FIELD OF INVENTION
The present invention relates to an improved test sample for charpy V-notch
impact toughness test of steel rebars and the process of testing where multi-layered
microstructure on the cross section during rolling and subsequent cooling in the plant.
The new sample geometry is supposed to overcome the limitations of measuring
impact properties in rebar using standard Charpy V-notch specimens.
Naturally hardened or microalloyed rebars are having uniform microstructure
throughout their cross-section and their impact toughness can be measured easily
using standard geometry (rectangular) in CVN test. High-yield reinforcing rebars
shows multi-layered microstructure on their cross-section, and thus, standard
specimens will not be representative for the actual microstructure of the rebar.
However, other sample geometries can be thought of by suitably machining steel
rebars so that samples for CVN testing represents the actual layered microstructure.
There are many studies available in literature, where direct measurement of impact
toughness of steel rebars have been reported [12]. The term "direct measurement"
means that V-notched specimens with original round section of rebars (and not with
the standard rectangular specimens) were used. The major demerits of this direct
measurement method comes from the variable size and depth of composite

microstructures of tempcore of rebars. Therefore, a new geometry for CVN test will be
necessary to correctly measure the toughness property of TMT (Thermo-mechanically
treated) rebar.
BACKGROUND OF THE INVENTION
The dynamic properties such as impact toughness of TMT steel rebar are very
much important factor for the design of structural constructions and they have been
extensively investigated in order to avoid the danger of premature failures [10].
Temperature and strain rate are the two important elements which affects the
fracture toughness of TMT rebars used in construction purpose under concrete. In this
respect, for investigation of impact toughness property of the reinforcing TMT steel
rebars, the transition temperature range from ductile-to-brittle fracture has to be
recorded properly. By definition, the toughness of a metal is the intrinsic ability to
absorb mechanical energy and to deform plastically before fracture under impact
loading.
The Charpy V-notch test is the standard procedure to evaluate the toughness
and the ductile to brittle transition temperature of steel rebar, because of easier
sample preparation and low cost, however, the energy absorption values cannot be
related directly to the structural design.
The use of high-yield reinforcing steel rebars in concrete construction has
been greatly increased in the current years.
As per as the current standard of notched bar impact testing of metallic
materials (ASTM E23-12c [1]) the standard sample size for Charpy impact testing is
10 mm x 10 mm x 55mm (with square cross-section) and the sample contains a notch
of 2 mm depth, having 0.25 mm notch root radius, Fig. la. Reinforcement bars in

general have round cross-section of variable diameter (<10 mm to ~40 mm) with a
hardened layer at the surface having thickness approximately 8-10% of the diameter
of the rebar and a softer core. The variable microstructure is the result of the
industrial Tempcore' process [2-3] and is absolutely necessary for achieving required
mechanical properties with lower alloying cost. These rebars cannot be machined into
a standard CVN specimen representing hardened surface and softer core together.
Standard geometry samples can only be used to measure the impact toughness of
naturally hardened rebars and microalloyed rebars (provided standard samples can be
machined), due to the uniform microstructure throughout the cross-section of those
rebars.
Due to the presence of complex microstructure on the round cross-section,
preparation of standard geometry Charpy V-notch specimens from TMT rebars
becomes almost impossible. Even if standard 'full-size' specimen or any other non-
standard 'sub-size' specimen (as proposed in ASTM E23-12c standard [1]) is prepared
from the rebars, those specimens may not represent the impact toughness of the
complex microstructure.
Lack of material availability, especially in case of smaller diameter rebars, and
the requirement of excessive machining for sample preparation impose further
challenges on impact testing of TMT rebars in industrial scale. Various difficulties
associated with the sample preparation could be the main reason why impact
toughness testing has rarely come up as a quality control measure for TMT rebars.
Considering the above aspects, present study proposes the use of direct V-notched
specimens of semi-circular cross-section for impact testing of TMT rebars. The
suggested specimen retains the original round face of the rebar in side, whilst a flat
face machined on the other side and the notch is prepared at the middle of the flat

