A Forming Limit Curve (FLC) is a map of strains indicating the onset of critical through-thickness necking for different linear strain paths. The FLC is dependent on the metal grade and the specific methods used in its creation.  When paired with the strains generated during forming of an engineered part, the associated Forming Limit Diagram (FLD) provides guidance on which areas of the part might be prone to necking failures during production stamping conditions that replicate those used in the analysis.

Several methods are available to measure the strains on formed parts. The earliest method is known as Circle Grid Strain Analysis (CGSA), with Dr. Stuart Keeler as its primary evangelist for nearly 50 years.  Dr. Keeler was the Technical Editor of these AHSS Guidelines through Version 6.0, released in 2017.

As the name suggests, a flat blank is covered with a grid of circles of precisely known diameter, typically applied by electrochemical etching. Forming turns the circles into ellipses, with the dimensions related to the major and minor strains.  Conventional measurement occurs after forming, and involves a calibrated Mylar™ strip marked with gradations indicating the expansion or contraction relative to the initial circle diameter. Typically, these are viewed through magnifiers, making it easier to discern the critical dimensional differences. Techniques and caveats are highlighted in Citations S-59 and S-60.

Instead of circles, most camera-based measurement techniques for analysis after forming use a regular grid pattern of squares or dots.  Forming turns the squares into rectangles, and the camera/computer measures the expansion or contraction of the nodes at the corners of the squares to determine the strains.  Similarly, forming changes the regular dot pattern, allowing for calculation of the strains.

These approaches determine only the strains after forming, and are constrained to assume linear strain paths.  An alternate approach based on Digital Image Correlation (DIC), where a camera tracks the movement of a random speckle pattern applied prior to forming,W-26, H-22, M-21 follows the strain evolution which occurs during forming and is not affected by non-linear strain paths.

Although DIC strain analysis is more accurate and informative, it is a higher-cost approach best suited for laboratory environments.  Circle-grid, square-grid, and dot-grid strain analysis are all lower cost options and readily applied on the shop floor.  Each of these in-plant techniques have different merits and challenges, including ease of use, accuracy, and cost.

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