El ensayo de tracción está considerado como uno de los más importantes para la determinación de las propiedades mecánicas de cualquier material. Los datos obtenidos se pueden utilizar para comparar materiales entre sí y para saber si una pieza de cierto material podrá soportar determinadas condiciones de carga. En el campo del estudio de la resistencia de materiales, se denomina carga a la fuerza aplicada a los materiales. Estas fuerzas se denominan fuerzas normales (son perpendiculares a la superficie) de tracción y compresión.

El ensayo de tracción consiste en someter una pieza cilíndrica o prismática (probeta) de dimensiones normalizadas a una fuerza normal de tracción que crece con el tiempo de una forma lenta y continua, para que no influya en el ensayo, el cual, por lo general, finaliza con la rotura de la probeta.

Durante el ensayo se mide el alargamiento que experimenta la probeta (ΔL) al estar sometida a la fuerza de tracción (F). De esta forma se puede obtener un diagrama fuerza-alargamiento, aunque para que el resultado dependa lo menos posible de las dimensiones de la probeta y que resulten comparables los ensayos realizados con probetas de diferentes tamaños, se utiliza el diagrama tensión-deformación:.

• - Tensión (σ): Fuerza aplicada a la probeta por unidad de sección. Si la sección inicial es S, la tensión viene dada por:.

donde σ se mide en pascales "Pascal (unidad)") en el SI.

• - * Carácter vectorial de la tensión: La tensión es una magnitud vectorial de la misma dirección y sentido que la fuerza aplicada a la superficie del material, aunque en el ensayo de tracción solamente interesa su módulo. La tensión se puede descomponer en un vector perpendicular a la superficie, tensión normal, y otro vector que es la proyección sobre la superficie, denominada tensión tangencial o cortante.

• - Deformación o alargamiento unitario en un instante del ensayo (∈): Cociente entre el alargamiento (ΔL) experimentado y su longitud inicial (L):.

La deformación es una magnitud que puede expresarse en tanto por ciento:.

Stress-strain diagram

In the stress-strain diagram you can see two zones:

• - Elastic zone (O-2): The deformation experienced by the specimen is not permanent. If the test is stopped at any point between 0 and 2, the specimen will recover its initial length.

• - Plastic zone (2-4): The elongations are permanent. If the test stops at point 3, the specimen recovers the elastic deformation () persisting at the end of a remaining or plastic deformation ().

Within the elastic zone there are:

• - Proportional zone: There is a proportionality relationship between the stress and deformation experienced by the specimen.

E being Young's modulus or longitudinal elasticity modulus, measured in N/m².

• - Non-proportional zone: The deformations are not permanent. If the test is stopped and a force is no longer applied to the specimen, it recovers its initial length. There is no proportionality relationship between stress and deformation.

Within the plastic zone we distinguish:

• - Zone of uniform plastic deformation: The curve becomes more pronounced, a high load increase is not necessary to achieve large elongations. The maximum force divided by the initial section of the specimen determines the tensile strength (), the point at which the plastic zone of uniform deformation ends.

• - Striction zone or localized plastic deformation: The deformation is located in a certain area of ​​the specimen, the tension decreases and the specimen ends up breaking said area.

If the test is restarted, the tensile curve will coincide with the unloading curve, and the elastic zone becomes larger. This achieves strain hardening.

En estos ensayos las cargas actúan instantáneamente para medir la resistencia al choque o la tenacidad de los materiales.

Shock tensile test

One way to measure the toughness of materials is to perform a simple tensile test in which the speed of force application is high. The area between the stress curve and the x-axis represents the work necessary to break the material. The work divided by the volume of the specimen used provides a measure of its impact toughness.

This type of testing is rare, since the preparation of the specimens is more expensive.

Shock bending test

The most used is the resilience test, also known as the Charpy test. In this test, a test piece with a square section of 10 x 10 mm and 55 mm in length is available, in the central part of which a 2 mm deep notch in the shape of a U or V has been previously made, with a cylindrical bottom of 0.25 mm radius.

The test consists of hitting the test piece on the side opposite to the notch with a pendulum that is dropped from a certain height.

Resilience "Resilience (engineering)") is calculated by dividing the energy consumed by the material at breakage by the section of the specimen and its notched part (). If the notch is U-shaped, KCU will be used, and if it is V-shaped, KCV will be used.

Resilience is measured in J/m².

Charpy pendulums are normalized. In the initial position they have an energy of 300 J, and at the moment of impact of the specimen they move at an approximate speed of 5 m/s.

If the test piece does not break, the resilience value will not be defined. If this happens, it must be indicated in the results obtained.

Defect control testing is the subjecting of a manufactured product to a series of tests and trials that guarantee the absence of defects. It aims to detect existing imperfections in the pieces. Apart from carrying out these tests, the control must be perpetuated while they are in operation, so that it is safe.

Currently the most used methods are:

• - Tightness tests: Penetrating gases are used to locate leaks.

• - Macroscopic tests: Penetrating liquids are used to locate cracks, pores, etc.

• - Optical tests: Information about cracks, pores, etc., is provided through the microscope.

• - Magnetic tests: They are used to detect continuity defects and surface condition.

• - Electrical tests: They are used to detect defects in railway tracks. They are based on the variation of electrical resistance.

• - Tests with ultrasounds of 105 to 107 kHz frequency: They detect continuity defects in the materials and the depth at which they are found.

• - Tests with x-rays and ɣ: X-rays and scintigraphy of the tested pieces are obtained, which reveal irregularities. X-rays are used for thicknesses up to 10 cm, and ɣ-rays for thicknesses up to 25 cm.

• - The industrial test.

