Final answer:
Discontinuities in materials, often seen as defects, are not necessarily flaws. They can be intentionally introduced to improve a material's properties, such as the electrical properties via doping in semiconductors, or to enhance ductility in metals.
Step-by-step explanation:
The statement that 'discontinuities are always flaw or defects' can be considered as somewhat misleading. While it is true that discontinuities in a solid generally refer to defects or imperfections, they are not inherently flaws. In the context of crystalline solids, defects such as vacancies, interstitial atoms, and substitutional impurities can be deliberately introduced to alter the material's properties favorably. For instance, doping is a process where impurities are added to a semiconductor material like silicon to improve its electrical properties. Therefore, not all discontinuities are detrimental; some are essential for technological applications. Another example involves the role of line defects or dislocations, which can enhance the ductility and malleability of materials like metals, facilitating plastic deformation. As such, the defects and their densities are crucial for understanding and harnessing the mechanical properties of structural materials.