On the effect of time and temperature on the strength of steel and iron

TA

- 8

YW02449

B 3 IDE MOD

EXCHANGE

<* McGILL UNIVERSITY

PAPERS FROM THE DEPARTMENT

ENGINEERING.

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y

— ON THE EFFECT OF TIME AND TEMPERATURE ON
THE STRENGTH OF STEEL AND IRON.

BY

ERNEST G. COKER, D.Sc.

[Reprinted from the Trans, of the Canadian Institute of Civil Engineers,
Vol. xv, Part I, Paper No. 158.]

MONTREAL, 1902.

W^r*X

I,1

ON THE EFFECT OF TIME AND TEMPERATURE ON THE
STRENGTH OF STEEL AND IRON.

BY ERNEST GEO. COKE# B.A. (CANTAB.), D.Sc.
Assistant Prof, of Civil Engineering, McGill University, Montreal.

INTRODUCTION

The behaviour of iron and steel under simple stresses of various
kinds has long been a subject of investigation by the Engineer.
and very complete data are available for use in designing structures
in metal.

When iron or steel is subjected to tensional stress sufficient to
cause a permanent stretch in the material, it is well known that
the ductility of the material is reduced, and that ultimately the
elastic limit rises.

As a first consequence, however, the elastic limit is first of all
lowered, in some cases to zero, but if the bar be left to itself the
elastic limit ultimately rises to a higher value than before.

In the laslt few years the phenomenon has received considerable at-
tention at the hands of Bauschinger*, Ewing| and MuirJ.

The present paper relates to a few experiments, which have been
made in the Testing Laboratory, McGill University, following the
lines indicated by the authorities mentioned above.

I. MEASUREMENT OF SMALL STRAINS.

The measurement of the change in length of a bar, by such a
small quantity as 1/50,000 inches has been rendered comparatively

* Unwin's Testing of materials of construction.

t Ewing "On the measurement of small strains in the Testing of materials and Struc
tures" Proc. R. S 1895.
j Muir "On the recovery of Iron from overstrain." Phil Trans 1899.

easy by aid of extensioaneters devised by Bauschinger, Unwin and
Ewing. The instrument mainly used in these experiments was one
constructed after Professor Swing's design, and has several novel
features. It is, moreover, especially adapted for work where several
settings o-f the instrument are necessary.

II. EWING EXTENSOMETER.

The instrument is shown in Fig 1, mounted upoin a test piece, and
consists of two clips, A and B, fixed upon the bar at a standard dis-
tance apart by set screws C, D. Upon the upper clip A, are two pins
E, P., upon which latter the frame G- is mounted. The frame carries
a reading misoros'cope H, and a micrometer screw I; this latter gears
with a conical hole in the lower clip B, and is kept in engagement
by a spring. The reading microscope is sighted upom a thick wire
mounted horizontally in a plate, J and illuminated from* behind by
a small mirror.

Hg I

The clips are made of gun-metal, and are formed so as to- give slight-
ly when the set screws engage the test piece, and thereby if a slight
diminution of section occurs in the bar, while the test is proceeding,
the screws do not become slack, and one sourse of error is thereby
avoided.

The clips A and B are each centred upon the bar, and each has
one degree of freedom with respect to it, viz., that of rotation, about
their respective set-screws, and by causing the micrometer screw
I to gear with the lower clip, both degrees of freedom are removed
and the apparatus becomes a rigid whole.

In setting a flat tension barf in the jaws of a testing machine, it
is difficult to avoid giving it a slight twist, but by pivoting the frame
G upon the upper clip in a transverse direction, a slight twist
will not cause error in the readings.

The micrometer screw and observation wire are set equidistant
from the set screws D upon the clip B, so that thei wire can be
moved up or down by turning the micrometer screw, and this move-
ment is measured by reference to a glass scale in thei eye piece of
the reading microscope.

In this manner the scale is calibrated by reference to the: micro-
meter screw, which latter has 50 threads to the inch, and the scale
is so arranged that a movement of the wire through 1-50 inch cor-
responds to a reading of 50 scale divisions. In actual woirking since
the micrometer screw engages the clip B, any extension between the
set screws will cause the relative movement between the reading
microscope and the wire to be doubled, so that the value of one
scale division =1-2 x 1-00 x 1-50 — 1-5,000 inch, and as 1-10
division can be estimated, a reading of 1-50,000 on an 8 inch length
can be measured.

