I just started reading a book about history of the jumbo, called "Boeing 747 - design and development since 1969" and it says:
"The third and sixth structurally complete 747s never flew but became the static- and fatigue-test airframes, respectively". Static, or "torture", testing began in early 1969 with the airframe being pulled, twisted, and stretched to see if anything "gave".
Finally, in Feb 1979, the starboard wing was deflected upwards 29 feet and snapped when stresses reached 116.7 percent of ultimate design load. [...] The fatigue airframe, located in an outdoor rig, began simulated flight cycles at the beginning of 1970"
"If the Wright brothers were alive today. Wilbur would have to fire Orville to reduce costs" (H. Kelleher)