Design Optimization Case Study: Car Structures Mark Carruth
1. INTRODUCTION Crash structures are used in transport vehicles such as cars and trains to protect the occupants and reduce the risk of serious injury in the event of a collision. The term crash structure typically refers to elements which are added to the main body of a vehicle specifically to provide crash protection, but nowadays the whole vehicle body is likely to play some role in crash performance. Before sale, vehicles (particularly road vehicles) must pass a series of crash tests/certifications, designed to ensure a certain level of protection in the event of different types of impact. Accurate prediction of the performance of a vehicle in a crash test typically requires advanced computer modelling or experiment. These experiments are expensive to perform, and simulations require large amounts of computer time. Without access to either experimental data or sufficient computer resources, what can we learn about the lightweighting of crash structures? Fundamentally, the key requirement of crash protection is to absorb and dissipate energy. In an impact, the crash structure must dissipate the energy of the impact whilst ensuring that the occupants of the vehicle are not subjected to excessive accelerations/forces, and that the survivable zone within the car remains intact (i.e. the crash structure does not ingress too far into the vehicle). This energy dissipation is achieved through plastic work done in deforming the material in the crash structure. Therefore, by comparing the capacity for energy dissipation of different materials, and considering their density, it is possible to assess which materials provide optimal energy dissipation per unit mass specific energy absorption. In this study, which focuses on cars, the comparisons will be based on a set of idealized assumptions about material deformation. Where possible, references will also be made to the literature on the design of crash structures and whole car bodies. The key aim of the study is to compare material performance in energy dissipation applications, the design possibilities for crash structures, and the potential for lightweighting of crash structures through the use of optimal designs and materials. 2. MATERIAL SELECTION FOR ENERGY ABSORPTION Crash structures dissipate energy through plastic work. An ideal material for a crash structure should have high plastic work capacity, whilst being as light as possible. The work done in plastic deformation can be calculated by the integral: where is the effective stress and is the effective strain. However, for some materials, such as foams, this method of calculating plastic work is inappropriate, as it is impossible to calculate the stress/strain state at each point within the material. In these cases, material properties are considered
only at a macroscopic level. When subjected to compressive crushing, foams typically exhibit a very flat (i.e. constant stress) stress-strain curve up to a strain known as the densification strain. At higher strains, the stress rapidly increases with very little increase in strain (Ashby, 2006). Therefore, in an idealized model, the energy dissipation per unit volume can be accurately predicted using the formula (Ashby, 2006): Where is the plateau stress, and is the densification strain. In this study, materials are compared using a set of highly idealized assumptions. For a block of material, it is assumed that the material yields perfectly under uniaxial compression (i.e. every point in the block experiences full plastic deformation to the point of failure), and the energy dissipation is calculated by multiplying the compressive strength and the fracture strain. The materials considered are: aluminium foams; ultra high strength steels; wrought aluminium alloy 5454; CFRP; and magnesium. Aluminium Foams Material Relative Density Density Energy Dissipation/ Unit volume (MJ/m 3 ) Energy Dissipation/ Unit mass (MJ/kg) Cymat Foam 0.2 560 6.3 0.01 Alulight Foam 0.35 1000 11.2 0.011 Alporas Foam 0.1 250 1.394 0.006 ERG Foam 0.1 250 2.7 0.011 Duracore Foam 0.35 1000 17 0.017 Ultra high strength steels Aluminium DP500-7800 70 0.009 DP600-7800 66.5 0.009 TRIP800-7800 132.3 0.016 CP-W800-7800 64 0.008 MS-W1200-7800 52 0.007 AA5454-H2-2680 26.5 0.01 AA5454-H4-2680 23.8 0.009 CFRP 1-1570 188.4 0.120 Magnesium (EA55RS) - 1945 55.35 0.028 Table 2.1. Comparison of energy dissipation of various materials 1 Due to the sensitivity of CFRP to loading conditions and fibre orientations, these numbers are taken from USDTRITA (2008). This value applies to progressive axial crushing the fibre orientation is unknown
