Name: how strain gauges measure deformation
Uploaded: 2024-05-23T12:59:55.000Z
Duration: 1 min 27 s
Channel: JoVE

La deformación mide la deformación o desplazamiento de un material. La deformación normal se calcula comparando las longitudes antes y después de aplicar una carga.
Un método más preciso para medir las deformaciones normales consiste en utilizar galgas extensométricas eléctricas que consisten en un cable delgado cementado a dos hojas de papel.
Cuando el material se alarga, la longitud del cable aumenta y su diámetro disminuye, lo que provoca un aumento de la resistencia eléctrica. La medición de este cambio en la corriente determina la deformación precisa.
Los componentes de la deformación en un punto dado se pueden determinar midiendo la deformación normal a través de los ejes dibujados a través de ese punto. Una tercera medición a lo largo de la bisectriz del ángulo formado por los ejes determina la deformación cortante.
Las mediciones de deformación se realizan a lo largo de tres líneas trazadas a través de un punto, formando ángulos respectivos con el eje horizontal.
Las mediciones de deformación correspondientes se sustituyen en la ecuación de deformación normal para formar tres ecuaciones de medición de deformación.
La disposición de las galgas extensométricas que se utilizan para medir las deformaciones se denomina roseta de deformación, normalmente en configuraciones de 45° o 60°.

medidas de deformación

Measurements of Strain

La deformación cuantifica la deformación de un material bajo fuerza, generalmente medida como deformación normal, que representa el cambio de longitud en comparación con la longitud original. Se utilizan galgas extensométricas eléctricas para mejorar la precisión. Estos dispositivos constan de un cable conductor montado sobre un soporte de papel que se adhiere a la superficie del material. Estos medidores operan con el efecto piezoresistivo, donde la resistencia eléctrica del cable cambia en respuesta a la deformación mecánica. El medidor de tensión se ajusta a medida que el material se alarga o comprime, alterando la resistencia del cable proporcionalmente a la tensión experimentada.

Para comprender de manera integral la deformación en un punto material específico, considere las deformaciones normales y de corte, que explican los cambios de forma a volumen constante. Este análisis requiere múltiples galgas extensométricas dispuestas en varias orientaciones. Las mediciones de deformación generalmente se realizan a lo largo de líneas establecidas en ángulos definidos con respecto a un eje horizontal, y estas lecturas se utilizan para calcular los componentes de deformación generales.

Una roseta de galgas extensométricas, que coloca varios galgas en configuraciones como 0°, 45° y 90°, o 0°, 60° y 120°, permite mediciones de deformación multidireccionales simultáneas. Esta disposición es fundamental para el análisis detallado de deformaciones en un punto y se emplea ampliamente en análisis estructurales, pruebas de materiales y validación de piezas. Estas metodologías son vitales para que los ingenieros evalúen el comportamiento de los materiales, mejorando el diseño y la seguridad de las estructuras de ingeniería.

Medidas de deformación

La deformación mide la deformación o desplazamiento de un material. La deformación normal se calcula comparando las longitudes antes y después de aplicar una carga.
Un ...

Education

Educación

JoVE Core

Mechanical Engineering

Ingeniería Mecánica

Unit 1

Unidad 1

Transformations of Stress and Strain

Transformaciones de esfuerzo y deformación

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

how strain gauges measure deformation

Strain describes how a material deforms when a force is applied. Normal strain compares the change in length with the original length. Engineers use this measure to track stretching or compression in materials.

For higher accuracy, they often use electrical strain gauges. A strain gauge has a conductive wire on a paper backing that sticks to the surface of the material. It works through the piezoresistive effect, which means the wire’s electrical resistance changes when the material is bent, stretched, or compressed.

As the material elongates or shortens, the strain gauge changes with it. That change alters the wire’s resistance in proportion to the strain. To understand strain at one point more fully, engineers also consider normal strain and shearing strain, which describe shape change at constant volume.

This more detailed analysis needs several strain gauges placed at different angles. Strain is often measured along lines set at defined angles to a horizontal axis. The readings are then used to calculate the overall strain components.

A strain gauge rosette makes this process easier by arranging multiple gauges together. Common layouts include 0°, 45°, and 90°, or 0°, 60°, and 120°. These setups allow simultaneous measurements in multiple directions and are widely used in structural analysis, materials testing, and part validation. Engineers use the results to study material behavior and improve the design and safety of engineered structures.

