BAA-RQKPD-2015-0001-Call7-Atch1.pdf

Composite Airframe Life Extension Project 7

Life Assessment Tools for Advanced Composite Structures.

Statement of Objectives

22 October 2019

The Air Force Research Laboratory Structures Advanced Technology Branch (AFRL/RQVS) intends to develop analytical models to assess the residual strength of advanced composite primary structure within the USAF fleet at any point during an aircraft’s service life, and to predict when such structure will no longer meet airworthiness requirements. With the aid of such tools, airworthiness authorities may be able to more effectively assess the potential to extend the service life of such aircraft, and may be able to do so without requiring additional extensive testing to validate service life increases. In support of this goal, AFRL intends to demonstrate the current capability of advanced composite life estimating methods, of all types, to predict the residual strength of advanced composite structural elements and joints subjected to spectrum loading in a service environment. The effort will test the performance of life prediction models by using such models to predict the residual strength of structure that has been subjected to repeated service loads, by performing realistic testing upon representative structure subjected to such service loads, and by comparing predictions of residual strength to the actual test results. This will allow AFRL, the Air Force Life Cycle Management Center, and US industry to assess the current capabilities, limitations and shortfalls of the models, refine the models as feasible, and identify development needs. These results will be documented for US Government and Industry.

1. BACKGROUND AND PROGRAM DESCRIPTION

The envisioned effort will perform an analysis of an article of structure designed to represent a typical, currently in-service structural component containing at least one structural element manufactured from advanced composites joined to another structural element. This component will be subjected to structural fatigue testing sufficient to represent at least one “design service life”. The load spectrum for this service life will be developed within the program, to produce appropriate aircraft lifetime service stresses, which will be applied, to the extent practical, within a realistic service environment. The analysis will predict the residual strength of the selected test structure as a function of the service life spectrum loads applied. The overall approach will be to predict when this structure has been so damaged by usage that it no longer meets residual strength requirements, and to validate this prediction through testing the joined structure. Multiple replicate testing will be performed using the same load spectrum as was used for design. If the predictions are sufficiently accurate, the methods used for the prior predictions, will be used again to predict the residual strength of the same structure subjected to different service life spectra. This second round of predictions will be followed by another series of tests to compare predicted performance to actual results. In all cases, predictions made prior to testing will be compared to actual test results to determine the current performance of the predictive methods relative to airworthiness needs, and identify development needed.

AFRL/RQVS will accomplish this through designing at least one Common Feature Test Component

(CFTC), manufacturing at least three replicates of each CFTC test article, conducting fatigue testing on those CFTC articles, and comparing test results to predictions of failure completed prior to testing. If

AFRL finds the predictions were sufficiently accurate or can be made sufficiently accurate with some revision to the predictive methods and approaches used, additional CFTCs will be manufactured and tested under different spectrum(s) and/or service conditions.

The term “Common Feature Test Component (CFTC)” was used in CALE BAA Call #3 to describe test articles that were to be designed for use in that effort to validate the progressive damage failure analysis methods developed within that effort. CALE BAA Call #3 resulted in Contract FA8650-17-C-2700

“Assessing the Durability and Damage Tolerance of Advanced Composite Structural Features.” This BAA

Call will make use of the same terminology, “Common Feature Test Component(s)”, or CFTC(s), to designate articles to be used for the same purpose – validation of the methods proposed and developed to achieve the objectives of this SoO. While the terminology is the same as that of the prior effort, this new SoO does not require the CFTCs designed and produced under contract FA8650-17-C-2700 be utilized in this research effort. The contractor is free to propose any CFTC configuration, within the objectives and requirements of this call.

Potential offerors should note that AFRL/RQVS is not specifically requiring the use of any form, class, or method of performing progressive damage fai lure analysis. Potential offerors should also note that this lack of a requirement for PDFA implementation does not indicate a lack of interest in demonstrating

PDFA methods as applied to this project. AFRL/RQVS is leaving it entirely to the offerors to select any and all methods of predicting residual strength, and/or assessing any damage initiation, damage propagation, etc. It is up to the offerors to propose the methods they feel best suited to analysis of the structural configurations they have selected, subject to other guidance and requirements stated within this SoO.

