BAA-RQKPD-2015-0001-Call6-Atch1.pdf

Composite Airframe Live Extension (CALE) BAA Call #6

Predicting Airframe Stresses due to Differential Thermal Strain

Statement of Objectives

6 May 2019

1.0 Background

Aircraft structure undergoes repeated variations in temperature during its service life. The temperature extremes of these variations themselves vary with respect to location in the aircraft. Prior to beginning a mission, aircraft structure equalized to ambient air temperature may vary between -30F and 115F depending on base location, and may exceed 115F due to exposure to solar heating. During the mission, the same structure may see temperatures ranging from -70F to 300F+, depending, again, on exact location in the airframe, proximity to sources of heating (e.g., engine combustion chamber, engine exhaust, auxiliary power subsystems, electronic equipment), and mission usage (e.g., aero-thermal heating due to aerodynamic drag). Hence, in a hypothetical mission originating near the arctic and ending in the tropics, an element of structure may start at -30F and increase in temperature up to 300F, cool to -70F, return to near 300F and then cool to 115F, with many more possible combinations due to mission profile and usage variability. This is only one of a near-infinite number of cycles that could realistically be expected, and such cycles are not necessarily described by smooth, continuous climbs and drops between the extremes.

Accounting for thermal loads in the aircraft structure design analysis is an integral part of proper aircraft design, and is required for Aircraft Structural Integrity Program (ASIP) execution. This includes loads used in the static strength analysis as well as repeated loads used in the durability and damage tolerance analysis (DADTA). Multiple paragraphs in ASIP MIL-STD-1530D specifically require some form of thermal analysis be performed during the design development validation. (See MIL-STD-1530D paras

5.2.2, 5.2.3, 5.2.5.1, 5.2.8, 5.2.9, 5.2.12.2, 5.2.14)

The Joint Service Specification Guide (JSSG) 2006 paragraph A.3.2.11 provides additional guidance on thermal loads for use in the static strength analysis.

In addition to the stresses imposed by absolute temperature, a second source of thermally induced stress is a strain that is caused by the difference in rate of expansion or contraction as the structural materials experience temperature changes. All material changes physical size due to temperature change, but materials do not change size at the same rate of change per degree temperature change. The rate at which the structural material increases or decreases in a given direction, e.g., length, with respect to temperature change is described by a material property called the Coefficient of Thermal Expansion

(CTE).

Many current USAF aircraft airframes are manufactured using advanced composite structure mechanically assembled to other advanced composite structure or mechanically assembled to metallic structure, usually aluminum and titanium. Aerospace grade aluminum alloys and advanced composite material systems have significantly different CTEs. This significant difference in thermal growth/contraction behavior causes stresses to occur in structures manufactured from these materials when these structures are subjected to temperature changes. This gives rise to the risk that as structural temperature changes, and the aluminum and composite materials expand or contract at different rates, local stresses that are significantly different than those predicted using analysis methods that did not account for such stresses will result. Due to a lack of validated and readily available models that fully account for CTE mismatch, there is no good way for USAF structural integrity experts to independently assess the extent of these risks in an efficient manner.

2.0 Statement of Need:

USAF structural integrity managers need a validated thermal analysis method for aircraft structural applications wherein structural members made of advanced composite material are joined with members made of aluminum and/or titanium alloys. The method should predict, with sufficient accuracy to assess structural strength, durability and damage tolerance, stresses in joined members due to both global and local thermal loads, and must specifically predict stresses due to strains resulting from differential thermal growth between joined members with significantly different Coefficients of Thermal Expansion. The method is intended for use by both US government and industry structural integrity experts. The analysis method is intended for use during both aircraft design development and sustainment phases for items such as repairs/modifications with the potential to impact the baseline thermal loads analysis. The analysis method developed is intended to be useful in predicting stresses and strains in structural joints that are already in-service in USAF aircraft, including some aircraft that may have been in service for many years. Therefore, it is important that the method rely on both input data and structural analysis models that have already been in use. Additionally, to help USAF and USAF contractors avoid the expense of developing and validating entirely new models and modelling approaches to predict stresses due to mismatches in CTE, AFRL intends to rely on validated analysis methods to the maximum extent practical. To make maximum use of existing airframe global FEMs and thermal models and data sets, it is highly desired that the method used to predict thermal stresses due to CTE mismatch related strains be compatible with these existing resources. The Air Force would like to avoid the cost of replacing existing analysis tools and models for in-service aircraft, therefore, approaches that will require creation of whole new airframe FEMs are discouraged. The envisioned general method of predicting stresses due to CTE mismatch should seek a balance between model complexity/cost and prediction accuracy. The model should be sufficiently detailed and have adequate degrees of freedom to predict overall stresses and strains, including the results of global and local temperature variations within differing elements having different CTEs, with the level of fidelity typically associated with aircraft-level models.

To meet this need, AFRL envisions modifications to the global aircraft finite element model that account for an appropriate level thermal strain- induced flexibility in the structural members themselves and at the joints.