The chief advantage of the instrument is that it can. be calibrated
in position, that the reading is a mean of two sides of the bar,
and that it can be quickly and accurately set on the specimen.

In order to set the clip accurately upon the specimen a distance
piece is provided, having V shaped recesses to receive corresponding
studs on the clips, and clamps to hold the clips in place, until these
latter are set on the bar.

The apparatus for applying tensional stress was a single lever
Wicks-tead testing machine, which was capable of exerting a pull
of 200,000 pounds. As the stresses used were comparatively small,
the weigh lever was) mounted to cause the jockey weight to travel
the whole length of the weighbeam for a range of 50,000 pounds,
and so render the measurement of applied stress as accurate as
possible.

III.— RECOVERY OF STEEL FROM TENSIONAL, OVERSTRAIN.

When a bar of iron or siteel is subjected to a tensional stresfc

sufficient to oause a permanent stretch in the material, the physical

3

character of the bar changes, and it becomes semi-plastic, while its
elastic limit generally reduces to zero, followed by recovery, wnich
is comparatively rapid! in the case of wrought iron, but less so in
the case of steel.

As an example of this, the case of a mild steel bar may be taken.
This bar, and others referred to later, were cut from a piece of
boiler plate, and afterwards- straightened and annealed. The bars
were machined to a breadth of 2.07 inches, and the thickness was
Iteft untouched, the mean thickness in the present case being 0.377
inches, and the length under test 8 inches.

The Extensometer used was of the EWing type, ome division cor-
responding to 1-50,000 inches.

A test was) first made to give the bar a permanent stretch, with
the following results: —

LOADS IN LBS.

READING OF
EXTENSOMETER.

DIFFERENCE.

0

400

—35

2,000

435

—35

4,000

470

—34

6,000

505

—35

8,000

539

—34

10 0

573

—35

12,000

608

—35

14,000

643

—36

16,000

679

—36

LS,000

715

—38

20,000

753

23,000

Went off scale.

Extended

length.

24,000

8.10 inches.

24,500

8.12 '

25,000

8.14 <

26,000

8.15 '

16,500

8.L>5 '

27 000

8.165 <

0

8.15 <

The measuring instrument was immediately re-set to the standard
distance of 8 inches, and a re-test was made, rising by increments
of 2,000 pounds to 24,000 pounds, and then to zero again by decre-
ments of 2,000 pounds. The following results were obtained: —

4

LOAD IN POUNDS.

READING OF

EXTENSOMETER.

DIFFERENCE.

0

400

—35

2,000

435

—34

4,000

469

—37

6,000

506

—37

8,000

543

-37

10,000

580

—42

12,000

622

—42

14,000-

662

—42

16,000

704

—40

18,000

744

-54

20,000

798

—48

22,000

846

—51

24,000

897

—35

22,000

8i2

—35

20,000

827

-37

18,000

790

—42

16,000

7^8

—39

14,000

709

—40

12,000

669

—40

10,000

629

-41

8,000

588

-42

6,000

546

—42

4,000

504

— 44

2,000

460

—45

0

415

A comparison of the readings obtained at once shows how differ-
ently the bar behaves after the tensile overetrain. There is now
no linear relation between the stress and the strain, but Itlie latter

5

rises irregularly as the stress increases; the differences are irregular
and do not agree except accidentally with thie differences obtained
as the load is removed.

The bar was now laid on one side, and a gradual recovery of its
elastic properties took place, which was shown by thei approach to
the first readings made. At the end of three hoiurs there was a
marked recovery, and the following reading® were taken: —

Load, Ibs.

Beading.

Difference .

0

400

—34

2,000

434

—34

4,000

468

—37

6,000

505

—37

8,000

542

—38

10,000

580

—39

12,000

619

—41

14,000

660

—40

16,000

700

—42

18,000

742

—45

20,000

787

—42

22,000

829

—46

24,000

875

—34

22,000

841

—35

20,000

800

—37

18,000

769

—38

16,000

731

—38

14,000

693

09

12,000

654

—41

10,000

613

—40

8,000

573

—41

6,000

532

—43

4,000

489

—43

2,000

446

—44

0

402

The tests of the bar were repeated at the end of one day, 3 days,
7 days and 28 days, and these showed a further recovery, whose
rate diminished with the time, but at the end of the month the
recovery was practically complete. The readings obtained are shown
in the annexed table: —

LOAD LBS.

