Material Classification & Properties | A3.1

Classification systems allow designers and engineers to organise the vast range of available materials into manageable groups, making comparison and selection practical. The first recorded material classification system is attributed to Aristotle (384–322 BCE), who grouped matter by elemental properties. Modern classification follows three broad property categories:

• Physical properties: measurable characteristics that do not involve a chemical reaction. Density, thermal expansion, thermal conductivity, melting point, electrical conductivity and resistivity.
• Chemical properties: how a material interacts with other substances. Corrosion resistance, reactivity with food, hygroscopy, flammability.
• Mechanical properties: how a material responds to applied forces. Strength, stiffness, toughness, hardness, malleability, elasticity, plasticity, ductility.

Advantages of classification:

• Enables systematic comparison across large numbers of candidate materials using databases and selection tools (e.g., CES EduPack)
• Allows designers to specify performance requirements in measurable terms that suppliers and manufacturers can match
• Supports decision-making across the full product lifecycle, from material extraction and processing to end-of-life recycling or biodegradation
• Provides a shared technical vocabulary that bridges design, engineering, and manufacturing disciplines

分类系统使设计师和工程师能够将大量可用材料组织成可管理的组，使比较和选择变得实际可行。第一个有记录的材料分类系统归功于亚里士多德（公元前384-322年），他按元素性质对物质进行了分组。现代分类遵循三大属性类别：

• 物理特性 — 不涉及化学反应的可测量特征：密度、热膨胀、热导率、熔点、电导率和电阻率
• 化学特性 — 材料与其他物质的相互作用：耐腐蚀性、与食物的反应性、吸湿性、可燃性
• 力学性能 — 材料对外加力的响应：强度、刚度、韧性、硬度、可锻性、弹性、塑性、延展性

分类的优势：

• 使用数据库和选择工具（如CES EduPack）对大量候选材料进行系统比较
• 允许设计师用供应商和制造商可以匹配的可测量术语来规定性能要求
• 支持整个产品生命周期的决策——从材料提取和加工到报废回收或生物降解
• 提供连接设计、工程和制造学科的共同技术词汇

Materials are grouped by their origin into natural (found in or derived from nature) and human-made (synthesised or significantly processed by people):

• Timbers: natural. Softwoods (pine, spruce, cedar) from coniferous trees; hardwoods (oak, teak, mahogany) from deciduous trees. Engineered wood (plywood, MDF, LVL) is human-made from timber by-products.
• Polymers: largely human-made. Thermoplastics (polyethylene, polypropylene, ABS, PET, all of which can be remelted and reshaped); thermosets (epoxy, polyester resin, permanently cross-linked and unable to be remelted); elastomers (rubber, silicone, flexible and able to return to shape).
• Metals: processed from natural ores. Ferrous (contain iron: mild steel, stainless steel, cast iron) and non-ferrous (aluminium, copper, titanium, brass, bronze).
• Glass: human-made from natural silica (SiO₂) with additives. Soda-lime glass, borosilicate glass, optical glass, toughened and laminated safety glass.
• Textiles: natural fibres (cotton, wool, silk, linen) and human-made fibres (nylon, polyester, Kevlar, carbon fibre).
• Composites: human-made combinations of two or more materials (covered in 3.1.7).
• Smart materials: human-made materials with properties that respond to environmental stimuli (covered in 3.1.8).
• Biomaterials / biodegradable materials: natural or human-made materials that break down biologically (covered in 3.1.9).

Structural forms and their material implications: Product structures are often classified as frame, shell, solid, or combination, and the material choice is directly linked to the structural form:

• Frame structures: load-bearing members (rods, beams, tubes) connected at joints; requires materials with high strength-to-weight ratio (steel, aluminium, carbon fibre)
• Shell structures: thin curved surfaces carry loads through their geometry; requires materials that can be formed into thin sheets (sheet steel, fibreglass, carbon fibre)
• Solid (mass) structures: material fills the entire volume; suitable where compressive strength is primary (concrete, stone, cast iron)
• Combination structures: use two or more structural forms together (e.g., a car body combining a steel frame with sheet metal shell panels)

