Below is a short summary and detailed review of this video written by FutureFactual:
Strain Gauges Demystified: How Resistance, Gauge Factor, and Wheatstone Bridges Measure Deformation
Short Summary
The video explains how strain gauges bonded to a surface translate deformation into changes in electrical resistance. It covers the definition of strain as delta L over L, how different materials affect sensitivity through gauge factor, and why Constantan is a popular gauge foil due to temperature stability. You will learn about the typical foil grid construction, gauge length considerations, and how resistance changes arise from cross sectional area, length, and resistivity effects. The talk then introduces Wheatstone bridge circuits used to detect tiny resistance changes, including quarter, half, and full bridge arrangements, and how these enable precise strain measurements. Temperature compensation, strain gauge rosettes, Moore's circle, and the calculation of principal strains are discussed, along with practical applications such as load cells and residual-stress measurements.
Introduction to Strain Gauges
From there, the discussion moves into the common electrical resistance strain gauge design. A typical gauge consists of a copper nickel alloy foil grid bonded to a thin insulating film, with protective layers and solder tabs for wiring. The grid is patterned to maximize stretch in the axial direction while minimizing transverse effects, ensuring that the measured resistance change corresponds closely to the axial strain of interest.
Material Choice and Gauge Factor
The video compares how different gauge materials respond to strain. It shows that materials with larger gauge factors produce larger resistance changes for the same strain, increasing sensitivity. Semiconductor gauges can have gauge factors around 150, but constantan gauges, which are copper-nickel alloys, are preferred for general usage due to good corrosion resistance, fatigue properties, and temperature stability. Temperature stability is highlighted as a key reason Constantin gauges are widely used in real-world measurements across wide temperature ranges.
Gauge factor K, defined as the relative change in resistance divided by the strain, is introduced as the slope of the resistance versus strain curve. The video provides numerical examples to illustrate how a 100 microstrain could translate into a measurable resistance change given a gauge factor and a baseline resistance.
The Wheatstone Bridge and Small Signal Detection
Measuring tiny resistance changes requires precision electronics. The Wheatstone bridge circuit balances four resistors so that, when the bridge is balanced, no current flows through the central segment. A strain gauge replaces one of the resistors; when the gauge's resistance changes under load, an imbalance appears, producing a small voltage that can be measured accurately. The video explains how to rebalance the bridge or compute resistance from measured voltages to determine the applied strain. Different bridge configurations are described: a quarter bridge with one gauge, a half bridge with two gauges, and a full bridge with four gauges, each offering different advantages for sensitivity and temperature compensation.
Temperature Effects and Compensation
Understanding Strain Directionality and Rosettes
A key limitation highlighted is that a single strain gauge measures normal strain along its installation direction only. Real materials experience a full strain state described by a tensor with components in normal and shear directions. To capture the full state at a point, multiple gauges are used in a rosette arrangement, typically at angles of 45 or 60 degrees. The video demonstrates how rosettes enable determination of principal strains and how Moore's circle can graphically represent the strain state. The transformation equations for a rectangular rosette with 45-degree gauge spacing are introduced, and the concept that double angles appear on Moore's circle is explained. Once the principal and directional strains are known, Hooke's law can be used to compute stresses in the material.
Practical Applications and Special Techniques
The discussion covers the use of strain gauges in load cells, where multiple gauges contribute to high-accuracy force measurements. The video also describes rosette designs for measuring shear on rotating shafts to infer torque, and a residual-stress rosette that measures strain relaxation after drilling a small blind hole. In these applications, strain gauges provide precise force, torque, and stress information that is essential for engineering design and quality control.
The video emphasizes the breadth of strain gauge applications, including structural health monitoring of bridges and high-speed machinery, along with material testing and experimental mechanics. It closes by noting the broader context of the field, including the interdisciplinary blend of mechanical engineering, electrical engineering, and materials science that makes strain gauges such powerful sensing tools.
