Blunt notch

In the laboratory the impact behaviour of a material could be examined by testing plain samples, but since brittle failures are of particular interest it is more useful to ensure that the stress system is triaxial. This may be achieved most conveniently by means of a notch in the sample. The choice of notch depth and tip radius will affect the impact strengths observed. A sharp notch is usually taken as 0.25 mm radius and a blunt notch as 2 mm radius. 

Fig. 2.79 shows the typical variation of impact strength with notch tip radius for several thermoplastics. The first important fact to be noted from this graph is that the use of a sharp notch will rank the plastics materials in a different order to that obtained using a blunt notch. This may be explained by considering the total impact strength as consisting of both crack initiation and crack propagation... 

It may be seen from Fig. 2.80 that some plastics experience the change from ductile to brittle behaviour over a relatively narrow temperature range. This permits a tough/brittle transition temperature to be quoted. In other plastics this transition is much more gradual so that it is not possible to attribute it to a single value of temperature. In these circumstances it is common to quote a Brittleness Temperature, rg(l/4). This temperature is defined as the value at which the impact strength of the material with a sharp notch (1/4 mm tip radius) equals 10 kJ/m. This temperature, when quoted, gives an indication of the temperature above which there should be no problems with impact failures. It does not mean that the material should never be used below Tb(1/4) because by definition it refers only to the sharp notch case. When the material has a blunt notch or is un-notched its behaviour may still be satisfactory well below Tb(1/4). 

The stresses near the root of a notch are extremely complex and the stress analysis becomes exceedingly difficult when the strain is large, as is the case when yield or failure is imminent. A sharp notch causes constraints and introduces a state of triaxial tension behind the root of the notch (5). This state of stress is consistent with LeGrand s observation of the growth of a flaw behind a notch in a bar of polycarbonate (4). A blunt notch causes constraints when the thickness of the specimen is large. Such a notch can also introduce a state of triaxial tension. While it is desirable to investigate the behavior of polymers in a well-defined state of triaxial tension, it is difficult to accomplish experimentally. However, as we demonstate below, a state of plane strain is relatively easy to produce. The relationship between plane strain and brittleness of plastics is the subject of our investigation. 

There are other cases when both crazes and shear bands are simultaneously observed in polymers. One such case occurs in the deformation of amorphous polymers at blunt notches " . In this case, discrete shear bands emanate from... 

Since the blunt-notched specimen undergoes the same viscoelastic deformation as the sharp-notched specimen, but without crack propagation, the crack initiation time can be identified as the time at which the two curves diverge. Determination of the subsequent crack growth is a somewhat more difficult task direct optical observation was not possible, due to the wrapping bag. An indirect method based on compliance analysis was therefore adopted. 

Deformation curves in Fig. 4 clearly show that materials cannot be considered linearly elastic, and therefore Eq. 2 cannot be used directly in this form. However, by subtracting from the sharp-notched specimen curve the flexural and shear deformation contributions given by the blunt-notched specimen curve, a curve accounting for the crack length contribution to the specimen compliance can be derived from the data of Fig. 4. Eq. 2 can then be rewritten as follows ... 

In several polymers, it is possible to induce brittle failures in impact with quite blunt notches and it is possible to give an apparent Gic, for these cases. Rigorously, of course, the use of Ki is not valid for anything other than a sharp notch. However, if ATjc is interpreted as a critical stress at a critical distance C we may write > ... 

FIGURE 3.39 Variation of impact strength with temperature for several thermoplastics with blunt notch. 

Figure 19.4 Schematic diagrams showing common fracture test geometries. Single edge-notched (a) three-point-bend, (b) tensile and (c) compact tensiem tests, each using specimens with razor-sharpened notches, are employed for fracture mechanics measurements of fracture toughness and fracture energy. The Izod (d) arxl Chatpy (e) tests are widely practised in industry to evaluate toughness, but use bluntly notched (or un tched) specimens and do not give fracture mechanics data...

Some of the more common mechanical properties are provided in Table 10.1 from IAEA (2009). Relative to various measures of toughness, some tests use a blunt notch, while others use a sharp crack to determine material resistance to crack initiation, crack propagation or both, and are conducted under either quasi-static (slow) loading or dynamic (fast) loading conditions. Of all these tests, the Charpy V-notch (CVN) impact test is the most commonly used. In addition to those mechanical properties, there are common reference fracture toughness indices used for RPV steels and many of those are also shown in IAEA (2009). These indices are important because they are used in various ways to normalize fracture toughness of RPV steels. 

Dynamic fracture toughness, Kimpact-response curve and the time-to-fracture was 6.2MN/m for PSZ and 1.7MN/m for Si3N4. The effects of the notch-root radius on static and dynamic fracture toughness were also evaluated. The specimens were blunt-notched with radii p = 25, 50, 100 and 150 pm. They were tested by instmmented CIT, applying the impact-response... 

FIG. 10—The frequency independence of CF life for blunt notch specimens of A517 steel, freely corroding in aqueous 3.5 % NaCI as a function of notch root strain range [4e[. (Initiation here is In fact crack formation plus early growth to a 1-mm depth.)... 

Figure 10 illustrates the minimal effect of loading frequency on CF crack formation and early growth for blunt notched steel specimens in aqueous chloride solution at free corrosion 46]. These data are notable for the rare combination of low f and high cyclic lives (175 days were required to obtain 3 x 10 cycles at 12 cpm), and for the lack of a frequency effect on the cycles required to produce 1 mm of CF cracking. This result indicates that one or more of the early... 

Vincent [110] and others have recognized that the impact strength depends on the geometry of the notch, which led Fraser and Ward [111] to propose that for comparatively blunt notches (i.e. those not introduced by a razor blade or a sharp cutting tool) failure occurs when the stress at the root of the notch reaches a critical value. This stress, which in a glassy polymer marks the stress required to initiate a craze, can be calculated by assuming that the deformation is elastic. On this hypothesis, the Charpy test, as undertaken in... 

It has been shown that the impact behaviour of blunt notched specimens of PMMA is consistent with a critical stress at the root of the notch [111], and similar considerations apply to polycarbonate [107] and poly(ether sulphone) [108] in the absence of shear lips. In these instances it appears therefore that the maximum local stress is the fracture criterion, independent of specimen geometry. 

Prior to fracture testing, the samples were notched as follows first a blunt notch was produced by using a machine with a V-shaped tool, and then a sharp notch of 0.2 mm depth was made by a razor blade fixed to a micrometric apparatus. The final value of notch depth was measured after fracture by using an optical microscope. 

The tails or ridges are formed due to the fact that the crack front movuig around each side of the particle never ends in the same plane. It was presumed that submicroii sized particles should improve the toughness when this mechanism prevails as more sites will act for crack pimiing. This is, however, not the case as shown later. The effect of the mean particle size on the dynamic fracture was moderate in an EP with 55 wt% filler. On the other hand, the impact energy, measured on blunt notched specimens, was remarkably reduced with increasing particle size [109]. 

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