What Parameters Are Monitored By High-Sensitivity Resistance Strain Gauges During Axial Compression?

High-sensitivity resistance strain gauges are deployed during axial compression testing to capture two specific, real-time deformation parameters: longitudinal compressive strain and lateral expansion strain. Attached directly to the outer surface of the stainless steel tube, these sensors provide the raw data necessary to determine how the tube physically deforms as it constrains the concrete core under load.

The simultaneous monitoring of longitudinal and lateral strain is not merely about measuring deformation; it is the fundamental method for calculating the confinement coefficient and tracking the evolution of the material's Poisson's ratio through different loading phases.

The Mechanics of Measurement

To understand the interaction between the steel tube and the lightweight aggregate concrete, you must isolate the specific directional forces at play.

Longitudinal Compressive Strain

This parameter measures the shortening of the stainless steel tube along the axis of the applied load.

As the testing machine applies vertical pressure, the gauges record how much the tube compresses. This data is critical for aligning the deformation of the steel shell with the compression of the concrete core.

Lateral Expansion Strain

Simultaneously, the gauges monitor the outward expansion, or "bulging," of the tube.

This measurement is the direct indicator of the confining force. As the concrete core attempts to expand outward under pressure, the steel tube resists this expansion, generating the confinement effect that enhances the structural integrity of the column.

Deriving Value from Strain Data

Collecting raw strain data is the first step; the deeper engineering value lies in what these two parameters reveal about the composite material's behavior.

Calculating the Confinement Coefficient

The effectiveness of a tube-confined concrete column depends on the confinement coefficient.

Researchers use the real-time data on lateral and longitudinal changes to calculate this coefficient. It quantifies exactly how much support the steel tube is providing to the core concrete at any given moment during the test.

Tracking Poisson's Ratio Evolution

The relationship between lateral expansion and longitudinal compression—known as Poisson's ratio—is not static in these materials.

High-sensitivity gauges allow you to observe the shift in this ratio as the material yields. You can track the material's transition from the elastic phase, where the ratio is approximately 0.3, into the elasto-plastic phase, where the ratio increases significantly, ranging from 0.3 to 0.7.

Understanding the Testing Context

While strain gauges provide the "eyes" for the experiment, the validity of the data relies heavily on how the load is applied.

The Role of Load Control

Strain gauges must respond to precise inputs. A microcomputer-controlled electro-hydraulic servo pressure testing machine is used to manage the loading rate.

The machine ensures a constant load rate initially, which is vital for accurate elastic phase readings. Without this stability, the strain gauge data could be erratic, leading to miscalculations of the initial Poisson's ratio.

The Necessity of Displacement Control

Once the specimen yields, the testing machine switches to high-precision displacement control.

This switch allows the strain gauges to capture the stress redistribution process and the residual bearing capacity. Relying solely on load control during this phase would result in a rapid failure that strain gauges might not capture with sufficient resolution.

Making the Right Choice for Your Analysis

When analyzing data from confined concrete testing, your interpretation of the strain gauge readings should depend on your specific engineering objectives.

If your primary focus is the elastic limit: Concentrate on the stability of the Poisson's ratio around 0.3 to verify the linear behavior of the composite material.
If your primary focus is failure mechanics: Analyze the rapid increase in lateral strain relative to longitudinal strain (Poisson's ratio 0.3–0.7) to understand the elasto-plastic capacity and the peak confinement provided by the tube.

Precise strain monitoring is the only way to effectively map the dynamic interaction between the stainless steel shell and its concrete core.

Summary Table:

Parameter	Direction	Purpose	Poisson's Ratio Range
Longitudinal Compressive Strain	Axial (Vertical)	Measures shortening under load	0.3 (Elastic Phase)
Lateral Expansion Strain	Radial (Horizontal)	Measures outward bulging/confinement	0.3 - 0.7 (Elasto-plastic)

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References

Ruiqing Zhu, Haitao Chen. A Study of the Performance of Short-Column Aggregate Concrete in Rectangular Stainless Steel Pipes under Axial Compression. DOI: 10.3390/buildings14030704

This article is also based on technical information from Kintek Press Knowledge Base .