1 DAY AFTER.

3 DAYS AFTER.

7 DAYS AFTEB.

'28 DAYS AFTEB.

Reading. A

Reading. A

Reading. A

Reading. A

0
2.000

400
434 £?

40')
434

400
434

400 oo
433 "g

4.000

469 ...

469 "S

468 •*[

467 "^*

6.000

504 ~™

504 "22

503 ~^J

500 "OR

8.000

539 "r°

539 "5J

538 ~^

535 o?

10.000

576 "SA

574 ~!^

574 *g

570 S

I2.oeo

14.000
16.000

614

654 Jy

695

en ~j:

648 "JJ

685

609 -g
646

684

605
640
676

18.000

736 ~4J

TOK ~4"

(ZO ,„

722 "2

712 ?

20.000

779 ~rl

7«r -40
A •)

761

747

22.000

822 ~4^

807

802

783 Xj

24.000

867 ~4£

851 ~44

847

818 S

22.000

832 "g

817 yf

813 ~:J*

784 oj

20.000
18.000
16.000
14.000

797 ~;j?
762 -*

724
686

782 -^
746
710 ~:^
672

779 ~34

34.

745
709 -*J

672 ~^

750 JJ
715
680 o?
645 "g

12.000

648

634 ~^

635

611 -?4

10.000

607

596

596 "g

576 "S

8.000

566

556 ~4"

558 ~'S

540 "2

6.000

525 ~*i

517 "5J

519

505 «

4.000
2.000
0

483
441
398

478
438 -4"
397

481 ~S
443 2
402

470 "iJ
435
399

In order to exhibit the changes in a manner which shows the
recovery in a striking manner, the results were plotted in Fig. 2
after the manner first suggested by Ptrof. Swing.* A number pro-
portional to the load was subtracted from each reading, and the
new reading obtained was plotted instead; this accentuates the
differences, and as each curve is plotted from a different initial
point, the recovery is exhibited in a form which can be readily fol-
lowed by the eye. In the present instance 25 divisions were sub-
tracted for each increment of 2,000 pounds.

Referring to the diagram, curve I. denotes the original stress
strain curve, only part of which is shown, as the total exension
Table I. is 8,250 divisions at a load of 27,000 pounds. Curve II.
and the succeeding curves show the result of the overstrain. The
stress-strain curve becomes a loop, and the area enclosed is a

#Muir on the Recorery of Iron from Overstrain . Phi]. Trans. , 1899.

2.EOOO

Z400C

2CCOG

I600C-

Fig. 2

I- Original Stress Strain Curve
E- Immediately afterwards

- 3 hours afterwards.
TSL- 1 day afterwards.
Y -3 days afterwards,
afterwards.
W-28 days

ISO 200

v » s s o N s
sure of the work done on the bar by the loading and unloading, and
therefore is a measure of the imperfectness of the elasticity.

The rest caused a gradual recovery of the metal, and showed itself
in the gradual diminuition of the enclosed area until at the end! of
28 days the recovery was very nearly but not quite perfect.

SHEAR STRAIN.

The recovery of iron and steel from shear strain appears to follow
the same general law as for tension. It will 'be sufficient to quote
the case of a turned specimen of wrought irom, having a diameter
of .0446 inch, and length under test of 4.00 inches. The specimen
was subjected to shear by a twisting couple, and the distortion be-
tween the sections was measured by a strain measuring instrument
of special design.* The bar was first subjected to torque of gradually
increasing amount until the elastic limit was passed. The readings
obtained were as follows, the calibration of the instrument being
sucTi that 12.8 divisions corresponded to 1 minute of arc.

*Coker «« on Instruments for Measuring Small Torsional Straics." Phil. Mag., 1899

8

Torque
inch Ibs.

Reading . Diff .

Torque
inch Ibs.

Reading. Diff.

