Adhesive Bond Process Qualification Protocol, Waruna Seneviratne, John Tomblin, and Upul Palliyaguru, Wichita State University
Development of certification road map for bonded structures involves demonstrating static strength, durability, environmental effects (including data scatter) and damage tolerance assessment of various bonded joint types. Adhesive properties (chemical and mechanical) are influenced by environment and bonded joints tend exhibit excessive data scatter. These factors influence standard industry practices for demonstrating compliance and may cause overly conservative designs. For example, large scatter in static/residual strength and fatigue SN data can produce unrealistic load-enhancement factors for durability and damage tolerance structural test substantiation. This task will expand the on-going efforts that primarily address lower levels of building-block protocols (bond-process qualification and adhesive characterization) into structural substantiation levels including strategies for developing protocols for environmental compensation factor (ECF) and load-enhancement factors (LEFs) for bonded structures. In addition to the development of certification road map for bonded structures, development of advanced material and process technologies is critical to adhesively bonded joints in aircraft primary and secondary structures. The quality assurance of bonded joints is a challenging task due to the lack of reliable methods for nondestructively determining the bond strength. Certification agencies and regulators often develop a mistrust on bonded structures because the strength and the life of the joint cannot be determine with confidence and historical bonded structural failures, primarily due to improper bond processes including unacceptable surface preparation practices. Therefore, developing adhesive bond process qualification protocol is a critical step towards ensuring structural integrity of bonded joints. Such protocols must include methods for determining integrity of materials prior to processing, surface preparation procedures, nondestructive inspection (NDI) of prebond surfaces, process control (traveler) coupons, post-bond nondestructive evaluation for manufacturing defects, quality control procedures, and training of personnel. Note that different qualification protocols must be developed for different adherend materials. For example, the surface preparation techniques for composites and metals are significantly different. Further, different aluminum alloys, steel, and titanium surface preparation techniques have significant differences. The primary goal of this research program is to develop a bond qualification protocol (BQP) for quality assurance of bonded joints. Secondary goal is to develop procedures for including adhesive test data generated following such protocols into shared databases such as Composite Material Handbook (CMH-17).
Advanced Fiber Reinforced Polymer Materials Guidelines for Aircraft Design Certification Process, John Tomblin, Rachael Andrulonis, Royal Lovingfoss, Wichita State University
As an advanced material of increasing interest to the aerospace community, thermoplastic composites have been selected for this initial phase of the Advanced Fiber Polymer Matrix Composites program. A framework for the qualification of thermoplastic continuous fiber composites has been developed and is currently being applied to a PAEK composite material. The material, Toray T700 12k/TenCate TC1225 unidirectional tape, was selected based on feedback from an Industry Steering Committee (ISC) survey at the start of the program. Throughout the duration of the program, the ISC has played an integral role by providing expertise in areas such as test method applicability, test temperature selection, test plan reviews, and pre-qualification screening data evaluations. Significant screening trial data and initial physical and mechanical qualification results have been generated. The trials included DMA and TMA, short beam strength, in-plane shear, and open hole compression (lamina and laminate) tests at several conditions from room temperature up to 450°F in both ambient and wet conditions. The final qualification test matrix includes several test temperatures from -65°F up to 450°F at both dry and wet conditions for select properties. An overview of all data generated under this program and transition plans for the data, specifications and guidelines to industry publications such as CMH-17 and SAE International will be presented. The expansion of the framework developed to other material systems as future work under this program will also be discussed.