face. Design of the proposed V-notch specimen is shown in Fig. lb. The direct V-
notch specimen requires less machining than full-size or sub-size specimens listed in
ASTM E23-12c moreover it also takes care of the diameter of the rebar as well as the
complexity of the microstructure in the cross- section of rebar.
a) Standard sample geometry used in prior studies:
Charpy impact test method is extensively used in the industries for the
evaluation of impact toughness of a variety of mass-produced sections such as, plate,
beam, bar and welded joints. ASTM standard ASTM E23-12c [1] and Indian standard
ISI1757 [11] describe the sample design for Charpy impact testing as shown in Fig. 2.
The pendulum type impact testing equipment is shown in Fig. 3, which is used to
perform the impact toughness testing of metallic materials in a three-point bending.
The tangential velocity of pendulum impact at the Centre of the strike lies in the range
of 3-6 m/s [ASTM E23] for which the strain-rate at the crack tip of the specimen can
reach as high as 6000-25,000/s [12]. The specimen is supported against the anvils,
the pendulum is released without vibration and the specimen is impacted by the
striker. During Charpy impact testing the registration of the load signal is carried out
by strain gauges positioned directly on the striker edge and arranged as a Wheatstone
bridge. The output is amplified by a two-stage amplifier. The measuring device was
able to register either load-time (F-t) diagrams or load-deflection (F-f) diagrams.
However, due to the round cross-section, variable diameter of the rebars and double-
layered microstructure on the cross-section of the rebars, preparation of standard
geometry specimen that can represent the impact toughness and transition behaviour
of the actual rebars, is a difficult task.

b) Non-standard sample geometry used in Prior studies:
Panigrahi et al. [10] has used 'sub-size' specimen of rectangular cross-section
on their work to study the impact toughness of Rock Bolt reinforcing steel rebars.
Sub-size, Charpy V-notched specimens (5mmxl0mmx55mm) were machined from the
sub-surface location of the 32 mm diameter TMT rebars in longitudinal fashion as
shown in Fig. 4 [10]. However, for smaller diameter rebars (say, 10 mm) preparation
of such specimen is difficult. Moreover, extensive machining is also needed for the
preparation of sub-sized rectangular samples, increasing the specimen cost and
specimen preparation time. Nikolaou and Papadimitriou [14] used a specimen of
round cross-section prepared from 12 mm diameter bars for both the Tempcore and
the microalloyed reinforcing steels. The sample geometry and notch design used by
them is shown in Fig. 5 [14]. The use of round cross-section is associated with the
difficulty of supporting the specimen against the anvil. As majority of the impact
testing machine has flat anvils on which round specimens cannot sit in a stable
fashion. For testing of round rebar specimens of different diameters variable round
grip will be required, which will be associated with additional expenditure and the
need for regular changeover of anvils for testing of flat and round specimens.
OBJECTS OF THE INVENTION
The main object of the invention is to propose an improved test sample for
charpy v-notch impact toughness test of steel rebar which is capable of measuring
impact toughness of thermo-mechanically treated (TMT) steel rebars using the Charpy
V-notch method, which will be able to account for the variable microstructure in the
rebar.

Another object of the invention is to propose an improved test sample for
charpy v-notch impact toughness test of steel rebar which is able to overcome the
major complications to measure the impact toughness encountered with the rebars
with variable diameters.
A still another object of the invention is to propose an improved test sample for
charpy v-notch impact toughness test of steel rebar which can address the merits of
using new specimen geometry (round or half-round specimens) instead of standard
geometry in CVN tests on the measurement of impact fracture toughness of the
reinforcing steel rebars.
A further object of the invention is to propose an improved test sample for
charpy v-notch impact toughness test of steel rebar which can establish ductile to
brittle transition curve for steel TMT rebars using new new geometry in CVN tests.
A still further object of the invention is to propose an improved test sample for
charpy v-notch impact toughness test of steel rebar which is able to identify the
factors effecting impact toughness and to establish an optimum combination of
factors, which increases energy density to highest level for rebars using new
geometry in CVN tests.
A still another object of the invention is to propose an improved test sample for
charpy v-notch impact toughness test of steel rebar which can reduce time for
measurement of impact toughness of TMT rebars.
A still another object of the invention is to propose an improved test sample for
charpy v-notch impact toughness test of steel rebar which is capable of avoiding
machining related defects of TMT rebars to make standard sample for the
measurement of their impact toughness.

