Beyond single-property retention: how multifunctional composites lose serviceability under irradiation

Retention of a single measured property does not, by itself, establish the serviceability of an irradiated multifunctional composite. This critical narrative review examines differential property evolution in radiation-shielding composites, silicon-carbide and carbon-based thermostructural composites, and fusion-magnet insulation. Comparisons are made within documented material and irradiation conditions, without treating absorbed dose, particle fluence and displacements per atom as a universal damage coordinate. Selected SiC/SiC studies show substantial thermal-transport degradation alongside retained flexural strength; C/SiC can stiffen while losing strength and deformation capacity. Cryogenic insulation illustrates a different limitation: a high measured breakdown voltage may coexist with structural damage, and the test medium can change the failure path. Direct post-ageing attenuation measurements in PP/PbO composites show reported dose-dependent differences between shielding, tensile and impact responses, but do not establish a universal failure order or component lifetime. Serviceability is defined at component level, relative to specified functions, loading and environmental conditions. A signed margin expresses each requirement relative to its own acceptance boundary; the first exhausted margin, not necessarily the fastest-changing property, limits serviceability under this convention. The review distinguishes measured response, validated surrogate inference and component qualification, and provides a reusable failure-route test matrix. Quantitative comparisons require appropriate uncertainty treatment, traceable irradiation histories and explicit post-irradiation conditioning. Where acceptance limits or adequate source statistics are unavailable, differential response remains informative but cannot be promoted to a demonstrated service-limit crossing.