The WorldAutoSteel Steel E-Motive program has been moving along now for nearly a year, and we’d like to share an update with you, our engineering colleagues, on some of the design decisions we’re facing. If you recall, the Steel E-Motive program is designing vehicle concepts for Mobility as a Service (MaaS), characterized by autonomous, electric, ride sharing vehicles.
Some Background
We partnered with Ricardo headquartered in the UK to conduct the design and engineering of the vehicles. Ricardo was selected for their well-known reputation for innovation, their demonstrated knowledge of vehicle powertrains and electrification and their commitment to sustainable transportation. Our steel members subject matter experts work with Ricardo via various teams and working groups to push the envelope of steel applications. And given our pandemic, all of this currently occurs via virtual meetings.
Targeting technologies available for deployment in 2030+, we are considering the impacts to vehicle manufacturers, fleet operators and the ride hailing customer, as MaaS inevitably leads to an increase in demand for vehicle sharing, rental models and ride-hailing services over the next decade. We can represent these requirements as shown in Figure 1. On the left you’ll see broad needs for the critical stakeholders including space efficiency, flexibility and total cost of ownership. Those requirements translate into 12 key considerations for the mobility service provider, shown on the right, aimed at delivering value to customers and a sustainable and profitable business model. These considerations then require innovative design, engineering and materials applications.
Figure 1: MaaS key attributes and functions.
There are four main phases to Steel E-Motive (Figure 2). Phase 0 was a 3-month pre-study, beginning 30 June 2020, to review and confirm vehicle targets, essentially defining the foundations, goals and approach for the project. On 1 October 2020, we entered the Phase 1 concept engineering, exploring the challenges and steel solutions for Level 5 autonomous vehicles. Essentially, we are designing the body structure in this Phase, utilizing CAE tools to guide us. Phase 2 focusses on further refinement and optimization of the selected body concept, and ensuring the design is fully validated as there will be no working prototypes or hardware produced in the project. Phase 3 will be the roll out and dissemination activities, although you will see from the Steel E-Motive website and blogs that we are continually releasing material throughout the project.
Figure 2: Project timing and key activities.
We’ll be disclosing detailed targets and specifications later in the program, but Figure 3 provides overall dimensions. Battery electric will be the primary propulsion with competitive range. There are two variants: urban for inner city and shorter journeys, and an extra-urban variant for longer city-to-city (or city-to-airport) journeys. With Level 5 autonomy, there are no direct driver interfaces such as the steering column and pedals. You can see from the Figure that the vehicles are fairly compact. The urban variant sits between a European B and C segment in size. The extra-urban vehicle has a stretched wheelbase and can accommodate up to six passengers and a greater luggage capacity.
Figure 3: SEM vehicle technical specification and dimensions – base vehicle geometry.
The vehicles will be engineered and purposed for global application; therefore, we are considering the major global crash and safety standards and load cases. High volume production is targeted, greater than 250,000 units per annum, and a hypothetical production date of 2030, which influences the steel grades and fabrication processes considered. Third Generation AHSS (3rd Gen AHSS) and press-hardened steels continue to evolve with higher strength and improved formability. Between these innovative product capabilities, we are addressing the challenges associated with Mobility as a Service and tackling geometries that otherwise would have been difficult to produce.
To further assist in the design and manufacture of efficient vehicle structures, there are many new manufacturing processes, such as roll forming and hot stamping, that help fabricate these stronger materials effectively, while often doubling material use efficiency. Figure 4 provides a list of technologies that will be considered for Steel E-Motive.
Figure 4: Steel technologies included in SEM’s portfolio.
With the portfolio of steel product and manufacturing processes already available and the addition of those forecasted for future commercial availability, we are expecting innovations that will be a roadmap for future mobility vehicle manufacturers.
Our end goal is to demonstrate multi-purpose opportunities for the vehicles via a modular architecture enabled by the application of innovative steel solutions. These solutions will help Steel E-Motive achieve a low environment footprint measured over the vehicle Life Cycle, and meet global crash standards while delivering the lowest Total Cost of Ownership (TCO).
