Elongation Requirements
for
Stainless Steel Rebar
Presented by:
Suresh Gupta
ASTM Committee Meeting A01.05
May 8, 2012
Purpose of Elongation
• Elongation is a measure of ductility, which
provides confidence that metals can be
formed, without cracking or fracturing.
• In rebar, it defines ductility so that bars can be
bent without cracking.
– If bars do not crack during fabrication, any excess
elongation is irrelevant.
Alloy Characteristics
With all other variables remaining same,
• Austenitic Alloys have highest elongation.
• Duplex Alloys have elongation in mid range
• Ferritic Alloys have lowest elongation
Measurement Methods and Facts
• % Elongation, when measured in a fixed gage
length, e.g. 8 inches, is lower than when
measured in a gage length, as a multiple of
diameter, e.g. 4 X diameter (4d).
(when 4d is less than fixed gage length).
• Elongation is generally lower as diameter goes
smaller.
• This fact is clearly described in ASTM E8,
Appendix XI
ASTM E8
Standard Test Method for Tension Testing of Metallic Materials
Appendix XI – Factors Affecting Tensile Test Results
• Comparison of elongation values determined using different
specimens requires that the following ratio be controlled:
Lo/(Ao)1/2
(XI.1)
Where:
Lo = original gage length of specimen, and
Ao= original cross-sectional area of specimen.
XI.4.4.1 Specimens with smaller Lo/(Ao)1/2 ratios generally give
greater elongation.
Note: (Ao)1/2 is proportional to diameter (d).
Therefore, for same ductility, larger diameter samples result in
greater elongation values, when measured in fixed gage length.
ASTM A955-9a
• Until revision 10a, elongation was not required for
– sizes # 3-5 Grade 75, probably because it was known that it is
unrealistic or not necessary.
– Grade 40 for sizes 7 and larger because, in any condition it will meet
the requirement, anyway.
% Elongation Requirements for
Rebar
Carbon and Alloy steels
• ASTM A615 requires 6 to 7% for Grade 75
• ASTM A706 requires 10 to 14% for all Grades.
Stainless Steels
• BS 6744 requires 14% minimum in 5d gage
length for Grade 500 (72.5 ksi).
• ASTM A955 requires 20% minimum in 8 inch
gage length for all grades, all sizes.
Manufacturing & Process Control
• Even with precise chemistry and process
control, duplex stainless steels meet the
elongation requirement very marginally.
– If rebar is supplied in coils, machine straightening
will cause work hardening and elongation will go
down even further.
–This may cause unnecessary dispute.
Rationale
• If 6% elongation in carbon steel is sufficient
• If 14% in 5d is sufficient for stainless steel
rebar per BS6744 in Europe,
–It should be sufficient for us as well.
FYI
• ASTM Specifications for plain smooth bars
require elongation in 4d
Proposal
• It is therefore recommended that,
– Using a foot note to Table 3, elongation
requirements be lowered to 14% for Grade 75,
• for all sizes of duplex grades and,
• all grades of stainless steels under 1.00 inch.