Table 4.4 : Fatigue lives : Number of Cycles to Failure for Matrix Aluminium Alloy LM 25. Stress level III 68MPa Life (Cycles)

Transcription

1 4.3: Fatigue Determination of Matrix Alloy LM 25 Fatigue test specimens of matrix alloy LM 25, were tested as described in section Test result of fatigue lives of 14 specimens at all the three stress levels are shown in Table 4.4. The average fatigue life at three stress levels is shown in Table 4.5. The S- N curve for matrix alloy LM 25 is shown in the Fig 4.3. Table 4.4 : Fatigue lives : Number of Cycles to Failure for Matrix Aluminium Alloy LM 25 Sample Number level III 68MPa Level I 122MPa Level II 95MPa Average Fatigue Table 4.5 : Average for LM 25 Aluminium Alloy Level(MPa) Average

2 S-NCurveofLM N, ( cycles) Fig 4.3: S-N Curve for Matrix AUoy LM 25 It is evident that the fatigue hfe increases as the stress level decreases. Fig 4.4 shows the fatigue life test data plotted on log-log co-ordinates for matrix alloy LM s. 100 s " ^ (A (A d> w (0 ** 10 tn N,IJfe( Number of cycles to failure) Fig 4.4: - Relation for Matrix Alloy LM 25 66

3 Best fit line is obtained by means of least square method. This line can be represented by, y = lxr^ (4.5) which has a regression correlation co-efficient R^ = where y = amplitude in MPa, X = 19520, p = X= Fatigue life in number of cycles. The above equation can be used to estimate fatigue life of LM 25 matrix alloy in the given range of stresses Scanning Electron Micrography Fracture surfaces of the failed LM 25 test specimens were comprehensively examined to both determine the macroscopic fracture mode and as also to characterize the fine scale topography and microscopic mechanisms governing fi-acture. The distinction between the macroscopic mode and microscopicfi"acturemechanism is based on the magnification level at which the observations are made as shown in Figure 4.5 and 4.6. In this context, the macroscopic mode refers to the nature of failure, while the microscopic mechanism relates essentially to the local failure processes like microscopic void formation, growth, coalescence and nature of cracking. The fatigue fi-acture specimens of LM 25 matrix alloy was investigated with the objective of determining the mode of fi-acture of the matrix alloy. Fig 4.5 shows the fatigue fi-acture specimen of a lower magnification of SO^m with a crack. Fig 4.6 also shows fatigue fi-acture specimen of a higher magnification of 3^m showing the formation of voids. The formations of voids result in weakening the matrix alloy resulting in poor stress bearing capability. This results in strain localization leading to the mismatch between the forces acting on the matrix and the self opposing forces of the matrix resulting in initiation of cracks. The initiation of cracks will result in the crack propagation in the path of least resistance. 67

4 MM^'^ EHT=15.00 ku ID: e06l'5!m^- FTG UD= 11 nn Hag= 1.88 K X Photo No.=4140 DetectGr= SEl Fig 4.5: SEM Micrograph of Fatigue Fracture Specimen of Matrix Alloy LM 25 Showing a Crack (Magnification=30^m) At higher loads, the matrix alloy fails to absorb the stresses resulting in lower fatigue resistance. However, at lower loads the matrix alloy is able to absorb the stresses being encountered due to lower internal strains resulting in higher fatigue resistance. Fig 4.7 shows SEM micrograph of tensile fracture specimen of matrix alloy LM 25 showing crack initiation. (Magnification=3^m). It is clear from the micrograph that the number of cracks is initiated simultaneously due to low stress bearing capability of the matrix alloy. At high stress levels these cracks does not find any suitable crack propagating inhibitors due to lack of any reinforcing particles. It will be easier for the cracks to propagate and therefore, the fatigue specimen fails at lower number of cycles. 68

5 EHT=15.00 kw 3\in 1 1 ID: 6061-LM25- FTG 11 m Hag= 4.ee K X Photo No.=4139 Detector= SEl Fig 4.6: SEM Micrograph of Fatigue Fracture Specimen of Matrix Alloy LM 25 Showing Voids (Magnification=3^m) Fig 4.7: SEM Micrograph of Tensile Fracture Specimen of Matrix Alloy LM 25 Showing Crack Initiation at Level 1. (Magnification=3^m) 69

6 4.3.2 Summary The fatigue behaviour of matrix alloy LM 25 was investigated at three different stress levels. The fatigue resistance increased with the reduction in stress levels. The scanning electron micrographs of the fatigue fractured specimens revealed the formation of voids at higher stress levels around which cracks initiate and fail faster and hence the matrix alloy has less fatigue life. 70

7 4.4: Fatigue Determination of Aluminium-Graphite Composites Aluminium-Graphite Composite specimens with different percentage reinforcement of graphite were tested in same manner as that of LM 25 specimens. Results are also analyzed on the same lines which are presented in the following section Aluminium-2.5% Graphite Composites Test results of Aluminium-Graphite composites with 2.5% Graphite reinforcement are given in Table 4.6. The average life for different stress levels are given in Table 4.7. S-N curve for this composite is shown in the Fig 4.8. Fig 4.9 shows the log-log plot of stress and fatigue life. Sample Number Table 4.6 : Fatigue lives : Number of Cycles to Failure for Aluminium -2.5% Graphite Composite Level I 106MPa Level II 83MPa level III 59MPa Average Fatigue life

8 Table 4.7 : Average for Aluminium - 2.5% Graphite Composite Level(MPa) Average S-N curve for Aluminium-2.5%Graphite composite T an ra CL s >^ w 60 w <D 1-4-I ( N, cycles Fig 4.8 : S-N Curve for Aluminium-2.5% Graphite Composite 72

9 1000.:;.:;::^:::1::;:^ ^ 100 CO I [--" (>.5(I46 (>.5(I46 y = 55404x1 R2 t n ftltr IX \0 m N,( Number of cycles to failure) Fig 4.9: - Relationship for Aluininium-2.5% Graphite Composite Least square model for fatigue life for Aluminium-Graphite is given by; y=>.ix-p, -(4.6) Which has a regression correlation co-efficient, R = Where, y = amplitude in MPa, Xi = pi = X = Fatigue life in number of cycles. Fatigue life of Aluminium-2.5% Graphite composite in the given range of stresses can be estimated from the above equation Aluininium-5% Graphite Composite Test results of Aluminium-Graphite composites with 5% Graphite reinforcement are given in Table 4.8. The average life for different stress levels are given in Table 4.9. S-N curve for this composite is shown in the Fig Fig 4.11 shows the log-log plot of stress and fatigue life. 73

