and at maximum load or failure. Sketch form of frac

ture.

Comparison of results with standard values. instructor.)

(See

COMPRESSION OF MATERIALS.

(Using Compressometer.)

(See also instructions 6a.)

Object.-In addition to determining the maximum strength in compression, as in other compression tests, it is intended in this experiment to find the strength at elastic limit, the modulus of elasticity, the modulus of elastic resilience and total rupture-work.

Operations in Testing.-Proceed as in other compression tests except that the load is applied in increments of pounds (about one twentieth of the probable maximum load) and the total amount of compression at each increment is measured by a compressometer. (Use slowest speed of the machine.)

Computations.-Plot points with load in pounds for ordinates and compression in inches for abscissæ. If possible draw a straight line averaging the points preceding the more rapid compression; and, tangent to this straight line, draw a smooth curve averaging the remaining points. (Consult instructor before inking in these lines.) Mark the points of maximum load (which latter is the point of tangency of the straight line and the elastic limit smooth curve) by x. Then draw a line through the origin parallel to the straight line previously drawn through the plotted points. (Do not continue this line parallel to plotted curve.) the point of elastic limit on corrected line.

Mark

The modulus of elasticity is calculated from the

formula E = where P and λ are the load in pounds FA

and compression in inches respectively, for any point on the corrected line; F is the square inches of crosssectional area of the specimen, and I is the gauge-length in inches of the specimen.

The moduli of elastic resilience and of rupture-work are the work done on each cubic inch of material in deforming it up to the elastic limit and ultimate strength respectively. These moduli may be obtained from the curve of plotted points by multiplying the area under the curve (up to the point considered) by the scale value of each unit area of the coordinate paper, and dividing the result by the volume of the specimen. This will be the modulus of elastic resilience or the rupture-work.

Report.-The report will contain (1) a brief and clear statement of the kind and condition of materials tested, (2) the methods of applying the loads and measuring deformations, (3) any peculiarity in the behavior of the specimen, and (4) a comparison of the properties of the material as obtained from the test with values recorded in reference books, etc. The character and form of fracture should be described and also shown by sketch.

The report will also contain a tabulation of results.

FLEXURE OF WOODEN BEAMS.

The purpose of the experiment are:

1. To obtain knowledge of strength and method of failure of materials.

2. A comparison of results with theoretical laws of flexure.

3. Practice in computing strength of beams. Preliminary.-Material will consist of three sticks of wood: (a) 2x 2 x 36 inches; (b) 2x4x36 inches; (c) 2 x 2 x 18 inches.

1. Note the serial numbers or marks on each specimen. 2. Measure and weigh each specimen and count the number of annual rings per radial inch.

3. Make sketches showing end views with direction of annual rings, sap and heart wood. Note any defects such as knots, season checks, crooked grain, rot, etc. 4. Mark the center lines and span lengths.

5. Place the beam upon the knife edges, using steel strips to prevent knife edges from crushing the wood, and rollers to prevent chain action. Balance the machine and apply a small initial load and place a deflectometer under the center of the beam or else hang a special deflectometer on pins in the neutral axis over the knife edges. For a considerable deflection a wire stretched between these pins and a scale attached to the beam at center will suffice. Bring wire to coincide with its image in scale to avoid parallax.

The Test.-Compute the probable breaking load and apply this in about twenty increments and read the total deflection at each increment. If care is exercised in keeping the beam balanced near point of failure, it will be possible to get the correct load and deflection for failure even though this does not take place at one of the regular load increments. Note the nature of the failure of the beam and sketch the failure.

Working up the Data.-(1) On section paper plot points using load in pounds for ordinates and deflection in inches for abscissæ. Use a separate sheet for each

curve. Choose scales such that the slope of the diagram near the origin shall be about sixty degrees. Each division of the paper must represent a decimal increment. Plot each point plainly. Draw a straight line averaging those points which precede the more rapid increase in deflection. Tangent to this line draw a smooth curve averaging the subsequent points. Mark on the curve the elastic limit and the load at failure, i. e., the maximum load. If the straight part of the curve does not pass through the origin, draw through the origin a dotted line parallel to the first. This line will represent the relation between the load and corrected deflection.

(2) Compute (a) the modulus of elasticity E by use of formula 4, page 254, Church's Mechanics of Engineering (d and P are taken from any point on the corrected line). (b) R", the fiber stress at elastic limit. (c) R, the fiber stress at rupture. (d) U", the modulus of elastic resilience. (e) U, the modulus of rupturework. To compute the modulus of resilience at elastic limit or rupture, divide the inch pounds of work done upon the beam up to that point by the volume in cubic inches of the part between the supporting knife edges. Also find the values of the exponents x, y, z and w in the following equations

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