Optimizing Lightweight Material Selection in Automotive Engineering: A Hybrid Methodology Incorporating Ashby’s Method and VIKOR Analysis

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1. Overview:

• Title: Optimizing Lightweight Material Selection in Automotive Engineering: A Hybrid Methodology Incorporating Ashby's Method and VIKOR Analysis
• Author: Edoardo Risaliti, Francesco Del Pero, Gabriele Arcidiacono, Paolo Citti
• Publication Year: 2024
• Publishing Journal/Academic Society: Preprints.org
• Keywords: Lightweighting; Redesign; VIKOR; ashby; automotive; material selection; environmental impact; MCDA methods

2. Abstracts or Introduction

The automotive industry is facing increasing pressure to reduce greenhouse gas (GHG) emissions. Lightweighting is a key strategy, and material selection plays a crucial role in achieving mass reduction, thereby decreasing fuel consumption and enhancing vehicle performance. This paper introduces an innovative methodology for material selection in the automotive sector, integrating the Ashby approach and VIKOR (Vise Kriterijumska Optimizacija I Kompromisno Resenje) analysis. The Ashby method is utilized as an initial filter based on mechanical performance coefficients, followed by the VIKOR method to rank materials considering multiple criteria, including mechanical behavior, cost, manufacturing process availability, and environmental impact assessed via Life Cycle Assessment (LCA). The proposed hybrid methodology aims to provide a balanced and sustainable approach to material selection, validated through a re-design case study of a motor bracket for a C-segment passenger electric car.

3. Research Background:

Background of the Research Topic:

The automotive industry is a significant contributor to greenhouse gas emissions, accounting for approximately 20% in Europe. To meet stringent GHG emission reduction targets, lightweighting has emerged as a critical design strategy. Mass reduction in vehicles leads to decreased fuel consumption and exhaust emissions, alongside improvements in vehicle performance, safety, and handling. Selecting materials that offer superior structural properties with reduced weight compared to traditional materials like steel is a significant challenge.

Status of Existing Research:

Current lightweighting strategies encompass three major approaches: utilizing lightweight materials such as aluminum [15][18], titanium [18], magnesium [19], High-Strength Steels (HSS) [20], and composites [21]; employing innovative manufacturing processes like additive manufacturing [27], advanced sheet compression molding [28], and injection molding [29]; and optimizing component design through topology optimization [33][34]. The Ashby theory [40][41][42] is a widely recognized method for material selection in automotive engineering, particularly for its effectiveness in selecting materials based on mechanical and structural integrity requirements. Multi-Criteria Decision Analysis (MCDA) methods are also increasingly applied to address the complexities of material selection involving multiple design criteria [45]. Common MCDA methods include TOPSIS, VIKOR, COPRAS, PROMETHEE, ELECTRE, and MOORA.

Necessity of the Research:

While the Ashby method is valuable for initial material screening based on performance indices, it has limitations in handling numerous selection criteria and lacks inherent material ranking capabilities. Furthermore, it does not comprehensively address trade-offs between technical, economic, and environmental aspects [43]. Existing applications of MCDA methods in material selection often rely on arbitrarily chosen parameters dictated by designer experience and may not systematically evaluate the underlying physical phenomena [44]. Therefore, there is a need for a more holistic and structured approach that integrates objective performance criteria with cost and environmental considerations for sustainable material selection in automotive lightweighting.

4. Research Purpose and Research Questions:

Research Purpose:

The primary research purpose is to explore and validate an innovative methodology for material selection in the automotive industry that promotes both lightweighting and sustainability. This methodology aims to overcome the limitations of traditional approaches by integrating the objective material performance criteria of the Ashby theory with the multi-criteria decision-making capabilities of the VIKOR method.

Key Research:

The key research focus is on the development and application of a hybrid methodology that combines the Ashby approach for initial material filtering based on mechanical performance indices with the VIKOR method for comprehensive material ranking. This integration allows for the simultaneous consideration of design, cost, and environmental aspects in the material selection process. The methodology is designed to emphasize the interaction between selection criteria and environmental impact, offering a holistic approach to material choice.

Research Hypotheses:

While not explicitly stated as formal hypotheses, the research operates under the premise that:

1. Integrating the Ashby method and VIKOR analysis will provide a more robust and comprehensive methodology for material selection compared to using either method in isolation.
2. The proposed hybrid methodology will enable a more balanced consideration of mechanical performance, cost-efficiency, and environmental footprint in automotive material selection.
3. Applying the VIKOR method with Design Index, Cost Index, and Environmental Index derived from Ashby theory and LCA will lead to the identification of optimal and sustainable lightweight material solutions for automotive components.

5. Research Methodology

Research Design:

The research employs a hybrid methodology design, integrating two established methods: the Ashby method and the VIKOR method. The Ashby method is used for initial screening of materials based on performance indices related to mechanical properties. Subsequently, the VIKOR method is applied to rank the pre-selected materials based on multiple criteria, including Design Index (mass), Cost Index (production cost), and Environmental Index (Life Cycle Climate Change impact).

