PRODUCTION FOCUS

By Andrew Halonen, Mayflower Consulting, LLC

CALM Studies Roof Lightweighting

Different concepts get different scores

64 Lightweighting worlD / SEP+OCT 2019 Lightweighting worlD / SEP+OCT 2019 65 Automotive lightweighting comes in weight reduction, though only the first many forms and is emphasized across concept, using advanced steels, met all the thousands of components that of the design criteria. make up the vehicle. Some areas of Background, Motivation the vehicle are more important than & Objective, Scope others. Wheels, for example, represent rotational mass, multiple per vehicle The emphasis on cost-effective weight and also unsprung weight. reduction is always top of mind for The roof is another, as it is the high- automotive manufacturers because the est point on the vehicle, and a heavy purpose of the vehicle and the con- roof raises the vehicle center of grav- sumer needs are evolving. The trend ity, reducing vehicle stability and ride now is on mobility solutions, and control. The Center for Automotive under that umbrella, there is ACES, Research, known as CAR, decided to which stands for automated, connect- study the vehicle roof and the oppor- ed, electric and shared vehicles. In ad- tunities to reduce weight, referred to dition, governments around the world as the Roof Lightweighting Study.1 and within municipalities are putting Working within its Coalition for restrictions on, even laws limiting, the Automotive Lightweighting Materials use of internal combustion engines (CALM2) group, which is made up of (ICE) within city limits and across more than 30 organizations, the team countries via greenhouse gas (GHG) formalized a plan to study the vehicle- regulations. roof construction and the means Much of the ACES vehicle is in de- to reduce weight, while taking into velopment. Yet the electrified portion account the cost, capital investment, is easy to understand in that there is a global material availability, ease of limitation in charging infrastructure, processing in the paint shop, integra- combined with longer than desirable tion to adjacent structure, field repair charging times. The need to extend the and recyclability. vehicle range is real and tangible. A The baseline roof is on a 2011 lower weight vehicle will travel further Honda Accord and is made up of on a charge. So, let’s lose weight! mild steel in its structure and the large In addition to the integration and sheet panel over the top. The group cost aspects already noted, the roof is down-selected to three concepts. The a Class A surface, which comes with first replaces mild steel with advanced stringent aesthetic requirements, and it high-strength steels. The second uti- plays a critical role in passenger safety lizes a mix of high-strength steel, alu- during rollover and side-impact events. minum sheet and polymer composite To complete a study like this, you roof bows. The third is an extension need ground rules to limit the number of the second, replacing the alumi- of options under consideration. One num sheet with carbon fiber weave. limit was on the manufacturing readi- Each concept demonstrates attractive ness level (MRL). The team selected

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FIGURE [ 1 ] / Baseline Roof – 2011 Honda Accord

Level 5 as the minimum, which is in After researching prior work on istics under non-linear loads as in a the pre-production phase, and states roof-weight reduction, the team side impact or a rollover event with “basic capability demonstrated.” selected the 2011 Honda Accord as three times the vehicle weight crashing The computer-aided engineering a baseline vehicle because its roof is down? The non-linear event challenges (CAE) analysis requires that each comprised of mild steel materials and the load cases, the boundary conditions concept match or exceed baseline presumably, in part because the vehicle and the material property data. performance of: information is readily available. Even For this study, the non-linear prop- • Roof crush the National Highway Traffic Safety erties of the materials were taken into Administration (NHTSA) provided account, with exception to the joints. • Bending crash-simulation models. Figure 1 Directional properties were used for • Torsion shows the baseline roof construction each layer of the composites. The • Dent The baseline details include the steel material is made up of multiple layers types and ranges of properties, thick- in 0 and 90 degree configuration. Out of the scope of this study was ness, and as you will see, the weight The three concepts are evaluated by physical testing, absolute cost and rede- of each component. The weight of the these baseline performance targets: sign of the supporting body structure. adhesive is negligible and not listed. The report also points to prior lit- erature on vehicle-roof structures, and The Engineering LOAD CASES BASELINE I am glad that these were presented The CAE tools today are really critical Mass 22.4 kg before reading the study. Typically, in to the rapid development of vehicles, Roof Crush 3.7 SWR, 62 kN technical papers, these are references and within the vehicle, the material, Frequency–Torsion 50 Hz listed on the bottom, and it takes a the design and the joining method Frequency–Bending 37 Hz great deal of attention to reach them between materials. The saying goes in Stiffness–Torsion 27.6 kN-m/deg and appreciate the authors and the Detroit automotive, “If we can’t model content. In this study, the reference it, we can’t use it.” That is easy to say, Stiffness–Bending 6.9kN/mm studies are readily available, coming yet not so easy to accomplish. Dent Resistance 1.2% from Lotus Engineering, EDAG, FEV Take the case of adhesives. The sup- (plastic strain) working for the EPA, a CAR survey ply base provides shear, peal and the and Politecnico Di Torino (Poly- common test samples and results. Yet, FIGURE [ 2 ] / Baseline performance targets technic University of Turin, Italy) on what are the load-carrying character- polymer composite roof structures.

