FATIGUE AND CREEP PERFORMANCE OF THERMOPLASTIC LEAF SPRINGS
7.3 FATIGUE PERFORMANCE OF THERMOPLASTIC LEAF SPRINGS
7.3.2 Spring Rate of Test Leaf Springs
Energy dissipation of test leaf spring is evaluated from the cyclic load-deflection plot.
To compare the leaf spring performance, the energy dissipation ratio at beginning (10th) and after finite number of cycle (3500th) was computed. The increase in energy dissipation ratio from the beginning to 3500th cycle for UFLS, SFLS and LFLS are found to be 1.40, 1.20 and 1.04 respectively. Earlier investigation (Section 6.7.4) under static condition also confirmed the enhanced energy dissipation behavior of SFLS than that of LFLS. This behavior is due to the presence of more fiber ends in SFLS than that of LFLS. In addition, the presence of high modulus long glass fibers reduces the energy dissipation significantly. Talib et al. (2010) also reported the reduced energy dissipation behavior due to the presence of high modulus carbon fiber in the composite springs.
These hysteresis loops move along the deflection axis throughout the life of the component. The shape and size of these loops, depends on the applied stress. Figures 7.3(a-c) show the cyclic load-deflection curve of long fiber reinforced leaf springs subjected to 0.8 Pmax , 0.9 Pmax and 1.2 Pmax at various period of fatigue life (Nf).
Cyclic load-deflection plot of test leaf springs exhibited significant increase in deflection with the increase in load during the fatigue loading. Since energy dissipation is associated with the spring rate reduction, further quantification of spring rate is of the practical importance to the suspension application.
very low (excessively flexible springs) than required, then the springs deflect drastically which contributes to the poor isolation of the vehicle from the road against the vibrations. Hence for the good suspension system, appropriate spring rate is required and to be retained during its service. Fatigue loading of leaf springs causes
material damage and reduces spring rate as the cycle progresses.
To enumerate the accumulated fatigue damage in the test leaf spring, relative spring rate (ratio of instantaneous spring rate (K) to the initial spring rate (Ko)) was measured and plotted as the performance index. The relative spring rate of test leaf springs at finite number of cycles at various loads were obtained from the cyclic load deflection curve. Figures 7.4 (a-b) show the relative spring rate of test leaf springs at three different loading levels (0.8 Pmax 0.9 Pmax and Pmax). Orth et al. (1993) also utilized the stiffness drop for carbon fiber reinforced spring retainer as a damage index under dynamic loading condition. Casado et al. (2006) adopted similar methodology for the performance investigation of railway fastening part made of short glass fiber reinforced nylon and reported a continuous stiffness drop and as well as fracture. Leaf spring fatigue failure is taken as spring rate drop/fracture as per SAE standard J 1528.
UFLS and SFLS exhibited failure by spring rate drop by 10 % whereas LFLS exhibited fracture failure. Unlike, SFLS and UFLS, LFLS do not exhibit appreciable spring rate drop initially but later significant drop in spring rate was observed, and similar kind of transition was observed for all the chosen loads.
Fig.7.4 Spring rate reduction in reinforced and unreinforced leaf springs for (a) Pmax and 0.8Pmax (b) 0.9Pmax
0.85 0.9 0.95 1
1 10 100 1000 10000 100000 1000000
Number of fatigue cycles (Nf)
Relative spring rate (K/ Ko).
.8PmaxSFLS PmaxSFLS .8Pmax LFLS
PmaxLFLS .8PmaxUFLS PmaxUFLS
Failure limit (a)
0.85 0.9 0.95 1
1 10 100 1000 10000 100000 1000000
Number of fatigue cycles (Nf )
Relative spring rate (K/ Ko).
