The Poisson’s ratio measures the length and width of a material undergoing transformation as it is stretched in a longitudinal direction. Application of the ISO 527-2 test standard helps determine the 527-2 Poisson’s ratio.
Testing and Simulation of Poisson's Ratio
When stretching an adhesive specimen along a longitudinal direction, it has a tendency to thin in a lateral direction. Measuring the relationship between how thin a material becomes during stretching and how far it stretches is a valuable metric that guides product development teams.
The Poisson’s ratio is predominantly applied by engineers to determine the precise amount of material that can be stretched or compressed before it reaches failure. This method is frequently used when designing structures to allow engineers to consider the expected dimensional changes of a given material when under load.
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Testing Procedure
Poisson’s ratio measurements are typically conducted in line with the ISO 527-2 tensile test method. The tensile test is carried out by positioning adhesive specimens in a universal testing machine and subjecting them to an increasing load until failure occurs, or until strain occurs past the point of no return (irreversible plastic deformation).
For practical reasons, strain is characterized as deformation of a solid as a result of an applied stress. A test speed between 5 and 10 mm/min is applied to determine the Poisson’s ratio value using a special biaxial extensometer.
Test Parameters
Specimens can be molded to fit the required dimensions, or they may be machined from cured products, such as cast sheets or moldings. The test specimens are called dog-bone geometries: either Type 1B (4 mm thick) or Type 5A (2 mm thick).
Selection is dependent on the specimen preparation method and material flexibility. At least five specimens are subjected to testing for each required orientation.
Data Provided
The Poisson’s ratio is expressed as: ѵ = - εt / εl
Signifying a measurement of transverse strain divided by axial strain, where ѵ represents the Poisson’s ratio, εt equals the transverse strain and εl is the longitudinal or axial strain.
It is characterized as the negative ratio of the relative contraction strain (transverse, lateral or radial strain) normal to the applied load – proportional to the extension strain (or axial strain) towards the direction of the applied load (dimensionless ratio).
A high Poisson’s ratio signifies a material that exhibits substantial elastic deformation, even when subjected to low strain, while a material with a low Poisson’s ratio demonstrates little deformation irrespective of the magnitude of applied strain.
Tips for Modeling and Simulation
When simulating a loading case where a temperature variation is used below and above the glass transition temperature of the material, Poisson's ratio should be used to calibrate the model as a function of temperature.
In the glassy state, Poisson's ratio is near constant and increases in a progressive manner before it plateaus in the rubbery state. The theoretical asymptotic limit for a vacant thermoset in the rubbery state is 0.5.
To model the transition zone between the values in the glassy and rubbery states, it is generally advisable to use Smoothstep functions (sigmoid-like interpolation).
The transition zone is characterized as the temperature range between Tg onset and Peak tan delta when determined using a DMA measurement.
This information has been sourced, reviewed and adapted from materials provided by Huntsman Advanced Materials.
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