Residual Performance of Structural Steel after Plastic
Deformation and Strain Aging
Shaoqi YANG
Candidate for the Degree of Master of Engineering
Supervisor: Dr. Taichiro Okazaki
Division of Architectural and Structural Design

Steel is very well suited for sustainable construction
owing to its recyclable and durable properties. Today,
nearly all steel products are reused after their service life.
However, because the recycle process requires abundant
energy consumption and associated carbon-dioxide
emission, sustainable steel construction can be further
promoted by establishing reuse: procuring steel
components as a structure is being demolished, refurbish
and then reuse in a new structure. To do so, theperformance of used steel must be evaluated. The effectof plastic work, due to earthquake or fabrication among
other causes, need to be quantified.
Therefore, research was conducted to understand theresidual performance of common Japanese structuralsteel after plastic deformation and strain aging. This
research program includes monotonic tension coupon
tests, cyclic tests and cyclic beam tests. At the time ofwriting, only monotonic tension coupon tests werecompleted. This thesis reports a background review,experimental methodology and conclusions at current
stage.

Literature Review
Bauschinger observed an increase in the maximum loadcarrying capacity of a piece of steel after it was tested toinelastic range and retested after some time [1]. Thisphenomenon, now known as strain-aging, issignificantly affected by chemical composition and themethod and duration of curing. Cottrell and Bilby [2]explained that strain aging of low carbon steel is causedby segregation of interstitial atoms, mainly carbon andnitrogen, to form atmospheres around dislocations. Onoet al. [3] highlighted the role of titanium on reducing
strain aging.
Based on literature review, Baird [4] summarizedthat strain aging results in increased yield strength,
recovery of a sharp yield point, and reduced ductility
and toughness as illustrated in Figure 1. Extensive study
on strain aging has been conducted in the field ofwelding and manufacturing. Based on a review of over
30 types of steel, Pense [5] stated that strain-aging canoccur in any steel. Inelastic strain introduced at ambient
temperature causes strain aging, but the effect may berelaxed depending on curing temperature, i.e., thetemperature maintained over the duration when strain
aging takes effect: temperatures up to 426°C has norelaxing effect; temperatures between 482 and 649°C
can relax ductility loss; temperatures above 649°C canrelax the change in yield and tensile strength andelongation but not to the original properties.
Table 1 listed experimental studies on strain aging
and tension properties [4, 6-9]. Wilson and Russell
reported for low-carbon rimmed steel that changes inyield strength and elongation is controlled by grain sizeand content of dissolved carbon and nitrogen and thatthe effect saturates after one week at 60℃ [4]. Succop
et al. [6] observed for pressure-vessel steel A533 andA516 that even a small, 1.25% pre-strain can cause 75%
increase in yield strength after 3,000 hours at 260℃.
They also suggested that 260 to 343℃ is the mostdetrimental curing temperature. Kaufmann et al. [7]studied the properties of the k-area of rolled I-sectionsof A572, A913, and A36 steel to address the change inmilling process from blast furnace to electric arc furnacein the 1980’s. In the range of 2 to 12%, larger pre-straincaused larger increase in yield strength, from 20 to 40%.
In steel cured after 12% pre-strain, which stressed thematerial to near the necking point, the yield plateau
reappeared but strain hardening was negligible. Yamada
et al. [8] observed for SS400 steel that strain aging may
saturate at 3 months under ambient temperature. They
also observed from cyclic-loading tests of beam-tocolumn connections made of SS400 steel that strain
aging can result in a favorable distribution of tension
properties to promote yielding in regions away fromcritical welds. Mojtaba [9] observed for SN490B steel
that increase in yield strength was independent of theamount of pre-strain and that strain hardening rate did
not change with aging.
4Engineering StressIntroduction

No curing: 1-2-6

25

6Pre-strain

Pre-strain (1-2-3), curing (3), retest (3-4-5)

103

Engineering Strain

Figure 1. Typical strain aging effectTable 1. Earlier experiments on strain agingAuthor
Wilson andRussell [4]SteelLow
carbonsteel

Pre-strain

Curing4%

60℃ / up to 2months
93 - 593℃ / up to
1.5 months

Succop et al. [6]

A533,
A516

1.25%

Kaufmann
et al. [7]A572,
A913, A36

2 - 12%

Yamada et al. [8]SS400

2.5 - 10%

Mojtaba [9]SN490B

0.8 - 10%93 - 204℃ / 10hours
Room Temperature/ up to 1 year
100 - 200℃ / up to10 hours

Experimental Method

Observations
Changes in yield strength and elongation wasinfluenced by grain size; Strain-aging saturatedat 1 week.
260 to 343℃ was the most detrimental curing
temperature; even small strain can be
detrimental if curing time is long enough.
Yield and tensile strengths were nearly
identical after curing if pre-strained 12%.
Strain-aging saturated at 3 months.
Increase of yield strength was independent ofamount of pre-strain.

evaluate the flange after 30 days and 90 days; andlocation C and E for the web after 30 days.

