A Comparative Study of Structural Changes in Conventional and Unconventional Machining and Mechanical Properties Evaluation of Polypropylene Based Self Reinforced Composites

2.1 Present State of Knowledge in advanced Machining

Advanced machining techniques improvement in mechanical properties of composites is quite successful for a wide variety of fiber composites including GFRP, Kevlar and CFRP laminates. AWJ machining and laser machining study on thermoset based composites are studied but thermos-plastic based laminates machining is lagging. Different loading parameters influence the chemical structure, morphology and thermos mechanical properties of the SRC’s [13, 14, 15].

CO₂ laser machining shown in Figure 1 (a) is used to reduce the electrical resistivity of PE/MNWT composite. An increase in the content of MWCNT leads to a reduction in electrical resistivity. FESEM reveals the effect of laser parameters on the resistivity of Polyethylene composite. HAZ values ranged between 0.75-1.7 mm and are at the entrance and 0.8 to 1.5 mm at outline is observed with an increase in line energy [16].AWJT process shown in Figure 1 (b) is a better machining method for LPDE material than the conventional machining methods. Minimum ASR of 1.67 lm and maximum MRR 14072.02 mm³/min were obtained respectively; at optimum parameters of 5 mm/min nozzle feed rate, 350 g/min abrasive flow rate and 2500 min–1 spindle speed [17, 18, 19, 20].

Figure 1

Diagrammatic representation of (a) Laser cutting process (b) AWJ machining

2.2 Paper Outline

Laser machining and AWJ machining on thermoset based composites are studied but thermos-plastic based SRPC laminates cutting is rarely studied. Hence in this work, a detailed investigation of cutting induced damage of self-reinforced composites has been carried out considering the structural changes that accompany the cutting processes. The effect is characterized by studying the surface chemistry and morphology using Scanning Electron Microscopy. Structural integrity, elemental aspect ratios, functional groups and damage are studied using the FTIR before and after machining. In addition, Tensile tests (ASTM 638), Flexural tests and Hardness tests (Shore D)were carried out to identify the cutting induced structural changes and their influence on the mechanical properties [9].

3.1 Experimental Procedure

The entire fabrication process is shown in Figure 2. The Raw Materials Isotactic Polypropylene fiber, polypropylene matrix of 1mm thickness (Korea E&T Co.Ltd) were used to prepare laminate. Isotactic Polypropylene fibre shown in Figure 3 (a) was used to fabricate laminate under consolidated temperature The Thermogravimetric (TGA) analysis and DSC (SDT Q600 V20.9 Build 20) investigation have been employed to acquire the thermal stability (consolidated temperature)of the polypropylene fibre and Polypropylene matrix samples. Initial temperature, residual mass and thermal stability were determined using TGA. An analysis was implemented initially with fibre and matrix of few milligrams at elevated thermal environments from 20 to 600 degrees and DSC analysis was also carried out at the same environmental conditions. The softening temperature determined from the DSC plot is used to fabricate SRC composite using the HOT compaction method [21, 22, 23].

Figure 2

Schematic diagram of fabrication and testing

Figure 3

(a) Polymeric structure of Polypropylene (b) Schematic diagram of Fibre lay up of HOT compaction method

The laminate is prepared using three polypropylene fibers and two high-density polypropylene matrix of size 500mm×500mm. The entire matrix and fabric layer assembly was hot compacted after slow heating to 164^∘C and held for one hour. Then the laminate was slow cooled in the oven over a few hours and de-moulded. The processing temperature for the Hot-compaction method was measured using the Differential scanning calorimetry (DSC) curve shown in Figure 4.

Figure 4

(a) DSC plot of Polypropylene (b) TGA plot of Polypropylene

The hot compacted laminate samples were cut using the conventional saw and unconventional laser and Abrasive water jet (AWJ) cutting. A Laser head speed is 420mm/min, the laser nozzle diameter size is 1.7mm and stand-off distance within the laser nozzle and the composite laminates surface was maintained at 15mm. The work table is CNC controlled, the focal length of the lenses is 200mm. The chiller unit is used to cool down the laser head and the laser source. The work area is enclosed by sliding doors. An abrasive water jet cutting system of the abrasive particle flow rate of 800g/min was used for cutting of Polypropylene SRC samples. The feed rate of the water jet is 1500m/s. the stand-off distance is 4mm between the surface and the nozzle. The water jet pressure of the system is maintained at 4000 bar. FESEM and FTIR spectroscopy studies were carried out to evaluate polymeric structural change, surface damage and Fibre failure.

3.2 Measurement procedures

The dimensions of the tensile specimens cut using conventionally and unconventionally cutting were 250mm×25mm×2mm based on the standard ASTM D 638. The analysis was implemented with a loading speed rate of 2mm/min using a universal testing machine (Instron 8801). The specifications of the Flexural specimen was 125mm×25mm×2mm based on the standard ASTM D 790 M. The Flexural test was carried out in the same Instron machine at a support span length to thickness ratio of 16:1. The loading rate of the central crosshead was 2 mm/min.

