Cortical bone gets weaker with aging, but it turns out that diaphyseal diameters can also change with aging, particularly in males, such that the safety factor of the diaphysis is approximately constant with
aging.The safety factor SF is defined as the failure stress divided by the functional stress for the bone tissue on the periosteal surface. If the tissue tensile strength of the cortical bone is decreased by 80 percent of its original (“young”)  value with aging, what is the change in periosteal diameter required to keep the SF constant at the periosteal surface?