Laboratory Manual of Testing Material

LABORATORY

MANUAL

OF

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LABORATORY

MANUAL

OF

TESTING

MATERIALS

BY

WILLIAM

KENDRICK

HATT,

C.E.,

Pn.D.

MEMBER AMERICANSOCIETYCIVILENGINEERS; PROFESSOROFCIVIL

ENGINEERING, ANDDIRECTOR OFLABORATORY FORTESTING

MATERIALS, PURDUEUNIVERSITY

AND

H. H.

SCOFIELD, M.E.

ASSISTANT PROFESSOROFCIVILENGINEERINGINCHARGE OF TESTING

MATERIALS, COLLEGE OFCIVILENGINEERING, CORNELL UNIVERSITY.

MEMBER: AMERICAN SOCIETY FORTESTING MATERIALS,

SOCIETY OFAUTOMOTIVE ENGINEERS

SFCOND EDITION

McGRAW-HILL

BOOK

COMPANY,

INC.

NEW

YORK:

239

WEST

39TH

STREET

LONDON:

6 & 8

BOUVERIE

ST., E. C. 4

COPYRIGHT, 1913, 1920, BY THE

MCGRAW-HILL BOOK

COMPANY, INC.

PREFACE

TO

SECOND

EDITION

The methods

of _{tests, specifications}

and

related data

have been revised

and

_brought

_up

to date. This has

been particularlynecessaryin thefield ofconcretewhere developments have been rapid in recent years.

It has beenthe intentionto

make

the

new

edition

use-ful in

more

than one institution

_by

_eliminating certain details of directions

and

_{apparatus which would}be

more

or less _{peculiar to}

_any

one laboratory. It is felt that the generalized directions _{while being} helpful as a guide

in class

work and

investigations, should also lead to a

more

_independentcharacterof

_{work upon}

the partof the student.

PREFACE

TO

FIRST

EDITION

This

manual

is the

outcome

of the _{operation, through}

eighteen years, of the Laboratory for _{Testing Materials}

of

Purdue

_{University. During} this time several

in-structors, temporarily serving the laboratory, have

improved its _{practice. Especial} mention should be

made

of the services of the late Professor Hancock,

whose

_{untimely death} deprived the science of _testing

materials of _{a very} able

and

patient investigator,

and

his _colleagues of a good friend. Professor

Yeoman,

now

of _{Valparaiso University,} did valuable service in the organization of the

work

of the cement _laboratory.

The

authors are indebted to Professor

Poorman

of

Purdue

_University, for valuable _suggestions.

Inits _original form the

manual was

published

by

the senior author of this

volume

; and later, with the

assist-ance of ProfessorScofield, the

manual was

enlarged,

and

was

found useful in several universities.

Now

the authors have availedthemselves of the assistance of the present publishers to _{enlarge the work.}

The

list of ex-periments has been increased,

and

a

more

complete treatment ofmachines

and

_{apparatus added.}

One

purpose of the

manual

is to relieve the instructor

from the_{necessity ofexplaining the}detailsofmechanical

procedure,

and

so to free histime for matters _{of greater}

educational importance. Itisalso_hoped

_by

the authors that the _practitioner will find the

volume

of convenient use.

LAFAYETTE, IND., August, 1913.

CONTENTS

PAGE

PREFACE

...

v

CHAPTER

I. GENERAL 1

CHAPTER

II. GENERALINSTRUCTIONS 4

Instructionsfor_WritingReports 7

CHAPTER

III. DEFINITIONS 10

Stress 10

Elasticity 12

Resilience 14

CHAPTER

IV. MATERIALS STRESSEDBEYONDTHEELASTIC

LIMIT 16

Fracturesunder Tension 17

Fracturesunder Compression 17

Fracturesunder Shear 19

CHAPTER

V. TESTINGAISDTESTINGMACHINES

...

20

Technical QualitiesofMaterials 20

Testing Machines 21

(a) Holdingthe Specimen 21

(6) MethodofApplyingLead 27 (c) Weighing Mechanisms 35 Extenso metersand Other Deformation Instruments . . 37

CHAPTER

VI. LIST OFEXPERIMENTS 44 ExperimentsforAdvanced Work 46

CHAPTER

VII. INSTRUCTIONS FOR PERFORMING EXPRRI

MENTS . 48

Article 1. _Testing Machines 48

A-l. _Studyof_Testing Machines 48

A-2. Calibrationof_Testing Machines 50

A-3. CalibrationofExtensometers 51

Vlll

PA.GK

Article2. Ironand Steel

...

52

B-l. Tension TestofIronand Steel

...

52

B-2. Tension TestofCastIron orSteelCastings. . 55

B-3. Autographic Tension TestofIron orSteel . . 50

B-4. Tension Test withExtensometer

...

57

B-5. ExperimentinTorsion

...

59

B-6. Testof Wire Cable

...

01

B-7. Compressionof_{Helical Spring}

...

02

B-8. EffectofOverstrainon Yield PointofSteel . 03 J3-9. FlexureTestofCast Iron orSteel

...

04

B-10. FlexureTestof Brake

Beam

...

05

B-ll. VibrationTest of_{Staybolt Iron}

...

05

Articles. TestofWood. _...

...

60

C-l. Instructions for Laboratory Exercise for the Identificationof Woods

...

66

C-2. CompressionofShort

Wood

BlocksParallel to Grain

...

67

C-3. Compressionof

Wood

_{Perpendicular}toGrain. 69 C-4. Testof

Wood

Columns

...

70

C-5. Flexure TestofSmall Wooden Beams

....

71

C-6. Flexure TestofLargeWooden Beams

....

73

C-7. ImpactTestof Wooden Beams

...

74

C-8. Abrasion Testof

Wood

...

....

75

Article4. TestsofCements

...

70

D-l. Testof Specific GravityofCement

...

82

D-2. TestofFinenessofGrinding

...

83

D-3. Normal Consistency

...

85

D-4. TimeofSetting

...

88

D-5. TestsforSoundness

...

87

D-0. Cement and Cement MortarsinTension

...

89

D-7. Cement and Cement Mortars in Compression 92 Articles. _StudyofAggregates

...

93

Noteson SamplingofAggregates

...

93

E-2. TestofSandforCleanness

...

90

E-3. Weightof_Aggregates . . . '

...

99

E-4. Specific GravityofVariousMaterials Used as an Aggregate inConcrete

...

102

E-5. DeterminationofVoidsinAggregates

....

105

E-6. Effectof Moisturein _{Aggregates on Per}Cent. ofVoids . . . 106

PAGE E-7. StudyofSieves 107

E-8. Sieve Analysisof_Aggregates 109

E-9. HandMixingof Concrete 113 E-10. _{Mixing Concreteby Machine} Mixer . . . .114

E-ll. Amountof Water Requiredfor_Mixing . . .115

Article 6. _{Proportioning} MortarsandConcretes . . . .119

E-12. _Theoryof_{Proportioning}Concrete 119 E-13. MixtureofFineand Coarse Material . . . . 121

E-14. _{Proportioning} Concrete by Method of Sieve

Analysis

...

