Why do objects break with straight or curved cracks?

[noagname]

sry but i am not sure what to put for the title
i am not sure exactly how to say this
but from what i have learned if you put more pressure on an object than it can handle it will break with straight lines.
forgetting about glass and other types of objects like that
i thought of this from two things
when you put a rivet into a piece of metal it creates tiny cracks
right
those cracks look curvy, jagged or even a lighting bolt shape.
right

now i was watching an airplane crash on TV and in the start of then investigation they thought it was fault of a giant screw or axle (i don't remember), later one guy had looked at it and said this can't be the cause because both of the halves have a straight line cuts, it isn't jagged. then he said that on impact when the plane had hit the ground that, that was it had broken

the object in this example is like a thin 2 by 4 of metal that is being hit by some type of knife
another example is if you look at each molecule as a strong guy all lined up and in the middle of the line you have a little kid(they are all holding each others hands)
now when something hits hits that object
the strong guys can move, transfer some of that energy left or right
so all of the strong guys move the energy to the little kid and he is not strong enough to hold on to the guy next to him so there is a break
now if you scale this up there will be a crack

so is anything i am saying true or am i just talking crazy

 

[mgb_phys]

Crack propagation depends on both factors in the material, inherent stresses and weakness and the direction and rate of the applied load - you can tell a lot from the crack but it is complicated.

 

[noagname]

ok so what i was saying was right depending on the material

 

[Danger]

What you were saying is neither right nor wrong; it's ambiguous.
As Mgb said, there are a lot of different factors that enter into it. One that he didn't mention is temperature. If you hit a bowl of Jell-O with a hammer at room temperature, you'll just end up with a big mess. After a quick bath in liquid nitrogen, however, that same Jell-O will shatter like glass.
The grain structure, size and orientation are critical, as is the molecular bonding strength of the material. I'll tell you one thing for sure, the NTSB guys are expert in what they do. If the guy said something on record, it's because he was dead-nuts certain that it was correct.

 

[Astronuc]

As mentioned, there are a number of factors affecting crack propagation - namely the physical properties of the materials (strength, ductility, . . .), the morphology (i.e. grain/crystal structure, phases, g.b. size and orientation, . . .), and the environoment (stress field, stress/strain rate, temperature, . . . .).

In a collision, parts encounter high loading rates/high strain rates, and there is strain rate hardening in addition to strain hardening. When one looks for a part which failed and was the root cause of the accident, one looks for corrosion products in the areas adjacent to the fracture surface and the for fatigue marks near the fresh crack surface.

The path of the crack (in Mode I) is normal to the stress field and the propagation takes the path of least resistance.

 

[Gokul43201]

Often, the straightness/crookedness of a crack within a given material depends on the strain rate (and hence the crack propagation speed). Cracks that propagate slowly (i.e., slower than diffusion rates in the material, for example, through fatigue) are more likely to be crooked than cracks that propagate rapidly (example: during catastrophic failure). Across materials, the crack propagation path is a very strong function of the microstructural details (precipitates, dislocation density, grain size, grain boundary segregates, etc.).

 

[FredGarvin]

The path of the crack (in Mode I) is normal to the stress field and the propagation takes the path of least resistance.

Excellent point. I forgot about that. My fracture mechanics is more than rusty.

 

[Astronuc]

Often, the straightness/crookedness of a crack within a given material depends on the strain rate (and hence the crack propagation speed). Cracks that propagate slowly (i.e., slower than diffusion rates in the material, for example, through fatigue) are more likely to be crooked than cracks that propagate rapidly (example: during catastrophic failure). Across materials, the crack propagation path is a very strong function of the microstructural details (precipitates, dislocation density, grain size, grain boundary segregates, etc.).

Good points. One has to differentiate between strain controlled crack propagation (limited stress - stress decreases with plastic deformation) and load controlled (unlimited or non-decreasing stress).

Cracks propagate at a maximum speed of approximately one-third the speed of sound in a material, which is about 1500 m/s in most structural metals.

The key phases to cracking are 1) nucleation, 2) sub-critical propagation to critical size and 3) crack propagation after critical size is achieved.

 

[noagname]

wow it sounds like everyone is saying the exact same thing i was saying but with bigger words
sorry i have not bee here in a while and i would just like to say hanks for all of your help