Civil Engineering Reference
In-Depth Information
data as it includes die contact and friction. For the testing, various sheet specimen
widths are tested (get varying strain paths) and the maximum height of the dome is the
LDH value. Thus, the higher the LDH value, the higher the formability of the material.
1.5.2.3 Hole Expansion Ratio
The hole expansion ratio (
λ
value) is a formability measure that describes the form-
ability of sheet metal near the edges of the part. This ratio is useful for determining
how a material will form in relation to the edges of the blank to be formed. A larger
ratio value is desired as there will be a higher capability for reaching higher strain
without material failure. This ratio and the results are very sensitive to the quality of
the edge and the microstructure of the material, thus testing should probably replicate
the same quality and microstructure that will be used for the actual forming process.
1.5.2.4 Forming Limit Diagram
The forming limit diagram (FLD) is a common conceptual tool used to represent
the forming behavior for a given material while describing the onset of sheet neck-
ing for varying loading configurations. On an FLD, the forming limit curve (FLC)
represents the maximum achievable major principle strains for a given minor prin-
ciple strain (Fig.
1.10
). The strain paths shown in the figure from left to right are
pure shear, uniaxial tension, plane strain, stretching, and balanced biaxial stretch-
ing. This concept was introduced by Keeler and Backofen in 1964 [
26
] for the
positive minor strains and by Goodwin in 1968 [
27
] for negative minor strains.
1.5.2.5 Uniaxial Tensile Test
The uniaxial tensile test is the most common test for determining the flow characteris-
tics and basic properties used for engineering analysis. Properties that are commonly
derived from this test include Young's modulus, yield strength, tensile strength, uni-
form elongation, total elongation, strain hardening value, and normal anisotropy.
Fig. 1.10
Schematic of a
Forming Limit Diagram
(FLD)
