U.S. Naval Aviation: operational airframe experience with combined environmental and mechanical loading

1 Background

With the increased complexity of newer systems and the demand for higher performance at less weight and cost, the objective of maintaining the intended level of performance throughout the aircraft life-cycle has become an increasingly difficult task. This is further challenged by concurrent design and production systems engineering plans, which have often led to de facto the finalization of materials and processes (M&P) much earlier in the acquisition life cycle and often at lower technology or manufacturing readiness levels than desired. Many recent examples have shown that the sustainment of these new systems is more challenging, especially when using the validation approach of designing to minimum system-level requirements. These examples show a trend in unexpected, unpredicted early failures due to material degradation and associated structural capability loss in the naval operational environment. The last 20 years have seen a systematic reduction in internal investment that coincided with policy changes that affected design validation approaches, with the unplanned result of expecting improved material durability and performance, without the necessary validation and risk mitigation associated with the consistent research, development, and characterization of material and structure design options. This scenario is coupled with increased reliance on performance-based requirements, stringent environmental and occupational safety regulations, and overarching drivers for full life-cycle affordability, safety, and readiness. As the U.S. Navy (USN) continues in an era of aging aircraft, reduced sustainment or upgrade budgets, and increasing multifunctional, multimission requirements, virtually every structural material and sustainment process developed and selected for naval aviation weapon systems must address the USN’s maritime operational environment. Historically, however, this has not been adequately or consistently conducted. The harsh corrosive environment, the structural load demands of carrier operations, and the inherent constraints placed on deployed fleet maintenance constitute formidable challenges to managing life-cycle cost (LCC), readiness, and performance risk. Airframe material and structure R&D objectives then should focus on opportunities to improve application-specific materials selection analysis and structural life prediction methodologies within a combined (environmental/mechanical) loading framework.

The current reality is that, across the variety of fleet platform types, Naval and Marine Corps Aviation is often experiencing in-service degradation that is both faster and more severe than what was predicted during mechanical structural analysis. This results in costing more in total LCCs, maintenance man-hours, and decreased operational availability (A_o) than originally predicted. There are complicated and interdependent reasons for this, not the least of which is the increased op tempo of the last decade; however, the general systemic trend stretching back to the 1980s has been for each successive platform to begin service as a more complex, higher-cost initial design that exhibits reduced tolerance for variation and decreased durability in operation. The estimated [Office of the Secretary of Defense (OSD) cost of corrosion study; LMI, 2008] naval aviation cost of corrosion, in terms of maintenance impacts, is that corrosion and the related structural repairs account for 1 in every 5 availability days lost on average and approximately one-third (32%) of all maintenance actions, making material degradation the single largest maintainability degrader.

There are several factors that play into this dynamic, such as increased regulatory restrictions on M&P for protection and maintenance, acquisition reform resulting in vague system-level “performance” metrics vice material-specific requirements and guidance, staffing trends that have resulted in replacing subject matter experts with general system or process engineers, and an overall design philosophy that emphasizes flyaway cost and out-the-door production schedule at the expense of in-service durability, which is evidenced by the observed negative effects on A_o and LCC. The majority of structural teardown findings are indeed systemic and not particular to a given platform or component.

2 Approach

USN airframe inventory management is based on the Safe-Life fatigue methodology. Historically, the USN’s employment of the Safe-Life approach was predicated on the austere carrier-based operating environment and the attendant impracticality of conducting frequent and intrusive major inspections aboard air-capable ships or in forward deployed expeditionary sites. Both the USN Safe-Life fatigue management approach and the U.S. Air Force damage tolerance approach rely on the analysis to show 2X planned fatigue life capability. The fundamental approach differs in the focus on the prognosis capability for Safe-Life and inspection capability and inspection frequency for damage tolerance.

In practice as design certification methods, both approaches will usually yield similar airframe structure and size. Naval aircraft are inherently oversized and heavier due to the extreme stresses experienced in ship-based operations. The USN approach also places heavier emphasis on low-speed stability and flight properties due to the increased risk during over-water to over-ship transitions. Some original equipment manufacturers use Safe-Life to determine proper airframe structural size and to spot-check certain areas using a damage tolerance methodology.

Under the USN Safe-Life approach, aircraft are presumed to be “crack free” when new, and are retired at crack initiation, before the onset of widespread fatigue cracking. The certification analysis must show that the time to crack initiation should not occur within two planned lifetimes. No preplanned major inspections should be required during the service life of the airframes. Conversely, under a damage tolerance approach, aircraft are presumed to have cracks from time of manufacture (assumed initial flaws) and are monitored for cracks from the time of service entry until the economic burden of inspections or the cumulative risk of missing a crack during inspection becomes prohibitive. The certification analysis must show that cracks should not grow to functional impairment within two planned service lifetimes.