122

K-15. Strengthin Relation to Density 127 E-16. SurfaceAreaof_Aggregates 128 E-17. Fineness ModulusofAggregates 130

Article7. TestsofConcreteandOther Brittle Materials 132

F-l. The ValueofaSandor _{Other Fine Aggregate} as ShownbyStrength Tests 132 F-2. CompressiveStrength ofConcrete 133

F-3. Compression Test Brittle Materials 134

F-4. Compression of Brittle Materials with Defor-mation Measurement 136

F-5. Reinforced Concrete

Beam

Test 137

F-6. BondStrength of Steel inConcrete . . . . .138

F-7. TestofConcreteReinforcing Fabric 139

F-8. Cross _Bending and _Compression Tests of

Building Brick 139

Article8. TestsofRoad Materials 140

G-l. RattlerTestofPaving Brick

...

140

G-2. _{Absorption Test} 142

G-3. Abrasion TestofRoad Materials 142 G-4. Cementation Test of Rock or Gravel or

MaterialsofLikeNature 143

G-5. Hardness Test of Rock Road Materials

Dorry Test , 145

G-6. _{Standard Toughness}TestforRoadRock. . . 146

APPENDIX

I.

COMMON

FORMULAS 148

APPENDIX

II. SpecificationsforMaterials 151

Steeland Iron 151

Methods for_{Metallographic}Testsof Metals.

...

155

x CONTENTS

PAGE A.R.E.A. Aggregates 159 Indiana State Highway Commission for Concrete

Roads ICO

APPENDIX

III. STANDARD FORMSOF TESTPIECES

...

163

APPENDIX

IV. STRENGTH TABLES 165

IronandSteel 166

Copper andAlloys 167

StructuralTimber 168

Stoneand Brick 166

LABORATORY

MANUAL

OF

TESTING

MATERIALS

CHAPTER

I

GENERAL

1.

The

student should obtain a_knowledgeofmaterials

by

handling

them and

watching their behavior under

stress.

From

the _appearance before

and

after test, he

is led to _{recognize the nature} of normal

and

defective samples. This knowledge will _give character to the

work

of_{engineeringdesign,}

and

willbeof service in

work

of inspection.

2.

A

_knowledge of _{the technique of testing} materials should be _gained,

_by

which he

_may

know

afterwards if

proper

methods

are _being usedin cases that

come

under

his _inspection,

_{and by}

which he

_may

_judge the signifi-cance of results of the tests of material submitted to him.

3.

A

_training should result in precise

methods

of observation.

4.

The

class-room instruction in _{Applied Mechanics}

which _{should precede} or

_accompany

this course, is rein-forced with concrete_knowledgeof _things

and

_properties,

which are _{otherwise only} words denned in text-books.

The

_application of _{theoretical analysis to} the _tests

per-formed in _{the laboratory}

becomes

of individualinterest

and

isfixed inthemind. Discrepancies between

theoret-icaldeductions

and

results of testsof actual material as 1

MATERIALS

supplied to the

market

also

become

evident.

_Many

of

the fundamental facts _relating to _metals, such as the relative stiffness of hard

and

_{soft steel,} the elevation of

the _{yield point,}

and

the _lowering of the elastic limit

through overstrain can also _{be brought} to the student's notice

_by

a few well-selected _experiments.

5.

The

_{report which accompanies} the investigational features of_{the work, affords practice in setting forth}

re-sults in a clear business-like

_way

and

should aid the

student in _forming at least the fundamentals of a style

which willbe later useful to him.

6.

The

student shouldrefertostandardtext-booksand

specifications to

compare

the results obtained with recorded data.

A

partial list of _{specifications} suitable for use is as follows:

Specifications in Year Book of American Society for _Testing

Materials.

SpecificationsinYear BookofStateHighwayCommissions.

MaterialsofConstructionbyA. P. Mills. MaterialsofConstruction_byJ. B. Johnson.

MaterialsofConstruction byThurston.

MaterialsofConstruction_byUpton.

MechanicsofMaterialsby M. Merriman. Applied Mechanicsby A. P. Poorman.

Concrete Plainand Reinforcedby Taylorand Thompson. Reinforced Concrete_by G. A. Hool.

Concrete_Engineers' Handbook byHool andJohnson.

Mechanical Properties of Woods grown in the United States Bulletin No. 566 U. S. _Dept. ofAgriculture.

7. Thesis

work

in testing materials presents a read}r

and

attractive

medium

_by

which students can receive

some

_trainingin _proper

methods

of_planning

and

execut-ing experimental investigations.

The

work

may

be individual, or performed

by

groups of _students,

and

the expense of material is small. If the _professor is interested in

some

one field _{of investigation}

and

system-atically plans for a term of years, thetheses intime are ofusein extendingknowledge.

The

method

of _{administering} thesis

work

in _general

involves the _following steps:

A

list of _problems, to which, on account of limitations of _equipment

and

the desire to _{concentrate, the}

work

of the laboratory should be confined, is _{prepared early} in the year. Theses

sub-jects are generally chosen from this list

_by

students.

When

_{a subject} is chosen

_by

_{a student, a} thesis outline is _prepared

_by

the _{professor in} consultation with the student, in which the _problem is _{clearly stated;} the authorities, if_any, cited; alistofliterature, or directions

to

main

source of information _given;

and

the

main

plan of attack _fairly definitely indicated. Details of apparatus, etc., are generally left to the student.

A

student

_may

_{present a subject} of his

own

choice.

The

written thesis covers a clear

and

_logical account of the

purpose of the _thesis; the material _tested; the

methods

and

machines, with a discussion or _error; the actual

results; the analysis

and

presentation of the _results;

CHAPTER

II

GENERAL

INSTRUCTIONS

Preliminary Notes. In all _tests, first carefully

ex-amine

the material. _Measure,

and

note characteristics

and

defects, if_any. Ifthis isnot donebefore the

speci-men

is _broken, the test is useless.

Note

the serial

number

if _any.

The

character of the _log sheet will be considered in

grading the report.

The

student should understand the _manipulation

and

readingof all instruments usedineach test.

Enough

readings shouldbe takento_accurately

deter-mine

the stress-strain curve. Calculate from table of

average strengths of materials, see appendix, the

incre-ment

of loading required to _give at least 18 readings.

When

quantities being

measured

are _changing rapidly, intervals

must

be shortened.

Specimens

upon

which further observations are to be

made

should be carefully

marked and

place in assigned

case.

