Steel is responsible for providing resistance to flexotraction to reinforced concrete structures. Steel is responsible for providing reinforced concrete structures with a flexural strength. For this reason, it is important to study its behaviour under different tensile states. This study used measuring equipment that was able to determine variations in magnetic properties of B500-SD steel bars during standard tensile tests. The magnetic field generated by a Helmholtz coil was collected through a secondary circuit. This enables the induced electromotive force to relate with the steel deflection stages when subjected to the tests. Moreover, it was possible to determine the variation of magnetic permeability when submitting 12mm and 16mm diameter bars to different tensile states. This method could prove extremely useful in determining the tensile state of ribbed steel bars that are embedded into the concrete structure.
El acero se encarga de dotar de resistencia a flexotracción a las estructuras de hormigón armado. Por esto, es imprescindible conocer cuál es su comportamiento cuando se encuentra sometido a distintos estados de tensión. En este trabajo se ha implementado un equipo de medida capaz de determinar las variaciones en las propiedades magnéticas de las barras de acero B500-SD cuando se someten a un ensayo de tracción normalizado. El campo magnético generado por una bobina Helmholtz ha sido recogido mediante un circuito secundario, permitiendo relacionar la fuerza electromotriz inducida con las etapas de deformación del acero durante el ensayo de tracción. Además, se ha podido determinar cómo varía la permeabilidad magnética al someter barras de diámetro 12 mm y 16 mm a distintos estados de tensión. Este método puede ser útil para conocer el estado tensional de las barras de acero cuando se encuentran dentro de una estructura de hormigón.
The reinforced concrete, which is mainly formed by concrete and ribbed steel bars, is one of the most used materials in the construction sector. For this reason, it is very important to study its behaviour and its components performance, under different types of stress during the building process (
Steel ribbed bars provide reinforced concrete structures with higher flexural strength. The study of these materials is crucial to determine the behaviour of structures during the building process (
For this study, and as we are dealing with steel alloys, with iron as the main component, we will work with ferromagnetic materials that are widely used in engineering because of their high magnetic susceptibilities. This means that the magnetic induction is practically proportional to the magnetization of the studied solid (
One of the most used configurations to reach a region of a relatively uniform magnetic field is the Helmholtz Coil. Helmholtz coils are two coaxial circular coils of the same radius. This radius is equal to the distance between the coil plains (
When using this type of coils in industry, it is essential to know its intensity and uniformity and to delimit the region where the generated magnetic field is uniform. One of the most important applications of these coils is the possibility of measuring the magnetic field generated by electronic devices, with the aim of studying its behaviour in wider systems (
When submitting ferromagnetic material to an external magnetic field such as the one generated by a Helmholtz coil, the variation of magnetic domains is affected by the microstructure of the material and its tensile state (
The objective of this work is to determine the variation of magnetization of construction steels under longitudinal tensile stress. By using Helmholtz coils, the challenge is to establish the relation between the applied tensile stress, the deflection rate and the variation produced in the magnetization of ferromagnetic material. To achieve this, two different diameters of steel bars and measuring equipment designed by the authors were used in the tests. This will enable to calculate with a high degree of accuracy the variations in the magnetic properties of the material.
For performing the testing and following the standards UNE 36065:2011 and UNE 36068:2011 about ribbed steels for concrete (
Yielding Limit |
Ultimate Tensile Stress |
Relation |
Elongation at maximum load |
Break Elongation |
---|---|---|---|---|
500 | 575 |
|
|
|
The B500SD steel has specific chemical characteristics referred to the analysis of the chemical composition of casting and the admissible values in the analysis of the product, as shown in
Element * | C (%) | S (%) | P (%) | N (%) | Cu (%) | C equivalent (%) |
---|---|---|---|---|---|---|
|
0.22 | 0.05 | 0.05 | 0.012 | 0.8 | 0.5 |
|
0.24 | 0.055 | 0.055 | 0.014 | 0.85 | 0.52 |
* Percentages are referred to maximum permissive content.
The equivalent carbon is calculated as follows (
where the symbols of chemical elements indicate its content in percentage in mass.
At the microstructural level, the B500SD steel is composed by a ferrite matrix with a substantial volume fraction of the pearlite micro-constituent (formed by a ferrite-cementite lamellar composite). The crystal structure of cementite is orthorhombic and ferrite in body-centred cubic, which are related with the magnetic domain structure on magnetization of the steel (
It is intended to show the variation of magnetic properties of concrete steels under mechanical stress. For such a purpose, it was necessary to design a coil capable of generating a strong enough magnetic field to delimit the test to the breaking zone in the ferromagnetic material.