SUMMARY OF INVENTION
The current invention is aimed to design a new geometry of testing samples for
the measurement of impact properties of steel rebars where multi-layered
microstructure on their cross section exist. Naturally hardened or microalloyed rebars
are having uniform microstructure throughout their cross-section and their impact
toughness can be measured easily using standard geometry (rectangular) in CVN test.
High-yield reinforcing rebars shows multi-layered microstructure on their cross-
section, and thus, standard specimens will not be representative for CVN tests.
Therefore, a new geometry for CVN test has been proposed and it will be better
suited for measuring impact toughness property for thermomechanically processed
(TMT) rebars. For the measurement of impact toughness of rebars of different sizes,
industrial TMT rebars of three different sizes (namely 10 mm, 12 mm and 16 mm)
have been chosen which were made of same steel composition. Those rebars were
manufactured using the thermo-mechanically treated (Tempcore) process and have a
complex 'multi-layered' microstructure on their cross-section.
The Charpy V-notch tests were perfomed using the newly proposed sample
geometry to evaluate the toughness, the ductile to brittle transition temperature and
the energy absorption of these steel rebars and the results were compared with the
standard sample geometry results. It has been found that although, the nature of the
impact energy transition curves obtained from the testing of newly designed
specimens are similar in nature to that obtained from the testing of sub-sized
specimens of rectangular cross-section (as mentioned in ASTM E23-12c standard), the
standard method was fail to discriminate the impact energy transition for variable
diameter of TMT rebars, whereas newly designed sample geometry has given the

right trend. However from the result it is also clear that the newly proposed sample
geometry provides a better representation of the impact property of the multi-layered
microstructure.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1: Schematic diagram showing the design of (a) standard geometry Charpy V-
notch specimen as per ASTM E23-12c standard [1] and (b) new sample geometry
direct V-notch specimen as proposed by the present study.
Fig. 2: Schematic diagram showing three common notch designs as used for Charpy
impact testing as per ASTM standard [13].
Fig. 3: Schematic diagram showing the Charpy impact testing machine and. the
hammer impact on the specimen [13].
Fig. 4: Schematic showing the design of sub-sized specimen machined from 32 mm
diameter TMT rebars as presented by Panigrahi et al. [10].
Fig. 5: Schematic showing the notch geometry in round specimen as used for impact
testing of 12 mm rebars by Nikolaou and Papadimitriou [14].
Fig. 6 : Macro-view of the cross-section of 12 mm diameter rebar showing inner core,
outer rim and transition zone (T.Z.).
Fig. 7: SEM micrographs of different regions of rebars of different sizes. (F: Ferrite, P:
Pearlite, B: Bainite and MT: Tempered martensite).