.9PmaxSFLS 0.9PmaxLFLS 0.9PmaxUFLS Failure limit
(b)
7.3.3 Fatigue Strength of Molded Leaf Springs
Based on the failure criteria as discussed before, the endurance curve of the thermoplastic leaf spring is shown in figure 7.5 for all the leaf springs. Endurance limit of the tested thermoplastic composite spring is taken as 2 x105 cycles (SAE J 1528, 1990) and the run out specimens were indicated by the arrow marks. The induced leaf spring bending stress Sb is computed using relation 7.1. Rajendran and Vijayarangan (2001) made use of the similar equation for designing the mono composite leaf spring for depicting the leaf spring bending stress estimation
e b
c
S = 3PL 2b t 2
(7.1)
where P is the applied load, Le is effective length of the spring, bc is the width at the center and t is the beam thickness at center. Test conditions were repeated for three times, and the whole set of data irrespective of load levels were used. A linear interpolation on the semi logarithmic plot (Smax - log (Nf)) was performed and the equation for the linear interpolation takes the form
Smax = So - B Log (Nf ) (7.2) This equation is rewritten as
Smax / So = 1 - m Log (Nf ) (7.3) where Smax is the maximum bending stress, So is the static strength and m is a constant parameter representing the fatigue sensitivity of material. The S-N curve of the test leaf springs shows the parameters of the interpolating line along with correlation index (figure 7.5). The values of correlation index were well within the limits. Linear interpolation was performed using the above equation (7.3) on the semi-logarithmic plot by various authors (Zhou et al., 1994; Allah et al., 1997). LFLS possessed
enhanced fatigue resistance in both the low and high cycle fatigue.
Fatigue performance of leaf springs was found to be improved with the increase in fiber length due to the enhanced load transfer from the fiber to matrix. LFLS showed higher sensitivity towards stress level than that of SFLS and UFLS. Sensitiveness towards stress level is due to reduced ductility of the matrix.
Fig. 7.5 Fatigue stress - life curve of reinforced and unreinforced leaf springs
UFLS and SFLS exhibited drop in spring rate and LFLS exhibited fracture for all the considered loads. Surface morphology of LFLS exhibited cracks on the tensile surface of the leaf spring as shown in figure 7.7. Figure 7.6a shows the schematic position of the cracks on the test leaf spring. Figures 7.6 (b-c) elucidate visible cracks in the transverse direction to leaf spring length and figure 7.6d shows the observed crazes near the cracks on the tensile surface of LFLS.
y = -6.5224Ln(x) + 107.08 R2 = 0.9607
y = -1.8111Ln(x) + 30.816 R2 = 0.9008
y = -1.0237Ln(x) + 17.381 R2 = 0.9582
0 10 20 30 40 50 60 70 80
1.E+02 1.E+03 1.E+04 1.E+05 1.E+06
Cycles to failure, Nf
Max bending stress Smax, (MPa)
SFLS LFLS UFLS
Fig. 7.6 Failure morphology of long fiber reinforced leaf spring (a) schematic cracks on the tensile surface (b) cracks in the direction transverse to the leaf spring length (c) cracks on the leaf spring (d) crazing on the leaf spring.
Cracks on the tensile surface
(a)
2mm
(b)
(c) (d)
Fig. 7.7 Schematic of the investigated fractured surface of leaf spring
Figure 7.7 shows the schematic cross section of investigated fractured surface of leaf spring, wherein both compressive and tensile stresses exist due to the bending load Figures 7.8 (a-b) show the fractured surface of the LFLS at different loading conditions, wherein two different failure features are observed. The left side of the figure 7.8(a-b) correspond to the leaf spring section which experiences tensile stresses and hence micro ductile/matrix fibrillation was observed. Right side of the figure 7.8 corresponds to the leaf spring section which experiences compressive stresses and hence brittle failure was observed.
Tensile stresses Load direction
Neutral axis
Compressive stresses
Fig. 7.8a Failure morphology of LFLS at 33.5MPa indicating ductile and brittle failure feature of matrix
t
Fig. 7.8b Failure morphology of LFLS at 45 MPa indicating ductile and brittle failure feature of matrix