To reach our goal of demonstrating steel innovation in this program, we are using a theoretical frameworkH-2as a guide, shown in Figure 5, considering innovation at an architectural level. That is, using body structure load paths shown in the vertical axis, and modular innovation for the major body components such as battery enclosure, side/crash rails, shown in the horizontal axis. Combining innovation levels and types of the two axes should enable us to demonstrate radical innovation in Steel E-Motive.
Figure 5: Steel E-Motive explores and demonstrates steel innovation. Exploring “modular” and “architectural” innovations for 2030 production.
Design Challenges
Figure 6 reveals an early or basic Steel E-Motive architecture. You can see that Level 5 autonomy creates both design freedoms that allow new occupant seating positions, while also creating challenges for short front and rear overhangs. We have an open pod-type structure with large door apertures for enhanced occupant ergonomics.
Figure 6: Challenges and opportunities of Level 5 autonomous MaaS battery electric vehicle.
Passenger comfort is key for MaaS vehicles. The open pod structure may give challenges with the air cavity mode coupling with structural modes. With occupants in different positions, we have different NVH source-path-receiver paths to consider. The larger door aperture gives us an inherent deficiency in overall body structure stiffness, for which we need to compensate. As with any BEV, the mass of the battery suspended on the lower structure may reduce body modes to frequencies that interact with other vehicle systems such as suspension modes. With a lot of emphasis on lower structure crash zones and battery protection, we may encounter some lower frequency upper body modes (such as lozenging), especially as we are targeting low overall body mass. These NVH risks and challenges are being addressed by taking a modal mapping approach, utilizing steel’s inherent high structural stiffness properties and undertaking thorough NVH simulation throughout the engineering phases.
Level 5 autonomy removes the requirements for driver vision and obscuration, but we do need to acknowledge passenger comfort issues, such as motion sickness. Consequently the designs consider a good level of outward visibility. However, we now have the freedom to place structure where we could not previously. We are using 3D topology FEA tools to help determine the optimum placement of structure in the body, and we have allowed the tool the freedom to place structure in the front and rear glazed areas. In the Figure 7 example, the software is recommending some structural members across the front windscreen, and further analysis shows that this has the potential to give us an overall Body-In-White weight saving as the load paths are more evenly distributed.
Figure 7: Level 5 autonomy removes the driver vision and obscuration requirements—an opportunity for new solutions.
To summarize the Steel E-Motive engineering activities, we are currently exploring the numerous challenges and opportunities that Level 5 autonomous BEVs provide us. We are in the concept phase, investigating both the overall body structure layout and load paths, as well as developing components and modules utilizing the unique properties of steel.
We expect to complete the program with full virtual concepts by December 2022. Our plans are to unveil more and more of the design concepts over the months to come, and we’ll be using virtual reality and other tools to communicate the concepts’ engineering. We invite you to subscribe at the website to receive all the news that will be coming out of the program, including more technical details as they become available. You can do that at www.steelemotive.world. We’re excited to share Steel E-Motive innovations in the future!
Modelling resistance spot welding can help to understand the process and drive innovation by asking the right questions and giving new viewpoints outside of limited experimental trials. The models can calculate industrial-scale automotive assemblies and allow visualization of the highly dynamic interplay between mechanical forces, electrical currents and thermal flow during welding. Applications of such models allow efficient weldability tests necessary for new material-thickness combinations, thus well-suited for applications involving Advanced High -Strength Steels (AHSS).
Virtual resistance spot weld tests can narrow down the parameter space and reduce the amount of experiments, material consumed as well as personnel- and machine- time. They can also highlight necessary process modifications, for example the greater electrode force required by AHSS, or the impact of hold times and nugget geometry. Other applications are the evaluation of whole-part distortion to ensure good part-clearance and the investigation of stress, strain and temperature as they occur during welding. This more research-focused application is useful to study phenomena arising around the weld such as the formation of unwanted phases or cracks.
