Fracture Mechanics Tests

A basic tenet of fracture mechanics is that the strength of most solids is governed by the presence of flaws. If stress is present, the flaws will tend to propagate until a critical condition is reached and rapid failure ensues. The theory attempts to describe the way in which flaws propagate under stress and to define the critical condition in terms of a failure criterion. If 

rapid failure, caused by the action of a high stress sufficient to overload the joint, 

slow failure, when the joint is subjected to a sustained load in an agressive environment, and 

Cases (2) and (3), dealing with slow failure, are of most interest in a discussion of joint durability. 

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Fracture Mechanics tests on a grade of ABS indicate Aat its K value is 2 MN and Aat under static loading its growA rate is described by the equation. 

Fracture Mechanics Tests One problem of both sustained load and slow strain-rate tests is that they do not provide a means of predicting the behaviour of components containing defects (other than the inherent defect associated with the notch in a sustained load test). Fracture mechanics provides a basis for such tests (Section 8.9), and measurements of crack velocity as a function of stress intensity factor, K, are widely used. A typical graph of crack velocity as a function of K is shown in Fig. 8.48. Several regions may be seen on this curve. At low stress intensity factors no crack growth is... 

Brocklehurst [37] has written an exhaustive review of the early work (prior to 1977) on fracture in polycrystalline graphite. Much of this work focused on the fracture behavior of nuclear graphites. In most investigations considered, conventional fracture mechanics tests and analysis were performed for macroscopic cracks. LEFM provided an adequate criterion for failure. Additionally, results on work of fracture, strain energy release rate, and fatigue crack propagation were reported. 

Moore, D., Pavan, A., and Williams, J. 2001. Fracture Mechanics Testing Methods for Polymers, Adhesives, and Composites. Elsevier, New York. 

Now the measured value of KIc or GIc is dependent upon the sharpness of the crack employed and an essential requirement of fracture mechanics testing is that a "naturally sharp crack is used for the experimental determination of K,c or G C. If a relatively blunt crack is used, an optimistically high value of KIc or G,c may be recorded. Suitable techniques include slow, controlled pressure applied to a razor blade, possibly having pre-cooled the specimen in liquid nitrogen and fatigue crack growth. 

The three principal forces to which adhesive bonds are subjected are a shear force in which one adherend is forced past the other, peeling in which at least one of the adherends is flexible enough to be bent away from the adhesive bond, and cleavage force. The cleavage force is very similar to the peeling force, but the former applies when the adherends are nondeformable and the latter when the adherends are deformable. Appropriate mechanical testing of these forces are used. Fracture mechanics tests are also typically used for structural adhesives. 

During a fracture-mechanical test performed in mode I, the crack propagates in this mode from a macroscopic point of view. But the crack can be deflected locally by the rubbery particles and can also propagate in mode II. As for isotropic materials, GIIc is generally higher than GIc an artificial increase of the macroscopic GIc value will be then evidenced. 

A further development of this procedure is fatigue crack propagation (FCP) experiments in the presence of a stress cracking environment. While in the fracture mechanics test methods described above the specimen is under constant load, in a FCP experiment the specimen is tested under cyclic loading conditions in the presence of a sensitizing medium. 

The processes at the crack tip of a notched specimen in a stress cracking liquid under mechanical stresses and possibly elevated temperatures can be regarded as local aging phenomena. Fracture mechanics, on the one hand, is a very sensitive method of detecting a polymer s behavior with regard to these influences. On the other hand, as demonstrated in the previous examples, FCP measurements can distinguish between small material differences. As depicted in Fig. 21, PE-HD shows the different behavior in static fracture mechanics tests with respect to water at a temperature of 80 °C for two different stabilizers. Stabilizer 2 leads to higher K values at comparable da/dt values and is therefore more appropriate for hot-water applications when oxidation processes also have to be taken into account [88]. 

Interestingly, the ductile-brittle transition observed for the MIM system provided an opportunity to assess the material fracture toughness, which was not possible using classical fracture mechanics tests due to the intrinsic brittleness of the MIM system. The measurement of the critical crack length, Lc, in the contact plane at the onset of brittle propagation allows estimation of a fracture toughness K C = a x+JnLc in the order of 0.85 MPa m1/2, i.e. much less than that of a poly(methylmethacrylate) homopolymer (1.20 MPa m1/2). 

Since crazing represents a cavitational form of plasticity, it is clear that crazes play an important role in the fracture of polymers. Crazing is, generally, involved when PC fails under plane strain conditions, e.g. in fracture mechanics tests on thick samples with sharp notches where high triaxial stresses are built... 

Data obtained from fracture mechanics tests provide useful information concerning the ability of different materials, or of different variants of the same material, to resist crack propagation and concerning the possible influence of existing cracks. 

FRACTURE MECHANICS TESTS TO CHARACTERIZE BONDED GLASS/EPOXY COMPOSITES ... 

Fracture Mechanics Tests to Characterize Bonded Glass/Epoxy Composites... 

Moore DR, Pavan A, Williams JG, eds.. Fracture Mechanics Testing methods for Polymers, Adhesives and Composites, ESIS Publication 28, 2001, Elsevier. 

Davies, P., B.R.K. Blackman, and A.J. Brunner, Mode II delamination, in Fracture mechanics testing methods for polymers adhesives and composites, D.R. Moore, A. Pavan, and J.G, Williams, Editors. 2001, Elsevier Amsterdam, London, New York. p. 307-334. 

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