Researchers from the United States Steel Corporation (USS) recently neutrons at Oak Ridge National Laboratory’s Spallation Neutron Source (earlier post) to better understand the properties of hydroformed AHSS and how it responds to residual stress introduced during manufacturing.
In an SAE International technical paper, the research team reported on their investigation of the residual stress distribution in a 980GEN3 steel part after hydroforming. They used the non-destructive neutron diffraction technique to reveal the residual stress profile across a 94-degree tube bending radius section of the hydroformed part.
They then conducted finite element analysis (FEA) to simulate the hydroforming and the subsequent unloading processes to predict the residual stress distribution within the same region.
Effective stress distribution contours in the regions of interest before and after springback. Huang et al.
The integration of AHSS in novel automotive structure design has introduced huge advantages in mass reduction while maintaining their structural performances, yet several concerns have been raised for this relatively new family of steels. One of those concerns is their potentially high springback after forming, which can lead to geometrical deviation of the final product from its designed geometry and cause difficulties during assembly.—Huang et al.
Because this is a new material containing retained austenite, we need to have a better understanding of its performance. A better understanding of how this material responds to different manufacturing processes like stamping or hydroforming will help us validate engineering models that will in turn make it easier to design and produce auto components that are lighter, stronger, and more durable.—Lu Huang, USS industrial research engineer
Huang said the instrument at SNS was the best tool for this research because of its ability to spatially resolve the residual stress in components. VULCAN’s sample environment can also accommodate large automotive components under observation in realistic operating conditions.
VULCAN is designed for deformation, phase transformation, residual stress, texture, and microstructure studies. Load frames, furnaces, battery chargers, and other auxiliary equipment for in situ and time-resolved measurements are integrated in the instrument. As a time-of-flight diffractometer at the world's most intense pulsed, accelerator-based neutron source, VULCAN provides rapid volumetric mapping with a sampling volume of 2-600 mm3 and a measurement time of minutes for common engineering materials. In extreme cases, VULCAN has the ability to study kinetic behaviors in sub-second time frames.
Based on the literature, we knew others had used the neutron diffraction line-scanning technique to study the residual stress in weldments for welding. We found that it is also very applicable when we look at as-formed parts, especially the ones formed with AHSS.—Lu Huang
Neutron diffraction provides nondestructive measurements of residual stress within the part formed with AHSS, making it possible to observe the intrinsic properties of the hydroformed parts and examine their emergence across various cross sections and lattices in great detail. This fundamental knowledge of the steel’s characteristics, Huang said, also laid the groundwork to probe the impact of residual stress in hydroformed steel parts on vehicle durability.
The researchers combed neutron data on residual stress distribution with computer simulations to see if they could improve the models that engineers use for designing and manufacturing auto parts.
A vehicle is constantly undergoing fatigue, or cyclic force, so auto companies want to make sure that the vehicle will be safe within a certain lifetime. Residual stress in the as-formed part may affect its fatigue performance. However, the effect of the residual stress on the fatigue performance is not usually incorporated in the simulation due to the lack of residual stress data or validated material models.—Lu Huang
We think our simulations and tests will help scientists and engineers to understand the fatigue performance of auto parts and how different manufacturing processes may affect it. The residual stress data from our experiment may also set a new precedent of accuracy and specialization for validations and selections of material models in residual stress prediction or modeling.—Lu Huang
Lu Huang’s coauthors include Xiaoming Chen from USS and Dunji Yu, Yan Chen, and Ke An from ORNL. The USS researchers received beam time at through the Industrial Applications Program at the Shull Wollan Center—a Joint Institute for Neutron Sciences.
Huang, L., Chen, X., Yu, D., Chen, Y. et al. (2018), “Residual Stress Distribution in a Hydroformed Advanced High Strength Steel Component: Neutron Diffraction Measurements and Finite Element Simulations,” SAE Technical Paper 2018-01-0803 doi:
Huang, L. and Shi, M. (2018) “Forming Limit Curves of Advanced High Strength Steels: Experimental Determination and Empirical Prediction,” SAE Technical Paper 2018-01-0804 doi: