This blog post covers the description and determination of Young’s modulus, tangent modulus, and chord modulus. These properties, commonly used for product and material specification, can be calculated by subjecting a specimen to uniaxial force, measuring its stress and strain properties, and generating a stress-strain curve. The accuracy of the modulus determination depends on the precision of the load and strain measurements. A testing machine equipped with the appropriate grips and fixtures and a strain measurement device is required to measure Young’s modulus, tangent modulus, and chord modulus.
Chord modulus- Slope of the chord drawn between any two specified points on the stress-strain curve below the maximum stress a material can withstand without permanent strain upon release of the stress (elastic limit). Chord modulus calculations are recommended for non-linear materials.
Chord modulus on an MTESTQuattro-equipped tensile system
Tangent modulus- Slope of the stress-strain curve at any specified stress or strain. Like chord modulus, tangent modulus calculations are recommended for non-linear materials.
Tangent modulus on an MTESTQuattro-equipped tensile system
Young’s modulus (E)- Ratio of tensile or compressive stress to the corresponding strain below the maximum stress a material can withstand without deviation from proportionality of stress to strain (proportional limit). Note that Young’s modulus in tension is different from Young’s modulus in compression.
A simplified test procedure includes the steps below:
Prepare the test specimens following the steps in a standardized test method of choice.
For tension testing, mount the sample into the tensile grips and clamp the extensometer or the strain measurement device.
For compression testing, a compressometer may be used (see ASTM C469).
For bend testing, place the sample on a bend fixture with the strain measurement device of choice.
Start the test. The speed of testing should be low enough to avoid thermal effects of adiabatic expansion or contraction and should be high enough that creep will be negligible.
Record and graph stress versus strain (include specimen number, test data, rate, and other pertinent information).
Modulus calculations can be performed by running static tests, dynamic tests, wave propagation methods, as well as nanoindentation. The obtained modulus value will differ based on the method used. This blog post covers static testing. Common test standards to measure modulus include:
When selecting test equipment, make sure a testing system with closed-loop control as well as a high-quality strain measurement device is selected for accurate modulus calculations.
Ensure measurement devices are properly calibrated and that the calibrated range covers the range that is used in the test.
Read more about ADMET ASTM Calibration How-To’s here.
Specimen Selection & Size
Most often, larger specimens are required for modulus calculations which may be hard to obtain.
Standardized test methods can be used for specimen selection criteria, specimen preparation, and the recommended dimensions.
Linear Portion of the Stress-Strain Curve
The straight-line or the linear part of the stress-strain curve, which is used to measure the modulus in a tensile test, may be hard to determine for some materials such as materials with high tensile residual stress with very low proportional limits.
Advanced testing systems with material testing software is available to automatically collect data from the linear portion of the stress-strain curve.
Test Method to Follow
There are multiple standardized test methods to choose from to run modulus testing. The scope of each test method should be taken into account first prior to committing to a test. For example, ASTM E8 covers tensile testing of metallic materials in room temperature only while ASTM E111 covers both tensile and compressive testing in room temperatures, elevated, and sub-zero temperatures.
In addition, as mentioned earlier, although this post covers static testing, there are other test types such as dynamic testing, wave propagation and the nanoindentation technique to measure modulus values. Calculated values will be different even if the same sample is tested. As an example, the modulus of elasticity values obtained will usually be less than the modulus derived under a dynamic test given other test conditions being the same.
Modified Environment Testing
Some methods require modification of the environment the specimen is loaded at. For example, if testing is run at high temperatures, it is important to ensure that an accurate value of the high-test temperature is kept constant throughout the testing.
eXpert 1600 Servohydraulic Testing Machines
eXpert 1600 universal testing machines are hydraulic testing systems that are used in high-capacity test applications. ADMET hydraulic testing systems are equipped with load cells that provide +/- 1% of reading down to 0.5% of capacity once calibrated per ASTM E4.
eXpert 1000 series servohydraulic testing system
eXpert 2600 Dual Column Testing Machines
eXpert 2600 dual column testing machines are electromechanical testing systems that can be used to test a variety of materials in different loading scenarios. These systems are available in table top or floor standing configurations from 2kN to 600kN.
eXpert 2600 UTM with wedge grips
Controller & Software
MTESTQuattro is ADMET’s highly versatile and capable PC-integrated software solution designed to achieve optimal performance from a materials testing system. MTESTQuattro was designed to accurately obtain results from tests requiring multiple steps such as modulus testing following standardized test methods.
Grips & Fixtures
The testing machine should be equipped with the appropriate grips and fixturing based on the test method, the application of the force, specimen dimensions, and the required force capacity. Tensile hydraulic and wedge grips, compression platens, shear, and bend fixtures may be used for static modulus calculations.