Modern Finite-Element resistance spot welding models account for electric heating, mechanical forces and heat flow into the surrounding part and the electrodes. The video shows the simulated temperature in a cross-section for two 1.5 mm DP1000 sheets:
First, the electrodes close and then heat starts to form due to the electric current flow and agglomerates over time. The dark-red area around the sheet-sheet interface represents the molten zone, where the nugget forms after cooling. While the simulated temperature field looks plausible at first glance, the question is how to make sure that the model calculates the physically correct results. To ensure that the simulation is reliable, the user needs to understand how it works and needs to validate the simulation results against experimental tests. In this text, we will discuss which inputs and tests are needed for a basic resistance spot welding model.
At the base of the simulation stands an electro-thermomechanical resistance spot welding model. Today, there are several Finite Element software producers offering pre-made models that facilitate the input and interpretation of the data. First tests in a new software should be conducted with as many known variables as possible, i.e., a commonly used material, a weld with a lot of experimental data available etc.
As first input, a reliable material data set is required for all involved sheets. The data set must include thermal conductivity and capacity, mechanical properties like Young’s modulus, tensile strength, plastic flow behavior and the thermal expansion coefficient, as well as the electrical conductivity. As the material properties change drastically with temperature, temperature dependent data is necessary at least until 800°C. For more commonly used steels, high quality data sets are usually available in the literature or in software databases. For special materials, values for a different material of the same class can be scaled to the respective strength levels. In any case, a few tests should be conducted to make sure that the given material matches the data set. The next Figure shows an exemplary material data set for a DP1000. Most of the values were measured for a DP600 and scaled, but the changes for the thermal and electrical properties within a material class are usually small.
Next, meaningful boundary conditions must be chosen and validated against experiments. These include both the electrode cooling and the electrical contact resistance. To set up the thermal flow into the electrode, temperature measurements on the surface are common. In the following picture, a measurement with thermocouples during welding and the corresponding result is shown. By adjusting the thermal boundary in the model, the simulated temperatures are adjusted until a good match between simulation and experiment is visible. This calibration needs to be conducted only once when the model is established because the thermal boundary remains relatively constant for different materials and coatings.
Figure 2: Temperature measurement with thermocouples during welding and the results. The simulated temperature development is compared to the experimental curve and can be adjusted via the boundary conditions.F-23
The second boundary condition is the electrical contact resistance and it is strongly dependent on the coating, the surface quality and the electrode force. It needs to be determined experimentally for every new coating and for as many material thickness combinations as possible. In the measuring protocol, a reference test eliminates the bulk material resistance and allows for the determination of the contact resistances using a µOhm-capable digital multimeter.
Finally, a metallographic cross-section shows whether the nugget size and -shape matches the experiment. The graphic shows a comparison between an actual and simulated cross section with a very small deviation of 0.5 mm in the diameter. As with the temperature measurements, a small deviation is not cause for concern. The experimental measurements also exhibit scatter, and there are a couple of simplifications in the model that will reduce the accuracy but still allow for fast calculation and good evaluation of trends.
Figure 3: Comparison of experimental and virtual cross-sections.F-23
After validation, consider conducting weldability investigations with the model. Try creating virtual force / current maps and the resulting nugget diameter to generate first guesses for experimental trials. We can also gain a feeling how the quality of each weld is affected by changes in coatings or by heated electrodes when we vary the boundary conditions for contact resistance and electrode cooling. The investigation of large spot-welded assemblies is possible for part fit-up and secondary effects such as shunting. Finally, the in-depth data on temperature flow and mechanical stresses is available for research-oriented investigations, cracking and joint strength impacts.
Note: The work represented in this article is a part a study of Liquid Metal Embrittlement (LME), commissioned by WorldAutoSteel. You can download the free report on the results of the LME study, including how this modelling was used to verify physical tests, from the WorldAutoSteel website.
