Geoscience Reference
In-Depth Information
• Crystal structure
• Strength
• Melting point
• Density
• Hardness
• Brittleness
• Ductility
• Modulus of elasticity
• Wear properties
• Coeficient of expansion
• Contraction
• Conductivity
• Shape
• Exposure to environmental conditions
• Exposure to chemicals
• Fracture toughness
Note: All of these properties can vary, depending on whether the force is crushing, corroding, cut-
ting, pulling, or twisting.
The forces an object can encounter are often different from the forces that an object can with-
stand. An object may be designed to withstand only minimal force before it fails (a toy doll may
be designed of very soft, pliable materials or designed to break or give way in certain places when
a child falls on it, thus preventing injury). Other devices may be designed to withstand the greatest
possible load and shock (e.g., a building constructed to withstand an earthquake).
When working with any material that will go in an area with a concern for safety, a safety factor
(SF) is often introduced. As defined by ASSE (1988), a safety factor is the ratio allowed for in design
between the ultimate breaking strength of a member, material, structure, or equipment and the
actual working stress or safe permissible load placed on it during ordinary use. Simply put, includ-
ing a factor of safety—into the design of a machine, for example—makes an allowance for many
unknowns (inaccurate estimates of real loads or irregularities in materials, for example) related to
the materials used to make the machine, related to the machine's assembly, and related to the use
of the machine. Safety factors can be determined in several ways. One of the most commonly used
ways is
Failure Producingload
Allowablestress
SF =
(11.1)
Forces on a material or object are classified by the way they act on the material. For example, a
force that pulls a material apart is called the tensile force . Forces that squeeze a material or object
are called compression forces . Shear forces cut a material or object. Forces that twist a material or
object are called torsional forces . Forces that cause a material or object to bend are called bend-
ing forces . A bearing force occurs when one material or object presses against or bears on another
material or body.
So, what is force? Force is typically defined as any influence that tends to change the state of rest
or the uniform motion in a straight line of a body. The action of an unbalanced or resultant force
results in the acceleration of a body in the direction of action of the force, or it may (if the body is
unable to move freely) result in its deformation. Force is a vector quantity, possessing both magni-
tude and direction (see Figure 11.1); its SI unit is the newton (equal to 3.6 ounces, or 0.225 lb).
According to Newton's second law of motion, the magnitude of a resultant force is equal to the
rate of change of momentum of the body on which it acts. The unit of force is the pound force in the
English or engineering system and is the newton in the SI system. The pound force is defined as the
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