Structured and unstructured mesh

No, a structured mesh can be composed of triangular elements.

In fact a mesh is said to be structured if one can found a primitive motif which is regular in space. This implies that for instance node connectivities has a fixed pattern. But the geometry may be not constant, for instance your mesh can be deformed from place to place (for instance to match a specific boundary). The only thing you know for sure when the mesh is regular is that the node (or elements) connectivty is always the same. This may be useful for algorithm design since you always have the same pattern.

However unstructured mesh are generally more flexible, since the connectivity pattern is not fixed, at the cost of having to store the connectivity.  There is no generic answer to your last question, since it depends strongly upon the application you have in mind.

I am doing my m.tech project on fracture mechanics..

I have to model orthotropic plate containing holes frm which 2cracks are emanating radially frm each hole.

my problem is i have to apply out of plane loading like bending moment and torsion.

So can any pls suggest for following problems

1.which element is suitable for above problem..?
2.how to model multiple cracks in ANSYS..?
3.How to apply moments over a plate..?

If u give suggestions it will be great helpful to me..

thanks in advance...

I do not know the  basic difference and hope that some people explain clearly！

Meshes are primarily composed of two distinct entities: the geometry (the position x,y,z of the nodes) and the topology (how the nodes are connected).

A mesh is said to be structured if its topology is regular, ie have a well known pattern (such as cubes in a sugar box). You can mesh complex objects with structured meshes, since the geometry of the nodes can be whatever you like, but it may give awful elements.
http://www.tecplot.com/Portals/0/images/showcase/plot-gallery/mesh/tecpl...

On the other hand unstructured meshes have a irregular topology, meaning that you have really to store the topology for each single node (or edges, facets, etc...). This specific feature makes unstructured mesh more flexible (and btw more memory demanding)

http://www.tecplot.com/Portals/0/images/showcase/plot-gallery/mesh/tecpl...

Meshes are said regular if their geometry have a pattern (regular cubes for instances with regular spacing dx, dy, dz between nodes). Their are seldomly used in mechanical engineering (difficult to create complex shape with this), but often used in cfd.

It is hard to say which of structured or unstructured is easier to create in general. Structured meshes are difficult for complex objects, and generating unstructured meshes is a very difficult task. The only thing very important is that, on complex objects, it is generally true that unstructured meshes create less degenerate elements.

There are important issues in mesh generation, and entire books are dedicated to it, so it is difficult to be specific (moreover I don't pretend to know much about it). I think it is a very complex field (or vast field to say the least) and it should not be considered as trivial in most real cases.

I hope it helps.

Thomas Laverne