Press hardening, as we know it today, was developed in Luleå, Sweden, by Norrbottens Järnverks AB (abbreviated as NJA, translated as Norrbotten Iron Works). The first patent application was completed in 1973 and awarded in 1977.N-23 The technology was first commercialized in agriculture components, where the high strength of Press Hardened Steels (PHS) was favored for wear resistance.B-45
In 1984, automotive applications of PHS started with the Saab 9000 side impact door beams, as seen in Figure 1. A total of 4 parts were used in this car.A-66 The uncoated blanks were almost half the thickness of a cold stamped beam.T-26
Figure 1: Door beams of the Saab 9000 (1984-1998): (a) A see-through car in Saab MuseumS-82, (b) the hot stamped part.L-42
The majority of the PHS parts were door beams through the mid-1990s, with approximately 6 million beams produced in 1996. By this time, the demand for bumper beams was also increasing.F-31 By the end of 1996, the European New Car Assessment Program (EuroNCAP) was formed, which increased the pressure on the OEMs for improved crashworthiness.T-26 In 1998, both the new Volvo S80L-44 and Ford Focus5 were equipped with Press Hardened bumper beams.
The year 1998 saw the development of one of the most important breakthroughs in Press Hardening technology. French steel maker Usinor developed an aluminum-silicon (AlSi) pre-coated steel, commercialized as Usibor 1500 (indicating the typical tensile strength, 1500 MPa.C-24, L-39 In 2000, BMW rolled out its new 3 series convertible. In this vehicle, the A-pillar is made from 3 mm thick uncoated, PHS sheet. This was BMW’s first PHS application, and one of the first PHS A-pillar reinforcement.S-83, S-84 Accra started delivering roll formed PHS components for the Volvo V70, initially an optional 3rd row seating support. Approximately 10,000 parts/year were supplied.G-28
AlSi coated steel was first hot stamped at a French Tier 1 supplier, Sofedit.V-15 This grade was first used in the front bumper beam of the 2nd Generation Renault Laguna (2000-2007). Laguna 2 was the first car to receive a 5-star safety rating from Euro NCAP.V-10 AlSi coated blanks were also used in PSA Group’s Citroën C5 (1st Gen: 2001-2007) in the front bumper beam, and the A-pillars. These three parts weighed a total of 4.5 kg, approximately 1% of the total BIW weight, Figure 2a. About one month later, PSA Group started production of the compact hatchback Peugeot 307, which had five hot stamped components (A- and B-pillars and rear bumper beam). Unlike the Citroën C5, these parts were uncoated. The total weight was 12 kg, corresponding to 3.4% of the BIW weight.R-17, P-27
Volvo started producing the XC90 SUV in 2002. The body-in-white with doors and closures weighed 531 kg.B-44 A total of 10 parts, weighing 37 kg are either roll formed or direct stamped PHS. This accounts for approximately 7% of the BIW weight.L-43 During its time, this was the highest use of PHS in car bodies. In Figure 2b, the Press Hardened components other than the 2nd row seat frame, which is a load bearing body part, are shown.
Accelerated Use and Globalization
The use of press hardened parts increased rapidly after the introduction of the VW Passat in 2005. This car had approximately 19% of its BIW (by weight) made from press hardened steels, Figure 2c. Some parts in this car saw the first use of varnish coated blanks in a two-step hybrid process. Three parts were produced using either an indirect or hybrid process, including the transmission tunnel.H-50
Following are a few highlights of PHS use in vehicle applications during this time period :
In 2006, the Dodge CaliberK-37 and BMW X5P-28 were among the first cars to have tailor-rolled and Press Hardened components in their bodies (Figure 3).
Figure 3: (a) Tailor Rolling ProcessZ-5, (b) B-pillar of BMW X5 (2nd Gen: 2006-2013).P-28
BMW 7 Series (5th Gen: 2008-2015) became the first car to have Zn-coated Press Hardened components in its body-in-white. The car also contained uncoated parts, as shown in Figure 4 (next page). The total PHS usage in this car was approximately 16%.P-20
Figure 4: PHS usage in BMW 7 Series (5th Gen: 2008-2015) (re-created using P-20).
