Materials Selection Criteria and Methods

Materials selection criteria and methods

Product design and selection methods

What is design? What do you do when you design things? It means making things starting from materials. There are products that are purely functional (we are not interested in them), but there are also products that are more symbolic and decorative. However, they are not too relevant for us. In contrast, there are products that combine both these parts, which are functional and symbolic or decorative, and we care about them.

Most products combine:

• Technical features (functionality)
• Aesthetical features (appearance)

The role of materials and technologies is to guarantee technical functionality and define the product personality. Within this course, we will learn how to properly select suitable materials and shaping technologies. This is a complex process based on multiple interactions: material science/research labs, material suppliers, and design/designers. We will also explore what information is required on the materials, how technology comes into play, and what selection methods are available.

Designers can usually operate on a product by following three methods: find new needs, find new responses to existing needs (create new products), or through the modification of existing products (shape/materials, adjust existing products).

Phases of the design process

In which phase of the design process do we choose the material? Let’s see the different phases of the designing process.

The first section of the process is about product design where both technological and aesthetical requirements are defined. It is composed of: concept (objectives/constraints), material screening, sketch, stress analysis, design and material selection (identify the best material).

The second one is about technology design where dimensions are defined. It is composed of: choice of shaping technology, equipment design, and set processing parameters.

The third section is about prototyping, while the last one is about production (molds and processing conditions + user feedback) and is composed of: executive design, equipment setup, production, and quality control. We should always consider the human factor within the design process since human beings are not reliable (they are not machines).

Concept

Important: remember that processes are associated with requirements while materials with properties. In the exam, the compatibility with food could be one of the requirements for the product. The professor will ask us, depending on a particular use, to rank some materials. In this course, we mostly care about the concept and the material screening phase.

The concept phase is about defining function (what is the product’s purpose? Rod: tensile loading, shaft: torsional loading, beam: flexural loading, or spring: storage of elastic energy), goals (what is to be minimized/maximized? Cost, weight, safety, or insulation) and constraints (what conditions have to be met? Are they mandatory? Are they a plus but not essential?).

Process of selection

The process of selection has four main phases:

• Screening: sorting out unsuitable alternatives. It means finding the limits on properties (go/not go)
• Ranking: seeking the best candidate. The designer does that by using material indexes or graphical methods
• Fine tuning: material producers, laboratory tests
• Local conditions: it’s necessary to check the local conditions and the know-how ability (environment affects materials and their behaviors)

Materials have, of course, different and specific properties, structures, processing, and applications. Some choice criteria could be the ideology, costs, recycling, or supplies. Through mechanical testing and optical measurement, the designer can define the general requirements which are physical phenomena described by mathematical equations.

A property is a characteristic/material parameter that relates an input (stress in this case) to an output (deformation). Behind Hooke's law (F = -kx), Robert Hooke, a British physicist who gave his name to it, demonstrated a relationship between the forces applied to a spring and its elasticity. Thanks to this experience, he noticed that the stress vs strain curve for materials such as skin has a linear region.

One century later, Hooke’s law could relate strain to stress in the linear elastic domain, thanks to a modulus named Young’s modulus (E, stiffness GPa). Young’s modulus is a numerical constant that describes the object’s resistance to being deformed elastically while an external force is applied to it. It quantifies the relationship between tensile/compressive stress (σ = F/A) (force per unit area MPa) and axial strain (ε= ΔL/Lo) (deformation) in the linear elastic region of a material and is determined using the formula: E=σ/ε. The modern theory of elasticity generalizes Hooke's law to say that the strain (deformation) of an elastic object or material is proportional to the stress applied to it (σ =E ε).

Information required to properly design a product

• Technical: using material technical data for physical properties, mechanical properties, thermal properties, or electrical properties, typically given as datasheets, graphs, or maps.
• User-related: aspects linked to product ease of use such as ergonomics and user interface. They are not specifically related to materials but more to design.
• Sustainability: aspects linked to the production of the material such as resources needed or available, energy consumption, emissions caused by the process or extraction, and the end of life (if the material or product is disposable).
• Sensorial: related to human perception. Touch (warm/cold, hard/soft, smooth/textured), sight (transparent, color, gloss), hearing (acoustic brightness and pitch), and smell/taste (food-drink packaging). They are strongly linked to individual perception.

How does technology come into play?

Both the technical (engineering) and aesthetical role (design) drive the creation of the final shape and joining of the product. Let’s see the different aspects of both fields.

Technology, engineering aspects: shaping, joining (mechanical joints, welding, or adhesives), surface and superficial finishing in order to enhance scratch/mar resistance, corrosion resistance, and visual or tactile perception.

Technology, design aspects: technology can modify the user’s perception of the product through both senses and expression (expression through shaping, joining, or surfaces).

What selection methods are available?

What is the purpose of a selection method? Three steps should be followed:

• Definition of product specifications
  • Product requirements: technical specifications which define how the product works and how the product is made (mechanical and thermal performance, durability, cost, etc.) and aesthetical specifications that define the impact of the product and the character of the product (visual and tactile features, feelings, and connections).
• Selection method
  • Selection by analysis: based on deductive reasoning, typical of engineering design. Input: function, objectives, constraints.
  • Selection by synthesis: based on inductive reasoning, based on previous experiences and analogy. Input: intuition, features, perceptions.
  • Selection by similarity: driven by the need to replace a material already in use (e.g., to comply with new regulations). Input: a previously existing solution.
  • Selection by inspiration: originated by interaction with a material, a product, an image. Input: curiosity, serendipity. Typically very bright ideas but the process is very hard to control. Living as an inventor is (even) harder than living as a designer!

Remember that different methods can (should) be combined for the best outcome.

Selection by analysis

• Definition of product requirements
  • Function: what is the product’s purpose? To bear loads? Store energy? Absorb impacts?
  • Constraints: mandatory conditions. Geometrical (shape/dimensions), structural (stiffness/integrity), environmental (operating temperature, agents), and annual production.
  • Objectives: properties to be minimized/maximized such as cost, weight, and environmental impact.
• Identification of material properties
  • Requirements must be associated with one or more relevant physical phenomena.
  • Physical phenomena can be described by mathematical models (Fourier’s equation).
  • Material properties relate physical variables (density, specific heat, thermal conductivity).
• Screening of candidate materials
  • It is important to source data on relevant properties such as material databases, technical/scientific literature, material data sheets (producers), and laboratory tests.
• Materials ranking
  • Remember that in general, the ideal material does not exist. One possibility we have is to select the material thanks to Pareto’s criterion. As shown in the picture, how can we choose between A, B, C, and D? The other two materials (and possibly A) are ruled out by Pareto’s criterion.
  • In contrast, when we have to compare non-homogeneous quantities (such as mass and cost) we can exchange rates, which depend on the specific application (mass, energy) and can be set by actual market prices or by existing regulations.
  • A more general method for materials ranking is needed to compare several specifications and properties at once. A score system can be used to compare different materials according to a set of relevant properties. For each property, minimum and maximum values are defined on a 0-10 scale. There are different ways to suitably identify minimum and maximum values:
  • Physical considerations: Tmax > 100°C for something that needs to be sterilized, transparent, or chemical resistant.
  • Order of magnitude: E in the MPa (rubber) or GPa (rigid plastic) range.
  • Existing solutions: values of materials currently used for the same product.
  • Relative comparison: according to min-max values of available materials.