Wind turbine blade stress. Mechanical engineers at the University of Newcastle (Australia) investigated the use of timber in high-efficiency small wind turbine LO9 

blades (Wind Engineering, Jan. 2004). The strengths of two types of timber—radiata pine and hoop pine—were compared. Twenty specimens (called “coupons”) of each timber blade were fatigue tested by measuring the stress

(in MPa) on the blade after various numbers of blade cycles.

A simple linear regression analysis of the data, one conducted for each type of timber, yielded the following results (where y = stress and x = natural logarithm of number of cycles):

Radiata Pine: yn = 97.37 - 2.50x, r2

= .84 Hoop Pine: yn = 122.03 - 2.36x, r2

= .90

a. Interpret the estimated slope of each line.

b. Interpret the estimated y-intercept of each line.

c. Interpret the value of r2 for each line.

d. On the basis of these results, which type of timber blade appears to be stronger and more fatigue resistant?
Explain.