4.1 Effect of Raster Angle
According to the ANOVA Table.7, the p-value of the raster angle is less
than 0.05, indicating that the raster angle is significant at the 95 %
confidence interval. Furthermore, as shown in Figure. 3, tensile
strength increases as the raster angle rises from 0° to 90°, as does the
number of shells. The increased number of shells in the printing process
may reduce the need for infill, allowing the outer shells to carry the
load. The shells of 3D-printed parts are typically overlapped to create
a stronger part. This overlap angle improves the bonding between the
shells and reduces voids between the layers. When the angle is increased
from 0° to 90°, stress is transferred from the inner layer to the outer
shells, increasing the tensile strength along with the angle and number
of shells while maintaining uniform layer thickness. A 90° angle forms
cross sandwiches and distributes the load to the outer shells, resulting
in greater ductility.
<<<Figure.3 Stress Strain Relationship
curve for Raster Angle (RA) a) 0°, b) 45° & c)
90°>>>
Because of the molecular bonding in the material, the virgin PLA
material has a high elongation at a 0° angle. With the increased bonding
created between different materials in the sandwiches, the composite
material showed less elongation than the composite sandwich material.
Because of the spaces between the layers and improper bonding, the
PLA/CF has the lowest tensile strength of all materials for all angles
of inclination. Short fibers, irrespective of stress, may result from
incorrect layer bonding, causing fibers to be pulled out during tensile
load application; this was observed by microscopic examination as shown
in figure.4. This can be prevented if continuous carbon fibers are used
to make the specimens. Premature failure was observed during testing due
to stress concentration at the fillet areas, as shown in Figure.5.
because the raster ends at the fillet profile in ASTM standard test
specimens. A number of researchers have observed the same type of
failure in the testing [37, 38, 39, and 40]. The greatest average
strength of the PLA material was 52 MPa at a 90° angle, and the minimum
average strength was 47 MPa at a 0° angle. The average tensile strength
of PLA material was increased from 0° to 90°. Because the overlapping
increases as the angle increases, the strength is improved. The tensile
strength of PLA/Cu composite material increased by 15.62 percent (27.67
to 32 MPa), PLA/Cu composites increased by 15.66 percent (27.66 to 32
MPa), and PLA/Al2O3 increased by 4.76 percent (42 to 44 MPa), but the
composite sandwich material’s average tensile strength remained
unchanged. But the tensile strength increases for all angles of
inclination in the material order of PLA/CF <PLA/Cu
<PLA/Al2O3 <PLA
<PLA-PLA/CF-SW <PLA-PLA/Cu-SW
<PLA-PLA/Al2O3.
When the angle changes from 0° to 90°, the failure mode in the specimen
changes from ductile to brittle. For an angle of 0°, the maximum and
minimum elongations of the material are 18% and 6.98%, respectively.
The maximum and minimum elongations of a 90° angle are 16 and 7.2
percent, respectively. When the angle is changed from 0° to 90°, it is
clear that the ductile failure transforms into brittle failure. It was
clearly observed from the figure.3 diagrams of stress-strain
relationships.
<<<Figure.4 Microscopic Examination on
Raster Angle 90° >>>
<<<Figure.5 Stress Concentration and
Failure in printed samples>>>