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Crash Management, structural performance
In addition to enhanced formability, Advanced High-Strength Steels (AHSS) provide crash energy management benefits over their conventional High-Strength Steel (HSS) counterparts at similar strength levels. Higher levels of work hardening and bake hardening at a given strength level contribute to this improvement in crash performance.
The energy required for plastically deforming a material (force times distance) has the same units as the area under the true stress-true strain curve. This applies to all types of plastic deformation – from that which occurs during tensile testing, stamping, and crash. The major difference between these is the speed at which the deformation takes place.
As an example, consider the press energy requirements of two grades by comparing the respective areas under their true stress – true strain curves. The shape and magnitude of these curves are a function of the yield strength and work hardening behavior as characterized by the n-value when tested at conventional tensile testing speeds. At the same yield strength, a grade with higher n-value will require greater press energy capability, as highlighted in Figure 1 which compares HSLA 350/450 and DP 350/600. For these specific tensile test results, there is approximately 30% greater area under the DP curve compared with the HSLA curve, suggesting that forming the DP grade requires 30% more energy than required to form a part using the HSLA grade.
Figure 1: True stress-strain curves for two materials with equal yield strength.T-3
The high degree of work hardening exhibited by DP and TRIP steels results in higher ultimate tensile strength than that exhibited by conventional HSS of similar yield strength. This provides for a larger area under the true stress-strain curve. Similarly, when panels are formed from these grades, the work hardening during forming leads to higher in-panel strength than panels from HSS of comparable yield strength, further increasing the area under the stress-strain curve, ultimately resulting in greater absorption of crash energy.
Finally, the high work-hardening rate better distributes strain during crash deformation, providing for more stable, predictable axial crush that is crucial for maximizing energy absorption during a front or rear crash event.
Many AHSS are bake hardenable. The relatively large BH effect also increases the energy absorption capacity of these grades by further increasing the area under the stress-strain curve. The BH effect adds to the work hardening imparted by the forming operation. Conventional HSS do not exhibit a strong BH effect and therefore do not benefit from this strengthening mechanism.
Figure 2 illustrates the difference in energy absorption between DP and TRIP steels as a function of their yield strength determined at quasi-static tensile testing speeds.
Figure 2: Absorbed energy for square tube as function of quasi-static yield strength.T-2
Figure 3 shows calculated absorbed energy plotted against total elongation for a square tube component. The absorbed energy remains constant for the DP and TRIP steels but the increase in total elongation allows for formation into complex shapes.
Figure 3: Calculated absorbed energy for a square tube as a function of total elongation.T-2
For certain parts, conventional steels may have sufficient formability for stamping, yet lack the required ductility for the desired crash failure mode and will split prematurely rather than collapsing in a controlled manner. AHSS grades improve energy absorption by restoring a stable crush mode, permitting more material to absorb the crash energy. The increased ductility of AHSS grades permit the use of higher strength steels with greater energy absorbing capacity in complex geometries that could not otherwise be formed from conventional HSS alloys.
Stable and predictable deformation during a crash event is key to optimizing the steel alloy selection. The ideal profile is a uniform folding pattern showing progressive buckling with no cracks (Figure 4).
Figure 4: Deformation after axial crushing.A-49
Achieving crack-free folds is related to the local formability of the chosen alloy. Insufficient bendability can lead to early failure (Figure 5). Research to determine proper simulation inputs with physical testing for verification.
Figure 5: Three-point bend testing of two DP980 products having different folding and cracking behavior resulting from different microstructures and alloying approaches.B-12
High Strain Rate Property Test Methods
for Steel and Competing Materials
Tensile testing occurs at speeds that are 1000x slower than typical automotive stamping rates. Furthermore, automotive stamping is done at speeds that are 100x to 1000x slower than crash events. Stress-strain responses change with test speed – sometimes quite dramatically.
