K. Doering, D. C. Van Aken, Missouri University of Science and Technology, Rolla, MO; R. L. Hanks, K. A. Young, R. J. Lederich, Advanced Manufacturing R&D, Boeing Phantom Works, St. Louis, MO
Machining of titanium to less than 0.080 thickness is required to take full advantage of titanium's specific properties and make it competitive with other materials. These pocketed machined parts are often composed of thin rib-stiffeners coupled to a thin floor or web. As the machined thickness is decreased, machining has the potential to alter both static and fatigue properties as a result of surface deformation and residual stress. In this paper, Ti-6Al-4V plate was machined to various thicknesses (0.020 to 0.120) to determine the change in static and dynamic properties. The most notable change in static properties with thickness is a loss of tensile ductility (~25% decrease) at gages thinner than 0.080. A loss in ultimate tensile strength was observed at a gage thickness less than 0.040. Fatigue performance was investigated using three different testing configurations on materials that had a nominal thickness of either 0.030 or 0.080. A rib-stiffener machining practice was used to produce the axial fatigue specimens. The fatigue strength at 10^6 cycles of the 0.030 thick gage was approximately 7% lower than the 0.080 thick gage specimens and the thinner gage specimens had lower cyclic life at the higher stress ranges, which corroborates the lower ductility observed in static tests. Bending fatigue studies using Krouse-type specimens are being conducted to better assess the effects of machining on the surface. These tests are being conducted using a servohydraulic test frame in load control. Sub-component testing on a 0.030 nominal thickness pocketed web and rib specimen with 0.120 fillet radius was also conducted to quantify the performance improvement resulting from hand blending the machined surface. Finite element analysis was used to determine the surface stress as a function of applied load; and, modeling and test results will be presented at the conference.
Summary: Machining of titanium to less than 0.080 thickness is required to take full advantage of titaniums specific properties and make it competitive with other materials. These pocketed machined parts are composed of thin rib-stiffeners coupled to a thin floor or web. As the machined thickness is decreased, machining has the potential to alter both static and fatigue properties as a result of surface deformation and residual stress. For example, the compressive residual stress of the machined floor is greater than the rib-stiffener, which is a result of a difference in cutting tool surface used during finish machining.
In this paper, Ti-6Al-4V plate was machined to various thicknesses (0.020 to 0.120) to determine the change in static and dynamic properties. A rib-stiffener machining practice was used to produce the various gage thicknesses. The most notable change in static properties with thickness is a loss of tensile ductility (~25% decrease) at gages thinner than 0.080. A loss in ultimate tensile strength was observed at a gage thickness less than 0.040.
Fatigue performance was investigated using three different testing configurations on materials that had a nominal thickness of either 0.030 or 0.080. A rib-stiffener machining practice was used to produce the axial fatigue specimens. Water-jet cutting was used to produce an hourglass shape with radii of 2 inches and gage width of 0.25 inches that resulted in an edge stress concentration factor of 1.04. All specimen edges were polished after water-jet cutting to remove surface damage from the abrasive cutting. Fatigue tests were performed in load control with R(Smin/Smax) = 0.1 and using a sinusoidal waveform at 10 Hz. The fatigue strength at 10^6 cycles of the 0.030 thick gage was approximately 7% lower than the 0.080 thick gage specimens and the thinner gage specimens had lower cyclic life at the higher stress ranges, which corroborates the lower ductility observed in static tests. Most of the axial fatigue failures initiated at the specimen edge, but approximately 35% initiated at the machined surface. Bending fatigue studies using Krouse-type specimens are being conducted to better assess the effects of fatigue crack nucleation on the machined surface. Sub-component testing on a 0.030 nominal thickness pocketed web and rib specimen with 0.120 fillet radius was also conducted to quantify the performance improvement resulting from hand blending the machined surface. Finite element analysis was used to determine the surface stress as a function of applied load; and, the results from modeling and testing will be presented at the conference.
