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了解强度与刚度与硬度的机械性能是机械工程中的基础，但是通常会混淆这些特性。这些属性是相关的，但是它们之间存在非常重要的差异：
- Stiffness是倾向于一个元素的一个指标return to its original form after being subjected to a force.
- 力量测量在元素永久变形或断裂之前，可以将多少应力施加到。
- 硬度measures a material’s resistance to surface deformation. For some metals, like18beplay下载 , hardness and tensile strength are roughly proportional (see ASTM A 370-68 Steel Tables).
For this article, we’re doing a refresh of the fundamentals, looking at some common pitfalls in identifying between mechanical strength vs. stiffness vs hardness, the technical differences between the three, and their relevance in the selection of材料s for product development.
Common Pitfalls: Strength vs. Stiffness vs. Hardness
It can be especially easy to confuse the difference between strength and stiffness. If a material is stiff, then it must be strong enough to support a large load, right? Therefore, aren’t strength and stiffness the same? The answer is a resounding no! A material can be strong and elastic, or strong and stiff (See section “Axial Loading on a Straight Member”), but strength and stiffness are two separate qualities.
Let’s break it down.
Since stiffness and strength are both related to the elastic modulus of a material (Young’s Modulus), it’s easy to look up the value in a material properties reference handbook and determine the strength and/or stiffness of given material. The elastic modulus measures the stiffness in a material, but strength is a function of the modulus. Both tensile strength and hardness are indicators of a metal’s resistance to plastic deformation.
The stress-strain curve (Figure 1) above is a good way of graphically demonstrating this. The ratio of stress to strain is the elastic modulus—stiffness, but the stress, and only the stress, defines the strength of the material. Remember, strength measures how much stress the material can handle before permanent deformation or fracture occurs, whereas the stiffness measures the resistance to elastic deformation.
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Understanding the Concepts of Stress and Strain
Stress is an internal force resulting from an applied load; it acts on the cross-section of a mechanical or structural component. Strain is the change in shape or size of a body that occurs whenever a force is applied.
力量，僵硬和硬度的重要定义
Here are a few important definitions to keep top of mind:
- 产量强度is used in materials that exhibit an elastic behavior. It’s the maximum tensile stress the material can handle before permanent deformation occurs.
- Ultimate strength指发生故障之前的最大压力。
- 断裂强度是对应于发生总失败的应力的值。
- Stiffnessis how a component resists elastic deformation when a load is applied.
- 硬度是对局部表面变形的抗性。
材料的强度可以指产生强度，最终强度或断裂强度。拉伸强度可以根据硬度计算，并且很方便，因为硬度测试（例如Rockwell）通常很容易做，便宜且无损。在样品上仅执行少量渗透。对于许多金属，随着硬度的增加，拉伸强度会增加。
Retrieving Mechanical Property Data
Mechanical, thermal, or electrical property data may be obtained for specific material types from the American Society for Testing Materials (ASTM), or consult the manufacturer’s website from which you’re planning to purchase materials.
*所有材料并非如此，而是许多金属材料，硬度与拉伸强度非常相关。有关详细信息，请参见ASTM A 370-68表。
A reliable online source iswww.matweb.com，可以快速获得准确的数据。
示例：力量与刚度
梁挠度
The image below is a finite element analysis (FEA) of a beam subjected to a loading. The beam’s response to the loading is a deflection resulting from internal bending and shear stress. If the internal stresses exceed the yield strength of the beam’s material, then you’d want a material with a higher Young’s modulus value—in other words, a stiffer material to resist the deformation and also higher yield strength to prevent permanent deformation and possible fracture.
Axial loading on a straight member
想象一下张力中的金属条，承受一定值的轴向负载。内部应力会导致金属以100 lbf破裂时的原始长度的变形为1％。接下来，考虑一些类似橡胶状的棒状聚合物，其遭受相同的力，但其原始长度变形为5％，并以100 lbf的形式断裂。关键点：两种材料都同样强，但是金属比聚合物更硬。
热应力
热应力also affects the deformation response of a component who’s already subject to some force. In this case, due to a temperature gradient within the member, the thermal stress induces additional strain to the strain already imposed by forces acting on it. Thermal stress has effects on the strength of materials. Computing thermal stress provides insight to select what values of stiffness and strength are appropriate for your design, assuming the temperature difference is not large enough that it will change the microscopic properties of the material!
设计强度和刚度的4种最佳实践
Here are 3 best practices to keep in mind throughout the design process:
1. Identify important components in the early stages of your design
在设计的早期阶段，例如构造CADmodel, determine which components will be critical to your design. Determine which ones will be subjected to impact loadings, uniform loadings, concentrated loads, constant loads, etc., so you get an idea of the stiffness or strength you need for your components. If needed, develop graphical representations of complex dynamical systems using bond graph or block-diagram modeling, and utilize software to model sophisticated systems.
2. Determine applied forces to each component
Calculate the expected stresses on mechanical components to find out where problems may arise or where forces may seem large. Measure how the design responds to various inputs using instrumentation techniques and perform data analysis to predict your model’s behavior. Consider creep and fatigue variables in subsystems and associated with the environment to which your system will be exposed.
3. Best practices for material selection
Metals, ceramics, polymers, and composites are material types with specific mechanical properties. Ceramics are usually brittle, meaning that there’s hardly any deformation before fracture; cracks may spread extremely rapidly, with very little accompanying plastic deformation. Metals exhibit two modes: ductile or brittle. A ductile material is accompanied by plastic deformation before fracture, while brittle materials are not.
延性金属的断裂过程通常在几个阶段发生 - 聚合物中的细胞 - 细胞或脆性行为取决于温度。对于原材料，确定哪些制造工艺将为您提供所需的材料特性。材料的测试必须符合ASTM标准；咨询工程手册以获取可靠的材料数据。
4.在原型制作之前验证您的设计
使用CAD软件评估和优化设计的几何形状。使用FEA分析，数值分析；并且在可行的情况下，执行手动计算确保结果在开始原型以节省时间和金钱之前达成一致。咨询其他工程师以进行审查。
主要要点
The proper use and implementation of stiffness, strength, and other mechanical properties throughout your design process will contribute to higher-quality products, since these properties characterize how a材料将响应施加的负载。对产品的预期使用的清晰了解是选择适当的材料特性的关键，这些材料特性将促进具有成本效益的设计，同时保持安全标准，结构完整性和实现所需的性能。
