CHAPTER 5
Testing Procedures
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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.
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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

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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.
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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

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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.
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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.
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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.
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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.
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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 timessometimes a millions of times. The most
important category of tests in this group is the
endurance or fatigue tests.
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Test Conditions-Temperature


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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.
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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.
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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.
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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.
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Selection of Test Specimens

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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.
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Preparation of Test Specimens-1
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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.
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Preparation of Test Specimens-2

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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.
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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.
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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
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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
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Sensitivity
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.
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ERRORS
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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
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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.
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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.
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