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May 23, 2025
Accurate measurements are crucial, especially in the material testing industry, where precise data is essential not only for validating produced materials but also for researching new ones. Understanding the mechanical properties of materials relies on the accuracy of these measurements, which in turn supports the development and improvement of new materials.
Measurement accuracy is influenced by several factors, including the precision of data acquisition devices that collect information from system sensors measuring applied loads or displacements. However, an often-overlooked factor is the proper placement of the sample or specimen within the testing machine.
In tensile or compressive testing, one common issue affecting measurement accuracy is the non-linear region that appears at the initial part of the stress-strain or load-deformation curve. This area, often referred to as the toe region or specimen slack, can lead to inaccuracies in data interpretation.
The toe region typically results from the specimen settling into place, misalignment, or the take-up of slack. During this phase, minimal or no load is recorded despite the movement of the crosshead. In tensile testing, this indicates that the specimen has not yet been stretched to its true gauge length, meaning the load applied is either nonexistent or minimal, often influenced only by the weight of the specimen itself.
Understanding and compensating for the toe region is essential for ensuring accurate, reliable measurements in material testing.
Toe region of the curve does not provide any information about the specimen properties, thus it should be eliminated to get correct results for the specimen measurements, for example for proper elongation and modulus calculations. If no toe compensation is performed for the stress-strain curve, elongation measurements can be highly incorrect. And actually, any measurement referring the specimen gauge length and thus strain data can have significant errors due to the toe curve artifact.
Several methods are used for the toe compensation to elimination of toe region from the stress-strain curve.
One of the methods is the preloading of the specimen, i.e. loading the specimen until a specified load value is reached. Preloading is done with predefined speed, called preload speed. The specimen should be loaded to such a load value that is high enough to remove the toe region, but at the same time low enough, to not affect any other measurements.
Preloading functionality is very common feature included in almost any material testing software applications. It allows to remove the slack, as the actual test and recording of the data happens only after the specimen is loaded and required preload value is reached. In this case usually position data is tared (reset to 0 value).
Other and more common way of toe region removal is done as a post-test processing. This method is defined in ASTM D638 and D790 standards, as an annex. These standards define a method for the evaluation of stress-strain curve with the purpose of finding in the curve initial non-linear region and remove it to perform toe compensation.
Depending on the specimen material the stress-strain curve can have a linear region, like for low carbon steel, or it may not. When the test is complete stress-strain or load-deformation curve is evaluated. If the curve has a linear region, straight line is drawn through the linear region, and the intersection of the straight line and the X axis, i.e. strain is taken as a new baseline point for any calculations referring to strain data. Based on the new baseline point also new gauge length is calculated, as a corrected gauge length.
If the specimen under test is not characterized by a stress-strain curve containing linear region, inflection point should be search for in the curve. The inflection point is defined as a point where the curve stops bending upwards and starts bending laterally. At the inflection point tangent is drawn, which intersection with the strain axis is taken as a new baseline for the strain.
One of the main obstacles in this method is the automatic definition of the linear region in the curve. Depending on the testing environmental conditions, specimen properties, load cell used, sensor wiring, data acquisition device’s noise immunity, etc. the curve may have artifacts, such has high noise level on the stress data, as well as on strain data, if bridge type extensometers are used. These issues are making it difficult to properly define a linear region in the curve. In most of the case proper data filtering should be performed, before the curve is evaluated for the linear region.
Noise artifacts can also hinder the correct identification of the inflection point, as spikes can obscure both the inflection point and the tangent line. To address this, various filtering mechanisms are used to remove noise and produce a clean curve for subsequent analysis and toe compensation.
All of the methods described are used to make accurate measurements and calculations for the testing specimens. The choice of method depends on the specific specimen, but the overarching goal is always to achieve precise and reliable test results.
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