A new testing machine has been developed at Northwestern University that will make an artificial leg "walk" at the rate of 3,000,000 steps in ten days--the equivalent of three years of normal wear.
The machine, called a fatigue tester, will permit faster testing and development of improved artificial limbs for this country's 20,000 veterans and 65,000 war workers who have had amputations.
A 300-lb. load, one and a half times that of the average man, is shifted continually from heel to toe of the foot of the artificial leg at a rate of four complete cycles per second--which means that the leg actually is "running" as fast as the swiftest athlete. Thus the amount of punishment it would take over a period of years is concentrated into a few days or weeks.
In one test the durability of artificial leg fittings used at U. S. Army amputation centers was demonstl"ated when a leg with the fittings completed 7,190,000 steps, or three weeks of continuous high-speed walking, without failing. This was equivalent to seven years of normal wear, which is computed to be one million steps per year.
Knee and ankle mechanisms of light metal alloys may be the parts most susceptible to wear, as evidenced by tests of another model in which the plastic shank remained intact after 4,800,000 steps, but the metal knee cracked after 600,000 steps and metal bearings in the ankle after 410,000 steps. An experimental plastic limb now under test has under~one 5,300,000 steps without failure.
Electrical wire strain gages that translate pressure into measurable electric current are attached to various parts of the limb to record the exact pressure and tension in each part. Although the over-all load is 200 to 300 lb., some parts of the knee and ankle mechanisms are so small that pressure concentrated on them is sometimes as high as 9,600 psi. Tl~e load on other parts, tests showed, is as low as 4 psi. A major problem is to design the limbs so that pressure is more equalized.
Preliminary tests indicate that a combination of riveting and bonding (plastic cementing) is the strongest means of connecting joints and that new plastic parts show promise of great strength. No plastic part of any limb has failed yet in the fatigue-testing machine.
In addition to testing, the Northwestern research group is studying all literature and patents on artificial limbs as far back as 1863. Their findings, which will include reports on legs, hands, hooks, and harnesses, and other suspension devices, will be presented in the spring to the National Research Council's Committee on Artificial Limbs. R. H. OPPERMANN.