The USN Safe-Life approach places stringent focus on appropriate criteria, as the goal is no fatigue cracks within the lifetime. As the take-off and landing operations place higher loads on the airframe, the potential consequences of an unexpected or missed fatigue crack failure are higher.

Damage tolerance methods are used by the USN in reaction to the finding of unpredicted cracks in fleet aircraft, until such time as fixes can be implemented, and, if necessary, to provide short-term service-life extensions beyond that defined by Safe-Life (provided the critical crack size is sufficiently large and inspection intervals are sufficiently long). The Safe-Life approach, however, remains the primary method for airframe inventory management now and for the foreseeable future. Consequently, it is imperative that the USN-sponsored science and technology (S&T) research be focused on a methodology consistent with the Safe-Life approach, specifically to enhance corrosion prediction and prevention efforts. This will ensure that the targeted product(s) are ready to be accepted by the USN’s structural integrity community and easily adoptable into the USN’s basing and life management concepts.

Safe-Life tracking (Hoffman, Rusk, Roerden, Modarres, & Rabiei, 2008) does not explicitly account for corrosion; corrosion effects are mitigated by the inspection and refurbishment of susceptible components and structural analyses are re-baselined against the teardown analyses of high-time aircraft (Figure 1). The maintenance policy requirements are to remove and repair all corrosion damage when found, coupled with time-based refurbishment limits for certain components. In practice, a knockdown factor of 2.0 is included in the USN Safe-Life methodology; Schütz (1995) recommended a 2.0–2.5 knockdown on a full-scale test life to account for corrosion effects on the external structure.

Figure 1:

Traditional cycle count crack initiation (i.e. Safe-Life) approach to structural fatigue life calculation for USN airframes, showing the methodology and inserted knockdown factor of 2 used to predict service life (Hoffman et al., 2008).

3 Findings

USN experience in ship-based aviation operations is that the current approach has not fully protected the fleet against corrosion-induced structural failures from pitting, crevice, or stress corrosion cracking (SCC) attack. A holistic look at metallic structure in-service durability, backed up by other DOD service evaluations, shows that the combined loading effects dominate the structural life capability. This was demonstrated experimentally in recent publications that evaluated the impact of simultaneous environmental and mechanical loading on endurance limit or cycles to failure for common structural metals (Palin-Luc & Bathias, 2010) (Figure 2).

Figure 2:

Ultra-high cycle fatigue (UHCF) testing of R5 steel showing the dramatic difference in material response between traditional precorroded mechanical fatigue testing and more realistic simultaneous corrosion-mechanical loading conditions.

The fatigue strength at 10⁸ cycles is significantly reduced by a factor of 74% compared to the virgin specimens and, more telling, by 71% compared to the precorroded ones. A dramatic reduction in strength is observed due to the simultaneous corrosion fatigue in the UHCF regime (Palin-Luc & Bathias, 2010).

A recent joint service, fixed-wing teardown analysis found that approximately 80% of observed structural cracks initiated from a corrosion feature (Fawaz, 2012); more telling is that just under half of these pits that initiated cracks were smaller than the current crack growth model’s defined a*. USN repair-depot structural engineering assessments of teardown have indicated the average carrier-based fixed-wing platform structural damage as a 60/40 split of SCC-to-fatigue (mechanical and corrosion), with up to 75–90% of structural failures initiated at dissimilar material (i.e. galvanically stressed) interfaces on some aircraft (Mendoza, 2012).

From these aircraft teardown analyses then, we can make a general statement that the two primary drivers that account for corrosion damage and repair are galvanic coupling and SCC. The galvanic acceleration of corrosion-initiated damage is estimated by depot structural engineers to be present for the significant majority of structural damage on USN ship-based platforms, with the remainder of the damage predominantly driven by SCC mechanisms. Again, this is anecdotally supported by the above-referenced teardown analysis of a high-altitude, land-based transport aircraft in which more than 75% of structural cracks initiated at a corrosion feature (Figure 3). Empirically, this makes sense, as galvanic interfaces are found at fasteners, faying surfaces, and electromagnetic interference-panel joints, where the highest mechanical stresses are also experienced. The edges of holes are typically at higher stress intensity levels (i.e. higher energy states), which translate into a higher potential for reaction (Kittur, 2012).

Figure 3:

Averaged data from end-of-life teardown analyses on five fixed-wing cargo aircraft.