An Engineering Approach for Damage Growth Analysis of Sandwich Structures Subjected to Combined Compression and Pressure Loading, Waruna Seneviratne, John Tomblin, Shenal Perera, Pirashandan Varatharaj, and Vishnu Saseendran, Wichita State University
Fluid-ingression phenomenon in composite structures is a concern for sandwich structural details. Inadequate design details and/or poor material selections can result in microcracks during ground-air-ground (GAG) cycling that consequently coalesce to form transverse matrix cracks that lead to moisture ingression into the subsequent composite and adhesive layers and finally into the core. Impact damages on sandwich structures exacerbate the fluid-ingression phenomenon as a result of localized transverse cracks, delaminations, disbonds, and core damages. Thermo-mechanical loads during GAG cycling could cause the local buckling on compression side of a sandwich structure that result in localized mode I stresses that may result in further delamination/disbond growth creating more passageways for fluid migration. Additionally, the trapped water in sandwich cells translate into vapor during high temperatures and increase the internal pressure and cause core disbond and/or fracture. In some cases, the damage growth due to the above-mentioned two mechanisms is stable and occurs over a period of several flights, but may not be readily detected on the ground, when the thermo-mechanical and internal vapor pressure loads are released. Although the damage size continues to grow in such cases, the structure continues to carry loads until it reaches a critical damage threshold (CDT), where the unstable damage growth triggers the catastrophic failure. Unless such damage is detected and repaired prior to reaching CDT, GAG effects will further the damage size and threaten the structural integrity and safety of the aircraft. Current phase of the research focus on investigating the effects of ground-air-ground (GAG) cycling on damage growth behavior of sandwich structures with synchronized temperature, pressure and mechanical loads; investigate the conditions for onset of damage growth and damage growth rates. Also, a standardized procedure and test apparatus for GAG testing for simulate damage growth due to mixed-mode stress state caused by pressure differential at high altitude coupled with in-plane mechanical loads is developed. Furthermore, predictive capabilities of onset of damage growth and progressive failure mechanisms using the cohesive zone modeling coupled with multi-scale material models are evaluated. An analytical model based on the Winkler foundation approach is shown to accurately predict the debonded facesheet. The mathematical model closely agrees with both numerical and experimental results. The information gathered through this research will be instrumental in developing analytical methods and validating finite element analysis procedures to further investigate the damage growth mechanics of sandwich composite structures.
Ceramic Matrix Composite Materials Guidelines for Aircraft Design and Certification, John Tomblin, Rachael Andrulonis, Matt Opliger, Wichita State U.
In order for ceramic matrix composite (CMC) materials to be safely qualified and certified, appropriate standards must be available to assure that the current level of safety that exists for traditional polymer matrix composite (PMC) materials is maintained. A framework has been developed, specific to CMCs, to provide consistent material and process control and a protocol to characterize the material at application relevant conditions. Under this framework, mechanical and physical test results, as well as lessons learned on processing sensitivities of CMC materials, test method applicability, and unique considerations for machining and testing at room and elevated temperatures have been generated. An overview of the JAMS qualification program that is currently validating this framework for Oxide/Oxide CMCs will be presented. The overview will include the primary goal of developing a framework for qualification of CMCs with specific task updates on the status of the qualification program in development with an Oxide/Oxide prepreg. Plans to transition the data, guidelines and specifications to the Composite Materials Handbook -17 (CMH-17) and SAE International will also be presented.
Certification by Analysis – Full Scale Ditching Analysis, Gerardo Olivares (PI), Luis Gomez, Armando Barriga, Chandresh Zinzuwadia, National Institute for Aviation Research, Wichita State University; Allan Abramowitz, FAA William J. Hughes Tech Center
The metallic and composite airframe crashworthiness research at NIAR has focused on developing a certification by analysis methodology that can be used in the future by the aerospace industry. Aerospace crashworthiness structural requirements were defined using detailed finite element models of a metallic narrow-body transport aircraft to study the crash worthiness response of a typical metallic aircraft structure during survivable impact on hard surfaces, soft soil, and water. Typical energy absorbing requirements, loading, and strain rates for various structural components were defined. At the coupon-level, several issues and challenges associated to the dynamic material response of composites materials were addressed by conducting coupon-level tests over a range of strain rates for which testing techniques have not been standardized yet. Component-level test were used to validate the coupon-level material data and the strain rate sensitivity at the component-level of various composite material systems. Joints and connections were evaluated at the element-level. A series of experimental tests at different loading rates were conducted to evaluate single point load transfer mechanisms between structural members. The results were used to evaluate the limitations and capabilities of various modeling techniques used to virtually join two components. At the full-scale level, the experimental and numerical best practices developed through the course of the research are used to develop a Finite Element Model of a representative Narrow Body Aircraft. The model is used to evaluate the ability of a full aircraft numerical model to predict the structural deformations and passenger injuries during crash events. The crash of Turkish Airlines Flight 1951 was selected for the evaluation. The structural response predicted by the model is compared with the crash data provided by the accident investigation team. The model provides researchers with comprehensive and thorough data on the behavior of the aircraft structure and passenger injuries when subjected to real crash scenario. This model will be used to further study different variety of crash conditions (e.g. ditching). This paper will summarize the response of a typical narrow body transport aircraft when subjected to survivable ditching events. This data will be provided to the ARAC Ditching working group to provide a better insight on the physics of typical ditching events and guidance on how to use analysis tools in the future to support the development and certification process of aircraft to meet ditching requirements.