The m-value is one parameter to characterize this effect, since it is a measure of strain rate sensitivity. Generally, steel has more favorable strain rate effect properties compared with aluminum, but this is also a function of alloy, test temperature, selected strain range, and test speed. L-20
These reasons form the background for the need to characterize the intermediate and high strain rate behavior of AHSS.
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Formability
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If all stampings looked like a tensile dogbone and all deformation was in uniaxial tension, then a tensile test would be sufficient to characterize the formability of that metal. Obviously, engineered stampings are much more complex. Although a tensile test characterizes one specific strain path, a Forming Limit Curve (FLC) is necessary to have a map of strains indicating the onset of critical through-thickness necking for different linear strain paths. The strains which make up the FLC represent the limit of useful deformation. Calculations of safety margins are based on the FLC (Figure 1).
Figure 1: General graphical form of the Forming Limit Curve.E-2
Picturing a blank covered with circles helps visualize strain paths. After forming, the circles turn into ellipses, with the dimensions related to the major and minor strains. This forms the basis for Circle Grid Strain Analysis.
Generating Forming Limit Curves from Equations
Pioneering work by Keeler, Brazier, Goodwin and others contributed to the initial understanding of the shape of the Forming Limit Curve, with the classic equation for the lowest point on the FLC (termed FLC0) based on sheet thickness and n-value. Dr. Stuart Keeler was the Technical Editor of these AHSS Guidelines through Version 6.0, released in 2017.
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Equation 1 |
These studies generated the left hand side of the FLC as a line of constant thinning in true strain space, while the right hand side has a slope of +0.6, at least through minor (engineering) strains of 20%.
Evidence accumulated over many decades show that this approach to defining the Forming Limit Curve is sufficient for many applications of mild steels, conventional high strength steels, and some lower strength AHSS grades like CR340Y/590T-DP. However, this basic method is insufficient when it comes to creating the Forming Limit Curves for most AHSS grades and every other sheet metal alloy. Grades with significant amounts of retained austenite experience significant deviations from these simple estimates.H-23, S-61 In these cases, the FLC must be experimentally determined.
Additional studies found correlation with other properties including total elongation, tensile strength, and r-valueR-8, P-19, G-23, A-45, A-46, H-23 with some of these attempting to define the FLC by equations.
Experimental Determination of Forming Limit Curves
ASTM A-47 and ISO I-16 have published standards covering the creation of FLCs. Even within these standards, there are many nuances left for interpretation, primarily related to the precise definition of when a neck occurs and the associated limiting strains.
There are two steps in creating FLCs: Forming the samples and measuring the strains.
Forming sheet specimens of different widths uses either a hemispherical dome (Nakajima or Nakazima method N-14) or a flat-bottom punch (Marciniak method M-22) to generate different strain paths from which critical strains are determined. The two methods are not identical due to the different strain paths generated from the punch shape. These differences may not be significant for many lower strength and conventional High-Strength Steels, but may deviate from each other at higher strengths or with advanced microstructures. Figure 2 highlights samples formed with the Nakajima method.
Figure 2: Forming limit curves can be created by deforming multiple samples of different widths. Narrow strips on the left allow metal to flow in from the unconstrained edges, creating a draw deformation mode leading to strains that plot on the left side of the FLC. Fully constrained samples, shown on the rightE-2, create a stretch deformation mode leading to strains that plot on the right side of the FLC.
Generally, strains are measured using one of two methods. The first approach involves covering the initially flat test samples with a grid pattern of circles, squares, or dots of known diameter and spacing to measure the strains associated with deformation. An alternate approach is 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 DIC methods are directly suited for use with stress-based FLCs.
Differences between Forming Limit Curves
and Forming Limit Diagrams
Often, the terms Forming Limit Curve (FLC) and Forming Limit Diagram (FLD) are used interchangeably. Perhaps a better way of categorizing is to define the components and what they encompass. This is somewhat of a simplification, since detailed interactions are known to occur.