材料按其来源分为天然材料（在自然界中发现或来源于自然）和人造材料（由人类合成或大量加工）：

• 木材 — 天然：来自针叶树的软木（松木、云杉、雪松）；来自落叶树的硬木（橡木、柚木、桃花心木）。工程木材（胶合板、中密度纤维板、单板层积材）是由木材副产品人工制造的。
• 聚合物 — 主要为人造：热塑性塑料（聚乙烯、聚丙烯、ABS、PET——可重新熔化和成型）；热固性塑料（环氧树脂、聚酯树脂——永久交联，不能重新熔化）；弹性体（橡胶、硅胶——柔韧，可恢复形状）。
• 金属 — 从天然矿石中加工：铁类（含铁——低碳钢、不锈钢、铸铁）和非铁类（铝、铜、钛、黄铜、青铜）。
• 玻璃 — 由天然二氧化硅（SiO₂）加添加剂人工制造；钠钙玻璃、硼硅酸盐玻璃、光学玻璃、钢化和夹层安全玻璃。
• 纺织品 — 天然纤维（棉、毛、丝、亚麻）和人造纤维（尼龙、聚酯、凯夫拉、碳纤维）。
• 复合材料 — 两种或多种材料的人造组合（见3.1.7）。
• 智能材料 — 具有响应环境刺激属性的人造材料（见3.1.8）。
• 生物材料/可生物降解材料 — 生物降解的天然或人造材料（见3.1.9）。

结构形式及其材料含义：产品结构通常分为框架、壳体、实体或组合结构——材料选择与结构形式直接相关：

• 框架结构 — 在节点连接的承重构件（杆、梁、管）；需要高强度重量比的材料（钢、铝、碳纤维）
• 壳体结构 — 薄曲面通过几何形状承载载荷；需要可以成型为薄板的材料（钣金、玻璃钢、碳纤维）
• 实体（块体）结构 — 材料填充整个体积；适用于压缩强度为主的场合（混凝土、石材、铸铁）
• 组合结构 — 使用两种或多种结构形式（如汽车车身结合钢框架和钣金壳体面板）

No single material excels in all properties. Material selection is a multi-criteria optimisation problem: the designer must identify the properties most critical to the product's function, environment of use, manufacturing process, cost constraints, and end-of-life requirements, then find the material (or combination of materials) that best satisfies those criteria simultaneously.

Key considerations in material selection:

• Functional requirements: what the material must do (support loads, conduct electricity, insulate heat, resist chemicals, flex without breaking)
• Manufacturing requirements: how the material will be shaped (casting, machining, forming, moulding, additive manufacturing); some processes require specific material characteristics (e.g., die casting requires materials with low melting points)
• Environmental conditions of use: temperature range, exposure to moisture, chemicals, UV radiation, or biological agents; outdoor use demands corrosion resistance; food contact demands chemical inertness
• Aesthetic characteristics: surface finish quality, colour, translucency/opacity, texture, perceived quality ("feel" and "weight" that communicate value to users); materials communicate brand identity beyond their function
• Cost and availability: raw material cost, processing cost, lead time; exotic materials with excellent properties may be impractical for high-volume production
• Sustainability: embodied energy, recyclability, biodegradability, source (renewable vs. finite), and life-cycle environmental impact

In practice, designers use material selection charts (Ashby charts) to plot two properties simultaneously (e.g., strength vs. density), allowing rapid visual comparison of material families and identification of candidates for more detailed evaluation.

没有任何单一材料在所有属性上都表现出色。材料选择是一个多标准优化问题：设计师必须确定对产品功能、使用环境、制造工艺、成本限制和报废要求最关键的属性——然后找到最能同时满足这些标准的材料（或材料组合）。

材料选择的关键考虑因素：

• 功能要求 — 材料必须做什么（支撑载荷、导电、隔热、耐化学品、弯曲而不断裂）
• 制造要求 — 材料如何成型（铸造、机械加工、成形、模塑、增材制造）；某些工艺需要特定的材料特性（例如，压铸需要熔点低的材料）
• 使用环境条件 — 温度范围、湿气、化学品、紫外线辐射或生物制剂的暴露；户外使用需要耐腐蚀性；食品接触需要化学惰性
• 美学特征 — 表面光洁度、颜色、半透明性/不透明性、纹理、感知质量（向用户传达价值的"触感"和"重量感"）；材料在功能之外传达品牌形象
• 成本和可用性 — 原材料成本、加工成本、交货时间；具有优异性能的特种材料对于大批量生产可能不切实际
• 可持续性 — 内含能量、可回收性、可生物降解性、来源（可再生与有限）以及生命周期环境影响

