How Materials Carry Load

 

Basic modes of loading a material: Tension, compression and shear

 

 

 

Stress:

 

Loads applied on a material are distributed over a surface. For example, the point load shown in the following figure might actually be a uniformly distributed load that has been replaced by its equivalent point load.

 

 

 

Stress is the load applied per unit area of the surface it is applied on. Normal stress is the stress normal to a surface and is denoted by the symbol "" (sigma). In the above figure the normal stress is uniform over the surface of the bar and is given by

 

 

 

Shear stress is the stress tangent to a surface. If in the following figure the shear stress (tau) that results in the shear load V is uniformly distributed over the surface, then the shear stress can be calculated by dividing the shear force by the area it is applied on.

 

 

 

The units of stress are the units of load divided by the units of area. In the SI system the unit of stress is "Pa" and in the U.S. system it is "Psi". Pa and Psi are related to the basic units through the following relations

 

 

 

Basic modes of deformation of a material: Extension, contraction, shearing

 

Material element can be extended, compressed, or sheared. The following figure shows how the square section to the left changes its shape during extension, contraction and shearing.

 

 

Strain:

 

Strain is the way engineers represent the distortion of a body. Axial strain (normal strain) in a bar is a measure of the extension of a bar per unit length of the bar before deformation. The following figure shows a bar of initial length lo that is extended by the application of a load to the length l.

 

 

The axial strain, denoted by (epsilon), in a homogeneously deforming bar is calculated by dividing the amount the bar extends by its initial length. This yield the equation

 

 

A positive axial strain represents extension and a negative axial strain represents a contraction. Strain has no units since it is one length divided by another length.

 

Shear strain, denoted by (gamma), is a measure of how the angle between orthogonal lines drawn on an undeformed body changes with deformation. In the following figure the square has been sheared into a parallelogram.

 

The shear strain is calculated from the equation

 

 

As can be seen from the following figure, the shear strain is equal to the tangent of the change in angle or the two orthogonal sides.

 

 

 

 

The difference between and becomes less and less as the angle (in radians) becomes small. This is since the tangent of an angle, given in radians, can be approximated by the angle for small values of the angle. In most structural materials the shearing is small and we can use the approximation

 

 

Tensile behavior different materials:

 

 

In a typical tensile test one tries to induce uniform extension of the gage section of a tensile specemin. The gage section of the tensile specimine is normally of uniform rectangular or circular cross-section. The following figure shows a typical dog-bone sample. The two ends are used for fixing into the grips, which apply the load. As can be seen from the free-body diagram to the right, the load in the gage section is the same as the load applied by the grips.

 

Using extensometers to measure the change of length in the gage section and a load cells to measure the load applied by the grips on the sample one calculates the axial strain and normal stress (knowing the initial gage length and cross-sectional area of the gage section). The result is a stress-strain diagram, a diagram of how stress is changing in the sample as a function of the strain for the given loading. A typical stress-strain diagram for a mild steel is shown below.

 

 

The different regions of the are response denoted by their characteristics as follows

 

Brittle versus Ductile behavior:

 

Brittle materials fail at small strains and in tension. Examples of such materials are glass, cast iron, and ceramics. Ductile materials fail at large strains and in shear. Examples of ductile materials are mild steel, aluminum and rubber. The ductility of a material is characterized by the strain at which the material fails. An alternate measure is the percent reduction in cross-sectional area at failure.

 

Different types of response:

 

Elastic response:

 

If the loading and unloading stress-strain plot overlap each other the response is elastic. The response of steel below the yield stress is considered to be elastic.

Elastic-plastic response:

 

After loading beyond the yield point, the material no longer unloads along the loading path. There is a permanent stretch in the sample after unloading. The strain associated with this permanent extension is called the plastic strain ( on the figure). As shown in the figure, the unloading path is parallel to the initial linear elastic loading path.

 

Viscoelastic response:

 

Most plastics when loaded continue to deform over time even without increasing the load. This continues extension under constant referred to as creep. If held at constant strain, the load required to hold the strain decreases with time. The decrease in load over time at constant stretch is referred to as relaxation.

 

Bearing Stress:

 

Even though bearing stress is not a fundamental type of stress, it is a useful concept for the design of connections in which one part pushes against another. The compressive load divided by a characteristic area perpendicular to it yields the bearing stress which is denoted by . Therefore, in form, the bearing stress is no different from the compressive axial stress and is given by

 

 

where F is the compressive load and A is a characteristic area perpendicular to it.

 

 

 

For example, if two plates are connected by a bolt or rivet as shown, each plate pushes against the side of the bolt with load F. It is not clear what the contact area between the bolt and the plate is since it depends on the size of the bolt and the shape of the deformation that results. Also, the distribution of the load on the bolt varies from point to point, but as a first approximation one can use the shown rectangle of area A=td to get a representative bearing stress for the bolt as

 

 


Mehrdad Negahban and the University of Nebraska, 1996-2000.

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Copy and distribute freely for personal use only

 

Department of Engineering Mechanics, University of Nebraska, Lincoln, NE 68588-0526

Last modified at: 3:07 PM, Wednesday, August 30, 2000