Compressive strength
It is opposed to tensile strength which withstands loads tending to elongate, resisting tension (being pulled apart).
The specimen, often cylindrical in shape, experiences both axial shortening and lateral expansion under the load.
In this region, the material deforms elastically and returns to its original length when the stress is removed.
Above this point the material behaves plastically and will not return to its original length once the load is removed.
When a uniaxial compressive load is applied to an object it will become shorter and spread laterally so its original cross sectional area (
Tests that measure the engineering stress at the point of failure in a material are often sufficient for many routine applications, such as quality control in concrete production.
[3][4] Friction at the contact points between the testing machine and the specimen can restrict the lateral expansion at its ends (also known as 'barreling') leading to non-uniform stress distribution.
For all practical purposes the volume of a high bulk modulus material (e.g. solid metals) is not changed by uniaxial compression.
As the load is applied, friction at the interface between the specimen and the test machine restricts the lateral expansion at its ends.
Round test specimens made from ductile materials with a high bulk modulus, such as metals, tend to form a barrel shape under axial compressive loading due to frictional contact at the ends.
where Note that if there is frictionless contact between the ends of the specimen and the test machine, the bulge radius becomes infinite (
For the curves where end restraint is applied to the specimens, they are assumed to be fully laterally restrained, meaning that the coefficient of friction at the contact points between the specimen and the testing machine is greater than or equal to one (μ ⩾ 1).
) approaches the value corresponding to frictionless contact between the specimen and the machine, which is the ideal test condition.
As shown in the section on correction formulas, as the length of test specimens is increased and their aspect ratio approaches zero (
However, conducting tests with excessively long specimens is impractical, as they would fail by buckling before reaching the material's true compressive strength.
To overcome this, a series of tests can be conducted using specimens with varying aspect ratios, and the true compressive strength can then be determined through extrapolation.
Otherwise, if the material is ductile yielding usually occurs which displaying the barreling effect discussed above.
If there is no constraint (also called confining pressure), the brittle material is likely to fail by axial splitting.
During axial splitting a crack may release that tensile strain by forming a new surface parallel to the applied load.
[6] The material now split into micro columns will feel different frictional forces either due to inhomogeneity of interfaces on the free end or stress shielding.
In the case of stress shielding, inhomogeneity in the materials can lead to different Young's modulus.
This will in turn cause the stress to be disproportionately distributed, leading to a difference in frictional forces.
In either case this will cause the material sections to begin bending and lead to ultimate failure.
[7] Microcracks are a leading cause of failure under compression for brittle and quasi-brittle materials.
Creating localized shear bands on which the material will fail according to deformation theory.
“The onset of localized banding does not necessarily constitute final failure of a material element, but it presumably is at least the beginning of the primary failure process under compressive loading.”[10] For designers, compressive strength is one of the most important engineering properties of concrete.
It is standard industrial practice that the compressive strength of a given concrete mix is classified by grade.
Test requirements vary by country based on their differing design codes.
When measuring the compressive strength and other material properties of concrete, testing equipment that can be manually controlled or servo-controlled may be selected depending on the procedure followed.
[17] Ultra-high performance concrete (UHPC) is defined as having a compressive strength over 150 MPa.
