**Strength of materials** is the scientific area of applied mechanics for the study of the strength of engineerig materials and their mechanical behaviour in general (such as stress, deformation, strain and stress-strain relations). Strength is considered in terms of **compressive strength**, **tensile strength**, and **shear strength**, namely the limit states of compressive stress, tensile stress and shear stress respectively.

Table of contents |

2 Stress - strain relations 3 Design terms |

## Definitions

### Stress Terms

**Compressive stress** (or compression) is the stress state when the material tends to compact (volume decrease). The simpler case of compression is the uniaxial compression induced by the act of opposites, pushing forces. Most of the materials can carry compressive stress, even the granulars such sands.

**Tensile stress** is a loading that tends to produce stretching on a material by the application of axially directed **pulling** forces. Materials can withstand some tensile loading, but if enough force is applied, they will eventually break into two parts. Steel is an example of a material with high tensile strength.

**Shear stress** is caused when a force is applied to produce a **sliding** failure of a material along a plane that is parallel to the direction of the applied force.

### Strength terms

**Compressive strength** is a limit state of compressive stress that leads to compressive failure in the manner of ductile failure (infinite theoretically yield) or in the manner of brittle failure (rupture as the result of crack propagation, or sliding among a weak plane - see Shear strength).

**Tensile strength** is a limit state of tensile stress that leads to tensile failure in the manner of ductile failure (yield as the first stage of failure, some hardening in the second stage and break after a possible "neck" formation) or in the manner of brittle failure (sudden breaking in two or more pieces with a low stress state).

### Strain - deformation terms

**Deformation** of the material is the change in geometry when stress is applied (in the form of force loading, gravitational field, acceleration, thermal expansion, etc.). Deformation is expressed by the displacement field of the material.

**Strain** or **reduced deformation ** is a mathematical term to express the trend of the deformation change among the material field. For uniaxial loadings - displacements of a speciment (for example a bar element) it is expressed as the quotient of the displacement and the length of the speciment. For 3D displacement fields it is expressed as derivates of displacement functions in terms of a senond order tensor (with 6 independent elements).

## Stress - strain relations

**Elasticity** is the linear response of materials in terms of stress and strain as described by Hooke's law (Some times elasticity has non-linear character as the recoverable stress-strain relation is a non linear-function). The simpler form of Hooke's law is the spring relation: F=k*Δx where is the spring constant. Elasticity describes the state where the work offered by the application of external agents (forces), is stored in the material in form of elastic energy and it is recovered in form of displacement when external agents are removed (see Elasticity_(physics).

**Plasticity** is the non-linear response of materials in terms of stress and strain. Plastic behaviour includes the irrevocably transormation of work offered by the application of external agents (forces) to forms of energy such as thermal energy or crack propagation - grow. When the agents are removed, peramanent deformation remains. Plastic behaviour is described by "Flow rules" like as the differential relations between stress state, stress change and strain change.

**Viscosity** is the non-linear time dependent response of materials in terms of stress and strain. The most known form of **viscosity** in solid mechanics is **creep**. **Viscosity** in solids may include elastic deformation (Viscoelasticity) or/and plastic deformation (Viscoplasticity).