Strain gauges are sensors that measure the deformation (strain) of an object when subjected to external forces. A strain gauge typically consists of a thin, flexible backing with a metallic foil, which deforms when the test subject deforms and is used to predict how long an object will perform under load before failure. As a type of transducer, strain gauges are used to convert physical stresses, such as pressure, torque, shear, tension and compression into a readable electrical signal.

This conversion is crucial for applications where understanding and monitoring structural integrity are necessary. Strain, a dimensionless measurement, is defined as the ratio of the change in length of a material to the original, unaffected length. Strain can be positive due to elongation or negative due to contraction. Stress is a measurement of the force applied divided by the initial cross-sectional area of an object, or the internal resisting capacity of an object. Stress (σ) and Strain (ε) are related by the Young’s Modulus (E), denoted by σ = Eε.

When an object stretches or contracts, its electrical resistance changes, and this change is measured using a Wheatstone Bridge circuit. By relating the output Voltage (VO), excitation Voltage (VEX) and gauge factor (GF), the strain response is obtained. By monitoring the strain over time with various loading cycles, the fatigue life can be predicted, making strain gauges indispensable in various fields of engineering and material science.

What is a Wheatstone Bridge?

A Wheatstone bridge is an electrical circuit used to precisely measure an unknown resistance by balancing two legs of a bridge network.

• It consists of four resistors arranged in a diamond shape.

• Two are known resistors, one is a variable resistor, and one is the unknown resistor (aka the strain gauge).

• A voltage source is connected across one diagonal, and a sensitive voltmeter is connected across the other diagonal.

• When the voltmeter reads zero, the bridge is “balanced”. This means the ratio of the known resistors equals the ratio of the other two, allowing you to calculate the unknown resistance with high accuracy.

• When the gauge changes resistance, the bridge unbalances, producing an output voltage proportional to the change.

It’s widely used in strain gauge applications for precise measurement of the resistance change of a strain gauge, because it is very sensitive to any changes in resistance caused by mechanical stresses acting on the gauge.

Wheatstone Bridge Configurations

A Wheatstone Bridge typically is set up in 3 configuration types – quarter bridge, half bridge and full bridge. These designations are defined by the number of active elements in the bridge, the orientation of the strain gauges, and the type of strain being measured.

A quarter bridge setup involves only one active strain gauge, whilst the remaining elements are completed with known resistors. This is the simplest way to use a strain gauge and is the method used when there is no temperature change or the temperature effects can be ignored.

The half-bridge configuration involves two active strain gauges and two known resistors. The second gauge can either be used as a “dummy” gauge, to compensate for temperature effects on an active gauge, or as another active gauge to isolate a component of strain.

The full bridge configuration involves four active strain gauges, giving a higher sensitivity, higher signal-to-noise ratio and excellent temperature compensation as compared to the quarter bridge and half bridge configurations.

Linear, Cross and Rosette Strain Gauges

By nature, a linear strain gauge measures strain in a single axis, the direction of the grid. This is perfect for cases where the loading axis is known; however, there are many applications where strain acts in more than one direction. In these cases, it is better to use either cross or rosette gauges, where multiple gauges are mounted on the same backing to measure strain in multiple directions.

Cross gauges consist of two foil grids mounted at 90° to each other at the same location. The purpose of these gauges is to measure strain in two perpendicular directions simultaneously or to allow for thermal compensation of one direction, removing any bonding directional errors.

Rosette gauges have three stacked gauges, often mounted at 0°, 45° and 90° at the same location. These gauges measure strain in three directions simultaneously, allowing principal strains and stresses to be found, even if the direction is unknown. There is also the added benefit that the elements come pre-aligned, so the error from misalignment is removed, and all of the gauges experience the same environmental conditions, bringing improved thermal compensation.

Strain Gauge Protection for Outdoor Testing

Strain gauges are commonly used in environments prevalent with dust, moisture, dirt, chemicals and varying temperatures. This, along with potential bumps, knocks and spills, could cause issues to the strain gauge and its accuracy. To address these challenges, several precautions can be taken to protect the gauge and ensure its longevity throughout the testing period.

The main methods of this are applying coatings/tapes or encapsulating the gauge. Coatings are selected for various protective properties and are applied directly over the gauge for environmental sealing. Some common types are:

• Polyurethane (PU): Flexible, good moisture protection, moderate chemical resistance, easy to apply.

• Epoxy Coating: Rigid, excellent adhesion, good chemical resistance, can handle higher temperatures.

• Silicone Rubber: Flexible, withstands a large temperature range, good moisture barrier.

• Acrylic Lacquer: Easy to apply, dries quickly, but has limited chemical resistance.

For even harsher environments, encapsulation of the gauge provides another degree of mechanical robustness against the test environment. There are a few types of encapsulation:

• Epoxy Potting: Fully seals and mechanically locks the gauge in place.

• RTV Silicone Encapsulation: Flexible, resists thermal expansion stresses.

• Filled Polyurethane: Adds impact resistance and chemical protection.

• Metal Tube: Often corrosion-resistant and weldable, suitable for high temperature environments

Types of Strain Measurement – Axial/Transverse

The purpose of Axial Testing with strain gauges is to measure the longitudinal strain and/or transverse strain. The longitudinal strain is measured by bonding a strain gauge parallel to the axis of loading (0° orientation), whereas mounting a gauge perpendicular to the loading axis (90° orientation) will allow for measurement of transverse strain.

With the longitudinal and transverse strains, the poisson’s ratio of the material can be calculated. These strains are typically used to validate material deformation under axial loads. By applying four gauges, with two in bending and two in compression, bending effects on a test subject can be cancelled, maximising the sensitivity to axial strains, whilst improving temperature compensation.

Strain Gauge Application: Axial Testing

With the development of new concrete-metal composites for structures, it is important to know the composite behaviour under loading stress. This is monitored by compressive testing using strain gauges on a smaller-scale sample of the composite. In applications with concrete, it is important to select a gauge of appropriate length such that the gauge length is greater than 4x the average aggregate size of the concrete.

With new concrete-metal composites, understanding their behaviour under loading stress is crucial. This is evaluated through compressive testing with strain gauges on smaller samples. In concrete applications, it’s essential to use gauges with a length greater than four times the average aggregate size.

Types of Measurement: Bending

When a test subject is bending, one surface will be in tension, and the opposite will be in compression. The “neutral axis”, is the centre line for symmetric cross-sections, which experiences zero strain. Strain varies linearly with distance from the neutral axis.

Strain gauges are typically paired when measuring bending strain, with one gauge in compression and one gauge in tension, on the opposite surface at the same longitudinal location. Averaging the two gauges cancels axial loading effects and isolates the bending strain. The difference in strain between the gauges gives pure bending strain.

Strain Gauge Application: Bending

Due to the bending forces exerted on aeroplane wings during flight, it is extremely important to evaluate structural strength, stiffness, and verify any design predictions before commissioning of these wings. The aerospace industry also has strict certification requirements, which can include validation tests using strain gauges. Gauges are mounted on both upper and lower surfaces to capture compressive and tensile forces, whilst often being arranged in half- or full-bridge configurations to measure bending strain while cancelling axial and thermal effects. The wings are rigged up with a bending force generally applied near the tip of the blade via a stinger and a fixed, rigid point at the base of the blade to simulate the fuselage attachment. The blade is run through a series of high-cycle tests to validate FEA models and load paths. This allows a verification of safety margins and can identify critical points where cracks or fatigue may initiate.