When a steel specimen is tested under tension, damage usually develops evenly all along the specimen, finally necking and leading to the typical cup-cone fracture surface. Nevertheless, some steels present an unusual fracture pattern consisting on a plane fracture surface with a dark region in the centre of the fracture zone. In this contribution, the authors analyse the evolution of the internal damage by using X-ray computed tomography (XRCT) on 3mm-diametre specimens of two steels. The specimens are tested in subsequent loading steps, after each of which it is unloaded and analysed with XRCT. This procedure helps to identify the evolution of damage developed inside each specimen at predefined strain levels. XRCT reveals a very high initial porosity in the material with the cup-cone fracture pattern and a very low initial porosity in the other. In the latter, fracture is triggered by a concentrated internal damage that can be seen as an internal notch which produces a stress concentration that leads to the eventual failure

Atienza Riera, José Miguel

Cendón Franco, David Ángel

Gálvez Ruiz, Jaime

Molina Aldareguía, Jon M.

Sket, Federico

Suárez Guerra, Fernando

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

               International Symposium on Notch Fracture (ISNF)   
               Santander (Cantabria, Spain), March 29-31, 2017 
 
 
 
 
ANALYSIS OF THE DAMAGE EVOLUTION IN STEEL SPECIMENS  
UNDER TENSION BY MEANS OF XRCT 
 
 
 
F. Suáreza*, J.C. Gálvezb, D.A. Cendónc, J.M. Atienzac, F. Sketd, J. Molina-Aldareguiad 
 
aDepartamento de Ingeniería Mecánica y Minera. Universidad de Jaén. Campus Científico-Tecnológico de Linares. 
Avda. de la Universidad (Cinturón Sur) 23700-Linares (Jaén) 
bDepartamento de Ingeniería Civil- Construcción. Universidad Politécnica de Madrid . E.T.S.I. Caminos, Canales y 
Puertos, C/ Profesor Aranguren s/n 28040 – Madrid, Spain 
cDepartamento de Ciencia de Materiales, Universidad Politécnica de Madrid . E.T.S.I. Caminos, Canales y Puertos, C/ 
Profesor Aranguren s/n 28040 – Madrid, Spain 
dInstituto IMDEA Materiales, C/ Eric Kandel 2, Tecnogetafe 28906 – Madrid, Spain 
 
* Contact person: fsuarez@ujaen.es 
 
 
 
ABSTRACT 
 
When a steel specimen is tested under tension, damage usually develops evenly all along the specimen, finally necking 
and leading to the typical cup-cone fracture surface. Nevertheless, some steels present an unusual fracture pattern 
consisting on a plane fracture surface with a dark region in the centre of the fracture zone. In this contribution, the 
authors analyse the evolution of the internal damage by using X-ray computed tomography (XRCT) on 3mm-diametre 
specimens of two steels. The specimens are tested in subsequent loading steps, after each of which it is unloaded and 
analysed with XRCT. This procedure helps to identify the evolution of damage developed inside each specimen at 
predefined strain levels. XRCT reveals a very high initial porosity in the material with the cup-cone fracture pattern and 
a very low initial porosity in the other. In the latter, fracture is triggered by a concentrated internal damage that can be 
seen as an internal notch which produces a stress concentration that leads to the eventual failure. 
 
KEYWORDS: Steel, XRCT, Damage Evolution. 
 
 
 
1.  INTRODUCTION 
 
Steel is one of the most used building materials and has 
been studied for a very long time. Tensile test [1] is the 
most used experimental procedure to characterise this 
type of materials, but it only allows obtaining the 
behaviour before necking develops, which usually leads 
to neglect the material behaviour after necking. In fact, 
some fracture mechanisms involved that lead to 
engineering problems in real life still remain unclear. 
 
It is commonly accepted that fracture in metals is the 
result of a process of nucleation, growth and 
coalescence of microvoids [2]. It is also usual to observe 
that cylindrical metallic specimens subjected to tensile 
loading tend to show a well-known cup-cone fracture 
surface [3, 4]; nevertheless, this is not always the case. 
For example, eutectoid steel bars that are used for 
manufacturing prestressing wires show a flat surface 
with a dark circular region around the centre of it. Both 
fracture patterns can be observed in Figure 1. Cup-cone 
fracture pattern has been deeply studied in the past and 
many numerical models have been developed, which
successfully reproduce the experimental observations 
[4, 5, 6, 7]. 
 
In previous works [8, 9, 10], the authors analysed the 
second fracture pattern considering that fracture was 
triggered by an internal damage zone. According to this 
hypothesis, the circular dark region observed in the 
fracture surface corresponds to the damage zone 
generated by nucleation, growth and coalescence of 
microvoids, which increases as applied load becomes 
higher. Once the damage zone gets a certain size, 
fracture develops in a brittle manner. This was analysed 
with tools usually applied in Linear Elastic Fracture 
Mechanics, like a modified cohesive model [9] and the 
formula by Guinea, Rojo and Elices [10], which 
predicts the size of an internal circular notch that 
triggers fracture for a certain load. Both approaches 
gave reasonable results. 
 