One approach envisioned by AFRL involves continuing to use fully rigid, “welded” joints within the aircraft level FEM, and adjusting the local strains, and resultant stresses within these models through the use of

“correction factors” to account for differential strains due to mismatched CTEs. The innovation in this approach will therefore be the approach to determining the correction factors to be used, applying them to the problem of interest, demonstrating that this method of finding and applying temperature/CTE correction factors is portable to other structural configurations and finite element modeling methods, and validating the accuracy of this approach with respect to predicting structural strains and resultant stresses.

A second approach envisioned by AFRL would be to implement flexibility within the aircraft level model by replacing the rigid / welded joints with spring joints, or by replacing rigid “clamped” fasteners to achieve such joints with spring fasteners. In this approach, the innovation would be the method of determining the correct flexibility of the joint. To continue the spring analogy, the innovation would be the method of determining the correct spring constant, K, to be used to represent the flexibility associated with differential thermal growth, applying this solution to the problem of interest, demonstrating this method of finding and applying K is portable to other structural configurations and finite element modeling methods, and validating the accuracy of this approach with respect to predicting structural strains and resultant stresses.

The two approaches described above are not intended to limit the scope of the solutions sought. Other approaches for predicting the effects of thermal strain mismatch within an aircraft level finite element model may be feasible, and appropriate to meet this need. Contractors are encouraged to propose such approaches.

Potential offerrors should carefully note that it is the intent of AFRL that any method or approach for predicting thermal induced stresses due to differing thermal strains resulting from this research effort be accepted by any and all potential airframe manufacturers. Potential offerrors are strongly cautioned not to propose the use of proprietary methods within their proposed solution approaches, nor should they propose the use of proprietary data for development, verification, or validation of methods within the proposed program as that will limit AFRL’s ability to transition the research and development deliverables to future Government needs.

Potential offerrors should note that AFRL does not own, and therefore cannot provide, any aircraft level finite element models for use during the performance of this effort.

3.0 Statement of Objectives:

The overall objective of this effort is to develop and demonstrate a method of predicting stresses that develop in structural assemblies of airframe materials having significantly different Coefficients of Thermal

Expansion (CTE) when subjected to significant temperature change. Of particular interest to this effort is the joining of advanced composite structure to aluminum structure via mechanical fasteners.

The temperature range of interest is from approximately -70F to approximately 300F, but there are no absolute requirements for temperature range or number of temperature cycles. The specific composite material types and the specific aerospace aluminum and titanium alloy are free for the proposer to select subject only to the requirement that they be those of the identified baseline structure. Said simply, the materials of interest are those of the selected baseline, and the temperature range of interest is the operating environment of the selected baseline aircraft. The contractor is encouraged to select a baseline structure that represents multiple, contemporary USAF aircraft for this demonstration program.

The contractor shall select a baseline structure, which will be used to both develop and validate the analysis methods developed within this program. The objective of the baseline is to provide both a realistic source of design requirements for wide ranging operational stresses, from maneuver and thermal conditions, and to provide a framework for assessment of the validity of the analysis methods developed through testing. The baseline structure shall be some section of a currently in-service aircraft, and it shall contain at least one mechanically fastened joint joining an advanced composite structural element to an aluminum structural element. Key features of the baseline should be: The structure should include structural elements of advanced composite structure and metallic structure joined together using multiple mechanical fasteners of any type. The structure should be exposed to significant temperature extremes during operation. These variations may be: global (entire structure sees the same temperature variation);

local (at least one part of the structure contains a “hot spot” that results in a different time-temperature variation profile); gradient (the entire structure experiences a temperature variation, but the temperature variation itself varies as a function of location); or a combination such conditions. The structure should represent structural configurations common to existing and anticipated future aircraft, and experience loading conditions commonly seen during aircraft operations. As much as possible within the baseline, a mix of fastener types is encouraged, as is a mix of loose fit, net fit, and interference fit tolerances. Other characteristics that may be desirable for the technology demonstration effort, but are not strictly required, include, but are not limited to; multiple metal alloys, including mechanical joints between dissimilar metals

(eg, aluminum fastened to titanium) and bonded structural joints between any type of structural material.

The physical size of the baseline structure is free with respect to all linear dimensions, weight, and volume, but the offeror is cautioned that the structure should be large and complex enough for significant thermal excursions to produce complex thermal strains and resultant stresses representative of major aircraft structure.

Strictly as an illustrative example, a baseline structure may be a section of an aircraft wing. The section chosen contains a stiffened composite skin mechanically attached to aluminum spars and ribs. The skin is exposed to solar heating while the aircraft is parked on the ramp, is exposed to exhaust gas heating while the aircraft engine is running but the aircraft not moving, and sees near atmospheric temperature while the aircraft is cruising at altitude. Other sources of localized temperature change may include aerothermal heating due to high speed cruise, localized heating due to electronic equipment, which may be constant or random/cyclic, or any other heat source or sink located near the structure. The above example is not meant to represent a set of requirements for the baseline, it is a notional example to describe the sort of structural configuration AFRL has envisioned as the baseline structure for this program.