Data Collection Method:

The study utilizes data from the Granta Selector Database [64] for material properties, industrial process parameters, and environmental impact data. A case study from existing literature [68], concerning the re-design of a motor bracket of a C-segment electric car, is adopted to validate the proposed methodology. Impact Reduction Values (IRV) for the use phase environmental impact are calculated based on geographical relevance and driving cycles, using data from [65] and [66].

Analysis Method:

The VIKOR method is the primary analysis method. It ranks design solutions by calculating a single score (Qi) based on group utility (Si) and individual regret (Ri). These scores are derived from normalized values of the Design Index, Cost Index, and Environmental Index. Weighting criteria are applied to prioritize cost, design, and sustainability aspects. The Design Index is calculated using Ashby performance indices to estimate component mass. The Cost Index is calculated considering raw material cost, manufacturing process costs, and tooling costs [Equation 16]. The Environmental Index is calculated using LCA principles, encompassing raw material acquisition, use phase, and End-of-Life (EoL) stages [Equation 17].

Research Subjects and Scope:

The research focuses on material selection for automotive components, specifically a motor mounting bracket for a C-segment electric car. The scope is limited to the selection of materials and primary manufacturing processes for lightweight re-design, considering design performance, production cost, and environmental impact (Climate Change). The analysis considers a range of materials, including steels, aluminum alloys, and cast irons, and compatible industrial processes available in the Granta Selector Database.

6. Main Research Results:

Key Research Results:

The application of the hybrid Ashby-VIKOR methodology to the motor bracket case study resulted in a ranked list of 1316 viable lightweight design solutions. The top-ranked solution is a low alloy steel (AISI 9255) processed by press forming. This solution achieves a 44% mass reduction, 75% cost saving, and 61% reduction in Climate Change impact compared to the reference AISI 304 stainless steel bracket. Wrought steels, particularly low alloy steels produced by press forming, dominate the top rankings due to their favorable balance of mechanical properties and cost-effectiveness. Aluminum solutions offer greater mass reduction but are ranked lower due to higher raw material costs and embodied energy, leading to a less significant overall environmental benefit in this specific case study. Cast iron alternatives, while offering substantial cost reduction, are limited by their lower mechanical properties and mass reduction potential.

Analysis of presented data:

Figure 1 illustrates Ashby diagrams used for initial material screening, plotting material index ratios relevant to tensile/compression and torque bending loads. These diagrams visually represent the material database and facilitate the selection of materials meeting initial performance criteria. Figure 2 presents a comparison of the Design Index, Cost Index, and Environmental Index ratios for the top 20 ranked solutions relative to the baseline design. This visualization highlights the trade-offs between different design solutions in terms of mass reduction, cost, and environmental impact. The analysis reveals that for the top-ranked solutions (low alloy steels), all three indices are relatively aligned, indicating a balanced improvement across all criteria. For aluminum solutions, the Design Index is lower (better mass reduction), but Cost and Environmental Indices are higher, reflecting the cost and environmental trade-offs associated with aluminum.

Figure Name List:

7. Conclusion:

Summary of Key Findings:

This study successfully developed and validated a hybrid methodology for material selection in automotive lightweighting by integrating the Ashby method and VIKOR analysis. The methodology was applied to a motor bracket re-design case study, demonstrating its ability to identify optimal lightweight solutions considering structural integrity, cost, and environmental sustainability. Low alloy steel processed by press forming emerged as the top-ranked material for the case study, offering significant improvements in mass, cost, and environmental impact compared to the baseline stainless steel design.

Academic Significance of the Study:

The research contributes to the field of material selection methodology by providing an innovative approach that bridges traditional performance-based selection with contemporary sustainability considerations. The integration of Ashby theory and VIKOR analysis offers a structured and objective framework for navigating complex material choices in eco-design. This hybrid methodology addresses the limitations of individual methods and provides a more holistic perspective on material selection in automotive engineering.

Practical Implications:

The developed methodology provides a valuable tool for automotive engineers and designers in the early stages of component re-design for lightweighting. It enables a systematic and data-driven approach to material selection, facilitating the identification of solutions that optimize not only component performance but also cost and environmental footprint. The methodology can be readily implemented using material databases and LCA tools, supporting informed decision-making in sustainable automotive design.

Limitations of the Study and Areas for Future Research:

The study is limited to the material selection phase and does not incorporate detailed design optimization. Future research could explore the integration of topological optimization with the proposed methodology to further enhance lightweighting potential and refine component design. Additionally, the economic and environmental analysis is limited to primary manufacturing processes and Climate Change impact; expanding the scope to include secondary processes, End-of-Life treatments in more detail, and a broader range of environmental impact categories would provide a more comprehensive assessment.

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9. Copyright:

• This material is Edoardo Risaliti, Francesco Del Pero, Gabriele Arcidiacono, Paolo Citti's paper: Based on "Optimizing Lightweight Material Selection in Automotive Engineering: A Hybrid Methodology Incorporating Ashby's Method and VIKOR Analysis".
• Paper Source: doi: 10.20944/preprints202411.1992.v1

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