0

0

—302

75

302

1489

—303

3(30

—81

150

605

1568

—303

375

—79

225

908

1666

—304

390

—98

. 00

1212

1808

—63

495

—142

315

1275

2038

—64

420

—230

330

1339

2548

—69 I 435

—510

345

1408

Specimen twisted

—81

450

through.

360

1489

40

Immediately after (the load was released and a new series of read-
ings obtained, the maximum torque applied being 375 inch pounds, and
similar observations were recorded at the end of one hour, 3 hours,
1 day, 4 days and 8 days, the last experiment showing practically
complete recovery.

The readings were as follows: —

Torque

Immediately

One hour

ihree hours

One day

* our days

Eight days

inch

after

a ter

after

after

after

after

Ibs.

I

I

I

I

I

I

Read-

Read-

Read-

Head-

Read-

Read-

in?. A

ing. A

ing. A

ing. A

T'g. A

ing. A

0

0

0 0

0

0

0

-305

—304

—301

—301

301

—302

75

305

304

301

301

301

302

—324

—306

—303

—302

303

—303

150

629

610

604

603

604

605

—324

—312

—307

—305

304

—304

225

953

922

911

903

908

909

—339

—319

—317

—314

307

—303

300

1292

1241

1228

1222

1215

1212

-361

—349

—322

—316

310

—305

375

1653

1590

1550

1546

1525

1517

-^3 3j -301

—301

—302

302

—301

300

1350

1289

1249

1244

1223

1216

—305

—307

—304

—303

303

—302

225

1045

982

945

941

920

14

—315

—312

— 109 —307

306

303

150

730

70

636

634

614

611

—325

—324

—316

—314

307

304

75

405

346

320

320

307

307

—338

—332

321

319

308

306

0

67

14

I

+1

1

1

The readings obtained are given in the above table, and are
plotlted on Fig. 3, after deducting 275 divisions for each increment
or decrement of 75 inch pounds of torque. The rapid recovery of
wrought iron is well shown by the looped stress-strain curves II.
to VII, and the rapid diminution off hysteresis in one hour after
the overstrain is very marked.

Fig. 3

I - Original Stress Strain Curve

II - Immediately afterwards.
Ill- / hour affer I.

IV - 3 hours after I.
- I day affer I.
affer I.
VH - 8 days affer I.

100 200 300 400 500 600

DIVISIONS OF ARC

of instrument - I minute of arc - 12-35 divisions)

THE INFLUENCE OF TEMPERATURE UPON THE RECOVERY
FROM OVERSTRAIN.

It is well known that if a bar of iron or steel be stressed beyond
the elastic limit in tension, it will regain its elastic properties if
raised to a dull red heat and allowed to cool slowly in air or in
lime, and this process may be repeated again and again without

10

Fig. 4

I -Initial Stress Strain Curve

ARer boiling in water for 10 minutes
ID- ....... Second boiling in inter for 10 minutes

BT ....... Hun/ » ...«

Y .._... fourth _

firth

600 800 1000 1200 1400

1 V I S I O N

(€00

11

seriously affecting the physical properties of the bar. Recently our
knowledge of annealing has been greatly increased by the discovery
of Muir* acting on a suggestion of Prof. Ewinig, that a compara-
tively low temperature, such as that of boiling water wild restore a
strained bar to its elastic condition. As an example of this a test
made in -the McGill Laboratory shows the effect of boiling water
upon an overstrained bar. The specimen was a flat 'bar of
boiler plate, width 2.07 inches, thickness 0.375 inch, and length
under test 8.00 inches. This specimen was similar to the one used
in the first experiment, and was annealed at the same time. The
Mstory of the bar is shown by aid of figure IV., in which Curve I.
shows the relation of stress to strain when the bar was subjected
to a gradually increasing stress. The first notable deviation from
perfect elasticity began w.hen a total stress of 20,000 was applied,
and a-ta 25,000 the extension amounted to 8.13 inches, an increase of
stress to 27,000 pounds, caused a further extension to 8.15 inches.
The load was then removed, and the bar was exposed for 10 minutes
to the temperature of 'boiling water and again re-tested. Curve II.
shows the resulting stress-strain curve, with a yield point at about
31,000 pounds, considerably above the first. The boiling was re-
peajted in the same manner, and a new yield point (Curve III.) was
obtained at 36,500 pounds, while further tests after annealing in
boiling water gave new yield points at 41,500, 45,600 and finality
48,100, when the bar broke, with a total extension of about 15
per cent. This extension is less by about 6 per cent, than would
have been obtained if the bar had been stressed to breaking with-
out any intermediate treatment. Even a temperature of 50 degrees C,
effects a considerable physical change in a bar of overstrained ma-
terial, restoring its elasticity and raising the yield point.