Simplified data averaging indicates several things, the most pertinent two of which are that a significant number of cracks were initiated from pits smaller than the currently defined crack initiation size in existing damage tolerance methods and that, even for aircraft that spend most of their service lives in relatively low maneuver flight profiles and in-land environmental conditions, environmental damage plays a significant role (Fawaz, 2012).

Galvanic stress (i.e. the distribution of current density across the component geometry, as the driving force for corrosion and degradation at dissimilar material contacts) should therefore be of the highest priority for investment into improved combined loading structural durability analysis.

Given the operational environment, sea-based aviation (SBA) demands structural and material requirements that are often unique or more demanding than the same land-based aviation requirements. A study conducted by the OSD Corrosion Prevention and Control Office over several years found that the average ship-deck environment was 4–8X more aggressive than the 95th percentile of aggressive land site conditions (Abbott & Jackson, 2004) (Figure 4). When coupled with the traditional approach to materials selection, based primarily on mechanical function and ease of purchase, and our lack of current modeling capability to assess the structural risk for location or relative severity of galvanic and chemical stress, this is not a positive trend when affordable readiness is a key objective.

Figure 4:

Weight loss specimens of AA6061T6 exposed at various land, sea-side, and aircraft carrier/naval vessel placements to compare the relative corrosivity of geographic location.

The specimens were exposed uncoated for 3 months, without applied mechanical load, and the expected weight loss was extrapolated to a year’s exposure and graphed. This shows that the 95th percentile (i.e. most aggressive land site tested) is equivalent to a below deck, mostly sheltered shipboard environment. The airframes stationed on deck are exposed to the most aggressive atmospheric environment of the normal military operational locations (Abbott & Jackson, 2004).

4 Summary

Naval aviation is in the forefront of the USN power projection mission, including our ability to successfully meet the acquisition development, operational performance, readiness, and affordability requirements.

The proposed strategy (Nickerson, 2012) for R&D and the associated engineering and technology transition should be to enable more robust aircraft design, to reduce maintenance and sustainment impacts due to design decisions driven by weight or environmental factors, to improve the fidelity of materials selection impact analysis and trade-off decisions, and to enable performance-driven properties such as weight/range/payload without reducing durability. A robust program that sustains a focused aircraft S&T activity is critical to the health of naval aviation. The SBA Structure and Material Program consists of two thrusts: advanced airframes and durable aircraft. These two thrusts cover technical areas, such as structural mode characterization; high-loading, lightweight structural and materials science; advanced structural concepts; material degradation and corrosion; and structural protection and maintenance. Few of the existing aviation materials and structural challenges are platform or design specific; rather, they are fully represented in both current and planned platform designs.

Some highlighted technical challenges that are relevant across manned and unmanned, fixed-wing and rotary, and both current and future designs are corrosion and material degradation S&T challenges including understanding the complex, multimaterial mechanisms; informing materials selection during design for impacts on LCC and A_o, combined environmental damage mechanisms of corrosion, erosion, thermal, moisture, and operational fluids; and predicting and analyzing sustainment risk during trade-off assessments.

The objective then of improved structural lifing prediction is to change the aircraft design and developmental paradigm to a higher-fidelity prognosis capability (i.e. to enable more durable aircraft by analysis advances). With the tools available today, aircraft are primarily designed for immediate mechanical performance and loads-only structural response. Galvanic management and corrosion-resistant materials selection have never been done systematically; the degradation of material properties is a seldom considered design criteria, and the cost of maintaining operational capability and readiness is often pushed off into the sustainment life-cycle of the aircraft.

Typical design specifications (Kittur, 2012) many times take the form, “Airframe that is free from corrosion.” This is a vague requirement, difficult to demonstrate achievement and further challenging to enforce compliance due to gaps in real-time, practical testing, and test methods. Within the model and predictive approaches, there is ample room to continue analyzing and corroborating via “build and break” tests while investing in more accurate or realistic SCC and corrosion fatigue test methods. The improved characterization results can then be configured into higher-fidelity multidamage models provided the dominant mechanisms and the co-dependent mechanical-chemical interactions can be adequately described.

Tied to this are opportunities for design strategy improvements, as the majority of the areas affected by corrosion and fatigue are at various types of holes and interfaces, such as fastener holes, lugs, and faying surfaces. Improvements such as bushing and flange modifications could potentially move potential areas for galvanic stress and crevice conditions farther away to a spatial location below the highest stress levels. M&P improvements such as surface treatments (e.g. shot peening and low plasticity burnishing) can leave compressive residual stresses at critical locations as well. Other M&P improvements take the form of coatings and metal finishing technologies for galvanic isolation or cathodic current suppression, allowing the component or airframe to more accurately be analyzed under current mechanics-only structural methods.

