Final fracture

Energy required to cause p/asfic deformation up to point of final fracture (plastic work at fracture)... 

At still higher true stress, do/de, the rate of work-hardening decreases further, becoming insufficient to maintain stability - the extra stress in the neck can no longer be accommodated by the work-hardening produced by making the neck, and the neck grows faster and faster, until final fracture takes place. 

The total elongation of the specimen just before final fracture was 16%, and the reduction in area at the fracture was 64%. 

Upon loading a void-containing material, a certain stress distribution in the sample will develop that proceeds and determines the following deformation. Typically the voids (or other dispersed phase) will tend to concentrate stresses to interphases between materials of different modulus. Even though no complete picture exists of what will happen upon deformation, such a stress description may give a better understanding of the relation between stress concentrations in the sample due to the voids and the final fracture behavior. 

Figure 4. Degradation in strength as a function of time for Nylon 6 samples in three different concentrations of NO, at 81° C. Final fracture was at room temperature. Concentrations are (a) 0.86% (b) 2.58% (c) 5.16%.

Figure 10 shows the notch tip zone in a SEN test specimen loaded to about 75% of maximum load. The transverse cracks are easily seen in this photograph. These cracks even occur at some distance away from the notch plane and the final fracture plane. These micrographs indicate that considerable damage occurs in the polyester composites prior to unstable fracture. It is thus important that the R-curve concept should be applied to these materials if their total fracture behavior is to be characterized. 

Post-failure studies of the fracture surface morphology of bulk semi-crystalline polymers are more difficult than those of amorphous materials due to the more complex multiphase structure associated with semi-crystalline materials However, it was shown that some fractographic details point to the formation of a stress whitened region ahead of a notch prior to final fracture of the material. In particular, stress-whitened regions were easily visible in semi-crystalline polymers such as LDPE and HDPE The resulting, macroscopically apparently brittle fracture... 

Inspite of a high band initiation stress Og, the resistance of PS to subsequent crack growth and final fracture is relatively low. The K -values of the crystalline polymers are much higher. A similar connection has also been established recently for various polymers under tensile test conditions... 

Consider now a polymer sample of current volume V responding by craze plasticity to an imposed current elongational strain rate e, developing a tensile (dilatational) flow resistance until a final fracture strain is achieved as shown in Fig. 1. The specific toughness W or the total deformational energy absorbed per unit volume for this polymer is the area under the stress strain curve, or... 

Figure 13 also shows a series of microphotographs of microtomed sections of notched PE in which much finer sphcrulites arc dispersed. The processes involved in craze nucleation and growth are fundamentally similar to those of PP, but a craze bundle in PE is composed of smaller individual crazes in length. As the load is increased, a concentrated craze line extends into the interior of the specimen a long the midplane. A crack initiates within this craze line and propagates stably with decreasing the applied load. Unlike PP. final fracture in PE is ductile. A higher magnification view of Fig. 13c is shown in Fig. 14. The damage zone near the notch root is composed...

Other difTcrent aspects of crazing and fracture are found in POM. Figure 16 shows the temperature dependence of bending moment-displacement relations of a notched POM sample. Fracture occurs in a nearly brittle manner for the samples loaded at 296 K (room temperature) and 313 K. At 333 K final fracture is also brittle after exhibiting considerable nonlinear deformation under nearly constant load. The... 

Fig. 23. Micropholograph of the microiomed stection of a notched PEEK sample unloaded just prior to final fracture...

Figure 14.2 Nominal stress, a , versus strain, for a ductile polymer and consequent change in the dimensions of the specimen. (X indicates final fracture.)...

This energy partition is usually done by considering Wy as the energy under the load-displacement curve up to the maximum load, and W ( as the energy from the maximum load up to final fracture [9,10.16,20,21,24,27]. Similarly to Eq. (4), the authors cited above expressed the variation of Wy and w , with the ligament length as... 

Flat tensile creep specimens were machined from the blocks so that the longitudinal specimen axes were either parallel to the plane containing the majority of the long axes of the fibres for the squeeze-cast composites or parallel to the extrusion direction for powder metallurgy materials. Constant stress tensile creep tests were carried out at temperatures from 423 to 523 K. The applied stresses ranged from 10 to 200 MPa. Creep tests were performed in purified argon in tensile creep testing machines with the nominal stress maintained constant to within 0.1% up to a true strain of about 0.35. Almost all of the specimens were run to final fracture. 

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