Question 1

What is a Forming Limit Diagram (FLD)?

Accepted Answer

A Forming Limit Diagram (FLD) is a plot used in sheet metal forming to show the combinations of major strain ε₁ (vertical axis) and minor strain ε₂ (horizontal axis) at which fracture or necking occurs. The boundary curve is called the Forming Limit Curve (FLC). Points below the FLC are in the safe zone; points above predict failure. Materials with a higher strain hardening exponent n have a higher FLC and better formability.

Question 2

How is FLC₀ (plane-strain forming limit) estimated using the Keeler-Goodwin method?

Accepted Answer

The Keeler-Goodwin empirical formula estimates FLC₀ (the forming limit at plane-strain, ε₂ = 0) from sheet thickness t (mm) and strain-hardening exponent n: FLC₀ ≈ (23.3 + 360·t)/100 · (n/0.21). A larger t or higher n raises the forming limit. On the negative ε₂ side (compression), the FLC rises as FLC₀ + 0.5|ε₂|; on the positive ε₂ side (tension), it rises as FLC₀ + ε₂.

Question 3

How does the strain path ratio β (dε₂/dε₁) affect formability?

Accepted Answer

The strain path ratio β defines the deformation mode: β = −0.5 is uniaxial tension, β = 0 is plane strain, and β = 1 is equibiaxial tension. The plane-strain condition (β = 0) corresponds to the lowest point of the FLC and is therefore the most critical path for necking and fracture. Equibiaxial stretching (β = 1) sits at the high end of the FLC, making it relatively safe for deep drawing. In press forming, the strain path varies across the part due to die geometry and blank-holder force.

Question 4

What is a safe target for safety margin in sheet metal forming?

Accepted Answer

The safety margin is defined as (FLC − ε₁_operating) / FLC × 100%. In production stamping, a safety margin of at least 20% is commonly recommended to account for material variability, tool wear, and lubrication changes. A margin below 10% is considered a risk zone. This tool colour-codes the margin: green (> 20%), amber (0–20%), and red (negative, i.e., failure predicted). Thickness reduction above 20–25% also correlates with elevated fracture risk in most sheet materials.

Forming Limit Diagram (FLD) & Strain Path Visualization

Material Selection

Material Parameters

Strain Path

Operating Point

Forming Limit Diagram (FLD) — ε₁ (Major Strain) vs ε₂ (Minor Strain)

Distance to FLC (Safety Margin)

Thickness Ratio t/t₀ vs ε₁ (Along Strain Path)