Strain Measurements by Means of an 8-Inch Metzger Extensometer

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Technical Note (National Research Council of Canada. Division of Building Research), 1954-09-13

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Strain Measurements By Means of an 8-Inch Metzger Extensometer

•

Many methods of measuring strains are in use. Some of these make use of electrically operated gauges, often consisting of special

applications of the electrical resistance wire strain gauge; others make use of mechanic·al extensometers.

The purpose of this Note, however, is to describe one method only, namely the measurement of surface strains by means of an 8-inch Metzger extensometer, shown in Figure 1.

An extensometer, in principle, is a mechanical device for measuring the change in distance between two gauge marks, which usually

consist of small holes drilled into steel. The extensometer itself consists of two conical steel points mounted on a metal frame which carries a dial gauge actuated by a lever. One of the conical points is fixed, the other movable (over a range of 3/100 inch). This movable point is part of the lever which actuates the dial gauge with a lever ratio of 1:10. Therefore each unit on the dial gauge which is graduated in 1/1000 inch, theoretically represents a movement of the movable

point of 1/10,000 inch. However, a correction factor has been experimentally determined for this particular extensometer, which is 0.85. With a gauge length of 8 inches and the correction factor each graduation unit of the dial corresponds to a strain of

1

10000

1 -6

x ｾ x 0.85 = 10.6 x 10 = 10.6 micro inches/inch,

or in steel with a modulus of elasticity of 30 x 106 psi to a stress of 10.6 x 106 x 30 x 106 = 318 psi.

A gauge length of 8 inches is used in this gauge; other extensometers with gauge lengths from about 1 to 30 inches are

commercially available. The choice of a particular gauge length results from a compromise between a desire for the higher accuracy of a longer gauge and a desire for a more accurate measurement of local stresses in areas of varying stresses.

2

Drilling of the Gauge Holes

For good accuracy of the readings, the steel surfaces at the two measuring holes should lie in the same plane and the holes

should be exactly perpendicular to this plane. Figure 2 shows a

template for drilling ｧ｡ｵｧｾ holes at the correct distance

perpendicular to a plane surface.

In preparing a round steel bar for drilJ.ing of gauge holes,

two small areas, 8 inches npart, should be ground arfilErlflat anisarrled.

Then the template bar should be fastened firmly to the bar, usj.ng

C-clamps, wooden wedges or VJhatever may be found suitable. A shalloW

V-shaped groove at the bottom of the template bn.r is provided to assist in keeping the template in place.

The small holes are then drilled, using a small portable

electric or hand operated drill and a

#54

template bar, if firmly attached, will ensure that the two holes are parallel to each other and properly spaced to be within the measuring range of the extensometer.

Before taking the first rending, the edges of the gauge holes should be slightly bevelled with a centre punch having a cone

with an angle of 60 to 90 degrees. A pierced wooden block to hold

the centre punch perpendicular to the surface of the rod during this

operation is helpful. By bevelling a drill hole in this way, the burr

is removed and the edge of the hole is cut and thus a better bearing

is provided for the conical points Ｈ｡ｮｾｬ･ of

45

extensometer and the danger of Q ｰｏｾＸｪ｢ｬ･ ｯｾｮｮｧ･ of reading by wear

of the hole is reduced.

Teohnique of ｅｸｴｾ｟ｾｳｯｭ･ｴ･ｲ Readings

For the readings, the gauge holes must be clean. Metal chips,

dirt and grease can best be removed by using a round wooden tooth-pick

or the sharpened end of a wooden match. When taking a ｲＹｾ､ｬｮｧ the

movable point of the extensometer should be set in one of ｾｾ gauge holes,

then the fixed point in the other gauge hole. Apply a firm Pl'''<:lsure

(roughly 10 lbs.) on the extensometer, using one hand at each enn. This

pressure must be applied perpendicularly to the structure, without a

component parallel to the extensometer. By slightly rocking the

instrument back and forth at right angles to its axis, no change of

reading should occur if the gauge holes are perfect. If this is not

the case, special attention should be paid to taking the reading only when the extensometer is perpendicular to thA surfaces of the rod at the gauge hole s.

3

It is good practice to repeat every reading by removing and replacing the extensometer. Even to remove only the fixed point part way is helpful. The accuracy of strain measurements depends to a great extent on a person's ability to obtain consistent readings, that is, to obtain closely the same value when the readings are

repeated. It is recommended to take three or more readings every time and record all of them; this will permit to jUdge the quality of the readings at a later time.

After completion of the reading the gauge holes should be protected by a bit of heavy grease until the time of the next readings. Temperature Effects

When making ｳｴｾ｡ｩｮ measurements the strains due to temperature changes of the structure as well as the effect of the temperature

expansion of the extensometer itself are included in the readings. The Metzger gauge is made of steel (not invar) and therefore is quite sensitive to temperature changes. In cold weather the heat of the operator's hands is quite sufficient to change the readings noticeably and the operator's breath when directed against the gauge can also have a marked effect.

If the assumption is made that the extensometer has the same coefficient of expansion as the structure, no correction of the readings is necessary for temperature effects, provided the

extensometer is at all times at the same temperature as the structure. As the coefficients of expansion of different steels do not vary

appreciably and in addition are fairly close to those of concrete, this assumption can be justified in many cases. A "standard bar;' for reference readings should be used in all cases. The standard bar, to permit complete temperature compensation, must have the same coefficient of expansion and must be at the same temperature as the structure at each point of measurement. These conditions are some-times difficult to satisfy in a strict sense.

It is also important to use a standard bar to be able to correct for any changes in the effective length of the extensometer which might be caused by accidental bending of the conical points, or by unavoidable adjustments, or by repairs to the gauge.

The punch bar which is supplied with each extensometer can be used as the standard bar by taking readings on the holes drilled for this purpose near the set-screws. The bar should be laid on the structure near the measuring points some time in advance of the readings to allow sufficient time for it to assume the temperature of the

4

struoture this may be difficult to obtain. The standard bar should be supported in suoh a way as to minimize bending under the foroe applied to it while readings are being taken.

Under normal conditions (fairly oonstant and uniform temperature of both structure and air), it is suffioient to take a reading of the standard bar at the beginning and at the end of a series of readings, provided, of course, that the extensometer and standard bar remain at the same temperature as the structure. It is neoessary therefore to prevent the extensometer from being warmed up by the operator1s hands. For this purpose 1/4-inch sponge

rubber pads can be glued on both ends of the extensometer to avoid direct contact between the fingers and the instrument. A record of the air temperature at the time of measurement should be kept.

Evaluation

The correction of a strain reading for a standard bar reading differing from that taken at the time of the zero reading, is done by adding or SUbtracting the same amount as is required to change the standard bar reading to its original value.

After the change in gauge length from the time of the zero reading has been established, the unit strain is computed by dividing

the change by the gauge length and mUltiplying with the correction factor.

The apparent stress is then determined by mUltiplying the unit strain by the modulus of elasticity of the material on whioh the measurements are made.

Example Reading 168.3 standard 117.0 Differenoe Indicated Change

=

After application of load

116.1 38.4

13 x 10-4 in/8 in. gauge 13 x 10-4 x 0.85 x 106

8

Pigllre 1

a-Inch Metzger Extenaometer

.J W (!J t-q .,