Nondestructive testing of defects in adhesive joints

As 'C' channel structures is non-symmetrical and has numerous discontinuities, mechanical behaviour of the
structure cannot be easily predicted. Due to its irregular features, number of stress concentrations points will be
present in it. Any improper fabrication or design will lead to catastrophic failure of the structure.
Analysis
The 3-D model developed was imported in FEM analysis software Ansys. Ansys was selected primarily of its
excellent structural analysis capabilities and ability to support orthotropic and anisotropic materials in analysis.
Von-mises stresses and Displacement Vector Sum were determined. Analysis was carried out for 3 different
polymer composites viz.
1. 30 % short glass fibre filled polycarbonate,
2. 40% Unidirectional glass fibre reinforced Epoxy, and
3. 40% Unidirectional carbon fibre reinforced Epoxy.
The properties of the same are tabulated in table 1. The failure criteria were also analysed to determine whether the
structure will withstand or fail while in operation.
The structure being air borne, it needs to withstand high inertial loads while in manoeuvring, take off and landing.
and deployed underwater from a flying platform has to withstand high inertial loads as well as hydrostatic pressures
on the system. While in analysis inertial load was taken into consideration. As the structure is to deployed
underwater hydrostatic pressure was also taken in account on complete structure.
The structure is mounted with sensors of weight 180 gm at 5 different mounting point along with 2 hinge points
and hydrostatic load of 4 MPa. The sensors are modelled as lumped mass for ease of modelling. A section of the
boundary condition is as shown in the figure 2. The red arrows show the hydrostatic pressure acting throughout the
structure while light blue arrows shows the hinge point area with zero degree of freedom. The yellow arrows
represent the sensor mass acting on the structure.
The structure was modelled using 10 node tetrahedral structural element referred at SOLID 187 in Ansys.
SOLID187 element is a higher order 3-D, 10-node element. SOLID187 has a quadratic displacement behaviour and
is well suited to modelling irregular meshes such as those produced from various CAD/CAM systems. The element
is defined by 10 nodes having three degrees of freedom at each node: translations in the nodal x, y, and z directions.
The element has plasticity, hyperelasticity, creep, stress stiffening, large deflection, and large strain capabilities. It
also has mixed formulation capability for simulating deformations of nearly incompressible elastoplastic materials,
fully incompressible hyperelastic materials and elastic orthotropic materials. [4,5]
Results and Analysis
The analysis carried out with various materials and their results are discussed below. Von-mises stress analysis of
the structure with 3 identified composites were carried out and results are shown in figure 3, 4 and 5 respectively
for 30 % short glass fibre filled polycarbonate, 40% Unidirectional glass fibre reinforced Epoxy and 40%
Unidirectional carbon fibre reinforced Epoxy.
The maximum stress obtained in three cases is in 320 MPa which well below the failure strength of carbon fibre
based composites. Similarly, for the case of glass fibre based composite and glass fibre filled polycarbonate does
satisfy the requirement of maximum breaking strength. Hence in terms of required strength, component with either
of the material will withstand the operating conditions.Next, Vector displacement of the component was analysed
and results are shown in the figures 6, 7 and 8.
The 30% short glass fibre filled polycarbonate shows 35 mm of deformation in the lower end of the structure. As
this much variation in the location of the mounted sensor will lead to erroneous result, it is unacceptable as suitable
material for fabrication of the component. Both Glass fibre and carbon fibre based epoxy composite shows much
less deformation and are suitable for the application. However, it is well known from the literature that glass fibre
undergo gelation when it is exposed to sea water for long period of time and hence high factor of safety is required
in design of the component. [6] As the component being modelled is limited by space constraint hence only
recommended material is Carbon fibre based composite for fabrication of component.
Conclusion
The modelling of a critical component used in air platform based underwater deployable system was carried out to
analyse the finalize the selection of suitable material of construction of the component using FEM software
ANSYS. The structure having various irregularities in the shape was modelled using solid elements and Von-mises
stresses and total vector displacements were analysed. The materials selected for analysis were 30 % short glass
fibre filled polycarbonate, 40% Unidirectional glass fibre reinforced Epoxy and 40% Unidirectional carbon fibre