Sayfadaki görseller

mix up some plaster of Paris and water to a very thick, creamy consistency. Spread a thin layer of this on paper placed upon the spherical bearing of the machine and cover it with a piece of tough sized paper, upon which the specimen should then be placed. The paper is to keep the water of the plaster out of the specimen. Upon another similar piece of paper a similar pad of the plaster should be spread covered with another piece of paper to form a pad, and the pad then placed upon the specimen. The head of the machine should then be run down rapidly until it presses upon the plaster sufficiently to cause it to flow, thus insuring a good bedding. With the trowels now fill up all the open spaces about the edges of the specimen near the faces of the machine. After letting the plaster set ten or twelve minutes, the specimen is ready to be compressed. The spherical bearing should also be used. Have the work inspected by the instructor before proceeding.

In the case of other materials see that the ends of the specimen admit of a good even bearing in the machine.

The Test.- Using the slowest speed available, now compress the specimen, meanwhile keeping the scale beam floating; and watch carefully the behavior of the specimen.

Computation.-Compute the stress in pounds per square inch at first crack, and at maximum load. Report.- Report should cover: Description of material. Method of test. Size of material.

Results. Results.-Load and crushing strength at first crack

and at maximum load or failure. Sketch form of fracture.

Comparison of results with standard values. (See instructor.)

(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.) Mark the point of elastic limit on corrected line.

The modulus of elasticity is calculated from the

PI formula E where P and 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 l 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.


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) 2 x 2 x 36 inches; (b) 2 x 4 x 36 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). (6) 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 pi h

Beams of equal length.
P. h

Beams of equal depth.
d h

For equal loads and equal lengths.
d h

For equal loads and equal depths.

= (..)


- (1)"

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