An Article to Help You Understand the Mechanical Properties of Materials: Strength, Hardness, Toughness, Brittleness…

In structural engineering, when we select the right material for a project or product, it is very important to choose this material, not that material, based on the mechanical properties of the material as the basis, this article will take you to understand the basic mechanical properties of materials: strength, hardness, toughness, brittleness…

★ Basic Concepts

Knowledge learning begins with concepts, which are the smallest units of knowledge. Understanding something, a subject, requires an understanding of many basic concepts. Therefore, to learn about the mechanical properties of materials, we need to first understand the relevant core concept and what this concept expresses. With this starting point, what follows will be much easier.

No.	Properties	Definition
1	Strength	The ability of a material to resist damage under the action of an external force.
2	Hardness	The ability of a material to resist local plastic deformation. The ability of a material to resist scratching, cutting, abrasion, indentation, or penetration.
3	Stiffness	Stiffness refers to the ability of a material or component to resist deformation under stress, which is a representation of the difficulty of elastic deformation and also the force required to cause unit displacement.
4	Flexibility	Flexibility, also known as slenderness ratio, is denoized as λ, which refers to the size of deformation along the vertical axis of the component under axial stress. It is the reciprocal of the stiffness.
5	Fatigue	Fatigue damage refers to the phenomenon of material failure under the stress which is far below the strength limit or even the yield limit of the material.
6	Toughness	Toughness, indicating the ability of a material to absorb energy during plastic deformation and rupture.
7	Brittleness	Brittleness refers to the property that material breaks under the action of external force (such as tensile impact, etc.) with only a small deformation.
8	Elasticity	Elasticity refers to the property that an object can recover its original size and shape after deformation, which is expressed by elastic modulus E.
9	Plasticity	Plasticity is the ability of an object to deform. When the external force is small, the object undergoes elastic deformation, when the external force exceeds a certain value, the object produces irrecoverable deformation, which is called plastic deformation.
10	Ductility	Ductility refers to the ability of a material or component to continue to carry after reaching a state of damage until it reaches its ultimate load carrying capacity. This is the ability to maintain deformation at a certain load carrying capacity.

★ Basic Characteristics

To better help you understand these mechanical properties, I have selected 10 common motion picture scenes from everyday work or life as a reference to further describe their basic characteristics, pass them on to your friends to learn from each other.

Strength: the material must be able to withstand the forces applied in the application scenario without bending, breaking, shattering or deforming.

Hardness: harder materials are generally more resistant to scratches, durable, and resistant to tears and indentations.

Stiffness: a material with good stiffness is less prone to deformation.

Flexibility: A greater degree of flexibility results in greater deformation and poorer stability of the component.

Fatigue: A material with high fatigue is of good quality and lasts longer.

Toughness: the tensile and impact resistance of the material, the better the toughness, the smaller the possibility of brittle fracture.

Brittleness: as opposed to toughness, the greater the brittleness, the material will be damaged at very little deformation.

Elasticity: The ability of a material to absorb force and bend in different directions and return to its original state.

Plasticity: Relative to elasticity, the better the plasticity, the deformation of the material will maintain the shape after deformation.

Ductility: The ability to be stressed and deformed in the direction of length. For seismic structures, materials with good ductility performance should be used.

★ Connections and Distinctions

After understanding the basic concepts and characteristics, it is even more important to understand the connections and distinctions between them in order to gain a deep understanding of the properties of materials or components and to better apply them to practical production life.

First of all, the specialties of different materials are different. In general, in material science, ceramic hardness is high, metal strength is high, polymer plasticity is good and so on, because they have different material structures (from microscopic to mesoscopic) and different chemical bonds, and there’s so much to talk about in that. You can see what is said in Fundamentals of Materials Science, which is written in great detail.

1 The relationship between strength and plasticity

Strength refers to the maximum amount of force that a material can withstand. Plasticity refers to the percentage of the material that can be deformed to a maximum. For example, if a steel bar can withstand a maximum force of 100Mpa, i.e. its strength is 100Mpa, and if under a force of 100Mpa it deforms by 20% and breaks, then its plasticity is 20%.

In industry, a typical situation where high strength and high plasticity are required is in the structural components of a car. On the one hand, we want it to be able to withstand more forces, and on the other hand, we want the structural components to be able to deform to a large extent in the event of a collision, so that they can absorb energy and protect the passengers. For example, we want a structural component to be able to withstand a pressure of 2,000MPa and at the same time deform by up to 60% without fracture. (Energy absorbed = force on the structural member x degree of deformation of the structural member) This is, in fact, toughness. Toughness is the amount of energy absorbed by a material during deformation and is usually represented by the integral under the curve in a tensile test diagram, i.e. the area, as shown below.

Generally speaking, the strength and plasticity of a material cannot be met simultaneously, they are like two sides of the same coin: an increase in strength usually leads to a decrease in plasticity. Research has shown that plastic deformation of metallic materials is usually achieved by dislocation slip. During work-hardening, the metal is plastically deformed, the grains slip, and dislocations become entangled, causing the grains to elongate, break and fibrilise, preventing further deformation and consequent failure and fracture.

2 Elasticity and plasticity are relative

Elasticity is simple, after the withdrawal of external forces deformation can be fully recovered; plasticity means that the material has plastic deformation, after the withdrawal of external forces deformation can not be fully recovered, there is residual plastic deformation. For example, the elongation index is used to evaluate the plasticity of steel. After a steel specimen is pulled off, the elastic deformation will recover, while the residual plastic deformation, so the elongation can be used to evaluate the plastic deformation capacity of steel.

3 Stiffness, ductility, and plasticity

Firstly all three are concepts that measure the degree of deformation. Stiffness is the value of load/displacement in the elastic phase, which is EI, a measure of softness and stiffness. Ductility and plasticity are deformations in the inelastic phase, the ductility coefficient can be calculated quantitatively and plasticity is a qualitative concept.

4 Toughness = strength + plasticity

Toughness refers to the energy absorbed by the material from force to fracture, the more energy consumed to make the material fracture, the better the toughness. The consumption of energy means that work is to be done on the material outside the system, which then indicates the presence of force and displacement (deformation). The ability to withstand stress is characterised by strength and the ability to deform is characterised by plasticity. So a ductile material has good plasticity.