10 Table 4.8: Fatigue lives : Number of Cycles to Failure for Aluminium-5% Graphite Composite Sample Number Level I 96MPa Level II 75MPa level III 54MPa Average Fatigue Table 4.9 : Average for Aluminium -5% Graphite Composite Level(MPa) Average

11 S-N curve for Aluminium-5%Graphite composite 120 nj Q. mo s 80 c (A 60 (A > (0 0) N, ( cycles) Fig 4.10 : S-N Curve for Aluminium-5% Graphite Composite 1000 «100 Q. 7^-0. 49?1 ::::y " 4220 f X ^o F r = o w ( N,( Number of cycles to failure) Fig 4.11 : - Relationship for Aluminium-5% Graphite Composite 75

12 Least square model for fatigue life for Aluminium-5% Graphite is given by; y=h\-^ -(4.7) Which has a regression correlation co-efficient, R^ = Where, y = amplitude in MPa, h = 42207, p2 = X = Fatigue life in number of cycles. The above equation can be used to estimate fatigue life of Aluminium-5% Graphite composite in the given range of stresses Aluminium-7.5% Graphite Composite Table 4.10 show the fatigue test results for Aluminium-7.5% Graphite composite. Table 4.11 show the average life of Aluminium-7.5% Graphite composite. Fig 4.12 show the S-N curve for Aluminium-7.5% Graphite composite. Table 4.10: Fatigue lives : Numbe r of Cycles to Failure for Aluminium-7.5% Graphite Com josite Sample Number Level I Level II level III 89 MPa 69 MPa 50 MPa Average Fatigue

13 Table 4.11 : Average for Aluminium 7.5% Graphite Composite Level(MPa) Average S-N curve for Aluminium-7.5%Graphite composite 100 'TO" 80 Q. s ^-^ u> > (0 ( N, (cycles) Fig 4.12 : S-N Curve for Aluminium-7.5% Graphite Composite Fig 4.13 show the stress-life relationship for Aluminium-7.5% Graphite composite. This figure shows the log-log plot of stress and fatigue life. 77

14 t - - _; 1:: i ^ 100 a. s. o) 10 "X-, - -., J ::: it S, ' I'le [^-0. 40^2 : ::- Vh y r^** I P' i^ Y Jl At K L/^iO lof N,( Number of cycles to failure) Fig 4.13 : - Relationship for Aluininium-7.5% Graphite Composite Least square model for fatigue life for Aluminium-7.5% Graphite is given by; y=^3 X -S (4.8) Which has a regression correlation co-efficient, R^ = Where, y = amplitude in MPa, l.^ = 11512, p3 = X = Fatigue life in number of cycles. The above equation can be used to estimate fatigue life of Aluminium-7.5% Graphite composite in the given range of stresses AIuminium-lOVoGraphite Composite Table 4.12 show the results of sample size for Aluminium-10% Graphite composite. Table 4.13 show the average life of Aluminium-7.5% Graphite composite. Fig 4.14 show the S-N curve for Aluminium-10% Graphite composite. 78

15 Table 4.12 :Fatigue lives : Number of Cycles to Failure for Aluminium-10% Graphite Composite Sample Number Level I 81MPa Level II 63MPa level III 45MPa Average Fatigue Table 4.13 : Average for Aluminium - 10% Graphite Composite Level(MPa) Average

16 ; : f=^ S-N curve for Aluminium-10%Graphite composite 100 ( (0 40 to 20 t -v,^ -_, N, cyles Fig 4.14 : S-N Curve for Aluminium-10% Graphite Composite Fig 4.15 show the stress-hfe relationship for Aluminium-10% Graphite composite. This figure shows the log-log plot of stress and fatigue life n L _,., j,.. ^ _, _j _ _, Z 100 s CO CO '^^"^^ r ^ 10 CO f=^ mi ;.9}.-0: 1^90 i «1 > J 1 R^ = 0.8i!41 ' 1 10( ( 3000 N,( Number of cycles to failure) Fig 4.15: - Relationship for Aluininium-10% Graphite Composite 80

17 Least square model for fatigue life for Aluminium-10% Graphite is given by; y=ux ^ -(4.9) Which has a regression correlation co-efficient, R = Where, y = amplitude in MPa, X4 = , P4 = X = Fatigue life in number of cycles. The above equation can be used to estimate fatigue life of Al-10% Graphite composite in the given range of stresses Analysis of Fatigue Behaviour of Aluminium-Graphite Composites Fig 4.16 shows the comparison of S-N curves for Aluminium-Graphite composite with different percentage reinforcements. Corresponding to a given fatigue life, as the percentage reinforcement of graphite is increased from 2.5% to 10%, the fatigue strength decreases. The decrease in fatigue strength can be attributed to the increased presence of soft graphite particulates. S-N Curves of LM 25 and Aluminium-Graphite composite -LM 25 -AI-2.5%Gr -AI-5%Gr -AI-7.5%Gr -AI-10%Gr N, in Cycles Fig 4.16: Comparison of S-N Curves for Aluminium-Graphite Composites with Matrix Alloy LM 25 for Different Percentage Reinforcements of Graphite 81

18 Due to the low strength at the interface between graphite particulates and Aluminium alloy and the presence of porosity at the interface the graphite particulates easily fracture [94]. During the production process of Aluminium alloy graphite particulate composites, clusters that form due to an uneven distribution of graphite particulates contain porosity which embrittles the material, causing crack nucleation. This may be one of the reasons for the reduction of fatigue strength of Aluminium- Graphite composites, when compared to matrix alloy LM 25 [27, 36]. The Young's Modulus of the composite also decreases with the amount of increase in graphite as reported in section leading to increased brittleness of the Aluminium-Graphite composite. The composite that is more brittle is easier to fracture. This may be one of the reasons for the decrease in fatigue resistance in this study, when the percentage reinforcement of graphite was increased from 2.5% to 10% [58]. One of the reasons why graphite negatively affects the fatigue strength of the alloy is that the liquid phase increases their porosity in proportion to the graphite concentration. Also, graphite reduces the load carrying capacity of the aluminium alloys. [60]. The decrease in tensile strength and hardness with increasing graphite as presented in section 3.3 and 3.5 is associated with increasing density of voids and cracks. As the percentage reinforcement of graphite increases, due to increase in the density of the material packing, the inter particulate spacing decreases, resulting in greater deterioration in tensile strength resulting in poor fatigue resistance [25]. Table 4.14 gives the tensile strength and fatigue strength values at cycles of Aluminium-Graphite composites with different percentage reinforcements. Fig 4.17 shows a plot indicating the dependence of tensile strength and fatigue strength on percentage graphite reinforcement. In this plot, fatigue strengths corresponding to a life of 300,000 cycles is considered for comparison purposes. It is seen that the tensile and fatigue strength decrease as percentage reinforcement increases and the reduction in fatigue strength is drastic than the tensile strength beyond 5% reinforcement. This is due to the increased percentage reinforcement of graphite particles which result in more number of voids. 82