实际上，设计师使用材料选择图（阿什比图）同时绘制两种性能（如强度与密度），从而快速直观地比较材料系列，并确定候选材料进行更详细的评估。

Physical properties describe what a material is. They can be measured without causing a chemical reaction or permanently altering the material's identity.

• Density (ρ): mass per unit volume (kg/m³ or g/cm³). A material's density determines structural weight and influences shipping cost, handling, and user experience. Aluminium (2700 kg/m³) is roughly one-third the density of steel (7800 kg/m³), making it attractive for weight-sensitive applications like aircraft and bicycle frames.
• Thermal expansion (coefficient of thermal expansion, α): the fractional change in length per degree of temperature change (K⁻¹). Engineering components operating across temperature ranges must accommodate differential expansion. For mild steel, α ≈ 11 × 10⁻⁶ K⁻¹, meaning a 1-metre steel rod expands by 0.011 mm for every 1°C rise. Bridges need expansion joints; pipelines need flexible connectors.
• Thermal conductivity (k): the rate at which heat flows through a material (W/m·K). High thermal conductivity (copper, aluminium) is needed in heat exchangers and cooking pans. Low thermal conductivity (wood, expanded polystyrene, aerogel) is required for thermal insulation in buildings and packaging.
• Melting point (T_m): the temperature at which a solid transitions to a liquid. A high melting point (tungsten: 3422°C) is needed for materials used in high-temperature environments. A low melting point is required for casting and soldering processes. Thermoplastics soften progressively rather than having a sharp melting point.
• Electrical conductivity (σ) and electrical resistivity (ρ_e): conductivity is the ease with which electrons flow through a material (S/m); resistivity is its reciprocal (Ω·m). Conductors (copper, aluminium, silver) have very low resistivity; insulators (rubber, ceramic, glass) have very high resistivity; semiconductors (silicon, germanium) fall in between and can be tuned by doping.

物理特性描述材料的本质——它们可以在不引起化学反应或永久改变材料性质的情况下测量。

• 密度（ρ） — 单位体积的质量（kg/m³或g/cm³）。材料的密度决定结构重量，影响运输成本、操作和用户体验。铝（2700 kg/m³）的密度约为钢（7800 kg/m³）的三分之一，使其在飞机和自行车框架等重量敏感应用中具有吸引力。
• 热膨胀（热膨胀系数α） — 每摄氏度温度变化的长度分数变化（K⁻¹）。在温度范围内运行的工程部件必须适应差异膨胀。对于低碳钢，α ≈ 11 × 10⁻⁶ K⁻¹，意味着每升高1°C，1米钢棒膨胀0.011 mm。桥梁需要伸缩缝；管道需要柔性连接器。
• 热导率（k） — 热量通过材料的速率（W/m·K）。热交换器和烹饪锅需要高热导率（铜、铝）。建筑和包装中的隔热需要低热导率（木材、膨胀聚苯乙烯、气凝胶）。
• 熔点（T_m） — 固体转变为液体的温度。在高温环境中使用的材料需要高熔点（钨：3422°C）。铸造和焊接工艺需要低熔点。热塑性塑料逐渐软化，而没有明显的熔点。
• 电导率（σ）和电阻率（ρ_e） — 导电率是电子通过材料流动的容易程度（S/m）；电阻率是其倒数（Ω·m）。导体（铜、铝、银）电阻率很低；绝缘体（橡胶、陶瓷、玻璃）电阻率很高；半导体（硅、锗）介于两者之间，可通过掺杂进行调节。

Chemical properties describe what a material does when it encounters other substances. They involve chemical reactions that alter the material's composition.