The relation in which flow density can be calculated in any point of space where the electric current circulates is given by Biot-Savart law (
where
The configuration of the current corresponds to the Helmholtz coil, that is, two circular coils of equal radius, with a common axis, separated at such distance that the second derivative of
The magnetic induction at a point P indicated in
where factor
The first derivative of
As it can be observed, it is cancelled for
for
Which is cancelled if
With the separation mentioned above, a magnetic induction is obtained in the midpoint of (
Nevertheless, it is convenient to know the value of the magnetic field at any point along the coil axis that is near the midpoint of the two coils and where the steel bar will be situated. In this way, the field
As the first three derivatives are cancelled, the fourth derivative
so for the region where
Thus, the configuration of the Helmholtz coil is presented as an ideal solution for establishing a specific area of a known and uniform magnetic field. In this research, this area corresponds to the breaking section of test specimens. The objective of that was to determine the influence of this mechanical stress in its magnetic properties.
For the convenience of analysis, for each bar of B500SD steel, instructions established by the standard UNE EN ISO 6892-1:2017 were followed. To define this model, it is necessary to use unit deflection in
and the elastic tension defined as (
where
Moreover, it is known that for unidimensional tensile states in which deflections are small, both expressions are related by the Hooke law (
where
Before the test, each bar was marked over its whole length with lines separated from each other by one centimetre distance. This will allow us to measure the last elongation after the tensile strength, elongation that was determined over the initial length of the specimen that was around 5 diameters (
where
These tests were performed using a hydraulic machine of IBERTEST series, with a testing capacity of 100 kN, according to the standard UNE-EN ISO 6892-1:2017. This mechanism was equipped with a extensometer of 50 mm of length that allows obtaining the stress-strain diagram.
Elasticity was controlled by the load, removing the extensometer when the deformation reached 2%. Once elasticity limit was reached, control was made by deflection. In this way, diagrams in which load was indicated in the Y-axis and elongation in the X-axis were obtained. Results also show the hydraulic press piston.
In addition, B500SD is classified as a ferromagnetic material. In these materials, the atomic magnetic moments can be ordered under the action of a magnetic field that is below its Curie temperature. This is due to the spins orientation of unpaired internal electrons.
As it can be observed in
where
This induced tension is collected by the integrating fluxmeter. In this way, the magnetic induction
where
If we consider as uniform the magnetization field in the transversal area of detection coil
where
In order to obtain the magnetic amplitude permeability
where
Moreover, the magnetic induction
In the following section, the results of the research and the discussion of these results are presented.
The break of the steel bars is cup-shaped on the furthest end from the break and truncated cone-shaped on the side corresponding to the point of rupture. This is related to the 45° plans theory of break with relation to the force direction, with the particular feature of the specimen having rotational symmetry.
At the atomic level, there are differences between the behaviour in the elastic zone and the plastic zone (
Thus, this defT produced during the test causes a variation of the induced electromotive force that is registered by the measuring equipment. The results corresponding to each bar are shown in
In
In the first area, there is an increase of the induced electromotive force, that corresponds to the rise of the magnetic permeability due to the tensile force. In the second area, there is a decline of the induced electromotive force until a change in the slope of the curve is produced. This change corresponds to the unit load in the point where the deflections of the sample are not proportional to the applied force, in other words until the yielding limit. This is the real yielding limit that can be defined as the maximum load with a momentary application that does not produce significant modifications in the dimensions of the bar nor in the physical or chemical properties of the steel.
The third area corresponds to the beginning of the plastic deflection, when the phenomenon of yield strength starts. In this area, a gradual decline of the induced electromotive force can be observed, what is related to the decline of the steel bar section. Finally, in the last part of the graphics, there is a rapid increase of the slope of the curve corresponding to the moment of the break. That finishes with the sample divided into two parts.
This study also determines the variation of the magnetic permeability of B500-SD steel specimens, for different tensile states under different magnetic induction states. For the measuring of magnetization field H, a Hall probe was used. It was also examined the effects of uniaxial tensile strength in the cycles of hysteresis of 12 mm and 16 mm diameter steel bars.
As illustrated in
The measuring equipment and the described method allow ascertaining effectively the elastic limit of a steel bar as well as its tensile state under a specific load. The latter is of particular relevance concerning bars that are embedded into reinforced concrete structures and it is impossible to reach them without destroying the containing surface.
Finally, as it can be seen in
The study of the magnetic behaviour of steel is a useful alternative method to obtain different mechanical behaviour and the yielding limit of steel under tensile stress. This research presents a measuring equipment able to register, through the variation of induced electromotive force, the rates of tension and deflection of construction steel B500-SD during a tensile test. Two commercial diameters of bars used in the building were implemented: 12 mm and 16 mm. The measurement method presented in this work could serve as a starting point to develop new sensors and equipment to determine the stress state of steel bars embedded in structural concrete without the necessity to extract it and test it in the laboratory. These techniques of inspection are of great technical interest for building assessments.
The magnetic test provides practical and valuable information for the evaluation of construction steel properties in its future mechanical uses. It was possible to verify how, as the applied axial tensile strength increases, the maximum magnetic permeability of the steel decreases. Moreover, when the tensile force rises, the magnetic field to be applied in order to obtain the maximum permeability of the material also increases. Finally, infrared thermography confirms that the force applied to the specimen gets transformed into deflection work, and a small part is used for sample heating.
Authors appreciate the collaboration of the professor Álvaro Gustavo Vitores González of the