Star Wars helped develop laser peening. Ronald Reagan’s Strategic Defense Initiative was nicknamed “Star Wars” after the 1977 episode release. The defense program “Star Wars” led to many technological investigations and discoveries; laser peening was one of those fledgling technologies that benefited. Battelle Columbus Labs had already patented “Laser Shock Peening” in the early 1970’s and had made some inroads to advance the field. But laser advancements made through the “Star Wars” initiative, which incorporated a focus on the use of high power lasers to protect America from incoming missiles, provided technology that could be used to design a high power laser for laser peening.
Battelle had a large high energy laser left over from a 1970s laser fusion program, with numerous laser amplifier stages (in total it was more than 50 feet long, the final stages with laser glass rods 120 mm diameter and almost a meter long). Many of LSP Technologies’ Senior Engineers were scientists for Battelle at the time, and they were using this system in single pulses of 100-1000 Joules at 20 ns, and up until this time the system was only capable of firing every 6 to 8 minutes. With the “Star Wars” effort, they modified the flashlamp pumping from short individual 400 µs pump pulses to much longer long flashlamp pump pulses in excess of 100 ms. By staggering the amplifier firing times, they extended the pumping period for the entire system to about a second. They then used a modified high repetition rate oscillator to inject a “pulse train” into the amplifier chain, creating a burst of laser pulses. This gave them the ability to create pulse trains of up to about 1000 Hertz for the short burst period. The system could be used to rapidly test laser/metal interactions for material improvements such as fatigue and damage tolerance increases (actually well in excess of modern industrial laser peening repetition rates).
All of the design modifications required to transform a huge, antiquated, single pulse, high energy laser into a 200 kW burst laser were provided by the SDI “Star Wars” initiative. This opened the path for Battelle to significantly shrink the size of the laser components and develop the first-ever high power laser system designed specifically for laser peening. That first laser shock peening system was a 50 J system that operated at 1 Hz on a table just bigger than a normal desk. Star Wars, the movie, as with many sci-fi movies, whetted the appetite of scientists and engineers everywhere, with Light Sabers, and all the other imaginative creations, which led directly to developments in real world technologies, helping laser peening jump from laboratory experiment to industrial process.
Why does fatigue cracking get so much attention?
Basic physics tells us that for every action, there is a reaction. When parts are manufactured and placed under a service load, the reaction is that the products will absorb the load (stress) and deform accordingly (strain). In many common, everyday applications, the amount of deformation that can be seen or observed is often imperceptible to sight or feel. For example, consider the floor in you house: the space and span of the floor joists installed per building codes make the deformation almost imperceptible as you walk around. While nearly unrecognizable, the floors do deform under the load but are operating at an extremely safe level. Even so all materials are capable of fatigue cracking, even wood and other polymers.
In many engineering applications, and particularly in critical load bearing elements, metal alloys are most commonly used. The design of these components is maintained within safe material margins of stress and strain. The process of fatigue cracking becomes troubling in that it may occur within this perceived safe margin. Fatigue cracking occurs due to cycling from thousands to millions of times at stress levels below what would be considered unsafe for the material. During cycling, the material develops microscopic material flaws leading to the formation of microscopic cracks. Once formed, these cracks will continue to grow until they are either identified and repaired or propagate to cause catastrophic failure. It is also worth noting that while designs can be made to reduce the probability of fatigue failure, other events may occur that dramatically increase the likelihood. These may occur due to corrosion, incidental damage, poor manufacturing and more. In a subsequent article, we will discuss some of the methodologies behind planning for fatigue cracking (damage tolerance, safe-life, law tolerance, etc…).
Laser peening increases fatigue life by minimizing the stress state causing the initiation and growth of the fatigue cracks. With proper application, the observed stress can be pushed to a level low enough to prevent cracking or extend the service life up to ten times the normal. The benefits of laser peening for fatigue applications can be developed in simulated service environments using equipment at LSP Technologies. Using our MTS 810 servo-hydraulic test unit capable of operation at up to 35 Hz (35 cycles per second), the fatigue tolerance of materials and geometrics can be tested and evaluated with and without laser peening. Take a look at our video for an MTS fatigue testing demonstration.
GE 7F and 9F turbine blades, as well as Rolls Royce Trent 60 turbine blades have an established record of applying laser peening as a surface treatment. GE and Rolls Royce use laser peening to address blade service life limitations from erosion, corrosion, stress corrosion cracking, and fatigue.
Reports provided at the 7F Users Group meetings firmly credit laser peening as a “key step” in enhancing the damage tolerance of 7F R0 turbine blades from erosion and resulting crack initiation and propagation.
Rolls Royce credits laser peening as a “face-hardening” process increasing the “erosion protection level by four to five times” which “boosts power output by 15-30%”.
The Electric Power Research Institute has several reports that credit laser peening as a surface enhancement solution that increases fatigue life significantly beyond the capabilities of conventional or alternative processes, such as shot peening.
Read the reports linked below for more details about the improvements provided by applying laser peening (also known as laser shock peening).
7F Users Group Meeting
Rolls Royce Article