Press hardening also allowed car makers to create unconventional cars. In 2011, Hyundai rolled out the 1st generation Veloster, a 3-door coupé (also known as 2+1, with one door on the driver side and 2 doors on the passenger side), and as such containing axisymmetric front doors. Thus, the car could not have a full B-ring, as illustrated in Figure 5a.B-14, R-19 Another unconventional design was the Ford B-Max subcompact MPV sold in Europe between 2012 and 2017. The car had conventional swing doors in the front and two sliding rear doors. A PHS B-pillar was integrated in the doors, providing ease of ingress. Its PHS components (integrated B-pillar in front and rear doors, door beams and cantrail) are shown with blue color in Figure 5b.B-14, L-45
Figure 5: Unconventional car designs with PHS: (a) Hyundai Veloster, asymmetric 2+1 doors coupé (re-created after Citation R-19), and (b) Ford B-Max, sub-compact MPV with integrated B-pillars in the doors.L-45
In 2013, the Acura MDX (3rd Gen: 2013-2020) became the first car to have a Hot Stamped door ring. The part was a tailor welded blank comprised of two sub-blanks, as shown in Figure 6a. The design saved about 6.2 kg weight per car and had high material utilization ratio thanks to sub-blank nesting optimization.A-67, M-46One of the most recent PHS applications was in 2017 Chrysler Pacifica with 5 sub-blanks, as shown in Figure 6b. This car also has a PQS550 sub-blank at the lower B-pillar region.D-28
Figure 6: Hot stamped door rings: (a) First application in 2013 Acura MDX had 2 sub-blanks, (b) a more recent application in 2017 Chrysler Pacifica has five sub-blanks with PQS550 at the lower B-pillar (re-created after Citations B-14, A-67, D-28).
Tubular hardened steels have been long used in car bodies, with minimal forming. Since 2013, a special 3-D hot bending and quenching (3DQ) process has been employed. One of the earliest uses of this technology was Mazda Premacy (known as Mazda 5 in some markets). The same process was also used in making the A-pillars of the Acura NSX (Honda NSX in some markets, 2016-present), as seen in Figure 7a.H-29 Since 2018, tubular parts formed with internal pressure, called form blow hardened parts, are used in the Ford Focus (4th Generation) (Figure 7b) and Jeep Wrangler (4th Generation).B-16, B-17
Figure 7: Tubular hardened steel usage in A-pillars of: (a) 2015 Acura NSXH-29, (b) 2018 Ford Focus.B-16
PHS Use in xEVs: Hybrid Electric, Battery Electric,
Plug-in Hybrid Electric & Fuel Cell Electric Vehicles
The first commercially available Hybrid Electric Vehicle (HEV) was the Toyota Prius (1st Gen: 1997-2003). The second-generation Prius (2003-2009) had very few Press Hardened components, as shown with red color in Figure 8a. This was the first time Toyota used hot stamped components.M-47 The third generation Prius (2009-2015) had approximately 3% of its BIW Press Hardened. In the 4th generation Prius released in 2015, the share of >980 MPa steels has risen to 19%.U-10 Figure 8b shows the Press Hardened parts in this latest Prius.K-38
Figure 8: PHS usage in Toyota Prius: (a) 2nd generation (2003-2009) and (b) 4th generation (2015-present) (re-created after Citations M-47 and K-38)
The 2012 Tesla Model S and Model X launched using aluminium bodies, with PHS reinforcements in the pillars and the bumpers. Model S is known to have a roll-formed PHS bumper beam. High volume Model 3 and Model Y have a significant amount of press hardened components in their bodies.T-35
In 2011, General Motors started production of its first Plug-in Hybrid Electric Vehicle (PHEV), the Chevrolet Volt (known as Opel Ampera in EU and Vauxhall Ampera in the UK). This car had six Hot Stamped components, including A and B pillars, accounting for slightly over 5% of the BIW mass.P-29
The smaller BEV Chevrolet Bolt, launched in 2017, had aluminum closures, but a steel-intensive BIW that is 80% steel, 44% of which is Advanced High-Strength Steels including 11.8% PHS. Figure 9.A-69
Figure 9: Chevrolet Bolt Body Structure and Steel Content.A-69
In December 2020, Hyundai announced their new electric platform, E-GMP. The platform will utilize Press Hardened steel components to secure the batteries.H-52
Automakers have turned to PHS to manage the extra load of Fuel Cell powertrains as well. The first-generation Toyota Mirai had only Press Hardened B-pillars, cantrails and lateral floor members.T-38 The second generation has a number of parts with PHS in its under body as well.T-39
In 2018, Hyundai Nexo became the first fuel-cell car to be tested by EuroNCAP, achieving a 5-Star rating. The car has PHS A- and B-pillars, rocker reinforcements, and several under body components, as seen in Figure 10.H-53
Figure 10: Press hardened steel usage in Hyundai Nexo Fuel Cell vehicle: (a) side view and (b) top view (re-created after Citation H-53).
