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P" and P" are loads at elastic limit.
d and d, must not be taken beyond the elastic limit, and must be taken from line of corrected deflections.
Compute the weight per cubic foot and the specific gravity, assuming that the per cent. of moisture on the basis of dry weight is 12 per cent.
The Report.-Give a brief description of the method of conducting the experiment. On following pages present the measurements, etc., of the beams; the sketches of cross sections and fractures; the data obtained; the diagrams, and then the computed result in tabular form.
EXPERIMENTS IN TORSION.
Object. The object of this experiment is to study the behavior of materials under torsion, and to obtain such data as will enable the shearing strength of the material and its modulus of elasticity in shear to be computed.
Material.— The material is to be supplied by the instructor, and may be steel, iron or wood, or other material.
Preliminary.-Carefully measure the dimensions of the cross section and the gauge length. Then adjust the specimen in the heads of the machine, being careful to have the jaws clamped tightly against the specimen, which should be fixed in the axis of rotation of the machine. Then apply the torsion troptometer to the specimen and adjust the clamps of the latter so that the center of the circle of the graduated arc will be in the axis of the machine. Apply a small initial moment of about one hundred inch pounds and set to zero the graduated arc, and also the permanent scale on the twisting head of the machine.
Experiment.- Apply the loads continuously in increment of -- inch pounds. Read on the graduated arc the movement of the pointer in inches for each increment. When the increase in the angle of torsion is found to be rapid, the elastic limit having been reached, the graduated arc and index should be removed.
If only the elastic properties of materials are to be determined the specimen may be removed. Ordinarily the tests are to be continued until the specimen is ruptured. The whole angle of the twist is read from the fixed scale on the movable head of the machine and should be read for even loads above the elastic limit. The scale should be kept balanced and the maximum load determined.
Computations.- Plot diagrams to suitable scales with the twisting moment in inch-pounds as ordinates and the angle of twist in degrees as abscissa. One of these curves will be drawn with the magnified abscissa and will show the points up to the elastic limit. The other curves will be on a small scale and will show the anglemoment diagram up to the rupture. As in other experiments, the straight line portion in the beginning should pass through the origin. If it does not, a straight line parallel to the straight line passing through the plotted points should be drawn through the origin and terminating at the elastic limit.
Mark the points corresponding to the elastic limit and maximum load on the curve. Compute (1) the shearing strength developed at the elastic limit and the maximum load, using formula
Calculate the modulus of elasticity in shear from the formula
aPs Ip* Pa
1 Using the coördinates of any point on the corrected curve of the magnified scale.
Compute also the modulus of elastic resilience and the modulus of rupture-work.
Report.— The report will contain (1) a brief description of the specimen and method of test, (2) a tabulated statement of the computed and observed results (rule a form to suit the requirements), (3) a description of the fractured specimen, (4) plotted curves on coördinate paper, (5) an ink copy of the running log of loads and deformation.
INSTRUCTIONS FOR TESTING CEMENT.
(Short Course.) The work will include two days in the laboratory, and during this time the student will determine the time of setting, the fineness on No. 100 and No. 80 sieve, and the strength of three to one mortar in the case of a standard brand of Portland cement.
Time of Setting.–Mix up neat cement with enough water to render a paste of such consistency that when placed on a glass plate it will retain its form, and at the same time by striking the glass against the hand the paste can be spread out without cracking on the suface.
* See Church's Mechanics of Engineering.
Trowel the paste on the glass to thin edges, and set the pat thus made aside in a damp chamber, and observe the time that elapses until (1) it will bear the quarter pound standard needle without appreciable indentation; (2), the weight of the one pound standard needle without appreciable indentation.
Tests for Strength.- Mix up the mortar consisting of three parts standard crushed quartz stone and one part Portland cement, proportion being taken by weight. Mix the sand and cement thoroughly until the mixture presents a uniform color. Gauge with about eight per cent. of water and work over the mixture thoroughly about six times. Tamp in molds in layers, and finish the surface of the briquette. Record the initials on one corner of the briquettes, and leave the briquettes in the molds. They will be taken therefrom by the instructor and placed under water after twenty-four hours.
SECOND DAY's WORK. At the laboratory period one week following, the briquettes are to be taken from the water and tested for strength.
Tests for Fineness of Grinding.–Sift about four ounces of cement through No. 100 and No. 80 sieve and weigh the residue left on the sieves.
The Report.- Each student should report the results of the tests made by him as an individual to determine the fineness of grinding, the time of setting, and the strength of the five briquettes. Reports should be made on blank forms provided for that purpose.
Special instructions are issued to civil engineers.
AN ELEMENTARY COURSE IN PROPERTIES OF
BY GEORGE L. CHRISTENSEN, Assistant Professor of Mechanical Engineering, Michigan College
Instruction in the properties of engineering materials should have the following objects in view:
1. To illustrate the behavior of materials under stress.
2. To establish clear and definite conceptions as to the meaning of such fundamental terms as elastic limit, yield point, ultimate strength, percentage of elongation, modulus of elasticity, resilience.
3. To familiarize the student as far as possible with the methods by which materials are tested to obtain numerical results indicating their qualities.
4. To fix in the memory a few of the average numerical values for the more common materials such as cast iron, wrought iron, steel, timber, stone and concrete; thus establishing a very convenient mental standard of reference for the more detailed study of these and kindred materials, and at the same time forming the basis for that ready judgment so essential to the engineer.
5. To illustrate the use of these numerical values in simple problems of designing, thus associating and connecting the material itself, and the mathematical considerations involved, and laying the foundation for that habit of thought which must ever recognize the material in its various strengths and elasticities, in every problem of design.