In this contribution, the authors study damage evolution 
in two cylindrical steel specimens under tensile testing, 
one of a material that presents the typical cup-cone 
fracture pattern and the other one the flat surface with
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the dark region mentioned before. X-ray computed 
tomography (XRCT) allows detecting internal voids or 
fracture regions inside a material, which here helps to 
identify the internal damage evolution. 
 
 
Figure 1. a) Cup-cone fracture pattern; b) Fracture 
pattern exhibited by eutectoid steel bars used for 
manufacturing prestressing wires. 
 
 
2.  EXPERIMENTAL WORK 
 
2.1. Materials 
 
a) Material 1 
 
To produce steel wires used in prestressed concrete 
structures, raw eutectoid steel bars are cold-drawn, 
reducing their section and improving some mechanical 
properties of the material, making it more suitable for 
their final use [11, 12, 13]. Nevertheless, this process 
induces residual stresses in the steel bar so, to avoid the 
uncertainties derived from them, the specimen used in 
this study was obtained from a raw eutectoid steel bar 
before cold-drawing. 
 
The chemical composition of this material can be 
consulted in Table 1. 
 
Table 1. Chemical composition of Material 1 
C 
(%) 
Si 
(%) 
Mn 
(%) 
P 
(%) 
S 
(%) 
Cr 
(%) 
Mo 
(%) 
0.83 0.25 0.72 0.012 0.004 0.24 <0.01 
 
Ni 
(%) 
Cu 
(%) 
Al 
(%) 
Ti 
(%) 
Nb 
(%) 
V 
(%) 
N 
(%) 
0.02 0.01 <0.003 <0.005 <0.005 <0.01 0.0097 
 
b) Material 2 
 
This material corresponds to standard steel used as 
passive reinforcement in concrete structures. The steel 
used here is referred to as B 500 SD, according to the 
Spanish standard UNE 36068:2011 [14], with high 
ductility and an elastic limit ft of 500N/mm
2. 
 
Table 2 shows the chemical composition of this 
material. 
 
Table 2. Chemical composition of Material 2 
C 
(%) 
Si 
(%) 
Mn 
(%) 
P 
(%) 
S 
(%) 
Cr 
(%) 
Mo 
(%) 
0.22 0.18 1.00 0.024 0.042 0.08 0.03 
 
Ni 
(%) 
Cu 
(%) 
Al 
(%) 
Ti 
(%) 
Nb 
(%) 
V 
(%) 
N 
(%) 
0.14 0.46 <0.003 <0.005 <0.005 <0.01 0.0113 
 
 
2.2. Specimens 
 
For both materials, 3mm-diameter specimens were 
tested; dimensions can be consulted in Figure 2. 
 
 
 
Figure 2. Specimens dimensions 
 
The use of such a small diameter facilitates the 
penetration of X-rays in the specimens to identify 
internal damage. 
 
2.3. Testing procedure 
 
For both specimens, the same procedure was applied. 
Firstly, the specimen was analysed with the XRCT 
(Nanotom 160NF, Phoenix) before testing in order to 
obtain an initial picture of internal voids which would 
help comparing later images. 
 
Then, the specimen was tested under tension up to the 
maximum load point in the load-strain curve (see 
Figures 3 and 5); once the maximum load was reached, 
the test was stopped and the specimen unloaded and 
analysed by XRCT. Afterwards, the specimen was 
tested under tension again up to a higher strain state, 
unloaded and anlysed by XRCT. This process was 
repeated for several strain states and until the specimen 
eventually failed. 
 
Tables 3 and 4 show the predefined strain levels used in 
the described process for both materials. The strain 
values correspond to engineering strain measured for an 
initial length of 12.5mm. centered in the necking zone. 
 
Table 3. Strain levels applied with Material 1 
Stage 1 2 3 4 
eng 0.076 0.104 0.118 0.124 
 
 
 
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Table 4. Strain levels applied with Material 2 
Stage 1 2 3 4 5 
eng 0.193 0.267 0.288 0.306 0.319 
 
The tomographic images were obtained at 80kV and 
140mA using a tungsten target; the tensile tests were 
carried out with low speed displacement control and 
using a Suzpecar universal testing machine with a load 
cell of 100kN.  
 
2.4. Results 
 
Figures 3 and 4 show the load-strain curve of Material 1 
with the analysed strain levels marked with blue points 
and the damage evolution observed by XRCT, where 
red areas correspond to low- density areas that can be 
identified as internal voids. 
 
The same results for Material 2 can be observed in 
Figures 5 and 6. 
 
 
3.  DISCUSION 
 
The initial tomographic image of both specimens, 
obtained before tensile testing, underlines the different 
nature of both materials. Material 1 presents almost no 
internal voids, whereas Material 2 has a high volume of 
them. It is also interesting to observe how these voids 
are lined up in the longitudinal direction of the 
specimen, probably due to the manufacturing process. 
 
The different nature of both materials can also be 
observed in how differently both materials behave, not 
just in terms of stress, but also in terms of strain; while 
Material 1 reaches a maximum strain of about 0.13, 
Material 2 deforms up to around 0.34. This is also 
evident in the fact that Material 1 does not neck much, 
while necking is pretty noticeable in Material 2. Figure 
7 shows the profiles of both specimens obtained by 
means of a Nikon V-12B profile projector. 
 