Figure 2.1: Specific energy absorption during progressive crushing (USDTRITA, 2008) The results of the material comparison are shown in Table 2.1. (The values are compiled from CES material selection software, Ashby et. al (2000), and data sheets from material suppliers). The properties of CFRP are very dependent on fibre orientation and method of loading; the values quoted here are taken from USDTRITA (2008). Table 2.1 shows a good degree of similarity in energy dissipation per unit mass between many of the different examples. Generally, aluminium foams perform slightly better than ultra high strength steels and aluminium alloys, but CFRP and magnesium stand out as being markedly superior to all other materials. Duracore foam, and TRIP800 steel perform particularly well, though still significantly worse than CFRP and magnesium. In Figure 2.1 (USDTRITA, 2008), the results of specific energy dissipation in a longitudinal crushing test are shown for several materials. Generally, these numbers show good agreement with Table 2.1, with the exception of foams, which in Table 2.1 are comparable to aluminium, but in Figure 2.1 are significantly worse. This discrepancy could simply be caused by the use of different foam properties; specific energy absorption of foams is very sensitive to the relative density of the foam. However, Ashby (2006) showed that for this specific type of crushing (progressive axial collapse), hollow steel tubes outperform foams in specific energy absorption, although not by the margin suggested by Figure 2.1, so this is unlikely to be the cause of the discrepancy. High strength steels are routinely used nowadays in automotive crash structures, and the development of these materials over the past decade has been driven partly with this application in mind (projects like ULSAB - Ultra Light Steel Auto Body - demonstrate this). Ongoing development seeks to find further improvements, and the TRIP (Transformation Induced Plasticity) materials are among those under ongoing development.
This analysis shows very similar performance between most aluminium foams, ultra high strength steels and aluminium, with the exception of the TRIP800 steel and Duracore foam, which are significantly better. This suggests that there may be weight benefit in switching to high performance aluminium foams, or more recently developed UHSS steels. However, switching to a CFRP or magnesium crash structure may, according to Table 2.1, allow significantly higher weight saving. This section has considered an idealized analysis of material selection for energy absorption applications. The next section looks in more detail at the design of the crash structures themselves, which strongly influences crash performance. 3. MATERIAL USE IN CRASH STRUCTURES Careful design of crash structures is vital if they are to provide effective protection during impact. Although the idealized analysis of the previous section is useful from a theoretical standpoint, in reality the design of the crash structure will greatly influence the degree of material deformation which occurs during impact, and hence the energy dissipation. Firstly, the direction of impact must be considered. For frontal impact in the direction of travel, crash structures typically take the form of long members which crush along their length during impact to dissipate energy ( progressive axial collapse ). For side impacts, in addition to the main car structure (e.g. the B-pillar), side intrusion beams are placed across doors, and typically dissipate energy through deformation in bending. Here the design of both front and side impact structures is considered, and alternative designs are compared to those in current practice. 3.1. FRONT IMPACT STRUCTURES Front impact structures absorb energy through progressive longitudinal deformation during impact. Typically thin walled tubes or prismatic sections loaded axially, the crushing force causes a progressive buckling of the walls during deformation, which proceeds down the length of the tube. Ashby (2000) compared the use of foam-filled core tubes with those of regular tubes in terms of specific energy performance (energy absorbed/unit mass). The results show that optimized tubes outperform foam filled tubes by a small but significant margin. Kim (2002) performed an optimization study on extruded aluminium sections to maximize the specific energy absorption by altering the cross-sectional shape. His results show that the specific energy absorption of a squaresection tube can be improved by 1.9 times by optimizing the cross-sectional profile. These results suggest that optimized design can offer large improvements in the specific energy absorption of front impact structures. Furthermore, the results of section 2 suggest that CFRP may be a very promising material for specific energy absorption, particularly for progressive axial crushing. Therefore an optimally designed CFRP crash structure would likely be significantly (perhaps 10x)
