What type of defect is detectable by dye penetrant inspection?

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Penetrant inspection works by letting a colored liquid enter defects that open to the surface and then drawing it back out with a developer to form a visible indication. Because the penetrant only reveals flaws that connect with the surface, it is especially good at showing surface-breaking defects such as cracks, laps, and seams. It won’t reliably detect flaws entirely internal, like internal porosity or subsurface delaminations, nor metallurgical zones that don’t create a surface opening. So the method best identifies surface-breaking defects such as cracks.