• Corrosion resistance: the ability to resist degradation by chemical reactions with the environment (oxygen, moisture, acids, salts). Stainless steel and aluminium resist corrosion through passivation: they spontaneously form a nanometre-thick chromium oxide (stainless steel) or aluminium oxide (aluminium) surface layer that is stable and adherent, blocking further oxidation of the underlying metal. This self-healing layer reforms if scratched. Iron and mild steel do not passivate; instead they form loose, flaky iron oxide (rust) that continues to expose fresh metal, causing progressive degradation.
• Reactivity with food (food safety): materials used in food contact applications must not release chemicals into food. Three relevant reactions are: migration (plasticisers or monomers diffusing from packaging into food), oxidation (fats and oils reacting with oxygen, accelerated by some metals), and hydrolysis (water breaking down polymer chains). Stainless steel, glass, HDPE, and food-grade polypropylene are used in food contact applications because they are chemically inert under normal conditions. Regulations (e.g., EU 10/2011, FDA CFR Title 21) specify migration limits.
• Hygroscopy: the tendency to absorb and retain moisture from the surrounding environment. Hygroscopic materials (wood, nylon, PVA, silica gel, salt) swell, weaken, or lose dimensional stability when they absorb water. Hydrophobic materials (wax, polyethylene, PTFE) repel water. Hygroscopicity is critical for packaging (a hygroscopic package protecting a moisture-sensitive product would be counterproductive), storage, and applications where dimensional accuracy is essential.
• Flammability: the ease with which a material ignites and sustains combustion. Measured parameters include the ignition temperature, rate of flame spread, and heat release rate. Materials are classified as flammable, combustible, or non-combustible. Designers specify flame-retardant additives or inherently non-flammable materials (ceramics, metals, glass) for products used near ignition sources or required to meet fire safety standards.

化学特性描述材料在遇到其他物质时会做什么——它们涉及改变材料组成的化学反应。

• 耐腐蚀性 — 抵抗与环境（氧气、水分、酸、盐）发生化学反应而降解的能力。不锈钢和铝通过钝化来抵抗腐蚀：它们自发形成纳米级厚的氧化铬（不锈钢）或氧化铝（铝）表面层，该层稳定且附着力强，阻止下层金属进一步氧化。此自愈层在划伤后会重新形成。铁和低碳钢不会钝化——它们形成疏松易碎的氧化铁（锈），持续暴露新鲜金属，导致逐步降解。
• 与食物的反应性（食品安全） — 用于食品接触应用的材料不得向食品中释放化学物质。三种相关反应是：迁移（增塑剂或单体从包装扩散到食品中）、氧化（脂肪和油脂与氧气反应，某些金属会加速此过程）和水解（水分解聚合物链）。不锈钢、玻璃、HDPE和食品级聚丙烯在食品接触应用中使用，因为它们在正常条件下化学惰性。法规（如欧盟10/2011、FDA CFR第21条）规定了迁移限制。
• 吸湿性 — 从周围环境吸收和保留水分的趋势。吸湿材料（木材、尼龙、PVA、硅胶、盐）吸水后会膨胀、变弱或失去尺寸稳定性。疏水材料（蜡、聚乙烯、PTFE）排斥水分。吸湿性对包装（用于保护对湿度敏感产品的吸湿包装会适得其反）、储存以及对尺寸精度要求高的应用至关重要。
• 可燃性 — 材料点燃和维持燃烧的容易程度。测量参数包括点火温度、火焰蔓延速率和热释放速率。材料分为易燃、可燃或不可燃。设计师为在火源附近使用或需要满足消防安全标准的产品指定阻燃添加剂或本质上不可燃材料（陶瓷、金属、玻璃）。

Mechanical properties describe what a material can withstand when forces are applied.

