General Concepts and Definitions
� Strength The ability to sustain load.
� Stiffness Push per move; the ratio of deformation to associated load level.
� Stability The ability of a structure to maintain position and geometry. Instability involvescollapse that is not initiated by material failure. External stability concerns the ability of a structure’s supports to keep the structure in place; internal stability concerns a structure’s ability to maintain its shape.
� Ductility The amount of inelastic deformation before failure, often expressed relative tothe amount of elastic deformation.
Strength Material strength is measured by a stress level at which there is a permanent andsignificant change in the material’s load carrying ability. For example, the yield stress, or the ultimate stress.
Stiffness Material stiffness is most commonly expressed in terms of the modulus of elasticity: theratio of stress to strain in the linear elastic range of material behavior.
Stability As it is most commonly defined, the concept of stability applies to structural elementsand systems, but does not apply to materials, since instability is defined as a loss of load carrying ability that is not initiated by material failure.
Ductility Material ductility can be measured by the amount of inelastic strain before failurecompared to the amount of elastic strain. It is commonly expressed as a ratio of the maximum strain at failure divided by the yield strain.
Mechanical properties of materials
A tensile test is generally conducted on a standard specimen to obtain the relationship between the stress and the strain which is an important characteristic of the material. In the test, the uniaxial load is applied to the specimen and increased gradually. The corresponding deformations are recorded throughout the loading. Stress-strain diagrams of materials vary widely depending upon whether the material is ductile or brittle in nature. If the material undergoes a large deformation before failure, it is referred to as ductile material or else brittle material.Stress-strain diagram of a structural steel, which is a ductile material, is given.
Initial part of the loading indicates a linear relationship between stress and strain, and the deformation is completely recoverable in this region for both ductile and brittle materials. This linear relationship, i.e., stress is directly proportional to strain, is popularly known as Hooke’s law.
s = Ee
The co-efficient E is called the modulus of elasticity or Young’s modulus.
Most of the engineering structures are designed to function within their linear elastic region only.After the stress reaches a critical value, the deformation becomes irrecoverable. The corresponding stress is called the yield stress or yield strength of the material beyond which the material is said to start yielding.
In some of the ductile materials like low carbon steels, as the material reaches the yield strength it starts yielding continuously even though there is no increment in external load/stress. This flat curve in stress strain diagram is referred as perfectly plastic region.
The load required to yield the material beyond its yield strength increases appreciably and this is referred to strain hardening of the material. In other ductile materials like aluminum alloys, the strain hardening occurs immediately after the linear elastic region without perfectly elastic region.
After the stress in the specimen reaches a maximum value, called ultimate strength, upon further tretching, the diameter of the specimen starts decreasing fast due to local instability and this p henomenon is called necking.
The load required for further elongation of the material in the necking region decreases with decrease in diameter and the stress value at which the material fails is called the breaking strength. In case of brittle materials like cast iron and concrete, the material experiences smaller deformation before rupture and there is no necking.