摘要:

AbstractMost aromatic polyimides and copolyimides show a β relaxation process below their α relaxation processes in dynamic mechanical and dielectric measurements. The origin of this β relaxation has been under active discussion. In order to gain a better understanding of the origin of the β relaxation process, the dynamic mechanical behavior of a series of segmented rigid‐rod polyimide and copolyimide films with different dianhydrides and diamines has been systematically studied. For some of the polymers, crystallinity and orientation effects on the β relaxation process were also investigated. It is found that the β relaxation process in the unoriented films can mainly be attributed to a relatively noncooperative motion in uncrystallized diamines. However, this noncooperative nature may be gradually lost by increasing the crystallinity and orientation. Molecular modeling and computation have led to a discussion of possible diamine motion in the β relaxation

ISSN:0032-3888    

DOI:10.1002/pen.760332102

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractBecause of the inability of existing automotive bumpers to absorb energy during impacts over the supports, energy absorbers have to be used to make bumpers effective in barrier impacts. A parametric analysis of a novel double‐beam bumper concept has shown that its performance, in terms of energy absorption and impact forces, is insensitive to the location of the point of impact. Physical realizations of this concept, which may not require more material than existing designs, and which are particularly suited to thermoplastic materials, have been develope

ISSN:0032-3888    

DOI:10.1002/pen.760332103

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractA test program was developed to compare the viscoelastic behavior of an untoughened epoxy (R922) with a toughened epoxy (R6376). Creep/recovery tests were performed on neat resin as well as [ ± 40° ]2scomposites using T500 or IM7 graphite fibers. The composite specimens were subjected to at least five cycles of load/unload at each stress level. The goal was to study the viscoelastic response of a flexibilized and unflexibilized epoxy and to try and observe any microstructural changes that might occur. Neat resin tests did not show a significant difference in behavior between the two systems. Single cycle behavior was found to differ significantly from multiple cycle behavior. Recovery strains were observed to be much higher than predicted from linear theory. Linear parameters were also seen to vary with stress level; however, when a plasticity‐based approach was used to model the creep behavior, the same parameters could then be used at all load levels. Creep and recovery behavior were seen to differ significantly. This observation leads to support for the theory of Schapery (1) that creep and recovery should be considered as different behavior in contrast to the assumptions of linear and nonlinear theory. In addition, the large difference between neat resin and composite data suggests that micromechanics models, which rely on constituent properties, will lead to large errors if neat resin properties are used to characterize composite viscoelastic behavior. Different fibers showed no influence on results. Scanning electron microscopy failed to reveal any microstructural chan

ISSN:0032-3888    

DOI:10.1002/pen.760332104

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractIt is a matter of fact that creep experiments can be conducted more easily and accurately than stress relaxation experiments, since it is easier to maintain a stress constant (for instance by a “dead weight”) than a strain constant. Nevertheless, in practice, structural parts made of plastics (which are nonlinear viscoelastic materials) are very often loaded under stress‐relaxation conditions. The present paper presents an approach to predict the behavior of a nonlinear viscoelastic material under stress‐relaxation‐type loading, based on data obtained from creep‐type experiments. The nonlinear creep compliance is described mathematically by an exponential series with a limited number of terms and a single nonlinearity function depicting the transient behavior. The nonlinear behavior of the material under constant strain (i.e., stress relaxation) is then obtained by dividing the considered time range into very short time intervals in which constant stresses are acting, while the different values of the applied stresses are chosen in a manner that guarantees the same stain at the end of each interval. In this way, one performs a numerical nonlinear superposition of the effects of the loadings in the various intervals, leading to the desired results under stress relaxation. A comparison of theoretical results with experiments conducted on some thermoplastic materials shows goo

ISSN:0032-3888    

DOI:10.1002/pen.760332105

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractStudies of the morphology of extruded polymer blend systems have shown that it is feasible to produce a laminar structure of an ethylene vinyl‐alcohol copolymer (EVOH) dispersed phase in a polypropylene (PP) matrix phase. The laminar structure forms in the core of the extrudate when a slit die is incorporated into the extrusion process. Morphological studies, including a study of morphology development inside the die and studies of the effect of processing conditions on the morphology of the final product, revealed that the laminar structure is a result of die design. Processing conditions influence mainly the shape and dimensions of the laminar core region of the extrudate. Oxygen permeation tests have shown that the blend exhibits lower oxygen permeability than pure PP, when EVOH is incorporated as a dispersed phase into the system. Oxygen transmission rates obtained with a blend system approach those obtained with a multilayer coextrusion product, although only at high EVOH concentrations. Comparison of experimental data with theoretical permeation predictions shows that, up to 20 wt% EVOH, the reduction in oxygen transmission rate follows the prediction for a homogeneous system. At 25 wt%, a considerable decrease in oxygen transmission rate is noticeable, and the trend for higher EVOH contents is towards the behavior of a multilayer syste

ISSN:0032-3888    

DOI:10.1002/pen.760332106

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe fatigue life of a linear low density polyethylene was measured as a function of stress, notch depth., and temperature under plane strain conditions. The fatigue life was precisely related to the stress intensity. The temperature dependence of the fatigue life corresponded to an activation energy of 72.5 KJ which was independent of stress level. A general equation for the fatigue lifetime was proposed for polyethylenes in terms of the material parameters, the mechanical parameters such as stress intensity, stress amplitude, frequency, and waveform.

ISSN:0032-3888    

DOI:10.1002/pen.760332107

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThis study focuses on the effect of various processing and cooling conditions upon the subsequent melting and solidification behavior of blends of poly(butylene terephthalate) (PBT) and high density polyethylene (HDPE). Differential scanning calorimetry (DSC) measurements have shown that the crystallization of reprocessed PBT is different from the crystallization behavior of virgin samples. Two melting endotherms for PBT were observed for reprocessed PBT and for PBT‐PE blends. The nature of each PBT peak is discussed in relation to processing history, solidification conditions, and composition. The presence of two crystallization peaks for the PE component in blends of PE and PBT are thought to be associated with the restriction of molecular motion of PE in the presence of the second component. The relative magnitudes of the two exotherms of PE vary with composition and cooling rate during solidificatio

ISSN:0032-3888    

DOI:10.1002/pen.760332108

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThree point bend tests of notched, polyetherimide (PEI) beams are performed for varying notch radii and beam thicknesses. Finite element stress results including pressure sensitive yielding effects are correlated with experimental load‐to‐craze observations to determine the three‐dimensional stress state at craze initiation. Three crazing criteria; maximum principal stress, hydrostatic stress, and the critical strain criterion of Oxborough and Bowden are examined for consistency. Over the range of geometries studied, the critical strain criterion provides the best fit to the experimental data. This criterion could be used as a brittle failure criterion to help understand and prevent brittle fai

ISSN:0032-3888    

DOI:10.1002/pen.760332109

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThis review article, supported by some original experimental results, explains the nature of the Cook‐Gordon (C‐G) mechanism of crack blunting. Examples are given of how the C‐G mechanism influences the toughness of several anisotropic materials: fibrillated polymeric films, composites based on oriented polymers, and beech and spruce woods. Moreover, the C‐G mechanism can explain the effects of orientation on anisotropic photo‐oxidative degradation of semicrystalline polymers, and also the sudden embrittlement of polymeric films exposed to UV light und

ISSN:0032-3888    

DOI:10.1002/pen.760332110

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

ISSN:0032-3888    

DOI:10.1002/pen.760332101

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY