again rise and should be kept floating up to the maximum load. At this maximum load the bar begins to neck in, the material becoming plastic at the point of the formation of the neck. Record in the report the yield point and the maximum load. IV. Measurements after Tests. Lay the broken ends of the bar together and determine the increase in elongation of the gauge length. Measure the dimensions of the fractured area. Determine the rate of descent of the pulling head of the machine. Describe the appearance of the fracture and determine the distance from the extreme gauge point. V. Calculations. Calculate the ultimate strength. maximum load Ultimate strength original area Calculate the yield point. Yield point load at the yield point original area Calculate the per cent. of elongation and per cent. of contraction of area. Per cent. contraction of area original area-area of neck X 100 original area By reference to various specifications on file in the laboratory determine the kind and classification of the material in the specimen. In case the fracture is outside the middle third of the gauge-length, the per cent. of elongation is to be computed on the assumption that the elongation is sym metrical on each side of the neck. This should be noted on the report. VI. Report. Report consists in filling out the blank form supplied for the test (except as indicated by the instructor.) TENSION TEST WITH HENNING POCKET RECORDER. Object.—To determine the properties of material from load-elongation diagram drawn by apparatus. Preparation.— Ascertain magnification of the lever in the apparatus. Set the lever stop with a clearance of 0.03 inches. Measure and lay off specimen as for a regular tension experiment. Place the test bar in the wedges of the machine and adjust the apparatus under the direction of the instructor. Wedges must be blocked to prevent their flying out upon rupture of the specimen. Experiment.-Run out the poise by steps of 4,000 pounds and allow the apparatus to draw a scale of loads on the card up to 50,000 pounds. Apply the load with the intermediate speed, keeping the beam floating until the specimen is broken. Note the load in pounds at the yield limit and the maximum limit. Computation. The card is completed by drawing the elongation axis through the zero of loads and at right angles to the load axis, and also a straight line from the rupture point perpendicular to the elongation axis. The load at yield limit and maximum limit are scaled from the card and checked by observations taken during the test. The per cent. of elongation obtained by the measurements on the fractured specimen is to be checked by scaling from the card. The work of deformation up to any point may be obtained from the area in square inches under the curve up to the point in question by multiplying this area by the scale value in inch-pounds of each square inch of card. This product divided by the volume of the specimen between the gauge points will be the “rupture-work” in inch-pounds per cubic inch. Compare the value of resilience by the card with the values as obtained from the following formulæ: T? 2E per cent. elongation (T" + 2T'') Rupture-work 300 Where T" and T' equal the stress in pounds per square inch at elastic limit and maximum load respectively, and E is the modulus of elasticity, E cannot generally be accurately determined from the card. (See instructor for assumed value.) N. B.-Be sure to record on the data sheet the serial number of the specimen before placing it in the wedges of the machine. Make sure that the magnification of the lever is taken account of in all measurements involving this magnification. The beam must be kept floating throughout the experiment even after the maximum load has been reached. TENSION TEST WITH EXTENSOMETER. Object.- In this experiment the elastic properties of metals in tension are determined. Method.-Carefully measure and prepare each specimen as for regular tension test (note the serial number on the specimen). Grip the piece in the wedges by applying a load of about one hundred pounds (the machine having been previously balanced). Apply and adjust the extensometer, and (after having had the apparatus inspected by the instructor) proceed with the test. Apply load in increments of pounds per square inch (an increment sufficient to produce fifteen measurements of extension up to the elastic limit) and measure the total elongation at each load increment. After reaching a sudden and large increase in elongation, remove the extensometer and apply load until specimen is ruptured. Note the maximum load. Computations.- Construct a diagram with load in pounds per square inch as ordinates and elongation in inches per inch as abscissæ. Draw a straight line averaging the points up to the more rapid increase in elongation, and, tangent to the straight line, draw a smooth curve averaging the remaining points. Ordinarily, the straight line of plotted points will not pass through the origin. Draw through the origin a line parallel to the straight line of plotted points. This line represents the true relation between stress and strain. Mark the elastic limit x on the two straight lines at the value of the ordinate at the point of tangency between the straight line of plotted points and the smooth curve. The modulus of elasticity is the stress in pounds per square inch divided by the elongation in inches per inch at any point on the straight line through the origin. It is most convenient to select an abscissa of elongation of one part in 1,000, and multiply the corresponding stress by 1,000 to obtain the modulus of elasticity. The modulus of elastic resilience is the amount of work done on each cubic inch of the specimen in deforming it to the elastic limit. It may be taken as the area (in inch-pounds) under the straight line up to elastic limit; or it may be calculated by the formula {T"e" where T'" and e" are the stress and relative elongation respectively at the elastic limit, and must be measured on the line passing through the origin. Per cent. of elongation and of reduction of area are determined as in the simple tension test. Report. The report will consist of (1) tension test blanks (furnished by instructor) properly filled out, (2) plotted curves with titles and scales shown, (3) ink copies of running log. Mention the form of extensometer used. Notes.- The extensometers are delicate instruments and must be handled carefully. Any roughness of usage or lack of delicacy in manipulation will result in unsatisfactory diagrams. Be sure that the test bar is straight. COMPRESSION TEST OF VARIOUS MATERIALS. The purposes of this experiment are: To obtain knowledge of the proper methods of testing materials in compression; of the crushing strength of such materials; and of the characteristic forms of fracture. Preliminary.- (1) Before testing any specimen carefully measure its height and cross section. (2) When brick, stone, concrete, or cement specimens are to be tested they should be carefully bedded either with blotting paper or with plaster of Paris. To bed a specimen with plaster of Paris, have the testing machine balanced, and the head down so far that it will clear the specimen only about one inch or two. Then |