Formability Testing

Formability is the materials ability to be shaped in to semi-finished and finished products by manufacturing techniques like rolling, forging, extrusion, deep drawing, etc. Many transportation equipment like cars, trucks, air planes, etc., use thin sheet metals generally steel and aluminum alloys formed in to respective shapes by processes such as deep drawing, bending, etc., and joined to each other to make an envelope. With increasing need for light weighting the equipment for fuel efficiency, many sheet metal producers are engaged in material developments like alloy development, coating development, etc., to improve the formability of high strength and light gage products. Both the sheet metal producer and fabricators need to know if a material can be formed in to the shapes required by traditional processes to form the body envelop of a car, truck or air craft, etc. These operations are done at ambient temperature or warm temperatures, but at high process speeds. This cold formability is very critical for component manufacturing. Equally several finished, semi-finished metal product manufactures like forging, extrusion companies and flat rolling companies would like to understand the formability of materials specifically metals and alloys. These operations are generally done at high temperatures. Thus, hot formability or warm formability of the materials is also important. These are assessed using simple tests like tensile tests, compression test, torsion test, bending test, drawing test, etc.

Types of Testing

Formability testing is divided in to two main groups namely cold formability, Hot and warm formability. Cold formability is important for majority of forming done at room temperature. Typical examples are can manufacturing, autobody sheet pressing, etc. It is generally assessed by simple tensile test or bend test. One can determine materials parameters like ductility, strain hardening exponent (n value), and draw ratio (r value) from a simple tensile test. Additional tests would involve cupping test or limiting dome height (LDH) draw test and forming limit diagram (FLD) test. In addition to it there are tests like guided bend test, stretch bend tests, etc., used to assess hemming and bending formability of sheet metals. These tests are limited to generally thin gage materials. In addition to assess bulk formability for hot forming tests like hot compression or torsion tests are done at a variety of test speeds.

Capability – At TTL, we have capability to run the following formability tests at ambient and non-ambient conditions.

• Tensile and Compression Testing
• Plane Strain Tension and Compression Testing
• Straight and Guided Bend Testing
• Draw Ratio (“r “value) Testing
• Strain Hardening Exponent (“n” value) Testing
• Ericksen Cup Test

Some relevant standards:

• ASTM E190 – Standard Test Method for Guided Bend Test for Ductility of Welds
• ASTM E290 – Standard Test Method for Bend Testing of Material for Ductility
• ASTM E517 – Standard Test Method for Plastic Strain Ratio r for Sheet Metal
• ASTM E643 – Standard Test Method for Ball Punch Deformation of Metallic Sheet Material
• ASTM E646 – Standard Test Method for Tensile Strain-Hardening Exponents (n-Values) of Metallic Sheet Materials