• Tensile strength (Ultimate Tensile Strength, UTS): the maximum stress a material can withstand before fracturing when pulled. Measured in MPa (megapascals). Steel structural members, cables, and tensioned fasteners require high UTS.
• Compressive strength: the maximum stress a material can withstand when compressed (squeezed). Concrete is strong in compression (≈ 20–40 MPa) but weak in tension; steel is strong in both (hence reinforced concrete combines the two).
• Stiffness (Young's modulus, E): resistance to elastic deformation under load; the ratio of stress to strain in the elastic region (GPa). A high Young's modulus means a material deflects very little under load. Steel (E ≈ 200 GPa) is far stiffer than rubber (E ≈ 0.01–0.1 GPa).
• Toughness: the energy absorbed per unit volume before fracture; the area under the full stress-strain curve. Toughness combines strength and ductility: a material can be tough by being strong (like high-strength steel) or by stretching a long way before breaking (like rubber). Measured by impact tests: Charpy (notched bar struck from behind) and Izod (notched bar struck from the side).
• Hardness: resistance to surface indentation or scratching. Tested by multiple methods: Brinell (steel/WC ball, best for castings); Rockwell (steel ball or diamond cone, measures depth); Vickers (diamond pyramid, standard for metals and ceramics); Knoop (elongated pyramid, for thin or brittle materials); Durometer (0–100 scale for polymers); Janka (half-ball penetration, for wood); Pencil test (for coatings); Shore Scleroscope (rebound height test).
• Malleability: the ability to be permanently deformed (plastically deformed) by compressive forces (hammering, rolling) without fracturing. Gold is the most malleable metal: it can be hammered into gold leaf 0.1 μm thick.
• Elasticity: the ability to return to original shape after the applied force is removed (elastic deformation). Occurs below the elastic limit (yield point). A rubber band is highly elastic.
• Plasticity: the ability to permanently deform without fracturing after the elastic limit is exceeded. Essential for forming processes (forging, rolling, drawing). Opposite of brittleness.
• Ductility: a specific form of plasticity: the ability to be drawn into wire or stretched into thin sections under tensile (pulling) forces without fracturing. Copper is highly ductile: it can be drawn into fine electrical wire.

力学性能描述材料在施加力时能够承受什么。

• 抗拉强度（极限抗拉强度UTS） — 材料在拉伸时断裂前能承受的最大应力。以MPa（兆帕）为单位测量。钢结构构件、电缆和拉伸紧固件需要高UTS。
• 抗压强度 — 材料在受压（挤压）时能承受的最大应力。混凝土抗压强度高（≈20-40 MPa），但抗拉强度低；钢在两方面都强（因此钢筋混凝土结合了两者的优点）。
• 刚度（杨氏模量E） — 在载荷下抵抗弹性变形的能力；弹性区间内应力与应变的比值（GPa）。高杨氏模量意味着材料在载荷下变形很小。钢（E≈200 GPa）比橡胶（E≈0.01-0.1 GPa）硬得多。
• 韧性 — 断裂前单位体积吸收的能量；完整应力-应变曲线下的面积。韧性结合了强度和延展性——材料可以通过强度高（如高强度钢）或断裂前拉伸很长（如橡胶）来表现出韧性。通过冲击试验测量：夏比（从后面击打缺口棒）和悬臂梁（从侧面击打缺口棒）。
• 硬度 — 抵抗表面压痕或划伤的能力。通过多种方法测试：布氏硬度（钢/碳化钨球，最适合铸件）；洛氏硬度（钢球或金刚石锥体，测量深度）；维氏硬度（金刚石金字塔，金属和陶瓷的标准）；努氏硬度（细长金字塔，适用于薄或脆性材料）；邵氏硬度（0-100标度，适用于聚合物）；詹卡硬度（半球穿透，适用于木材）；铅笔测试（适用于涂层）；肖氏回跳硬度（回弹高度测试）。
• 可锻性 — 在不断裂的情况下被压缩力（锤击、轧制）永久变形（塑性变形）的能力。金是最具可锻性的金属——它可以被锤打成0.1μm厚的金箔。
• 弹性 — 撤去外力后恢复原始形状的能力（弹性变形）。发生在弹性极限（屈服点）以下。橡皮筋具有很高的弹性。
• 塑性 — 超过弹性极限后在不断裂的情况下永久变形的能力。对于成形工艺（锻造、轧制、拉拔）至关重要。与脆性相反。
• 延展性 — 塑性的一种特定形式：在拉伸（拉力）下不断裂地拉伸成细丝或细截面的能力。铜具有高延展性——它可以拉伸成细电线。

Composite materials combine two or more constituents, a matrix (binder/continuous phase) and a reinforcement (dispersed phase), to produce a material with properties superior to either constituent alone. The three main categories are:

1. Particle-reinforced composites: Hard particles distributed in a softer matrix. The particles resist deformation and wear; the matrix transfers loads and holds particles in place.

