Browsing by Author "LEE, CB"

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A NEW APPROACH IN J-INTEGRAL METHOD BASED ON HYSTERESIS ON DUCTILE POLYMERIC MATERIALS
(1992-04-05)
LEE, CB
;
LU, ML
;
CHANG, FC
;
應用化學系
;
Department of Applied Chemistry
FRACTURE-TOUGHNESS OF ACRYLONITRILE-BUTADIENE-STYRENE BY J-INTEGRAL METHODS
(1995-09-01)
LU, ML
;
LEE, CB
;
CHANG, FC
;
應用化學系
;
Department of Applied Chemistry
The fracture toughness of acrylonitrile-butadiene-styrene (ABS) was determined by three J-integral methods, ASTM E813-81, E813-87, and by hysteresis. The critical J values (J(1c)) obtained are fairly independent of the specimen thickness, ranging from 10 to 15 mm. ASTM E813-81 and hysteresis methods result in comparable J(1c) values, whereas the ASTM E813-87 was similar to 40% to 50% higher. The critical displacement determined from the plots of hysteresis (energy or ratio) and the true crack grow length vs. displacement are close. This indicates the critical displacement determined by the hysteresis method is indeed the displacement at onset of crack initiation, and the corresponding J(1c) represents a physical event of crack initiation. The elastic storage energy, the input energy minus the hysteresis energy, is the most important factor in determining the onset of crack initiation. The critical elastic storage energy (at the beginning of crack growth) was found close to the J(1c) obtained from the E813-81 or the hysteresis method.
FRACTURE-TOUGHNESS OF HIGH-IMPACT POLYSTYRENE BASED ON 3 J-INTEGRAL METHODS
(1993-03-10)
LEE, CB
;
LU, ML
;
CHANG, FC
;
應用化學系
;
Department of Applied Chemistry
Three J-integral methods and their modified versions have been used to characterize the fracture toughness of high-impact polystyrene (HIPS) with different thicknesses. The Jc values obtained were highest from the E813-87 method, followed by the E813-81 method, then by the hysteresis method. The hysteresis method based on the steep rising of hysteresis energy under constant displacement-controlled loading in Jc determination has many advantages over the ASTM E813-81 or the E813-87 method. The requirement of crack growth length measurements is no longer necessary and the controversial issue on the crack blunting line can also be avoided. The E813-87 method-resulted in significantly higher Jc values for polymers' but the modified version of E813-87, by moving the offset line from the original 0.2 to 0.1 mm, resulted in comparable Jc values. Since crazes as the main failure mechanism for HIPS, well-defined crack blunting does not expect to occur and the Jc obtained by the original E813-81 based on the theoretically predicted blunting line is indeed slightly higher. The modified version of E813-81 by neglecting the blunting line in Jc determination is believed to be more reasonable for HIPS. The nature of polymers will determine whether the crack tip will be blunted, partially blunted, or not blunted. ASTM E813-81 is appropriate for those polymers with a well-defined blunted crack tip (such as elastomer-modified poly-carbonate), whereas the modified version of ASTM E813-81 seems better for those polymers with craze as the main failure mechanism (such as HIPS). Experimental results indicated that this hysteresis method is able to inherently adjust the crack blunting effect and therefore can be applied to any type of ductile polymer.
TOUGHENING BEHAVIOR OF ELASTOMER-MODIFIED POLYCARBONATES BASED ON THE J-INTEGRAL
(1992-06-01)
LEE, CB
;
CHANG, FC
;
應用化學系
;
Department of Applied Chemistry
The J-integral method to determine the fracture toughness of tough and ductile polymeric materials previously developed has been applied to the elastomer-modified polycarbonates. This investigation compares three different methods to obtain Jc: the conventional crack growth length, the stress whitening zone, and the newly developed hysteresis method. Jc values obtained from these three comparative methods are fairly close. The hysteresis method has the advantage over the other two methods of obtaining Jc without requiring the measurement of the crack growth length or the stress whitening zone, therefore avoiding the controversy in defining crack blunting. Results also indicate that the effect of elastomer quantity in polycarbonate on Jc is insignificant as long as the crack is in a stable condition. Higher elastomer contents in polycarbonate result in higher dJ/d-DELTA-a, dJ/d-DELTA-l, and tearing modulus (Tm). This indicates that the elastomer toughening mechanism is due to the increase of the energy required for crack growth extension. The hysteresis loss energy is directly related to the size of the crack tip plastic zone, and the presence of more elastomer indeed increases the crack tip plastic zone, thus making the polycarbonate tougher. Besides, the presence of elastomer tends to increase the crack initiation displacement and shift the failure modes from an unstable fracture. Jc and the criterion for crack initiation based on rate change of hysteresis energy are discussed in detail.