Book Description
Ultrasonic Nano-Crystal Surface Modification (UNSM) is a relatively new material processing technology to enhance the operating service lives, or fatigue life, of engineering components. There is an increasing interest in extending this technology to metal parts such as aircraft engine turbine blades, compressor blades and medical implants such as spinal rods. In this process a ball made with tungsten carbide generates 20 - 40 KHz strikes of a few hundred Newton on the specimen surface. It works as a cold forging process; however the small ball that works itself across the specimen surface has a dynamic load added to the normal static load. The total striking force, feed, ball radius, amplitude of dynamic load and speed can vary to yield different results. UNSM induces severe plastic deformation and deep compressive residual stresses to increase surface hardness, improve surface roughness, and introduce nano-crystallization near the specimen surface. Currently there is no systematic approach to predict the material response under UNSM. Therefore the objective of this thesis is to develop a process model for predicting the material response as a result of the UNSM process. Before developing the model, experimental data is extracted from two UNSM treated coupons, one of Ti-6Al-4V and the other IN718 SPF. First a MATLAB code is developed to define the displacement history of the carbide ball on the surface of the specimen during the UNSM process. For titanium alloy (Ti-6Al-4V) a temperature, pressure, and rate dependent constitutive material model for is established to account for the high strain rates associated with UNSM. A semi-implicit forward tangent modulus algorithm is developed to implement the material and damage model, and this is linked with the FEM software LS-DYNA through a user-defined material subroutine. We use the Johnson Cook material model already within the LS-DYNA software to simulate IN718 SPF. The residual stress obtained from the simulation is compared and verified with experimental results. To further understand the UNSM process, the residual stress results are compared with Laser Shock Peening (LSP) to note the differences.