How Strain Gauges Measure Deformation

transformation of plane stress

Plane Stress Changes with Rotation

Plane stress changes with rotation in a material element. The analysis looks at a prismatic element inside a cube under plane stress. As the element turns, the normal stress and shearing stress on its faces change in both size and direction. These changes are described with trigonometric functions of the rotation angle. The forces on the rotated faces are compared with the forces on the original faces that are perpendicular to each principal axis. The equilibrium equations use only the forces...

principal stresses

Stress Circle for Principal and Shear Stress

The stress circle shows how normal stress and shearing stress change with angle on a transformed plane. Its center, C, lies on the vertical axis and represents the average normal stress. The radius shows the amount of stress variation as the plane rotates. Points A and B, where the circle crosses the horizontal axis, mark the maximum and minimum normal stresses. At these points, shearing stress is zero. These points define the principal stress planes, where only normal stress, called principal...

principal stresses: problem solving

Finding Principal Planes and Stress Limits

Principal planes and principal stresses are found by analyzing two planes that meet at right angles under shearing, tensile, and compressive stresses. The goal is to use the given stress values to describe how the material is loaded and to locate the planes where the stress state changes. To find the principal planes, use the formula that relates the plane angle to twice the shearing stress divided by the difference between tensile and compressive stresses. After substituting the known stress...

mohr&#39;s circle for plane stress

Reading Stress Components with Mohr's Circle

Mohr's circle is a graphical way to read the stress state at a point in a material under plane stress. It turns normal stress and shearing stress into a two-dimensional picture. This makes stress transformation easier to analyze. The method uses Cartesian coordinates. The horizontal axis shows normal stress, written as σ, and the vertical axis shows shearing stress, written as τ. Two points, A and B, are plotted from the stresses on the element. Point A is at (σx, -τxy), and point B is at (σx,...

general state of stress

Stress Tensor and Principal Stresses

The stress tensor describes the internal forces inside a material. It is used when a material is under external force or deformation. At a point such as O, the tensor can show the stress components on any plane through that point. A tetrahedral element helps show how these forces act. One face, XYZ, is perpendicular to the line OA. The other faces line up with the coordinate axes, and point O is the origin. The stress on the tetrahedron is split into normal and shear components on each face. <br...

yield criteria for ductile materials under plane stress

yield-criteria-for-ductile-materials-under-plane-stress

Tresca and Von Mises Yield Criteria

Ductile materials can yield when they are loaded beyond a safe stress level. Engineers use yield criteria to check whether structural elements and machine parts can handle the expected load without permanent deformation. The yield point is first found with a tensile test, which measures how the material responds to uniaxial stress. A tensile test is not enough when a part faces biaxial or plane stress conditions. In those cases, more advanced criteria are needed to predict failure. One common...

transformation of plane strain

Plane Strain Under Rotated Axes

Plane strain changes when the coordinate axes are rotated. This matters for long bars and similar elongated structures that are loaded uniformly on their sides. In these members, one dimension is much larger than the others, so deformation is mainly studied in a single plane. At a point such as O, the key strain components lie along the x and y axes. The strain along the z-axis is negligible in plane strain. Together, the x and y strain components describe the deformation state in the xy-plane.

mohr&#39;s circle for plane strain

Principal and Shear Strains on Mohr’s Circle

Mohr’s circle for plane strain shows how normal strain and shear strain change on a graph. The horizontal axis is normal strain, written as ε, and the vertical axis is shear strain, written as γ. It uses two plotted points, X and Y, to represent the strain state. Point X has coordinates (εx, -γXY), and point Y has coordinates (εY, γXY). The center of the circle, O, gives the average normal strain. The radius comes from the relationship between normal strain and shear strain, so the circle...

three-dimensional analysis of strain

Principal Strain Directions in 3D

Three-dimensional strain analysis shows how a material changes shape when it is loaded. It is especially useful for elastic, homogeneous materials. A small cubic element inside the material can expand or contract along principal stress axes. That change turns the cube into a rectangular parallelepiped, which makes the deformation easier to picture. The principal stress axes are orthogonal, or at right angles to each other. Along these directions, the stress does not create shear inside the...

measurements of strain

Strain describes how a material deforms when a force is applied. Normal strain compares the change in length with the original length. Engineers use this measure to track stretching or compression in materials. For higher accuracy, they often use electrical strain gauges. A strain gauge has a conductive wire on a paper backing that sticks to the surface of the material. It works through the piezoresistive effect, which means the wire’s electrical resistance changes when the material is bent,...

key terms and concepts

Learn how strain gauges measure deformation, from normal and shearing strain to rosette layouts used in structural analysis and materials testing.

How Strain Gauges Measure Deformation | JoVE K12

JoVE Core Mechanical Engineering offers concise, easy-to-understand animated videos covering key topics such as equilibrium, forces, truss analysis, and structural mechanics. Explore their real-world applications, and witness these concepts in action through scientist-led experiments.

mechanical engineering

How Strain Gauges Measure Deformation

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