Nondestructive testing of defects in adhesive joints

Nondestructive testing of defects in adhesive joints
P.N. Vinod, Reji John, Shiv Kumar, K. Shajahan, C.G. Padma Kumar and RMR Vishnubhatla
Naval Physical and Oceanographic Laboratory,
Defence Research and Development Organisation,
Thrikkakara P.O, Cochin-682 021
Abstract
Email: tsonpol@vsnl.com
Rubbers are generally used for encapsulating underwater components to prevent moisture
ingress. Reliability aspects of these components are very important while considering the performance
of the sensor. The sensor is deployed in underwater for quite long period of time and has no option for
monitoring the structural health and functionality. The sub-surface defects such as micro-cracks,
voids, porosity and delamination are generally introduced during the encapsulation or damage may
create due to handling or when service stress exceeds design stresses. Nondestructive test (NDT)
techniques such as infrared thermography (IRT) and micro-focal x-ray radiography techniques have
been applied as an inspection methodology for detecting these defects in the encapsulants. The main
advantage of the NDT techniques is that reliable and quantitative information about the hidden defects
or flaws can be generated with high precision without sacrificing or damaging the entire structure of
the device or its components. The bright spots and its dimensions observed on the IR images of the
partially debonded samples correspond to the location as well as the relative size of the delamination
presents in the sample. The x-ray image of the spiced cable joints shows that the internal structure
with cracks or voids at the cable joints.
Keywords: encapsulants, NDT, infrared thermal imaging and microfocal x-ray radiography.
1. Introduction
Rubbers are generally used for encapsulating underwater components to prevent moisture
ingress. The sensor is deployed in underwater for quite long period of time and has no option for
monitoring the structural health and functionality on a regular basis. The generally observed defects of
underwater components are delamination of adhesive joints or at the cable-spliced joints, cracks in
ceramics, porosity and pin holes in transducer encapsulations and rubber moulded junction boxes,
which are seriously affecting the reliability of the sonar systems. Hence, detection and localization of
the above types of defects and monitoring the health of the components are the prime important
factors to improve the reliability of the device and in predicting the life of the components, thereby
eliminating the possibility of catastrophic failure of sonar. These defects are so tiny that visual
inspection is not practically feasible. Consequently, there exists an increasing demand for reliable and
effective non-destructive evaluation (NDE) for the wet-end components of the encapsulants. A
number of different nondestructive test (NDT) techniques have also been developed as an inspection
methodology and applied successfully to identify and characterize the defects [ 1- 2]. No single
method can detect all forms of flaws in all materials. Hence, suitable NDT inspection methods have to
appropriately select for each case. In the present study, the delamination and cracks in the
encapsulants of the wet-end components of the underwater sensors have been detected and
characterized using the NDT technique such as micro-focal x-ray radiography and IR thermal imaging
techniques. These techniques are applied due to their reliability and high sensitivity in detecting the
defects [ 3].
1. (a) The principle of Infrared thermography
Infrared thermography is a non-contact sensing method concerned with the measurement of
radiated thermal or infrared radiation from a surface in the infrared region. This involves the use of an
IR camera to capture the evolution of surface temperature profiles of the test object after being