radiographic testing
Radiographic testing is a medical X-ray for engineering parts: it shines penetrating radiation through an object onto film or a digital detector, and the shadow it casts reveals what is inside. Where the part is solid, less radiation gets through and the film stays light; where there is a void, a crack, or a low-density inclusion, more radiation passes and that spot shows up dark. You are, quite literally, looking at the object's internal shadow.
A source on one side — an X-ray tube, or a gamma-ray source like iridium-192 for field work — sends rays through the part to film on the far side. The amount absorbed depends on thickness and density, so a gas pore (less material to absorb) prints as a dark spot, and a denser inclusion prints lighter. Crucially, radiography sees a flaw best when it has depth ALONG the beam — a rounded pore images clearly, while a tight planar crack shows up strongly only if the beam runs along its plane, and can be missed if it lies across the beam.
Radiography gives a permanent, area-wide picture of internal soundness — it is the classic way to qualify welds, castings and pressure parts, and its film is a durable record. Honest limits and cautions: it is exactly opposite in blind spot to ultrasonics (great on volumetric pores, weak on flat cracks not aligned with the beam), it gives little depth information (the shadow is a flattened projection), and it uses ionising radiation, so it demands strict radiation-safety controls and exclusion zones.
A radiograph of a weld shows a string of round dark spots along the seam — trapped gas porosity — plus one sharp dark line where the beam happened to align with a crack; the film becomes a permanent record of the weld's internal quality.
A penetrating-ray shadow prints internal voids dark on film — best for volumetric flaws.
Radiography and ultrasonics have opposite blind spots: radiography excels at rounded voids but can miss a tight crack lying across the beam, while ultrasonics is the reverse. And it uses ionising radiation, so it is never casual — safety zones and dosimetry are mandatory.