It is well known that real life engineering components are most often subjected to a multi-axial loading conditions and under such cases, the crack propagates under mixed mode loading conditions.
Although, an extensive number of experimental and numerical studies have been conducted in the last five-six decades, but it can be observed that most of the engineering components designed for damage tolerance incorporates only mode I fracture toughness KIc in their designs. On the other hand, the use of other mode fracture toughness, i.e., KIIc (mode II), and KIIIc (mode III), are rarely seen.
Why use of mode II and mode III fracture toughness parameter of materials are less popular while designing real damage tolerant engineering components and structures?
Is there any example where mode II and mode III fracture toughness values are being incorporated widely?
Hi Shiv Sahaya Shukla ,
I think your question is very relevant.
As far as I know, when considering homogenous and isotropic materials crack propagation would occur in mode I only; or, in other words, any crack that is subject to any loading mode different from mode I will kink and propagate aligning itself such that propagation occurs under pure mode I. As a consequence, for these materials (that represent a significant portion of those used in engineering applications) the mode I fracture toughness is the most relevant to consider.
Fracture toughness relative to other modes becomes relevant when propagation of the crack under those modes can occur in practice. This is the case in nonhomogeneus materials. Interlaminar fracture (delamination) of fibre reinforced polymers, FRPs, is one example. Interlaminar cracks may be bounded by the neighbouring layers and thus be constrained to propagate along the interlamina, even under mixed mode conditions (or even pure mode II... mode III is kind of a difficult story I would say :D). Hence there is a huuuuge interest in charactersing properly mode II and mixed mode fracture toughness in this case and using them for the design of structures made of FRPs.
Hope this helps ;)
Kind regards,
Torquato
Thanks Torquato Garulli for putting it in a very simple way.
If I have understood correctly, we can say that a crack in isotropic homogeneous material is not constrained enough to continue to propagate under mixed mode conditions.
Similar to composites, if we can define a specific path for a crack to progress in an isotropic material then the study of other mode fracture toughness will be more relevant.
What is you view on this? Can you please suggest any such real life applications where such situation arises in isotropic materials.
Dear Shiv Sahaya Shukla
I would imagine the only way to control/define a path for crack propagation in material would be microstructural modification, implying creating inhomogeneity and anisotropy.
I can't think of any way to control the crack propagation path in an isotropic material, and am prone to thinking that, if the material is truly isotropic at the scale at which you are defining your crack propagation and fracture toughness, then there would be no way.
But of course I might be wrong.
Kind regards.
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