A1-in-diameter solid round bar has a groove 0.1-in deep with a 0.1-in radius machined into it. the bar is made of aisi 1020 cd steel and is subjected to a purely reversing torque of 1800 lbf ? in. for the s-n curve of this material, let f 5 0.9. (a) estimate the number of cycles to failure. (b) if the bar is also placed in an environment with a temperature of 750°f, estimate the num- ber of cycles to failure.