THE EFFECT OF TEMPERATURE ON OVERSTRAIN PRODUCED

BY SHEAR.

The annealing effect of boiling water is equally marked upon
material overstrained by shear. As an example, a test was made
upon a turned steel bar of 0.425 inch diameter, and having a length
of 4 inches under test. On each occasion the stress was carried
well beyond the yield point, and was followed by immersion in
boiling water for 10 minutes. The curves obtained are plotted in
Fig. 5, the unit of strain being such that 12.85 units correspond
to an angular distortion of 1 minute of arc. Curve I. shows the
initial stress-strain curve, and curve II. represents the behaviour of
the bar after 10 minutes' immersion in water at 100 degrees: C.
The remaining curves III. to IX. show the relation of stress to

* Muirloc. cit.

12

1200

»050

900

750

U

Z

- 600

-u 450

»

car
a

o 300!

I-

150

L

7"

200 400 600 800 100 1200

DIVISIONS

1 12 85 Divisions =• I minufe)

1400

1600

strain after each subsequent annealing in boiling water. After
each stress and subsequent heating, the recovery was practi-
cally complete, and the yield point rose after each annealing. At
the end of 9 separate twists the bar exhibited no change externally.
A test of a steel bar was also carried out for me by Messrs.
Elanchard, Clement and G-agnon, Fourth Year students in the Civil
Engineering Department. A turned steel bar of similar quality to
the previous one was chosen, having a diameter of 0.757 inch, and
a length under Itest of 8.00 inches. The strain measuring instru-
ment was a simple graduated circle divided to 15 minutes, and
readings could be estimated to 1 minute of arc. Each separate appli-
cation of twisting moment was carried beyond the yield point,

13

and was followed by immersion in water at 50 degrees C. for 15
minutes. Even' this low temperature was sufficient to cause a very
effective recovery to the elastic condition, the recovery being in
all cases accompanied by a rise in the yield point. The data ob-

Fig. 6

O" 4 8* 12* l«f V? Ut Z?

tained are exhibited in Fig. 6, and the history of the treatment of
the bar is told by the curves marked I. to XV., I. being the initial and
XV. the final curve of the series. Even at the end of the fifteenth
twist, when the total permanent twist had reached .270 degrees, there
seemed no probability of the bar fracturing.

The remarkable effect of a slight increase in temperature in re-
storing an overstrained bar to approximately its primitive condition
may ultimately prove to be of considerable practical importance,
as it affoirds a means of annealing, which, although not as perfect
as that accomplished by the blacksmith, has the advantage that
polished surfaces of iron and steel retain their polish and are not
distorted by the process. In bridge structures and the like, exposed
to repeated stresses and corresponding fatigue of the metal, the
suggestion that this fatigue may be partially counterbalanced by
the heat of a summer's day may not be so improbable as it at
first sight seems to be.

THE EFFECT OF LOW TEMPERATURE! UPON THE
RECOVERY FROM OVERSTRAIN.

It is interesting to determine what action low temperature, such
as that afforded by a Canadian winter, has upon the behaviour of
steel after overstrain, and in order to contrast this with recovery at

the temperature of the Laboratory, viz., about 68 degrees Fahr., a
specimen was taken similar to the one described with, reference
to Fig. II. and the accompanying tables. The bar was 2.07 inches
in breadth, and had a mean thickness of 0.378 inch, the length
under test being as before, 8 inches. The specimen was first per-
manently stretched in exactly the same manner as in. the previous
case, and a comparison of the appended table with that on page 4
shows a very close agreement.

December 15th, 1900.

Load,
pounds.

Reading.

