Sayfadaki görseller
PDF
ePub

6. To study the processes of manufacture, in so far as these give name to the product or modify its qualities.

7. To study methods of protecting and preserving materials under conditions of use.

The class-room instruction should be fundamental, keeping before the student the underlying principles and truths, and guiding him through the maze of experimental facts to habits of clear, discerning, independent thought; the laboratory, a means of illustration, going hand in hand with the class-room, supplementing, reviewing, clearing up, fixing the ideas. It is the purpose of this paper to outline briefly some features of the instruction in this subject as developed at the Michigan College of Mines under the increasing influence of this idea of using the laboratory as a means of illustration in conjunction with the class-room work. The course as given runs through twenty-three weeks, three recitations of fifty-five minutes each per week. The text-book is Johnson's “The Materials of Construction.” The class, numbering sixty students, is divided into three sections of twenty each for recitations, and each of these sections is again divided into two laboratory divisions of ten students each, making in all six laboratory divisions. During a portion of the course, the first recitation period of each week is set aside for laboratory illustration work, when, instead of the whole section meeting in the class-room, one of the small laboratory divisions of ten students meets in the laboratory, the other division taking some other hour of the same day. With this organization of the class it is possible to undertake a series of tests, the whole class being made familiar with the plan and scope, but each laboratory division doing only a small part of the work.

Beginning the course, the first week is devoted to a class-room consideration of definition of terms and the behavior of materials under tensile stress. In the laboratory period of the second week, tensile tests are made of two specimens both cut from the same wrought iron or mild steel bar, one being rough, one inch in diameter, and the other turned down to a smaller size (Fig. 1). The group of ten students gathers around the testing machine-there is room enough for all to note everything that takes place—and each student is

[graphic][subsumed][subsumed][merged small]

required to keep full records just as though he were making the test individually. This being the first test, the construction and operation of the testing machine is explained. The rough bar is tested first. It is marked off in the presence of the class, with punch marks an inch apart, and placed in the machine. As the test proceeds, attention is first directed to the detection of the yield point by noting the drop of the beam, then the breaking down action is watched as shown by the loosening of the mill scale, then interest centers in the rapid stretching of the bar, readily noted with a pair of dividers, and finally, in turn, the ultimate load, the 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][subsumed]

Fig. 2. 1 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][ocr errors]

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

[graphic][ocr errors][ocr errors][merged small]
« ÖncekiDevam »