• Concrete: Gravel or crushed stone particles in a cement paste matrix. High compressive strength. Combined with steel rebar (reinforced concrete), it also gains tensile strength.
• Cemented carbide (tungsten carbide + cobalt binder): WC particles provide extreme hardness for cutting tools; the cobalt matrix provides toughness to prevent brittle fracture of the tool edge.

2. Fibre-reinforced composites: Fibres embedded in a matrix (typically epoxy resin). Fibres are excellent in tension but cannot resist compression or shear without the matrix. The matrix glues fibres together, transfers load between them, and prevents buckling.

• Carbon fibre reinforced plastic (CFRP): Carbon fibres in an epoxy matrix. Used in the Boeing 787 Dreamliner (over 50% composite by weight), high-performance sports equipment, and racing cars. Lighter than aluminium, stiffer and stronger than steel for the same mass.
• Fibreglass (GFRP): Glass fibres in polyester or epoxy matrix. Used in boat hulls, car body panels, bathroom fixtures. Lower performance than CFRP but much cheaper.

3. Laminar (layered) composites: Layers of different materials bonded together.

• Plywood: Thin wood veneers glued with alternating grain directions, giving balanced strength in both directions and resistance to splitting. Far more stable than solid timber.
• Laminated glass: Two glass panes with a polyvinyl butyral (PVB) interlayer. When broken, the PVB holds the fragments together, preventing dangerous flying shards. Used in car windscreens and building façades.
• Cardboard: Corrugated paper sandwiched between flat paper liners, forming a structural sandwich panel that uses minimal material but achieves high bending stiffness for its weight.
• Sailcloth: Polyester, carbon or Aramid fibres laminated with polymer film in multiple orientations to withstand wind loads from all directions.

复合材料将两种或多种成分——基体（粘合剂/连续相）和增强体（分散相）——结合起来，产生性能优于任何单一成分的材料。三种主要类别是：

1. 颗粒增强复合材料：硬颗粒分布在较软的基体中。颗粒抵抗变形和磨损；基体传递载荷并将颗粒固定到位。

• 混凝土：砾石或碎石颗粒在水泥浆基体中。高抗压强度。与钢筋结合（钢筋混凝土），还获得了抗拉强度。
• 硬质合金（碳化钨+钴粘合剂）：WC颗粒为切削工具提供极高硬度；钴基体提供韧性，防止工具刃口发生脆性断裂。

2. 纤维增强复合材料：嵌入基体（通常为环氧树脂）的纤维。纤维在拉伸方面表现出色，但没有基体就无法抵抗压缩或剪切。基体将纤维粘合在一起，在纤维之间传递载荷，并防止屈曲。

• 碳纤维增强塑料（CFRP）：环氧基体中的碳纤维。用于波音787梦幻客机（按重量超过50%为复合材料）、高性能运动器材和赛车。比铝轻，同等质量下比钢更硬更强。
• 玻璃纤维（GFRP）：聚酯或环氧基体中的玻璃纤维。用于船体、汽车车身板和浴室设备。性能低于CFRP，但价格便宜得多。

3. 层压（分层）复合材料：不同材料的层粘合在一起。

• 胶合板：薄木单板以交替纹理方向粘合，在两个方向上提供均衡强度，并抵抗开裂。比实心木材稳定得多。
• 夹层玻璃：两块玻璃面板之间有聚乙烯醇缩丁醛（PVB）夹层。破碎时，PVB将碎片固定在一起，防止危险飞溅碎片——用于汽车挡风玻璃和建筑外立面。
• 纸板：瓦楞纸夹在平面纸衬之间——一种结构夹心板，使用最少的材料，却能实现高重量比弯曲刚度。
• 帆布：聚酯、碳或芳纶纤维以多个方向与聚合物薄膜层压，以承受来自各方向的风载荷。

Smart materials respond to a change in their environment (mechanical stress, temperature, light, electric or magnetic field) by significantly and reversibly changing one or more of their properties. This allows products that adapt to conditions without complex external control systems.

