Toughness measurement

We can mount samples either vertically or horizontally. When Ave mount the sample vertically, the pendulum strikes the upper half of the sample on the notch side, as shown in the upper part of Fig. 8.7. To mount a sample horizontally, we place it on a pair of L -shaped supports, as shown in the lower part of Fig. 8.7, vith the notch pointing away from the pendulum, which strikes the sample in the middle of the opposing side. 

We test the impact resistance of polymer plaques using the configuration shown in Fig. 8.8. We subject circular injection molded plaques to the shock of a falling weight with a hemispherical impacter. This test is also known as the Dart Drop Test . Samples can fail in a brittle or a ductile manner. Brittle samples often shatter. Ductile samples can split or a small disk may be punched out of their center. This test provides results that are analogous to those obtained from un-notched impact beam testing. 

When the probe makes contact wth the film, it generates a radial stress field arotmd the point of contact. If the film is isotropic, it deforms in a uniform ring around the probe, as shoAvn in Fig. 8.11 a). If the film is oriented, it deforms in a non-uniform manner. When the film is mildly oriented, the deformation area becomes ellipsoidal, as -we see in Fig. 8.11 b), wth its long axis 

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In tougher materials the minimum thickness required by equation 15 can become excessive. In such cases the /-integral test is an attractive alternative. Because this test considers the stress distribution around the crack inside the plastic 2one, it can be used to obtain a vahd toughness measurement in a thinner specimen, because more extensive plastic yielding does not invaUdate the analysis. The equivalent of equation 15 for the /-integral test is... 

T. D. Burchell, I. Oku and M. Eto, A Comparison of Fracture Toughness Measurement Techniques as Applied to Nuclear Graphites, Extended Abstracts and Program, International Carbon Conference, 1990, pp 278 279... 

T.D. Burchell and. T. Oku and M. Eto. A comparison of fracture toughness measurement techniques as applied to nuclear graphite. In Proc. Carbone 90, Paris, France. Pub. Groupe Francais D etude des Carbones, 1990. pp. 278 279. 

MacLean, 1954 Stamm, 1956). Hardwoods exhibit higher strength losses than softwoods when treated under the same conditions. For example, Chang and Keith (1978) found that elm and beech were more susceptible to thermal degradation compared to aspen and maple, as determined from toughness measurements of the thermally modified samples. 

Fracture cracks often propagate in an uncontrolled manner. Hence, adhesion tests should utilize cracks which are grown carefully and slowly, as is the case with the procedures commonly used for fracture toughness measurements (ASTM D3433-85). 

The flaw spectrum approach may be capable also of correlating failure data obtained under conditions which appear to give different material behavior. For example, tensile, Izod, and dart drop impact tests all give different impact energies or toughness measures. In part, this may be explained by the simple idea that a different fraction of flaws is active in each test. Equal biaxial loading (as in dart drop) and uniaxial tension (as in tensile impact) give rise to different flaw activities thus it is to be expected that the number of crazes produced in each test will be different. 

Computer-generated contour plots of the elongation at break and the toughness, measured by the area under the stress-strain curve, exhibit... 

Der, V.K., Holloway, D.C. and Kobayashi, T. (1978). Techniques for Dynamic Fracture Toughness Measurements. Prepared for the National Science Foundation by Photomechanics Laboratory, Mechanical Engineering Department, University of Maryland, College Park Campus. 

An experimental investigation was performed on a laminate and separately on the individual components of the laminate. Peak load, energy dissipation at onset of fracture and at fast fracture were investigated. Fracture toughness cannot easily be defined because of very large scale yielding. Therefore an alternative toughness measurement was proposed. 

The characteristic values of the crack initiation and propagation parameters (Jic, Kjc and Tr) are listed in Table IV for the different ternary blends. In all cases the requirements for valid fracture toughness measurement were fulfilled. These results confirm that type B fibres are better suited as reinforcement for rubber-toughened nylon materials. But the marked reduction... 

Parvin and Williams made fracture toughness measurements on HIPS between —120° and 20 °C. Some of the measurements were made on 150 mm wide single edge (SEN) specimens, but the study concentrated on fracture in 6 mm thick... 

Fig-3- Schematic of the asymmetric double cantilever beam geometry used for fracture toughness measurements... 

The fracture toughness measurements, combined with the post-mortem surface analysis of the fracture surfaces, allow us to develop some simple micromechanical models to account for these three situations. 

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