- The Forming Limit Curve is a material parameter reflecting of the limiting strains resulting in necking failure as a function of strain path. It is a function of the metal grade, thickness, and sheet surface conditions, as well as the methods used during its creation (hemispherical/flat punch, test speed, temperature). It is applicable to any part shape.
- Deforming a flat sheet into an engineered stamping results in a formed surface with strains as a function of the forming conditions like local radii, lubrication, friction, and of course part geometry. These strains are essentially independent of the chosen metal grade and thickness. Plotting these strains allows for a relative assessment of which strains are higher than others, but no judgment can be made on “how high is too high?”
- The Forming Limit Diagram is a combination of the Forming Limit Curve (a material property) and the strains (reflecting part geometry and forming conditions). The FLD provides guidance on which areas of the formed part requires additional attention to achieve robust stamping conditions. Creating a subsequent FLD may be warranted when conditions change, since changes to the sheet metal properties (FLC) and the forming conditions (radii, lubrication, beads, blank size) will change the FLD, potentially affecting the conclusions.
Key Points
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- Conventional Forming Limit Curves characterize necking failure only. Fractures at cut edges and tight bends may occur at strains lower than that suggested by the Forming Limit Curve.
- Differences in determination and interpretation of FLCs exist in different regions of the world.
- This system of FLCs commonly used for low strength and conventional HSLA is generally applicable to experimental FLCs obtained for DP steels for global formability.
- The left side of the FLC (negative minor strains) is in good agreement with experimental data for DP and TRIP steels. The left side depicts a constant thinning strain as a forming limit.
- Determination of FLCs for TRIP, MS, TWIP, and other special steels typically requires an experimental approach, since conventional simple equations do not accurately reflect the forming limits for these advanced microstructures.
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Formability
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.
Joining
The unique physical characteristics of Advanced High Strength Steels (AHSS) present some challenges to welding and bonding processes. AHSS differ from mild steels by chemical composition and microstructure, and it’s important to note that their microstructures will change from welding operations. For example, intensive localized heat associated with some welding processes causes a significant change in the local microstructure, and hence affect properties. Due to fast Cooling Rates (CR) typical in welding, it is normal to see martensite and/or bainite microstructures in the weld metal and in the Heat-Affected Zone (HAZ).
When joining AHSS, production process control is important for successful assembly. Manufacturers with highly developed joining control methodology will experience no major change in their operations. Others may require additional checks and maintenance. In certain instances, modifications to equipment or processing methodologies may be required for successful joining of AHSS.
The coating methods for AHSS are similar to those for mild steels. Welding of either AHSS or mild steels with coatings will generate fumes. The amount and nature of fumes will depend on the coating thickness, coating composition, joining method, and fillers used to join these materials. The fumes may contain some pollutants. The chemical composition of fumes and the relevant exhaust equipment must meet appropriate regulatory standards. Thicker coatings and higher heat inputs cause more fumes. Additional exhaust systems should be installed. While welding AHSS (with or without metallic or organic coatings and oiled or not oiled) gases and weld fumes are created similar to mild steels. The allowed fumes or gases must comply with respective national rules and regulations.
Joining Processes
Considering recent developments of hybrid approaches to welding, there are now over 100 types of welding processes available for the manufacturer or fabricator to choose from. The reason that there are so many processes is that each process has its list of advantages and disadvantages that make it more or less appropriate for a given application. Arc welding processes offer advantages such as portability and low cost but are relatively slow and use a considerable amount of heating to produce the weld. High energy density processes such as laser welding generally produce low heat inputs and fast welding speeds, but the equipment is very expensive and joint fit-up needs to be ideal. Solid-state welding processes avoid many of the weld discontinuities produced by those requiring melting (fusion), but they may be expensive and often are restricted to limited joint designs. Resistance welding processes are typically very fast and require no additional filler materials but are often limited to thin sheet applications or very high-production applications such as in the automotive industry.
