Notch concept

Fig. 1. Artificial notch concept real fracture (left) is replaced with artificial notch (right) [2].

The concept of a ductile-to-brittle transition temperature in plastics is likewise well known in metals, notched metal products being more prone to brittle failure than unnotched specimens. Of course there are major differences, such as the short time moduli of many plastics compared with those in steel, that may be 30 x 106 psi (207 x 106 kPa). Although the ductile metals often undergo local necking during a tensile test, followed by failure in the neck, many ductile plastics exhibit the phenomenon called a propagating neck. Tliese different engineering characteristics also have important effects on certain aspects of impact resistance. 

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. 

Notch cleavage (114), has provided encouraging results. Although the inhibitory potency of NSAIDs as a class (micromolar) does not recommend them as clinical candidates, they represent a pre-clinical proof of concept that it may be possible to design highly specific A/14 2 inhibitors. 

In postyielding fracture mechanics, the EWF concept has been successfully employed to characterize the fracture toughness of ductile polymers and tough composites due to its simplicity over conventional 7-integral analysis (70-75). The EWF approach involves the determination of the total fracture energy (Wf) of notched specimen. It can be divided into two components ... 

The DT test [ ] is similar in concept to the CVN test. The simplicity of a three-point bend specimen has been combined with an energy criterion for fracture resistance. In this test, a 16 mmx38 mmx 181 mm specimen with a sharp notch is fractured on a large pendulum or drop-weight impact machine, and the fracture energy is measured. 

Available results show that proportionality crmstants jS, s and are material-dependent They also indicate that specimen dimensions required for acquiring fracture mechanics values ruider impact loading are maintained for 6 = 4 mm and a/W > 0.2 in the LEFM concept the MC and ST /-integral estimation methods and in the CTOD concept, if the notch part of critical crack-tip-opening displacement is used. 

FIGURE 15.20 The basic concept of notch waveform isolation. In the isolation process, the working point of precursor ions is moved first to a j-value at which the secular frequency of precursor ions lies inside the window of the notch waveform. Subsequently, the notch waveform is turned on to eject all unwanted ions. Due to the limited duration of application of the notch waveform, the window is not a perfect box as is seen in the bottom picture of this figure. 

Theoretical investigations into phenomenon of the notch sensitivity of composites have tended to rely on classical fracture concepts. ITiese efforts have taken two forms micro-and macro-mechanical representations. In die micro-mechanics format, local... 

This test method parallels the use of F 519, specimen Type le in specimen design and concept. The steel specimen has been modified to have a lower hardness of 51 1 HRC, and the notch root radius is 20-mil instead of 9-mil in order to reduce the stress concentration. The modifications were made with the intent of making the specimen less sensitive because the application was lower hardness fasteners (33-44 HRC) for automotive applications as compared to the higher hardness (45-52 HRC) steel used in aerospace apphcations. 

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