19 Table 4.14: Comparison of Tensile Strength and Fatigue Strength of Aluminium-Graphite Composite SI.No. Percentage Reinforcement Tensile strength(mpa) Fatigue strength corresponding to 300,000 Ratio of tensile strength to fatigue strength cycles(mpa) 1 2.5% % % % Comparison of tensile strength and fatigue strength of Aluminium-Graphite composites 140 n i^^^-^j lo Q. 1-^ ^"*"^*'*4-^,..,^ 1 rmj^^^ 2 80 i Tfc I 60 S \ - Tensile strength of Al-Gr l\ i - Fatigue strength of Al-Gr 0 1 i i i ( ) Reinforcement percentage Fig 4.17: Comparison of Tensile and Fatigue Strength of Aluminium-Graphite Composites 83

20 4.4.6 Scanning Electron Micrography The following figures show the general morphology and crack pattern for the Aluminium-Graphite composites. The discontinuous Graphite particulate reinforcement phase in the aluminium alloy matrix were of fairly uniform size with narrow particle size distribution, dispersed within the alloy matrix. The micrographs of Aluminium- Graphite composites revealed lack of wettability between the matrix and the graphite particulates. In the Fig 4.18 thefi-acturesurface of the composite showed the formation of number of cracks of varying magnitude which resulted in the fatigue failure of the specimens. EHT=15.00 kv 20\im ' UD= 10 mm Photo No. Mag= 1.50 K X Detector= SEl Fig 4.18: SEM Micrograph of Fatigue Fracture Specimen for Aluminium- 5% Graphite Composite Showing General Morphology. (Magnification=30^m) As the percentage reinforcement of graphite increases from 2.5% to 10%, reinforced particulates agglomerate, resulting in poor diffusion and adhesion. Thus the graphite particulate weakens the structure of the matrix alloy LM 25 which leads to reduction in fatigue resistance [34]. 84

21 From the study of the micromeehanism of fracture, it was found that the cast graphite composites contained a number of cracks and particle matrix separation at the interface. Fig 4.19 shows the graphite particle in the vicinity of which a number of cracks have been developed in the matrix. ID: AL-7.SX EHT=15.80 ku UD= 20 nn Mag= 508 X 30^m i \ Photo No. =4129 Detector^ SEl Fig 4.19: SEM micrograph of Fatigue Fracture Specimen for Aluminium- 7.5%Graphite Composite Showing Voids and Cracks (Magnification=30fim) Matrix cracks were invariably noticed especially in the case of higher percentage reinforcement of graphite (Fig 4.20, Fig 4.21 and Fig 4.22). Also, the graphite particles were pulled out from the matrix leaving behind cavities. The separation at the graphite particle-matrix interface and the matrix cracks in the vicinity of particles is believed to be instrumental in decreasing the strength properties of the cast Aluminium- Graphite composites. The matrix cracks combined with the cracks over the particulate phase in higher percentage reinforcement of graphite composite have possibly led to drastic decrease in strength properties in the Aluminium-Graphite composites. 85

22 EHT=15.e0 kv 3Mia I 1 UD= 20 nn Photo No.=4130 Mag= 4.00 K X Detector^ SEl Fig 4.20: SEM Micrograph of Fatigue Fracture Specimen for Aiuminium- 7.5%Graphite Composite Showing a Fractured Surface. (IVIagnification=3^m) EHT=15.00 kw 10pin i I UD= 23 nn Photo No.=4133 Ma6= 1.00 K X Detector= SEl Fig 4.21: SEM Micrograph of Fatigue Fracture Specimen Aluminium- 10% Graphite Composite Showing General Morphology and Crack, (Magnification=20^m) 86

23 Low wettability seen in Aluminium-Graphite composites results in weak interphase between the matrix and the reinforcing graphite particles resulting in easier and faster crack propagation. This drastically decreases tensile strength which has eventually resulted in decrease of fatigue properties. EHT=15.00 ku 3nn I i WD= 13 mm Photo No.=4147 Mag= 4.00 K X Detector= SEl ID: Al-10%-Gr-FTG Fig 4.22: SEM Micrograph of Fatigue Fracture Specimen Aluminium- 10% Graphite Composite Showing a Craclc.(Magnification=3^m) Summary The investigation into the fatigue behaviour of Aluminium-Graphite composites for different percentage reinforcement of Graphite particulates revealed the reduction in fatigue resistance with increase in percentage reinforcement when compared to matrix alloy LM 25. The SEM micrographs of the fatigue failed specimens revealed lack of wettability for Graphite particulates resulting in poor interphase between the matrix and the Graphite reinforcement. The weakening interphase between the matrix and the Graphite particulates result in easy formation of voids which eventually result in easy formation of cracks. The cracks fail to stabilize due to weak matrix-reinforcement bond, leading to easier propagation resulting in faster failure of the composite. The matrix 87

24 cracks combined with the cracks over the particulate phase in higher percentage reinforcement of graphite composite have possibly led to drastic decrease in fatigue strength of the Aluminium-Graphite composites. 88

25 4.5 S-N curves for Aluminium-Silicon Carbide Composites Aluminium-Silicon Carbide Composite specimens were tested in the same manner as that of LM 25 specimens. Results are also analyzed on the same lines which are presented in the following section Aluminium-2.5% Silicon Carbide Composite The fatigue test results of Aluminium-Silicon Carbide composites with 2.5% Silicon Carbide are given in Table The average life for different stress levels are given in Table S-N curve for this composite is shown in the Fig Fig 4.24 shows the log-log plot of stress and fatigue life. Table 4.15 : Fatigue lives : Number of cycles to failure for Aluminium- 2.5% Silicon Carbide Composite Sample Number Level I 133 MPa Level II 104 MPa level III 74 MPa ^ Average Fatigue

26 Table 4.16 : Average for Aluminiuin-2.5% Silicon Carbide Composite Level(MPa) Average S-N curve for Aluminium-2.5%Silicon carbide composite n Q. s w le 0) OT 80 ( N, in cycles Fig 4.23: S-N Curve for Aluminium-2.5% Silicon Carbide Composite 1000 n S. 100 s (0 (0 «"- 10- (0 1 r,, -iiqtqotiv"^ 52?t=^= J ciiy^tiy I :ao:rx_: ] -fr r ^ n ^ ^ " " ^ " ^ ^ " ^ " ^ ^ ^ 1 J L L 1 J 1 J ^^., 1 1 ^ r 1 r r - 1 t I I I t 1 ; I I I --^--l- 1 ; 1 1 -i, N,( Number of cycles to failure) Fig 4.24 : - Relationship for Aluminium-2.5% Silicon Carbide Composite 90

