Sayfadaki görseller

necking down action, and the breaking load are noted. The broken specimen is passed around, it is handled and examined. They note that it is warm. They see the form of fracture and compare it with that of specimens pictured in the text-book. The diameter of the broken section is measured, and, finally, the broken ends are placed together and the elongation in lengths of

[graphic][ocr errors][subsumed][ocr errors][subsumed][subsumed][subsumed][merged small]

2, 3, 4, 5, 6, 7 and 8 inches is measured with a pair of dividers, always keeping the broken section as near the middle of the measured length as possible. The test of the other specimen is then run through rapidly, noting all the results the same as for the first. A neatly-written report of these tests with computed significant results, and a diagram (Fig. 2), showing the variation in percentage of elongation when based upon different lengths, is required as a part of the next lesson; and the broken specimens find a place upon the class-room table where they remain within easy reach, to be appealed to most effectively in many a class-room discussion in the weeks to come. These simple tests, readily carried out within the limits of the hour, have kept the little group of students keen with interest, have thrown new light upon many points discussed in the text, have appealed to the eye, the ear, and the sense of touch as no amount of class-room discussion

[graphic][merged small][merged small]

could; and, finally, have added a little of that time element often necessary to the full comprehension of a new subject.

The third week finds the class considering the behavior of materials under compressive stress. Accordingly, the laboratory period is devoted to tests in compression of wrought iron and mild steel, as types of ductile materials and sandstone, sand-lime brick, and cast iron as types of brittle materials (Fig. 3). The wrought iron and mild steel specimens are cut from the same bars as the tensile specimens (Fig. 1) tested the previous week, thus enabling a comparison of the yield points in tension and compression.

After this preliminary study of definitions, prin


l'ig. 4.

ciples and materials, the class is ready for the more specific consideration of each material by itself. It must not be forgotten in this connection that nearly all our computations regarding stresses in materials are based upon the principles of elasticity with stresses below the elastic limit. It is not sufficient, then, to consider only ultimate strengths. The elastic limit, and the more or less perfect elasticity of the material, as well as its behavior throughout the whole range of stress, should be made the subject of careful study. The student needs to be brought face to face with these principles and qualities, again and again, until they become a part of his unconscious possession, guiding his judgment, giving form to his expression.

Timber seems to offer many advantages as the material first to be considered. It is one of the most common materials of construction. The specimens are easily prepared and cheap. The deformations under stress are relatively great, and, therefore, readily measured. The strengths and elasticities vary in different directions with reference to the grain, thus requiring fuller investigation as well as clear ideas as to direction of stress. The strengths are subject to many variables, and yet, by controlling these variables, or by recognizing their presence, consistent and highly instructive results may be obtained.

Beginning the subject of timber, the first laboratory period is devoted to the examination of the minute structure of wood as revealed by the microscope in transverse, radial and tangential sections of the common hardwoods and softwoods; this being the basis for explaining so many of its strength, grain and shrinkage

properties. For the strength tests, a clear, straightgrained, well-seasoned, white pine plank, 16 inches broad and 13 inches thick, having a little sap-wood in two outer corners, was used (Fig. 4). Specimens from this plank were tested under three conditions of moisture, viz.: “Dry,” specimens dried over a boiler one week; “Normal,” specimens tested as cut from the plank; and “Wet,” specimens kept under water one week. There being six laboratory divisions, the plank was cut so as to give six endwise compression specimens in its width (Fig. 4). Thus, specimens Nos. 1 Dry, No. 7 Normal, and No. 13 Wet, representing the three moisture conditions, and all from similar positions with respect to the center of the tree, were tested in the first laboratory division (Fig.5). The next three specimens, in the second division (Fig. 6), and so on (Figs. 7, 8, 9 and 10). These three tests, with full data as to loads and shortenings, were easily finished within the hour. The shortening was measured with a Brown and Sharpe test indicator as shown in Fig. 11. This instrument reads directly to thousandths of an inch, but ten-thousandths may be estimated with fair degree of accuracy.

The method of measuring between compression blocks proves sufficiently accurate if a small initial load be applied before taking the first reading

In same manner, during the laboratory periods of the following two weeks, the remainder of the tests (Fig. 4), consisting of across grain compression (Figs. 12, 13 and 14), endwise tension, across grain tension, and along the grain shear, were completed; each student following a strip of the plank throughout its length through all these tests.

« ÖncekiDevam »