If_{the reports are}tobewrittenin_{the laboratory during} the _assigned period, all notes

and

data, together with unfinished reports,

must

beleft inthe filing case.

Operation of

Machines

During Test. Preliminary to every test, each student should

become

familiar with

the operation

and

mechanism

of the testing

machine

to beused.

Balancepoise at zerowithtestpiece inthe

machine

but freefrom the pulling head in every way.

All _readings

_upon

test piece for a certain load should

be taken

when

the

beam

is balanced atthe load

and

at

no

othertime.

The

finger

may

beusedtoliftthe

beam

slightly

and

so give warning which will _prevent the loads being exceeded.

The

speed of _{applying the load should} be such that the

beam

_may

be kept balanced, otherwise the readings

will be of _{doubtful accuracy.}

Often after the elastic limit hasbeen reached a faster speed

may

be used

and

much

time saved.

_Any

consist-ent speed is allowable if _{the load readings are accurate,}

except in _experiments for which the

machine

speed to

be used is stated in the instructions. Students should

be sure that

machines

_{are properly} thrown out of _gear

when

the test isfinished.

CAUTION. Testing machines have

upon

different occasions been left

_by

_{the operators with countershaft}

running

and

the friction clutches

thrown

in, so that the

machines continued running.

The

result has usually

been that

some

part of the

machine was

broken.

The

operatorwill takeespecial care that this does not occur

with machines for which he is _responsible.

He

will be charged with all the repairs

made

necessary

by

careless handling.

Note

_Keeping.-

The

standard data sheets will in

most

cases be used for _{note keeping.} In the case of

most

of the _experiments the original log of the test will

be _required as _{a part} of the report.

Carbon

copies of the _originallog

made

in_{the laboratory}willbeacceptable in casethe

work

is of such a _{character that only} one of

two

men

can record the data obtained.

Neat

and

orderly notes

and

test _logs will be insisted _upon. Reports. Reports will be written in ink or type-written on one side _only of the standard 8"

X

LABORATORY MANUAL OF TESTING MATERIALS

paper

and

placed in _{the regular} manila cover.

On

the outside ofthe coverwill_{appearthe required information}

in _lettering.

Clearness

and

orderof statement, legibility of writing,

lettering

and

neatness will receive due attention in

marking

the report.

In _plotting stress-strain curves, select such a scale as can easily be read

by

inspection, in decimals. State plainly the scale of coordinates.

Use

the

bow

pen to

circle _{the points plotted,}

and

draw

curves with

instru-ments.

Use

India ink for curves

and

lettering on the curve sheets.

Avoid

lines from point to point.

Max.

Y.P.

^ Deformation > > Deformation ^

FIG. 1. FIG. 2.

FIGS. 1 AXD 2. Stress diagrams

The

generalform of load-deformationcurve should be

noted. Figure (1) represents a characteristic curve of ductile materials where the curve is

drawn

a _straight

line to the elastic limit _{averaging the plotted points.}

Figure (2) represents the curve for brittle materials. Select scales of coordinates so that the _slope of the portion below the elastic limitis about

60.

As

in Fig. 1

when

curve does not start at origin

draw

a parallelline

throughorigin totheelastic limit.

The

title of the curve sheet should be placed in the

lower _right-hand corner

and

should contain such

experiment, glancing at the curve sheet, would grasp the

main

facts without aid of the written report.

The

student will note carefully

any

characteristics of the curve that are peculiar

and

state reasons for their

appearance

and

how

they can be avoidedif_{they seem} to

be errors.

INSTRUCTIONS

FOR

WRITING

REPORTS

The

clerical part of the report should be suitable for

submissiontoa practising engineer,

who

would

naturally judgeof the qualifications of the writer

by

the neatness

and

_system of the report.

The

_sequence

and

the form in which the results are presented

must

be suchas toenable abusy

man

to ascer-tain in a few

moments

_just

what was

done

and

what

was

determined.

A

careful study should be given to

economy oflanguage in writing the report.

SUGGESTED

FORM

OF

REPORTS

Title.-

The

title should indicate briefly

what

is

covered

_by

the report.

Purpose.

Under

this _heading the purpose, or pur-poses, of thetests should be_{concisely stated.}

-MaterialsTested.- Itis _importantthat the materials tested should be _concisely

and

definitely described. It is _ordinarily sufficient to define the material as to its

kind, size, shape

and

condition, although

any

other

essential_{descriptive facts}shouldbestated.

Dimensioned

sketches are _{frequently necessary} to _properly describe the test _specimens. Reference should here be

made

to

any

sketches, descriptive of _{the material, appearing} in

an appendix or elsewhere in the report. Give page. Apparatus. Important special apparatus, only, should belisted

and

_concisely

and

_{accuratelydescribed.}

8 LABORATORY MANUAL OF TESTING MATERIALS

Diagrammatic

sketches are _{frequently necessary} to

properly describe apparatus asused. Reference should

be here

made

to diagrams or sketches _{descriptive of}

apparatus appearingelsewherein,thereport.

Method

of Test.

_Only

the essential details of the testing procedurewill beconcisely

and

accurately given.

It is _{not usually necessary} to include in this division the unessentialdetails of_manipulation

and measurement

which are

more

or less

common

to all _experimental

work

of this nature. It is _{important, however,} to use

good

judgment

in the selection

and

statement of such

details of _procedure, as to afford a _proper

and

clear interpretation

and

comprehension of the results obtained.

Results ofTests.

The

summarized

results ofthetests should be _given under this _heading.

The

results are usually

most

conveniently

and

clearly

shown

in

sum-mary

tables. Care should be taken that

summaries

should be accurate

and

concordant with the facts

shown

elsewhere in the report. Averages should be

accompanied

by

a statement as to

how

_many

and

what

data areincluded.

Discussion

and

Conclusions. _Explainthesignificance of the curves used to _{present graphically the} results of

the tests. _(It should be here _emphasized that curve

sheetsshouldbeclear

and

_{as far as possible}

self-explana-tory.)

Where

there are standard specifications for the

material

and

test, a comparison with these standards should be shown.

The

standards should be quoted

and

authority

and

reference given.

The

specifications of

the

American

_Society for _{Testing Materials}

and

other

national societies should be consulted.

Comparisonswith publisheddata

by

other _authorities, along similar_lines, are_{generally necessary}for_{the proper}

interpretation of the test results. Give references.

Attention should be called to the factors affecting the accuracy, uniformity

and

validity oftheresultsasshown.

The

significance

and

importanceofthe test

and

results should bediscussed.

The

conclusions

drawn

should take all, of the above considerationsinto_account, particularlythosewhichare affected

_by

the limitation ofthe testing apparatusused, the personal factor involved in _ijie

methods

and

special or unusual characteristics, or defects, in the test

speci-mens

visibleeither before or after the completion of the

CHAPTER

III

DEFINITIONS

As

supplementingthe text-books inmechanics the

fol-lowingdefinitions are recordedfor reference:

For formulas

and

_symbols see _Appendix.