Fig. 8 : Schematic showing the design of Charpy impact testing specimen as
proposed by the present study, (where "D"=diameter of rebar, "d"=notch depth).
Fig.9: (a) Instrumented Charpy impact testing machine, (b and c) placement of direct
V-notched specimen of proposed design over the flat anvil of instrumented impact
testing machine.
Fig.10: Impact energy transition curves of 10 mm, 12 mm and 16 mm diameter TMT
rebar samples determined using direct V-notched specimen, as proposed by the
present study.
Fig.ll: Fracture appearance transition curves of 10 mm, 12 mm and 16 mm
diameter TMT rebar samples determined using direct V-notched specimen, as
proposed by the present study.
Fig.12: Instrumented Charpy impact data showing load vs. displacement curves of
10, 12 and 16 mm dia rebar samples tested at (a, d, g) lower shelf temperature (-
196°C), (b, e, h) transition temperature (-50ºC) and (c, f, i) upper shelf temperature
(+50°C) regions. The symbols are defined as: Pgy, general yield load, Pmax, maximum
load, Pi, load at initiation of unstable crack propagation, and Pa, load at end of
unstable crack propagation (arrest load).
Fig.13: Charpy sub-size specimen geometry fabricated from 10, 12 and 16mm
diameter TMT rebars.
Fig.14: Impact energy transition curves of 10 mm, 12 mm and 16mm diameter TMT
rebar specimens obtained from Charpy impact testing of sub-size specimens.

Fig.15: Fracture appearance transition curves of 10 mm diameter TMT rebar samples
determined using Charpy sub-size specimens.
Fig.16: Instrumented Charpy impact data showing load vs. displacement curves of 10
mm rebar samples using Charpy sub-size specimens, tested at (a) lower shelf
temperature (-196°C), (b) transition temperature (-50°C) and (c) upper shelf
temperature (+50°C) regions. The symbols are defined as: Pgy/ general yield load,
Pmax, maximum load, Pif load at initiation of unstable crack propagation, and Pa, load
at end of unstable crack propagation (arrest load).
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
1. Experimental Materials:
For the measurement of impact toughness of rebars of different sizes,
industrial TMT rebars of three different sizes (namely 10 mm, 12 mm and 16mm)
have been chosen and those rebars were made of same steel composition as given in
Table 1. Those rebars were thermo-mechanically treated and have a complex 'multi-
layered' microstructure on the cross-section. Macro-view of the cross-section of 12
mm rebar having different microstructural regions is shown in Fig. 6 and high
magnification SEM micrographs taken from the core, rim and transition region of the
rebars are given in Fig.7. Average depth and area of the rim, measured by image
analysis for different rebar sizes, are listed in Table 2.


SEM (Scanning electron microscope) micrographs in Fig.7 show ferrite-pearlite
microstructure at the core and tempered martensite microstructure at the rim for all
the rebars investigated during this work. At the transition region the microstructure is
comprised of different constituents such as, ferrite, pearlite, bainite and martensite.
2. Proposed new sample Geometry:
In the present investigation new direct V-notch specimens were fabricated,
having semicircular cross-section i.e. half of the total rebar cross-section (0.5xnR2,
where 'R' represents the radius of the rebar), i.e. the sample retains the original round
face of the rebar in one side with a flat face on the other side, having a V-notch
prepared on the flat face as shown in Fig. 8. Flat base was prepared at both ends of
the specimen so that the specimen can sit steadily on the anvil.

In the direct V-notched (DVN) specimen for different diameter TMT rebars, the
notch should have a minimum depth in order to induce a triaxial state of stress in
front of the notch. Insufficient depth prevents breaking, leading to general yielding
throughout the crosssection of the specimen [13]. According to the previous study,
varying the notch depth of direct V-notched specimens (of round cross-section)
introduces different states of stress in front of the notch. The effect of notch depth on
the impact toughness can be studied from 'Notch Depth Ratio' (NDR).
Notch depth ratio, NDR, can be defined as :

where, 2R is the rebar diameter and 'd' is the notch depth.
As the notch depth ratio (NDR) increases the fracture energy decreases continuously
and a value is attained where the fracture energy remains relatively constant [14].
This occurs beyond a critical notch depth ratio, i.e. for d > R [14]. At this stage, a
local triaxial state of stress approaching plane strain condition is attained and yield is
localized at the notch-root. The concentration of plastic strain below the root of the
notch raises the effective strain rate and causes the yield strength to increase by
strain hardening. In the present study, for the samples of different diameters (10,
12, 16mm), the notch depth ratio was kept constant, i.e.
in order to maintain constant stress-triaxiality at the notch
root.
Therefore, with increasing the bar diameter, though the notch depth increases,
yet the constraint at the notch root remains the same. Therefore, the effect of notch-
depth on the absorbed impact energy is minimized as far as possible.