27 Least square model for fatigue life for Aluminium-2.5% Silicon Carbide is given by; y=h X "5 -(4.10) Which has a regression correlation co-efficient, R^ = Where, y = amplitude in MPa, h = 97937, pj = X = Fatigue life in number of cycles. The above equation can be used to estimate fatigue life of Aluminium-2.5% Silicon Carbide composite in the given range of stresses Aluininium-5%Silicon Carbide Composite The fatigue test results of Aluminium-Silicon Carbide composites with 5% Silicon Carbide are given in Table The average life for different stress levels are given in Table S-N curve for this composite is shown in the Fig Fig 4.26 shows the log-log plot of stress and fatigue life. Sample Number Table 4.17: Fatigue lives : Number of Cycles to Failure for Aluminium- 5% Silicon Carbide Composite Level I 145 MPa Level II 113 MPa level III 81 MPa Average Fatigue

28 Table 4.18 : Average for Aluminiuin-5% Silicon Carbide Composite Level(MPa) Average S-N curve for Aluminium-5%Silicon carbide composite ns Q (fi ^ 80 I : ^ \ ; ^ - \1^ N, in cycles Fig 4.25: S-N Curve for Aluminium-5% Silicon Carbide Composite 92

29 1000 " " _ " :- : I. _. --_... 1.: :! 1 "Ta 100 D. s (0 (0 a> k. 4-1 (0 V) 10 :: : 1 y t^,jgj-^-0,5354:;:::: r _i C , Number of cycles to failure Fig 4.26 : - Relationship for Aluininium-5%Silicon Carbide Composite Least square model for fatigue life for Aluminium-5% Silicon Carbide is given by; y=^x-''6 -(4.11) Which has a regression correlation co-efficient, R^ = Where, y = amplitude in MPa, h = , pg = X = Fatigue life in number of cycles. The above equation can be used to estimate fatigue life of Aluminium-5% Silicon Carbide composite in the given range of stresses Aluminium-7.5% Silicon Carbide Composite : The fatigue test results of Aluminium-Silicon Carbide composites with 7.5% Silicon Carbide are given in Table The average life at different stress levels are given in Table S-N curve for this composite is shown in the Fig Fig 4.28 shows the log-log plot of stress and fatigue life. 93

30 Table 4.19 : Fatigue lives : Number of Cycles to Failure for Aluminium- 7.5% Silicon Carbide Composite Sample Number Level I 158 MPa Level MPa level III 88 MPa Average Fatigue Table 4.20 : Average for Aluminium -7.5% Silicon Carbide Composite Level(MPa) Average

31 S-N Curve for Aluminium-7.5%Silicon carbide composite 170 D c 130 (A 110 (A <1> 4-> 90 w _ 70 w N, in cycles Fig 4.27: S-N Curve for Aluminium-7.5% Silicon Carbide Composite 1000 'S' 100 a. S (0 (0 a> 4^ ^ (0 w" 10 rrztzz EEE^ENIEEEEEEEEEEEEEEE: s 7JiQ y= Jl-^v"* R' = 0. di }^ 7 s 7JiQ Jl-^v"* N,( Number of cycles to failure) Fig 4.28 : - Relationship for Aluminium-7.5%Silicon Carbide Composite 95

32 Least square model for fatigue life for Aluminium-7.5%Silicon Carbide is given by; y=>^7x-''7 -(4.12) Which has a regression correlation co-efficient, R^ = Where, y = amplitude in MPa, h = , py = X = Fatigue life in number of cycles. The above equation can be used to estimate fatigue life of Aluminium-7.5% Silicon Carbide composite in the given range of stresses. 4,5.4 Aluminium-10% Silicon Carbide Composite The fatigue test results of Aluminium-Silicon Carbide composites with 10% Silicon Carbide are given in Table The average life for different stress levels are given in Table S-N curve for this composite is shovi^n in the Fig Fig 4.30 shows the log-log plot of stress and fatigue life. Table 4.21 : Fatigue lives : Number of Cycles to Failure for Aluminium- 10% Silicon Carbide Composite Sample Number Level I 152 MPa Level II 118 MPa level III 85 MPa Average Fatigue

33 Table 4.22 : Average for Aluminium -10% Silicon Carbide Composite Level(MPa) Average S-N Curve for Alumlnium-10% Silicon carbide composite 160 m Q (A «100 V) N, in cycles Fig 4.29: S-N curve Comparison of LM 25 and Aluminium- 10% Silicon Carbide Composite S. 100 s «(0 o ^ V) 10^ }.682 fi y. [c1f+ )b)r ^=i r.97' f < N. ( Number of cycle StO fc lilure) Fig 4.30 ; - Relationship for Aluminium-10% Silicon Carbide Composite 97

34 Least square model for fatigue life for Aluminium-10%Silicon Carbide is given by; y=>.8 X - "s - (4.13),2 Which has a regression correlation co-efficient, R = Where, y = amplitude in MPa, h = , pg = X = Fatigue life in number of cycles. The above equation can be used to estimate fatigue life of Aluminium-10% Silicon Carbide composite in the given range of stresses Analysis of Fatigue Behaviour of Aluminium-Silicon Carbide Composites Fig 4.31 compare the S-N Curves of Aluminium-Silicon Carbide composites having percentage reinforcement of Silicon Carbide ranging from 2.5% to 10% with S-N Curve of matrix alloy LM 25. Corresponding to a given life, fatigue strength of these composites increase as the % Silicon Carbide reinforcement increases. This is very much true up to 7,5% Silicon Carbide reinforcement. However, beyond 7.5% Silicon Carbide reinforcement there is no improvement in the fatigue strength with increased % Silicon Carbide reinforcements. This is evident from the Fig 4.31 where S-N Curve of Aluminium-7.5%) Silicon Carbide and of Aluminium-10% Silicon Carbide almost overlap. The fatigue strength of the Aluminium-Silicon Carbide composite increased with increasing reinforcement content only as long as the composite was able to exhibit enough ductility to attain full strength. As the amount of Silicon Carbide reinforcement exceeded 7.5 % reinforcement, the matrix probably did not have sufficient internal ductility to redistribute the very high localized internal stress [43]. It can be concluded that the results of the present study generally agrees with research conducted by most of the investigators [50, 52]. The fatigue resistance increases with increase in percentage reinforcement from 2.5 to 7.5, compared to the matrix alloy LM 25. However, the fatigue resistance shows no improvement for 10 % reinforcement when compared with reinforcement of 7.5%. 98

