Stress means:additional internal forces that per unit area bears.The formular is:
Among which, �� is stress; ��Fj indicates the direction of the force on j; ��Ai is the direction of the force in the I area.
Because the area and force are vectors,if the forced area and the stress in the same direction,as shown in Figure1, ��x and ��y; If the forced area and the stress orthogonal to each other is shtress.Shown in Figure 1 ��xy or��yx.
��Internal stress[1]�� refers to a single structure composed of different materials will cause various types of stress because of the different ways of deformation.

When the material under the external force can not occur displacement, its geometric shape and size will change, this deformation is called strain.The internal occur the same size and opposite reaction against external forces when the deformtion appears. The distribution of internal forces at a point known as the set degree of stress.the product of stress and the micro-area is Micro internal force.Or because of external factors (stress, humidity, etc.) that objects appear deformation and among various parts of the objects produce internal forces, to resist this external effect, and endeavor to make the object from the position after deformation returned to pre- position. In the study of a point on the internal force per unit area in cross-sectional is called stress. In accordance with the direction of stress and strain relations, stress can be divided into the direct stress �� and shear stress ��, the direction of direct stress is parallel to strain directions, and the direction of shear stress and strain is vertical.In according to the different form of load, the stress can be divided into tensile compressive stress,bending stress and torsion stress.

Stress classification

Normal stress and sheat stress

Direct stress and shear stress
The same vertical section known as normal stress, with the same tangenting section known as the shear stress.Stress will increase with the increase of external force. For a certain material,the increase of stress is limited,the material ill be damaged once over the limit .A certain material ,the limit that the stress can reach is called ultimated stress. The ultimated value can be measured via mechanical testing. Making an appropriate reduce on measured ultimated stress,and provided the maximum security of the material,this is allowable stress. To use the materails safe,the internal stress will lower than ultimated stress when using the material,or the matrial will be damaged.

Some materials in the work, the external force have no change over time, then the size of its internal stress have no changes, known as the static stress; there are some other materials, external forces suffered periodic change with time, then internal stress also changes periodically with time, known as the alternating stress.Materials under alternating stress as fatigue damage. Usually the alternating stress is much smaller than its ultimate strength under static load, the damage may occur. In addition, the material will enlarge internal due to the section size change. This phenomenon called stress concentration. For the uniform and brittle materials, the stress concentration will greatly reduce the intensity of component,which should pay special attention to the component.

When the objects be forced will occur deformation, the inner deformation of each point generally not the same. To describe the point on the degree of deformation of the mechanical quantity is the point of strain.Therefore, we can compare the size and shape with before.

Linear strain

Shear strain

Units of two mutually perpendicular edge, right-angle change value after deformation, defined as the angle strain or shear strain, with �� said.The shear strain of a point in the x-y direction,y-z direction and z-x direction.respectively ��xy����yz����zx. To reduce the shear angle is positive, otherwise negative.

Point of the strain state

When known the point of the strain component ��x, ��y, ��z, ��xy, ��yz, ��zx, the point in any direction in the linear strain, and any two line segments through the point of change in volume between the right angle, can according to the coordinate transformation formula of the strain components obtained. The strain state of this points is fixed.
The strain component of strain state of��x, ��y, ��z, ��xy, ��yx, ��yz��zy, ��zx, ��xz composed of strain tensor,
In the right of the shear strain tensor with ��xy, ��xz, --- that apply to the use standard presentation of label tensor
Left in the shear tensor with ��xy, ��xz, --- that is the custom representation

Measuring Tools

Strain gauge is an instrument to measure transparent objects that due to internal stress bring the birefringence phenomenon.The source of the birefringence is due to the uniform cooling or external mechanical effects.