Egyptian obelisk engineering mystery (Al Bakun / Wendell Wilkenning fracture analysis)

Wendell W. Wilkening's 1976 MIT PhD thesis. With 1976 knowledge of stone toughness, an obelisk being raised from horizontal should fracture under its own weight from natural flaws. Wilkening found micro-cracking ahead of the crack tip explains how it didn't.

[Tom produces a granite fracture toughness specimen.] This is a fracture toughness specimen I retrieved from Wendell W. Wilkening, doing his PhD thesis here in 1976 when I came back on the faculty. He was trying to understand why the Egyptian obelisks were able to be raised up. Obviously they didn't cut them to shape in the vertical position — they hauled them out of granite in the horizontal position, and they had to raise them up. In 1976, with our current knowledge of the toughness of stone — granite is a composite of three different types of minerals, and you can see them, white, gray, and black, in that particular granite — the current knowledge was that if you tried to raise it up like this it should fracture under its own weight with just the natural flaw sizes there. What he found was that you get micro-cracking ahead of the crack tip. That's got a slot cut in it; you grab it by the two holes and you pull it. We do the same thing with metal specimens.

Mid-1970s MIT thesis (Wendell Wilkening under Al Bakaven). Fracture mechanics predicts the obelisk should snap under its own weight during raising; reality is the three-phase granite microstructure forms toughening microcracks at flaw tips, giving 2–3× the toughness predicted from single-phase rocks.

This is a fracture toughness specimen made out of granite. I got this the first week I was here as an assistant professor, from Wendell Wilkening who was finishing his doctoral thesis. Al Bakaven was actually one of the brighter faculty members I ever knew here, but he was sort of quirky. Al Bakaven was curious about how the Egyptians were able to build the obelisks.

A 1970-era fracture mechanics calculation said obelisks couldn't be raised intact. Tom's office-mate (Wilkening, working under Backofen) discovered microcrack toughening at the crack tip in three-phase granite (silica, feldspar, third phase), giving 5-10× higher toughness than expected. Resolved the engineering paradox and seeded 1980s tough-ceramics research.

[Tom hands a granite fracture-toughness specimen around the class.] When I first took over as an assistant professor here, in the little office around the corner, 8-137, I shared it with a graduate student who was working on the fracture toughness of granite. This is a fracture toughness specimen — you can pass it around. No one had really looked at fracture toughness of granite. Why was he looking at it? Professor Backofen, who taught mechanical properties of materials in course 3, was interested because if you actually did some calculations estimating the toughness of granite and other stones, there was no way to raise the Egyptian obelisk. One of the engineering marvels is the Washington Monument, which they actually built going straight up. But it had always been assumed that the Egyptians carved the stone, because their obelisks were one monolithic piece of stone. The Washington Monument is a bunch of bricks. If you did the calculation you would predict that the bending load would break the obelisk in two when you started to lift it up.