Measuring and testing – Specimen stress or strain – or testing by stress or strain...
Reexamination Certificate
2000-11-17
2003-07-15
Williams, Hezron (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
Reexamination Certificate
active
06591690
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to materials testing machines, particularly to such machines which are used to measure engineering properties, and most particularly to machines for measuring uniaxial and axisymmetric triaxial loading configurations.
BACKGROUND OF THE INVENTION
Materials which have isotropic material properties, and those which have transversely isotropic properties, are often tested to quantify those engineering properties using a right circular cylindrical specimen loaded with a “triaxial” (axisymmetric) pressure cell. This type of testing is quite useful, in particular for geotechnical materials such as solid rock, soils, stabilized soils, aggregates, asphalt concrete and portland cement concrete. The concepts are applicable to a range of other materials as well, including plastics, composites, and any materials which do not have a homogeneous structure at the scale of the test specimen dimensions or which rely on external boundary conditions to retain their shape. In order to perform tests on materials such as pavement materials, one must do more than hydrostatic or proportional loading such as that suggested in U.S. Pat. Nos. 4,615,221 and 5,493,898 in terms of both loading and instrumentation.
U.S. Pat. No. 4,579,003 to Riley illustrates a useful device which combines triaxial loading with the capability to introduce direct shear to the specimen. However, in composite materials such as asphalt, the maximum size and size distribution of the aggregate strongly affect shear measurements because of their interaction with the specimen geometry, and many shear devices generate stress fields during loading that are functions of the material properties of the specimen which are the subject of the testing, and therefore cannot directly produce accurate measurements of those properties. While “confined” (i.e. pressurized) and “unconfined” (i.e. unpressurized) tension and compression tests e.g. those tests in which the stress applied along the direction of the axis of the cylinder is greater than the all around confining pressure, a condition which results in the engineering terminology describing the axial stress being the “major principal” stress, the cyclic portion of which is also termed “deviatoric”, and the radially inward stress resulting from the simultaneous application of confining pressure being termed the “minor principal” stress, are the most common types of tests conducted in the axisymmetric triaxial configuration, it is obvious that it is difficult, if not impossible, to conduct tension tests on materials with little or no cohesion.
In the compression test, the stress is applied toward the mid-height of the specimen, but in the tension test, the stress is applied away from the mid-height which requires some technique for attaching the loading system to the ends of the specimens and such attachment is virtually impossible for many soils and aggregates. For these materials, which include many soils and aggregates, another type of test in which the major principal stress direction is changed from being applied along the axial direction, e.g. vertical in the usual orientation, to the horizontal direction, e.g. radially inward toward the center of the specimen under test, is useful. This test is referred to as an “extension” test in engineering terms and is conducted with the major principal stress direction radially inward (or horizontal in the usual configuration) and the minor principal stress direction is applied in the compression direction along the axis of the specimen. The extension test often yields engineering property data which might help understand tension behavior of the material without actually conducting a tension test, and anisotropic behavior without incurring the complexity of true three-dimensional testing on a prismatic specimen.
The instrumentation in the standard geotechnical triaxial cell has been a persistent problem in the prior art. While useful for rock specimens, instrumentation solutions such as that given in U.S. Pat. No. 4,587,739 do not work for low cohesion materials such as soils, road base materials and hot asphalt concrete in part because the devices often have high localized stress fields at the specimen contact points in order to support the mounting system. Ultrasonic testing systems such as that given in U.S. Pat. No. 5,741,971 suffer from difficult analysis procedures required to accurately quantify the properties of particulate materials, and the extrapolation of the results from the test's high frequencies down to relevant frequencies for time-dependent and stress-dependent pavement materials is sometimes ineffective.
Some cohesive and engineered geotechnical materials are also tested using beam flexure and indirect tension loading (e.g. Standard Test Method for Indirect Tension Test for Resilient Modulus of Bituminous Mixtures by ASTM in ASTM D4123 (Apr. 30, 1982), Resistance of Compacted Bituminous Mixture to Moisture Induced Damage by AASHTO in AASHTO T283 (1989), and Resistance to Plastic Flow of Bituminous Mixtures Using Marshall Apparatus by AASHTO in AASHTO T245 (1994)), both of which general categories of test are usually performed without confining pressure.
There are two major categories of axisymmetric triaxial pressure cells. In the first type of triaxial cell (e.g. Standard Test Method for Determining the Resilient Modulus of Soils and Aggregate Materials by AASHTO in AASHTO TP46 (T294) (1994), Resilient Modulus of Subgrade Soils and Untreated Base/Subbase Materials by AASHTO in AASHTO T292 (1991)), the specimen is capped, placed in a rubber membrane, and is surrounded on all sides, top, and bottom by a confining medium inside the cell (usually air, water, or oil, the fluid obviously being selected based upon the necessary thermal, mechanical, and electrical conductivity requirements of the particular application). This configuration is termed the “standard geotechnical” configuration throughout the instant disclosure. When pressure is applied all around the specimen through the confining medium alone, a hydrostatic stress state is said to exist.
In order to evaluate material properties at a constant confining pressure, the stress along the axis of the specimen is usually changed dynamically (i.e. cyclically) through a sealed linear bearing by an actuator shaft reacting against a load frame external to the triaxial cell. This dynamic action is usually referred to as a deviatoric stress and the direction of application is usually the major principal stress direction. In some systems, the confining pressure may also be changed dynamically. Usually, the specimen deformation instrumentation associated with this configuration is either outside the cell or referenced to some relatively rigid component of the cell such as the base plate.
DISADVANTAGES OF THIS BASIC CONFIGURATION INCLUDE
1. the direction of the major dynamic deviatoric stress cannot be altered from the axial direction without sophisticated analysis and control systems which enable the axial actuator to counteract the change in the axial component of the dynamically changing hydrostatic pressure, also taking into consideration (a) the frictional effects of the pressure sealed linear bearing which is necessary if the load measuring device is located outside the triaxial cell and (b) the applicable cross sectional area of the loading shaft and platen assembly, factors (a) and (b) above being addressed to a certain extent by U.S. Pat. Nos. 4,679,441 and 5,435,187;
2. measurement devices which bear on the confining medium side of the membrane must have their measurements adjusted for the expected deformation of the membrane when subjected to a pressure change;
3. local specimen inhomogeneities (e.g. rocks with adjacent voids), normal specimen bulging in compression and normal necking in tension or extension cause inaccuracy with externally referenced instrumentation devices (i.e. the deformation along the sensitive axis of the instrumentation sensor cannot be separated from the deformation in another direction that occurs du
Dickens Charlene
McHale & Slavin P.A.
Williams Hezron
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