Effect of Surface Contamination on Composite Bond Integrity and Durability, Brian Hernandez Dwayne McDaniel and Benjamin Boesl, Florida International University
Proper surface preparation of adhesively bonded composite joint surfaces is critical in maximizing bond performance. What is less understood, are the effects of undesirable conditions on surfaces that are to be bonded. Contamination on surfaces has been significantly reported in the literature as a critical factor of these undesirable conditions that affect adhesion. For this study, a uniformly distributed contamination approach was evaluated to determine its effects and further assess techniques to mitigate contaminated surfaces in order to return the bond performance to nominal bond strength levels. The same approach was also used to investigate bond durability of the contaminated specimens.To evaluate the process of the uniformly sprayed contamination, DCB specimens were manufactured using baseline conditions along with low-, mid-, and high-level of contamination (Frekote) applied using an automated spraying machine developed at FIU. Methods to mitigate the contamination included sanding and solvent wiping. In addition, specimens representing each surface condition were aged in an environmental chamber for 4 weeks to assess the effects on durability. Bond strength and fracture mechanisms were evaluated at both macro and micro scales using traditional testing methods and in situ testing within an electron microscope. Results of DCB testing showed that while the solvent wipes led to minimal statistical change on fracture toughness, the wipe-sand-wipe procedure led to a significant increase and recovery of bond strength. It was also found that the 4-week period of environmental exposure had no significant effect on bond integrity for each surface condition. Furthermore, micro-scale fracture via micro DCB testing, on pristine and contaminated specimens, revealed the mechanisms of deformation in each condition and locations of initial damage. To further evaluate the durability of the adhesively bonded joints, initial strength and strength after cyclic loading will be measured for specimens with mid-level uniformly distributed contamination and specimens with the mitigation procedures applied.
Evaluation of Parameters used in Progressive Damage Models, David Plechaty and John Parmigiani, Oregon State University
Understanding damage initiation and propagation behavior is critical to the design of effective and safe load-bearing carbon fiber composite components. The ability to predict this behavior through finite element simulations is particularly important for optimizing designs and minimizing costly experimental validations. In order for these simulations to be of greatest use they must include progressive damage models which accurately model the initiation and propagation of damage. Such damage is often categorized by the loading mode causing it resulting in fiber tension, fiber compression, matrix tension, and matrix compression progressive damage models. Of these, significant published research exists for fiber tension, fiber compression, matrix tension. However, prior to recent work at Oregon State University (OSU), very little published work existed for matrix compression. The work at OSU has resulted in the creation of a test specimen capable of isolating matrix compression damage initiation and propagation for both commercial carbon fiber material (GRAFIL TR50S/NEWPORT301) and Boeing proprietary carbon fiber material. This specimen is an all-90°-ply compact compression specimen with a reduced-thickness region where matrix compression is isolated. Determination of an accurate matrix-compression progressive-damage model was begun using this specimen for both commercial and proprietary materials. Very promising preliminary results were obtained, however manufacturing defects consisting of cracking in the reduced-thickness region and ply delaminations inhibited testing the required large numbers of specimens. Also, testing parameters such as notch length and testing speed needed to be determined. The manufacturing defects were addressed through several changes: fixed steps on the layup plate were replaced with floating shims, a caul plate, was added, the type of mold release was changed, and the type of removal tool was changed. Suitable testing procedures were identified though testing notch lengths of 0.25 inches to 1.875 inches at displacement rates of 0.004 in/min to 0.2 in/min. The changes resulting in significant improvements. Prior to the manufacturing modifications, defect rates were up to 90%, afterwards defect rates were consistently well below 50%. The investigation of notch length and testing speed showed that shorter notches and speeds of 1 mm/min gave the best results. With effective manufacturing procedures and testing parameters, determination of matrix-compression material model can proceed.
