Laser shock peening is the next generation of material improvement. This powerful surface enhancement process improves metal fatigue strength up to twenty times – providing invaluable service life extensions for critical components. Though traditionally applied to steels, titanium, and other metals, laser peening is now proving a valuable application for a new class of material: engineered ceramics.
The NASA Space Shuttle incorporated ceramic tiles for high heat resistance
Ceramic engineering involves manufacturing objects from inorganic, non-metallic materials like silicon or zirconia. Ceramic parts offer several key advantages over their metal components in that they are lighter, noncorrosive, and offer superior heat resistance.
Ceramics in Aerospace
Ceramic materials have been used in targeted aerospace applications for years, providing thermal protection and insulation for the Space Shuttle to withstand the extreme temperatures of atmospheric reentry. As ceramic engineering has evolved, these versatile materials have been employed in applications ranging from brake pads to ball bearings to bulletproof vests. Continue reading
Beneficial Compressive Residual Stresses
Fatigue strength improvements are proportional to the magnitude and depth of induced compressive residual stresses present in a component. Deeper compressive residual stresses provide superior resistance to crack propagation and fatigue failure. Laser peening typically produces compressive residual stresses up to 10X deeper than competing surface enhancement processes.
So how does laser peening produce effective compressive residual stress profiles? LSPT engineers control the depth and magnitude of compressive residual stresses by increasing the power density and coverage of the laser pulse.
Compared to shot peening, laser peened parts retain deeper compressive residual stress profiles in high-temperature environments. Laser peening’s superior heat resistance results from the lower percentage of cold work involved, producing deep compressive residual stresses that remain stable at elevated temperatures.
Surface enhancement techniques like shot peening and laser peening rely upon plastic strain to produce beneficial compressive residual stresses within a material. Laser peening is a mechanical process that utilizes a high-energy, pulsed laser to generate a compressive stress wave at the surface of a part. The compressive stress wave induces plastic deformation as it propagates into the material, causing dislocations in the microstructure that enhance the strength and fatigue resistance of the part. Compressive residual stresses inhibit the initiation and propagation of fatigue cracks.
Metal Fatigue Prevention
Metal fatigue failure typically results from microscopic crack formation at the surface of the material. Cracks may initiate due to cyclic loading, foreign object damage, or corrosive environments, and cracks will propagate with successive loads or continued exposure until failure occurs.
Enhancing materials through the introduction of compressive residual stresses inhibits the initiation and propagation of fatigue cracks. Compressive residual stresses provide a counterbalance to the tensile stresses present at the part surface, and the compressive environment slows the propagation of microcracks. Deep compressive residual stresses provide enhanced fatigue resistance, and can significantly extend the fatigue life of a metal part.
Figure 1: Illustration of pilgering process courtesy of thefabricator.com
Cold pilgering is an important metalworking process for producing seamless pipes and tubes for critical high stress applications. The process involves feeding prefabricated tubes through rotating steel dies to reduce their diameter and wall thickness (Figure 1). Cold pilgering is applied to many different metals – steel, copper, titanium, etc. – and has applications across industries ranging from zirconium tubes in nuclear power plants, hydraulic tubes in aircraft, umbilical tubes for offshore oil platforms, and high-performance golf club shafts.
Pilger dies are subjected to cyclic loading at significant pressures (up to 1500 MPa for some alloys) and are thus prone to fatigue failures within the die grooves. When a pilger die begins to fail, small cracks are introduced into the forming groove surface requiring post-process sanding to remove the defect. These cracks will eventually grow to the point where tube defects can no longer be eliminated, forcing the die to be removed from service.