EYE-BOLT STRESSES AS DETERMINED BY PHOTOELASTIC TEST 250068
Principal stresses in one type of eye-bolt have been determined in the laboratory of photoelasticity at the Massachusetts Institute of Technology by the photoelastic method. In the test, an eye-bolt, designed in accordance with a method suggested for circular eyes in a course in machine design by the Institute, was made of celluloid 0.25 in. thick, 1 in. wide on either side of the eye, with a 1.405-in. diameter of eye, and a 1.333-in. width of shank. Steel loading-plates were pinned to the broadened end of the shank and a load of 100 lb. was suspended from the bolt, which gave a mean stress of 300 lb. per sq. in. in the shank. Plain polarized light was passed through the celluloid model and the isoclinic lines, or lines of equal inclination of principal stress, were observed and recorded. Two families of lines of principal stress, designated as P and Q stresses, were determined graphically from these isoclinic lines. The difference in these principal stresses was determined by circularly polarized light, the transparent model being placed in suitable apparatus and loaded and the image on the screen becoming highly colored. These color bands are isochromatic lines that indicate an equal value of difference of the principal P and Q stresses.
For a complete determination of the stress, it was necessary to record the isochromatic lines and to measure the value of the difference of principal stress at a sufficient number of points in the model. The isoclinic and isochromatic lines, the lines of principal stress and the points at which the value of the difference in principal stresses were measured, are shown in drawings. Values of the principal stress at several sections through the model are plotted.
The test shows that the maximum stress of 862.44 lb. per sq. in. occurs at a section on either side of the eye at the inner boundary 0.115 in. below the line from the center of the eye to the outer boundary of the bolt perpendicular to the axis of the shank. The principal tension and compression stresses per square inch at many points in the model under an applied load of 100 lb. are given in tabular form, in which the P and Q stresses along the interior boundary and at the sections of maximum stress are also given separately.
Conclusions deduced from the test are (a) that, in an eye-bolt of the type tested and bearing on a pin that fills the eye, a maximum stress equal to 2.87 times the mean stress applied to the shank must be expected at the interior boundary of the eye and (b) that the type of pin or support used will affect the stress distribution considerably, so that, in subsequent tests, it would be advisable to reduce the pin diameter successively, observing the effect upon the stress.