Fatigue Damage Growth Analysis of Composites under Variable Amplitude Fatigue, Waruna Seneviratne and John Tomblin, Wichita State University
Due to the anisotropic and heterogeneous nature of composites, fatigue damage growth characteristics of composites are complex and predictive analysis methodologies are at their infant stages. Therefore, overly conservative assumptions are made for life assessment without taking full advantage of fatigue capabilities of composites. In order to design efficient composite structures, a greater understanding of fundamentals of fatigue damage initiation and growth characteristics of composite is needed. NIAR researchers are developing a database of fatigue damage growth data using X-ray computed tomography or XCT that will include fatigue damage growth data under constant amplitude fatigue as well as realistic fatigue spectra representing fighter, transport, and bomber operations. High-fidelity inspection results can then be used for validation of high-fidelity analysis methods that can capture complex damage evolution of composites. One example of a finite element analysis method is an USAF developed mesh-independent b-spline analysis method or BSAM, which uses regularized extended finite element modeling (RX-FEM) techniques. This analysis technique is capable of modeling matrix cracks, delamination, and fiber breaks. XCT is one of the high-fidelity inspection techniques that can provide volumetric details of composites so that the damage evolution and interaction of different failure modes can be observed with clarity. Damage growth of composites is typically a mixture of several failure modes including matrix cracks, delaminations, and fiber breaks. Understanding of the interactive nature of these mix-failure modes under variable amplitude fatigue loading is essential for fatigue analysis of airframe structure under spectrum loading. Since these failure modes are not readily detectable, damage growth characterization is challenging. A technique known as 4D XCT, a collection of three-dimensional XCT data over fatigue lifetime, is being used so that the damage evolution under cyclic loading can be captured with ply-by-ply detailed information for validating high-fidelity analysis predictions. This research focuses on developing a quantifiable, risk-based assessment methodology for determining the service life of advanced composite structures aided by high-fidelity damage modeling and high-resolution damage growth inspections using XCT. Life prediction tool based on residual strength degradation that will be validated with these experiments under this task can then be used for enhancing structural substantiation during certification and for prognosticating component life and developing suitable fleet maintenance strategy to minimize the risk of structural failure without the heavy burden of routine inspections.
Guidelines for Characterization of Repair Materials, John Tomblin, Rachael Andrulonis, Royal Lovingfoss, Jeff Gilchrist, Wichita State University
In order for new repair material platforms to be safely qualified and certified, the part must be proven to be well-designed for operation, safety and durability. The primary goal of this JAMS program is to develop a qualification framework, specific to the repair material platform, which considers the variability of the process and outlines parameters needed for process control. The material selected for validation of this framework, based on industry feedback through a survey and discussion with industry members, is Solvay 5320-1 T650 PW with FM300-2 adhesive. A unique test plan focusing on repair specific qualification tests was developed in conjunction with the Industry Steering Committee and the SAE Commercial Aircraft Composite Repair Committee (CACRC). As the material selected has previously been qualified by the National Center for Advanced Material Performance (NCAMP) in a non-repair process, the test plan includes an equivalency portion of non-repair specific tests, as well as a set of repair qualification tests, all utilizing a new repair processing specification. An overview of program, the results of the equivalency, qualification methodology selected and current status including the qualification mechanical test results, will be presented. In addition, transition plans to submit the data and specifications to CMH-17 and SAE International will be discussed.
Guidelines for Formulating and Writing Process Control Documents (PCD) and Process Specifications for Advanced Materials, John Tomblin, Royal Lovingfoss, Rachael Andrulonis, Wichita State University
The Federal Aviation Association (FAA) and the aviation industry (OEMs and their suppliers) both require guidelines concerning how documentation should be formulated and written for the processes surrounding the manufacturing of advanced materials for use in aviation related programs. Currently only a limited number of guidelines exist for certain material processes associated with specific material types. No documentation exists that gives overarching instruction on the information that should be included in material process control documents (PCD) and specifications called out in such documents. This JAMS project is generating a guidelines document that describes the documentation and key process parameters that must be in place to ensure that the material being manufactured has the appropriate pedigree and controls to be considered feasible for use as part of an FAA certification program. Material types covered in the guidelines include thermoset composites, thermoplastic composites, RTM composites, ceramic matrix composites, adhesives, and polymer based additive manufactured materials. An overview of the general guidelines as well as the material specific sections will be presented.