35 S-N Curves of LM 25 and Aluminium-Silicon Carbide composite ^ i S 105 (0 w DC? N, in Cycles 4 LIVf25 - -AI-2.5%SiC A-Al-5%SiC -B-Al-7^%SiC ) ^AI-10%SiC Fig 4.31: Comparison of S-N Curves for Aluminium-Silicon Carbide Composites with Matrix Alloy LM 25 for Different Percentage Reinforcement of Silicon Carbide Table 4.23 gives the tensile strength and fatigue strength values at cycles of Aluminium-Silicon carbide composites with different percentage Silicon carbide reinforcements. Fig 4.32 shows a plot indicating the dependence of tensile strength and fatigue strength on percentage Silicon Carbide reinforcement. It is seen that the tensile and fatigue strength increases upto 7.5 % reinforcement. Beyond 7.5% there is no considerable increase in tensile and fatigue strength which can be attributed to increased strain localization due to increase in density of reinforcement particles which result in the loss of load bearing capability of the Silicon Carbide reinforcement. 99

36 Table 4.23: Comparison of Tensile and Fatigue Strength for Aluminium- Silicon Carbide Composite Sl.No. Fatigue strength Ratio of tensile % Tensile corresponding to strength to Reinforcement strength(]vipa) 500,000 fatigue strength cycles(mpa) 1 2.5% % % % Comparison of tensile strength and fatigue strength of Aluminium-Silicon carbide composite (0 zuu Q i lur''^^'^^^^ \ ^4"?i i 1,1 i MM 'L/^\ \'\\pf\\ M M 1 - Tensile strength? 80 J= 60 ( n i ' i U T ( D Percentage Reinforcement - Fatigue strength Fig 4.32: Comparison of Tensile Strength and Fatigue Strength of Aluminium- Silicon Carbide Composite 100

37 4.5.6 Scanning Electron Micrography Fig 4.33, 4.34 and 4.35 shows the microstructure of the Aluminium-Silicon Carbide composites for different percentage reinforcements. The discontinuous Silicon Carbide particulate reinforcement phase in the LM 25 aluminium alloy matrix were of uniform size with narrow particle size distribution. At frequent intervals an agglomeration or clustering of the Silicon Carbide reinforcements was observed resulting in particulate rich and particulate depleted regions. An agglomerated site consisted of uniformly small and regularly shaped Silicon Carbide with the matrix material. The beneficial effect of increasing percentage Silicon Carbide reinforcement on fatigue life was observed. Increasing the percentage Silicon Carbide resulted in higher fatigue resistance. (Taken as the highest number of cycles at which the specimen failed). In particular, the highest percentage reinforcement of 7.5% Silicon Carbide and 10% Silicon Carbide showed noticeable improvement in fatigue resistance compared to the unreinforced matrix. The constraints in mechanical deformation arising as a result of the hard, locally brittle and globally ductile elastically deforming Silicon Carbide particulates in a soft, ductile and plastically deforming aluminium alloy metal matrix and the development of resulting stress state aids in limiting the overall flow stress of the composite matrix. This favors void initiation and growth in the matrix, and failure by both cracking of the Silicon Carbide and separation at the particulate-matrix interfaces as shown in the fig Furthermore, as a direct consequence of the deformation constraints a higher applied maximum stress is required to initiate microplastic deformation in the composite. This translates to higher young's modulus of the composite material. That may be the reason, for the Aluminium-7.5% Silicon Carbide composite, the fatigue resistance has been found to be highest. Examination of the fracture surface of the composites revealed damage to be localized at the reinforcing phase through cracked particulates and failure by cracking and separation at interfaces with the matrix, with little evidence of microscopic void formation away from the cracked particulate as shown in the micrographs. This suggests that microplastic deformation become localized during the early stages of higher stress levels. The intrinsic brittleness of the reinforcing Silicon Carbide coupled with the propensity for the fracture due to localized deformation resulted in both cracking and interfacial failure. loi

38 M' m^h r EHT=15.00 ku 30^lm I UD= 13 nn Mag= 1.00 K X Photo No.=4144 Detector= SEl ID: Al-5% Fig 4.33: SEM Micrograph of Fatigue Fracture Specimen for Aluminium-5% Silicon Carbide Composite (Magnification=20nm) Showing Overall Morphology The drop in tensile strength coupled with damage to the composite microstructure arising from the conjoint influence of particulate cracking and interfacial failure is responsible for the inferior fatigue resistance of the Aluminium-10% Silicon Carbide composite over the unreinforced alloy. Neglecting any contribution from the interfaces, the stresses induced during loading favor limited growth of the microscopic voids in the matrix of the composite. The limited growth of the microscopic voids coupled with lack of their coalescence, as a dominant fracture mode for the Aluminium-Silicon Carbide composite, clearly indicates that the deformation properties of the aluminium alloy matrix are significantly altered by the presence of the discontinuous Silicon Carbide particulate reinforcements. In Silicon Carbide particulate rich regions of the matrix, fracture occurred early and the damage propagated rapidly among the particulate agglomerate. However, the reinforcement lean region, i.e. the metal matrix, aids in retarding the progression and linkage of damage. 102

39 Fracture of the brittle Silicon Carbide exacerbates the local damage and serves as a starting point for the formation of a microscopic crack. This in conjunction with failure of the surrounding matrix results in the formation of fine microscopic voids cx the particulate-matrix interfaces. Very few of the fine microscopic voids coalesce and surround the cracked particles. With an increase in reinforcement content, microscopic fracture was dominated by cracking of the particulates on account of their intrinsic brittleness as shown in the fig Fig 4.34: SEM Micrograph of Fatigue Fracture Specimen for Aluminiuin-7.5% Silicon Carbide Composite (Magnification=3^,m) Showing Microcrack Initiation at Region of the Particle Agglomeration The overall damage resulfing fi-om mechanical deformation of the Aluminium- Silicon Carbide composite can be associated with failure of the discontinuous reinforcement through (a) Cracking and (b) separation at interfaces with the matrix. The broken Silicon Carbide particulates elevate the local stress, which raises the probability of failure of the matrix and other particulates. Final fi-acture is achieved by damage propagation through the matrix between particle rich regions. The macroscopic voids were intermingled with fine microscopic 103

40 voids. Few of the voids generated by particle cracking did not grow extensively which is generally the case in ductile fracture of the unreinforced aluminium alloys. A large mismatchjn stress carrying capability exists between the brittle and elastically deforming reinforcing Silicon Carbide and the soft and plastically deforming aluminium alloy metal matrix. This result in a large concentration of stress near the reinforcing Silicon Carbide causing them to crack and the adjacent matrix to fail by ductile fracture which results in formation of voids as shown in Fig Failure through the Conjoint influences of separation at the matrix- reinforcement particulate interfaces, referred to as decohesion, and cracking of the reinforcing Silicon Carbide suggests the exacerbation(to increase the intensity) of microplastic deformation in regions containing a high percentage reinforcing particulates of Silicon Carbide. This may be one of the reasons for the decrease in fatigue resistance of Aluminium-10% Silicon Carbide composite. EHT=15.00 ku UD= 13 nn Photo No.=4145 Mag= 500 X Detector= SEl ID: Al-10%- Fig 4.35 : SEM Micrograph of Fatigue Fracture Specimen for Aluminium- 10% Silicon Carbide Composite (Magnification=20 ^m ) Showing Voids 104