Materials
Three different structural steel grades, JIS SS400,
SN400B, SN490B, were selected for testing. Table 2 lists
the mechanical properties and carbon content reported in
the mill-test reports. Table 3 summarizes the nitrogencontent measured based on the thermal conductimetricmethod according to JIS G 1228: Iron and steel –Methods for determination of nitrogen content and theglow discharge optical emission spectroscopy (GD-OES)
method. Since the GD-OES can only measure the relative
composition, the nitrogen content listed in Table 3 wasbased on the carbon content listed in Table 2. The GDOES results suggested a higher nitrogen content
compared to the thermal conductimetric method. Thediscrepancy may be attributed to the sampling procedureused for the GD-OES.

Table 2. Material properties from mill test report
Steel Type
SS400 SN400B SN490B
Yielding Strength [MPa] 310
314
364
Tensile Strength [MPa] 454
469
531
Yield Ratio
0.68
0.67
0.69
Elongation [%]302928Carbon [wt%]
0.15
0.16
0.16
Table 3. Measured nitrogen content [wt%]Analysis Method
SS400
SN400
JIS G 1228
0.00730.0031GD-OES
0.01000.0073SN490
0.00280.0072

Monotonic Tension Coupon Tests
JIS 1A tension coupons were sampled from H-500
×200×10×16 hot-rolled sections at the locations shown in
Figure 2. An extensometer shown in Figure 3 wasdevised to measure engineering strain over the standard
gauge length of 200 mm. The extensometer was fixed tothe coupon at each gauge marks through four springs.
Engineering strain was calculated as the average
displacement measured by two transducers divided by
the original gauge length. The accuracy of strain
measurement in the elastic range was confirmed bycomparison against measurements from strain gauges
attached to the middle of the gauge length.
Coupons were pre-strained to three different values,
2, 4 and 8%, and then cured at ambient temperature for
30 or 90 days. After the designated curing duration, thepre-strained coupons were tested to fracture. A fourth
coupon was tensioned to fracture with no pre-strain toprovide data on properties without strain aging. Coupons
sampled from location A, B, F and G were used toFigure 2. Sample Location

Figure 3. Coupon with extensometer and buckling
restrainer
The tests were conducted using a 1,000-kN capacity
universal testing machine. The strain rate was controlled
at 0.00005 /s until the start of strain hardening, 0.00025

/s until strength drop (tensile strength was measured), andthen 0.0005 /s to fracture.

aging for three steels may saturate by 90 days at ambient
temperature.

Results and Discussion

Future Works

Figure 4 shows the strain-stress relationships obtained
from all six cases, three steel grades and two sample
locations, of monotonic coupon tests. The yield strength
was established based on the segment of the yield plateau
where the loading rate was 0.00005 /s. Change in tensionproperties was noticed in all six cases as re-appearanceof distinguishable yield plateau, rise in yield strength, risein tensile strength, and reduction in elongation withrespect to the original properties listed in Table 4. For all
steel, larger pre-strain resulted in larger change in tension
properties. Eight-percent pre-strain resulted in recoveryof upper-yield point and yield plateau, and 40 to 60%
increase of yield strength. Little change in tensile
strength was observed: 8% pre-strain resulted in 4%
increase in SS400 and smaller increase in SN400B andSN490B. Little change in elongation was observed: 8%
pre-strain resulted in 15% reduction in SS400 but nosignificant change in SN400B and SN490B. it wasnoticed that yield and tensile strengths were nearly
identical after curing if pre-strained 8% which suggested
a total loss of ductility. The result of longer curing times
indicates very small difference. The yield strength was
further increased around 5% from 30 days to 90 days. The
result agrees with Yamada et al. [8] who reported thestrain aging of SS400 steel may saturate at three months
in ambient temperature.
Among the three grades of steel, SS400 saw the mostprofound change in tensile properties, while SN400B andSN490B saw very little change. The result correlatedwith the nitrogen content listed in Table 3: According to
the thermal conductimetric method, the nitrogen contentof SS400 of 0.0073% was 2.5 times that of SN400B orSN490B. The change in yield strength and elongation for
SS400 steel was consistent with the report by Yamada etal. [8]. The rise in tensile strength was notably smaller
than the results reported by Yamada et al. [8] and Pense[5].

