MEASUREMENTS ON FINGERNAILS 421 crosshead speed of 0.2 in./min with the machine calibrated to measure forces in the range from 0 to 20 lb. In these initial tests, a total of 196 samples were tested. The tensile strength values determined from these samples ranged from 4,464 to 17,081 lb/in. 2 TEARING TEST APPARATUS The objective of the tearing test is to simulate the action that occurs when a fingernail is torn. In order to gain a better understanding of the mechanisms involved, a "living" fingernail was torn in situ, and the results were carefully observed. As a result of the in situ tearing experiment, we concluded that the action involved closely resembled that of a piece of paper being held in both hands and torn. Once the initial break has occurred, the stresses concentrate at the point of failure and propagate the tear along the path of least resistance. In order to translate this action to a physical test, it was evident that the clamping jaws must be free to rotate so that the stresses could follow the line of the tear. This was achieved by attaching the jaws by means of a pin whose axis coincided with the edge of the nail where the tear would originate. Figure 5 shows the clamping jaws with i jaw open to reveal the clamping "teeth." During the actual test, stress is applied to the edge of the nail until failure begins to oc- cur. As the test proceeds, the jaws rotate concentrating the stress at the point where failure is occurring. Figure 6 shows the apparatus modified for tearing tests. The tests were conducted at a crosshead speed of 0.05 in./min with the machine calibrated to measure forces in the range from 0 to 20 lb. TEARING TEST EXPERIMENTS Specimens for the tearing experiments are prepared in the same way as those em- ployed for the flexural strength test. In this case, however, there were no standard Figure 5. Clamping jaws used to secure tensile and tearing specimens for testing

422 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 6. Apparatus for performing tearing tests on fingernails methods to use as a guide, and we were forced to develop our own criteria for sample evaluation. Before beginning the test, each fingernail specimen was carefully measured. The sam- ples were then subjected to stress until failure occurred, and the force necessary was recorded. The results were plotted as tearing resistance in pounds per inch of nail thickness. A total of 15 fingernail samples were tested, and the values obtained ranged from 274.3 to 676.2 lb/in. IMPACT ABSORPTION TESTER The objective of the impact absorption test is to determine the effect of the impact of a swinging pendulum on a rigidly clamped sample. As with the previous tests, we found