Mode I fracture

The mode I fracture toughness, KIc, for nearly all nuclear graphites falls in the range 0.75 MPa in to 1.5MPa m. 

Historically, most attention has been devoted to mode I delamination growth to assess defects critically, as the first-generation composite systems exhibited low mode I fracture toughness when subjected to interlaminar normal stresses. The... 

Damage zone size in mode I fracture and corresponding interlaminar fracture toughness values, G j. and Gji,.. of carbon fiber epoxy matrix composites. ... 

Daghyani. H.R.. Ye. L. and Mai. Y.W. (1995b). Mode I fracture behaviour of adhesive joints, 2, Stress analysis of constraint parameters. J. Adhesion 53, 163-172. 

Fig. 20 Calculated maxima in the mode I stress intensity factor as a function of crack depth for a DGEBA/IPD epoxy material under cyclic loading. The dotted line corresponds to the experimental value of the mode I fracture toughness...

In, the central zone, the regular distribution of transverse cracks shows that the induced strain is rather homogeneous. Consequently, many analytical models can be applied in order to determine the intrinsic parameters of the coatings. The critical cracking energy and the mode I fracture toughness of the deposited silicon carbide film were assessed by means of the model presented previously. It should be remembered that it was first established and developed for composite materials based on research by Kelly and subsequently by Hu ", that when the stress normal to the coating reaches a critical value... 

Whilst test methods to measure the mode I fracture resistance of polymer fibre-composites [1] and stmctural adhesive joints [2,3] have now proceeded to full standards, the efforts to standardise a mode II method have been beset by a number of problems. Firstly, there exist a number of competing tests e.g. the end-loaded split (ELS), the end-notched flexure (ENF), the four point end-notched flexure (4-ENF) and the stabilised end-notched flexure (S-ENF) have all been proposed and have been subjected to various inter-laboratory evaluations. However, the results of these programmes have always revealed a very large scatter in the values of Gnc measured e.g, [4], and this scatter has been variously attributed to friction... 

As shown in Fig. 15, the mode II fracture toughness, G], was more than twice as high as the mode I fracture toughness, G q. However, the difference was much smaller than that of the other materials such as polymer matrix composite materials. This would be the consequence that the microscopic fracture of the present specimen was not pure mode II owing to the discrepancy between the macroscopic crack face and microscopic crack path even though the mode II loading was applied macroscopically, as suggested by the results shown in Fig. 14. 

The mixed mode fracture toughness of the acrylic resin adhesive interface was obtained by applying the proposed method. The mode II fracture toughness was more than twiee as high as the mode I fracture toughness. 

Mode I fracture tests were performed in three-point bending (SE(B)) at a low (lOmm/min) and a high (Im/s) loading rate according to the testing protocols that were developed by the European Structural Integrity Society - Technical Committee 4 (ESIS TC4) to determine fracture toughness in plastics by a linear elastic fracture mechanics approach, which have become now international standards [15,16]. 

Mode I and II interlaminar fracture toughness was investigated with unidirectional CF/epoxy and ALF/ epoxy laminates in RT-air and 77K-LN2- A common epoxy matrix was used for both laminates. Mode I fracture toughness of the neat resin was also measured for comparison. The results are summarized as follows ... 

The mode I fracture toughness of the neat resin increased by 1.8 times changing the environment from RT-air to 77K-LN2-... 

Blackman, B.R.K., Brunner, A.J., (1998) Mode I Fracture Toughness Testing of Fibre-Reinforced Polymer Composites Unidirectional versus Cross-ply Lay-up , Proceedings 12" European Conference on Fracture ECF-12 Vol. Ill, Fracture from Defects, EMAS Publishing, pp. 1471-1476. 

Bohse, J., Krietsch, T., Chen, J., Brunner, A.J., (2000) Acoustic Emission Analysis and Micromechanical Interpretation of Mode I Fracture Toughness Tests on Composite Materials , Proceedings ESIS Conference on Fracture of Polymers, Composites and Adhesives, ESIS Publication 27, pp. 15-26, Elsevier, Oxford. 

G1C Critical strain energy release rate (mode I fracture)... 

This fact supports the availability of the analysis of a Mode I fracture mechanics problem. 

It is considered that some fractures opened with increasing pore-pressure maybe after shear slip because the maximum wellhead pressure is about lOMPa. However, Mode I fractures can not radiate seismic events having enough energy, and we can not detect these seismic events. Therefore, we consider only shearing fractures for estimation of critical pore-pressure using induced microseismic events. 

Other variables investigated with respect to fracture toughness include processing parameters, e.g.. production schedules and postcure cycles [166.171] for thermosets, and cooling rates for thermoplastics [171] matrix composites, and moisture content. A study [171] on composites based on epoxy and vinyl ester matrices has shown that whilst matrix plasticization improves mode I fracture toughness, mode II fracture toughnc.ss deteriorates due to interface degradation. The sensitivity of (Jut to water absorption has been demonstrated even for matrices that absorb very small amounts of water such as pol propy lene [168]. 

Once the performance of the interface bond was established by the previous delamination (durability) tests, contoured double cantilever beam (CDCB) specimens were designed to conduct mode-I fracture tests. In this study, bilayer CDCB specimens (see Fig. 6) were designed by the Rayleigh-Ritz method [8] and used for fracture toughness tests on bonded FRP wood interfaces under both dry and wet conditions. The critical strain energy release rate, G/c, which is a measure of the fracture toughness, is given as ... 

The linear-slope CDCB specimens shown in Figs. 13 and 14 were used for mode-I fracture tests on FRP-wood bonded interfaces under both dry and wet conditions. A typical specimen under mode-I fracture load is shown in Fig. 15 for phenolic FRP wood interface under dry conditions, and the corresponding critical tip load versus crack opening displacement is plotted in Fig. 16. As an illustration, the fracture surfaces of epoxy FRP-wood with HMR and RF primers on wood substrates are shown in Figs. 17 and 18,... 

Finally, the tapered double cantilever beam specimen [8] described in this study can be effectively used to obtain interface mode-I fracture toughness values under various environmental loadings (e.g., under dry and wet conditions [5,14,15]) and cyclic load conditions (fatigue fracture [7]) these data can be implemented in practical applications to assess the potential growth of a delamination crack at the FRP wood interface. 

Kusaka T, Hojo M, Mai YW, Kurokawa T, Nojima T, Ochiai S. Rate dependence of mode I fracture behaviour in carbon-fibre/epoxy composite laminates. Compos Sci Technol 1998 58 591-602. http //dx.doi.org/10.1016/S0266-3538(97)00176-0. 

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