How to conduct strain testing?

 

Principles, architecture, and key practical points

 

Keywords: Strain testing, Temperature drift, PCB BGA chips, Signal conditioning.

 

"How to eliminate temperature drift in strain gauges?" and "How to measure strain during the lamination process of PCB BGA chips?" Strain testing is a crucial method in engineering measurement for assessing the stress state of materials and structures, widely used in materials testing, structural verification, fatigue analysis, and equipment monitoring.

Its core concept involves converting the minute deformations of a material under stress into measurable electrical signals using sensing elements, and then analyzing these signals through signal conditioning and data acquisition systems.

This article explains the basic definition of strain, the principle of strain gauge measurement, system architecture, measurement process, sources of error, and engineering applications, serving as a reference for technical evaluation and practical implementation.

 

Preface

The core purpose of strain testing is to convert the minute deformations produced by a structure under stress into analyzable electrical signals to support design verification, reliability assessment, and failure analysis. In practical engineering, strain measurement is not only applicable to materials testing but is also frequently used for machine safety testing, fatigue life assessment, and structural health monitoring. Therefore, establishing correct measurement methods and a stable system architecture is crucial to ensuring the reliability of the results.

 

Basic Definition and Measurement Principle of Strain

Key mathematical models include the Exponential Distribution. Under the assumption of a constant failure rate λ, the reliability function is given by R (t) =e^ {-λt}, where the Mean Time to Failure (MTTF) is 1/λ. In addition, the Weibull Distribution is commonly used to describe failure patterns at different stages. Its shape parameter β determines the failure characteristics: β < 1 decreasing failure rate over time; β= 1 constant failure rate; β> 1 increasing failure rate over time.

 

 

A strain gauge is a sensing element that measures strain by utilizing changes in resistance. When the strain gauge is attached to the surface of a specimen and deforms with the structure, the length and cross-sectional area of its metal conductor change (ɛ), resulting in a minute change in resistance (ΔR/R). Because the resistance change is extremely small, a Wheatstone bridge is used in practice to convert the resistance change into a voltage change, which is then amplified, filtered, and compensated by a signal 2 conditioner. The accuracy of this measurement method is affected by the strain gauge specification (120 Ω/350 Ω/1000 Ω), attachment quality, temperature, and wiring conditions.

 

 

Where GF stands for Gauge Factor.

 

Strain Testing System Architecture

A complete strain testing system typically includes the following units:

• Sensing Terminal:
Strain gauge, adhesive, and protective layer, used to accurately transmit specimen deformation to the sensing element.

• Bridge Circuit:
Converts minute resistance changes into a readable voltage signal.

• Signal Conditioner:
Performs amplification, zero-point adjustment, filtering, and compensation.

• Data Acquisition System:
Responsible for digitizing and recording the analog signal.

• Analysis Software:
Performs waveform display, data processing, and report output.

Figure 1: Data Acquisition System for Strain Testing

 

Since the resistance change (ΔR) is typically only in the milliohm (mΩ) range, it must be amplified to a voltage signal in the millivolt (mV) range using a Wheatstone bridge.

The strain measurement function of the DAQ-9600, combined with appropriate multiplexers (DAQ-900/DAQ-901), allows engineers to choose from three bridge configurations depending on the measurement objective:

 

Table 1: Bridge Configurations for Strain Testing

 

Test Procedure and Key Operational Points

The standard strain gauge test procedure can be divided into the following steps:

• Define the test purpose:
Confirm whether the measurement type is static, dynamic, fatigue, or monitoring.

• Select the measurement location:
Determine the strain gauge placement based on the force direction and structural analysis results.

• Surface treatment and attachment:
Clean, degrease, level, and fix to ensure strain transfer efficiency. (See Figures 2a and 2b)

• Wiring and protection:
Reduce the impact of wire pulling, moisture, and external interference. (See Figures 2c and 2d)

• Zeroing and calibration:
Confirm bridge balance and baseline stability.

• Loading and data acquisition:
Gradually load and record data simultaneously according to test conditions.

• Data analysis:
Check the strain-load relationship, peak value, residual strain, and cyclic behavior.

In practice, attachment quality and installation conditions often have a more direct impact on measurement results than instrument specifications.

 

Figure 2: Strain gauge installation workflow

 

Applications

The main applications of strain testing include:

• Material testing:
tensile, compression, bending, and torsion testing.

• Structural verification:
stress analysis of bridges, supports, frames, housings, and moving parts.

• Fatigue testing:
observing strain accumulation and degradation under cyclic loading.

• Equipment monitoring:
used for condition monitoring of machines, pipelines, tracks, and large structures.

• Sensor development:
extending to derivative sensing applications such as torque, pressure, and load.

 

Sources of Error and Compensation Methods

Common sources of error in strain gauge measurements include temperature drift, poor adhesion, wire resistance, electromagnetic interference, and measurement direction deviation.

Temperature effects are commonly caused by the difference in thermal expansion between the strain gauge body and the measured material. If not properly compensated, this can easily lead to zero-point drift and false signals.

Common compensation methods include:

• Half-bridge or full-bridge configuration:
Improves temperature and common-mode interference suppression.

• Dummy gauge:
Uses a strain gauge unaffected by mechanical strain but affected by temperature for compensation.

• Soft temperature compensation:
Corrects for thermal output characteristics.

• Proper grounding and shielding:
Reduces external noise coupling.

 

Conclusion

Strain testing is a fundamental and critical engineering measurement technique. Its reliability depends on the complete measurement chain, from sensing elements, attachment quality, bridges, and temperature compensation to back-end analysis. In engineering interpretation, strain data must be comprehensively analyzed in conjunction with load conditions and structural behavior, rather than just looking at single-point peak values.

Structural condition and signal authenticity are assessed by observing the linearity of strain and external force, residual strain, and dynamic signal quality (such as sampling rate and filter settings). By establishing standardized procedures and properly managing errors, robust, repeatable, and high-value measured data can be provided for material analysis, structural verification, and reliability assessment.

 

 

DAQ-9600 Data Acquisition System See more

 

 

 

 

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Diana

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E-mail: diana@goodwill.com.tw