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G.F. Burnside
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Fatigue characterization of fiber-reinforced polymer composites is slow and costly, which motivates the search for faster screening tools. This thesis assesses whether Dynamic Mechanical Analysis (DMA), a quick measurement of viscoelastic properties, can indicate the fatigue performance of such composites, and what a DMA-based screening method would require. Two unidirectional glass/epoxy materials, denoted KS02 and KS03, made from the same fiber and resin but differing in glass transition temperature, were characterized by DMA temperature and frequency sweeps and compared against existing transverse-tension fatigue, hysteresis, and static tensile data. When the DMA and fatigue results proved difficult to reconcile, the scope was extended to Differential Scanning Calorimetry (DSC) and optical microscopy.
DMA separates the two materials clearly in their glass transition temperatures, which differ by roughly \SI{20}{\celsius}, but their fatigue lives overlap across most of the stress range, so this thermal difference does not carry through to fatigue life. The pristine DMA damping metrics, room-temperature $\tan\delta$ and loss modulus $E''$, instead track the character of damage accumulation: the more dissipative KS03 shows wider hysteresis loops, a more gradual early-life loss of stiffness, and a more textured, deflected fracture path, while the stiffer KS02 retains its stiffness until abrupt, brittle failure. DSC indicates that the two materials differ in more than cure state, most likely in resin-to-hardener mixing ratio, leaving cure state and composition confounded. The results are consistent with an interpretation in which the matrix governs the character of damage while the fiber-matrix interface governs fatigue life. DMA is therefore found to carry information about the character of fatigue damage but not, for this material pair, about fatigue life, and a calibrated screening method cannot be built from two materials whose S--N curves overlap and confound several processing variables. A calibration dataset varying one processing variable at a time, with the composition difference first confirmed by Fourier-transform infrared spectroscopy, is identified as the necessary next step. ...
DMA separates the two materials clearly in their glass transition temperatures, which differ by roughly \SI{20}{\celsius}, but their fatigue lives overlap across most of the stress range, so this thermal difference does not carry through to fatigue life. The pristine DMA damping metrics, room-temperature $\tan\delta$ and loss modulus $E''$, instead track the character of damage accumulation: the more dissipative KS03 shows wider hysteresis loops, a more gradual early-life loss of stiffness, and a more textured, deflected fracture path, while the stiffer KS02 retains its stiffness until abrupt, brittle failure. DSC indicates that the two materials differ in more than cure state, most likely in resin-to-hardener mixing ratio, leaving cure state and composition confounded. The results are consistent with an interpretation in which the matrix governs the character of damage while the fiber-matrix interface governs fatigue life. DMA is therefore found to carry information about the character of fatigue damage but not, for this material pair, about fatigue life, and a calibrated screening method cannot be built from two materials whose S--N curves overlap and confound several processing variables. A calibration dataset varying one processing variable at a time, with the composition difference first confirmed by Fourier-transform infrared spectroscopy, is identified as the necessary next step. ...
Fatigue characterization of fiber-reinforced polymer composites is slow and costly, which motivates the search for faster screening tools. This thesis assesses whether Dynamic Mechanical Analysis (DMA), a quick measurement of viscoelastic properties, can indicate the fatigue performance of such composites, and what a DMA-based screening method would require. Two unidirectional glass/epoxy materials, denoted KS02 and KS03, made from the same fiber and resin but differing in glass transition temperature, were characterized by DMA temperature and frequency sweeps and compared against existing transverse-tension fatigue, hysteresis, and static tensile data. When the DMA and fatigue results proved difficult to reconcile, the scope was extended to Differential Scanning Calorimetry (DSC) and optical microscopy.
DMA separates the two materials clearly in their glass transition temperatures, which differ by roughly \SI{20}{\celsius}, but their fatigue lives overlap across most of the stress range, so this thermal difference does not carry through to fatigue life. The pristine DMA damping metrics, room-temperature $\tan\delta$ and loss modulus $E''$, instead track the character of damage accumulation: the more dissipative KS03 shows wider hysteresis loops, a more gradual early-life loss of stiffness, and a more textured, deflected fracture path, while the stiffer KS02 retains its stiffness until abrupt, brittle failure. DSC indicates that the two materials differ in more than cure state, most likely in resin-to-hardener mixing ratio, leaving cure state and composition confounded. The results are consistent with an interpretation in which the matrix governs the character of damage while the fiber-matrix interface governs fatigue life. DMA is therefore found to carry information about the character of fatigue damage but not, for this material pair, about fatigue life, and a calibrated screening method cannot be built from two materials whose S--N curves overlap and confound several processing variables. A calibration dataset varying one processing variable at a time, with the composition difference first confirmed by Fourier-transform infrared spectroscopy, is identified as the necessary next step.
DMA separates the two materials clearly in their glass transition temperatures, which differ by roughly \SI{20}{\celsius}, but their fatigue lives overlap across most of the stress range, so this thermal difference does not carry through to fatigue life. The pristine DMA damping metrics, room-temperature $\tan\delta$ and loss modulus $E''$, instead track the character of damage accumulation: the more dissipative KS03 shows wider hysteresis loops, a more gradual early-life loss of stiffness, and a more textured, deflected fracture path, while the stiffer KS02 retains its stiffness until abrupt, brittle failure. DSC indicates that the two materials differ in more than cure state, most likely in resin-to-hardener mixing ratio, leaving cure state and composition confounded. The results are consistent with an interpretation in which the matrix governs the character of damage while the fiber-matrix interface governs fatigue life. DMA is therefore found to carry information about the character of fatigue damage but not, for this material pair, about fatigue life, and a calibrated screening method cannot be built from two materials whose S--N curves overlap and confound several processing variables. A calibration dataset varying one processing variable at a time, with the composition difference first confirmed by Fourier-transform infrared spectroscopy, is identified as the necessary next step.