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:
- 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].
- 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.