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Compressive Stress Comparison

This specimen was evaluated by Case Western Reserve University

The industry over the past 22 years has come to know the benefits of compressive stress to help fight steel fatigue.  The MetaLLifeÒ process is the accepted method of choice for extending the life of perishable tooling that fails due to fatigue or thermal fatigue.  Now on the horizon rises a new star.

After 3 years in laboratory and development research, Laser peening has successfully emerged as a viable commercial technology.  The compressive residual stresses that are created are approximately 4 times that of conventional peening.  Although still in its infancy due to high costs, commercial aerospace applications such as in the aviation sector, have found a ROI that justifies a laser peening process and technology.

Laser peening (Laser*Life for tooling) involves impinging an intense beam of light on an ablative surface.  The shock wave created by the laser beam impresses a deep but highly controlled level of residual compression into specific applied areas of the selected material without any heat.  These compressive values and depth are 4 times those of conventional methods.

The basics of laser peening involve an ablative thermal insulating layer, water, and a beam that has roughly a 25J at 25 ns output from a Nd:glass laser.  The area to be processed is covered with an ablative thermal insulating layer.    The leading temporal edge of the laser impacts the ablative layer which is saturated with water to confine the energy transfer. As the laser vaporizes the ablative material, the plasma pressure builds to approximately 1,000,000 psi or 100 kBar with the water serving to confine the inertial pressure.  The rapid rise in pressure creates a shock wave that penetrates into the metal plastically straining the near surface layer.  This results in the creation of residual compressive stress that penetrates to a depth of 1mm to 8mm depending on the material and process conditions.  All of this without rolling critical sharp corners.  Sharp edges are a standard required configuration of many perishable tools.

Currently the technology is being applied to jet engine components, aircraft structures, military, automotive, medical and energy systems.

Again back to the basics, metallugically "a crack cannot propagate into or through a layer of compression unless the yield strength is exceeded".  Imagine, if you will,  inducing a compressive layer that is 4 times that of MetaLL ifeÒ and you can see why the aircraft industry is so eagerly embracing this new technology.

Although the ROI of doing it to tooling at this time discourages this method of preventative maintenance, in the future this will  become the accepted method of the choice as the cost comes down.  We can all remember how much a CD burner use to cost.

Tests are still in the initial stage at Case Western Reserve University. To date measurement comparisons of peening v/s laser have been completed to measure both the compressive levels and length of extended life with regard to die cast Hot Work H-13 materials.

We are proud to be a participant of this new technology that was developed at Livermore Labs in conjunction with an established peening industry leader.