Stress is the _internal, equal

and

opposite, action

and

reaction between

two

_portions of a deformed body. Stress is a distributed force,

and

is _expressed in force units.

As

in case of _{deformations,} stresses are: ₍₁₎ normal, S, either tensile

(+) accompanying

a lengthen-ing of a bar, or _compressive ( )

accompanying

a shortening of a _bar;

and

(2) tangential or shearing,

S

v,

when

_{acting parallel} to a section

and

_accompanying

achange of _angle of the faces.

Uniformly distributed normal stress _accompanies a load thatacts _{along ageometric} axis ofabar.

Unitstress_{(pounds per square inch)} is the

amount

of

stress_{per unit}of areaof surface.

The

word

strainisoftenusedto _express both

deforma-tion

and

stress.

When

usedbelowit

means

deformation.

Stress-strain _{diagrams (Fig. 3)} are

drawn

from data

obtainedin testsofmaterialsinwhich _gradually increas-ing loads are _applied from zero until rupture occurs.

Unit deformations are

shown

as abscissae,

and

unit stresses as _ordinates, tension

(+)

and

_compression

( ).

Such

diagrams display the

most

important mechanical_{properties of materials.} Figure 3shows

sev-eral such _diagrams.

The

tension deformationsat small loads are

shown

in magnified scale to the _right

and

the

12 LABORATORY MANUAL OF TESTING MATERIALS

relative values of

S

p for various materials to the left.

The

stresses are not actual but nominal, because the load is divided

_by

the _original,

and

not

_by

the actual,

deformed area.

ELASTICITY

Elasticity is the _tendency of deformed bodies to

resume their formershape.

Elastic Limit is the limit of stress within which the

deformation completely disappears after the removal of

the stress.

As

measured in tests,

and

used in design

this term refers to the _proportional elastic _limit,

S

_p

,

which is the unit stress within which stresses

and

defor-mations are directly proportional.

At

the Commercial ElasticLimitor Yield Point,

S

v,

some

materials experience

a sudden

and

large increase of deformation without

in-crease of stress.

S

p is from 0.75

S

y (hot-rolled steel) to

0.90

^^(annealed steel). Location of

S

y depends

upon

speecffrfstress.

KEY

TO CURVES IN UPPER LEFT CORNER FIG. 3.

CurveNo. CurveNo. CurveNo. CurveNo. CurveNo. CurveNo. CurveNo. annealed CurveNo. CurveNo. CurveNo. Curve No. CurveNo. Curve No. Curve No. Curve No. Curve No. 11

A

SoftO. H.Steel, asrolled.

B

SoftO. H.Steel, oil_{tempered and} annealed.

C

SoftO. H. Steel, oil_tempered.

A

AxleSteel,asrolled.

C

AxleSteel,oil_tempered

A

O. H. Steel, forgeddisc.

B

O. H. Steel, forged disc, oil _tempered and

C

O.H. Steel,forgeddisc, oil tempered.

B

HeavySteel_Casting, annealed

A

Cast Copper, annealed.

B

Cast Copper.

C

Hard Rolled_Copper.

Iron.

;eel Casting,rimof smallgear.

ChromeTungsten.

Hooke's

Law

states that, within the elastic limit, the

deformation produced is _proportional tothe stress.

NOTE.

Unless modified, the deduced formulas of

me-chanics _apply only within theelastic limit.

Beyond

this

the formulas aremodified

_by

experimentalcoefficients, as

for instance,

modulus

of _rupture.

Modulus

_{of Elasticity (pounds} per square inch) is the

ratio of the incrementof unit stress to incrementof unit

deformationwithin the elastic limit.

The

Modulus

of Elasticity in Tension, or Young's

Modulus, E, is _graphically measured

_by

the _{slope of}

OP

(Fig. 3); the compression

modulus by

the slope of

OP'.

The

inverse values of

E

for several materials express the relativeunit deformations of these materials

under the

same

unit stress.

E

is a measure of stiffness.

Modulus

of Elasticity in Shear, F, is the _shearing

unitstressdivided

_by

the angleof distortion

exps^ed

in radians. Theoretically,

F

=

n/2E(n

-f 1);

and

when

n

=

3,

F

= %E.

(SeePoisson'sratio.)

Bulk

Modulus

of Elasticity, B, is the ratio of unit

stress, applied to all side faces of _{a cube,} to the unit

change of volume. Theoretically,

B

=

^

En/(n

2);

and

when

n

=

3,

B

-

E.

Lateral Deformation, e'', accompanies longitudinal

de-formation, e. Poisson's Ratio, m, is the ratio ofe' to e.

The

inverse value of

m

isdenoted

by

n,thatis,

n

=

l/m.

Values of

m

_given

_by

Unwin

are: _{Flint Glass, 0.244;} Brass, 0.333; Copper, 0.333; Cast Iron, 0.270;

Wrought

Iron, 0.278; Steel,0.303; Concrete, accordingto_Talbot,

0.10.

Change of volume, under

_long^J^nal

deformation.

/, d, b,

=

length, width

and

thicflHs;

m

=

Poisson's

ratio; s

=

unit deformation.

Deformed volume

-

_sms)b(l

14 LABORATORY MANUAL OF TESTING MATERIALS

Fractional _change of

volume

=

(1 2m)s.

When

m

is

less _{than J^} the

volume

is increased in tension

and

decreased in _compression.

For

_steel,

_(m

=

_}),

_change

of

volume

is _{about J^ooo part} at the elastic limit.

A

barunderstress does notat once assume_{the length}

due to its

modulus

of _elasticity E. Deformation

pro-ceedsfor_days

and

weeks. These

_phenomena

ofresidual

elasticityorelasticafterworkingdonot

seem

tobeof

prac-tical _importance.

RESILIENCE

Resilience,

K,

(inch-pound) is the _potential elastic

energy stored

up

in a _{deformed body.}

For

_instance, a

falling weight compresses aspring; the stored energy, or

resilience, of the material is a source of

work and

will

produce returnmotionin _{the weight.}

The

amount

ofresilience i _equal to the

work

required

to deform the

volume

of material from zero stress to stress S.

For longitudinal deformation _^Fig. 3),

P

load,

e

=

_deformation,

S

unit stress,

E

=

modulus

of

elasticity, I

=

length,

A

=

area cross _section,

V

=

volume. Resilience

=

work

of deformation

=

_average

force

X

deformation

=

_%Pe

=

^AS

Sl/E, or

K

=

_vy

₂

_&/E.

The

resiliencefor

_any

otherkindof stresssuchas

shear-ing, bending, torsion, is the

volume

times a constant

C

times one-half the square of the stress divided

_by

the appropriate

modulus

of _elasticity.

