10:50   Session 12: Fatigue life enhancement and repair solutions II Chair: Takao Okada 10:50 20 mins Bonded prestressed method for fatigue crack repair Wandong Wang Abstract: Fatigue life extension methods and fatigue crack repair methods have always been a vibrant research topic, seeking effective methodologies to significantly prolong the service life of aircraft. Bonding fibre reinforced epoxy composite patches have delivered superior fatigue crack growth life extension in comparison with mechanically joining metal patches. The longer fatigue life extension of bonding composite patches results from the bridging mechanism and the circumvention of new fatigue hot spots. However, tensile residual stresses in the parent metal structure due to mismatch of thermal expansion coefficients of composites and metal structures have adverse effects. A state-of-art fatigue crack life extension methodology is to bond prestressed patches. There are three fatigue crack retardation mechanisms in this system, namely added load path, bridging mechanism and compressive stresses due to prestressing. The emergency of an iron-based shape memory alloy (Fe-SMA) that exhibits excellent shape memory effect and generates notably high recovering stresses makes the bonded prestressed method for fatigue crack repair feasible to achieve. An experimental campaign has been carried out to study the feasibility of developing such a bonded prestressed fatigue strengthening method and to demonstrate its repair efficiency. The specimen configuration consists of a metal plate with a central crack repaired with Fe-SMA patches on both sides. Different activation methods to generate compressive stresses around the crack area in the metal plate using the shape memory effect of Fe-SMA were investigated. Different patch sizes were also studied. The repaired specimens were then subjected to tensile-tensile fatigue tests with beach marking technique. The a-N data were analysed and the role of bridging mechanism and prestressing were discussed. The experimental results demonstrate the superior repair efficiency the newly developed repair solution where the Fe-SMA is bonded and activated. The fatigue life can be extended several times and even complete crack arrest can be achieved. The great potential of bonded prestressed fatigue strengthening method to significantly prolong the service life of fatigue damaged aircraft structures is substantiated in this paper. 11:10 20 mins Tools and methods for landing gear fatigue analysis with surface treatment effects Rob Plaskitt, Michelle Hill, Andrew Halfpenny, Ben Griffiths, Andrew Clark, Ben Madsen Abstract: Landing gear manufacturing and overhaul/maintenance processes alter surface material properties that influence fatigue life. The effect of these processes was not accurately accounted for in legacy United States Air Force (USAF) landing gear designs. As aging aircraft in the USAF fleet continue to be pushed beyond their originally intended service life, it has become increasingly more critical to characterize the effect of specific surface processing conditions on fatigue life. For this reason, USAF Landing Gear Systems (417 SCMS/GUEA), Select Engineering Services (SES), General Atomics Systems Integration (GA-SI), and the Hottinger Bruel & Kjaer (HBK) Advanced Materials Characterization & Test facility (AMCT) are conducting research/testing to develop material fatigue data, tools and methods that incorporate surface treatment effects for USAF landing gear fatigue models. Surface treatment factors (K factors) from this strain-life fatigue testing programme are used to modify baseline strain-life curves to account for material surface conditions during fatigue prediction calculations from finite element stress analyses. HBK and SES have tested and characterised >20 fatigue curve datasets to derive multiple surface treatment K factors for 3 common landing gear materials; 300M steel, 4340 steel and 7075 aluminium alloy. Surface treatment conditions include chrome plating, anodising, shot peening and combinations and/or repetitions of these, for example, “shot peen, chrome, strip, chrome” to represent repeated landing gear overhaul/maintenance processes. SES has developed material assessment and predictive analysis tools for these fatigue test data for surface treatment K factor calculation, visualisation, and integrated application into fatigue simulations. The presentation will: • introduce the project and relation to USAF landing gear surface treatment procedures • provide an overview of material and surface treatment fatigue tests and characterizations • describe surface treatment K factor raw fatigue data and fitted strain-life curve tools • describe integrated application into fatigue simulation and predictive analysis tools Positive outcomes and conclusions resulting from completed surface treatment research and analysis include: 1. Increased confidence in life extension for aging aircraft landing gear components 2. The timely removal of landing gear components from service to decrease the risk of failure 3. The potential for improvements to repetitive overhaul processes that will reduce their negative impact on fatigue life 11:30 20 mins Assessment of chromate free alternatives as paint primers Jay Patel Abstract: Chromate-based primers are widely used on aircraft to protect the aluminium structure from corrosion. Strontium chromate compound has been authorised under REACH legislation but the authorisation is time limited to 7 years, from the sunset date of January 2019. A search for chromate-free alternatives is known to be conducted by the various aircraft manufacturers. However, this effort is considered to be largely confined to new rather than legacy aircraft. A suitable alternative found may be unique for the aircraft manufacturer and which may not be appropriate for the military fleet that operates aircraft from several different manufacturers. The current UK National Standard for procuring paint primers, BS 2X 33, specifies a strontium chromate formulated product. In addition, the corrosion test specified in BS 2X 33 is also considered to be affected by REACH legislation. It is also widely considered that the set of standardised current test methods would not adequately predict the performance of the novel chromate-free primers. A new or revised standard is required for chromate-free primers that is acceptable nationally. There