Peening Process

  • What is peening ?

    Peening is a cold working process where media is propelled onto the surface of components to induce a thin layer of compression to reduce the initiation and propagation of crack thus improving the fatigue life of the components.

  • Why peening ?

    Peening increase fatigue life of components without increasing their weight

  • What is fatigue life of a component ?

    Number of low level loading cycles that a component can sustain until a crack initiates and grows to a critical levels.

  • What is the difference between peening, flapper peening, roto peening, needle peening ?

    These peening processes have the same objective to induce a thin layer of compression on the surface of components but use different methods to do this.  Conventional Peening uses air pressure or wheel motion to propel the media.  Flapper peening, also known as roto peening, uses rotating flaps with rows of tungsten carbide balls to strike the surface.  Needle peening uses tungsten carbine needles that are moved up and down perpendicularly to impact the surface.

  • What material can be peened ?

    All metal components can be peened. Aluminum, steel, hardened steel, titanium, Inconel, etc.

  • What components can be peened ?

    Most metal components can be peened although peening is often reserved for critical structural members. Example include Lower Wing Skins, Landing Gears, Wheels, Critical Structural Members, Ribs, Spars, Flap Tracks, Engine Components, Disks, Blisks, Blades, and many more

  • What is peening intensity ?

    Peening intensity is a measure of the force of impact of the media against the surface of the component.

  • How do I measure intensity ?

    A method to measure peening intensity was defined many years ago by a man named Almen.  This method uses the deformation, or arc height, of thin strips of metal called Almen strips to measure the force of impact of the media. This deformation of the thin strip depends on many factors such as hardness of the shot peening media, the velocity of the shot peening media, the angle of impact of the shot peening media, the flow rate of the shot peening media (amount of impacts).  Intensity is found by generating a saturation curve and calculating the intensity from the curve.  Almen Strips are most commonly used to find intensity but new methods are now available to measure the velocity of the media (Velocity Meter) or to measure the impact force directly (E-Strip®).

  • What is an Almen Strip ?

    Almen strips are made of SAE1070 spring steel and are classified into three types based on thickness: A, N, and C.  An Almen strip is peened on one side only. The residual compressive stress from the peening will cause the Almen strip to bend or arc towards the peened side.

  • What is a saturation curve ?

    A saturation curve is a method to determine peening intensity. Almen strips are exposed to a shot stream for varying amounts of time with all other parameters fixed. The arc height of the almen strips are plotted verses exposure time. A best fit curve is generated through the points. The first point on the line (not necessarily a data point) when time is doubled at which the arc height increases by 10% is called the saturation point and is defined as the intensity.

  • Curve Solver (Shockform)?

    The Shockform Saturation Curve Solver (SSCS) is a user friendly application that will quickly and easily plot your saturation curve data and find the peening intensity using the 10% rule.  No more confusing Excel spreadsheet to fiddle with!  The SSCS can be used for conventional peening and for rotary flapper peening.

  • Why sometimes we see 8A and 20A ?

    Intensity is often measured in thousands of inches (0.008A) but is often called out as 8A.  This can lead to confusion since measurements can also be made in millimeters for the metric system where an 0.008A inch becomes 0.203A mm.

  • What is coverage ?

    Coverage of the surface of a component is defined as the percentage of the surface impacted or dimpled by the shot peening process. In the shot peening process, a full or 100% coverage is often requested although it is only a theoretical limit. A coverage of 98 % is defined as full coverage and if often mandatory.

  • How can I measure coverage ?

    Coverage is determined by visual inspection of the surface using a 10x, 20x or 30x magnifier.

  • Why do some drawings asks for 100% coverage and other 200% ?

    A 100% coverage is usually required for fully automated peening while 200% coverage is required for manual peening.  A 200% coverage does not change the depth or properties of the compression layer.  It is more like a safety factor when peening.

  • How do I calculate 100% or 200% ?

    Coverage to 100% is determined by visual inspection of the surface using a 10x, 20x or 30x magnifier.  A 100% coverage is defined as complete dimpling of the surface. To reach 200% coverage, the time required to reach 100% is noted and applied again to the surface.  For example, if 2 minutes of peening were required to reach 100% coverage as inspected using a 10x, 20x or 30x magnifier then an additional 2 minutes of peening would be required for 200% coverage.

    2 minutes of peening = 100% coverage

    2 minutes + 2 minutes = 100% + 100% = 200% coverage

  • Why is certain drawings call for time and other coverage ?

    All recent specifications and drawings define coverage by surface inspection using a 10x, 20x or 30x magnifier. When the inspection of the peening surface with magnifier reveals full coverage, peening is complete.  Some older specifications determined coverage time by factoring the time to reach intensity on the Almen strips.  This method falsely assumes that the component peened is made of the same material as the Almen strip and will therefore require a time proportional to the peening time on the Almen strip.

  • Why is time not the reference ?

    Time to reach intensity is no longer used to define coverage because the material of the Almen strip is seldom the same as the material of the component.  If the component material is softer than the Almen strip material, the component will be over peened and if the component material is harder than the Almen strip material, the component will be under peened.  The method where a magnifier is used will result in the proper coverage for the component material.

  • How do I prepare the surface before peening? Can I peen over paint ?

    Peening can only be performed on clean surfaces without paint, coatings, grease, dirt or oil.  The surface must also be free of corrosion and damage for maximum efficiency.

  • Where can we use flapper peening ?

    Flapper peening is often used to re-peen small areas where the surface compression from the original peening was damaged or removed.  Flapper peening can be used on most metallic components as long as the tungsten carbide balls on the flaps can properly impact the surface of interest. The best tool for flapper peening is the FlapSpeed® PRO developed by Shockform.

  • Where can we use needle peening ?

    Just like flapper peening, needle peening was developed to re-peen small areas where the surface compression from the original peening was damaged or removed.  The difference is that needle peening does not produce FOD and can be used on fully assembled aero-engines.  Needle peening can be used on most metallic components as long as the tip of the tungsten carbide needles can properly impact the surface of interest. The best tool for needle peening is the Spiker® developed by Shockform.

  • Can I use flapper peening or needle peening to peen large surfaces ?

    Peening larges surfaces with flapper or needle peening tools is not recommended because it would take too much time. That being said, peening a fully assembled component using loose media with compressed air of wheel methods requires bagging to avoid media flying everywhere, masking to peen only the area of interest and cleaning after peening to remove all loose media.  These three steps often require more time than flapper peening or needle peening.

FlapSpeed® Controller

Spiker® Needle Peener