A

0

400

- 34

2,000

434

-36

4,000

470

—36

6,000

506

—35

8,000

541

—33

10,000

574

—34

12,000

608

—34

14,000

642

—35

16,000

677

-34

18,000

711

—33

20?000

744

—42

22,000

786

—82

24,000

868

26.000

8.08"

27,000

8.12"

27,500

8.16"

0

b.155"

The specimen was then exposed to the outside temperature for
3 days, during which the temperature varied from a minimum of — 2.5
degrees Fahr. to a maximum of 11.2 degrees Fahr., ami it was then
subjected to a cycle of stress ranging from zero to 24,000 pounds.
The results are recorded in Fig. VII., Curve I., and a comparison
with Fig. II shows it to have a fair agreement with Curve III. ol
that figure. The bar was again exposed to the outside temperature,
which had a mean value during the nexft four days of 18.52 degrees
Fahr., a minimum value of — 2.5 Fahr., and a maximum value of 31.72

15

Fig. 7

29000

250

50 100 150 200

SCALE DIVISIONS

degrees Fahr. A second test, on December 18th, showed1 very
little change in the condition of the bar, as will be seen by com-
paring the figures in Table III. with those previously obtained,
and the bar was therefore given a long rest through the re-
mainder of December and the whole of January. The mean tem-
perature for the remaining days of December was 25.64 degrees
Fahr., the highest 39 degrees Fahr., and lowest 0.6 degree

16

Fahr. During January the mean temperature was 12.75 degrees
Fahr., the lowest temperature recorded being — 16.7 degrees Fahr.,
and the highest 39.6 degrees Fahr. The bar was tested again on
February 1st, see Table III. and Curve III., Fig. VII., and, as will be
seen, it exhibited some slight indications of recovery, but very small
in comparison with the recovery which would 'have been effected by
the ordinary indoor temperature. A final test was made on Fe-b. 26th,
the mean temperature during the interval being 12.46 degrees Fahr.,
with a minimum of — 4.0 degrees Fahr., and a maximum of 28.4 de-
grees Fahr. The recovery during this period was practically nil. It ap-
pears, therefore, that low temperatures tend to retard the recovery
of steel, which has been subjected to overstrain in quite a marked
degree.

TABLE III.

LOAD LBS.

DEC. 18, 1900.
I.

DEC. 22ND.

11.

FEB'YIST.
III.

FEB'Y 26TH.
IV.

Reading. A

Reading. A

Reading. A

Reading. A

0

400 o.

400 _u

400

400

2,000

434 X7

4.) 4 qq

432 -11

431 ~H

4,000

469 22

467 £

464 ~*t

463 -;"

6.000

502 ~f£

502 ~q7

498 ~™

496 "g

8,000

538 _™

536 ""JJ

531

530 "J*

10,000
12,000

611 "I**

571 SJ2

607 3°

562 ~3J
599 "'H

564 "tj
598 ~?J

14,000

648 "ii

645

633 ~t*

633 a!

16,000

685 3<

683 -;j°

670

671 •**

18,000

724

722 -JJ

708 3*

706 ~S

20,000

763

762 ~f

746

744 ~q®

22,000

805 44

804 :«

785

783

24,000

849 -44

847 j£

828 "|f

825 "SJ

22,000

816

815 ~QO

797 •«*

794 -**

20,000

782 -?£

783

765 -^

762 "g

18,000

749

750 -£

733 -g

729 „

16,000

713 *JJ

716 "g

699 -f*

696 qq

14,000

679 "5|

682 ^

667 ~»J

663 qj

12,000

642 "Si

646

632 "^

629 ~qV

10,000

605 £'

608 ~^

599

594 '2

8,000

567 "2

573 g

561 -g

558 ^

6,000

529 -;£

535 ,"

527

522 "^

4,000

491 3°

496

491 ~f

475 "2f

2,000
0

453 OQ
414

456
412

454 "J*

413

440 q?
403

In conclusion, the author desires to thank Professor Bovey, Past-
President of the Institution, who placed the Testing Laboratory of
McGill University at his disposal; Prof. McLeod, Secretary of the
Society, who kindly supplied the information regarding the tempera-
ture variations from the McGil Observatory records, and Messrs.
Blanchard. Clement and Gagnon. who made all the observations re-
corded on Fig. 6.

17

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