Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Patent
1991-06-28
1993-12-14
Myracle, Jerry W.
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
G01N 1100
Patent
active
052691900
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an apparatus for the performance of rheological measurements on viscous, viscoelastic and purely elastic materials for the determination of the rheological properties of such materials. Such instruments are generally designated as rheometers, most of which belong to two main categories: Rotational rheometers and axial rheometers. Rotational rheometers measure the rheological properties of materials through a rotational action, which in some cases may be combined with a normal force measurement in the axial direction of the apparatus, while axial rheometers measure the properties exclusively through axial motion of the mechanical measuring components.
The rheological properties of purely elastic materials are usually given in terms of the modulus (stiffness) or, alternatively, in terms of the compliance. In case of simple liquids, the rheological properties are given in terms of the viscosity.
Polymeric materials are viscoelastic, i.e., they exhibit properties which are characteristic of liquids and solids, both. This means that time and shear rate together play important roles in the measurements and in the reporting of the rheological properties of polymeric materials.
Polymeric materials exhibit a stiffness which decreases with time at fixed deformation. This property is given in terms of the Stress Relaxation Modulus, G(t), which therefore is a decreasing function. Correspondingly, it has been found that polymeric materials creep under a fixed load, i.e., the deformation increases with time. This is given in terms of the Creep Compliance, J(t), which therefore is an increasing function. These properties may also be given in terms of the dynamic-mechanical properties for which the properties are given as functions of the angular frequency .omega.. The stiffness properties are given by the Storage Modulus G'(.omega.) and the Loss Modulus G"(.omega.) while the creep properties are given by the Storage Compliance J'(.omega.) and the Loss Compliance J"(.omega.). G"(.omega.) and J"(.omega.) are measures of the viscous properties of the material.
The properties are linear at small deformations and small rates of deformation, i.e., the moduli are independent of the magnitude of the deformation and the viscosity is independent of the shear rate, thus making it possible to calculate one type of property from another type of property. This is, however, not the case for large deformations and/or high rates of deformation.
It is common to distinguish between the following main types of rheological measurements:
Stress-Strain measurements which for simple sample geometries may be used to calculate a modulus. In some cases a flow limit is observed. This may be given in terms of modulus and deformation at the onset of flow. Some rheometers may further allow determination of stress and strain at the break point for solid-like materials. Several test geometries for the performance of stress-strain measurements are being used.
Shear viscosity measurements which usually are perfomed at varying shear rates. This type of measurement is typically performed by shearing the liquid between two plates which rotate relative to each other (rotational viscometry) or by applying a pressure to force the liquid through a capillary (capillary viscometry).
Elongational viscosity measurements which usually are performed by stretching of a highly viscous cylinder which consequently decreases in diameter during stretching.
Stress relaxation measurements during which the decrease in stress is measured as a function of time at a maintained deformation. Several test geometries are being used.
Creep measurements during which the increasing deformation is measured as a function of time at a maintained load. Several test geometries are being used.
Dynamic-mechanical measurements during which the properties are measured as a function of frequency. Several test geometries are being used.
It is often important to be able to determine the properties of viscous, viscoelastic as well as purely elastic materials on very
REFERENCES:
patent: 2325027 (1943-07-01), Anway
patent: 4383450 (1983-05-01), Pringiers et al.
patent: 4848141 (1989-07-01), Oliver et al.
Kramer Ole
Winther Grethe
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