Wind turbine blade stress. Mechanical engineers at the University of Newcastle (Australia) investigated the use of timber in high-efficiency small wind turbine blades (Wind Engineering, January 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: y^ = 97.37 - 2.50x
Hoop Pine: y^ = 122.03 - 2.36x
(a) Interpret the estimated slope of each line
(b) Interpret the estimated y-intercept of each line
(c) Based on these results, which type of timber blade appears to be stronger and more fatigue resistant? Explain.