AFRL intends to distribute the results of this research effort, including lessons learned and capability assessments, to other US Government agencies and industry as appropriate. The contractor is cautioned to avoid the use of proprietary methods, approaches, and data when performing the design of the CFTC and predicting the residual strength and/or onset and propagation of damage under spectrum loading. Use of commercially available proprietary vendor software for modelling damage initiation and/or propagation is not subject to this caution.

Potential offerors should note there is no requirement to accurately reproduce any specific, existing aircraft mission-design-series load spectrum when developing the service life spectrum. The objective herein is to apply a set of varying loads, within a varying environment, that applies realistic aircraft operational stresses to the CFTC in a cyclic manner. The magnitude of the design stresses should be representative of stresses occurring in aircraft currently operational in the USAF fleet, and the selected design load spectrum should be representative of real aircraft service load spectra. AFRL/RQVS strongly recommends that the CFTC structure configuration and the design load spectrum represent the same type aircraft applications. (As an example, if the proposed CFTC represents fighter structure, then the load spectrum should be representative of fighter design spectrums. But it is not necessary to replicate the service load spectrum of any specific existing fighter aircraft.)

AFRL/RQVS intends that failure of the CFTC during structural durability testing occur within at least one composite element of the CFTC, after the end of the first design lifetime, and before the end of the second design lifetime. This is an unlikely result for a properly designed structure subjected to the design load spectrum. Therefore, AFRL anticipates, but does not require, the use of a severe test spectrum to induce such failure. If such a test spectrum is proposed for use in th is effort, AFRL strongly cautions against the use of severe overloads that would result in non-typical damage. (Damage not to be expected from extended service.) AFRL recommends that a test spectrum be developed using higher frequency of the larger loads within the design load spectrum than occurred in the design spectrum.

AFRL will also consider use of load enhancement factors in the test spectrum subject to the caution stated earlier. AFRL anticipates working with the selected contractor to develop the test spectrum during the execution of this program. Other approaches to promoting failure in the CFTC between the first and second design life may be feasible, and potential offerors are welcome to propose such.

Block cyclic loading may be used to apply a spectrum of loads to the actual test articles, for both analysis and test purposes. If block loading is used, it is strongly recommended that the load blocks vary significantly in direction and magnitude of loads overall, vary significantly in R ratio overall, and also vary significantly from block to successive block. (A load scheme using “Big swings” from block to block is preferred over gradual changes from block to block.)

The Air Force Research Laboratory intends to perform CFTC testing required to complete Objectives 2 and 3 using test facilities managed by AFRL/RQVV. The cost of such testing at AFRL must be accounted for in developing the program proposals, the cost of the proposed contractor effort plus the estimated cost of executing the required test plan at AFRL, including inspection and examination during testing, test set-up and tear down, and data reduction as required, must be within the program budget.

Contractors are encouraged to work with AFRL to develop these estimates. Points of contact will be provided.

2.0 STATEMENT OF OBJECTIVES

2.1 Objective 1 – Select a Service Life Load Spectrum and Design Common Feature Test Articles to

Achieve Required Life Within Service Life Load Spectrum and Environment. The contractor shall determine a design load spectrum to be used in the design of the Common Feature Test Component

(CFTC) and for durability/damage tolerance/residual strength testing of manufactured CFTCs. The contractor shall identify relevant service environment considerations that should be accounted for during durability testing. The contractor shall design at least one CFTC such that it contains realistic structural features commonly found in airframes containing advanced composite structural elements.

The CFTC shall contain at least one multi-fastener joint and at least one advanced composite structural element. The inclusion of metal structural elements in the CFTC is encouraged, but is not required.