Inspection and Teardown of Aged In-Service Bonded Repairs, Waruna Seneviratne, John Tomblin, and Brandon Saathoff, Wichita State University
In-service repairs performed with trained technicians and qualified materials and processes must restore the original structural capability with minimal alteration to the original load path to ensure the safe operation of aircraft throughout the remaining life of the component. Therefore, long-term durability under operational environments and GAG loading must be understood and the aging mechanism must be investigated to support maintenance practices and to establish criteria for structural retirement. Detailed nondestructive inspections (NDI), teardown inspections, and laboratory testing of bonded repairs on aircraft components that have been retired from service provide vital information related to the aging mechanism and any undetected material degradation. The current research program intends to conduct aging studies with the help of industry to gain the related background data on design, service history of individual component and relevant repair information (ex., known service problems related to fluid ingression or disband). Several decommissioned structural members, both metal and composites, with multiple repairs will be subjected to detailed teardown inspections and limited cyclic loading on selected articles, where applicable, in order to determine the remaining life of those repairs; the primary focus of the research will be on the teardown inspections and bond integrity evaluation. Background data on each repair such as repair process and historical flight data will be correlated to test observations on bondline integrity and durability of repairs. The main goal of this research program is to evaluate bondline integrity and durability of in-service repairs on composite structures in commercial aircraft in order to provide guidance into AC 65-33 (Development of Training/Qualification Programs for Composite Maintenance Technicians) and AC 43-214 (Repairs and Alterations to Composite and Bonded Aircraft Structure). In addition, the research scope is increased to identify a "fleet leader inspection approach" to evaluate structural bonding through teardown and detailed inspections for "aged general aviation aircraft structures" and establish protocol for such an evaluation that will facilitate future advances for new products.
Polymer-Based Additive Manufacturing Guidelines for Aircraft Design and Certification, John Tomblin, Rachael Andrulonis, Royal Lovingfoss, Paul Jonas, Wichita State University
A standardized qualification framework and database for advanced materials, such as additively manufactured materials, is critical to the design and insertion of these emerging materials in aerospace or other applications. Leveraging the experience and lessons learned from the National Center for Advanced Material Performance (NCAMP) and the Composite Materials Handbook – 17 (CMH-17) qualification databases, the first qualification program for polymer additive materials has recently been completed as a JAMS project. This program utilized several currently available standards and test methods as well as modified methods to adapt to the unique nature of AM. As part of this collaborative research effort in conjunction with America Makes, new material and process specifications were developed to fit the unique needs of polymer AM materials. A summary of completed qualification and equivalency results for the selected material and process, Stratatsys ULTEM 9085 manufactured on a Fortus 900MC machine, along with an overview of the published framework documents, specifications and test plan for polymer additive materials, will be presented. Transition plans for the data generated and industry specifications, future work including ongoing research in the areas of test method applicability, new material qualifications, and factors affecting qualification and certification, will also be discussed.
Standardization of Incandescent Ignition Source Detection Methodology for Composite Structure Lightning Testing, Billy Martin, Alyssa Gonzaelz; Wichita State University
This project involves the development of a procedure to be used in the detection of incandescent ignition sources in composite fuel tanks due to low level lightning currents. Currently the methods used results in a number of edge glow false positive failures that occur with current photographic method, which deems any light (above the threshold determined through calibration) a failure. A round robin test procedure has been developed, in conjunction with the SAE AE-2 and Eurocae WG-31 Lightning committees. This procedure has been distributed to multiple USA and European lightning labs to conduct testing, with NIAR tabulating the results, finalizing the procedure and incorporating this procedure into the current SAE and Eurocae standards, which are referenced within FAA/EASA advisory material. It is also possible that this procedure, or at least references will be included in CMH-17.