Conditional Sensitivity and Structural Identifiability of a PLA/GNP Interlayer Strength Surrogate

Strength-based calibration of an interlayer welding model can constrain a combined response while leaving its physical cause unresolved. This study examines a conditional PLA/graphene nanoplatelet surrogate combining empirical reinforcement, transient wall heat transfer and assumed healing kinetics. The analysis converts a multiplicative identifiability result into an experimental exclusion rule: with shared contact and mobility, a fixed exponent and unsaturated healing, adding temperature or return-time conditions cannot separate those factors. An independent contact or rheological observable is needed within this parameterization. A separate resistance calculation tests whether a composition ranking survives uncertainty in the temperature assigned to welding kinetics. At equal contact and mobility, the perfect-contact reference gives a strength surrogate ratio near 0.83; material-side temperature brackets cross unity even when neat and filled interfaces have equal nonzero resistance. Refining both materials changes hot-side exposure by up to 7.15% and bracket endpoints by up to 1.80%, without changing their classification relative to unity. This comparison is not an error bound. The contribution is a set of limits on calibration and ranking decisions within the surrogate. Conductivity, kinetics, porosity and contact inputs remain scenario assumptions; no new experimental measurements or validated PLA/GNP performance estimates are supplied.