3. Charpy Impact Testing with proposed sample geometry:
Instrumented Charpy impact testing was carried out on the proposed rebar
samples (direct V-notched specimens) having semi-circular cross-section over a wide
range of temperature (+100°C to -196°C). Sample temperature was maintained by
immersing the samples either in hot water (for higher temperature testing) or in a
f mixture of liquid nitrogen and methanol used at different proportions (for low
temperature testing). Specimens were soaked at least for 15 minutes in the bath
before testing. A thermocouple remained inserted into the samples over the entire
testing process for correctly monitoring the samples temperature. The time span
between removing the specimens from the bath to the hammer impact was
maintained within 5 s as instructed by ASTM E23-12c [1] standard. All Charpy testing
was conducted in an Instron 400J impact machine (Model: SI-1C3), attached with an
Instron Dynatup Impulse Data acquisition system. The placement of direct V-notch
specimen over anvil of the instrumented Charpy impact testing machine are shown in
the Fig. 9.
3.1 Determination of impact energy transition curves:
The impact transition curves were drawn based on the impact energy absorbed
by the test specimens of proposed design at different test temperatures in
instrumented Charpy impact test. The impact energy transition curves of 10 mm, 12
mm and 16 mm rebars are presented in Fig. 10. The transition curves are drawn in
between the scattered data points by fitting a curve using a hyperbolic tangent curve
fitting method [11]. The energy transition temperature (ETT) corresponds to the
impact energy value halfway between the upper shelf energy (USE) and the lower
shelf energy (LSE). The ETT of the investigated steel lies in the range of -50 to -55°C-

The upper shelf energy of investigated rebar samples are different due to the
different diameters of the tested rebars. It is found that larger the diameter of the
rebar i.e. higher the sample cross-section area, higher are the absorbed USE values.
3.2. Determination of the fracture appearance transition curves:
Impact tested samples were subjected to extensive fractographic study using
SEM and the nature of the fracture surface was investigated. Complete (100 %)
fibrous fracture surface, indicating stable-ductile crack propagation was observed at
higher test temperatures (in the upper shelf region) and complete cleavage fracture
surface, indicating unstable-brittle crack propagation was found at lower test
temperatures (in the lower shelf region). Partial fibrous and cleavage fracture was
observed on the fracture surface over the transition temperature regions and the
relative fraction of both type of fractures varied depending on the test temperatures.
Fracture appearance transition curves of the investigated rebar samples obtained from
the present study are given in Fig. 11. Similar to energy transition temperature, the
fracture appearance transition temperature of the investigated steel lies at ~-50°C.
3.3. Determination of load-displacement and the fracture mechanism:
The load vs. displacement plots obtained from the impact testing of 10, 12, and
16 mm diameter rebar samples (of proposed design) at very low temperature (-
196°C), intermediate temperature (-50°C) and high temperature (+50°C) are given in
Fig. 12 (a - i)
The Load vs. displacement plots obtained from the instrumentation system of
Charpy impact testing, were analyzed using the load diagram approach'. Unstable

