a) Explain in detail how forming limit diagram is produced and how can it help a fabricator to design the sheet metal forming process? (20 points) minimum 15 bullet points / b) What causes deflection of the rolls in the rolling process? (2.5 points) How can it be reduced? ( 2.5 points)
The Correct Answer and Explanation is:
(a) Forming Limit Diagram (FLD)
- Definition: A Forming Limit Diagram (FLD) is a graphical representation that shows the strain limits of sheet metal before failure during forming processes.
- Purpose: It helps predict the formability of sheet metal and identifies zones of safe deformation, necking, or failure.
- Preparation of the Specimen: Flat sheet metal specimens of specific dimensions are prepared with thickness, surface conditions, and material type representing actual forming conditions.
- Marking Grids: A grid of small circles (typically 2-5 mm diameter) is etched or printed on the sheet to measure strain distribution after deformation.
- Testing Setup: Specimens are deformed using a hemispherical punch under different conditions to simulate real forming processes.
- Stretching Under Varying Strain Paths: The specimens are stretched with varying conditions to create different strain paths (e.g., uniaxial, biaxial, and plane-strain deformation).
- Measurement of Strain: After deformation, the elongation of grid circles is measured using optical or scanning techniques to determine principal strains.
- Strain Plotting: The principal strains (major and minor) are plotted on an FLD with the x-axis as minor strain and y-axis as major strain.
- Forming Limit Curve (FLC): The curve divides the safe deformation region from necking or failure regions.
- Zones on the FLD:
- Below FLC: Safe deformation.
- On FLC: Onset of necking.
- Above FLC: Risk of fracture or failure.
- Material-Specific: FLDs vary depending on material properties like thickness, composition, and strain hardening.
- Application in Design: It aids fabricators in selecting appropriate forming processes, die designs, and material specifications.
- Troubleshooting: FLD helps identify problem areas in forming processes where material might fail.
- Reducing Waste: Using FLD minimizes material waste by predicting failure zones before production.
- Enhancing Productivity: It optimizes processes, improving production rates and reducing downtime.
(b) Causes and Reduction of Roll Deflection in Rolling Process
- Causes of Roll Deflection:
- High Rolling Forces: Excessive forces during rolling lead to bending of rolls.
- Roll Diameter and Material: Smaller diameter rolls or softer materials are prone to deflection.
- Non-uniform Distribution of Force: Uneven contact pressure between rolls and metal.
- Reducing Roll Deflection:
- Using Crowned Rolls: Rolls with a slight convex shape counteract bending.
- Roll Bending Mechanisms: Applying opposing forces to rolls during operation.
- Using Back-Up Rolls: Larger rolls support smaller working rolls, reducing deflection.