Shore D Hardness Test was performed using Durometer which measures the depth of a depression developed by a force utilizing regulated presser foot on the sample. This depth is reliant on the shape of the presser foot, Visco and elastic properties, the hardness of the material. ASTM D2240 durometers are allowed to measure the primary hardness of the sample and concluding hardness after a duration of a time period [24, 25]. By applying the consistent force, without shock, hardness (depth of indentation) can be measured. Force is applied for timed hardness up till the required time and then interpret. The thickness of the material used for this analysis is 6.4 mm (0.25 inches).

4.1 Thermal stability evaluation

The thermal stability of polypropylene fibre and matrix is investigated performing DSC and TGA analysis. DSC curves of Polypropylene obtained during the second stage of the process of heating are represented in Figure 4 (a). Transformations comprised of DSC plot show that change in enthalpy related to change of material from glassy is around 58 ^∘C, exothermic solidification at 80 ^∘C, polymer matrix softening at 164^∘C and level of crystallinity is 50% as shown in Figure 4 (a).

The TGA plot of Polypropylene exhibits density loss contour at distinct temperatures, as represented in Figure 4 (b). The initial level of density loss was observed at a range of temperatures around 30–100^∘C, in representation with a weight loss of 7% of moisture. A non-oxidative thermal degradation was identified at a temperature range of 300–480^∘C, which signifies the de-propagation of the Polypropylene chain. The chemical changes at the initial debonding stage of the Polypropylene chain lead to evaporation and elimination of explosive products. The deterioration of Polypropylene is increased beyond 300^∘C lead to the development of a cyclic and crosslinked design. The deterioration of Polyethylene is increased beyond 300^∘C lead to the development of cyclic and cross-linked chains. Further degradation leads to the formation of methyl ester and alkyl free radicals. A carbonize develops after 480^∘C and decay occurred at around 600^∘C, remaining debris of around 1.6% w/w at the thermal condition of 750^∘C.

4.2 Mechanical characterization

The influence of cutting on the Tensile strength of polypropylene SRC laminate is illustrated in Figure 5. The Ultimate tensile strength (UTS) of Polypropylene SRC laminates increased ~20% after advanced cutting. This increase in tensile strength shown in Figure 5 (b) and Figure 5 (c) is probably because of the chemical changes that occurred after advanced cutting, Continuous depreciation of the hydrates enhances the tensile characteristics of the laminate. Moreover, this increasing inclination in tensile strength also results from enhancing plasticity and adhesion between the fibre and matrix due to advanced machining. Because of the formation of C-N, C==C bonds after advanced cutting the tensile and flexural strengths of the polypropylene composite exhibit an improvement.

Figure 5

Tensile load plots of PP (a) Conventional samples (b) laser samples (c) AWJ samples (d) Average modulus and load plots

Results displayed in Figure 5 and Table 1 states that the laser cut polypropylene samples have higher load-bearing capacity compared to conventionally cut samples. In the case of ultimate tensile stress, the abrasive water cut samples exhibit a higher value than a conventionally cut sample and laser-cut sample because of the definition of cut and brittleness of the fibre. A similar trend is followed in the young’s modulus. Cutting surface damage of AWJ samples is less compared to the laser cut samples. It is observed that the maximum tensile load and ultimate tensile strength of conventional samples are 1209N and 21 MPa respectively. After laser cutting the ultimate tensile strength is increased to 27MPa due to the formation of C-N bonds which exhibit strong stretching and increased ductility compared to conventional cut samples.As the cutting definition and ductility of the sample are increased after AWJ machining, the observed formation of stiff C=C double bonds exhibits an increase in the tensile load compared to laser machining thereby achieving a tensile strength of 30 MPa. C=C double bonds are stronger than C-N single bonds as the bond energy of the former is 612 KJ/mol and that of the later is 315 KJ/mol. The extent to which these bonds are formed also contributes to the strength [26].

Figure 6(a) infers that the flexural strength of the laser cutting is high compared to conventional cutting by 38%. The aromatic and nitro bonds formation after advanced machining is the reason behind the increase in the flexural stress of the Polypropylene SRC laminate.

Figure 6

(a) Flexural stress and load plots (b) Hardness values of Polypropylene SRC laminate for different machining parameters

The flexural stress and load-bearing capacity of laser samples were 59 Mpa and 62 N respectively because of the formation of C-O bonds and C-N bonds. Nitro Bonds exhibiting brittleness and stiffness in the sample as shown in Figure 6. Flexural strength of the abrasive water jet samples was also high compared to conventional cut samples by 35% as a result of the formation of C=C bonds as shown in Figure 6(a) [27].

