LASER PEENING TERMS

Glossary of Terms

  • Residual stresses are stresses that remain in a solid object after external forces have been removed. These are stresses within the material that often result from manufacturing processes like welding or casting.
  • Residual stresses come in two forms: tensile and compressive. Tensile stresses try to stretch the material and pull it apart, while compressive stresses try to squeeze the material and compact it.
  • Tensile stresses lead to premature component failure because the stretching effect within the material exacerbates cracks and accelerates crack growth.
  • Compressive stresses protect against component failure by inhibiting crack propagation and reducing the magnitude of applied tensile stresses on the part surface.
  • Laser peening produces compressive residual stresses deeper and at greater magnitudes than shot peening, providing superior component cracking resistance that shot peening cannot achieve.
  • When forces act upon a solid object, the object can deform in a variety of ways.
  • Elastic deformation is a temporary change in the object's shape that disappears when the forces are removed. For example: a rubber band can be stretched wide before returning to its original shape.
  • Plastic deformation is a permanent change in shape resulting from external forces. For example, a paperclip twisted out of shape will not return to its original form when released.
  • Laser peening produces plastic deformation in an object, stretching the surface layer with a powerful shockwave. This material stretching is permanent, and forces the surrounding material to constrain the expansion, resulting in a layer of compressive residual stress.
  • Metal fatigue is the weakening of a metallic object due to repeated loading at stress levels below the material's fracture threshold.
  • For example, a metal bar may be able to easily lift a 100-pound load without breaking. However, after lifting that 100-pound load one million times, the bar will fail due to accumulated wear in the material.
  • Metal fatigue typically manifests as small cracks that develop in the material. These cracks begin as microscopic defects at areas of high stress concentration and grow over time with successive loads. Eventually, a crack will reach a critical size that weakens the structure and causes it to fail suddenly.
  • Metal fatigue characteristics vary by material and can be exacerbated by environmental factors like corrosion, oxidation, or extreme temperatures.
  • Fatigue strength is a measure of how much stress an object can withstand over a given number of cycles without breaking.
  • Fatigue strength is usually expressed in megapascals (MPa) over a large number of cycles. For example, a titanium alloy may have a fatigue strength of 250 MPa at 107 cycles. This means the material can withstand repeated loading of 250 MPa up to ten million times without failing.
  • After ten million cycles, repeated loads of 250 MPa could be expected to cause material failure due to an accumulation of fatigue cracks.
  • Laser peening increases the fatigue strength of metal components, enabling them to bear heavier loads without succumbing to premature failure.
  • Fatigue life is the number of stress cycles a metal object can be subjected to before failing. Fatigue life for a given component varies with different stress magnitudes, but it provides a rough estimate of operational lifetime when stress magnitude is fixed.
  • For example, an engine component subjected to consistent stress loads may have a fatigue lifetime of 30 million cycles. This means that after 30 million cycles, the component is in danger of failing due to accumulated fatigue cracks and other material defects.
  • Laser peening extends the fatigue life of metal components by slowing the growth of fatigue cracks and delaying part failure. For example, an engine component with a fatigue life of 30 million cycles could be extended to 60 million cycles with an application of laser peening.
  • A microcrack is a microscopic crack in solid material. These cracks are not visible to the naked eye, and often appear deep beneath the material surface.
  • Microcracks do not immediately undermine the material, but they act as initiation points for larger defects that can lead to component failure.
  • Over time, repeated stress loading causes microcracks to propagate and grow into larger, macro-scale cracks. These cracks weaken the component and may cause sudden, catastrophic failure.
  • Laser peening inhibits the growth of microcracks by generating a protective layer of deep compressive residual stresses. Compressive stresses slow the growth of microcracks and reduce the tensile strain of repeated stress loading.
  • Power density is a measure of power delivered per unit area. It indicates the intensity of a laser pulse as it irradiates the component surface and can be adjusted by manipulating laser parameters like energy and beam diameter.
  • Power density is typically represented in Watts per square centimeter (W/cm2), but laser peening generates such enormous power that associated power densities are often calculated in Gigawatts (GW/cm2).
  • Each laser peening application is performed at a specific power density to optimize the residual stress profile. Softer metals might be peened at 4 or 5 GW/cm2, while harder metals like titanium require power densities as high as 10 GW/cm2 - ten billion Watts per square centimeter.
  • Pulse width is the duration of a single laser pulse. It may also be referred to as pulse duration or pulse length.
  • Laser peening is pulsed laser application. Rather than a continuous beam, laser peening is performed with short pulses of energy that are applied in predetermined patterns across a target surface. A single laser peening application may involve hundreds or even thousands of individual pulses.
  • Laser peening pulse widths are very short, typically in the 8-20 nanosecond range.
  • A nanosecond is one billionth of a second. Laser peening is a short pulse application and pulses are typically measured in nanoseconds.
  • A 20-nanosecond laser pulse irradiates the target surface for 20 billionths of a second. Light travels about one foot in a nanosecond, making the typical laser peening pulse 20 feet long or less.
  • Plasma is an ionized gas consisting of positive and negative particles. It is considered a unique state of matter (along with solid, liquid, and gas) and is most abundant in stars.
  • Laser peening generates a plasma burst on the surface of a metal component. The laser pulse vaporizes a thin layer of surface or overlay material, and superheats this vapor into a plasma state.
  • This plasma contains a lot of energy and expands rapidly, creating enormous pressure on the component surface. A transparent overlay (typically flowing water) confines the plasma, amplifying the pressure pulse on the surface.
  • The plasma burst is really the mechanism that does the work of laser peening. The high-pressure plasma shocks the component surface and deforms the material, producing compressive residual stresses.
  • The shock wave is a propagation of energy that performs work in a material.
  • During laser peening, the shock wave is produced by high-pressure plasma expansion on the part surface. This sudden pressure rise shocks the material surface, and the energy propagates into the material as a high-amplitude wave.
  • If the magnitude of the shock wave exceeds the dynamic yield strength of the material, plastic deformation occurs, and the material is permanently altered. This deformation produces beneficial compressive residual stresses that improve fatigue strength and damage tolerance.
  • As the shock wave propagates through the material, it loses energy to the work of plastic deformation. The wave gradually attenuates until it falls below the material's dynamic yield strength and no longer induces plastic deformation.
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