• Piezoelectric materials: generate an electric charge when mechanically stressed (direct effect), and conversely change shape when an electric current is applied (inverse effect). Applications: ultrasonic transducers for non-destructive testing (a piezoelectric probe sends sound waves into a pipe; returning echoes stress the crystal, generating a voltage that reveals internal flaws); pressure sensors; sonar; medical ultrasound imaging; loudspeakers and microphones.
• Shape memory alloys (SMAs): return to a pre-programmed shape when heated after being plastically deformed at a lower temperature. Nitinol (nickel-titanium alloy, discovered at the Naval Ordnance Laboratory) is the most common SMA. Applications: coronary stents (inserted compressed in martensite phase at room temperature, expand to programmed diameter at body temperature in austenite phase, opening blocked arteries without surgery); eyeglass frames that spring back to shape after bending; orthodontic wires that apply continuous gentle force as they return to shape. Pseudo-elasticity (superelasticity): some SMAs return to shape immediately on stress removal at constant temperature (no heating required).
• Photochromic materials: darken proportionally to UV light exposure through a reversible photochemical reaction. Applications: photochromic (transition) lenses, where glass lenses contain silver chloride (AgCl) molecules; UV exposure causes a reversible oxidation-reduction reaction forming colloidal silver that absorbs up to 80% of incident light; the lens clears when UV exposure decreases. Polymer lenses use organic photochromic dyes with similar reversible reactions.
• Electro-rheostatic (ER) fluids: suspensions of particles that form chain-like structures and become semi-solid (increase in viscosity by several orders of magnitude) when exposed to a strong electric field. The change is nearly instantaneous and fully reversible.
• Magneto-rheostatic (MR) fluids: suspensions of iron particles in oil that stiffen dramatically when exposed to a magnetic field. Applications: automotive suspension systems (2002 Cadillac Seville STS; Audi TT second generation), where varying the magnetic field strength changes the fluid's viscosity in milliseconds, stiffening for cornering and softening for comfort; seismic and wind dampers installed throughout the National Museum of Emerging Science in Tokyo.
• Thermoelectric materials: generate a voltage across a temperature gradient (Seebeck effect), enabling conversion of waste heat to electricity. The reverse (Peltier effect) uses electricity to create a temperature difference for solid-state cooling (no moving parts). Applications: industrial waste heat recovery; wearable sensors powered by body heat; solid-state refrigerators and CPU coolers.

智能材料通过显著且可逆地改变一种或多种特性来响应环境变化（机械应力、温度、光、电场或磁场）。这使得产品无需复杂的外部控制系统就能适应条件。

• 压电材料 — 受到机械应力时产生电荷（正压电效应），相反，施加电流时发生形变（逆压电效应）。应用：用于无损检测的超声波换能器（压电探头向管道发送声波；返回的回声使晶体受力，产生揭示内部缺陷的电压）；压力传感器；声呐；医学超声成像；扬声器和麦克风。
• 形状记忆合金（SMA） — 在较低温度下发生塑性变形后，加热时恢复预设形状。镍钛诺（镍钛合金，在海军武器实验室发现）是最常见的SMA。应用：冠状动脉支架（在室温下以马氏体相压缩形式插入，在奥氏体相体温下扩展至预设直径，无需手术即可打开堵塞的动脉）；弯曲后恢复形状的眼镜框；恢复形状时施加持续轻柔力的正畸钢丝。伪弹性（超弹性）：某些SMA在恒定温度下去除应力后立即恢复形状（无需加热）。
• 光致变色材料 — 通过可逆光化学反应与紫外线照射成比例地变暗。应用：光致变色（变色）镜片——玻璃镜片含有氯化银（AgCl）分子；紫外线照射引发可逆氧化还原反应，形成吸收高达80%入射光的胶体银；紫外线照射减少时镜片变清。聚合物镜片使用具有类似可逆反应的有机光致变色染料。
• 电流变（ER）液体 — 粒子悬浮液，在强电场作用下形成链状结构并变成半固态（粘度增加几个数量级）。变化几乎是瞬时的且完全可逆的。
• 磁流变（MR）液体 — 铁粒子在油中的悬浮液，在磁场作用下急剧变硬。应用：汽车悬架系统（2002年凯迪拉克STS；奥迪TT第二代）——改变磁场强度在毫秒内改变液体粘度，弯道时变硬，舒适行驶时变软；安装在东京国立新兴科学博物馆的抗震和抗风阻尼器。
• 热电材料 — 在温差下产生电压（塞贝克效应），实现废热到电力的转换。反向应用（珀尔帖效应）使用电力产生温差，用于固态制冷（无运动部件）。应用：工业废热回收；由体热驱动的可穿戴传感器；固态冰箱和CPU散热器。

