DAMPING CHARACTERISTICS OF THERMOPLASTIC LEAF SPRING MATERIAL
5.7 HYSTERETIC HEATING
Loss factor of chosen material decreased with the increase in frequency (figure 5.12a).
Sato et al. (1992) also reported similar behavior and this is due to the fact that increased frequency decreases viscoelastic strain decreases and pays the way for increased stiffness (Crawford, 1998; Rosato et al., 2000). Loss factor of all the chosen test materials was found to increase with the increase in temperature (figure 5.12b) and this behavior was due to the matrix softening which resulted in increased molecular mobility at elevated temperature. When the test material temperature increases, the free volume, i.e., space for the internal movement of the molecule increases resulted in drop of storage modulus and hence, rise in loss factor was visualized. The increase in damping factor with increase in temperature is less for LFPP due to the reinforcing efficiency of longer glass fiber polypropylene composites. Thomason and Groenewoud (1996) reported the reinforcing efficiency of longer glass fiber polypropylene composites in retaining the modulus at higher temperatures. Damping factor (ξ) was calculated using the relation 5.5 (Landro and Lorenz, 2009)
ξ = tanδ
2 (5.5) The damping factor of LFPP, SFPP and UFPP were 0.022, 0.030 and 0.032 respectively. It is to be noted the variation in damping factor obtained through dynamic mechanical analysis and forced vibration was due to the change in geometry of test specimen, type of load, tightening torque (clamping effects) and the effect of air damping (Menard,1999).
hysteresis heat generation. Damping affects the amount of hysteresis heat generation during service (Senthilvelan and Gnanamoorthy, 2006). Rise in temperature due to the material hysteretic heating depends strongly on the damping properties of the material (ASM International, 1988; Kultrural and Eryurek, 2007). Thus, evaluation of internal friction generated in the form of heat for the chosen leaf spring material during cyclic loading is of practical importance.
To investigate the influence of fiber length on the hysteretic heating for leaf spring materials, injection molded tensile test specimens pertaining to ASTM D 638 standard were used. Test specimens were subjected to finite number (7200) of fatigue cycles using servo hydraulic fatigue testing machine(Instron 8801) at 1 Hz and R = 0 (stress ratio) under constant displacement mode. The load required for 1 mm deflection under cyclic sinusoidal loading was measured for the chosen test materials.
During testing, specimen temperature was measured using non-contact infra red temperature sensor (Raytek MID, 0.1 C accuracy). Sensor was positioned in such a way to measure the temperature at the middle of the gauge length of tensile specimen.
The temperature rise under cyclic load was observed for all the three test materials.
The generated heat exceeds the heat transferred to the surroundings because of poor conductivity of the material resulted in specimen heating. During cyclic loading some part of the mechanical work was spent on irreversible molecular processes leading to various microscopic deformations; crazes, shear bands, voids and micro cracks and the other polymeric material, resulting with rise in specimen temperature. In general, fatigue tests are being carried out to evaluate the endurance strength of a material. In this study, the heat generated during cyclic loading was measured and attempted to correlate with the hysteretic heating behavior of the material and hence, material damping.
Fig. 5.13 Hysteretic heating of unreinforced and reinforced polypropylene specimens during fatigue
Figure 5.13 shows the measured surface temperature of test materials during fatigue testing. The rise in temperature for unreinforced PP was due to the hysteretic heating behavior of the polypropylene matrix material and the measured surface temperature was less than reinforced PP. In the case of reinforced PP, presence of fibers and fiber matrix interface causes more internal friction resulted in increase of heat generation.
Short fiber reinforced PP exhibited higher temperature than that of the long fiber reinforced PP due to the higher internal collision because of its higher fiber ends. For a given volume of material, short fiber reinforced material have more fiber ends than long fiber reinforced material (figure 5.2). Hence, when the short fiber reinforced materials are subjected to cyclic loading, presence of more fiber ends cause more internal friction, resulted in higher heat generation. Kultrural and Eryurek (2007) also
0 5 10 15 20
0 2000 4000 6000 8000
Number of cycles (Nf) Rise in specimen temperature ( ο C ).
SFPP
LFPP
UFPP
reported similar behavior, wherein higher material temperature was observed when the percentage of calcium carbonate filler in polypropylene was higher under fatigue testing.
Fig.5.14 Load drop during constant deflection mode for unreinforced and reinforced polypropylene specimens
Figure 5.14 shows the load required for maintaining 1 mm deflection of test materials under fatigue loading. Due to the rise in material temperature and material visco- elastic behavior, load required for the fixed amount of deflection decreased with the progression of cycles. Among reinforced materials, short fiber reinforced material exhibited higher load drop than that of long fiber reinforced material. This behavior was due to the increased rise in material temperature during fatigue as well as weaker fiber matrix bond in the short fiber reinforced material .When the fiber matrix bond is weaker, load drop was observed to be significantly larger due to the slip between fiber and matrix.
0.00 0.25 0.50 0.75 1.00 1.25
1 10 100 1000 10000
Number of cycles (Nf)
Load (kN)
LFPP
SFPP UFFPP