is therefore a requirement to (a) evaluate suitable test methods and (b) to use the developed test protocol for evaluating chromate-free primers for aerospace purposes. For this purpose, a draft test protocol was agreed with stakeholders including aircraft manufacturers’, paint manufacturers’ and interested members of British Standard Committee responsible for paints for aerospace purposes. The test programme comprised of tests for assessing colour, gloss, artificial and natural weathering, adhesion, flexibility, fluid resistance and corrosion resistance properties. Some of the corrosion tests were conducted with exposure periods up to 6000 hours and included the chromate containing primer that was used as a control, to compare the results from the chromate-free products. The candidate products were applied onto un-treated 2024-T3 and 7075-T6 aluminium as well as commercially available chromate-free pre-treatments for aluminium alloys. The laboratory work is nearing completion with a report being prepared. Simultaneously, the test programme and the results obtained are expected to be used for drafting a new national standard for chromate-free primer paint system for aerospace purposes. 11:50 20 mins Optical simulation of scratch repair in F/A-18 transparences Matti Okkonen, Aki Mäyrä, Aslak Siljander, Mika Siitonen Abstract: An on-aircraft system for assessing scratches and dents in F/A-18 transparencies (windshield and canopy) and applying optical simulation for optimizing their repair is presented. The motivation was to automatize the detection and quantification on transparency defects and to assist the manual repair process to minimize induced optical distortions by replacing visual inspection and subjective decision making with automatic, repeatable and objective solution. The information can be applied to optimize the whole logistics chain of the repair process and to maximize aircraft availability. The system developed consist of three consecutive measurement tasks: First a machine vision-based scratch/dent detection and mapping enabling detecting and classifying defects for further inspection. Second, the dimension of the chosen defects are measured. Third , based on the defect location and dimension, optical simulation is applied to estimate the effect of the repair process, and a most suitable one is chosen regarding optical distortion. The above on-aircraft machine vision system developed for defect detection automatically scans and subsequently maps all defects on the transparencies and provides estimates of their scattering from the pilot perspective. The resulting map enables automatic classification of the scratches and dents found, based on user requirements. It also enables monitoring of the transparency defects’ evolution through time, thus providing a modern tool for the transparencies’ life cycle management. In addition to the above, a manually operated optical micrometre was developed for measuring the depth of the scratch or dent, based on 3D imaging. Last but not least, optical simulation models were developed for connecting the optical distortion induced by the material extraction in the repair process and vice versa. With the simulation, the effect of the repair can be mapped into standardized optical distortion measurement for evaluation. The performance of the defect mapping and depth estimation was assessed by real transparent samples and reference measurements. The simulation models were validated by comparing them with a real defect repair with before and after measurement of optical distortion. The developed system will be fielded to operational use within the Finnish Air Force. 12:10 20 mins Using digital twins to accelerate qualification and certification of fatigue critical components Gary Whelan, Jiadong Gong, Greg Olson Abstract: Fatigue of engineering alloys is a major challenge in aerospace applications. Developing and qualifying fatigue critical components for use in aerospace requires manufacturing and testing of a significant number of test coupons. This process is highly time-consuming and expensive. QuesTek’s ICMD® modeling software can provide reliable property predictions that can significantly lower the amount of testing required while still yielding a robust material property dataset. Utilizing integrated computational materials engineering methodologies, QuesTek has been developing microstructure sensitive fatigue models that can account for both intrinsic (e.g., grain size, grain morphology, phase fractions, crystallographic texture, etc.) and extrinsic (e.g., inclusions, surface roughness, porosity, etc.) features that drive fatigue life in engineering alloys. QuesTek uses a fatigue modeling framework combining crystal plasticity finite element method to predict fatigue indicator parameters at the mesoscale, with microstructurally small crack propagation and physically long crack propagation algorithms, to predict the full fatigue life from incubation to ultimate fatigue failure in both high and low cycle fatigue. This approach depends on both microstructure characterization used to generate the inputs for a microstructure digital twin, as well as a select few experiments to calibrate the physics-based models. By simulating the majority of loading scenarios of interest, testing can be targeted to just the most informative experiments during qualification. This approach offers three key benefits when compared with traditional design of experiment approaches; (1) decreased cost of qualification, (2) decreased time for qualification, and (3) improved mechanistic understanding of the key driving features for fatigue failure in a given alloy system, enabling optimization and improvements. QuesTek has recently made major strides in improving this modeling approach by decreasing the computational cost of simulations, making the approach feasible on QuesTek’s cloud-based software, and by incorporating key microstructure features of interest for additively manufactured alloy such as anisotropic grain morphology/texture, porosity, and surface roughness. While this toolkit can be applied to traditionally manufactured alloys, it is particularly impactful for additively manufactured alloys due to their complex microstructures which result in difficult and expensive qualification processes when microstructure sensitive models are not utilized.

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