Recent developments in highspeed aircraft subject personnel to unusual shocks and impacts in ordinary flight as well as in accidents. This has made it important to develop safety devices for protection of the human body and to establish logical criteria for the design of these devices.
When it was realized that investigation of the problem involved close cooperation between the engineering and medical professions, the National Bureau of Standards was requested by the Naval Medical Research Institute to assist with the mechanical problems.
Because the human skeleton provides support for the body and protection for the vital organs, Bureau scientists decided to begin a study of the mechanical properties of human bones and joints. Results are to be used in the design of simplified mechanical models of the human frame that will make it possible to estimate the forces set up in the body during impacts of various types. Preliminary data indicate that bone may be considered an elastic, brittle material, having about one-fourth the compressive strength of cast iron and more than twice that of hickory wood.
The study was initiated with measurements of the mechanical properties of the structural members of the skeleton, beginning with long bones, and was designed to establish average values on a statistical basis. The initial work involved 17 tests on specimens made from bone of the compact type removed from near the middle of the bones of the extremities of humans and monkeys. Fourteen of these tests were made in compression with the direction of the load parallel to the fibers of the bone. One was made in compression with the direction of the load perpendicular to the bone fibers, one in bending, and another undertorsion load. Test specimens consisted of sections removed from the bones and of portions machined into plates and hollow cylinders.
The results of the tests showed the average ultimate strength of the 14 compression specimens to be about 23,000 psi. The fifteenth specimen, in which the load was applied at right angles to the bone fibers, had about the same ultimate strength, indicating that the effect of the direction of loading on the bone is small. The strength of the bone in bending, that is, extreme fiber stress at failure, was slightly lower than the strength in compression. The computed ultimate shear stress for the torsion to failure was about 15 per cent of the compressive strength.