Resilience of solids of _varying section cannot be ex-pressed per unit _ojayolume.

Modulus

of rWMfence,

K

p (inch-pounds per cubic

inch), or unitresilience, is theelasticenergy stored

up

in

deformation

K

_p is _graphically measured

_by

the area

OPP".

(Fig. 3.)

K

p

=

H

S*/E.

RESILIENCE PER UNIT OF VOLUME

K

S= _longitudinalstress,Sv= shearingstress,E = tensionmodulusofelasticity, F = shearingmodulusofelastfcity

The

unit resilience stored

_up

at a stress _{beyond the}

elastic limit is measured

_by

the area of the _triangle

S"SS

f

(Fig. 3).

Unit _{rupture work,}

K

r, sometimes called Ultimate

Resilience, ismeasured

_by

the areaofthe stress-deforma-tion _diagram to _rupture,

OPYMRR',

_{(Fig. 3).}

K

r

=

_y$ eu (Sy

+2S

m)

....

approx. (G)

Here

eu

=

the unit elongation at _rupture.

27

Forstructural _{steel,for instance,}

K

r

=

%

nT (35,000

+2

X

60,000)

=

13,950 (inch-pounds per cubic inch).

CHAPTER

IV

MATERIAL STRESSED

BEYOND

THE

ELASTIC

LIMIT

Beyond

elastic limit in tension

S

p, ductile materials

like steel enter a semi-plastic stage.

The

material

stretchesfrom

Y

to

L

_{(Fig. 3)} without increase ofstress,

and

the scale

beam

of the _testing machine _"drops,"

and

thehardmill-scalefallsfrombar. _{Yielding proceeds}

from eithershoulder of barto center. Steel shows both

S

p

and

S

v; cast iron, neither

S

y nor

S

p;

wood and

hardened steel,

S

P) but no

S

y.

The

_shapeofthe _{diagram from}

S

pto

S

vdepends

upon

speedofstress.

_Ewing

foundfulllinefor_fast,

and

dotted

line, forvery slow, speed (Fig. 3).

After

L

semi-plastic

and

semi-elastic deformations pro-ceed.

One

orseveral contractions of cross section occur

depending

upon

homogeneity of metal.

The

maximum

load is reached at

M

_{(Fig. 3)}

when

the metal at one of these _{contractions begins} to flow.

The

contraction or

"neck"

proceedsuntil_{bar ruptures}atR.

The

character

of the _{metal changes under} this excessive deformation, and, therefore, the actualstress

on

_{the ruptured} section

isnot _{of practicalimportance,}

and

is not observed.

Ultimate strength

S

m

= maximum

load/original area.

The

_elongation

and

contraction after rupture are observed.

The

load

_may

be released at intervals to observe the

set,

OS"

(Fig. 3).

Fracture under tension indicates quality of

mate-rial, butisinfluenced

by

speed

and

method

ofproducing

fracture,

and by

shape of test piece.

A

metal that is

tough

and

fibrous

_may

_appear crystalline if broken

quickly at a nicked section. Contraction is _greatest in _tough

and

ductile,

and

least in brittle, materials.

The

shape of fracture is _{usually a center} flat surface of

failure in tension surrounded

_by

a rim

on

which the

metalshears.

The

extentoftherim is

more

_pronounced

when

the ratio of _shearing to tensile strength is _less;

is

more

_developed in soft _steels; becomes a _complete

cone in _very soft _materials;

and

vanishes in cast iron.

The

color, grain

and

shear of the fracture are

insignifi-cant.

Forms

offracture intension are

shown

in Fig. 4.

Terms

_{describing fracture} are: silky, dull, granular,

crystalline, fibrous.

Failure under _{compression depends} on _material,

slenderness of _specimen,

and

restraint at ends or sides.

Short blocks of brittle materials in compression like cast

iron, stone, cement,

when

unconfinedatthe _sides, fail

_by

slidingalong inclined planes.

The

_angle of _{these planes} is a function of _{the shearing} stress

and

of the coefficient

of friction ofthematerial.

The

theoretical_angleof frac-ture,with thecrosssectionis_?r/4 -f- 0/2 where < is_angle of_reposeofmaterial. Internal cones,with theirbasesat the pressureheadsofthe_{testingmachines,}

and

pyramids,

form in cylindrical

and

rectangular specimens

respectively.

Ductile or soft material, like copper

and

soft steel,

cannot be ruptured in _compression.

_They

_bulge,

in-crease in diameter under _{increasing stress,}

and

_finally

become

plastic at the stress_offluidity.

That

is, the

de-formation _proceeds under a constant actual _{stress, per} unitof deformed_area,

and

is_permanent.

18 LABORATORY MANUAL OF TESTING MATERIALS

The

_productofdeformation

and

load (ornormalstress) is then _constant,

and

is _expressed

_by

a _rectangular hyperbola.

The

outer portions of the stress strain

diagram forlead

and

_copperin Fig. 3 are_{approximately} hyperbolic.

Unwin

_quotes the _following values for _pressure of

fluidity in pounds persquareinch.

Mild

steel, 112,000;

Copper

54,000;

Lead

1700. Experiment

by Hatt

gives

for _{wrought iron, 76,000.}

The

strength of materials of

medium

ductility, like

steel

and

_wrought iron, in compression is _generally to

be taken at the yield point of thematerial.

r\r\

^m

U

I / Copper Wrought Cast Stone Wood

Iron Iron Steel

FIG.4. Characteristic fractures of materials in tension and compression.

Strength under compression depends on ratio of length to diameterof specimen.

The

_cornpressive strength of stone

and

concrete is

about 10times the_{tensile strength.}

Fracture forms for several materials in _compression

are

shown

in Fig. 4.

Failure_{under pure} shearis difficult toproduce.

The

common

form of test introduces _bending stresses.

Ratioof_{shearing strength} to_compressive strength is

notwelldetermined.

For

concrete

and

brittlematerials this ratiois_reported to_{vary from} 0.32to 1.25.

The

_{ultimate shearing strength} of steel

and

iron is

nearly three quartersofits_{tensile strength.}

Hancock's Tests (Proc.

Am.

Soc. Test.

Mat.

1908,

p. 376)

show

that the shearing elastic limit ofsteel

and

iron, determined in torsion, is 0.50 to 0.57 _the

propor-tional elastic limit in tension.

Failure of steel at or abovetheelastic limit is

accom-panied

by

appearance of

Hartmanris

lines onpolished surfaces. These lines

make

an angle, with the axis of a tension _{bar, of} 63 for soft_steel,

and

58 forannealed

steel.

_They

indicate a _{slippage along} the cleavage

planes of the crystals ofthe metal. Steel consists of

an

aggregation of _{crystalline grains separated}

_by

films or

membranes

of material of different _{compositions.}

Un-der this view failure in tension or _compression is essentially a failure under shearing stress modified

_by

internalfriction.

