Al-CNT Composites Lab Report

Stress relaxation is an intrinsic property of a material, in which, the material decreases the applied load held at constant strain with passage of time. The total strain remains constant but a fraction of the elastic strain transforms into plastic strain [3]. Stress relaxation characteristics of a material become vital when designing mechanical fasteners, riveted or bolted assemblies, shrink or press-fit components, solderless electrical connectors, etc. Moreover, stress relaxation data could be beneficial to predict necessary heat treatment to relief the residual stresses in castings, forgings, cold rolled or welding assembles [4].
Previously, many researchers investigated stress relaxation in pure metals for example: aluminum [5], copper
The results of pure aluminum are comparable with the mechanical properties of standard AA1199-O4, with minor deviations. The deviations could be attributed to the processing conditions and/or purity level. To have a justified comparison, the processing conditions were kept same for the fabrication of Al-CNT composites. In Al-CNT composites’ specimens, the yield and tensile strengths increased from 29 MPa and 56 MPa to 97 MPa and 109 MPa, respectively. However, a decrease in elongation from 22.6 % to 7.4 % was observed. It is believed that the amelioration of mechanical strengths is a synergistic effect of the strengthening by the nanotubes, increased lattice strain and refinement in crystallite size. Table-2 shows the results of tensile testing along with percent change in various mechanical
in the beginning of tensile curve, Figure-1) matrix (aluminum) and strengthening material (MWCNTs) deform elastically and it continues till the yield point of the matrix (i.e. 29 MPa). If the composite is unloaded in this stage, the elastic strain (εe) and plastic strain (εp) will be zero. Therefore;
Et=0 (2)
However, at stress levels above the yield point (>29 MPa), matrix deforms plastically, whereas the nanotubes still deform elastically. It is stage-II, here the composite behaves in quasi-elastic way, therefore, on unloading;
Et=Ep (3)
The unloading has effectuated elastic retention of the CNTs and compression strains within the matrix. Stage-II persists until the yield point of the composite, beyond which both aluminum and the nanotubes undergo plastic deformation. It is stage-III and it features delamination of the Al-CNT interphase. Therefore, unloading of the composite will demonstrate permanent deformation attributing plastic strains of the matrix (εpm) and the reinforcement (εpr);
Et=Epm+Epr (4)
Finally at stage-IV, crack initiation a growth occurs, which continues until the fracture