Aggelis, Barkoula, Matikas and Paipetis (2012) showed that combined acoustic-emission (AE) and ultrasonic-transmission monitoring tracks the damage sequence — transverse cracking, delamination, fibre rupture — in fatigued glass/epoxy cross-ply laminates, including a counterintuitive early increase of ultrasonic pulse velocity caused by delamination-driven isolation of the stiff outer plies, reproduced by two-dimensional elastic finite-difference simulations with idealized open air-gap cracks. Open directions implied by the paper's explicitly stated limitations — two-dimensional, perfectly elastic (lossless) simulations with open air-gap crack and delamination inclusions, a mesh comparable to the damage dimensions, deterministic crack arrangements whose alternatives the authors concede may give different results, and demonstration on a single glass/epoxy cross-ply configuration — include incorporating contact, frictional and partially bonded interface physics and three-dimensional geometry into the wave simulations, and mapping the dependence of the acoustic-emission and ultrasonic signatures on realistic stochastic spatial distributions of cracks and delaminations. These directions remain unachieved. No existing work establishes a quantitative, physics-based mapping from a stochastic distributed damage state with realistic contact, frictional and partially bonded interfaces to the measured AE and ultrasonic-transmission signatures, validated against measured data.

Background

Fibre-reinforced composite laminates are stacks of thin plies whose fibres are oriented at chosen angles; in a cross-ply laminate, such as the glass/epoxy [04∘/904∘]s[0^\circ_{4}/90^\circ_{4}]_{s} layup studied by Aggelis, Barkoula, Matikas and Paipetis (2012), plies with fibres aligned with the load alternate with transverse plies. Under quasi-static or fatigue loading these laminates damage in a characteristic sequence: matrix (transverse) cracking of the 90∘90^\circ plies, then delamination — separation between adjacent plies — and finally rupture of the load-bearing 0∘0^\circ fibres. Two acoustic monitoring techniques follow this evolution in situ. Acoustic emission (AE) is the passive detection, by surface-mounted piezoelectric sensors, of the transient stress waves that each microscopic fracture event radiates; a 'hit' is one such detected transient, the MARSE energy (Measured Area under the Rectified Signal Envelope) is the energy proxy of a hit, and the RA value (hit rise time divided by amplitude) is a waveform-shape index in which low RA indicates tensile (opening-mode) sources such as matrix cracks and high RA indicates shear-dominated sources such as delamination. Ultrasonic (UT) testing is the active counterpart: a pulser launches an ultrasonic pulse that traverses the specimen between sensors, and the pulse velocity (path length over first-arrival time) and transmitted amplitude report on the state of the material along the path. In homogeneous solids cracking lowers the pulse velocity, so the 2012 study's observation that fatigue damage first raised the surface pulse velocity by about 10% — before a plateau and a 6–11% drop at failure — ran against bulk intuition. The authors explained it by a waveguide-isolation mechanism: accumulated transverse cracks and delaminations progressively decouple the stiff outer 0∘0^\circ plies, which then act as faster guiding layers for the surface-measured pulse. Two-dimensional time-domain finite-difference simulations (Wave2000) of a three-layer elastic model with matrix cracks and delaminations represented as 0.2 mm-thick open air inclusions reproduced the trends, including the velocity increase and its asymptotic approach to the single-ply value as total delaminated length grows. The supporting simulations, however, assumed perfectly elastic (lossless) materials with perfectly debonded, frictionless, open crack faces, used a mesh (0.5 mm) comparable to the ply thickness and damage dimensions, explored only a limited set of deterministic crack/delamination arrangements, and the experimental signatures (RA thresholds, velocity plateaus) were demonstrated only for one glass/epoxy cross-ply system under incremental step loading and tension–tension fatigue at a fixed stress level. Later work advanced the pieces separately. Three-dimensional finite-element simulation of guided (Lamb) waves — ultrasonic waves guided by the plate surfaces — was validated against laser vibrometry for scattering at a single artificial delamination in quasi-isotropic carbon laminates (Hervin, Maio and Fromme 2021), and earlier for pristine anisotropic aerospace composites (Leckey, Rogge and Parker 2014); those models still treat the delamination as a zero-volume or open gap without contact and dissipation. Effective spring boundary conditions, which substitute a distribution of micro-cracks or a partially closed delamination by compliant interface springs, brought partial contact into scattering models (Golub, Doroshenko and Gu 2023). On the prognosis side, data-driven frameworks now convert AE and fused monitoring data into life predictions: adaptive probabilistic prognostics and SHM data fusion for in-situ life prognosis of composites (Eleftheroglou and co-workers 2018, 2020), AE-based remaining-useful-life prognosis of aeronautical structures under compressive fatigue (Galanopoulos and co-workers 2023), and AE-driven damage-evolution equations for laminate life prediction (Dong, Li and Li 2025, 2026); these frameworks are empirical, calibrated to specific materials, layups and loading spectra, and contain no wave-scattering model of distributed cracking and delamination. Reviews of AE descriptors in fibre-reinforced plastics (Barile and co-workers 2020) consolidate what has been demonstrated per configuration. What no existing study provides is the physics-based prediction of what AE and UT instruments measure for a realistic, spatially stochastic population of cracks and delaminations whose interfaces carry contact, friction and partial bonding, quantitatively validated against measured AE and ultrasonic data on damaged laminates.

Problem Statement

For fibre-reinforced composite laminates undergoing progressive fatigue damage, establish and validate a quantitative, physics-based mapping from an evolving, spatially stochastic population of transverse cracks and delaminations whose interfaces exhibit realistic contact, friction and partial bonding (rather than the idealized open, lossless air gaps used in the source study) to the acoustic-emission indices (hit statistics, per-hit energy, RA-type waveform parameters) and the ultrasonic transmission and pulse-velocity signatures that surface sensors would measure. The modelling must remove the source study's acknowledged idealizations (two-dimensional geometry, perfectly elastic lossless materials, open air-gap crack and delamination inclusions, deterministic crack/delamination arrangements, and a mesh comparable to the damage dimensions). The predicted acoustic-emission and ultrasonic signatures must be quantitatively compared with measured acoustic-emission and ultrasonic data on damaged laminates. Resolution requires at least one spatially stochastic damage state for which the model reproduces, within tolerances the solver states and justifies against the measured data, the measured ultrasonic pulse-velocity trend and the measured acoustic-emission parameter distributions — quantitatively, not merely as qualitative trends; the idealizations listed above are required of that model as a whole rather than being separate deliverables.

References

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