brittle fracture occurred at the lower shelf region, as indicated by the sharp load
drop, Fig. 12a, 12d and 12g. At the transition-temperature range, crack propagation
starts in stable fashion, following by unstable crack propagation as indicated by load
drop from Pi, to Pa, Fig. 12b, Fig. 12e and Fig. 12h. Fracture initiated at the root of
the notch by fibrous tearing. At a short distance from the notch, unstable crack
extension occurs (at Pi) and the fracture mechanism changes from fibrous to
cleavage (mixed mode, or another low energy fracture mode), which often results in
distinct radial markings in the central portion of the specimen (indicative of fast,
unstable fracture) [Heartzburg book]. After several microseconds the unstable crack
extension arrests (at Pa). Final fracture occurs at the remaining ligament and at the
sides of the specimen in a stable manner. As shear-lips are formed at the sides of
the specimen, the plastic hinge at the remaining ligament ruptures. In the ideal
case, a 'picture frame' of fibrous (stable) fracture surrounds a relatively flat area of
unstable fracture. At high test temperature, stable crack extension occurred
throughout the specimen as indicated by load vs. displacement curves, Fig. 12c, 12f
and 12i.
Chaypy impact testing over a temperature range, using the proposed sample
design, therefore, identified the impact transition behaviour for a range of rebar sizes.
In order to show the validity of the present results, impact testing needs to be carried
out on the same rebars using a different sample design.
4. Charpv impact testing using 'sub-size' standard geometry f ASTM E23-
12c):
In order to confirm the impact transition behaviour of the investigated rebar
steel, the instrumented Charpy impact testing was also carried out by using sub-size
Charpy V-notch (5 mmxlO mmx55 mm) longitudinal specimens machined from the
sub-surface location of 10, 12 and 16 mm diameter TMT rebars, as shown in Fig. 13.

Such specimens have been used before for impact testing of rebars [7, 12]. The
impact energy transition curves for different rebar sizes as obtained by using sub-size
Charpy impact specimens are presented in Fig. 14. In order to study the fracture
mode and energy absorption capacity of the investigated steel as the function of test
temperature, load-displacement diagrams obtained from testing of sub-sized
specimens are plotted in Fig. 15 and Fig. 16.
The impact energy transition curves, fracture appearance transition curves and
load vs. displacement curves in Fig. 14, Fig. 15 and Fig. 16 show clear impact
transition behaviour. The energy transition temperature lies ~ -50°C, as obtained
from the impact testing of direct V-notched specimens of proposed design. The critical
demerit of the standard method is that it does not capture the impact energy
transition with variable diameters, whereas, the specimens of proposed design
successfully picked up the impact energy transition behaviour of the TMT rebar having
different diameter. It must be noted that, as the constraint acting in sub-size
rectangular specimen and direct V-notched semicircular specimen is different, the
absolute values of absorbed impact energy of both specimens cannot be compared. In
addition, the proposed sample design is capable of comparing the impact energy
transition behaviour of different grades and variable diameters of TMT rebars and can
be used for quality control measure. Another biggest advantage of the proposed
direct V-notched specimen is the ease of sample preparation with the requirement of
less machining than the preparation of sub-sized rectangular specimens.

WE CLAIM
1. An improved test sample for charpy V-notch impact toughness test of steel
rebars comprising;
a fabricated V-notched specimen (S) having flat base (B) prepared at both ends
of the specimen (S) for allowing the said specimen to sit steadily on an anvil of a
charpy impact testing mavhine;
characterized in that,
the said specimen has a circular cross-section where the specimen/sample
retains the original round face (R) of the rebar in one side with a flat face (F) on the
other side having a V-notch (V) prepared on the flat face (F) wherein the notch has a
minimum depth for inducing a triaxial state of stress in front of the notch for
preventing breaking and avoiding yielding throughout the cross-section of the
specimen (S).
2. An improved test sample as claimed in claim 1, wherein notch depth (d) is
defined as Notch depth ratio (NDR),

where, 2R is the rebar diameter and 'd' is the notch depth.

3. A process of testing with the improved test sample claimed in claim 1
comprising;
maintaining sample temperature by immersing the sample in hot water for
higher temperature testing or in a mixture of liquid nitrogen and methanol for low
temperature testing;
soaking the specimen for at least 15 minutes in a bath;
inserting a thermocouple into the specimen over the entire testing process for
correctly monitoring the sample temperature;
wherein the time span between removing the specimen from the bath to the
hammer impact on the anvil of the impact machine is maintained within 5 seconds.
4. The process as claimed in claim 3, wherein the range of temperature of the
specimen (S) is maintained at +100°C to -196° C.