Longitudinal top-down crack propagation in asphalt runway pavement under realistic aircraft landing-impact loading: mixed-mode stress intensity factors, Paris-law fatigue life, and full-scale validation

Flexible asphalt runway pavements develop surface-initiated longitudinal cracks (top-down cracks) that are driven, in part, by transient loads imposed during aircraft landings. A 2015 fracture-mechanics framework (Modarres and Shabani) coupled a 3D finite-element pavement model of a Boeing 777-300 gear with a one-degree-of-freedom, critically damped landing-gear idealization and uniformly distributed tire contact pressures, extracted mixed-mode stress intensity factors, fitted equivalent-SIF functions of crack depth, and integrated the Paris crack-growth law to estimate fatigue life — but explicitly left open whether the predicted crack-propagation parameters survive realistic multi-degree-of-freedom gear dynamics, measured non-uniform tire contact pressures, mix-specific and temperature-dependent fracture calibration, configurations where mode III matters, and validation against full-scale crack measurements. The open problem is to determine and validate, with full-scale evidence, the longitudinal top-down crack propagation parameters of asphalt runway pavement under realistic landing-impact loading, and to delimit the regime in which the mechanistic-empirical stress-intensity-factor shortcut of the NCHRP (the U.S. National Cooperative Highway Research Program) pavement-design methodology is valid for such transient loads.

Background

Asphalt (flexible) runway pavements support aircraft through multi-wheel landing gears whose loads are far heavier and more tire-pressure-intensive than highway traffic, and landings add a short transient impact pulse on top of the rolling load; measured pavement responses to hard landings of heavy aircraft were reported as early as Kuo, Mahgoub and Holliday (2004). A dominant distress of thick asphalt layers is top-down cracking (TDC): cracks that initiate at the wearing surface, typically as longitudinal cracks aligned with the wheel path, and grow downward through the asphalt layer. Because the crack driving force is concentrated near the surface, fracture mechanics is the natural predictive tool: the stress intensity factors $K_I$ (opening mode), $K_{II}$ (in-plane shear mode) and $K_{III}$ (out-of-plane shear, or tearing, mode) characterize the near-tip crack-driving field, and fatigue crack growth per load cycle is modeled by the Paris–Erdogan law $da/dN = A[\Delta K(a)]^n$ , where $a$ is crack depth, $N$ the number of load repetitions, $\Delta K$ the range of an equivalent mixed-mode SIF, and $A$ , $n$ material constants obtained from laboratory crack-growth tests on the asphalt mixture. Modarres and Shabani (2015) built the first framework of this type for runway impact loading: a three-dimensional ABAQUS finite-element model with a linear viscoelastic asphalt layer (a Prony-series representation, i.e. a sum of exponential relaxation terms fitted to the creep master curve — the time- and temperature-shifted curve of the asphalt's slow deformation under sustained load), stress-dependent nonlinear resilient unbound layers (granular base and soil layers whose repeated-load stiffness, the resilient modulus, depends on the applied stresses) calibrated to resilient-modulus tests at the U.S. Federal Aviation Administration (FAA)'s National Airport Pavement Test Facility (NAPTF, the full-scale instrumented test facility in Atlantic City where aircraft gear loads are applied to instrumented pavement sections), and a transient landing-impact load generated for a Boeing 777-300 gear layout. The load came from a one-degree-of-freedom spring–dashpot landing-gear model (gear mass 167000 kg, stiffness 700000 N/m, damping ratio assumed 1, i.e. critical damping), applied as identical half-sine pressure histories uniformly distributed over each tire contact patch. For seven longitudinal crack positions they extracted $K_I$ and $K_{II}$ at several initial depths, combined them into an equivalent stress intensity factor $K_{eq}$ (a Tanaka-type fourth-root combination, a standard mixed-mode formula that folds the individual mode SIFs into one scalar crack-driving force), dropping $K_{III}$ , which they computed to be roughly an order of magnitude smaller in their specific simulations, fitted quadratic $K_{eq}(a)$ relations per weather state (dry/wet/snow, coupling surface friction and asphalt stiffness), and integrated the Paris law — with constants $A = 3.44\times10^{-6}$ and $n = 2.71$ adopted from Jacobs' laboratory tests on other asphalt mixes — to obtain fatigue lives, validating only the model's global responses (surface stress and displacement, subgrade displacement) against NAPTF measurements. They also compared the finite-element SIFs against the mechanistic-empirical (mechanics-based calculation calibrated against field observations) shortcut used in the pavement-design methodology of NCHRP (the U.S. National Cooperative Highway Research Program) Project 1-37A — an empirical formula that converts the stress at the crack-tip depth into a scalar SIF via creep-compliance-derived conversions — and found it calibrated toward cold, high-stiffness conditions. The authors stated that the framework cannot be considered comprehensive until its load and material idealizations are replaced by realistic ones and validated, and this is precisely the unresolved target: no later work has yet redone the crack-propagation prediction with realistic gear dynamics and measured contact pressures, mixture-specific fracture calibration, or full-scale crack measurements.

