Page 43 - MetalForming Magazine March 2022
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Metal Matters By Daniel J. Schaeffler, Ph.D.
Strength and Stiffness are Not the Same
Strength and stiffness represent important properties of engi- neered components, but they have different meanings and describe different phenomena. Using these terms interchangeably may lead to incorrect conclusions.
Simply put: Strength is a plastic response to loading, while stiffness is an elastic response.
Strength
Strength measures the stress that a material can withstand before either permanently deforming or breaking. Calculate stress by dividing applied load by the cross-sectional area, leading to the units of lb./in.2 (psi).
Yield strength describes the level of stress where the material begins to deform plastically and permanently. If the applied stress is less than the yield strength, then removing the load allows the sheet metal to return to its original shape. After exceeding the yield strength, the permanently deformed material cannot return to its original shape even after removing the applied load. However, removing this load allows for relief of the elastic stress- es—the basis for springback.
Dr. Danny Schaeffler, with 30 years of materi- als and applications experience, is president of Engineering Quality Solutions (EQS) and Chief Content Officer of 4M Partners. EQS pro- vides product-applica- tions assistance to mate- rials and manufacturing
companies; 4M teaches fundamentals and practi- cal details of material properties, forming tech- nologies, processes and troubleshooting needed to form high-quality components. Schaeffler is the Metallurgy and Forming Technical Editor of the AHSS Application Guidelines available from World- AutoSteel at AHSSinsights.org.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
Tensile strength describes the stress at which a material fractures, and, therefore, also is a response to plastic deformation.
Strength, a property of the sheet material and a function of composition and processing, is independent of part design. Carbon, for example, is an effi- cient strengthener, where only 0.10 percent can dramatically increase strength. Processing comes in many forms, including rolling in the produc- tion mill to plastically deform the sheet and increase material strength. After cold rolling, a heat treatment known as annealing relieves internal stresses and reduces the strength while restor- ing ductility.
Forming also permanently deforms the sheet metal, strengthening it through a phenomenon known as work hardening. Certain applications may call for applying a stress-relief heat treatment to the formed component to reduce the strength and improve ductility.
Processing variables make it impos- sible to state that one metal alloy or one composition always will be stronger than another. We typically associate higher carbon and man- ganese content with higher-strength steels, but there are many routes to increase strength. In
addition, some alu-
minum grades are
stronger than steel
grades, but they gain
that strength from
different processing
routines and rarely
are they intended
for the same appli-
cations.
sents the measure of a formed com- ponent’s ability to return to its original shape after removing an applied load. It is a function only of the elastic mod- ulus, thickness and part geometry; microstructure and mechanical prop- erties have little impact on component stiffness.
The elastic modulus—the slope of the initial linear portion of a stress- strain curve obtained during tensile testing—represents the resistance of a material to elastic deformation under load, which is fully recoverable upon removing the load. The yield strength marks the deviation from this linear region and indicates the start of per- manent plastic deformation.
All steel grades have approximately the same elastic modulus, yet these grades come in many different strengths depending on alloying com- positions, rolling practices and heat treatments. Similarly, all aluminum alloys have approximately the same elastic modulus, approximately one- third that of steel. This difference in elastic modulus means that for the same sheet thickness and part design, a part made from an aluminum alloy will deflect three times as much as a steel part subjected to the same loading conditions. Achieving the same stiff-
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Stiffness
Stiffness, an elas- tic response, repre-
Ribs and other geometric features in body-structure members increase part stiffness.