A New Software Application for Thin Wall Section Analysis
Advanced High-Strength Steel (AHSS) grades offer increased performance in yield and tensile strength. However, to fully utilize this increased strength, automotive beam sections must be designed carefully to avoid buckling of the plate elements in the section. A new software application, Geometric Analysis of Sections—GAS2.0, available through the American Iron and Steel Institute, is a tool to aid in this design effort.
Plate Buckling in Automotive Sections
To understand how plate buckling affects the strength of a thin walled beam consider Figure 1. A square beam is made of four identical plates connected at their edges. Under an axial compressive load each plate may buckle. Considering just one of the plates, the stress that will cause buckling depends on the ratio of plate width and thickness (b/t). Thinner wider plates with large b/t ratio will buckle at a lower stress than thicker narrower plates.
Figure 1: Plate Buckling Behavior.
Now consider a plate of mild steel (200 MPa yield stress) which has been designed to buckle just as yield stress is reached, Point A in Figure 2. The plate would have a b/t ratio of approximately 60. This design is taking full advantage of the yield strength of the material.
Now consider the same plate but substituting an AHSS grade (600 MPa yield stress) as shown in Figure 2. The plate will buckle at the same 200 MPa before reaching the material’s potential, Point B in the figure. To take advantage of this materials yield strength, the proportions of the plate will need to be changed, Point C. This illustration demonstrates the need to consider plate buckling particularly in the application of AHSS grades.
Figure 2: AHSS Substitution in a Plate.
Moving from a single plate to the more complex case of a beam section of several plates, consider Figure 3. On the left is the beam made of four plates with a compressive load causing the plates to just begin to buckle. However, this condition does not represent the maximum load carrying ability of the beam. The load can be increased until the stress at the corners of the buckled plates are at the material yield stress, center in Figure 3. Note that in this condition the stress distribution across the plate is nonlinear with lower stress in the center of each plate. One means to model this complex state is by using an imaginary Effective Section. Here the center portion is visualized as being removed and the remainder of the section is stressed uniformly at yield. The amount of plate width to be removed is determined by theory.W-21, A-42, Y-9, M-18 The effective section is a convenient way to visualize the efficiency of a section design given the material grade and provides an estimate of the maximum load carrying ability of the beam.
Figure 3: Concept of Effective Section.
Geometric Analysis of Sections – GAS2.0
Geometrical Analysis of Sections software determines the effective section for complex automotive sections. Figure 4 illustrates the GAS2.0 user interface. The user has the ability to construct sections or to import section data from a CAD system. Material properties for 63 steel grades are preloaded with the ability to also add user-defined steel grades. Two types of analysis are available. Nominal analysis, which provides classical area properties of the section, and Effective analysis which determines the effective section at material yield. Figure 5 summarizes both the tabular results and graphical results for each type of analysis.
Figure 4: GAS2.0 User Interface.
Figure 5. GAS2.0 Analysis Results.
Figure 6 illustrates an example of an Effective Analysis for a rocker section. In the graphical screen, the effective section is shown in green. Ideally, the whole section would be effective to fully use the materials yield capability. Also shown in the graphical screen are the section centroid, orientation of the principle coordinates, and stress distribution. In the right text box are tabular results. At the bottom of the tabular results is the axial load that causes this stress state and represents the ultimate load carrying ability of this section.
Figure 6: GAS2.0 Graphical Results.
It is clear that much of the material in the section of Figure 6 is not fully effective. GAS2.0 allows the user to conveniently modify the section. For example, in Figure 7 a bead has been added to the left side wall increasing its bulking resistance. Note that the side wall is now largely effective, and the ultimate load at the bottom of the text box has increased substantially.
