Final answer:
The dark spots observed in the radiograph of a welded joint likely indicate surface porosity, which are small cavities caused by trapped gases and appear darker due to less attenuation of X-rays.
Step-by-step explanation:
The radiograph showing numerous round and elongated dark spots in the weld and the parent material adjacent to the weld is likely to indicate surface porosity. This is a common defect in welding, characterized by the presence of small cavities or pores resulting from gases trapped in the molten weld metal. The appearance of these dark spots on the radiograph is due to the lower density of the porosity compared to the surrounding material causing less attenuation of the X-rays and hence appearing darker on the film.
Surface porosity can adversely affect the mechanical properties of the weld, such as its strength and toughness. The high melting point of tungsten, its use in X-ray tubes due to its ability to withstand high temperatures, and the need for significant accelerating voltage to create inner-shell vacancies in heavy elements like tungsten are factual aspects relevant to understanding radiographic imaging but do not directly describe the defects found in welds.