CHAPTER 5 Testing Procedures.

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Presentation transcript:

CHAPTER 5 Testing Procedures

5.1 Types and purposes of mechanical tests One of the important considerations in the design of a structure is strength, a characteristic that enables the device to serve its function safely and well. Strength refers to the ability of a structure to resist load without failure, which may occur by rupture due to excessive stress or may take place owing to excessive deformation. The properties of materials that are of significance are the mechanical properties.

Test Procedures In order to approximate the conditions under which a material must perform in service, a variety of test procedures are necessary. The relation between different test procedures can be made evident by an orderly classification of test variables. loading methods Test Variables test conditions

Loading Methods Mechanical tests are classified according to the method of loading: the type of stress induced, the rate at which the load is applied, the number of times the load is applied.

Type of Stress Induced tension, compression, direct shear, torsion, flexure In tension and compression tests, an axial load is applied to a test specimen to obtain uniform distribution of stress over the critical cross section

In direct shear tests, one attempts to obtain uniform distribution of stress, but this ideal condition is never satisfied in practice. Because of: the way the shear stresses are developed within the body under direct shear loads, incidental stresses set up by the holding devices.

Torsion tests have an advantage over direct shear tests in that strains can be determined by measurement of the angle of twist. In flexure tests, deflections are measured directly, and the modulus of rupture can be calculated.

Rate of applied load-1 Static Tests: the load is applied over a relatively short time and yet slowly enough that the speed of testing can be considered to have a negligible effect on the results. For compressive strength testing of concrete, this rate is ~ 2 kgf/cm2 /sec Such tests may be conducted over periods ranging from several minutes to several hours. The majority of the tests fall in this category.

Rate of applied load-2 Dynamic tests: The load is applied very rapidly as by striking a blow, the test is called an impact test. Long time tests: The load is sustained over a long period, say months or even years, the test is a of which creep tests are a special type.

Number of times the load applied 1. A single aplication of load; this grup includes most of all tests made 2. The load is applied several times- sometimes a millions of times. The most important category of tests in this group is the endurance or fatigue tests.

Test Conditions-Temperature The majority of tests are carried out at normal atmospheric or room temperatures. Some tests are carried out at very low temperatures, in order to determine the properties of materials, such as brittleness of steel. Some tests are carried out at temperatures higher than ordinary temperatures. These tests may be done in order to determine some properties of materials at elevated temperatures such as determining the fire effect on some materials. Elevated temperatures may also accelerate the chemical reactions, thus, decrease the time required for testing, such as alkali aggregate reaction testing conducted at elevated temperatures.

Test Conditions-Moisture Conditions The mechanical properties of some materials are affected by moisture conditions. e.g., the strength of materials such as concrete, brick, stone, and wood is markedly influenced by the moisture in the material. The standard tests on concrete are made in a saturated condition (SSD), whereas those on brick are made on oven-dry specimens. Tests on wood may be made on specimens either in a green or in an air-dry condition, but the moisture content at the time of the test is always determined. Known moisture conditions are required for a standard test so that the test results obtained by different operators will be comparable.

Test Conditions In research investigations, the procedure in finding the effect of given variables is to maintain all conditions constant except those under investigation. In designing, conducting, or reporting tests, significant test conditions must be specified, controlled or known.

5.2 Selection and Preparation of Test Specimens Specimens must be selected and prepared so as to give reliable indication of the properties of the materials or parts they represent. RANDOM Many specifications for particular materials contain sampling requirements.

Selection of Test Specimens In testing material from metal plate, the direction of rolling is an important parameter. With specimens that are molded (such as concrete, mortar, rubber and plastics), care must be taken that molding conditions do not cause defects in the specimen. Curing conditions may also influence the results of the tests. Specimens of stone (or aggregate) should be selected with regard to homogeneity of the deposit, as well as with regard to the direction of the bedding planes. Specimens of wood should be selected with due regard for the direction of the grain, the density of the wood as influenced by the rate of growth, the proportion of springwood to summerwood and the presence of defects.

Preparation of Test Specimens-1 In preparing metal specimens, if a rough “blank” is taken by shearing, punching or flame-cutting, make sure that the finished specimen does not contain any of the damaged metal. Care should be taken not to bend the piece, because working of the metal tends to change its properties. Specimens of concrete, mortar etc. have to be molded while in plastic state. The sawing, coring or grinding of stone, concrete and ceramic materials should receive attention with respect to flatness and squareness of bearing surfaces and adequate smoothness of lateral surfaces. Wood specimens should be sawed, planed, or turned in such a way as to avoid defects on critical surfaces.

Preparation of Test Specimens-2 The size of the finished specimen is generally governed by the size of the piece from which it is taken and by the capacity of the testing machine available to test it. In some cases, the homogeneity or uniformity of the structure of the material may dictate the size of the finished specimen. e.g, the diameter of a concrete specimen should be 3-4 times the diameter of the largest aggregate used. Attention should be given to the marking and identification of test specimens and relating the test specimens to the lot of the material they represent.

5.3 Testing Apparatus The selection of apparatus for a particular test involves considerations of: 1. The purpose of the test, 2. The accuracy required, 3. Convenience or availability, 4. Economy.

5.4 Measurements Some of the basic quantities of materials to be measured are; lengths, pressures, volumes, angles, masses, time intervals, forces, temperatures, electric currents, voltages and resistances

Calibration The error in the readings of a measurement instrument is controlled by a process called calibration. The true value is obtained through; some fundamental method of measurement, by comparison with readings of an instrument of known accuracy. Calibration of a device accuracy

Sensitivity & least reading presicion Sensitivity, is the smallest value of the quantity to be measured that induce a response on the indicating device of the measuring instrument. An instrument that requires a relatively large change in magnitude of the quantity being measured is said to lack sensitivity. The least reading is the smallest value that can be read from an instrument having a graduated scale. Sensitivity & least reading presicion A sensitive instrument may not be necessarily accurate.

ERRORS Systematic errors Accidental errors

Systematic Errors Systematic errors are illustrated by the speedometer of a car that indicates 90km/h whenever the car is actually traveling at a speed of 87 km/h. Some are natural errors, such as will occur when corrections for the influence of the thermal expansion on the length of a steel tape are not made, or when abnormal humidity conditions influences the instrument or the specimen. Instrument errors are caused by constructional imperfections of an instrument, such as poor optics of a microscope. Personal errors are slowness of reaction that may cause the observer to push the button on a stopwatch a bit late

Accidental Errors Accidental errors tend to fall randomly on the plus or minus side of the correct measurement. They are generally caused by the inability of the observer to match the precision of an instrument. Even if we have a perfectly accurate ruler, it would be impossible to obtain the same length of the brick with a precision of 1 mm.

Systematic vs. Accidental Systematic errors are often cumulative and care is needed to detect and reduce them. Accidental errors are self-compensating if a series of measurements are taken. In contrast to systematic errors, accidental errors cannot be corrected. Instead, statistical methods are used to analyze them. e.g, we might measure the brick 10 times and average the lengths, expecting to obtain a more reliable result. Outlier in a series of measurements may also be the result of an accidental error.