Abstract:
Temperature distribution in structural elements in practical cases usually changes in two or three
directions. Based on such facts, aiming at more effectiveness, a functionally graded material
(FGM) coating, whose properties change in two directions called bi-directional FGM was
introduced. This thesis aimed to model and investigate a mode I surface cracked coating-substrate
problem with a bi-directional coating bonded to a homogeneous substrate under linear thermal
loading and uniform stress. Sequentially Coupled three-dimensional thermoelastic analysis was
used to investigate the stresses and the stress intensity factor (SIF) at various crack geometry
dimensions under thermomechanical loads. The extended finite element method was utilized to
model the problem in ABAQUS software. FORTRAN user subroutines for the FGM properties
were developed and implemented into the ABAQUS package. The thermomechanical stress and
stress intensity factor was solved by considering different parameters i.e., gradient index, cack
aspect ratio, crack length, and temperature gradient. The applied remote stress was 100 MPa with
maximum values of temperature employed ranging from 400 ℃ to 800 ℃. Thermo-mechanical
stress field was evaluated and extracted from the crack tip for radian value and the stress intensity
factor along the crack front was calculated for the crack with those parameters.
The numerical results showed that the stress intensity factor was affected by the gradient. The
maximum value of stress intensity value decreased from value for n = q = 0 to value for n = q = 1
by 52.56% for a/c = 0.7 and by 63.84% for aspect ratio a/c = 1. The decrease showed that the
maximum stress intensity factor value of functionally graded coating was reduced compared to
the same crack geometry and loading in a homogeneous material. For linear bidirectional coating
types (n = q = 1) mode I stress intensity factors were obtained maximum at the symmetry surface
while it was at free surface for homogeneous material coating. The crack stress intensity factor
was also affected by thermal the loading and the gradient index that Maximum Stress intensity
factor increased from value for n = q = 1 to value for n = q = 2 by 22.97% at 400 ℃ while by
23.6% at temperature 800 ℃. The results generally showed that the crack stress intensity factor
can be minimized by controlling the loading and the gradient index for the crack in functionally
graded coating.
