The macroscopic yield behaviour of polymers

A yield criterion, not previously compared with the actual macroscopic behaviour of polymers, is herein compared with the pressure-modified octahedral shear stress criterion earlier suggested by others. This new relation, which is a version of the von Mises criterion, accommodates differences in tensile and compressive yield strengths and accounts for any dependence of yielding on the hydrostatic component of the applied stress state.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44784/1/10853_2004_Article_BF00550671.pd

Determination of the mechanical properties of amorphous materials through instrumented nanoindentation

A novel methodology based on instrumented indentation is developed to determine the mechanical properties of amorphous materials which present cohesive-frictional behaviour. The approach is based on the concept of a universal hardness equation, which results from the assumption of a characteristic indentation pressure proportional to the hardness. The actual universal hardness equation is obtained from a detailed finite element analysis of the process of sharp indentation for a very wide range of material properties, and the inverse problem (i.e. how to extract the elastic modulus, the compressive yield strength and the friction angle) from instrumented indentation is solved. The applicability and limitations of the novel approach are highlighted. Finally, the model is validated against experimental data in metallic and ceramic glasses as well as polymers, covering a wide range of amorphous materials in terms of elastic modulus, yield strength and friction angle

Application of activated barrier hopping theory to viscoplastic modeling of glassy polymers

YesAn established statistical mechanical theory of amorphous polymer deformation has been incorporated as a plastic mechanism into a constitutive model and applied to a range of polymer mechanical deformations. The temperature and rate dependence of the tensile yield of PVC, as reported in early studies, has been modeled to high levels of accuracy. Tensile experiments on PET reported here are analyzed similarly and good accuracy is also achieved. The frequently observed increase in the gradient of the plot of yield stress against logarithm of strain rate is an inherent feature of the constitutive model. The form of temperature dependence of the yield that is predicted by the model is found to give an accurate representation. The constitutive model is developed in two-dimensional form and implemented as a user-defined subroutine in the finite element package ABAQUS. This analysis is applied to the tensile experiments on PET, in some of which strain is localized in the form of shear bands and necks. These deformations are modeled with partial success, though adiabatic heating of the instability causes inaccuracies for this isothermal implementation of the model. The plastic mechanism has advantages over the Eyring process, is equally tractable,and presents no particular difficulties in implementation with finite elements.F. Boutenel acknowledges an Erasmus Programme Scholarshi

EFFECT OF THERMAL HISTORY ON β RELAXATIONS AND COMPRESSION YIELD BEHAVIOR OF POLYCARBONATE

Les pics de pertes mécaniques mesurés à l'aide d'une pendule de flexion, dans le domaine de la transition β du Polycarbonate, sont relevés pour des échantillons ayant un passé thermique différent. Aucune influence de ce passé thermique n'a pu être mise en évidence, comme c'était d'ailleurs le cas pour les pics mesurés en torsion. L'effet du passé thermique sur la limite d'écoulement en compression est étudié dans un grand domaine de températures à vitesse constante. Les résultats sont analysés en faisant l'hypothèse que deux processus activés α et β nécessitant des contraintes additives σCα et σCβ sont impliqués dans la déformation au seuil d'écoulement. Suivant un traitement proposé antérieurement, une corrélation est établie entre le pic β et la contrainte σCβ pour des échantillons vieillis ; elle est en bon accord avec les résultats expérimentaux. Ces résultats confirment que seule l'entropie configurationnelle du processus α est affectée par le vieillissement tandis que la pression hydrostatique influence l'enthalpie d'activation des deux processus α et β. Ils permettent de cerner la nature du volume libre impliqué dans la déformation au seuil d'écoulement.The loss curve related to Young's modulus of samples of Polycarbonate characterized by different thermal histories is measured in the β transition range. Peaks are found to be insensitive to thermal histories as was the case for the shear loss curve. On similar samples, the compression yield behavior is investigated at constant strain rate over a wide range of temperatures. Assuming that two activated processes α and β requiring additive stresses σCα and σCβ are involved at yield, and using a treatment previously proposed, a good correlation is found between the β peak and σCβ related to aged samples. These results confirm that only the configurational entropy of the α process is affected by aging while the hydrostatic pressure influences the activation enthalpies of both α and β processes. They allow to discern the nature of the free volume involved in yield deformation