By Srinivas Reddy
Hadley Industries manufactures cold-roll formed products, primarily for the building and construction industries. The company developed and patented a cold-forming process known as UltraSTEEL, which significantly improves mechanical and structural properties of strip steel by imparting a dimpling pattern prior to the roll forming operation.
UltraSTEEL uses a pair of roll tools with special teeth that form the dimple shape from both sides of the plain sheet prior to the cold-roll forming process. Dimpled steel products are increasingly used in a wide range of applications including wall studs, framing and roofing members, corrugated panels, vineyard posts, door reinforcements and many other applications.
Before committing to dimpled steel products, customers need to investigate their performance by estimating section properties such as stiffness and load-carrying capacity in order to make a buying decision. However, finding a means for estimating dimpled steel’s properties proved elusive when Hadley first developed it because of the differences between UltraSTEEL and standard cold-roll formed products.
Standard cold roll formed products have a uniform cross-sectional geometry, so theoretical calculations can be relatively easily performed to determine their section properties. The UltraSTEEL process, however, creates more complicated material properties that vary over the geometry. Interrelated nonlinear changes in contact, geometry and material properties occur during production and subsequent section forming and secondary operations. Simplified theoretical calculations cannot accurately analyze the performance of such a complex material.
While physical testing is a possible solution, it is prohibitively expensive with tooling costs running anywhere from $30,000 to $150,000 per part being a major obstacle to dimpled steel’s adoption.
Digital simulations replace costly physical testing
Based on past experiences with simulation, Hadley started using nonlinear finite element analysis (FEA) software to predict the changes in geometry and material properties that occur during the UltraSTEEL process, cold roll forming and secondary processes.
The challenge is accurately simulating the dimpled steel production processes and applications under loading. Accurate prediction of the final product behavior requires simulations that connect to previous or subsequent simulations while taking into account the changes in geometry, material and structural properties of the materials.
Hadley is able to simulate the UltraSTEEL process from start to finish in MSC Software’s Marc non-linear analysis solution, with the following steps:
1) the dimpling process that deforms a flat steel strip into a dimpled strip
2) the cold roll forming process that produces the desired section, and
3) additional processes, such as shear cutting and virtual testing of products under tension, bending, compression loads, etc.
The geometry and material data of the dimpled strip is transferred from one process to the next in a closed loop. This approach is practical for small sections of dimpled products and optimizing the dimpling process itself.
For larger models with tens of millions of elements, Hadley Group uses a simplified method to compute the properties of dimpled products. Engineers simulate the deformation of a flat steel plate by producing a single dimple. This general dimple geometry is used to generate a dimpled strip; 3D elements are used for smaller products and shell elements are used for larger products.
Only the geometry of the dimple is transferred from the dimpling process. The material properties of the dimple are provided from a separate tensile test on a dimpled steel sample. This test can be quickly and inexpensively performed without tooling. The next step is simulating the cold-roll forming process that shapes the dimpled strip into the desired product, other processes and applications.
In an example of the simulation process, a CAD model of the top and bottom rolls was imported into the Marc software. These rolls were modeled as rigid bodies that rotated around their central axes.
The plain strip was generated in Marc and placed in a pre-defined position between the two rolls. An elastic-plastic material model was used with a Young’s modulus of 205 GPa and a Poisson’s ratio of 0.30. Five layers of elements were used through the strip thickness to model both bending and stretching phenomena. Two different meshes were evaluated, one with 37,980 solid elements and the other with 149,810 solid elements. The dimpling simulation showed that the maximum plastic strain and stress developed in the two meshes differed by less than 7 percent and 4 percent respectively so the coarser mesh was used for the balance of the study.
The top and bottom rolls had an overlapping gap of 0.40 mm between the mating teeth. The sheet was fully fixed at one end and initially fed to the rotating rolls with a velocity equal to the linear velocity at the tip of the roll teeth. When the roll teeth just grasped the strip, the fixed end was released and the strip was deformed by the rotating rolls.
The original plain sheet and the dimpled sheet were used in tension and bending simulations. The engineering stress and strain data of the plain steel sheet were obtained from tensile tests. The dimpled sheet was merged into the new model in order to start the new analysis and the pre-state option was employed to directly transfer result data from the previous dimpling process into the tensile and bending simulations.
Simulation results confirm UltraSTEEL’s technical superiority
Both simulation approaches have been validated by experimental results comparing the 3D geometry of the predicted shape with scanned data of the physical sample. The geometry of the predicted dimpled sheet differed from the actual process by less than 1.7 percent.
Accurate simulations have enabled Hadley to substantiate technical superiority about dimpled steel produced through the UltraSTEEL process. With the simulations providing customers with the data they need to make buying decisions, Hadley has forecast an eventual sales volume of more than $4 million for dimpled steel products. It has also generated additional revenue by licensing the process to other manufacturers.
Bio:Srinivas Reddy is a senior product marketing manager at MSC Software Inc. and is currently responsible for the FEA product line. Prior to joining MSC, he worked as an engineer at Hindustan Aeronautics Ltd, India, and as a consulting engineer, helping engineers at automotive, oil & gas, and biomedical industries analyze their products with simulation software. At MSC Software, he has held several roles including technical consultant, application engineer and product manager. Reddy has a BS degree in Aerospace Engineering from Indian Institute of Technology, Chennai, India, and a PhD in Engineering Science & Mechanics from The Pennsylvania State University.
MSC Software Corp.
Filed Under: TECHNOLOGIES + PRODUCTS, Design World articles, Software
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