lighter than a conventional steel structure currently in use. However, CFRP is significantly more expensive, and the majority of car manufacturers do not have manufacturing capabilities for this material. 3.2 SIDE IMPACT BEAMS The behaviour of side impact beams during deformation is markedly different to that of front impact beams. Rather than deforming longitudinally along the axis of the beam, side impact beams, which are typically mounted at the edges of the door frame, deform mainly in bending. Nowadays these beams are typically made of ultra high strength steels, but other materials are also used, including aluminium extrusions and cold-formed steels. As mentioned in section 3.1, foam filled tubes perform worse than hollow tube cylinders during longitudinal deformation, but for oblique impacts such as this, the situation is different. Foams have the advantage that their properties are isotropic, so will perform in the same way regardless of the direction of impact, whereas a steel tube will perform considerably worse in an oblique impact compared to a longitudinal impact. Foam sandwich panels may provide a lighter weight alternative to side intrusion beams. Ashby (2000) describes an aluminium foam system produced by the German company Karmann GmbH. These products use an aluminium foam core, roll bonded to two aluminium face sheets. Karmann claim that these parts are up to ten times stiffer and 50% lighter than equivalent parts made of steel, though no further evidence could be found for these claims. The results of section 2 suggest that foam may have a small advantage over ultra high strength steel; however, these sandwich panels have the added benefit of using the body skin panel as part of the structural system of the car, which is not usually the case, and would provide additional weight saving. This is an example of functional integration, where multiple functionality is integrated into a single component. However, this integration can lead to quality issues, as the bonding of the foam to the body skin panel makes the body panel more susceptible to accidental damage and denting. The use of CFRP in side impact crash structures may be more difficult. As side impact behaviour is dominated by bending, rather than progressive axial crushing, the specific energy absorption is likely to be significantly less than that shown in Figure 2.1. However, the use of CFRP combined with other materials, may still offer a lightweight alternative to side impact beams. 4. POTENTIAL FOR MASS SAVING The analysis of the previous sections suggests that there may be lightweighting potential for crash structures through both optimized design and material substitution. CFRP, in particular, performs extremely well in front impact, though may be less beneficial for side impact structures.
This section seeks to estimate the possible mass saving, based on the material analysis from section 2, but also by examining the available literature on lightweight automotive design. 4.1 ESTIMATED MASS SAVINGS FROM MATERIAL DATA Using the idealized material properties from section 2, a theoretical estimate of the mass saving potential of material substitution can be obtained. It is assumed that DP600 is representative of a steel which might be used in a modern crash structure. Mass savings are then measured relative to this material, shown in Table 4.1. Material Aluminium Ultra High Aluminium CFRP (front Magnesium Foam Strength Steels impact) Mass Saving over DP600 Cymat: 24% Alulight: 24% Alporas: -53% ERG: 21% Duracore: 50% DP500: 5% TRIP800: 50% CP-W800: -4% MS-W1200: -28% AA5454-H2: 14% AA5454-H4: 4% 93% 70% Table 4.1: Estimated mass savings from material data relative to DP600 steel (note negative numbers represent a mass increase) As these numbers are based on highly idealized assumptions, it is impossible to make an accurate estimate of the weight saving potential of material substitution in crash structures relative to real world cars. However, the results are sufficiently clear that the following general conclusions can be drawn: Magnesium and CFRP may offer weight savings in crash structures of over 50% compared to the materials which are currently used Modern UHSS steels such as TRIP800, and efficient aluminium foams such as Duracore offer weight improvements relative to older UHSS steels, but are not as good as CFRP or magnesium Given the large additional cost of using and manufacturing CFRP and magnesium components, and the large emissions associated with producing aluminium foams, further developments in UHSS steels may offer the most attractive means of weight saving 4.2 ESTIMATED MASS SAVINGS FROM LITERATURE Less theoretical estimates of weight saving potential can be found in the literature. These estimates are often from car manufacturers or material suppliers, and although they are likely to give a more accurate estimate of possible weight savings over current designs, they are harder to verify, as they rarely contain detailed technical information.