The naval aviation structural challenges are not the same as land-based aviation requirements, and in many cases, aviation materials are fundamentally different in composition, strength-weight properties, mechanical loading and fatigue, and degradation mechanisms when compared with ship or automotive structures. Persistent issues in aviation structures and materials science are in understanding and predicting the impact of material property degradation, shipboard maintenance and repair methods, airframe structural life prognosis and sustainment planning, materials selection, and high loads associated with shipboard launch and recovery. From a fracture mechanics view, the research direction needed is on the combined effects of structural analysis (i.e. the ability to conduct airframe risk and reliability analysis for multimaterial, multiscale service-life prediction that accounts for both environmental and mechanical stresses). The disparate approaches need to be brought together under a fracture mechanics-like framework to create a material degradation, structural damage, and lifing integrated toolkit. Functional analysis tools outputs should focus on platform life-cycle assessment supported by materials selection for durability as part of the overarching structural engineering process. The development and validation of airframe service-life methods hinges on the inclusion of corrosion fatigue and SCC modeling capability that currently does not exist in any useful functional format, which is critical in the maritime aviation operational environment.

These interdependent technical subfocus areas can also be broadly grouped according to the operational life-cycle transition opportunities, such that broad categories, although not comprehensive, can provide an overview of the approach being sought by the USN. The general flow of RDT&E and Tech Transition can be approximated as (1) analyze and predict; (2) test and characterize; (3) validate and verify; (4) optimize and insert novel materials; and (5) then inspect, repair, and maintain.

From the perspective of the naval aviation enterprise, the ultimate end goal would be a durability and service-life prediction capability, enabled by environmental and chemical loading mechanics to define and standardize material characterization for nonmechanical property performance, incorporated into a philosophy for new or existing platforms of materials selection based on usage profiles, combined mechanical-environmental loading, operational environment durability, and true service-life maintenance interval prediction that can account for degradation and damage accumulation to produce actionable decisions.

5 Recommendations

Today’s trend is towards more complexity and less durability. The goal is to deliver more durable, lower maintenance-burden airframes to the fleet. The challenge is to add durability into the trade space without impacting affordability or operational performance. The path forward is to leverage S&T with engineering functions to address in-service, life extension, and new design improvements simultaneously. Tomorrow’s capability must be multimaterial, predictive, and high fidelity. Loading history and damage accumulation will be coupled with sea-based environmental degradation impacts. Materials selection for durability will be an integrated component of the structural analysis and lifing prediction process. This will enable increased availability and readiness through improving the accuracy of structural prognosis and reducing unexpected, early failures associated with combined loading damage mechanisms. These advances in engineering analysis are needed to address the fleet’s desire to transition to condition-based maintenance (CBM). The goal of CBM is to reduce the frequency of recurring, preventive maintenance actions that do not directly contribute to durability and readiness and to tie scheduled maintenance to the actual material condition of the system being maintained. A major limitation in the application of CBM approaches to structural integrity is the inability to predict, with any degree of accuracy, the level of environmentally driven damage that an airframe structure will see in service. The use of onboard damage detection sensors can increase knowledge of current material condition for large damage events such as cracks, loose fasteners, and impacts. However, there are currently no onboard sensors that can reliably detect local environmental material degradation on airframe components (Burns & Gangloff, 2011). This becomes an issue when attempting to grant maintenance credits for component inspection and overhaul intervals based on CBM, because the increased risk associated with environmental damage mechanisms cannot be adequately quantified. The future application of CBM to existing airframe structural maintenance and logistics practices may also eventually result in fewer opportunities for preventive maintenance actions, such as material condition inspections and nondestructive inspection, which will continue to be required to assess the ongoing state of environmental damage on a particular airframe. Several research and engineering focus areas of interest arise from this discussion, such as the following recommendations:

1. Focus on informed materials selection and galvanic corrosion risk assessment during new or rework design to select material systems for both mechanical and environmental responses.

2. Focus on structural mode characterization advances, especially to analyze for combined mechanical/environmental stress analysis and corrosion cracking behaviors, integrating risk and reliability for airframe structures in the USN operational environment.

3. Focus on formulating advanced protective measures, such as next-generation coating systems for galvanic isolation and specifically corrosion cracking resistance.

4. Focus on transitioning mechanistic understanding into improved specifications and methods; standardizing testing, specimen configurations, and providing verified quality data and best practices to industry and Department of the Navy engineering organizations.

5. Focus on a standardized, best state-of-the-art M&P and engineering tools usage as the only acceptable minimum baseline while investing in accelerating S&T solution transitions through advanced analysis tools to the engineering commands and advanced M&P to the operational fleet.