I look after technical sign-off for a fabricator, and every supplier hands me a glossy datasheet with exactly the numbers I want to see. When does trusting that stop being good enough and I actually have to pay for independent testing?
A datasheet is usually a fair starting point, but the figures on it are generated under controlled lab conditions on the manufacturer's own samples, which may not match your substrate, build-up or service environment. For indicative properties and early comparison between products, it is generally fine to trust it. Independent testing earns its cost the moment the result becomes load-bearing in a safety, warranty or regulatory sense, so structural capacity, reaction to fire, or anything an insurer or building control will hold you to. It also matters when the datasheet value was measured to a different standard, or on a different build-up, than the one you are actually specifying. If the claim carries real consequence, the safer move is to get a representative sample tested rather than lean on the sheet alone. Materials testing can confirm the specific properties that matter for your job.
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