Most welding processes produce a weld (metallic bond) using some combination of heat, time, and/or pressure. Those that rely on extreme heat at the source such as arc and high energy density processes generally need no pressure and relatively small-to-medium amounts of time. This section introduces joining processes that are applicable to HSS automotive applications, while describing unique process attributes.
About
Dr. Stuart Keeler, Sc.D.
We were honored at WorldAutoSteel to have Dr. Stuart Keeler as the author of our first five volumes of AHSS Application Guidelines and an Advisor on Version 6. His knowledge and experience were instrumental in creating a repository of best practices that is one of the best in the world. He was a joy and privilege to work with, and many of us will always fondly remember our interactions with him in developing each version of the Guidelines. It was a great sorrow to us all when he passed away in 2019. It is to him we dedicate this first online edition of the AHSS Application Guidelines. May it carry on and facilitate his pioneering efforts to bring science to the press shop floor for the continual improvement and innovation of steel’s use in the manufacturing of vehicles and other equipment.
As a commemoration of Dr. Keeler’s long service to the steel industry, his dedication to the Guidelines and our own personal love and appreciation of his friendship, we share with you the details of his long and distinguished career. We hope it will inspire.
STUART KEELER, Sc.D.
Dr. Keeler began his extensive sheet metal career at the Massachusetts Institute Technology, where he received his Doctor of Science Degree in Mechanical Metallurgy. Dr. Keeler’s doctoral research on sheet metal forming limits led to his development of the Forming Limit Diagram, now used throughout the world to predict forming severity and tearing of sheet metal stampings.
A two-year stint at the Detroit Army Tank Automotive Command as an Ordnance Corps First Lt. in the Metallurgical Research Section followed graduation. During his tour of duty, Dr. Keeler started taking his exams for Professional Engineer – Metallurgy in the state of Michigan. In 1965, he completed his mandatory experience period and passed his final exam.
The next 24 years were spent at National Steel Corporation – primarily as Manager of Automotive Research in Detroit. During his stay at National Steel, he pioneered the notion of bringing science to the press shop, creating circle grid analysis, thinning strain analysis, statistical deformation control (SDC), and many other press shop troubleshooting procedures. In the early 80’s Dr. Keeler conceived, created, and helped equip the National Steel Product Application Center in Livonia, Michigan, which became a benchmark for customer technical assistance by other steel companies. His last major project for National Steel was to research and write a state of the art book for the American Iron and Steel Institute entitled Automotive Sheet Metal Formability – Report AU89-1.
In March of 1987, Dr. Keeler joined the Budd Company Technical Center as Manager of Sheet Metal Technology. As a Tier 1 supplier to the automotive OEMs, Dr. Keeler was responsible for all research, development, and technology transfer for sheet metal stampings and castings. His major accomplishment was the implementation of virtual sheet metal forming (computerized die tryout). Dr. Keeler also became trained in the Shalnin Problem Solving Process, after which he became The Budd Company corporate trainer. He then trained and coached 30 teams within the company divisions. On 1 October 1999 he retired from The Budd Company as Manager of Technical Development to continue his sheet metal forming work as Keeler Technologies LLC.
In recognition for his work in bringing science to the press shop, Dr. Keeler was elected a Fellow in both the Society of Automotive Engineers (FSAE) and the American Society for Metals (FASM). ASM International also presented Dr. Keeler with the 1992 William Hunt Eisenman Award for his contributions to the practical application of science and materials engineering. He was elected to two-terms during 1968-1972 and one term during 1988 to 1990 to the presidency of the International Deep Drawing Research Group. Dr. Keeler also served in several positions within SAE – last as Editor for SAE Transactions for Materials and Manufacturing.
Dr. Keeler has conducted courses, seminars, and lectures throughout the world. All of Dr. Keeler’s presentations — for the university or the press shop — were dynamic, interactive, down to earth, and even entertaining. He also has a 50-year history of writing practical and understandable articles and papers read by persons in both steel mills and press shops. Given a choice, however, he would be found on the press shop floor.