41 4.5.7 Summary The increase in percentage reinforcement of Silicon Carbide particulates resulted in improvement of fatigue properties of Aluminium-Silicon Carbide composites upto 7.5 % reinforcement when compared to matrix alloy LM 25. However, at 10 % reinforcement the fatigue properties exhibited no improvement. The constraints in mechanical deformation arising as a result of the hard, locally brittle and globally ductile elastically deforming Silicon Carbide particulates in a soft, ductile and plastically deforming aluminium alloy metal matrix and the development of resulting stress state aids in limiting the overall flow stress of the composite matrix. As a direct consequence of the deformation constraints a higher applied maximum stress is required to initiate microplastic deformation in the composite. This translates to higher elastic constant and tensile strength of the composite material. This results into higher fatigue resistance for the Aluminium-Silicon Carbide composites upto 7.5 % reinforcement. However at 10 % reinforcement, due to decohesion and cracking of the Silicon Carbide, result in increasing the microplastic deformation in regions containing high percentage reinforcement resulting in no improvement in fatigue resistance. ' 105

42 4.6 S-N Curves for Hybrid Aluminium-Silicon Carbide-Graphite Composites Hybrid Aluminium-Silicon Carbide-Graphite Composite specimens with different combined percentage reinforcements were tested in the same manner as that of LM 25 specimens. Results are also analyzed on the same lines which are presented in the following section S-N Curves for Hybrid Aluminium-1.25% Silicon Carbide-1.25% Graphite Composite The fatigue test results of Aluminium-1.25% Silicon Carbide-1.25% Graphite composites are given in Table The average life for different stress levels are given in Table S-N curve for this composite is shown in the Fig Fig 4.37 shows the log-log plot of stress and fatigue life. Table 4.24 : Fatigue lives : Number of Cycles to Failure for Aluminium- 1.25% Silicon Carbide-1.25% Graphite Composite Sample Number Level I 117 MPa Level II 91 MPa level III 65 MPa Average Fatigue

43 Table 4.25 : Average for Hybrid Aluminium- 1.25"/o Silicon Carbide-1.25% Graphite Composite Level(MPa) Average S-N Curve for Hybrid Aluminium-1.25% Silicon carbide- 1.25% Graphite composite I'fU 120 re E 80 g 60 ^ ( ! ' 1 ) ( 3000 N, Number of cycles 1 Fig 4.36: S-N Curve for Aluminium-L25% Silicon Carbide- 1.25% Graphite Composite 107

44 1000 -:-:--:-: ' : :^ ^ (0 (0 CO ja V _=_70^*^H^ 0.5( T 1 R^ = 0.96^ N,( Number of cycles to failure) 1 - Fig 4.37: - Relation for Hybrid Aluminium- 1.25% Silicon Carbide-1.25% Graphite Composite Least square model for fatigue life for Aluminium-1.25%Silicon Carbide-1.25% Graphite is given by; y='hx-% (4.14) Which has a regression correlation co-efficient, R = Where, y = amplitude in MPa, h) = 70359, p9 = X = Fatigue life in number of cycles. The above equation can be used to estimate fatigue life of Aluminium-1.25% Silicon Carbide-1.25% Graphite composite in the given range of stresses S-N curves for Hybrid Aluminium-2.5% Silicon Carbide- 2.5% Graphite Composite The fatigue test results of Aluminium-2.5% Silicon Carbide-1.2.5% Graphite composites are given in Table The average life for different stress levels are given in Table S-N curve for this composite is shown in the Fig Fig 4.39 shows the log-log plot of stress and fatigue life. 108

45 Table 4.26 : Fatigue lives : Number of Cycles to Failure for Aluminium- 2.5% Silicon Carbide-2.5% Graphite Composite Sample Number Level I 130 MPa Level II 101 MPa level III 72 MPa Average Fatigue Table 4.27 : Average for Aluminium- 2.5% Silicon Carbide-2.5% Graphite Composite Level(MPa) Average

46 S-N Curve for Hybrid Aluminium-2.5%Silicon carbide- 2.5%Graphite composite re Q. 100 S _C 80 (A (A d) 60 4-( V) > N, Number of cycles Fig 4.38: S-N curve for Aluininium-2.5% Silicon Carbide- 2.5% Graphite Composite 1000 Q s (0 CO 0) ( ^^( t^-oi! MHN -- Y' - 7d;j»3. c ::_: 4::: D? n_q r4l K l/.3 i9t<» 100( DOOO N, (Number ()f cycle!s to f( 3ilur( 0 Fig 4.39: - Relation for Hybrid Aluminium-2.5% Silicon Carbide- 2.5% Graphite Composite no

47 Least square model for fatigue life for Aluminium-2.5% Silicon Carbide-2.5% Graphite is given by; y=x,o X - P,o (4.15) Which has a regression correlation co-efficient, R^ = Where, y = amplitude in MPa, A-io = 73999, Pio = X = Fatigue life in number of cycles. The above equation can be used to estimate fatigue life of Al-2.5% Silicon Carbide-2.5% Graphite composite in the given range of stresses S-N Curves for Hybrid AIuminium-3.75% Silicon Carbide-3.75% Graphite composite The fatigue test results of Aluminium-3.75% Silicon Carbide-3.75% Graphite composites are given in Table The average life for different stress levels are given in Table S-N curve for this composite is shown in the Fig Fig 4.41 shows the log-log plot of stress and fatigue life. Table 4.28 : Fatigue lives : Number of Cycles to Failure for Aluminium-3.75 % Silicon Carbide-3.75 % Graphite Composite Sample Number Level I 136 MPa Level II 106 MPa level III 76 MPa ^ Average Fatigue

48 Table 4.29 : Average for Aluminium- 3.75% Silicon Carbide-3.75% Graphite Composite Level(MPa) Average S-N Curve for Hybrid Aluminium-3.75% Silicon carbide- 3.75% Graphite composite re 0. (A (O ( H 4- I 1 J 4-1 I _. _. -^ - -,- -_ N.LIfe in Cycles Fig 4.40: S-N Curve for Aluminium-3.75% Silicon Carbide- 3.75% Graphite Composite 112

