PSI - Issue 14

Sarthak S. Singh et al. / Procedia Structural Integrity 14 (2019) 915–921 Author name / Structural Integrity Procedia 00 (2018) 000–000

917

3

machine is 25 kN. At first, the compression fixtures are aligned without the specimen and loaded in compression upto 20 kN. The force vs. deformation curve thus obtained is used to estimate the compliance of the machine ( M C ). Such tests are performed at least 4 times to get an average value of the machine’s compliance. In order to conduct the compression experiments on the actual test samples, silicon grease is applied onto the compression fixture to minimize the friction between the specimen and the fixture. Care is taken to mount the specimen at the middle of the fixture. The deformation data obtained from the cross displacement is corrected using the following relation:     S T M PC , (1) where  S : Deformation of the specimen,  T : Total deformation recorded by the machine from the cross head displacement. This includes the deformation of the fixture and the specimen, and P : Load corresponding to T  .

Rupture of material

5

Strain hardening region

4

Yield stress

Strain softening region

1

3

2

Plateau region

Figure 1. Stress vs. strain curve of neat epoxy under quasi-static compression highlighting various phenomena associated with the respective segments of the curve

The representative stress vs. strain curve for the neat epoxy specimen is shown in Fig. 1. Initially, the stress increases linearly with deformation till the yield strength of 115 MPa is attained (Segment: 0-1). This is followed by a sharp decrease in the stress level to 95 MPa (Segment: 1-2). This region is known as strain softening. Further, the material starts to deform under a constant stress of 95 MPa till 40 % of strain value (Segment: 2-3), beyond which the stress rises exponentially upto 220 MPa with increase in the strain values (Segment: 3-4). This is attributed to the strain hardening process. Finally, specimen ruptures at ~ 62 % of strain value. This is depicted as point 5 in Fig. 1. 4. Effect of filler volume fraction on the mechanical behaviour of compsoites The effect of filler volume fraction on quasi-static compression behavior of glass filled epoxy composites is analyzed by plotting stress vs. strain curves in Fig. 2. All composites exhibit the mechanical behaviour similar to what is observed in the neat epoxy system. The strain softening, region of constant stress and strain hardening in post-yield regime is clearly evident from the figure although these zones appear to have ben influenced by the filler volume fraction. The post-yield stresses (in both plateau and strain hardening regions) show a monotonic increase in the stress levels with increasing filler volume fraction. The stress in the plateau region increases from ~ 90 MPa for neat epoxy case to ~ 120 MPa for 10% filler case. Interestingly, all composites (including neat epoxy) exhibit yielding at ~115 MPa, indicating that the yielding process is matrix dominated. This is attributed to the large inter particle distance at low filler volume fractions considered in this investigation due to which the contribution of filler particles towards resistance to deformation remains negligible. It is also anticipated that the spherical particles (at