Biodegradable materials are broken down by microorganisms (bacteria, fungi, algae) into water, carbon dioxide or methane, minerals, and organic matter: non-toxic substances that can re-enter natural cycles. This contrasts with conventional synthetic plastics, which persist in the environment for hundreds of years.

Examples include natural materials (wood, cotton, wool, paper, food) and engineered biodegradable polymers such as PLA (polylactic acid), derived from corn or sugarcane starch, which is used for biodegradable packaging, cutlery, and medical sutures.

Biodegradable materials and the circular economy: The circular economy's biological cycle depends on biodegradable materials re-entering natural systems safely after use. Designers who specify biodegradable materials are:

• Designing out waste: products that biodegrade do not accumulate as persistent pollution in landfills or oceans; they return material value to the soil as nutrients
• Closing the loop: materials flow: product → use → composting/biodegradation → soil nutrients → new plant growth → new material → product again
• Reducing dependency on finite feedstocks: biodegradable polymers sourced from renewable crops (corn, sugarcane, cassava) reduce reliance on fossil-fuel-derived plastics

Design considerations for biodegradability:

• The end-of-life pathway must match the material: some biodegradable plastics (PLA) require industrial composting conditions (high temperature and specific microbial environment) and will not break down in a home compost pile or a landfill within a reasonable timeframe
• Products labelled "biodegradable" must be accompanied by clear instructions for correct disposal; otherwise the benefit is not realised
• Biodegradable materials must still satisfy functional requirements during the product's useful life: premature degradation from heat, moisture, or UV exposure is a design failure
• Using biodegradable materials alongside non-biodegradable components (e.g., a biodegradable package with a plastic window) creates end-of-life complications that defeat the purpose

可生物降解材料被微生物（细菌、真菌、藻类）分解为水、二氧化碳或甲烷、矿物质和有机物——可以重新进入自然循环的无毒物质。这与传统合成塑料形成对比，后者在环境中持续存在数百年。

例子包括天然材料（木材、棉、羊毛、纸、食物）和工程可生物降解聚合物，如PLA（聚乳酸），来源于玉米或甘蔗淀粉，用于可生物降解包装、餐具和医用缝合线。

可生物降解材料与循环经济：循环经济的生物循环依赖于可生物降解材料在使用后安全地重新进入自然系统。指定可生物降解材料的设计师正在：

• 设计出废物 — 生物降解的产品不会在垃圾填埋场或海洋中作为持久污染物积累；它们以养分的形式将材料价值归还土壤
• 闭合循环 — 材料流：产品→使用→堆肥/生物降解→土壤养分→新植物生长→新材料→再次产品
• 减少对有限原料的依赖 — 来源于可再生农作物（玉米、甘蔗、木薯）的可生物降解聚合物减少了对化石燃料衍生塑料的依赖

生物降解性的设计考虑因素：

• 报废途径必须与材料匹配：一些可生物降解塑料（PLA）需要工业堆肥条件（高温和特定微生物环境），在合理时间内不会在家庭堆肥堆或垃圾填埋场中分解
• 标有"可生物降解"的产品必须附有正确处置的清晰说明；否则无法实现其优势
• 可生物降解材料在产品使用寿命期间仍必须满足功能要求——由于热、湿气或紫外线辐射导致的过早降解是设计失败
• 将可生物降解材料与不可生物降解组件一起使用（例如，带塑料窗口的可生物降解包装）会产生报废复杂性，从而破坏其目的