What characterizes internal flaws in a material?

Internal flaws in a material are characterized by defects that are located within the bulk of the material, which are not visible on the surface. These flaws can arise from various factors, such as inclusions, voids, micro-cracks, or changes in the material's structure that occur during the manufacturing process or throughout the material's life. Because they are internal, these defects may significantly affect the performance and integrity of the material under stress, yet they cannot be detected simply by inspecting the outer surface.

This understanding is crucial as internal flaws can lead to issues such as failure under load, fatigue, or other forms of structural compromise that may not be evident until the material is subjected to real-world conditions. Recognizing this allows engineers to incorporate non-destructive testing methods to identify and assess these internal flaws, ensuring the reliability and safety of the materials being used in engineering applications.