Figure 7: Improved Design Concept.
Role of GAS in the Design Process
GAS2.0 can play a significant role in early stage design, see Figure 8, by quickly creating initial designs which are more likely to function and to ensure that adequate package space is set aside for structure. This will result in fewer problems to fix later in the design sequence. During the detail design stage, GAS2.0 can supplement Finite Element Analysis by identifying problems earlier, and by screening design concepts for those with the greatest promise prior to more detailed analysis by FEA.
Figure 8: Role of GAS2.0 in Design Process.
GAS2.0 is available for free download at www.steel.org, Included in the resources at steel.org is an American Iron and Steel Institute introductory webinar conducted by Dr. Don Malen on 16 June 2020, as well as a number of GAS2.0 tutorials and training modules.
You are most likely wondering why WorldAutoSteel is writing a blog about a bicycle. It is because when we talked to Jia-Uei Chan, Regional Business Development at our member company, thyssenkrupp Steel Europe (TKSe), about the journey of inventing the world’s first Advanced High-Strength Steel road bike, we were incredibly inspired. This is more than a story about a steel bicycle. This is the story of steel innovation, conceived in a WorldAutoSteel members workshop to brainstorm ideas on transforming steel’s image to the sophisticated and advanced material it is. Their journey led to new steel applications, patentable processes, and in the steelworks bicycle, ideas that we think can inspire new automotive applications as well. And anyway, who doesn’t like an inspiring story?
Bikes of this genre have some of the same requirements of modern vehicles: lightweight, strength and durability, affordability, and high performance. To achieve these, the thyssenkrupp steelworks team developed what they called inbike® technology, which combines high-strength steel, half-shell technology and automated laser welding.
How it was made
The bike frame is made from DP 330/590 steel, used for its cold forming abilities, stamped as thin as 0.7mm. The steel blanks are pressed into a die to form two half-shells in a deep-drawing process.
A major challenge was to bring these two half shells together in such a way that minimized gaps and achieved a tight fit, enabling automated laser welding (this process requires no gaps over 6 meters of contact length), while ensuring that the frame achieves an elegant, seamless look. Enter innovation.
At the stamping plant, the half-shells were fitted with “dimples,” (See Figure 1) tiny bumps on the welding flanges that create channels at the weld seam for the zinc, preventing vaporized zinc from remaining trapped in the seam during subsequent welding. The half shells were then clamped in a special device and shipped to the laser specialist (See Figure 2).
Figure 1: Tiny bumps prevent vaporized zinc from remaining trapped in the seam during subsequent welding.
Figure 2: Frame half-shells clamped in the device for laser welding.
The particular challenge lay in the reliable processing and fusing of both frame halves by means of automated laser welding in such a way that no damage to the frame would occur, while also ensuring the weld seam lay as close as possible to the bend radius of the frame halves. The complex frame shape is welded by following a sophisticated trajectory in a 3-D space. After countless continuous improvement exercises, the steelworks team was able to achieve a very flat, elegant weld seam design. This translates into a very stable bike, with a frame that has the needed rigidity in the bottom bracket area to enable high biomechanical power transmission, but with high elasticity in the seat tube configuration to make for an unusually comfortable ride. In comparison, aluminium and carbon fiber bikes are very stiff and characteristic of an unpleasant ride experience.
Inventing the possibility
Tackling a project that is such a reach beyond the norm is never easy. The thyssenkrupp steelworks team repeatedly heard from qualified experts that the project was actually not feasible. At the same time, they had partners who were so fascinated by the challenge that they wanted to make it possible. Chan related to WorldAutoSteel that there were many times when giving up was the more attractive option. Endurance won out. And as it turns out, the half-shell technology invented out of necessity for this bike could find an application in the tough requirements of an electric vehicle battery case.
Says Chan, “We genuinely believed that steel is the perfect material for a road bike. And we wanted to break with convention and make the most out of steel with high-tech engineering.” Have a look at steelworks.bike, and you will undoubtedly agree they did just that.
Dr. Donald Malen, College of Engineering, University of Michigan, reviews the use of two recently developed Powertrain Models, which he co-authored with Dr. Roland Geyer, University of California, Bren School of Environmental Science.
