressure which recedes to zero upon the completion of each cycle, the alternating stress will be equal to the mean stress. The second alternative does not promote fatigue longevity. Compare these two scenarios on the following Goodman diagram of acceptable fatigue life.

maximator-test-goodman-diagram-pressure-fatigue Proper application of autofrettage can result in an increase in cyclic pressure carrying ability of 30% to 200% depending on initial stress risers and surface finish of your particular product. Looking at this another way, the increase in pressure carrying capability is equivalent to gaining multiple orders of magnitude of operating cycles at a given pressure level. maximator-test-auto-frettage-pressure-table
Under working pressure the stresses inside the component do not move between 0 and a maximal value anymore, but start in the pressure area below 0 and end at a much lower maximal value. maximator-test-auto-frettage-stress-loads The crack will propagate to a certain length and will then be stopped by the pressure internal stresses that are inside the wall where the internal stresses “press” the crack together.

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How Do You Autofrettage?

The best way to get started is to work with experts from in the field of autofrettage like Maximator Test to develop a simple, low-cost method to autofrettage your product to quickly and easily maximize pressure fatigue life. This can make it effortless to gain the benefits of this highly effective method with very little work and investment on your part. Maximator Test can assist in this regard by working with your engineers and help develop the best strategy to improve your products. The first concern becomes “What autofrettage pressure should I use”? The answer to this question is complicated, but it can be answered fairly simply. Generally, higher pressures offer greater benefits, but the designer needs to be cautious to avoid applying pressures sufficient for static burst. The second concern is to eliminate any deformation to the seals that will be used during the installation of the product during its normal service life. Autofrettage should use fittings designed to seal at locations other than those just mentioned, or they should spread out the load over larger areas to minimize any undesirable deformation. In some cases product design may be influenced.

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Why Autofrettage – The Benefits of Autofrettage?

  1. First, there is a hindered or slowed crack initiation phase. In most cases, the crack does not develop due to the lack of sufficient tensile hoop stresses at the bore during peak pressure. In some cases, an already developed crack (or defect acting as a crack) may become arrested and grow no further with repeated cyclic pressure loading.
  2. Second, with autofrettage the bore of the pressure vessel experiences alternating stresses with a superimposed mean compressive stress. This means that the oscillating pressure allowed can approach its maximum potential. In the case where there is no autofrettage and the part is exposed to a cyclic pressure which recedes to zero upon the completion of each cycle, the alternating stress will be equal to the mean stress. The second alternative does not promote fatigue longevity. Compare these two scenarios on the following Goodman diagram of acceptable fatigue life.
  3. The service life of a component can be increased.
  4. The working pressure, the pressure that could be held by the Autofrettaged components without failure, increases. e.g., with pipes up to 1.8 times higher and with components with cross boring up to 2.5 times higher
  5. The sensitivity of the notch is significantly reduced.
  6. If the working pressure remains the same the thickness of wall can be reduced offering both cost and weight savings in addition to using cheaper alternative materials. In cases where higher cost materials with higher strengths and/or lower inclusion levels are specified, often commercially available materials with greater availability at lower cost are more durable at given pressure cycle applications with the use of simple, proper autofrettage procedures.
  7. The possibility exists to increase the working pressure.
  8. In special cases, the finishing of borings and surfaces can be reduced and sometimes eliminated.
  9. Autofrettage can making better use of a material’s structural properties.
  10. The Autofrettage process can reduce fatigue concerns in pressure pulsating applications. Estimates are that autofrettage post processing increases the ability to withstand pulsating pressure spikes by a factor of 1.8 in tubes with substantial thicknesses. This gives engineers the leeway to reduce thickness without sacrificing performance.
  11. Another benefit of autofrettage is minimizing the effects of poor surface finish on pressure fatigue life. Every Engineer knows the detrimental effect of rough surface finish on the fatigue life of a product. Autofrettage has the ability to impart a residual compressive stress that is deeper than the worst-case defect of a machined or otherwise as-formed ID surface. This can result in a surface that has very little to no detrimental impact on fatigue life. Engineers also may know that most commercially available processes to improve the surface finish of the inner bore of a product are either very costly or otherwise experimentally prohibitive, but autofrettage may provide a simple, cost-effective solution.
  12. Either changes in cross-section, cross holes, or irregular ID features can be serious trouble when it comes to pressure fatigue life. Autofrettage can successfully eliminate the concern for many of these unavoidable design features. One of the most advantageous results of autofrettage is that its life-enhancing effects are best realized at ID stress risers. Residual, life-enhancing compressive stresses after autofrettage will be the greatest at these stress riser features.

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Maximator Engineers are Experts in the Field of Autofrettage

With over 50 years’ experience in numerous industries ranging from Automotive, Aerospace, Oil & Gas and Deep Sea applications, Maximator Engineers are experts in the field of Autofrettage and would be delighted to discuss how the autofrettage process can add value to your products.

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