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A short-beam shear fracture approach to measure the mode II fracture toughness of materials with preferred interfaces
Increasing complexity of numerical simulations has made it important to obtain reliable experimental data for a variety of mechanical properties. Fracture properties are complex to measure but accurate data is often desirable for several challenging problems. We have developed a novel short-beam shear fracture approach in order to measure the mode-II fracture toughness properties of materials with 'preferred' interfaces.
Prof. Roy Xu has already discussed some elements of this approach in this comment. Our paper has been published in the International Journal of Fracture [pdf] recently. Fig.1 shows a schematic of the short-beam shear specimen with different possible modes of failure. Of course, for a pure mode-II criterion to be satisfied, the intial crack must not kink from the crack tip.
In our paper, we discuss the implementation of this approach, provide experimental data for bonded polymer specimens and also show that this method can be extended to unidirectional fiber composite materials. Our approach eliminates friction between the crack faces, provides a clear load-drop and thereby provides an accurate value of mode-II fracture properties. We provide calibration charts for composite materials and validate our method by comparing it with four-point bending.
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From Southwest B737 airplane cracks to B787 composite fracture
Recent news about crack propagation inside metal Boeing 737 fuselage gave us a clear warning: well-established elastic-plastics fracture mechanics still could not prevent unexpected fatigue crack initiation and then a dynamic fracture process, see this article “Boeing didn't expect 737 cracks so soon (link)”. These cracks inside metals are mainly mode-I opening cracks.
However, new Boeing 787 aircraft’s half structural materials are composite materials. Then the mode-II in-plane shear crack will be another major failure mode, especially along the interfacial joints of composites and metals. Therefore, our paper is very important since B 737 represents the past 30 years, while B787 will dominate the airlines in the next 30 years. Why should we study composite failure? Simply “protect yourself, family, friends and more….” Hope someone would recall my suggestion after 20 years if composite structure failure did occur inside one B787 aircraft.
interesting thought
The incidence of B737 happened right in time for my class on Aerospace Materials. The topics I am supposed to discuss in this class are composite materials, fracture, and fatigue in the next three weeks. I guess I could use this incidence to motivate the undergraduate class majoring in Aerospace Engineering. Traditionally, however, our students are more interested in designing cool aircraft or even satellites without bothering much to understand the material behavior (including failure) or any fracture mechanics.
On the other hand, I am not sure if this incidence indicates "well-established elastic-plastics fracture mechanics still could not prevent unexpected fatigue crack initiation and then a dynamic fracture process". Is elastic-plastic fracture mechanincs really well-established? Most studies on fracture mechanics to date adopt the concepts from linear elastic fracture mechanics (e.g., stress intensity factors, energy release rate, etc.). Some may be extended to nonlinear elastic or elastic-plastic fracture, but with significant limitations. Fatigue, as I will discuss in my class (S-N curve, Paris law, etc.), is essentially an empirical engineering practice, with little to no fundamental understanding. I would never trust any fatigue lifetime prediction without a large safty factor, and I am not surprised that Boeing engineers did not expect the crack. As for composites in B787, I can only hope that frequent inspections (much more than current practice) could prevent similar incidences.
RH
Your discussion
Rui,
Thanks for your in-depth discussion. Compared to fracture mechanics of composite materials, elastic-plastics fracture mechanics for metals is quite established since it’s hard to define a single crack inside composites. Anyway, your work and my research provide a completely theoretical and experimental failure mechanics understanding for future safe vehicles.
Roy