Final answer:
The yield strength of an orthodontic wire is higher than the proportional limit, which is the stress level beyond which permanent deformation begins. Work hardening increases yield strength, contradicting the idea that it decreases it.
Step-by-step explanation:
The yield strength of an orthodontic wire is an important concept in materials engineering. It refers to the stress level at which a material, such as the wire used in orthodontics, begins to deform plastically. When a load increases from zero, the stress and strain are directly proportional, which is described by Hooke's law until a certain limiting value, known as the proportional limit. Beyond this point, the material will undergo permanent deformation.
The correct answer to the question 'The yield strength of an orthodontic wire is' would be D. higher than the proportional limit. This is because, beyond this point, the material no longer behaves elastically and begins to deform permanently. It is worth noting that work hardening, which involves strengthening a material by plastic deformation, can increase the yield strength of a material. Therefore, option B is incorrect as it suggests the opposite effect. Additionally, the yield strength is different from the stress at fracture (ultimate stress), eliminating options A and C.