66 Lightweighting worlD / SEP+OCT 2019 Lightweighting worlD / SEP+OCT 2019 67 FIGURE [ 3 ] / Concept 1

Performance Criteria concept utilized consistent A-, B- and like Faurecia, Gestamp, IAC, Magna, C-pillars, with the pillars carrying signifi- Martinrea and Shiloh. An impressive The roof crush test is based on Federal cant loads. The future effort plan includes list with considerable capability.2 Motor Vehicle Safety Standard (FM- the B-pillar and its optimization with VSS) 216. A design of experiments Three Concepts to Reduce intention to enable Concepts 2 and 3 to was performed based on the surface Roof Weight meet the torsional stiffness target. response sensitivity using these pa- Concept 1 is all-steel like the base- rameters: Materials line, yet because it replaces mild steel • Gauge or Thickness There is a large variety of materi- with press hardened steel (PHS 1500 • Material Grade als available to the automobile roof Mpa) or Gen3 980 Mpa, considerable designer, from structures underneath efficiencies were achieved. The base- • Shape or Cross Section to the large panel to the attachment line had four roof bows, whereas the The roof crush is most affected by the method. However, for this particular conversion to high-strength steel only front header and the nearest roof bows. study, Motivation #2 is to provide requires one roof bow. Interestingly, the roof rails are not rank- CALM members a platform to show- Almost 20 percent of the 4.91 kg ing high in the sensitivity plot. case their material and manufacturing saved came from the 0.65-mm thick Though the only system studied was technology to the automakers. 490 Mpa roof panel, the large sheet the roof, clearly the load cases take the The CALM members include over the top. Joining the parts is the entire vehicle into account. lightweight-material producers such same even with the change in the Coincidently, the BIW stiffness-tor- as AK Steel, Arconic, BASF, DuPont, steels. Spot welds are used for the roof sion loading was most difficult to meet Henkel, NAGASE, SABIC and U.S. bows to the roof rails, and adhesives as only Concept 1 using optimized steel Steel. Fastening and joining solutions are used to secure the roof panel. passed. The other two concepts missed from 3M, ARaymond, Coherent, Dow, The total weight savings was 22 per- the target by 22 percent and 26 percent DuPont, Eastman, Henkel, KOBEL- cent from the baseline, and it passes respectively. In all the tests, each roof CO, PPG and Sika. Tier 1 integrators all the design criteria. The cost is a

The BIW Stiffness-Torsion loading was most difficult to meet as only Concept 1 using optimized steel passed.

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FIGURE [ 4 ] / Concept 1– Design, Manufacturing and Supply Chain Impact

10-percent increase over mild steel, (four), though three are steel, yet at a mm thick, delivering a whopping 64.2 well within the normal targets for lower strength than in Concept 1. The percent weight reduction! weight reduction. As Figure 4 shows, fourth bow is a ribbed, glass-fiber PA6 Concept 2 meets all performance Concept 1 is a very robust solution. nylon with a glass-fiber unidirectional besides the torsional stiffness as The joining is well understood. There tape (GF UD). The front and rear shown (see Figure 6). is a global supply chain. There is low headers are also made of this glass- The authors included a note stating, impact on the processes in the assembly fiber composite, with 0 and 90 degree “If our design space was bigger, we plant. The repair facilities know how layers, with the GF UD tape. could meet target torsional stiffness to fix these materials, and there are A complication with the compos- by taking a small hit on the percent- existing recycling processes in place. ite is the means to attach it to the age of mass reduction achieved.” It is An optimized steel roof is an elegant side rails. The composite roof bow not known how much mass would be solution because it is simple, cost effec- is attached to a steel end connector, added nor if this roof concept would tive, easy to implement. It met all target presumably with adhesive. The end pass all criteria without redesigning performance criteria, and it meets most connectors are then MIG welded to other body structures, such as the B- weight-reduction initiatives. the steel rails. The end connectors add pillar, which is not within the scope of Coincidently, the 2018 Honda additional weight, and when com- this effort. 3 Accord also used an all-steel design, bining the weight reduction using Concept 3 is the final concept, and though its design looks to be even composites, the net weight reduction also a mixed-material solution that is more conservative than Concept 1, as is just 0.7 kg. With a cost delta of $4X very similar to Concept 2, except that it is closer to the baseline. The 2018 for composites over mild steel, only the aluminum sheet is replaced by a Accord has four roof bows, though premium vehicles would find it worth carbon fiber roof panel. using 1500 Mpa in the center arch, the effort. A carbon fiber roof is pushing the 780 Mpa on the front header and 340 The big weight savings in Concept innovation button, yet there is not a Mpa on the roof panel. 2 comes from the panel conversion whole lot of weight saved compared to Concept 2 is a mixed-material solu- to 6022-T43 + PB aluminum sheet the aluminum roof in Concept 2. Just 4 tion, and the design returned to the with properties of 130 Mpa YS, 250 0.2 kilograms are reduced with the number of roof bows in the baseline Mpa UTS, 26 percent elongation, 0.9 carbon fiber roof. This would bring