El ensayo de tracción está considerado como uno de los más importantes para la determinación de las propiedades mecánicas de cualquier material. Los datos obtenidos se pueden utilizar para comparar materiales entre sí y para saber si una pieza de cierto material podrá soportar determinadas condiciones de carga. En el campo del estudio de la resistencia de materiales, se denomina carga a la fuerza aplicada a los materiales. Estas fuerzas se denominan fuerzas normales (son perpendiculares a la superficie) de tracción y compresión.

El ensayo de tracción consiste en someter una pieza cilíndrica o prismática (probeta) de dimensiones normalizadas a una fuerza normal de tracción que crece con el tiempo de una forma lenta y continua, para que no influya en el ensayo, el cual, por lo general, finaliza con la rotura de la probeta.

Durante el ensayo se mide el alargamiento que experimenta la probeta (ΔL) al estar sometida a la fuerza de tracción (F). De esta forma se puede obtener un diagrama fuerza-alargamiento, aunque para que el resultado dependa lo menos posible de las dimensiones de la probeta y que resulten comparables los ensayos realizados con probetas de diferentes tamaños, se utiliza el diagrama tensión-deformación:.

• - Tensión (σ): Fuerza aplicada a la probeta por unidad de sección. Si la sección inicial es S, la tensión viene dada por:.

donde σ se mide en pascales "Pascal (unidad)") en el SI.

• - * Carácter vectorial de la tensión: La tensión es una magnitud vectorial de la misma dirección y sentido que la fuerza aplicada a la superficie del material, aunque en el ensayo de tracción solamente interesa su módulo. La tensión se puede descomponer en un vector perpendicular a la superficie, tensión normal, y otro vector que es la proyección sobre la superficie, denominada tensión tangencial o cortante.

• - Deformación o alargamiento unitario en un instante del ensayo (∈): Cociente entre el alargamiento (ΔL) experimentado y su longitud inicial (L):.

La deformación es una magnitud que puede expresarse en tanto por ciento:.

Stress-strain diagram

In the stress-strain diagram you can see two zones:

• - Elastic zone (O-2): The deformation experienced by the specimen is not permanent. If the test is stopped at any point between 0 and 2, the specimen will recover its initial length.

• - Plastic zone (2-4): The elongations are permanent. If the test stops at point 3, the specimen recovers the elastic deformation () persisting at the end of a remaining or plastic deformation ().

Within the elastic zone there are:

• - Proportional zone: There is a proportionality relationship between the stress and deformation experienced by the specimen.

E being Young's modulus or longitudinal elasticity modulus, measured in N/m².

• - Non-proportional zone: The deformations are not permanent. If the test is stopped and a force is no longer applied to the specimen, it recovers its initial length. There is no proportionality relationship between stress and deformation.

Within the plastic zone we distinguish:

• - Zone of uniform plastic deformation: The curve becomes more pronounced, a high load increase is not necessary to achieve large elongations. The maximum force divided by the initial section of the specimen determines the tensile strength (), the point at which the plastic zone of uniform deformation ends.

• - Striction zone or localized plastic deformation: The deformation is located in a certain area of ​​the specimen, the tension decreases and the specimen ends up breaking said area.

If the test is restarted, the tensile curve will coincide with the unloading curve, and the elastic zone becomes larger. This achieves strain hardening.

En estos ensayos las cargas actúan instantáneamente para medir la resistencia al choque o la tenacidad de los materiales.

Shock tensile test

One way to measure the toughness of materials is to perform a simple tensile test in which the speed of force application is high. The area between the stress curve and the x-axis represents the work necessary to break the material. The work divided by the volume of the specimen used provides a measure of its impact toughness.

This type of testing is rare, since the preparation of the specimens is more expensive.

Shock bending test

The most used is the resilience test, also known as the Charpy test. In this test, a test piece with a square section of 10 x 10 mm and 55 mm in length is available, in the central part of which a 2 mm deep notch in the shape of a U or V has been previously made, with a cylindrical bottom of 0.25 mm radius.

The test consists of hitting the test piece on the side opposite to the notch with a pendulum that is dropped from a certain height.

Resilience "Resilience (engineering)") is calculated by dividing the energy consumed by the material at breakage by the section of the specimen and its notched part (). If the notch is U-shaped, KCU will be used, and if it is V-shaped, KCV will be used.

Resilience is measured in J/m².

Charpy pendulums are normalized. In the initial position they have an energy of 300 J, and at the moment of impact of the specimen they move at an approximate speed of 5 m/s.

If the test piece does not break, the resilience value will not be defined. If this happens, it must be indicated in the results obtained.

Defect control testing is the subjecting of a manufactured product to a series of tests and trials that guarantee the absence of defects. It aims to detect existing imperfections in the pieces. Apart from carrying out these tests, the control must be perpetuated while they are in operation, so that it is safe.

Currently the most used methods are:

• - Tightness tests: Penetrating gases are used to locate leaks.

• - Macroscopic tests: Penetrating liquids are used to locate cracks, pores, etc.

• - Optical tests: Information about cracks, pores, etc., is provided through the microscope.

• - Magnetic tests: They are used to detect continuity defects and surface condition.

• - Electrical tests: They are used to detect defects in railway tracks. They are based on the variation of electrical resistance.

• - Tests with ultrasounds of 105 to 107 kHz frequency: They detect continuity defects in the materials and the depth at which they are found.

• - Tests with x-rays and ɣ: X-rays and scintigraphy of the tested pieces are obtained, which reveal irregularities. X-rays are used for thicknesses up to 10 cm, and ɣ-rays for thicknesses up to 25 cm.

• - The industrial test.