The assessment of bonded joints

Lead Research Organisation: University of Bristol
Department Name: Mechanical Engineering


New methods of construction are needed to advance the capability of aircraft structures,
meet business demands & reduce manufacturing costs. One is the use of bonded
assemblies for primary structure. FAA currently requires that unless the strength of the
structure can be proven to match or exceed the design requirements, it must have
mechanical fasteners to prevent critical failure. Bonded structures allow a significant
reduction in weight due to using thinner skins. If 'Chicken rivets' are mandated, then this
negates any benefit in terms of weight saving. The only current reliable way of testing the
bonded assembly strength is proof loading - expensive & unrealistic for testing every part.
An NDE method of testing a bonded assembly is needed to allow the use of lightweight
bonded primary aerostructures. This must allow assessment that the desired strength is
achieved, or identify areas of a weak-bonded area to allow repair. The project will
investigate the application of phased array inspection approaches to determine bond
strength, firstly studying current best practise with linear phased arrays and moving on to
compare this to the recently developed nonlinear phased array approach.


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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509085/1 01/10/2015 31/03/2021
1652620 Studentship EP/N509085/1 01/10/2015 30/11/2019 Yasen Polihronov
Description Compromised interface bonds in high value components can lead to premature loss of structural integrity. These types of defects are very difficult to detect using traditional acoustic inspection techniques. Here we present results of novel inspection methodologies which allow for high confidence measurement of bond integrity. We assess the ability of linear ultrasonic non-destructive testing (NDT) methods to detect weak-kissing bonds and/or differentiate between weak, kissing, degraded or contaminated bonds from healthy ones. Ultrasonic transducer array(UTA) have been selected as the vehicle of choice, since they allow for increased informational content. Despite the considerable research that has been carried in the area of weak bonds detectability, this remains as a significant challenge for the industry and more studies are required for a better understanding and characterisation of bond quality. Our research addresses these issues in UTA measurement techniques, specifically FMC/TFM1,2, Scattering Matrix(derived) 3,4 extraction in time domain, and other related or derived methods. We have developed an iterative testing methodology, in which the experimental variation in each of the following steps is significantly reduced;
Sample Preparation; Data Acquisition; Imaging1,2; Feature Analysis3,4;Characterization/Classification; Interpretation.
Using imaging optimisation routines, the coherence and quality of the image is increased up to 120% depending on imaging distance. This improves localisation of points of interest, allows automation of the process and increases separability of the classification data. The final quality metric can be compiled from both linear and non-linear data. The methodology is versatile, allows for metal and CFRP bonds to be tested, which is a particular interest of the automotive and aerospace industries.

1. J. N. Potter et. al. Nonlinear Ultrasonic Phased Array Imaging. 144301, 1-5 (2014).
2. Zhang, J., Drinkwater, B. & Wilcox, P. D. Comparison of ultrasonic array imaging algorithms for nondestructive evaluation. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 60, 1732-1745 (2013).
3. Hunter, A. J., Drinkwater, B. W. & Wilcox, P. D. Autofocusing ultrasonic imagery for non-destructive testing and evaluation of specimens with complicated geometries. NDT E Int. 43, 78-85 (2010).
4. Bai, L., Velichko, A. & Drinkwater, B. W. Ultrasonic characterization of crack-like defects using scattering matrix similarity metrics. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 62, 545-559 (2015).
Exploitation Route The findings are directly applicable in a very specific testing applications in the manufacturing industry. (see the report above)
Sectors Aerospace, Defence and Marine,Manufacturing, including Industrial Biotechology

Title IETO - Interface Examination Tool 
Description IETO ( Interface Examination TOol) is a collection of functions/ libraries for TMC(Full Matrix Capture) and TFM(Total Focusing Method) extraction and processing for the express purpose of interface examination of bonded assemblies using ultrasonic data from array controller. The software features automatic TFM optimization routines. Scattering Matrix extraction and processing routines. Classification and Characterization routines, and parameter sweeps along with reliability estimation. The tool is capable of post-processing, as well as real time acquisition and analysis of the ultrasonic data. 
Type Of Technology Software 
Year Produced 2018 
Impact The software is yet to be released to the general public, but preliminary outcomes suggest the tool has potential to be deployed beyond the research phase once sufficient testing on samples has been conducted. The intended target audience is aircraft and vehicle manufacturers. 
Description Anglo-French Physical Acoustics Conference (AFPAC) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact The scope of the conference is kept intentionally wide so as to encourage cross-fertilisation of ideas across different application areas in the field of Physical Acoustics. Presentations in the past have covered: theoretical developments (all methods); experimental measurements; transduction; linear or nonlinear elastic or acoustic (bulk, surface, guided) wave radiation, propagation and scattering; application to material characterisation, imaging (non-destructive testing, medical, underwater, seismology), ultrasound therapy, SAW devices etc.
Organised by the Physical Acoustics Group

My role was to present novel results related to recent outcomes form my research work. The conference was three days long and represents is annually attended by the most prominent academics,researchers and professionals in the are of acoustics.
The outcomes were positive, there was specific interest form other parties whose work relates to the scope of mine.
Year(s) Of Engagement Activity 2010,2018