Core Viewpoint - The article provides a comprehensive guide on material selection based on various mechanical properties such as Young's modulus, strength, and cost, emphasizing the importance of choosing the right materials for specific applications. Group 1: Young's Modulus and Density - When hard materials are needed, such as for top beams or bicycle frames, materials at the top of the chart should be selected [2] - For low-density materials, such as packaging foam, materials on the left side of the chart are recommended [2] - Finding materials that are both rigid and lightweight is challenging, and composite materials are often a good choice [3] Group 2: Young's Modulus and Cost - For hard materials, the top materials in the chart should be chosen for applications like top beams and bicycle frames [14] - For low-cost materials, those on the left side of the chart are preferred [14] - If a cheap and hard material is required, materials in the upper left corner of the chart, mostly metals and ceramics, should be selected [15] Group 3: Strength and Density - The strength indicated in the chart refers to tensile strength, with ceramics showing compressive strength [26] - High-strength and low-density materials are located in the upper left part of the graph [26] - Strength is a critical indicator of a part's ability to resist failure under load [26] Group 4: Strength and Cost - The strength indicated is tensile strength, except for ceramics which indicate compressive strength [38] - Many applications require materials with high strength, such as screwdrivers and seat belts, but these materials are often expensive [38] - Only a few materials can meet both strength and cost requirements, typically found in the upper left part of the chart [38] Group 5: Strength and Toughness - The strength indicated is tensile strength, while ceramics indicate compressive strength [50] - Typically, materials with poor toughness also have low strength; increasing strength may reduce toughness [50] - Strength measures a material's ability to resist external forces, while toughness measures its ability to absorb energy before failure [50] Group 6: Strength and Elongation at Break - Ceramics have very low elongation at break (<1%); metals have moderate elongation (1-50%); thermoplastics have high elongation (>100%) [61] - Rubber exhibits long-term elastic elongation, while thermosetting polymers have low elongation (<5%) [61] Group 7: Strength and Maximum Working Temperature - The chart applies to components used in environments where working temperatures exceed room temperature, such as cookware and automotive parts [73] - Polymers have lower maximum working temperatures, metals have medium, and ceramics can withstand very high temperatures [73] Group 8: Specific Strength and Specific Stiffness - Specific strength is defined as strength divided by material density, while specific stiffness is stiffness divided by material density [84] - High strength and high stiffness usually coexist, as they largely depend on the bonding forces between atoms [84] Group 9: Resistivity and Cost - The chart is primarily for selecting materials that require low prices and good electrical insulation or conductivity [97] - Good electrical conductors are typically good thermal conductors, while good electrical insulators are good thermal insulators [97] Group 10: Recyclability and Cost - The chart identifies materials' recyclability features, especially for expensive and recyclable materials [108] - Metals are particularly suitable for recycling due to ease of sorting and remelting, while ceramics are rarely recycled [108] Group 11: Production Energy Consumption and Cost - The energy consumed in producing a material is a factor in raw material costs, with most materials located in the low-cost/low-energy or high-cost/high-energy quadrants [121] - Metals often require significant energy for extraction, such as aluminum production consuming a substantial portion of total energy in the U.S. [123]

Core Viewpoint - The article discusses the historical development of aviation, highlighting key figures and technological advancements that have shaped the industry over time. Group 1: Historical Milestones - The ancient Chinese had dreams of flying, as evidenced by historical artifacts like the Dunhuang murals [4] - The first powered flight was achieved by the Wright brothers in 1903, marking the beginning of modern aviation [7] - Chinese aviator Feng Ru created two aircraft models and conducted test flights in China, but tragically died in a crash in 1912 [12] Group 2: Engineering Challenges - Early aviation safety considerations focused on structural integrity, such as wing strength and fuselage durability [13][17] - Engineers used sandbags to simulate aerodynamic loads during ground tests due to the lack of advanced technology [16] - The concept of strength refers to a structure's ability to resist failure under load [17] Group 3: Aircraft Design and Safety - Lift is generated by the pressure difference between the upper and lower surfaces of the wing as it moves through the air [20] - Aircraft can experience vibrations and structural deformation during high-speed flight, necessitating a focus on wing stiffness [26][28] - Historical aircraft like the DC-3 and the Comet faced structural failures due to material fatigue [29][31] Group 4: Material Science and Testing - The introduction of fatigue testing and damage tolerance concepts has improved aircraft safety [59][62] - Modern aircraft design utilizes advanced materials like carbon fiber composites, which offer high strength-to-weight ratios [78][79] - Testing methods have evolved to include simulations and dynamic load tests to ensure structural integrity [88][90] Group 5: Human Factors and Operational Safety - Human error accounts for 70% to 80% of aviation accidents, highlighting the importance of training and technology in mitigating risks [93] - Modern aircraft employ advanced flight control systems that reduce the likelihood of pilot error [97] Group 6: Future of Aviation - The future of aviation is expected to integrate air and space travel, with advancements in high-speed vehicles that could redefine distance and travel time [130][131]