Materials of construction
Author: Katie Johnson [2015]
Stewards: Jian Gong and Fengqi You
Material Properties
Before choosing a material, the following properties should be known about it. Note that these properties for different common materials are often already collected and are available in various forms from manufacturers or in books.
Tensile Strength
The tensile strength, or tensile stress, of a material is the maximum amount of stress it can withstand before fracture. Proof stress is similar, but measures that maximum amount of stress a material can withstand before deformation becomes permanent. There are standard tensile tests that measure tensile strength; however, strength is a common material property that is often already tabulated. [1]
In addition to considerations such as the pressure of the process, there are often guidelines that specify maximum allowable stress. One such set of guidelines is laid out by ASME in their Boiler and Pressure Vessel Code.[1] This should be consulted while designing pressure vessels. There are also equations that can estimate these values. The maximum pressure that a cylindrical vessel can withstand is given by the following equations, where t is shell thickness, p is pressure, R is the inside vessel radius, and S is the allowable tensile stress:
There are tabulations of S for various metals found in Perry’s Handbook. [2]
Modulus of Elasticity
The modulus of elasticity of a material, sometimes called its stiffness, measure the amount a material deforms when a certain amount of stress is placed on it. This measure applies when elastic deformation occurs, that is, when all deformation is reversible and is linearly proportional to stress.[3] This is important because it measures the resistance of a material to bending and buckling.[1]
Ductility
Ductility measures the amount a material will deform before it fractures.[1] When a material has very low ductility it is defined as brittle. Some materials have a ductile-brittle transition points at low temperature. While these materials generally exhibit ductile properties, at low enough temperatures, they will not deform and will exhibit brittle fracture.[4] This is something to be aware of when design a process , especially at low pressures.
Hardness
The material’s ability to resist plastic deformation such as dents.[1] There are many simple and relatively inexpensive tests, such as Rockwell Hardness Tests and Brinell Hardness Tests, which can determine that hardness of a material. Hardness is a use property of a material to be aware of because it can be used to predict other mechanical properties such as tensile strength.[3]
Fatigue Resistance
Fatigue is failure of a material that can occur when there is cyclic loading on equipment, for example, in pumps. It can also occur if there are cycles of temperature or pressure.[1] When there is cyclic loading, failure can occur at lower stress levels than the normal tensile strength.[3]
Creep Resistance
Creep is the amount a material deforms while it is under constant tensile stress over long periods of time. This generally is only an issue for metals at high temperatures; however, for some materials such as lead, creep can occur at more moderate temperatures.[1]
Other considerations
Other considerations include the ease of fabrication, including welding ability and flexibility, the availability and cost of material, thermal conductivity (which is especially important for equipment like heat exchangers), electrical resistance, and magnetic properties for certain cases.[1]
References
- ^ a b c d e f g h Towler, G.P. and Sinnot, R. (2012). Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier.
- ^ G.D. Ulrich, A Guide to Chemical Engineering Process Design and Economics, Wiley: New York, 1984.
- ^ a b c Callister, William and Rethwisch, David. Materials Science and Engineering, Wiley: New York, 2011.
- ^ M.S. Peters, K.D. Timmerhaus, Plant Design and Economics for Chemical Engineers, 5th Ed., McGraw-Hill: New York, 2003.