ISSN:0032-3888    

DOI:10.1002/pen.760181402

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractFor polymer melts and concentrated solutions highly nonlinear effects dominate all aspects of both the transient and steady state mechanical behavior. The first part of this paper will review the salient features of the experimental results and focus attention on those common features which should be understood in a coherent manner. The various constitutive equations, especially the integral representations that have been proposed to describe polymer melts will also be discussed. They fall roughly into three classes: those where the relaxation function depends on the deformation rate, deformation, or stress history. Evidence will be presented which indicates that the stress history rather than the deformation history may be the more relevant variable to account for the non‐linearities. The last part of the paper will briefly review the theories that have been proposed to understand the mechanism for non‐linear behavior at the microscopic level and the influence of molecular weight and concentrat

ISSN:0032-3888    

DOI:10.1002/pen.760181403

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractDeformation processes in viscoelastic polymers are examined in terms of an energy balance involving work, strain energy, viscous dissipation and damage. The concepts of time dependent damage and reversible cracks are presented. The latter, while giving rise to novel mechanical behavior in viscoelastic materials, are important considerations in temperature dependent recovery phenomena of deformed polymers. Constitutive equations are derived for systems with reversible cracks analogous to Maxwell and Kelvin viscoelasticity models. Retractive forces arising from crack closure occur by conversion of surface energies to strain energy and subsequently to mechanical work. This effect is investigated during contraction of hard‐elastic polypropylene fibers under constant load at different temperature

ISSN:0032-3888    

DOI:10.1002/pen.760181404

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractThe fibrous material obtained by drawing or extrusion exhibits a more than linear increase in the axial elastic modulus with draw ratio, a much slower increase in tensile strength and a decrease in strain to break. The reason for such a behavior is found in the existence of structural defects of fibrous material, which is composed of very long and narrow microfibrillar elements. At their ends the axial connection by taut‐tie molecules is almost completely interrupted. Hence the transfer of tensile stress through such a defect is only possible by lateral shift of the load to adjacent microfibrils. The incompleteness of this transfer, the excess straining of the material in the defect area, and the relatively easy axial displacement of the ends of microfibrils favor the local formation of microcracks which grow and coalesce with close‐by microcracks until a critical size crack is formed and the strained sample fails. The process is favored by a high elastic modulus of microfibrils which is practically identical with that of the drawn or extruded sample. Hence the fibrous polymer fails at lower strain the higher its axial elastic modulus, i.e., the higher the draw ra

ISSN:0032-3888    

DOI:10.1002/pen.760181405

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractA numerical simulation of ballistic impact and penetration on woven textile panels is described which can easily incorporate a wide variety of realistic constitutive and fracture models. The use of this model in assessing Viscoelastic relaxation effects is illustrated, and is further extended to include non‐linear Viscoelastic effects. Since a variety of non‐linear models is presently available and there is insufficient evidence to indicate the superiority of any single one in this instance, the Eyring non‐linear model was chosen arbitrarily to indicate the ease with which these models may be implemented into the numerical treatment. The results obtained using the non‐linear model are compared with comparable computer experiments using linear elastic and linear Viscoelastic

ISSN:0032-3888    

DOI:10.1002/pen.760181406

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractNon‐linear stress relaxation data were obtained for polycarbonate and polystyrene as a function of temperature, strain magnitude and thermal history. Data were analyzed according to the previously proposed free volume model to account for the shift of relaxation times with strain and thermal history. The result of the analysis is shown to be in close agreement with the tensile stress‐strain data. The yield phenomenon is analyzed as a rate dependent strain‐induced glass to rubber trans

ISSN:0032-3888    

DOI:10.1002/pen.760181407

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractThe microscopic flow and failure processes, the structural parameters controlling these processes, and how such processes are modified by the service environment are presented for three classes of polymer glasses: polycarbonate, polyimides, and epoxies. The microscopic flow characteristics of polycarbonate are controlled by (a) the ease of shear band deformation which depends on free‐volume and previous thermal history and (b) surface crazes whose characteristics depend on exposure to organic compounds, thermal history, and to surface crystallization and fabrication stresses. Polyimides deform and fail in the bulk by crazing with extensive fibrillation. The growth and development of shear bands from microvoids in polyimides are discussed. Epoxies predominantly deform and fail by crazing with regions of high crosslink density remaining intact during the flow processes. Craze cavitation and growth are enhanced in epoxies by the presence of absorbed moistur

ISSN:0032-3888    

DOI:10.1002/pen.760181408

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractThe relaxation behavior of poly(methyl methacrylate) in the hard region is caused by hindered rotations of the methoxycarbonyl side groups of the molecules. Measurements of the Young's modulus and the shear modulus at different frequencies, temperatures and strains lead to the following conclusions: each moving side group is surrounded by the elastic matrix of mean chains, The elasticity of this matrix depends on volume and temperature. Also the activation energy of the moving side group depends on volume and temperature, because the activation energy is the sum of an intramolecular and an intermolecular component. Due to the statistical entanglement of the molecules, the intermolecular contribution to the activation energy has a statistical distribution depending on volume. The reaction rate theory is of value for interpreting the viscoelastic behavior in the linear and non‐linear range if several considerations are followe

ISSN:0032-3888    

DOI:10.1002/pen.760181409

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractAlthough crosslinked networks are commonly used as adhesives and composite matrixes, structure‐property relationships are not as well established as with thermoplastics. For this reason, an extensive study was begun to systematically examine effects of stoichiometry, morphology, and distribution of crosslink density on viscoelastic behavior and ultimate properties. The system selected was based on a bisphenol‐A‐type epoxy cured with methylene dianiline. This paper describes and discusses results obtained for resins in which the amine/epoxy ratio ranged between 0.7 and 2.2. In agreement with reports by others, the tensile strength, modulus, and ultimate elongation were relatively insensitive to stoichiometry but did not show slight maxima or minima when the amine was somewhat in excess. Impact strengths, tensile energies‐to‐break and fracture toughness were, in contrast, quite sensitive, though the patterns of each differed significantly. Both fracture toughness and the stress intensity factor required to drive the crack at a given rate varied directly with the amine/epoxy ratio, as did estimates of the characteristic flaw size. Fatigue striations were observed on the fracture surfaces and corresponded to the incremental advance of the crack in one load

ISSN:0032-3888    

DOI:10.1002/pen.760181410

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractWe consider the analysis of data obtained in one‐dimensional acceleration wave experiments on a non‐linear viscoelastic material and discuss how such data can be used to characterize the dynamic response of the material. To begin, we review briefly the general theory of one‐dimensional motions in viscoelastic materials and the methods employed to generate and observe acceleration waves in a material sample. We then discuss two methods of data analysis, a wave front analysis and a wave profile analysis, and indicate the type of information each analysis provides with regard to the dynamic properties of the material. Finally, using a known model for poly(methyl methacrylate) in a finite‐difference, Lagrangian wave‐propagation code, we calculate acceleration wave profiles at several locations in the sample and, treating these profiles as “experimental” data, we then illustrate how data from three or more acceleration wave experiments can be used to formulate a specific viscoelastic constitutive model for

ISSN:0032-3888    

DOI:10.1002/pen.760181411

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY