Structural Integrity & Materials Selection

Structural integrity

• the ability of a structure to support a designed load without bending, collapsing, or breaking

Structural failure

the loss of structural integrity,

• created when the material is stressed to its strength limit, thus causing fracture or excessive deformations.
•  

Common types of failure:

1. Fracture:

Brittle vs. Ductile

(a) Very ductile, soft metals (e.g. Pb, Au) at room temperature, other metals, polymers, glasses at high temperature.
(b) Moderately ductile fracture, typical metals
(c) Brittle fracture, cold metals and ceramics.





*brittle fracture → rapid run of cracks through  stressed material.  
*very little plastic deformation
*No warning → worst type of fracture
*Amorphous microstructures (glass) produce shiny brittle fracture surfaces



In crystalline materials:
 transgranular fracture - travels through the grain of the material

  intergranular fracture - crack traveling along the grain boundaries
 2. deformation 

Elastic Deformation:

 - object returns to its original shape

 - Not permanent





Plastic Deformation:

- object becomes permanently deformed

3. Fatigue

The weakening of a material caused by repeatedly applied loads.

 

 

* microscopic cracks form around small discontinuities at the surface & near grain boundaries. 

* cracks eventually reaches a critical size, then suddenly propagates through the remainder of the solid.

 

 

 

 

Fatigue life depends on:

 

• Temperature
• surface finish
• atomic microstructure

 

 

Things to avoid:

 Sharp corners & bends - make everything smooth!

 

 

Example:  90° corner:

Use the probe to point out where the max stress is:

 

 

 

Same loading force, comparison of max stress with and without fillet:

 

 

 

1.411 vs. 0.58 → stress along 90° corner is ~ 2.5 times larger than the stress along the fillet. 

 

 

 

Find where stress is concentrating on your bridge for various loading conditions, then redesign joints to more homogeneously distribute the load.

 

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Fatigue rules of thumb:

 

It can be hard to measure, and get repeatable results on. 

Fatigue usually applies to tensile stresses but can also occur under compressive loads.

 

↑stress = shorter life.

Damage is cumulative & usually permanent.

 

 

Design Criteria:

 

Localized failure should not cause immediate or even progressive collapse of the entire structure.

 

 

 

 

 http://gawker.com/thousands-of-bridges-in-u-s-could-collapse-if-only-sin-509937766

 

 

Test for "critical" spots on your bridge.

Run a FEA sim where you apply a forces to critical spots (at supports and joints).

 

 

Modal Analysis

 

Earthquake? Wind? Vibrating machinery? Vibrating cars & trucks?

 

Modal Analysis solves for natural vibrational tendencies of a structure in the form of mode shapes and frequencies.

 

 

 

https://www.youtube.com/watch?v=Ug2u7sTWrcw&feature=youtu.be

 

 

 

Try just applying one fixed constraint, and then run the simulation without any other applied forces.

 

Use the animate tool to watch it vibrate!

 

 

Display the position of your center of gravity, and notice the influence of the center of gravity on the vibrational patterns in your bridge:

 

View → Center of Gravity

 

 

 

 Notice how everything vibrates around the center of gravity. 

 

 

Further Reading:
Mechanical resonance
Skim through:
List of bridge failures

Think about the main causes of bridge failure, and then decide how to modify your bridge to avoid these types of failures.

Find the mass, volume, center of mass etc. of your part:

 

 

 

 

The larger the volume, the more expensive it is to make. 


What on your bridge is over-designed?  Where can you remove a little bit of material to reduce costs?