Synopsis :A new tensile test method to obtain a true stress -true strain curve was developed, in which a CCD camera and an image analysis device were equipped to enable the in-situ measuring of the change in shape of test specimen until the very end of the fracture. A smooth round bar tensile test specimen was adopted. The dynamic change in the necking geometry was observed continuously with a CCD camera and the measured data was simultaneously processed by an image analyzer. Tensile tests were carried out using SS400 steel and 5.0%Mn steel. The data of the radius of cross sections perpendicular to the specimen axis around the necking portion, the radius of curvature on the longitudinal section of necking surface, and the load applied to the specimen were obtained continuously until the specimen ruptures. The change in the minimum radius of the neck and the necking curvature were calculated from the data obtained, and they were confirmed to be accurate. According to the Bridgman's model, true stress -true strain curve after necking can be obtained from the minimum neck radius, the neck curvature and the load. The data was analyzed based on the Bridgman's model. True stress -true strain curves of SS400 and 5.0%Mn steels were determined by this method. True stress -true strain behavior after necking was thus elucidated. This newly developed test method is confirmed to be applicable for the accurate measurements of true stress -true strain curves until up to fracture of specimen.
SummaryThe network model developed in a previous paper is applied to the simple shear flow of polymer melts. The constitutive equation obtained consists of two terms. One of them describes the stress due to the network strands which exist at the onset of the deformation, dissociate during the deformation and result in a single integral constitutive equation with a strain dependent damping function. The formulation of the damping function in invariant form seems to be almost impossible.The second normal stress difference N2 of the model is not zero,but has negative value~. According to our model this is a consequence of the deformation dependence of the disentanglement process. The theory is compared with experimental data for a LDPE melt. It is found that the model explains the main features of the shear flow behaviour of the LDPE melt investigated preciously. ZusammenfassungDas Netzwerk-Modell, das in einer vorangegangenen Arbeit entwickelt wurde, wird fiir die einfache Scherstr6-mung yon PoIymerschetzen angewendet. Die abgeleitete rheologische Zustandsgleichung besteht aus zwei Gliedern. Das erste beschreibt die Spannung-Dehnung-Beziehung der Kettensegmente, die zu Beginn der Deformation existieren und w~ihrend der Deformation aufgel6st werden. Es hat die Form der einfachen Integralbeziehung mit einer Ged~chmis-funktion. Es ist kaum m~Sglich, die dabei erhaltene Ged~chtnisfunktion als Funktion der Invarianten der der Tensoren darzustellen. Die zweite Normal-Spannungsdifferenz N2 des Modells ist nicht Null und hat einen negativen Weft. Dies ist nach unserem Modell eine Folge der Deformationsabh~ingigkeit des Entschlaufungsprozesses. Die Theorie wird mit dem experimentellen Daten fiir eine LDPE-Schmelze verglichen, wobei sich zeigt, dag das Modell die wesentlichen Merkmale des Scherverhaltens der LDPE-Schmelze gut erkl~rt.
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