68 Lightweighting worlD / SEP+OCT 2019 Lightweighting worlD / SEP+OCT 2019 69 FIGURE [ 5 ] / Concept 2

LOAD CASES BASELINE CONCEPT 2 MEETS/EXCEEDS (TARGET PERFORMANCE BASELINE PERFORMANCE) PERFORMANCE (within 5%) Mass 22.4 kg 13.6 kg Roof Crush 3.7 SWR, 62 kN 3.8 SWR, 63 kN YES Frequency–Torsion 50 Hz 50 Hz YES Frequency–Bending 37 Hz 37 Hz YES Stiffness–Torsion 27.6 kN-m/deg 21.4 kN-m/deg 78% of baseline Stiffness–Bending 6.9 kN/mm 7 kN/mm YES Dent Resistance (plastic strain) 1.2% 1% YES

FIGURE [ 6 ] / Concept 2 results

FIGURE [ 7 ] / Concept 3 Roof Construction

68 Lightweighting worlD / SEP+OCT 2019 Lightweighting worlD / SEP+OCT 2019 69 PRODUCTION FOCUS The most prominent aluminum-body vehicle on the planet, the Ford F-150, converted a three-piece the total weight savings to 9 kilo- and cost effective. If they want more grams, which is a 40-percent total roof weight reduction, they will replace steel stamped front roof weight reduction—a big headline, yet the steel panel in Concept 1 with with the asterisk for not passing the aluminum, keeping the efficient high- header to a one-piece torsional stiffness requirement. Like strength steel support structure. aluminum extrusion and 2, it is not known how much weight Some OEMs are more conservative would be added to pass all the criteria. than others. Yet, we have already seen cut a whopping 2.9 kg. Future Work in the 2018 Honda Accord where Honda redesigned the 2011 Accord The future work identified by the more conservatively than Concept 1. CALM Group: The most prominent aluminum- • an all-aluminum roof concept body vehicle on the planet, the Ford been reduced by Concept 1? Unfortu- • to test all concepts under a side- F-150, converted a three-piece steel nately, as it is, the media doesn’t read impact load stamped front roof header to a one- the fine print, and they run to social • to evaluate modifications to other piece aluminum extrusion and cut a media and their newsletters to publish structural components, such as the whopping 2.9 kg. A huge amount of results that are really incomplete. lw B-pillar, to enable Concepts 2 and 3 to weight savings on the header, yet no meet the torsional requirements extrusions were proposed in the three • to evaluate the materials for a pan- concepts because there are CALM REFERENCES oramic sunroof members who produce aluminum ex- 1. www.cargroup.org/publication/ These all sound very good, though trusions. This will be studied in other roof-lightweighting-study/ after seeing Tesla transition from the 2. www.cargroup.org/calm-program/ CALM efforts. calm-roster/ all-aluminum body on the Model S to Concept 3 was largely an extension 3. www.repairerdrivennews. a mixed-material body on the more of 2, yet utilized fewer composite roof com/2019/06/26/37520/ affordable Model 3, I think the future bows, more high-strength steel, and 4. www.manualzz.com/doc/9466000/ 6022-spec-sheet is really in mixed materials. it replaced the aluminum sheet with carbon fiber composite. The resulting Closing Thoughts ABOUT THE AUTHOR 1-percent improvement in weight sav- CALM’s intent is to provide an ings doesn’t move the needle for the Andrew Halonen is president of Mayflower unbiased study for OEMs. For this additional cost and complexity. Consulting, LLC, a light- reason, they do not provide conclu- The goal of this study was to assess sion statements or recommendations. weighting consultancy that multiple materials to reach a weight- provides strategic marketing and The purpose of this study is ultimately savings goal according to a set of per- business development for high-tech to drive change. Will the OEMs design formance metrics. The first two parts clients. The focus is on mobility for the next-generation vehicle with a were met. There were mixed materials, defense ground vehicles, commercial trucking and automotive markets. lower weight roof, and will they use and there was weight savings. The last Mr. Halonen works with castings, materials and concepts proposed in item was left incomplete. Only the this study? extrusions, composites, brakes, Concept 1 reached all target metrics. additive manufacturing and new My opinion is that there are many This begs the question,if the tor- alloy development programs. areas of the vehicle that could use a sional stiffness target was reduced by www.lightweighting.co weight-reduction effort. The OEMs 26 percent such that all concepts could will use Concept 1 first because it easy reach it, how much weight would have

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