Abstract
Finite element simulations of bonded repair technology can greatly reduce the cost of repairing ageing and damaged aircraft structures. In this study, finite element simulation and analysis are performed for several bonded repair techniques of damaged aircraft structures with cracks. The simulations start from fatigue damage accumulation, crack initiation, crack repair, to fatigue crack re-initiation until structural failure. The effectiveness of bonded repair techniques is assessed by comparing the service lives of no repair, patch-bonded repair (live repair), stop-drill repair, and damage removal repair. It is found that the load attraction by repair patch can greatly sustain fatigue crack growth, leading to more than at least 2 times longer service life before the skin structure needs to be replaced. Damage removal bonded repair can further extend service life by more than 20 times comparing to no repair, benefiting from the fatigue damage tolerant service life extension. Along with the service life comparison, we also established a simulation framework that lays out the groundwork to perform aerostructure bonded repair effectiveness evaluation. The results demonstrate that finite element analysis can be efficiently used to simulate the various forms of bonded repairs and effectively evaluate fatigue crack growth and service life with structural damage. Such a rigorous simulation framework enables the future design of new repair techniques for aircraft structures.
Keyword : aircraft structures, finite element simulations, bonded repair, fatigue crack growth, surface crack, service life
Introduction
Bonded repair technology is an essential and vital component in aging military and commercial aircrafts [1]–[3]. As aircraft structures age, they can become progressively more susceptible to fatigue cracking and other forms of structural damage, thus can significantly impact their service life [4]. Bonded repairs can economically repair aging and damaged aircraft structures, often without removing components from the aircraft. Externally bonded composite patches are an effective method of repairing cracked or damaged structural components [5]. The bonded repair methodology was first used to repair cracks in military aircraft, it has recently been applied to civilian aircraft [6]–[9]. However, based on the Delegated Engineering authority (DEA), currently conducted repairs are heavily based on historical, experimental data (in accordance with T.O. 1-1A-81 [10] and T.O. 1-1-6912 [11]), which lacks a process that allows repeatability and further hinders the developments of new bonded repair techniques. Bonded repairs can involve bolted patch repair, adhesive bonded patch repair [12], scarf, and etc. A simulation study has shown that bolted patch repair and adhesive bonded patch repair can have different efficiencies and effectiveness [13].
Baker and Jones [1] enumerated that the conventional approach to through-life-support for aircraft structures can be divided into the following phases: (i) detection of defects, such as cracks and damage, (ii) diagnosis of their nature and significance, (iii) forecasting future behavior-prognosis, and (iv) prescription and implementation of remedial measures including repairs. Considerable scientific efforts have been devoted to the development of science and technology for the first three phases. Among them, analytical analysis of fracture mechanics in predicting residual strength in the presence of cracks (damage tolerance) and rate of crack propagation under service loading has been a major focus. Intensive effort is currently being devoted to developing similar approaches for fiber composite structures, to assess damage tolerance and durability in the presence of delamination damage [14]. Until recently there has been limited attempts to develop a process for the last phase, with respect to the evaluation of repairs. Most of the analysis tools have been focused on empirical relationships between crack growth and stress concentration factor [15]. Finite element modeling and simulation have shown promise in analyzing behaviors of aircraft structure repairs [9], [16]. However, rigorous approaches are required to allow assessment of the type and magnitude of defects amenable to repair and the influence of the repair on the stress intensity factor and most importantly the extension on their service life [17]. Such an approach is also required for the development and design of optimum repairs and for assessment of their durability.
This paper attempts to design and develop a simulation approach to address the above-mentioned technical challenges by setting up simulation framework using FEA software, Ansys [18]. Although Ansys has capabilities to perform stress analysis and even predict crack development [19], the process of correlating stress field, damage, crack initiation, crack propagation and ultimately service life estimation, is not straight-forward and requires further exploration and development of a repeatable and deployable analysis process for bonded repair applications in Damage Tolerance Analysis (DTA) on aircraft structures. Therefore, there is a critical need [10] to design and develop capabilities and a workflow that allow repeatable process where finite element analysis tools/packages, can be effectively and efficiently deployed to determine the fatigue characteristics on aircraft structures prior to, and after bonded repairs. The development of such methodology is to enable a robust and repeatable FEA engineering process so that: (i) the Bonded Repair Center of Excellence (COE) can more effectively evaluate the quality of both common fatigue driven wing plank repairs on aircraft and eventually unconventional repairs on weapon systems, (ii) the COE will be better equipped to develop a vast array of repairs with a higher degree of confidence and accuracy, (iii) the COE will be more confident in establishing maintenance service schedule.
In this study, aspects of the design process as well as the results of a constant-amplitude fatigue test program are outlined. The results of a three-dimensional finite element analysis, of both the repaired (of varying techniques) and unrepaired specimens, are presented with predictions of crack-growth rates and service life. Our developed framework can generate common and unconventional damage scenarios. To successfully develop a simulation workflow of bonded repair, we intend to address the following objectives in this study:
  1. Develop a proof-of-concept “Bonded Repair Simulation Workflow” (BRSW) with Ansys suite, which carries out bonded repair design process using Finite Element (FEA)-based Damage Tolerance Analysis (DTA). This FEA process allows users to effectively and repeatedly evaluate bolted bonded repair design effectiveness by employing various tools offered by the existing Ansys software suite. The solution addresses the unique challenges in the bonded repair DTA analysis with advanced capabilities which are based on fatigue and fracture mechanics theories [18].
  2. Validate BRSW with use-cases on predicting fatigue damage evolution, crack initiation and propagation with service life estimation, and bonded repair effectiveness. This study presents a bolted patch repair use-case, where the implemented BRSW FEA repair process can effectively quantify the damage, crack initiation, fatigue crack propagation, service life and repair effectiveness in repaired damaged aircraft structure. The BRSW process will be applied to repaired and un-repaired cracked aircraft skin to evaluate and compare their remaining service life. We first perform a COE “standard” repairing technique, namely bolted bonded patching, using BRSW process. Patch design with Titanium material is considered. The planned case studies for bolted bonded repair technique will be on a cracked pressurized fuselage structure [20].
This paper is organized as follows: in Section 2, we will introduce our proposed FEA solution workflow for aircraft damage tolerant repair using Ansys. In Section 3, we will present the case studies for a bolted bonded repair with crack initiation, fatigue crack propagation, and service life quantification with and without crack repairs. Specifically, three crack repair techniques will be examined, live crack repair, stop-drill crack repair, and damage-removal crack repair. Finally, conclusions and future work are discussed in Section 4.
Simulation Methodology and Workflow
In this section, we will present a FEA simulation methodology and workflow on bonded repair of an aircraft structure experiencing an initial crack. The simulation procedure is carried out using Ansys suites functions. But the workflow is suitable and can be easily adopted using any existing FEA simulation software.