CHAPTER

V

TESTING

AND

TESTING

MACHINES

Technical qualities of materials

_may

_{be grouped} as: Technological, having to do with _{manufacturing}

re-quirements, such as malleability, fusibility,forgeability,

bendingto_shape.

Physical, such as _specific gravity, plasticity,

homo-geneity, durability, structural characteristics, including fibrous, crystalline.

Mechanical properties examined in tests. These are

listed in table below, together with criteria

commonly

used.

Plasticity...!Theabsence of elasticity. Defornj without Lead,

return or rupture. 20

TESTING MACHINES

Testing Machines

must

be accurate

and

sensitive.

Accuracy depends on correct lever _{proportioning,} condition of knife edges,

and

stiffness of levers. (See

Experiment A-2.)

Forsensitiveness knife _{edges should} be of small_radius,

and

_straight,

and

stiff. Knife edge machines_{are usually}

more

sensitive than necessary.

Machines

should be

examined for clearance of levers from frame of machine.

(See Experiment A-2.)

Specimens under test should not be subject to shocks or vibrations arising from the

power

elements of the machine, as forinstance inertia of levers

when

_specimen

takessuddenelongation, or

by

the action ofthe

_pump

in setting a

body

of liquid inmotion.

Machines

_vary in _capacity from wire tester of 600 Ib.

capacity to U. S. Govt. machines of _10,000,000 Ib.

capacity in _compression. For _{ordinary use} in a

com-mercial _laboratory, a machine of _200,000 Ib. _capacity is

most

suitable. For instruction ofstudents a machine of 30,000 Ib. _capacity is

most

convenient.

The

present limit of _screw-power scale

beam

machines for tension

and

compression is _1,000,000Ib.

The

_following table of _{large capacity testing} machines

istakenfromanarticle

_by

E.L.Lasierin_{the proceedings}

of the

American

_Society of _{Testing Materials,} 1913.

Static testingmachines

must

_{provide proper}

means

for

(a) holding specimen, (6)applying load,

and

(c) weighing

the load. _{Mechanical problems} of gearing, absorbing shock, and oiling

must

besolved.

(a)

HOLDING

THE SPECIMEN

Tension

and

_Compression. It is _{important in testing}

22 LABORATORY MANUAL OF TESTING MATERIALS

OSO ffiO 'O 00is.

b->-i O O

OC

O

00O O5O Gi GS C^

fit

<uoo>

III

es^sessssss&s

sa

CuCCCGflCCCMC

csaJco3c8c8o3cSc8ci3a!a3 13 13 "3 la Is 13 Is"c13S13 . -S^"3 S 2S

U'U

MWW

WfflW W 333 ^ ^ 222 2 jooooo;

lllll 111

:* : _:S : 88rf ^ -CO

^-11

ill

ill

;IM22fiiiIl!*

;<s

21:

S 5-2ci ssls-s^ ^n..j" 5

MACHINES 23

FIG.5. Universaljoint forholding screw endtestpiece in tension,

HeadofMachine

'A

Bedding

\S

V >/ %?%!%%!%%%^^ BaseofMachine

Ifia.6. Spherical bearingplateand methodof_supporting test piece in

24 LABORATORY MANUAL OF TESTING MATERIALS

is _applied as _{nearly as possible in} the axis of the test

piece. This implies that the _specimen should be

ac-curately centered in the testing machine.

The

common

method

of _{holding the} test bar is

_by

serrated _wedges

with flat _face, which is suitable for ductile materials.

Forbrittlematerials, or short specimens, it is _customary

to use a _{spherical, or universal, joint} betweenthe

speci-men

and

the testing machine. These if _correctly

designed do

away

with _bending

and

buckling in the specimen.

Incorrect

Correct

FIG.7.' Methodofgrippingtestspecimenin tension.

Fig. 5

and

6 illustrate _{the adaptation} of the universal joint to tension

and

compression tests.1

Fig. 5 shows a convenient _apparatus for _gripping the standard short screw-end test piece

and

Fig. 6 shows the

common

method

of _{supporting concrete} or other like material in

compression.

A

bedding of Plaster of Paris or blotting

paper is used between the _specimen

and

the surfaces of the _machine, to _give an even _application of the load. Fig. 7 indicatesa correct

and

anincorrect

_way

to grip a _specimen in tension.

The

test _piece should extend

1_NOTE

: For compressiontests ofconcrete the spherical bearing

MACHINES 25

through the _grips and these should have their full

bearing overtheir entirelengthintheheadsofthetesting

machine.

FIG.8. Anapparatusfor testing smallbeamsinflexure.

FIG.9. Amethodof testing largebeamsinflexure.

Flexure tests _require freedom of _specimen to

bend

26 LABORATORY MANUAL OF TESTING MATERIALS

specimen. In case an auxiliary straining

beam

in used,

rollers are especially necessary to _prevent

_compounding

of straining

beam

and

_{specimen through} the horizontal shearat _{the loading points.}

Figs. 8

and

9

show

apparatus as used

_by

the Forest Serviceinthetestsof timber, InFig. 8,thesupporting

beam

is _conveniently laid across the weighing table of a small capacitytestingmachine.

The

rollers

and

plates

FIG.10. Ashearingapparatusforwood.

between supports

and

specimen prevent local failure

and

binding at _{the supports.}

Intesting largebeams, Fig. 9, it is_customaryto_apply the loads at the third points

by means

of

an

_auxiliary

beam

with rollers

and

plates.

The

knife edge supports

are, in this case, so constructed that theyrock freely as

the

beam

bends

downward.

ShearTests. _{Shearing shackles}

and

tools_should

pro-duce pure shear without bending. Fig. 10 shows a simple

and

reliable _shearing tool for tests of timber.

testing

machine and

the plunger

comes

in direct contact with the underside of the

movable

head.

; (b)

METHOD

OF

APPLYING THE

LOAD

Loads

are applied mainly

by

screw

power

(Olsen or

Riehle); or (2) hydraulic

power (Emery

or _Amsler).

Fio. 11. Diagrammatic view ofa Riehld testing machine. (Modified frojn

Marten'sHandbookofTestingMaterials.)

Screw

Machines.-

American

machinesare

_commonly

of screw power. Old screw machines should be

ex-amined

to see if the wear of the threads _{produces an}

oscillatorymotion ofthetestingheads.

Fig. 11 represents

by

diagram a Riehle testing

ma-chine. This is a two-screw machine.

_By

rotation of

28 LABORATORY MANUAL OF TESTING MATERIALS

the screwsabouttheir axes,thepullingheadis

moved

_up

and

down. Suitable gearinggives a variety ofspeedsin

either direction.