49 I 100 m (A ^ 10 CO 1 1 J AAA U -(^-ssi 1: : :V = 1239 IbX / R =0.9711?i^ N,Ufe( Number of cycles to failure) Fig 4.41: - Relation for Hybrid Aluininium-3.75% Silicon Carbide- 3.75% Graphite Composite Least square model for fatigue life for Aluminium-3.75% Silicon Carbide-3.75% Graphite is given by; y=x X - P (4.16) Which has a regression correlation co-efficient, R^ = Where, y = amplitude in MPa, l^ = , pn = X = Fatigue life in number of cycles. The above equation can be used to estimate fatigue life of Al-3.75% Silicon Carbide-3.75% Graphite composite in the given range of stresses S-N Curves for Hybrid Aluminium-5% Silicon Carbide-5% Graphite Composite Test results of Aluminium-5% Silicon Carbide-5%o Graphite composites are given in Table The average life for different stress levels are given in Table S-N curve for this composite is shown in the Fig Fig 4.43 shows the log-log plot of stress and fatigue life. 113

50 Table 4.30 : Fatigue lives : Number of Cycles to Failure for Aluminium- 5% Silicon Carbide-5% Grphite Composite Sample Number Level I 136 MPa Level II 106 MPa level III 76 MPa Average Fatigue Table 4.31 : Average for Aluminium- 5% Silicon Carbide-5% Graphite Composite Level(MPa) Average

51 S-N curve for Hybrid Aluminium-5%Silicon carbide- 5%GrapliJte composite c 80 (A 60 ^ B^^^ ^^^m^ ( ) E+06 1E+06 N, in Cycles Fig 4.42: S-N Curve for Aluminium-5% Silicon Carbide- 5% Graphite Composite 1000 Z 100 to tn o k. ^ 10 j\ 5713 ]f 35186x' 5713 ' 1 r = T).966< 1 d N,( Number of cycles to failure) Fig 4,43: - Relation for Hybrid Aluminium- 5% Silicon Carbide-5% Graphite Composite 15

52 Least square model for fatigue life for Aluminium-5% Silicon Carbide-5% Graphite is given by; y=>^i2x-p,2 -(4.17) Which has a regression correlation co-efficient, R^ = Where, y = amplitude in MPa, >.i2 = , pi2 = X = Fatigue life in number of cycles. The above equation can be used to estimate fatigue life of Aluminium-5% Silicon Carbide-5% Graphite composite in the given range of stresses Analysis of Fatigue Behaviour of Hybrid Aluminium-Silicon Carbide- Graphite Composites Fig 4.44 shows the S-N curves of Aluminium-Silicon Carbide-Graphite composites having combined percentage reinforcement of Silicon Carbide and Graphite particulates in the range of 2.5% to 10%. S-N Curves of LM 25 and Hybrid Aluminium-Silicon carbide-graphite composite (0 Q N, in Cycles -LM 25 -AI-1.25%SiC-1.25%Gr -AI-2.5%SiC-2.5%Gr -AI-3.75%SiC-3.75%Gr -AI-5%SIC-5%Gr Fig 4.44: Comparison of S-N Curves of Matrix Alloy LM 25 and Hybrid Aluminium-Silicon Carbide-Graphite Composite 116

53 These curves are compared with S-N curve of matrix alloy LM 25. As shown in the figure, S-N curve of matrix alloy LM 25, overlaps on the S-N curve of Al-1.25% Silicon Carbide-1.25% Graphite composite. This is due to the fact that the contribution made by Silicon Carbide particulates in improving the fatigue strength at a given life is neutralized by the graphite particulates which have a general tendency to reduce the fatigue strength. However, as can be noticed from the S-N diagrams the fatigue strength at a given life increases as the combined percentage reinforcement of Silicon Carbide and Graphite particulates increases. This is true for Aluminium-2.5% Silicon Carbide-2.5% Graphite and Aluminium-3.75% Silicon Carbide-3.75% Graphite composites. This is due to the fact that the positive influence of hard Silicon Carbide particles on fatigue strength is more than the negative influence of soft graphite particulates. Fatigue strength of Aluminium-5% Silicon Carbide-5% Graphite composite is less than that of Aluminium-3.75% Silicon Carbide-3.75% Graphite composites. This is because of the fact that the influence of each Silicon Carbide and Graphite particulates on fatigue strength is to decrease the fatigue strength with their increased amounts of presence in the composite, as discussed in earlier sections (a) Comparison of Tensile Strength and Fatigue Strength of Hybrid Aluminium-Silicon Carbide-Graphite Composite Table 4.32 and Fig 4.45 compare the tensile strength and fatigue strength at a cycles of hybrid composites Table 4.32: Comparison of Tensile and Fatigue Strength for Hybrid Aluminium- Silicon Carbide-Graphite Composite Sl.No. Percentage Reinforcement Tensile strength(mpa) Fatigue strength corresponding to. 500,000,, ' V V ' cycles(mpa) Ratio of tensile strength to fatigue strength 1 2.5% % % %

54 Comparison of tensile strength and fatigue strength of hybrid composites -Tensile strength of Hybrid composite Fatigue strength of Hybrid composites Percentage reinforcement 10 Fig 4.45: Comparison of Tensile Strength and Fatigue Strength for Hybrid Aluminium-Silicon Carbide-Graphite Composite It is seen that both tensile and fatigue strengths tend to decrease beyond 7.5% reinforcement. This may be attributed to the increased percentage of graphite reinforcement Scanning Electron Micrography of Hybrid Aluminium-Silicon Carbide- Graphite Composites The beneficial effect of increasing Silicon Carbide+Graphite percentage reinforcement on fatigue life was observed. Increasing the Silicon Carbide+Graphite percentage reinforcement resulted in higher fatigue resistance upto 7.5%, beyond which the fatigue resistance began to show a down ward trend. In particular, the highest volume fraction of 3.75% Silicon Carbide+3.75% Graphite and 5% Silicon Carbide+5% Graphite showed noticeable improvement in fatigue resistance compared to the unreinforced aluminum alloy and Aluminium-Silicon Carbide-Graphite composites with lower levels of reinforcement. 118

55 The macroscopic or low magnification observations, revealed the fracture surface to essentially comprise features reminiscent of brittle (microscopic and macroscopic cracks) and local cracking mechanisms. The microscopic cracks were found in large numbers in the sample defomied to failure at moderate stress levels and resultant enhanced fatigue life compared to Aluminium-1.25% Silicon Carbide-1.25% Graphite. The microscopic voids, of varying size were distributed randomly through the fracture surface as shown in Fig Fig 4.47 shows the SEM micrograph of fatigue failure specimen of hybrid Aluminium-1.25% Silicon Carbide-1.25% Graphite composite fracture surface showing cracks.,1.; :^;^; UD= 12 nn Photo No.=4058 Mag= 2.00 K X Detector= SEl ID: 2.5X IIB FTG Fig 4.46: SEM Micrograph of Fatigue Failure Specimen of Hybrid Aluminium-1.25% Silicon Carbide-1.25% Graphite Composite Showing Microscopic Crack and Voids Fig 4.48 shows SEM micrograph of fatigue failure specimen of hybrid Aluminium-2.5% Silicon Carbide-2.5% Graphite composite with a crack. The macroscopic and fine microscopic voids were (a) surrounded by cracked Silicon Carbide+Graphite particles and (b) isolated regions of shallow voids. A large portion of the fatigue fracture occurred 119