Additionally, more than one structural joint and more than two composite elements are also encouraged, but are not required. The contractor shall design the CFTC such that failure will occur within at least one composite element of the CFTC, or within a joint between at least one composite element of the CFTC and another element. The contractor shall design the CFTC such that failure will occur after completion of one lifetime of test spectrum loading but prior to completion of a second lifetime of test spectrum loading. Within the limitations of their analysis, the contractor shall provide

AFRL with predictions of failure location, load, initiation and propagation, and other factors relevant to both discovery during test and assessing future airworthiness for actual aircraft use. The contractor shall consult with the Air Force to develop a test plan to apply a test load spectrum to the CFTC. The purpose of the test load spectrum is to initiate and propagate the failure, within relevant and reproducible environmental conditions, such as moisture, temperature, and other environmental conditions relevant to the baseline for the CFTC, within the second design lifetime. The magnitudes of loads within the test load spectrum shall not exceed loads from the design load spectrum, but such magnitudes may be applied with greater frequency as required. The objective of the structural tests to be conducted will be to accurately detect loss of residual strength, failure initiation, propagation of damage and any other relevant test data indicating a reduction in the airworthiness of the CFTC . The contractor shall plan the test load spectrum such that multiple values of R, positive and negative, are within the test spectrum. The contractor shall plan to complete residual strength testing of each article after the completion of the first lifetime of spectrum loading. The contractor shall select the number of cycles to complete before residual strength testing, i.e, the point in the second lifetime of spectrum loading at which residual strength testing shall be performed. The contractor shall predict the extent of damage or degradation at that point in the testing, and/or the residual strength at that point in the testing, the failure mode, and any other failure characteristics the contractor considers significant to validating the predictive methods. The contractor shall conduct the Objective 1 Assessment Review to present the results of the Objective 1 technical effort to AFRL.

2.2 Objective 2 – Demonstrate the Accuracy and Performance of Design and Life Assessment Tools

Through Test. The contractor shall manufacture at least three replicates of each CFTC designed during objective 1. The contractor shall send these test articles to AFRL for testing in accordance with the test plan developed in under Objective 1. The contractor shall assess the performance of the selected life assessment tools by comparing test results to the life and failure predictions developed under Objective

1. The contractor shall make recommendations to the AFRL program manager for revisions to the tools, methods and approaches of prediction, as necessary.

2.3 Objective 3 - Demonstrate the Robustness of the selected Life Assessment Tools. After receiving approval to proceed from the AFRL program manager, the contractor shall use results obtained from the spectrum testing of Objective 2 to revise their selected life assessment tools, methods, procedures, etc as required. The contractor shall consult with AFRL to develop two new, spectra of fatigue test loads, subject to the same requirements as the spectrum of Objective 1. The contractor shall use the same methods and processes as used to predict prior CFTC failure, revised as necessary, to predict the initiation of damage and the propagation of such damage within the CFTC for the original set of fatigue loads as used in Objective 1 and for the two new sets of fatigue loads. The contractor shall provide these predictions to AFRL prior to the initiation of CFTC testing in support of this objective. The contractor shall plan to complete residual strength testing of each article, the contractor shall select the number of cycles to complete before residual strength testing, and shall predict the extent of damage and the residual strength, and the failure mode, at that number of cycles. For each CFTC design, the contractor shall manufacture at least three more test articles, identical to those designed through objective 1. The contractor shall send the three articles to AFRL for fatigue testing in accordance with the revised fatigue test plans as developed under this objective. The contractor shall assess the performance of the revised durability and damage tolerance models by comparing test results to the predictions of damage initiation and propagation developed under this objective, and make recommendations for improvement.

2.4 Objective 4 – 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, to include assessing results, documentation of lessons learned, and observations related to capabilities and remaining technical challenges. The contractor shall prepare and present a Contract Kickoff Review, and a test planning and coordination review, to be presented to AFRL and other government attendees at AFRL. The contractor shall prepare and present a

Final Technical Review of the research results to US Government and US Industry at AFRL. The contractor shall prepare for one technical interchange meeting at an appropriate time, as directed by

AFRL, during the execution of the effort. 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 via technical conference or government and industry workshop, and shall consult with AFRL/RQVS regarding opportunities and venues to do so.

2.5 Security - 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 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.