Cyclic Coupon Tests

A total of twelve H-500×200×10×16 beam specimens
were fabricated and will be tested to evaluate residual
performance from component level. The steels camefrom exactly the same heat with the three studied incoupon tests. Detail of the specimen is shown in Figure 5
and test setup is shown in Figure 6. The beam specimens
were bolted into a real beam-to-column connection andtested by a revised AISC protocol. Similar to coupon tests,beam tests were conducted in two stages. The beamspecimens first went through certain portion of theloading protocol until a designated plastic deformation;
cured 90 days under ambient temperature; and tested
again after curing period. A 3D reconstruction method
using images was developed using software Agisoft
PhotoScan. This method will be used to quantify the local
buckling of specimen flange.

Conclusion

Reference

A literature review on the effect of strain aging on tension
properties of steel was reviewed. Strain aging in Japanese
structural steel SN400, SN400B, and SN490B wasexamined through tensile coupon tests conducted in twophases. The change in tensile properties was influenced
by the extent of pre-strain and nitrogen content. Amongthe three steel grades, SS400 with nitrogen content of0.0073% saw the most profound change in tension
properties, while SN400B and SN490B with nitrogencontent of 0.003% saw very little change. The strain[1] A. Momtahan, R.P. Dhakal, and A. Rieder. Effects ofstrain-aging on New Zealand reinforcing steel bars.
Bulletin of NZSEE, Vol. 42, No. 2, September 2009.
[2] A.H. Cottrell, and B.A. Bilby. Dislocation theory ofyielding and strain ageing of iron. Proc. Physical Soc.,
62(1-349 A), pp. 49-61, 1949.
[3] T. Ono, K. Murayama, K. Masuda, A. Sato and T.
Yokokawa. The experimental study on the effect oftrace elements onn toughness of steel plate forbuilding structure. J. Struct Constru Eng, AIJ, No.536,
pp.157-162, 2000 (in Japanese).

Coupons sampled from location A, B, F and G will be
used to evaluate residual performance of flange whensubjected to cyclic and cured 90 days at ambient
temperature. A buckling restrainer as shown in Figure 3
was devised to prevent buckling of the coupon about it
minor axis during compression. The tests will beconducted by a 750kN dynamic actuator following adisplacement control, quasi-statically cyclic loading
protocol. It has an initial amplitude of 0.5% and theamplitude increases 0.5% every two cycles. Coupons will
be loaded until finish three designated strain levels, ±1,±2 and ±3%, and then cured at ambient temperature for
90 days. After the designated curing duration, the prestrained coupons were tested to failure. A fourth coupon
will be loaded to failure with no pre-strain to provide dataon properties without strain aging.

Beam Tests

Table 4. Measured mechanical properties
Steel Type
SS400
Location
Flange (A) Web (C)Yield Strength [MPa]300332448462
Tensile Strength [MPa]Yield Ratio
0.67
0.72
Elongation [%]
31.3
28.4

SN400B
Flange (A) Web (C)
284
322
457
472
0.62
0.68
32.9
29.0SN490B
Flange (A) Web (C)
344
358
521
522
0.66
0.69
28.5
25.3

600
(a)

(b)

(c)

(e)

(f)

500

Engineering Stress [MPa]400
As received
30 days
90 days

300
200
600 00.10.20.3(d)500

400

300
00.1

0.2

0.3

00.1

0.2

0.3

00.1

0.2

0.3

Engineering Strain

Figure 4. Strain-Stress Curve After One-Month Curing: (a) Flange of SS400; (b) Flange of SN400B; (c) Flange ofSN490B; (d) Web of SS400; (e) Web of SN400B; (f) Web of SN490B.

Figure 5. Beam specimen (dimensions in mm)[4] J.D. Baird. The Effects of Strain Aging due toInterstitial Solutes on Mechanical properties ofMetals. Met. Rev., 5, pp. 1-18, 1971.
[5] A. Pense. HPS corrugated web girder fabricationinnovations, Final report, Part 4: Literature andexperimental study of strain aging in HPS and otherbridge steels. Center for Advanced Technology,Betlehem, PA, 2004.
[6] L.N. Succop, A.W. Pense, and R.D. Stout. The effectof warm overstressing in pressure vessel steel
properties. Welding J., 49(8), pp. 354-364, 1970.

Figure 6. Beam tests setup[7] E.J. Kaufmann, B. Metrovich, and A.W. Pense.
Characterization of cyclic inelastic behavior onproperties of A572 Gr. 50 and A36 rolled sections.
ATLSS Report No, O1-13, August 2001.
[8] S. Yamada, A. Takeuchi, S. Kishiki, and A. Watanabe.
Hysteresis behavior of steel affected by strain aging.
Summary of Technical Papers of Annual Meeting,
AIJ, pp.927-930, 2006 (in Japanese).
[9] M.A. Mojtaba. Static strain aging in low carbon
ferrite-pearlite steel: Forward and Reverse Loading.
Electronic Theses and Dissertations (ETDs),
University of British Columbia, 2015.