The use of Advanced High-Strength Steel (AHSS) grades offer a means to lightweight a vehicle. Among the benefits of this lightweighting are less fuel used over the vehicle life, and better acceleration performance. Vehicle designers as well as Greenhouse Gas analysts are interested in estimating these benefits early in the vehicle design process. G-13
Models are constructed for this purpose which range from the use of a simple coefficient, (for example fuel consumption change per kg of mass reduction), to very detailed models accessible only to specialists which require knowledge of hundreds of vehicle parameters. Draw backs to the first approach is that the coefficient may be based on assumptions about the vehicle which do not match the current case. Drawbacks to the detailed models are the considerable expense and time needed, and the lack of transparency in the results; It is difficult to relate inputs with outputs.
A middle way between the simplistic coefficient and the complex model, is described here as a set of Parsimonious Powertrain Models. G-10, G-11, G-12 Parsimony is the principle that the best model is the one that requires the fewest assumptions while still providing adequate estimates. These Excel spreadsheet models cover Internal Combustion powertrains, Battery Electric Vehicles, and Plug-in Electric Vehicles, and predict fuel consumption and acceleration performance based on a small set of inputs. Inputs include vehicle characteristics (mass, drag coefficient, frontal area, rolling resistance), powertrain characteristics (fuel conversion efficiency, gear ratios, gear train efficiency), and fuel consumption driving cycle. Model outputs include estimates for fuel consumption, acceleration, and a visitation map.
Physics of the Models
Fuel consumption is determined by the quantity of fuel used over a driving cycle. The driving cycle specifies the vehicle speed vs. time. An example of a driving cycle is the World Light Vehicles Test Procedure (WLTP) cycle shown in Figure 1.
Figure 1: Fuel Consumption Driving Cycle (WLTP Class 3b).
Given the velocity history of Figure 1, the forces on the vehicle resisting forward motion may be calculated. These forces include inertia force, aerodynamic drag force, and rolling resistance. The total of these forces, called tractive force, must be provided by the vehicle propulsion system, see Figure 2.
Figure 2. Tractive Force Required.
Once vehicle speed and tractive force are known at each point of time during the driving cycle, the required torque and rotational speed may be determined for each of the drivetrain elements, as shown in Figure 3 for an Internal Combustion system, and Figure 4 for a Battery Electric Vehicle.
Figure 3. Internal Combustion Powertrain.
Figure 4. Battery Electric Vehicle Powertrain.
In this way, the required torque and speed of the engine or motor may be determined. Then using a map of efficiency, shown to the right in Figures 3 and 4, the energy demand is determined at each point in time. Summing the energy demand over time yields the fuel used over the driving cycle. The reader is referred to References 1 and 2 for a much more in depth description of the models.
Example Application
As an example application, consider the WorldAutoSteel FutureSteelVehicle (FSV).W-7 The FSV project, completed in 2011, investigated the weight reduction potential enabled with the use of AHSS, advanced manufacturing processes and computer optimization. The resulting material use in the body structure is shown in Figure 5.
This use of AHSS allowed a reduction in the vehicle curb mass from 1200 kg to 1000 kg. What are the effects of this mass reduction on fuel consumption and acceleration performance? The inputs required for the powertrain model are shown in Table 1 for the base case.
Table 1: Model Inputs for Base Case.
The results provided by the powertrain model are summarized in the acceleration-time vs. fuel consumption graph of Figure 6. Point A is the base case at 1200 kg curb mass. The lightweight case with same engine is shown as Point B. Note the fuel consumption reduction and also the acceleration time reduction. Often the acceleration time is set as a requirement. For the lighter vehicle, the engine size may be reduced to achieve the original acceleration time and an even greater reduction in fuel consumption as shown as Point C.
Figure 6. Summary of results of base vehicle and reduced mass vehicle.
Using the parsimonious powertrain models allows such ‘what-if’ questions to be answered quickly, with minimal data input, and in a transparent way. The Parsimonious Powertrain Models are available as a free download at worldautosteel.org.
