ABSTRACT
This paper presents the results of series of isothermal and thermomechanical fatigue
(TMF) tests of unburnished and burnished 7075-T6 Al specimens. A designed roller
burnishing tool was employed to improve the strength of the surface layer of the
fatigue specimens. The fatigue stresses were developed in the specimens by
combining constant amplitude rotating bending stresses along with constant
temperature variation. Isothermal (ITF) and TMF tests were conducted on a rotary
bending fatigue testing machine. In ITF tests, two different constant temperatures
were used namely: 523 and 623 K. For TMF tests, a constant temperature variation
between 523 and 623 K was applied. All these tests were conducted at a constant
operating speed of 1200 rpm.
The present results revealed that roller burnishing processes have played a
significant role in increasing the fatigue lifetimes for both ITF and TMF specimens.
The enhanced fatigue strength of the burnished specimens was attributed to the
overall increase in the surface layer strength which may delay fatigue crack growth
from the surface. Two distinct fatigue fracture regions were observed: region I and
region II. In region I, the fracture surface is associated with the formation of fatigue
striations. In region II, the fracture surface is covered with surface dimples. This
indicates that local strain softening mechanism has dominated the final stage of
fatigue failure. Extensive effort has been paid at investigating the fracture surface of
ITF and TMF specimens.