It is not necessarily the material that has the highest tensile strength that is the best for a product. A material used to form a lightweight cover to be carried, shaped into a complicated configuration and used externally, might be strong enough to carry the applied load but might be too heavy, difficult to make, susceptible to corrosion, or more expensive than required. The goal of material selection is not to find the highest number on the list of material properties but to select the material that has a combination of properties required for the service conditions.
Select an everyday component, such as a food container, bicycle frame, saucepan, mobile phone case or window glazing. List the functional requirements or duties of the component when it is performing its function. Include the types of loading it experiences, temperature range, humidity, acceptable mass, surface wear, necessary shape and probable manufacture. Only then should a choice of candidate materials be undertaken. By performing the functional requirements first, it prevents the single attractive property in a material from dictating the decision.
Strength is defined as resistance to yielding or fracture under a specific type of loading and does not necessarily encompass all aspects of useful property. A structural component that is expected to remain stiff may need to be not just strong but also stiff enough. A component that must protect its surroundings may have to be tough so that it can absorb energy without brittle failure. A sliding component may need to be hard and wear resistant. A component subject to contact with water or a corrosive environment may need to be corrosion resistant even though the mechanical loading may be relatively low.
Even strength depends on other properties. For example, while steel is stronger than aluminum, an aluminum alloy can provide a useful combination of strength and light weight for an application that is sensitive to weight. The lightest material on the list is also not the correct choice for an application. For example, some low-density plastic will creep or soften with sustained load or high temperature, whereas a more expensive metal may maintain its shape and not expand as much as adjacent components. The question is not whether to pick a stronger material or the highest density material, but whether to choose the material that provides adequate strength without causing another issue.
Manufacturing or processing of a material also influences the final product selection. A high-strength material may be selected after a specific heat-treatment process, but more manufacturing steps and tighter tolerances will then be needed. Materials that are hard, thermally stable and wear resistant, such as ceramics, may be too brittle or too difficult to machine to be the appropriate choice. The ease of manufacture of polymers in complex shapes and the ease of forming metals may lead to a final product that performs adequately. A material made up of a polymer matrix and fiber, such as a composite material, can have a directional characteristic or may require special repairs.
Material property data helps to guide a material selection process, but it is necessary to be aware that the value in each property column may vary depending on other factors. Consider the units used in each column, alloy type, processing conditions, temperature used during testing, direction of loading, type of specimen used and range of values presented. Each value in a property chart may represent a range of alloys or processing conditions, each leading to different grain structures, porosity levels or orientations of fiber reinforcement in the case of a composite material. This means that a material property in a single value should not necessarily be representative of all examples of the manufactured material.
Material selection at a fundamental level will often result from an elimination process rather than selecting one material over another for its superior properties. List the performance requirements needed to fulfill the function, examine several property values for each possible material, consider processing methods, service environment and acceptable trade-offs. When an exceptionally high value appears on the list of material properties that suggests a superior choice, take a closer look at what condition this property value may not cover.