Cheah (2010) summarized examples in the literature where claims are made about possible weight savings, repeated partially in Table 4.2. Concept/Source Main Material Year Car Type Weight Saving 1 Future Steel Vehicle Steel 2009 Compact car 13% 2 Future Steel Vehicle Steel 2009 Midsize Car 15% 3 Lotus study Steel 2010 CUV 21-38% 4 SuperLIGHT-CAR Aluminium 2009 Compact car (201kg) 5 Lotus APX Aluminium 2006 SUV 28% Table 4.2: Examples from the literature of possible weight savings (Cheah, 2010) The numbers in Table 4.2 refer to whole car weight savings rather than specific parts, so it is difficult to generalize these results. However, the results for the Future Steel Vehicle (WorldAutoSteel, 2009) break down the weight saving by different areas of the car, and for the body/crash structure, estimate a saving of 11% compared to a modern (2010) vehicle, by substituting existing materials with higher strength steels. This suggests that the estimates of lightweighting potential made in section 4.2 are too high, most likely because modern vehicles are already using steels superior to DP600, which was assumed as the baseline case in that analysis. However, Lotus (2010) estimate possible weight savings of 21-38% compared to a 2010 Toyota Venza, and show that DP590 steel is used in large parts of the side body structure (which is important for crash performance), which has similar performance to the DP600 steel used as the benchmark in section 4.1. The Lotus study uses two new designs to estimate mass savings, the first of which is based on upgrading to higher strength steels, and the second of which is based on substituting steels with other high performance materials (glass fibre composites, magnesium, and aluminium). In the first case, a mass saving of 15.9% is estimated, and in the second a mass saving of 42.2%. Although these results differ qualitatively from section 4.1, they reinforce the following key points: For steel vehicles, substitution of existing materials with higher strength steels enables a mass saving of ~15% (for aluminium vehicles, using higher strength alloys may offer similar mass savings) Larger savings of ~40% can be achieved by substitution with other high performance materials, such as magnesium and fibre composites (although this is likely to increase cost).
4.3 SUMMARY OF POSSIBLE MASS SAVINGS Sections 4.1 and 4.2 made quantitative estimates of the potential for mass saving through material substitution. These are summarized here, combined with other examples mentioned in the literature: Lightweighting Measure Estimated Saving Replace steel with higher strength steels 10-15% Replace steel with other materials (e.g. aluminium, magnesium, composites) 20-40% Optimized design of front impact structures 47% (Kim, 2002) Aluminium Foam Sandwich Panels Up to 50% (Karmann GmbH) Table 4.3: Summary of mass saving possibilities for crash structures 5. CONCLUSIONS The analysis of this working paper leads to the following conclusions: Further developments of high strength steels may enable mass savings of 10-15% for car crash structures Substitution of steels with higher performance materials, such as aluminium, magnesium or CFRP may enable higher weight savings of 20-40% Optimized design, or the use of alternative structures such as foam sandwich panels, may enable weight savings of up to 50% Using both material substitution and optimal design may enable higher weight savings These estimates are in-line with the aims of car manufactures for weight saving, for example Jaguar Land Rover, who are targeting a 30% reduction in door weight 6. REFERENCES Ashby, M.F., Evans, A.G., Fleck, N.A., Gibson, L.J., Hutchinson, J.W., Wadley, H.N.G. (2000), Metal Foams: A Design Guide, Butterworth Heinemann. Ashby, M.F. (2006), The properties of foams and lattices, Philosophical Transactions, Series A, Mathematical, physical and engineering science, 364(1838), 15-30.
Cheah, L.W. (2010), Cars on a Diet: The Material and Energy Impacts of Passenger Vehicle Weight Reduction in the U.S., PhD Thesis, Massachusetts Institute of Technology Kim, H. (2002) New extruded multi-cell aluminum profile for maximum crash energy absorption and weight efficiency. Thin-Walled Structures, 40(4), 311-327 Lotus Engineering Inc. (2010), An Assessment of Mass Reduction Opportunities for a 2017-2020 Model Year Vehicle Program, submitted to The International Council on Clean Transportation US Department of Transportation Research and Innovative Technology Administration (USDTRITA) (2008), A Summary of Proceedings for The Safety Characterization of Future Plastic and Composite Intensive Vehicles (PCIVs) WorldAutoSteel (2009), Future Steel Vehicle Phase I Executive Summary. 7. ACKNOWLEDGEMENTS I would like to thank Rebecca Lees and Martin Halliwell of Jaguar Land Rover for their useful discussions on this topic.