摘要:

AbstractIn this short summary reference will be made to some recent molecular level studies of strength. Especially the role of chains and of their entanglements in stress transfer and energy dissipation and the influence of rate of loading will be referenced.

ISSN:0258-0322    

DOI:10.1002/masy.19910410103

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractFracto‐emission(FE) is the emission of particles and photons during and after fracture of materials. The observed emission includes electrons, negative and positive ions, neutral species in both ground states and in excited states, and visible photons. This emission can often serve as a sensitive probe of crack growth and may prove to be a useful tool for investigating molecular and microscopic events accompanying crack growth and for studying the details of failure modes in a variety of materials. Here, we examine photon and electron emission during failure of axially loaded stainless steel fixtures (e.g., rods) embedded in epoxy. These experimental data provide time resolved information on the sequence of events leading to interfacial fracture proceeding pullout. In addition, we examine the emissions during frictional pullout which over a large range of strain rates exhibits stick‐slip behavior. Relations of these emissions to the fracture and pullout processes are discus

ISSN:0258-0322    

DOI:10.1002/masy.19910410104

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractElectron emission was detected during deformation of both carbochain and heterochain polymers in vacuum. It was found that the features of emission are similar to those observed in molecular scissions under drawing of unoriented and oriented polymers. This fact indicates that there is a relationship between the fracture process and electron emission under deformation of polymers. This relationship is also obvious from the experiments with interruption of loading when electron emission during the repeated loading does not begin immediately at the moment of load application, but can be recorded only at the degree of deformation which is higher than that reached during the first loading. The interconnection between deformational electron emission and molecular scissions allowed visualization of the fracture process in the subsurface layers of polymers using an electron‐optical convertor which gives a mechanoemission image of a stretched sample. It is supposed that the deformation‐induced electron emission of polymers is caused by ionization of stressed macromolecules resulting from tunnel transitions of electrons into deep traps. During deformation, the traps are destroyed and a part of electrons escapes in vac

ISSN:0258-0322    

DOI:10.1002/masy.19910410105

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractCrazes or craze‐like deformation bands occur in many different polymers. Mechanisms of initiation and growth of the crazes have been studied using different techniques of electron microscopy. Examples of modifying amorphous and semi‐crystalline polymers are given to demonstrate to which extent the macroscopic toughness of the polymers is affected by changes in the microscopic structure and by the amount of cra

ISSN:0258-0322    

DOI:10.1002/masy.19910410106

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractSlow crack growth in a wide variety of polyethylenes have been investigated. The quantitative relationship between: (1) the external variables such as stress, temperature, stress intensity and J‐integral; (2) the morphological variable such as density and crystal size and (3) molecular variables such as molecular weight, branch density and branch distribution and the rate and time to failure for slow crack growth will be presented. The kinetics of slow crack growth is correlated with the microstructural changes. A fundamental description of slow crack growth is given in terms of tie molecules and the stress field on the boundary of the craze from which fracture initiate

ISSN:0258-0322    

DOI:10.1002/masy.19910410107

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractThe mechanism of plastic flow of isotactic polypropylene is considered in terms of Eyring's equation. It is demonstrated that to explain the dependence of the yield point on temperature a dependence of activation volume on overcooling (Tm‐T) must be admitted. The effects of drawing rate and temperature on the ultimate elongation of polypropylene are discussed in the case of two‐step drawing. The critical conditions of extrusion in solid state are considered. Criteria of transformation of a steady flow into unsteady state are suggested. Multiple fracture of semicrystalline polymers under critical conditions of plastic is descri

ISSN:0258-0322    

DOI:10.1002/masy.19910410108

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractCrazing and/or shear yielding mechanisms in multiphase polymer alloys play a critical role in toughening. The present paper describes the use of finite element models (FEM) to simulate the crazing and shear deformation behaviour around the particles embedded in brittle or ductile matrices. The FEM simulation results on the stress distribution reveal that the dilatational stress within the rubber particles is high enough to cavitate. The stability of craze growth can be reached when the compliant particle is incorporated in a brittle matrix. On the other hand, shear yielding around the particle occurs in the equator of the particle/matrix interface when the stress locally exceeds the yield stress of the matrix. This yield‐initiation stress increases with the increase in the elastic modulus and Poisson's ratio of the particles. The toughening mechanism, that cavitation occurs first followed by shear yielding to form a neck between the particles, is discussed based on the simulation results for the two‐particle mo

ISSN:0258-0322    

DOI:10.1002/masy.19910410109

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractThe time of expectation for breakdown (life‐time, τ) was measured as a function of temperature (T) and tensile stress (σ) for a variety of polymers, including nylon‐6, polyethylene, poly(ethylene terephthalate) and poly(tetrafluoroethylene). At moderate temperatures (above ∼200 K), the τ(1/T)σdependences are exponential (of the Boltzmann‐Frenkel type). The elementary acts of polymer fracture occur at these temperatures through the thermal‐fluctuation above‐barrier transitions. At low temperatures (below ∼200 K), τ is T‐independent (an athermic plateau) and, therefore, the above‐barrier mechanism is ruled out. It is concluded that the elementary acts of fracture proceed at low temperatures via the tunnel below‐barrier mechanism. Possible types of tunnel processes (tunneling of atoms or

ISSN:0258-0322    

DOI:10.1002/masy.19910410110

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractBehavior of polymers in creep, stress relaxation, stress‐strain curve determination, impact tests and under dynamic periodic loads are analyzed. Rapid crack propagation and slow crack propagation are considered in relation to service performance and life time. All essential aspects of mechanical behavior of polymeric materials and components have a common denominator: competition between chain relaxation capability (CRC) and destructive processes. For a given force (magnitude, rate, direction, duration) the outcome of this competition depends largely on the amount of free volum

ISSN:0258-0322    

DOI:10.1002/masy.19910410111

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractPolymers based on epoxies are widely used in the industrial production of materials for different applications. The crosslinking is often done with hardeners like aliphatic or aromatic amines, with dicarboxylic acid anhydride or anionically. The resins containing mineral fillers possess similar thermal mechanical properties. On thermal oxidative ageing, however, the systems behave very differently. An account is given of the thermal mechanical behaviour and of the weight loss on ageing above the glass transition temperature. By using semiempirical molecular orbital calculations we have been able to identify a possible reason for the different thermal oxidative stability of epoxy resins crosslinked with different hardeners.

ISSN:0258-0322    

DOI:10.1002/masy.19910410112

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY