摘要:

Acoustic emission (AE) has been coupled with slow crack propagation experiments in polystyrene to probe the mechanisms of damage and fracture. The single AE active mechanism recorded during slow crack growth is probably caused by failure of the craze fibrils in the craze zone at the propagating crack tip. The AE event rate and the AE energy release rate varied linearly with the crack speed indicating that the AE depends only upon the amount of fracture surface created independent of the crack speed. Since the duration of the AE was less than 1% of the total time of the experiment, it is concluded that crack propagation is discontinuous even at the highest crack speeds investigated.

ISSN:0021-8979    

DOI:10.1063/1.337090

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A statistical consideration based upon a theory of microscopic molecular kinetics is developed to describe the time‐dependent fracture process for oriented polymers. The interrelation between the macroscopic fracture strength and the microscopic behaviors is formulated with the aid of a double Fourier series expansion. The microscopic behavior in the vicinity of a point is expressed in the form of a spherical function which can be expanded into a series of symmetric tensorial terms. Subsequently, the fracture strength of polymers is evaluated in terms of both time and molecular orientation. By taking account of the individual values of the directional fraction of integrity, the time to fracture and the most probable direction of fracture within any volume element in a medium are predicted.

ISSN:0021-8979    

DOI:10.1063/1.337091

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The two‐stage process discovered in the recovery of deformed amorphous polymers (PS, PMMA, and PC) is verified in this study. The first stage which takes place belowTgreleases part of the stored energy (part 1) and recovers the volume. The second stage which takes place aboveTgreleases the rest of the stored energy (part 2) and recovers the dimension. The relation between the part 2 stored energy and dimensional changes during deformation is the same as that during recovery. In this study, the dimension is constrained not to change during recovery. As expected, the part 2 stored energy is not released but the part 1 stored energy is released as usual. Furthermore, if in a second scan the constraint is removed and the dimension is allowed to recover, the part 2 stored energy is then released. These results confirm the unique relationship between dimension and the part 2 stored energy and the separability of the two‐stage recovery process.

ISSN:0021-8979    

DOI:10.1063/1.337092

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Audio frequency electrical conductivity and relaxation studies have been carried out on Parel 58 elastomer and Parel 58 elastomer complexed with a variety of lithium salts. The measurements have been carried out in vacuum over the temperature range 5–380 K and at pressures up to 0.65 GPa over the temperature range 230–380 K. Both the electrical conductivity for the complexed material and the electrical relaxation time associated with the &agr; relaxation in the uncomplexed material exhibit VTF or WLF behavior. From a VTF analysis for both the vacuum electrical relaxation time and electrical conductivity,Eais found to be about 0.09 eV andT0is found to be about 40 °C below the ‘‘central’’ glass transition temperature. In addition, it is found that the activation volumes for the electrical relaxation time and the electrical conductivity are the same when compared relative toT0. These results imply that the mechanism controlling ionic conductivity is the same as that for the &agr; relaxation, namely large‐scale segmental motions of the polymer chain.

ISSN:0021-8979    

DOI:10.1063/1.337093

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The influence of electrical conductivity on the piezoelectricity of polymers was studied using a three‐phase model. The model consists of a continuous nonpiezoelectric phase (phase 1) surrounding a spherical piezoelectric phase (phase 2), which is itself bounded by an electrically conductive shell (phase 3). The piezoelectric constants for the system can be expressed asd=d2KPKTande=e2KPKS, whered2ande2are the piezoelectric strain and stress constants of phase 2, respectively, andKPis a function of the dielectric constants of phases 1, 2, and 3.KTandKSare expressed as functions of the elastic constant of each phase. The piezoelectric constantsdandedecrease with decreasing elastic constant of phase 3 and with increasing dielectric constant of phase 3. The three‐phase system shows a Maxwell–Wagner piezoelectric relaxation with a relaxation time given by &tgr;=(2&egr;1+&egr;2+&eegr;&egr;3)/(2&sgr;1+&sgr;2+&eegr;&sgr;3), where &eegr;=2&dgr;/a. In these expressions &egr;iand &sgr;iare the dielectric constant and the electrical conductivity of phasei, respectively,ais the radius of the sphere (piezoelectric phase), and &dgr; is the thickness of the electrically conductive shell around the piezoelectric phase. When &tgr;1(=&egr;1/&sgr;1)>&tgr;2[=(&egr;2+&eegr;&egr;3)/(&sgr;2+&eegr;&sgr;3)], the functional dependence ofKPon frequency implies the occurrence of a relaxation process. Conversely, when &tgr;1<&tgr;2, a retardation process is observed.

ISSN:0021-8979    

DOI:10.1063/1.337094

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Thermal treatment of polymers has been found to have a pronounced effect on the residual stress state and the impact properties of these materials. Researchers have found that compressive residual stresses can be used to bring about a transition from a brittle to higher‐energy absorbing impact failure in polycarbonate and have subsequently proposed that this transition is predominately a function of the local stress state at the notch tip. In this study the impact strength of polycarbonate has been measured as a combined function of the residual compressive stress in the material, the thickness of the material, the radius of the notch tip and the location of the notch relative to the residual stress field. The transition in the energy absorption capability of the material was found to be highly dependent on these local variables at the notch tip.

ISSN:0021-8979    

DOI:10.1063/1.337095

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Observed phases and transition temperatures in mixtures of poly(ethylene oxide) and sodium thiocyanate (PEO–NaSCN) are described by a binary phase diagram. The components of this system are semicrystalline PEO which can dissolve salt up to a mole fractionXs=0.023, and a crystalline complex of composition PEO3.5NaSCN (Xcc=0.22). The salt‐deficient PEO component is normally formed by a eutectic reaction with eutectic temperatureTe=63 °C. Conductivity of PEO–NaSCN mixtures is calculated assuming that only ions in the amorphous, liquidlike regions contribute to charge transport. This model predicts a positive discontinuity in conductivity on eutectic melting, and a sharp maximum in conductivity at low salt concentrations, in agreement with observed behavior.

ISSN:0021-8979    

DOI:10.1063/1.337781

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The Curie transition in a VF2‐TrFE random copolymer of fixed chemical composition (60/40 mol %) has been investigated as a function of processing history. Both solution cast and melt crystallized materials were investigated using differential scanning calorimetry and wide angle x‐ray diffraction. Results indicate that due to the higher solubility of TrFE units in dimethylformamide, dimethylacetamide, and cyclohexanone, solution cast films of VF2‐TrFE yielded crystalline regions deficient of TrFE units. Upon annealing, a redistribution and equalization of the TrFE comonomer units within the crystalline regions was observed to occur resulting in a lowering of the Curie point. Melt crystallization, on the other hand, provided a more uniform distribution of TrFE units within the crystalline and amorphous regions, each having nearly the same proportion as in the overall composition. Subsequent annealing caused negligible changes in the Curie temperature.

ISSN:0021-8979    

DOI:10.1063/1.337096

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The shear creep and recovery behavior of narrow molecular weight atactic polystyrene (APS) (6.7×105Mw) solutions (27.3 wt. %) in two solvents (n‐amyl andn‐butyl acetate) were studied at various temperatures down to −83 °C above and below reported thermoreversible gel points (∼−50 °C). In both cases viscous flow was measured above and below the reported gel points. The resulting compliance curves for two solutions (n‐amyl andn‐butyl acetates) were successfully reduced toT0=−82 and −61 °C, respectively, indicating viscoelastic liquid response down to −82 °C. These solutions were clear. A solution of 18.7% APS (3.9×105Mw) in iso‐amyl acetate was also studied. It responded similarly to the first solutions untilT=−49 °C when it became cloudy, indicating phase separation. At lower temperatures the viscosity increased dramatically, suggesting gel‐like behavior.

ISSN:0021-8979    

DOI:10.1063/1.337097

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

We calculate the spectrum of a signal derived from a continuous signal by ‘‘sample and hold’’ random sampling. It is seen that an undistorted spectrum is obtained only if the mean sampling frequency is more than an order of magnitude greater than the highest frequency in the original signal. The cases of signal‐independent and signal‐dependent sampling probabilities are investigated. We show how to obtain a good estimate for the spectrum of the unsampled signal and determine the limitations of this form of processing.

ISSN:0021-8979    

DOI:10.1063/1.337098

出版商:AIP    

年代:1986

数据来源: AIP