The surface roughness values of AWJ samples are less compared to Laser-cut samples as shown in Table 2. The surface quality was observed to be good after AWJ cutting and lesser waviness was obtained compared to that of laser cutting. The abrasive water cut specimen exhibits good ductility which is the reason for low hardness. The hardness value is decreased by 48% in the AWJ cut samples as seen in Table 1. It appears that the laser cut samples thermally exhibit a temperature rise beyond the melting point of PP and quench down within minutes to the ambient temperature. The zone surrounding the heat-affected zone exhibits an adiabatic characteristic as the sudden temperature rise and subsequent quenching occur within minutes. This is seen to produce an increase in hardness due to quenching effects as in martensite formation in steels. The surface roughness is also seen to be higher than those of conventional or AWJ cut samples. Good mechanical properties can be associated with a smoother surface finish in the AWJ cut samples. Laser cutting is seen to be followed by a quench which is more of a reason in the improved mechanical properties than the influence of higher surface roughness.

4.3 FT-IR Spectral analysis

FT-IR spectroscopy of Polypropylene SRC laminate originated for conventional cut, laser-cut and abrasive water jet cut are shown in Figure 7 and Figure 8. The band originated around 1560 cm⁻¹ assigned for the nitro compound is missing in the polypropylene SRC laminate fabricated, but the peak gradually appeared as the laminate is cut using AWJ jet cutting as inferred from Figure 8(b). The absorption spectrum at 1730.15 cm⁻¹ is the typical peak of Polypropylene SRC laminate which denotes the C=O group during conventional cutting is not visible after advanced cutting.

Figure 7

FTIR Plot of PP Conventional cut sample

Figure 8

FTIR Images of PP (a) laser sample (b) AWJ cut sample

The absorption spectrum at 1165 cm⁻¹ is the–C–N stretching vibration visible after laser cutting due to the reaction of open-air during cutting as seen in Figure 8 (a). The spectrum around 1452.40 cm⁻¹ characterized by the bending oscillation exhibited by the covalent (C–H) chain of the methyl group. Spectra at 1579 cm⁻¹ and 1259 cm⁻¹ attribute to the C-O aromatic ester and N-O stretching. An absorption band of 972cm⁻¹ is visible after AWJ machining shows alkene group which exhibits strong stretching and bending vibration. The C-C bonds have been converted into different bonds like C=C, C=O, C-F and C-N after laser machining. The resultant bonds exhibit greater strength and stiffness than the C-C bonds that existed before machining.

The C=C bonds and the C=O bonds are more than twice as strong as carbon-carbon single bonds which can be the appropriate reason for an increase in the mechanical properties. All the samples exhibit similar spectra after ~1 mm of case depth. Chemical interaction and reaction occur only up to ~1mm thickness as depicted in Figure 8. A cross-sectional area for the surrounding 1 mm case depth of altered chemistry is seen to be responsible for the difference in mechanical properties. As tension is a cross-sectional property the percentage difference for the same sample is seen to be different compared to flexure which is due to the maximum stress in the outer fibres. A favourable chemical change at the surface would affect flexure more than tension provided the material is more brittle and less ductile. A more brittle material would show a good influence on the flexural strength but a flexible and ductile material would not benefit from a subsurface level structural change as its neutral axis would shift towards the tensile face during flexure tests, thereby reducing its strength. However, the tensile strength depends more on the cross-section of the case depth up to which a structural change is observed.

4.4 Fractographic analysis

Morphological studies of the polypropylene composite for different cutting conditions were shown in Figure 9. Before the advanced cutting process, PP composite had limited porosity within the composite as seen in Figure 9 (a). The gap between the PP fibre and matrix keeps them from bonding well with each other in the conventional approach. Fibre debris is low after conventional cutting. FE-SEM image of the Polypropylene SRC laminate laser cut sample concluded that carrying out the laser cutting process on Polypropylene SRC laminate samples improved the mechanical characteristics of the composite.

Figure 9

FESEM images of PP (a)Conventional sample (b)laser sample at 100X Magnification (c)laser sample at 500X Magnification factor (d) AWJ sample

Observed occasional fibre breakage after laser cutting. PP is less ductile, but adhesion is good as shown in Figure 9 (b) and Figure 9 (c). Here, an interdiffusion of the matrix and fibre appeared to occur due to the thermal effects associated with laser cutting, these probably improve the tensile strength after laser cutting. Besides, the formation of stronger bonds as described earlier was also another reason for the improvement in strength due to laser cutting. Our own earlier supporting evidence of different polymeric systems is included in the following references [26, 27] and [28].

Also, advanced cutting exhibit chemical reactions which affect interfacial bonds and unity of the fibre to the matrix. It is noticeable from Figure 9(d) that Polypropylene SRC laminate samples exhibited the best property against delamination damage after AWJ cutting than Laser cutting. Microfiber fibrillation is a dynamic, high strain rate damage of the fibers due to dynamic cutting speeds. Where the polymer fiber absorbs the energy initiated by the cutting forces and dissipates the same by fibrillation mechanism. The single fiber axially splits into many individual fragments which are described as fibrillation. This was observed to a large extent in the AWJ cutting due to the high localized cutting force. The surface finish of brittle materials was exhibiting low roughness values as the laminate fracture was sharp, smooth, straight perpendicular, whereas ductile materials exhibit good surface finish with the broom, rough fracture.