56 along the Silicon Carbide+Graphite reinforced rich region as evidenced in the micro and macroscopic view of the fi"actured composite microstructures.»jf^g«^«r-' EHT=15.80 ku 30[in I 1 13 nra Photo No.=4057 Mag= 588 X Detector= SEl ID: 2.5% HB FTG Fig 4.47 : SEM Micrograph of Fatigue Failure Specimen of Hybrid Aluminium-1.25% Silicon Carbide-1.25% Graphite Composite, Showing Cracks Fig 4.48: SEM Micrograph of Fatigue Failure Specimen of Hybrid Aiuminium-2.5% Silicon Carbide-2.5% Graphite Composite Showing a Crack 120

57 High magnification observation of the sample revealed the following: (a) An overall rough fracture surface. (b) Evidence of gross brittle failure through failure of a large number of the reinforcing Silicon Carbide+Graphite particulates by both cracking and decohesion at the interfaces as shown in Fig (c) Traces of failure through the formation and presence of shallow voids surrounding the cracked Silicon Carbide+Graphite particulates. Fig 4.49: SEM Micrograph of Fatigue Failure Specimen of Hybrid Aluininium-3.75% Silicon Carbide-3.75% Graphite Composite Showing a Crack At higher levels of particulate reinforcement i.e. for Aluminium-5% Silicon Carbide-5% Graphite composite, large mismatch in stress carrying capability exists 121

58 between the brittle reinforcing Silicon Carbide+Graphite and the soft and plastically deforming aluminium alloy metal matrix. This results in a large concentration of stress near the reinforcing Silicon Carbide+Graphite particulate causing them to crack as shown in Fig 4.50 and the adjacent matrix to fail by fracture, thus resulting in lower fatigue life compared to Aluminium-3.75% Silicon Carbide -3.75% Graphite composites. Fig 4.50: SEM Micrograph of Fatigue Failure Specimen of Hybrid Aluminium-5% Silicon Carbide-5% Graphite Composite Showing a Cracked Surface Careful microscopic observation of the micrographs reveal failure through the separation at the matrix- reinforcement particulate interfaces, referred to as de-cohesion or conjoint influences, and cracking along regions containing high percentage 122

59 reinforcement of the reinforcing particulates of Silicon Carbide+Graphite as shown in Fig Fig 4.51: SEM Micrograph of Fatigue Failure Specimen of Hybrid Aluininium-3.75% Silicon Carbide-3.75% Graphite Composite Showing General Morphology Summary Examination of the fracture surfaces revealed damage to be localized at the reinforcing phase through cracked particulates and failure by cracking and separation at interfaces with the matrix, with little evidence of microscopic void formation away from the cracked particulate. This suggests that microplastic deformation become localized during the early stages of higher stress levels. The intrinsic brittleness of the reinforcing Silicon Carbide and low modulus of elasticity of Graphite coupled with the propensity for the fracture due to localized deformation resulted in both cracking and interfacial failure. The constraints in mechanical deformation arising as a result of the hard, brittle and elastically deforming Silicon Carbide and soft, ductile Graphite particulates in a 123

60 soft, ductile and plastically defomiing aluminium alloy metal matrix and the development of resulting stress state aids in limiting the overall flow stress of the composite matrix. This favors void initiation and growth in the matrix, and failure by both cracking of the Silicon Carbide+Graphite and separation at the particulate-matrix interfaces. Furthermore, as a direct consequence of the deformation constraints a higher applied maximum stress is required to initiate microplastic deformation in the composite. This translates to higher elastic constant and higher tensile strength of the composite material. That may be the reason, for the Aluminium-3.75% Silicon Carbide-3.75% Graphite composite, having maximum tensile strength of 151 MPa aivd highest Young's modulus of 74.5 GPa among hybrid composites, the fatigue resistance have been found to be highest, compared to Aluminium-1.25% Silicon Carbide-1.25% Graphite and Aluminium-2.5%) Silicon Carbide-2.5% Graphite composites. The tensile strength of 151MPa and Young's modulus of 74.5GPa for Aluminium-3.75%) Silicon Carbide- 3.75% Graphite is highest among the different percentage reinforcement of composites. This clearly indicates that eventhough the soft graphite particulates reduces the strength of the composite, Silicon Carbide particulates will offset this decrease in the overall strength of the composite resulting in combination of superior hybrid composite which incorporates better tribological properties as well as superior strength. The stresses induced during loading favor limited growth of the microscopic voids in the matrix of the composite. The limited growth of the mircroscopic voids coupled with lack of their coalescence, as a dominant fi-acture mode for the Aluminium- Silicon Carbide-Graphite composite, clearly indicates that the deformation properties of the aluminium alloy matrix are significantly altered by the presence of the discontinuous Silicon Carbide+Graphite particulate reinforcements. Fracture of the brittle Silicon Carbide and ductile Graphite exacerbates the local damage and serves as a starting point for the formation of a microscopic crack as shown in Fig This in conjunction with failure of the surrounding matrix results in the formation of fine microscopic voids at the particulate-matrix interfaces. Very few of the fine microscopic voids coalesce. The lack of formation of ductile dimples in the matrix, as a dominantfi-acture mode, is attributed to the constraints on plastic flow caused by the presence of Silicon Carbide i.e. the deformation incompatibility between the plastically deforming metal matrix and the elastically deforming reinforcement phase, and not due to high ductility of the aluminium alloy metal matrix. With an increase in reinforcement 124

61 content, microscopic tracture was dominated by cracking of the particulates on account of their intrinsic brittleness. Fig 4.52: SEM Micrograph of Fatigue Failure Specimen of Hybrid Aluminiuin-2.5% Silicon Carbide-2.5% Graphite Composite Showing a Crack and Voids The overall damage resulting from mechanical deformation of the Aluminium- Silicon Carbide-Graphite composite can be associated with failure of the discontinuous reinforcement through (a) Cracking and (b) separation at interfaces with the matrix. The overall fracture of the composite resulted due to the combination of ductile and brittle failures. The broken Silicon Carbide+Graphite particulates elevate the local stress by not absorbing the stresses distributed by the matrix which raises the probability of failure of the matrix and other particulates. Final fracture occurred by damage propagation through 125