Problem Statement

Determine, and validate against full-scale measurements, the longitudinal top-down crack propagation parameters of flexible asphalt runway pavement under realistic aircraft landing-impact loading, as an extension of the validated-prediction objective of Modarres and Shabani (2015). Specifically: (1) quantify how the mixed-mode stress intensity factor histories $K_I$ , $K_{II}$ (and the equivalent SIF entering the Paris integral) and the resulting Paris-law fatigue life $N_f$ for longitudinal top-down cracks beneath a multi-wheel gear (the Boeing 777-300 gear layout) change when the one-degree-of-freedom, critically damped gear idealization is replaced by realistic multi-degree-of-freedom landing-gear dynamics with actual damper characteristics and gear/airframe mass distribution, and when the assumed uniform tire contact pressures are replaced by measured non-uniform contact pressure distributions; (2) identify the geometries, wheel offsets, and transient shear conditions under which the mode-III stress intensity factor $K_{III}$ becomes non-negligible in the equivalent-SIF combination (non-negligible meaning that retaining the mode-III term changes the equivalent SIF $K_{eq}$ , and hence the Paris-law fatigue life $N_f$ , by more than the relative tolerance adopted for the mixed-mode combination, which the analysis must state); (3) recompute fatigue lives using Paris-law constants $A$ and $n$ calibrated for the actual runway asphalt mixes as functions of temperature and loading spectrum, rather than constants adopted from unrelated laboratory mixtures; (4) validate the predicted crack-propagation behavior against full-scale runway measurements (instrumented sections such as NAPTF or equivalent) covering varied tire contact pressure distributions and landing conditions, including the dynamic nature of the landing load; and (5) delimit the temperature/stiffness regime in which the NCHRP mechanistic-empirical stress-intensity-factor shortcut is a valid substitute for full finite-element mixed-mode SIF extraction under aircraft-impact transients (valid meaning that over that temperature/stiffness range the shortcut stress intensity factor agrees with the extracted mixed-mode SIF to within the relative accuracy tolerance adopted for the crack-propagation prediction, which the analysis must state). Elements (1)–(5) are jointly required: none of them alone resolves the task, and the task is resolved only when all five are addressed. The required target level is the general case rather than the named instances alone — any multi-wheel commercial-transport gear layout (with the Boeing 777-300 layout as the reference configuration that must be analysed) and any full-scale instrumented runway section of NAPTF-grade instrumentation (the NAPTF sections being the reference source). These elements serve one objective — replacing the idealized 2015 framework with a realistic, experimentally validated predictive capability for landing-impact-driven top-down cracking.

References

Amir Modarres, Hamed Shabani (2015). Investigating the effect of aircraft impact loading on the longitudinal top-down crack propagation parameters in asphalt runway pavement using fracture mechanics. Engineering Fracture Mechanics 150:28–46. doi:10.1016/j.engfracmech.2015.10.024, https://doi.org/10.1016/j.engfracmech.2015.10.024
Mohsen Alae, Hamzeh F. Haghshenas, Yanqing Zhao (2019). Evaluation of top-down crack propagation in asphalt pavement under dual tire loading. Canadian Journal of Civil Engineering 46(8):704–711. doi:10.1139/cjce-2018-0432, https://doi.org/10.1139/cjce-2018-0432
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