Material Selection and Mechanical Properties for Industrial Use
Materials and Industrial Processes
In any industry, a specific project must be developed before its execution. It is within this project that the materials to be used are chosen. Several criteria should be considered for material selection:
- Properties: Intrinsic characteristics of the material.
- Aesthetic Qualities: Visual and tactile appeal.
- Manufacturing Process: Assess the availability of necessary machinery, operator skill, and required techniques for working with the material.
- Cost: Not all materials are abundant or easy to obtain, which significantly impacts their cost.
- Availability: Consider market supply and the material’s expected lifespan.
- Environmental Impact: Upon the product’s end-of-life, evaluate its potential for reuse or recycling, and the ecological consequences of its disposal.
Mechanical Properties of Materials
Mechanical properties describe how materials behave under the application of external forces. Each material exhibits unique behavior when specific external forces are applied. Understanding these properties is crucial for selecting the most appropriate material for any given application.
Mechanical Strength and Tensile Testing
Mechanical strength is a material’s ability to withstand forces or deform without fracturing. Loads can be applied to a material in various ways:
- Tension: Pulling forces that stretch the material.
- Compression: Pushing forces that squeeze the material.
- Flexion: Bending forces.
- Torsion: Twisting forces.
- Shear: Forces that cause parts of the material to slide past each other.
The shape of a material also significantly influences its mechanical strength. Different shapes are more suitable for supporting specific types of stress.
Deformation Models and Mechanical Behavior
When a material is subjected to stress, it can undergo either temporary (elastic) or permanent (plastic) deformation. If the deformation is temporary, we speak of elastic deformation. If it is permanent, we refer to it as plastic deformation.
Some materials exhibit brittle behavior, breaking without experiencing significant deformation. In contrast, materials with ductile behavior deform considerably before fracture.
In structural design, it is crucial to ensure that a material will not exhibit brittle behavior. Deformation prior to breaking or cracking serves as a vital warning, allowing for the detection of potential structural collapse and the prevention of accidents.
Tensile Test Concepts
To ensure that the values obtained from tensile tests depend solely on the material and not on the dimensions of the specimen, concepts like stress (unit effort) and strain (uniform elongation) are used.
Stress (Unit Effort)
Stress is the relationship between the applied force (F) and the cross-sectional area over which it is applied.
Strain (Unit Elongation)
Strain is the ratio of the change in length (elongation) to the original length (L₀) of a specimen.
Stress-Strain Diagram (Tensile Diagram)
The stress-strain diagram is generated from tensile tests. Standardized specimens are subjected to increasing tensile forces in laboratory machines until fracture.
Elastic Zone
Deformations produced in this zone are elastic, meaning the material returns to its original shape once the load is removed. The value of the elastic modulus (Young’s Modulus) can be interpreted as the stiffness of the material.
Plastic Zone
Beyond the initial elastic deformation, permanent deformations begin. Point B on the diagram represents the elastic limit, which is the maximum stress a material can withstand without experiencing any permanent deformation.
The maximum stress value used in the design of a component is often known as the yield strength or maximum working stress.
In the section from the elastic limit (Point B) to Point C, the material undergoes significant elongation with little increase in stress, often referred to as yielding or plastic flow. In the section between points C and D, strain hardening occurs, where the material requires increased stress to continue deforming. Deformations in this section are always permanent.
Point D marks the ultimate tensile strength (UTS) or tensile strength, which is the maximum stress the material can withstand before necking begins and eventual fracture. As the specimen begins to neck (localize deformation and thin), the apparent stress required to continue deformation decreases until Point T, where the sample finally fractures. The total elongation at fracture is the total permanent deformation the sample experiences just at the moment of break.