Page 72 - Zmist-n4-2015

Ô³çèêî-õ³ì³÷íà ìåõàí³êà ìàòåð³àë³â. – 2015. – ¹ 4. – Physicochemical Mechanics of Materials

UDK 620.193:691.328
ANALYSIS OF THE PLASTICITY CHARACTERISTICS
OF PROGRESSIVELY DRAWN PEARLITIC STEEL WIRES

1 2 1
М. І. HREDIL , J. TORIBIO , H. M. NYKYFORCHYN
1
Karpenko Physico-Mechanical Institute of the NAS of Ukraine;
2
University of Salamanca, E.P.S., Zamora, Spain
Changes in the plasticity characteristics in air and in a hydrogenating environment of
prestressing steel wires due to cold drawing process are investigated on the basis of slow
strain rate tests on smooth specimens. The tested pearlitic steel is highly susceptible to
hydrogen embrittlement at all stages of cold drawing. The inconsistency is revealed
between the changes of two plasticity characteristics: reduction in area and uniform elon-
gation. The obtained results are analysed distinguishing the contribution of resistance to
crack initiation and crack propagation. Susceptibility to crack initiation increases as a
result of cold drawing with simultaneous improvement of the crack propagation resistance.
Keywords: prestressing steel, plasticity parameters, crack initiation and propagation.

Prestressed concrete structures are a very important part of infrastructure of any
country. They include buildings, transportation network, energy production, nuclear
power plants, water and waste-water treatment systems etc. There is a lot of long-term
service objects that have been in operation for 50 years and more. Their increasing age,
the environmental impact, increasing traffic loads may change their serviceability [1].
Damage during service, frequently appearing after several years or decades of exploi-
tation, is usually most consequential. The lack of sufficient protection resulting from
the alkaline nature of the cement matrix in as-received state or the loss of alkalinity due
to carbonation and/or depassivation caused by the chloride attack are the major causes
of corrosion initiation. Failures of prestressing steel in structures are predominantly
attributed to hydrogen-induced stress corrosion cracking [1] under simultaneous action
of aqueous corrosive environment and static tensile stresses. In such conditions not
only concrete coating fracture occurs but also prestressing steel wires could suffer in-
service degradation similarly to other steels [2–5].
Mechanical behaviour and fracture peculiarities of prestressing steels under diffe-
rent environmental conditions have been studied thoroughly using both notched and
pre-cracked specimens [6–9], paying attention to the stress state in the vicinity of the
crack tip [7–9]. Meanwhile, there is a lack of data concerning smooth specimens and
they are not clear/monosemantic due to a big scatter of results [6, 10]. This paper is
aimed at fulfilling this gap and finding out some regularities of hydrogenation effect on
cracks initiation and propagation in the prestressing steel wires without pre-cracking.
Materials and methods. Materials used were cylindrical specimens of cold drawn
pearlitic steel wires from different steps of the real manufacturing process, and also the
initial hot rolled steel bar. The steel conventionally called E, which passes through
seven cold drawing steps has been studied. The chemical composition of the steel is the
following (mass.%): 0.88 C, 0.69 Mn, 0.22 Si, 0.010 P, 0.024 S, 0.239 Cr, 0.076 Ni,
0.010 Mo, 0.129 Cu, 0.118 V, Fe is balance. Plastic strain ε P = 2ln(D / D accu-
)
accum 0 i
mulated by steel at each stage of the cold drawing process and corresponding diameter

Corresponding author: M. I. HREDIL, e-mail: mysya@ipm.lviv.ua
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