Title: Summary Part Mechanical Properties

Choose the correct statement.

- The tensile test gives the most basic mechanical characteristics, including modulus of elasticity, yield strenght, tensile strength, ductility and toughness. (correct)
- The tensile test gives the elastic properties of materials, including modulus of elasticity and Poisson's ratio. Hardness tests are required to measure plastic properties such as strength.

- The fundamental mechanism of elastic deformation is the stretching of atomic bonds. (correct)
- The fundamental mechanism of elastic deformation is the movement of dislocations.
- The fundamental mechanism of elastic deformation is opening and closing of defects in crystals (grain-boundaries, vacancies etc.)

- The hardness test is a simple alternative to the tensile test. Several hardness tests are available to determine the resistance of a material to permanent indentation or to scratching. Hardness is related to strength and wear resistance of a material, but hardness is not a fundamental property. (correct)
- The hardness test is a simple alternative to the tensile test. Several hardness tests are available to determine the resistance of a material to permanent indentation or to scratching. Similar to strength, hardness is a fundamental materials property.

- Plastic deformation in metals takes place by a slip mechanism. Although the theoretical shear stress required to cause slip is very high, actual required stresses are much lower because of the presence of dislocations. (correct)
- Plastic deformation in metals takes place by a slip mechanism. Because the theoretical shear stress required to cause slip is very high, metals and other crystalline materials such as ceramics are very strong.

- Dislocations play a critical role in the plastic deformation of metals. They are one-dimensional defects that facilitate atom displacement by slipping in high-density atomic planes along high-density atomic directions. (correct)
- Dislocations play a critical role in the plastic deformation of metals. They are vacancies (point defects) in the crystal lattice that facilitate easy movement of atoms as they provide space for atom-hopping.

- Simple and symmetrical crystal structures with many high-density atomic planes (slip systems) – such as the face-centered cubic (fcc) lattice structure – are more ductile than more complex structures with less high-density atomic planes – such as the hexagonal close-packed (hcp) structure or many ceramic structures. (correct)
- Most crystal structures feature several high-density atomic planes (slip systems) where dislocations can move. The difference in ductility between the different metals and between metals and ceramics is mainly owed to the presence of defects (many defects: brittle, less defects: ductile).

- Dislocations can become entangled with one another or are impeded by barriers such as grain boundaries, impurities, and inclusions. As a result, the shear stress required to cause further slip is increased. Consequently the overall strength of the metal is increased. (correct)
- Dislocations can move relatively easy and freely on their slip systems. As the dislocations hardly interact with defects in the crystal lattice, most metals are strong even though the crystal lattice is often full of defects.

- Ceramics and glasses are characterized by brittle fracture. Nevertheless, they are relatively strong in compression, but relatively weak in tension. (correct)
- Ceramics and glasses are characterized by brittle fracture. Thus, although their atomic bonds are amongst the strongest (covalent and ionic), they are relatively weak both in tension and compression.

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