Strength of materuals

Building science is the collection of scientific knowledge that focuses on the analysis and control of the physical phenomena affecting buildings. The practical purpose of building science is to provide predictive capability to optimize building performance and understand or prevent building failures. One of the fields of building science is the strength of materials. Its purpose is to determine the dimensions of the constructions in order to resist to strains which they have to withstand, or to check if a specific construction is able to withstand certain strains. The strength of materials also gives the value of the bearing reactions of the hyperstatic structures. It enables to ensure the good performance of the beams under the permanent and service loads. Furthermore, this science studies the mechanical properties of materials used in the construction industry. The strength of materials is also the study of dimensions and choice of materials to implement in a construction. To design a mechanical part, a structure, it is initially to imagine the forms and geometrical skeleton which fulfil the specific functions; then, it is to determine the quantities of matter necessary and sufficient to achieve these forms and to ensure a resistance without damage to the object with all the strains it will be subjected to during its service. This dimensioning calls upon calculations that forecast the performance of the object whose design must combine the best conditions of security, economy, and esthetics.

In general, when an external force is applied to a body, a deformation takes place; this is called the strain. This deformation causes forces to be set up in the internal structure of the material, and the material is then said to be in a state of stress. The strain increases until the resulting stresses are sufficient to neutralize the applied force when the body is again in a condition of equilibrium. Deformation of the material is the change in geometry when stress is applied in the form of force loading, gravitational field, 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.

There are three kinds of stresses: compressive, tensile and shear. Compressive stress is the stress state caused by an applied load that acts to reduce the length of the material in the axis of the applied load, in other words the stress state caused by squeezing the material. Compressive strength for materials is generally higher than that of tensile stress. Tensile stress is the stress state caused by an applied load that tends to elongate the material in the axis of the applied load, in other words the stress caused by pulling the material. The strength of structures of equal cross sectional area loaded in tension is independent of cross section geometry. Shear stress is the stress state caused by opposing force acting along parallel lines of action through the material.

There are some strength terms in the strength of materials. Yield strength is the lowest stress that gives permanent deformation in a material. Compressive strength is a limit state of compressive stress that leads to compressive failure in the manner of ductile failure or in the manner of brittle failure. 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 some pieces with a low stress state). Fatigue strength is a measure of the strength of a material or a component under cyclic loading, and is usually more difficult to assess than the static strength measures. Impact strength is the capability of the material in withstanding by the suddenly applied loads.