 
Figure 7. Specimens shapes after fracture; shape for 
Material 1 specimen on the left and shape for Material 
2 specimen on the right. 
 
In Material 1, under maximum loading, some small 
voids can be noticed. In further strain states new voids 
appear, some of them apparently in a random manner 
but some others interestingly aligned in the loading 
direction. The most remarkable observation in this 
material has to do with the last obtained picture, 
corresponding to stage 4, an instant very close to failure. 
Before this image, the voids have appeared somehow 
independently, not connecting to each other, but here 
internal cracks seem to have formed, especially a large 
one in the center of the necking region, generating a 
large plane crack.  
 
This observation agrees well with the hypothesis posed 
by the authors in former works [8, 9, 10]. It suggests 
that fracture is initiated by an internal crack that, once it 
reaches a certain size, acts as an internal notch that 
triggers a brittle fracture process. This is interesting 
since it allows assuming some models and tools that 
belong to the Linear Elastic Fracture Mechanics field, 
which work reasonably well, as [9] and [10] proves, for 
a material that is clearly elastic-plastic. 
 
Regarding Material 2, as mentioned before, a high 
number of voids are present in the beginning of the test, 
even before any load is applied. As load becomes 
higher, voids increase in number and in volume. In 
comparison with Material 1, it is interesting to notice 
that no void coalescence can be observed. It is not clear 
if this means that coalescence takes place just instants 
before failure happens or if the test could not be stopped 
close enough to the failure moment, so internal fracture 
surfaces could be observed. According to the results by 
Bluhm and Morrisey [15], it could be expected to see a 
fracture plane perpendicular to the loading direction, 
which would form the flat surface of the cup-cone 
fracture pattern, and some inclined fracture planes, 
related to the shear planes of the cup-cone shape. 
 
 
4.  CONCLUSIONS 
 
In this paper, the authors have analysed two steels with 
distinct failure patterns, one of them corresponding to 
eutectoid steel used for manufacturing wires for 
prestressing (Material 1) and another one to standard 
steel used as reinforcement in concrete (Material 2). The 
study has been carried out on 3mm-diameter cylindrical 
specimens, which have been tested under tension in 
subsequent incremental strain steps up to failure. With 
the use of XRCT the internal damage evolution has 
been identified. 
 
Material 1 specimen presents a plane fracture surface 
with a circular dark region in the center of the fracture 
area, whilst Material 2 specimen presents the classical 
cup-cone fracture pattern. 
 
Material 1 has almost no initial voids and a slight 
internal porosity is developed through the tensile test. 
90
  
 
 
Figure 3. Load-strain curve of the specimen of Material 1. The load stages used to study the 
damage evolution are represented with blue circles. 
 
 
 
 
 
Figure 4. Damage evolution in the specimen of Material 1. Image a) correspond to the specimen 
before testing and images b)- f) to stages 1-4, respectively. 
91
  
 
 
Figure 5. Load-strain curve of the specimen of Material 2. The load stages used to study the 
damage evolution are represented with blue circles. 
 
 
 
 
 
Figure 6. Damage evolution in the specimen of Material 2. Image a) correspond to the specimen 
before testing and images b)-f) to stages 1-5, respectively.
92
  
Just before failure, a large penny-shaped internal crack, 
perpendicular to the loading direction, is formed. This 
large internal crack acts as an internal notch which 
triggers fracture, which agrees well with previous works 
by the authors [9, 10]. 
 
Material 2 has a high initial porosity, probably due to 
the manufacturing process, which increases as the 
tensile test is carried out. In this case, no internal 
fracture planes have been observed, although, according 
to the observations by Bluhm and Morrisey [15], the 
progressive formation of the cup-cone fracture surface 
should be expected. This could be due to a very fast 
formation of this surface before fracture. 
 
 
AKNOWLEDGEMENT 
 
The authors want to express their gratitude to Luis del 
Pozo and Luisa Villares, from Emesa Trefilería, S.A. 
(Arteixo, La Coruña) for supplying the steel wires, as 
well as for providing their useful comments. 
 
F. Suárez also wishes to express his gratitude to the 
Fundación Agustín de Betancourt for the grant 
provided. 
 
 
REFERENCES 
 
 [1] ISO 6892-1:2009, Metallic materials. Tensile 
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fundamentals and applications, second edition. CRC 
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 [3] Mirza, M.S., Barton, D.C., Church, P., Sturges, 
J.L., Ductile fracture of pure copper: an experimental 
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896, 1997. 
 
 [4] Tvergaard, V., Needleman, A., Analysis of the 
cup-cone fracture in a round tensile bar, Acta metall. 
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model and data for metals subjected to large strains, 
high strain rates and high temperatures, Proceedings of 
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 [12] Toribio, J., Ovejero, E., Effect of cumulative 
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Analysis of the Damage Evolution in Steel Specimens under Tension by Means of XRCT

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Analysis of the Damage Evolution in Steel Specimens under Tension by Means of XRCT

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