Fig. 12 shows an Olsen machine. This is _generally

a four-screw _{machine, sometimes} three. In this _type,

the _pulling head is

made

to

move

_up

or clown

_by

the rotation _{of large} geared nuts stationary in the base of

FIG. 12. DiagrammaticviewofanOlsen machine. (Modified from Marten's

HandbookofTestingMaterials.)

the machine.

_Through

these nuts pass the

main

screws to which is attached the _pulling head.

The

screws do

not turn on their axes.

Hydraulic

Machines

Hydraulic machines,

now

becoming

more

popular, have

many

advantages in

power

plant for laboratory! These sometimes involve friction of _{packing in cylinder,} which is not important

in large machines,

and

which is obviated in small

ma-chines (Amsler)

by

useofa_{floatingpiston}

and

a viscous

fluid likecastoroil. _{Proper provision should}be

made

to

hold a steady load.

Fig. 13 shows the Amsler machine of small capacity.

FIG.13. _DiagramofAmslertestingmachine. (FromWawrziniok.)

As

_shown, the machine is _equipped with a

hand

_pump

and

a _mercury

column

load _{indicating device.}

To

the right is

shown

the

more

common

pendulum method

of indicating load.

Fig. 14 is a _diagrammatic sketch of a vertical

_Emery

testing machine.

The

hydraulic cylinder

A

is fixed to

30 LABORATORY MANUAL OF TESTING MATERIALS

the frame

D

ofthe machine

_by

the

main

rodsSS.

The

plunger

B

is attached to _{the gripping apparatus.}

The

weighing system is _independent of the

_power

_system.

Impact Machines.-

Impact

testing machines are (1)

with vertical guides, or (2) of

pendulum

type. Pro-vision should be for

meas-uring energy remaining in

hammer

after _rupture of specimen.

For

this

pur-pose, the

hammer may

fall

on _{a spring (Fremont}

ma-chine),or,apencil attached tothe

hammer

_may

writea velocity d

isplacement

curve on _{a revolving}

drum

(Turnermachine).

Method

of release

and

hoist

and

mechanical details differ.

Machines

should be

ex-amined

for: _(a) fit of

ham-mer

in _{guides; friction;}

proportion of _height of

hammer

to clearwidth

be-tween_guides;proportionof

weight of

hammer

to foun-dation; perfection of release; shape of striking edge.

Hammer

should deform specimen as a whole. Loss of

energy at surface of _impact

and

in foundation due to inertia of _specimen should besmall.

Fig. 15 illustrates the Turner vertical _type of _impact

machineasused

_by

theForest Servicein_{impact bending}

of _{small specimens.}

The

pencil on the falling

hammer

makes

a record on the revolvingdrum.

The drum

record, Fig. 16, represents a test of a spcci-FIG. 14. DiagramofaverticalEmery

men

broken in seven blows of _{increasing heights} of

hammer

drop.

The

record gives the rebound heights

and

the deflection

and

set ofthe

beam

foreach blow.

The drum

record, Fig. 17, represents a test ofa

speci-men

broken ina _singleblow of the

hammer.

The drum

velocity is _given

_by

_{the tuning fork record}

T-T.

The

FIG.15. TheTurner impact machine.

datum

line

0-0

is the position of the

hammer

at instant of striking the specimen.

The

velocity of the

hammer

at _{that point}

_may

be obtained from the slope of the curve at _{the point of crossing.} Distance

_up

from the

datum

line_representsfreefallofthe

hammer.

Distance

down

from the

datum

line to _point of _rupture

C

repre-sents restrained fall of the

hammer

and

deformation

32 LABORATORY MANUAL OF TESTING MATERIALS

of _{the specimen.}

Below

the point of _{rupture, the}

ham-mer

has free fall and the residual _energy

_may

be

com-puted from the _{velocity as given by} the curve_slope.

Endurance

Machines.

The

ability of metals to

withstand _{a rapid reversion} of stress is an important

property. Heat-treated automobile or other machine

Fro. 1C. Drum record, Turnerimpactmachine. Specimenbrokenby seven

blowsofhammer.

parts are tested for this quality

by means

of the

endur-ancetesting machine as

shown

in Fig. 18.

The

test piece A, accurately

machined

to size

and

shape, is _{rigidly gripped} in _{the pulley B.}

The

pulley

isseated

and

free toturnin ball bearingsin theframeF.

FIG. 17. Drum record, Turnerimpact machine. Specimenbroken in single blow.

34 LABORATORY MANUAL OP TESTING MATERIALS

standards, a load

may

be applied to each end of the

test _specimen.

The

pulley is

made

to rotate

_by

belt connection toa motor.

The

speedof rotationis _usually

1300 r.p.m.

The

stress in _{the top}

and bottom

is

alter-nately tension

and

_compression in _rapid succession.

Complete

failure occurs at the edge of the fillets after a

varying

number

of revolutions

and

at a stressbelow the

elastic limit ofthe metal.

Special Test. Hardness is not well _defined, nor

uniformly measured.

Unwin

defines it as resistance to

permanent

or _plastic deformation. In this definition

it is _{distinguished} fromresistance to abrasion which is a

compound

of other _qualities.

The

best test forhardness is theBrinell Ball test.

A

hardened spherical ball (10

mm.

diam.), is forced into a

flatsurfaceunderastatic_pressureof3000kg.

and

500kg. for soft metals.

The

time of _pressure should be at

least 30 seconds.

The

_specimen is 10

mm.

thick

and

35

mm.

wide. Hardness

=

pressure / curved area of de-pression.

A

fixed ratio existsbetweenhardness

and

ten-acity ofsteels.

Martens

uses the_depthofthe impression.

With

balls 5

mm.

in diameter the ratio of load to

depthisconstant within a_{depth range}of0.05

mm.

The

hardness

number

is_{the load necessary}toindent material to 0.05

mm.

For

instance, Brinell hardness

number

is as follows.

Acid Bessemer _steel, carbon 0.10, hardness

number:

rolled, 100; annealed, 96. Basic Bessemer _steel, carbon

0.12, hardness

number:

rolled, 76; annealed, 81.

A

more

recently perfected

method

for determinations of hardness is

_{by means}

of the Scleroscope. In this instrument the height of rebound of a small

diamond

pointed tup istaken as a measureof the hardnessof the surface

_upon

which it is caused to fall.

The

_height of

drop is a fixed distance.

The

area of contact of the

diamond

point is so small that the metal

_upon

which it

strikesis stressed

_beyond

the elastic limit.

The

tensile strength of steel in _{pounds per} square

inchis obtainedfrom the following equations.

TABLE 1. EQUATIONS CONNECTING

MAXIMUM

STRENGTH AND BRINELL NUMBERS

Kindof steel Equations

TABLE 11. EQUATIONS CONNECTING

MAXIMUM

STRENGTH AND SCLEROSCOPE NUMBERS

Kindofsteel _Equations

M

is

maximum

_strength in 1000

_pound

_{per square}

inch units.