3.1 Objective 1 – Develop and Validate Methods to Adjust Airframe Model Details to Account for

Local Stresses due to Differential Thermal Strain. Develop stress analysis tools that predicts stresses within airframe members and mechanical joints between typical aerospace advanced composite and metallic structural elements subjected to significant temperature variation. These adjustment tools are intended to be accurate at the feature level.

The contractor shall consider the problem of predicting the static strength, durability, and damage tolerance of structural joints composed of typical advanced composite materials and typical aerospace grade metallic members that have been mechanically fastened together using typical aerospace fasteners and assembly methods. The contractor shall make use of the baseline structure and structural design requirements, including thermal environment and design load spectrum to predict the stresses near and at critical design details that are typically (or actually) considered in the static strength, durability, and damage tolerance analysis. (The contractor may perform these predictions using a conventional simple to complex building block iterative development approach at their option.) The contractor shall plan for feature-based testing to assess the accuracy of the predictions and the performance of the modelling approach and correction factor method(s). The contractor shall provide the test plan, along with initial prediction of test results, to AFRL for review/concurrence prior to initiating testing. After receiving AFRL concurrence, the contractor shall execute the test plan. The contractor shall report the results of the testing to AFRL.

The specific structural materials and structural configurations are not constrained, the contractor is encouraged to consider this as a general class of problems. The objective herein is to devise an approach to predicting the state of stress in the structure subjected to a spectrum of flight loads within a spectrum of thermal fluctuations. The key predictions of interest are the far field stress within the structural members away from the joints and the stresses within the joints themselves along the length of each structural member. This would include stresses within and around multi-fastener and multi-material joints subjected to flight loads including stresses in the components arising from differential strains due to significant mismatches of material CTEs.

3.2 Objective 2 – Verify Analysis Tools Predict Airframe Stresses due to Differential Thermal

Strains at an Aircraft Level Component. Verify the analysis tools developed through achieving

Objective 1 can be combined to accurately model stresses due to differential loads in a complex structure. This effort will be initiated at the discretion of AFRL, based on outcome of the technical activity to achieve Objective 1. The contractor shall apply the tools, methods, lessons learned from the technical effort to achieve Objective 1 to predict the stresses of the selected baseline, including static and repeated loads, within a spectrum of varying thermal loads, for an extended service life. (The intent here is to represent a Service Life Extension type application.) The contractor shall provide these predictions to

AFRL prior to initiating validation testing. The contractor shall plan to verify the accuracy of the prediction through full scale testing of the baseline structure subjected to the static, repeated, and thermal load spectrum used to generate the prediction. The contractor shall provide this test plan to AFRL for approval.

Upon receipt of approval from AFRL, the contractor shall execute the verification test plan.

The intent herein is to verify the analysis methods and tools involved to the greatest extent feasible, it is not necessarily to verify the life extension of the actual structure. (Verification of the tools is the intended outcome of this objective, not an increase in service life.) The contractor should view Objective 2 as a

“graduation exercise” devised to verify the method of modelling stresses due to differential thermal strain, combined with other stresses, produce accurate results within the airframe finite element model. As such, the contractor should also demonstrate that their method of predicting the stresses due to differential thermal strain effectively addresses the Air Force needs as described in the Statement of Needs. To reduce verification test complexity and cost, the contractor may select to perform this testing against some significant, complex structural element of the baseline structure. (Full scale section of the baseline article, but not the entire baseline article.) With respect to achieving the goals of Objective 2, and at

AFRL’s option, a different structure than the contractor’s originally chosen baseline may be selected for use in the verification effort.

3.3 Objective 3 – Document and report results to relevant US Government Agencies and US

Industry. The contractor shall prepare quarterly technical and final technical reports documenting technical activity conducted during this program. The contractor shall prepare and present a final review of the research results to US Government and US Industry as directed by AFRL. The contractor shall plan for reporting technical results, lessons learned, and observations related to capabilities and remaining technical challenges to US industry and government at least twice per calendar year via technical conference or government and industry workshop, and shall consult with AFRL/RQVS regarding opportunities and venues to do so.

4.0 Operations Security (OPSEC) Operations Security (OPSEC) must be an integral part of our daily activities. As we maintain security on our future technologies that are vital to national interest, we must recognize and prepare for the threat poised against our technology. Department of Defense policies mandate a high degree of security throughout the acquisition process. However, heightened security awareness and threat-based countermeasures are particularly essential during the research and development phase when our technology is most vulnerable to espionage, sabotage, or exploitation. It is the obligation of each employee or persons involved on this contract to be constantly aware of and strictly adhere to security requirements designed to protect sensitive unclassified and other information and resources produced by acquisition, research and development, and technological security efforts outlined in this SOO. The contractor shall ensure employees receive training and follow appropriate Operations

Security (OPSEC) measures during the performance of the contract.