Nondestructive evaluation methods for composites focus on “seeing inside” the material without cutting it open, each using a different physical signal to reveal hidden defects. Ultrasonic C-scan sends high-frequency sound into the laminate and builds a map of the interior, making delaminations and weak interfaces visible as variations in the image. Infrared thermography looks at how heat flows through the part; defects like delaminations or poor bonding disrupt heat transfer, so thermal patterns appear as anomalies on the surface or in a thermal image. X-ray or CT imaging uses penetrating radiation to view the internal structure in 2D or 3D, exposing voids, porosity, inclusions, or misaligned fibers that aren’t seen from the outside. Acoustic emission monitors the high-frequency elastic waves produced as the material experiences damage, providing real-time indications of crack initiation and growth or fiber breakage under load. The other statements don’t align with what these techniques actually detect—color imaging, surface roughness, moisture, magnetic fields, density, or electrical conductivity aren’t the primary measurements for these NDE methods in composites.

Tsai-Hill is a ply-level, stress-based failure rule for laminated composites. It uses the actual in‑plane stresses in a ply (normal and shear) and combines them in a quadratic form with the ply’s strength limits in tension, compression, and shear. The result is a single, dimensionless failure index: when this index reaches 1, the ply is predicted to fail under the given loading. This makes it useful for assessing how different stress components interact under combined loading, rather than judging each stress component in isolation. It’s specifically about predicting ply failure due to the stress state inside the laminate, not about resin cure temperatures, fatigue of matrix cracking alone, or hydrostatic pressure failure.

Adhesives in composites act as the bonding medium that holds layers and components together so the structure behaves as a single unit. By gluing the plies or parts, the adhesive transfers loads between layers, helps resist delamination, and allows joining different materials without fasteners that can create weak points. The adhesive must be compatible with the materials and environment to maintain strength and stiffness, distributing shear and peel stresses across the joint during bending, impact, and temperature changes. This bonding function is why the primary purpose is to hold materials together. It isn’t primarily about conducting electricity, increasing weight, or dissolving resin.

Curing residual stresses come from the mismatch between how the matrix and the fibers shrink or expand during cure and cooling, and they are set by how the part is processed. The key factors shaping these stresses are the cure temperature, how fast you cool, and the boundary constraints from the layup. Higher cure temperatures create a larger temperature difference from the end of cure to room temperature. Since the resin and fibers respond differently to temperature and have different shrinkage behaviors, a bigger temperature swing leads to a greater differential strain that becomes locked in as residual stress when the laminate solidifies and is restrained by the surrounding material. Cooling rate matters because it controls how much time the material has to relax stresses before the network becomes rigid. Slower cooling allows more viscoelastic relaxation and rebalancing of strains, reducing residual stresses, while rapid cooling traps more of the differential strain as stress in the laminate. Layup constraints, including the stacking sequence, fiber directions, and tooling boundaries, dictate how the differential strains are constrained within the laminate. Boundaries that resist transverse or in-plane movement convert those differential strains into residual stresses, especially where plies with different CTEs are bonded together and cannot freely contract. Why the other ideas don’t fit: humidity during service isn’t a curing factor and won’t create curing residuals. Resin viscosity at room temperature isn’t controlling the cure-stage mismatch that locks in residual stresses. Making plies thicker doesn’t eliminate residual stresses and can actually influence them through greater constraint and different stress distribution.