Damage Assessment for Composite Sandwich Structure

Walker, T.H., Graesser, D.L., Ward, S.H., Floyd, J.F., Razi, H., Ploubis, V.

AIAA 2003-1596, 44th AIAA/ASME/ASCE/AHS Structures, Structural Dynamics, and Materials Conference, Norfolk, Virginia, 7 – 10 Apr 2003.

Allowable Damage Limits (ADLs) incorporated in a commercial aircraft Structural Repair Manual (SRM) reflect the maximum damage that can be sustained at a specific location of a structural member without reducing strength capability below regulatory load-carrying requirements. These limits are influenced by many material, structural, damage, and loading variables. Large ADLs are desirable to minimize the maintenance burden of the aircraft operator.

A comprehensive methodology for assessing in-service damage to light-gage composite sandwich panels is presented. The methods can be used to explicitly predict the residual strength of a known damage, or to determine Allowable Damage Limits (ADLs) for inclusion in the Structural Repair Manual (SRM). The approach uses damage metrics compatible with airline operator inspection methods, and addresses all in-service damage types for a wide range of configurations typically found in fairing and flight-control-surface applications on current Boeing airplanes. Specifically, the approach addresses sandwich facesheets less than 0.15″ thick, fabricated from standard 250° F and 350° F thermoset epoxy systems with carbon, fiberglass, and/or aramid fibers, all used in conjunction with Nomex® core. However, some generalized strength results allow estimating the residual strength response of other materials, as might be desired in preliminary design. The methodology represents an improvement over past methods through higher-fidelity analyses and consideration of damage severity. This latter consideration avoids a major conservatism often included in past approaches, where delaminations, dents, and punctures are all treated as holes.

A major aspect of the effort was developing a semi-empirical characterization approach for the residual strength of individual materials with penetrations and impact damage. The semi-empirical prediction method is based on a power-law relationship between strength and damage size, and relies heavily on a large database of notched and impact damaged strengths of thin-gage honeycomb configurations collected at Boeing since the 1980’s. Continuous functions were developed to capture the effects of layup and damage severity on residual strength (using orthotropic stress concentration factors and damage-diameter-to-depth ratios, respectively), leveraging data trends from all materials. Regression analyses were then used to characterize the response of each material. These results provide, for a specific material/layup combination, the residual strength as a function of damage size and severity.

Applying these characterizations to realistic damages and structures present additional challenges. Statistics, environment, core thickness, and finite-width effects are addressed through the use of correction factors. Additional approaches are employed to address facesheet thickness, combined loading, non-circular damage, hybrid layups, facesheet springback, and multi-site damage. These correction factors and approaches leverage data, where available, and use conservative assumptions where uncertainties exist.

For damage scenarios with delaminations, but little or no dent, failure modes associated with outward buckling of the delaminated plies must be considered in addition to those for damage with dents. Existing analysis methods form the basis for predicting (a) sublaminate buckling, (b) facesheet fracture due to the buckled sublaminate, and (c) static delamination growth, all as a function of damage size and delamination depth. These predictions are compared to the applied loading to preclude static failure and fatigue delamination growth. Conservative techniques are employed to address limitations of in-service inspection methods relative to identifying delamination depth and the existence of multiple delaminations.

An implementation strategy to support in-service use of these strength predictions was developed. A number of damage size and spacing measurements are necessary to support the above methods. SRM allowable damage data for one zone of a specific aircraft part include one plot addressing single-site damage, while a second plot provides a correction factor for multi-site damage scenarios.

The single-site ADL curve results from the hole/impact residual strength capability, as well as consideration of delaminations without dents. The left-hand portion of the curve is based on the impact and hole residual strength predictions. The curve is generated by determining the damage size that results in failure at the applied Ultimate strain from the residual strength curves for the specific layup. The right-hand portion of the curve reflects the results for delaminations with no dent. These constant values are a function only of delamination depth. Each line represent the smallest of the damage sizes associated with the failure modes related to sublaminate buckling (facesheet fracture, static and fatigue delamination growth) for the applied strains and the delamination depth indicated.

The central portion of the single-site ADL curve results from consideration of facesheet springback, which results when a relatively undamaged facesheet either partially or fully returns to its initial position, despite the damaged core. Evaluations of impact damage surveys indicate that springback occurs only in less-severe damage scenarios, and that the severity level below which springback might occur is a function of facesheet thickness. The residual strength at this severity threshold is used as an upper limit for less severe damages. However, damage depths associated with facesheet springback might also relax over time when subjected to environmental and mechanical cycling associated with service. Damages where this is considered to be a possibility are therefore conservatively treated as a delamination with no dent, except in cases where intermediate inspections can and will be performed to monitor the damage for growth.

Multi-site corrections are based on single-site strength reductions due to interactions between pairs of damages. Specifically, isotropic stress concentration factors were found to reasonably predict the interactions observed in test data, and are therefore used to define strength knock-down factors based on damage spacing, relative damage size, and loading orientation. The correction factors for damage size are determined from these strength-based knock-downs, and are dependent on layup due to the dependence of the notch sensitivity on layup. Severity differences between damages within a pair are treated conservatively by assuming both damages are of the greater severity of the two damages.

In-service usage is necessarily simple. The operator obtains the required damage size and spacing measurements. For single-site damage, the damage severity (d2/y) is calculated, then the allowable damage limit is obtained from the upper curve. If the maximum size of the actual damage is less than the ADL, and all spacing requirements are met, it is acceptable. Otherwise, the damage must be repaired.

For multi-site damage, each unique damage pair are treated separately. The severity of both damages in a pair are calculated, and the smaller of the two is used to determined a single-site ADL from the upper curve. Spacing and diameter ratios (S/D3 and d3/D3, respectively) are calculated, and used to obtain the multi-site diameter correction factor (MDCF) for each of the smaller and larger damages from the lower plot. The multi-site ADLs are determined for each damage as the product of the single-site ADL and the respective MDCF. If the maximum size of the actual damages are both less than their respective ADLs, and all spacing requirements are met, they are acceptable. Otherwise, both damages must be repaired.