|
|
Material: Glossary
The following material parameters are used in the Material Database:
- Strain
- Stress
- Young's Modulus
- Yield Stress
- Poisson's Ratio (ρ)
- Mass Density
- K-Value
- Ultimate Tensile Stress (UTS)
- Stress-Strain Curve
- Forming Limit Curve/ Diagram
- n-Value
- r-Value
Strain
Strain can be defined simply as a measurable deformation of the metal. In other words, metal must be strained in order to change its shape. Strains can be positive (pulling the metal apart, or tension) or negative (pushing the metal together, or compression). Strains also can be permanent (plastic) or recoverable (elastic). The result of elastic straining commonly is referred to as springback, or elastic recovery.
Every metal type wants to return to its original shape when it's deformed. The amount the metal springs back is a function of its mechanical properties. When engineers refer to part areas that are high strain, they typically are referring to areas that have been subjected to substantial stretch or compression.
Note: When there is NOT a lot of strain (like in brackets, etc), changing material (even from steel to aluminum) will have almost no affect on the BLANK size. However, when there is much strain (deep drawing, etc), the BLANK size may be different when changing materials.
Stress
Stress is the result of straining the metal. When subjected to stress, metal incurs internal changes that cause it to spring back or deform non-uniformly. Trapped stresses within a part often result in a loss of flatness or other geometric characteristics. All cut or formed parts incur stress.
Young's Modulus
Young's Modulus, also known as the elastic modulus, is a measure of the stiffness of a solid material.
Yield Stress
The stress at which plastic (permanent) deformation begins and elastic deformation stops.
Poisson's Ratio (ρ)
Poisson's Ratio is the ratio of the lateral contraction of a material to the longitudinal extension. A material tends to expand in directions perpendicular to the direction of compression. Conversely, if the material is stretched rather than compressed, it usually tends to contract in the directions transverse to the direction of stretching.
Mass Density
Mass Density is a expression of the amount of mass contained per unit volume. The standard unit is kilograms per meter cubed (kg/m3).
K-Value
Is the material strength or stress coefficient is used in the power law relationship to determine the shape of the plastic portion of the stress/strain curve.
Ultimate Tensile Stress (UTS)
The maximum stress that a material can withstand without fracture.
Stress-Strain Curve
The relationship between the stress and strain that a material displays is known as a Stress-Strain curve. It is unique for each material and is found by recording the amount of deformation (strain) at distinct intervals of tensile or compressive loading. These curves reveal many of the properties of a material (including data to establish the Modulus of Elasticity, E).
Forming Limit Curve/ Diagram
Circular grid patterns on the surface of a sheet metal part deform differently based on the type of loading. There exists a relationship between the distortion of the circle and the type of stressing. It is possible to find combinations of maximum strain and minimum strain for which neither necking nor fracture occurs. These results can be plotted in the shape of a diagram, known as the forming-limit diagram (FLD). These diagrams are valid for a definite sheet-metal-quality and define the following zones:
-
Fracture
-
The beginning of necking (deformation)
Necking is a type of deformation where large amounts of strain are localized in a specific region of the material.
n-Value
The work hardening exponent determines the rate at which a material hardens and describes the ability of the material to stretch.
r-Value
The coefficient of anisotropy or r-bar is used to measure the differences in material properties through the thickness of the sheet versus the plane of the sheet and describes the drawability of a material.
The metal's r-value is defined metallurgically as the plastic strain ratio. To understand this concept, you must clearly know the difference between stretching and drawing. Stretching is a metal forming process in which the metal is forced into tension. This results in an increase in surface area. Items such as most automobile hoods and fenders are made using this process.
Drawing is the displacement of metal into a cavity or over a punch by means of plastic flow or feeding the metal. Items such as large cans, oil pans, and deep-formed parts usually are made using this process.
The metal's r-value can be defined simply as the metal's ability to flow. It also is expressed numerically using a value from ~0.5 to 2. The greater the r-value is, the more drawable the metal.
|
