Browsing by Author "CHU, LH"

Now showing 1 - 4 of 4

COEXISTENCE OF DUCTILE, SEMI-DUCTILE, AND BRITTLE FRACTURES OF POLYCARBONATE
(1992-03-25)
CHANG, FC
;
CHU, LH
;
交大名義發表
;
應用化學系
;
National Chiao Tung University
The ductile-brittle transition behavior of polycarbonate and methylmethacrylate-butadiene-styrene (MBS) elastomer modified polycarbonate has been investigated in terms of notch radius and temperature. At -40-degrees-C and 21-mil notch radius, polycarbonate fractures in three possible modes, ductile (25%), semi-ductile (50%), and brittle (25%). This semi-ductile mode fracture has never been reported previously with brittle characterization, but to a greater extent in localized shear yielding on the fracture surface and intermediate toughness. A two-dimensional fracture mode diagram in terms of temperature and notch radius has been constructed to interpret the observed phenomena. This diagram can also predict the existence of other conditions under which the triplet fracture modes may also occur. Another unstable zone has also been identified where the fracture occurs in either ductile mode or brittle mode over a broad temperature range, instead of the narrow temperature range typically observed for polycarbonate. A model based on the excessive precrack strain just below yielding due to the greater notch radius is proposed to explain such observed semi-ductile mode fracture.
DUCTILE BRITTLE TRANSITION BEHAVIOR OF POLYCARBONATE - EFFECT OF MOLECULAR-WEIGHT, TEMPERATURE, STRAIN RATE, AND ELASTOMER
(1989-04-09)
CHANG, FC
;
CHU, LH
;
應用化學系
;
Department of Applied Chemistry
FRACTURE AND IMPACT PROPERTIES OF POLYCARBONATES AND MBS ELASTOMER-MODIFIED POLYCARBONATES
(1992-01-25)
CHANG, FC
;
WU, JS
;
CHU, LH
;
交大名義發表
;
應用化學系
Mechanical properties of polycarbonates (PCs) and elastomer-modified polycarbonates with various molecular weights (MW) are investigated. Higher MW PCs show slightly lower density, yield stress, and modulus. The ductile-brittle transition temperature (DBTT) of the notched impact strength decreases with the increase of PC MW and with the increase of elastomer content. The elastomer-modified PC has higher impact strength than does the unmodified counterpart if the failure is in the brittle mode, but has lower impact strength if the failure is in the ductile mode. The critical strain energy release rate (G(c)) measured at -30-degrees-C decreases with the decrease of PC MW. The extrapolated zero fracture energy was found at M(n) = 6800 or MFR = 135. The G(c) of the elastomer-modified PC (MFR = 15, 5% elastomer) is about twice that of the unmodified one. The presence of elastomer in the PC matrix promotes the plane-strain localized shear yielding to greater extents and thus increases the impact strength and G(c) in a typically brittle fracture. Two separate modes, localized and mass shear yielding, work simultaneously in the elastomer-toughening mechanism. The plane-strain localized shear yielding dominates the toughening mechanism at lower temperatures and brittle failure, while the plane-stress mass shear yielding dominates at higher temperatures and ductile failure. For the elastomer-modified PC (10% elastomer), the estimated extension ratio of the yielding zone of the fractured surface is 2 for the ductile failure and 5 for the brittle crack. A criterion for shifting from brittle to ductile failure based on precrack critical plastic-zone volume is proposed.
THE LONG-TERM RELAXATION AND BUILDUP TRANSIENT OF PHOTOCONDUCTIVITY IN SI1-XGEX/SI QUANTUM-WELLS
(1995-06-05)
CHU, LH
;
CHEN, YF
;
CHANG, DC
;
CHANG, CY
;
電子工程學系及電子研究所
The long-term relaxation and build-up transient of photoconductivity has been observed in Si1-xGex/Si quantum wells. The long-term relaxation behaviour pelt) of photoconductivity can be described by a stretched-exponential function, I-pc(t) = I-pc(0) exp[-(t/tau)(beta)] (0