Reference. Proceedings of the

American

_Society for Testing Materials, 1915.

(c)

WEIGHING MECHANISMS

1.

The

most

common

_type is the lever _system.

These _{are illustrated in Figs. 11}

and

_{12, diagrammatic}

LABORATORY MANUAL OF TESTING MATERIALS

All lever _bearings

and

connections are hardened steel

knife _edges

and

plates.

Load

is indicated

_by

the posi-tion ofthepoise p on the_{weighing beam.}

2.

A

_gage or manometric

column

indicates the pres-surein_{the cylinder}ofhydraulic machines.

Sometimes

a

pendulum

lever is

_employed

to serve the

same

purpose.

These _types are

illus-trated in Fig. 13 in the

diagram of the Amsler machine.

3.

The

pressure from the weighing table is

transmitted to a hy-draulic _{pad H,} in _Fig.

14,

and

thencetoa

mano-metric

column

or as in

the

_Emery

machine,toa separate lever _system the fulchra of whichare

elastic plates.

4.

A

less

common

_type is that in which the de-formations of _{a portion} of the frameof the

ma-chine are

measured.

FIG.19.

The

loadisindicated

_by

a _measuring apparatus

or

_by

the _change in

volume

of a _hydraulic

chamber

which inturn isrecordedonafluidcolumn.

A

diagram-matic view of thisis

shown

_{in Fig.} 19.

The

viewshows one of the

main

rods

B-B

of the machine. This is fixed totheframe

A

ofthe machine.

As

the load

comes

on B, the deformation of

B

_changes the

volume

of the

TESTING AND TESTING MACHINES

other convenient means.

The

load value of the column

is determined

_by

calibration.

EXTENSOMETERS AND OTHER DEFORMATION

INSTRUMENTS

For the determinations of yield point of tension bars

and

deflection of beams, instruments reading to 0.01 in.

are sufficient.

But

for the determination of elastic

FIG.20. Yale-Richl6 extensometer. FIG. 21. Johnsonrollercxtensomcler.

limit

and modulus

of elasticity

and

for

many

other

purposes, deformation instruments reading to 0.0001 in. are required*

Extensometers

may

be classified:

1. Micrometer screw, see Fig. 20. These are very

38 LABORATORY MANUAL OF TESTING MATERIALS

v\

FIG.22. Marten's mirror extensometer.

used_{for practical} work.

The

contact ofthe micrometer

pointsis best determined

by means

of an electric circuit

and

_{a telephone} receiver in series.

2. Roller dial _(Johnson type), See Fig. 21.

A

simple

EnlargedHoUforBerrySt"

Gage,inSteelBar

FIG. 23a.

ert iuConcretet'erCompression

FIG. 235.

A

type for _laboratory use but unreliable

on

account of slippage at the roller. Thereisalso errorintroduced

_by

the wear of the roller.

The

_{readings are given} direct

on the dials.

3. Roller mirror, see _Fig. 22.

Thisis_verydelicate

and

reliable,

butunsuitedtostudentorroutine

work

of thelaboratory.

The

in-strument

must

be used where

there islittle vibrationorjar. 4. Lever _{type,examples}ofthese are the Olsen

_{Ewing and}

_Berry.

The

last of these is _particularly

well_adapted to a_largevarietyof practical

work

both in the lab-oratory

and

inthefield. InFig.

23 is

shown

this instrument as

made

_{with contact points}foruse inreinforced concrete

work and

with adjustable gage length.

The

contact points are placed in small counter-bored holes in the test

piece. In

some

cases, the instrument

may

be fixed

and

held in _{testing position throughout} the loading,

FIG. 23c. Extensometer

40 LABORATORY MANUAL OP TESTING MATERIALS

but generally it is

removed

after _{each reading} is taken. In using the instrument, proper correction

must

be

made

for _changes in _temperature.

The

instrument reads directly to 0.0002 in.

and

closer

_by

estimation _{of parts} of a _graduated division.

The

following precautionsaretakenin_{using theBerry} strain _gauge:

1.

The

instrument is seated in the hole 5 timesforan

observed _length.

2.

A

_preliminary series is taken in

new

holes.

FIG. 24. Diagrammaticrepresentationofan Olsendeflectioninstrument. 3. After five sets _{of readings,} the instrument is read

ona standard bartocheck

up

changesinthe instrument.

4.

To

reduce observations for _{temperature changes,} readings are

made

on a _{specimen which} is without _load,

and

whichis_exposedinthe

same

manner

asthe_specimen

undertest.

5.

A

_single observer should

make

any

set of _readings

and

_alwaysin the

same

order.

6.

The

instrumentshouldbe_applied at right anglesto the bar

and

with uniform pressure.

Compressometers.

The

simplest type of

compres-someter is

shown by

_diagram in _{Fig. 24;} this is a _good practical instrument for rough work. It reads _directly to 0.01 in.

and

_by

estimation to 0.001 in.

Fig. 25 illustrates an instrument designed for closer

more

accurate determinations. This compressometer

needs_very carefulhandlingfor the bestresults. Better service

_may

_{be accomplished}

_by

_{replacing the} electric bell

_by

a telephone receiver, to give the micrometer contact.

The

instrument reads directly to 0.0001 in.

FIG. 25. TheOlsen compressometer.

A

compressometer for short columns oftimberwhich has been _{found very} useful for student

_{work by}

the authors, is

shown

_{in Fig.} 26.

Two

_yokes bearing

Ames

dialsareattached tothe_specimen

_by

fourcontact screws each.

As

the _pieceis _deformed, the readings are given

on both sides

_by

the

Ames

dials. These read direct to 0.001 in.

and by

estimation to 0.0001.

42 LABORATORY MANUAL OF TESTING MATERIALS

FIG. 26. Author's compression instrumentfor shortwoodcolumns.

Autographic

Recording

Apparatus.

A

drum, Fig

27, is fixedon the frame ofthe testing

machine

and

put

in _gear with the _specimen so that the rotation of the

drum

is_proportionalto thestretch of_{the specimen.}

A

pencil in gear with the poise

moves

parallel to the axis of thedrum.

As

the test _{proceeds, the diagram on} the

drum

hasabscissae of deformation

and

ordinatesof load. In

some

types, the poise is _replaced

_by

_{a spring} at the

end of the scale beam.

As

the load is_{applied the}scale

beam

rises

and

_{the spring} measures the load.

The

rise

ofthe

beam

actuates the_pencilonthedrum. _{This type}

yields delicate load measurements.

A

home-made

simple autographic device is

made

of a steam-engine

indicator on_{this plan.}

Autographicrecorders are notsuitable for thedelicate

measurements

for elastic constants

and

are of limited