What is Strain?

Strain is used to describe the measurement of the deformation of a material. The material of a certain component or object can be elongated (tractioned) or contracted (compressed), thus experiencing strain due to the following factors:

• the effect of an applied external force (mechanical strain)
• the influence of heat and cold (thermal strain)
• internal forces from the non-uniform cooling of cast components, forging, or welding (residual strain)

Why is Strain Measured? 

Most commonly, strain is measured to determine the level of stress on the material – Experimental Stress Analysis. The absolute value and direction of the mechanical stress is determined from the measured strain and known properties of the material (modulus of elasticity and Poisson’s ratio). These calculations are based on Hooke’s Law. In its simplest form, Hooke's Law determines the direct proportionality of the strain ε [m/m] and the stress σ [N/mm²] of a certain material using its elasticity or Young's modulus E [N/mm²].

σ = ε⋅E

How is Strain Measured?

To understand how strain can be measured, one must first understand the effects of strain on the material. A specimen without strain has a base length of l_o. If strain ε is applied to the object, its length will change by a certain amount Δl as per the relationship given below:

Strain is a non-dimensional value that represents the change of length of a material relative to its initial length. Since the changes of length are usually very small, the standard fractional prefixes of the SI system are used. For strain, micrometer per meter (μm/m = 10^-6 m/m = ppm) is generally used.

There are several different types of gauges and sensors that can be used to measure strain. Strain gauges are the most common devices used. HBM offers both electrical strain gauges and optical strain sensors, among other products for strain measurements.

Technologies for Measuring Strain: Electrical or Optical?

Learn all there is to know about measuring with electrical or optical strain gauges, such as the basics, installation procedures, avoiding failures, and selecting the right strain gauge, by clicking on the headers below.

Measuring with Electrical Strain Gauges	Measuring with Optical Strain Sensors
The most commonly used instruments to measure strain are electrical strain gauges. These are known as conventional strain gauges or foil strain gauges. HBM has been offering these sensors for more than six decades with proven quality and proficiency.	Fiber optic strain sensors are also a significant solution offered by HBM. To meet the high standards of the company, optical sensors based on Fiber Bragg Grating technology are available through HBM FiberSensing.
Strain gauges are usually used in Experimental Stress Analysis (ESA), durability testing, and transducer manufacturing.	Optical sensors are commonly used for structural monitoring, where long-distance transmissions are easily found, and for material testing, where high strain and high-fatigue limits are present.
HBM's range of conventional strain gauges comprises of an extensive assortment for widely differing strain measurement applications. Over 2000 types of linear strain gauges and rosettes are available.	A single optical fiber can hold various sensors (with different measurands) in complex sensing networks. Available in configurable versions of ready-to-install sensor chains, HBM optical strain sensors can be bonded, spot welded, attached, or directly cast in different materials, structures and components.
Find out more about measuring with electrical strain gauges	Find out more about measuring with optical strain sensors

Both options can be purchased separately or in a combined sensing network.