Structures 2 Flashcards by Matthew Gregoire

Tensile Fracture (Brittle)

*Very little necking or deformation
*Fracture surface normal to tensile strength
*Bright sparkly granular surface
*Failure is at a 90 degree angle to the applied tensile load

Small 45 shear lip all around perimeter

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Tensile Fracture (Ductile)

*Classic “Cup-Cone” fracture
*Equalized dimples are a classic example
*Points back to the center where the fracture started

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Chevron Marks

Point back to the origin/initiation site (both ductile and brittle)

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Beachmarks vs Striations

Beachmarks - can be seen by naked eye; point where the crack was arrested after propagating (towards initiation)

Striations - microscopic; crack growth

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Compression Failure (Brittle)

Can’t fail due to (-) normal stress, but can slip on planes 45 degrees to normal stress - fails on SHEAR plane

Pure shear transforms to compression and tension… and the tension can cause cracks PARALLEL to the compressive strength

Key clue: “blossomed” out (wood is normally brittle)

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Compression Failure (Ductile)

*GPD - cross section area increases - specimen mashes down/squashes/swells up
*Slight trace marks around perimeter
Thin walled structures will BUCKLE

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Shear Failure (Brittle)

Material will fail on one of two 45 degrees

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Shear Failure (Ductile)

Appreciable GPD - shear deformation (not bending)

Fracture surface will be smooth, almost machined looking

Failures in the vertical direction

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Torsional Stress

Primary stress is from shear

Secondary stress is tension and compression normal stress at 45 degrees to shear stress

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Torsional Fracture (Brittle)

*Max normal stress plane is oriented 45 degrees to angle of shaft on outside
*Fractured surface is Helix - like a screw thread
*Use FBD to determine which end of object failed first

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Torsional Fracture (Ductile)

*GPD: twisting of shaft
*Fracture surface is flat, normal to axis of shaft, and smooth, almost machined looking
*“Trace marks” are concentric
*Small projection in center due to zero shear at that point

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Bending Fracture (Brittle)

*Results from positive (tensile) normal stress
*Crushing due to high compressive stresses is possible
*Chevrons determine direction of crack propagation
*Shear lip tells us we have bending instead of tensile fracture

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Bending Failure (Ductile)

*GPD: bending deformation, necking on tension side, “orange peeling”, bulging on compression side
*Chevrons in middle
*“Brinelling” - shiny spot as pieces touch and rub when coming apart

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Buckling

A process occurring at or above a “critical buckling load” in which relatively large, non-proportional changes in stresses and deflections result from small changes in load or in the effective point of load application

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What is required for buckling to occur?

Compressive Normal Stress

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Buckling Boundary Conditions

*Pinned-Pinned - rotating ends (L)
*Fixed-Fixed - non-rotating ends (1/2L)
*Fixed-Free - Flagpole, single locked (2L)

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What kind of stress do aircraft panel carry?

Shear stress (generally)

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Column behavior with buckling

Column bows to the side - will not accept more load after buckling

Plate behavior with buckling

Still can carry load after buckling (skin on aircraft, panel)

Shell behavior with buckling

Shed the load after buckling

Where does buckling occur during torsional loads?

Across the compressive stress (driveshaft, soda can)

Stress Concentration

*Commonly called a “Stress Riser”
*A condition in which high localized stresses are produced as a result of an abrupt change in geometry
*Anything that puts a defect in material (either by design or by accident (nicks and dings))

Stress Risers

*Holes and cutouts (windows/doors, rivets)
*Toolmarks (nicks, dents, scratches)

Macroscopic Stress Risers

*Corrosion pits
*Abrupt cross-sectional changes (sloped better than right angle)

Stress Concentration Formula

Local Stress = Normal Stress * Stress Concentration Factor (K) K=3 for circles

Elliptical Crack Theory

Crack parallel to tension (narrow width) - a=0, K=1 Crack perpendicular to tension (wide width) - a/b=infinity, K=infinity

Ways to reduce Stress Concentration Factors

*Radius of curvature - make as large as possible *Reduce the abruptness *Stop Drilling

Which direction do you blend nicks in blades?

Always blend with tension field

Fatigue definition

Progressive, localized, permanent damage to a structure due to tensile strains

When do fatigue cracks typically appear?

40-60% of fatigue life

What causes fatigue?

*Vibrations *Rotating parts/cyclic loads

Requirements for Fatigue

*Cyclic Stress *Tensile Stress *Plastic Strain

S-n Diagrams

Predict how many cycles it takes to reach prescribed fatigue stress levels

Endurance Limit

A stress level below which the fatigue life is infinite Steel and Titanium - Endurance Limits Aluminum and Copper - no endurance limit

Stages of Fatigue

*Crack Initiation - point of maximum stress, usually 45 degrees to normal stress *Crack Propagation - parallel to max normal stress, smooth surface but not shiny *Instantaneous/Overload Zone - zone where the load divided by the remaining area exceeds the materials ultimate strength

Service Life Extension Program (SLEP)

Fatigue test aircraft to 2x the requested extension Must lower aircraft limits to meet desired extensions

Stop Drilling

Fill a hole with a rivet; reduce concentration load from 3 to 1.3

Brittle vs Ductile Failure

Brittle- fail when normal stress is exceeded; fails on max shear plane Ductile- fail when shear stress is exceeded; fail on max normal shear plane

Ways to prevent corrosion

*Calendar day inspections *Sealant: primer - paint *Electroplating *Cladding

Types of corrosion

*General - metal becomes oxidized (gives up electrons to oxygen) ie. rust *Electrochemical (Galvanic)

Electrochemical Corrosion requirements

*Must have two dissimilar metals *Must be a conductor to carry electrons from anode to the cathodes (closer together on the Galvanic chart, less chance for corrosion)

Conditions for corrosions

1. There must be something to corrode (anodic medal) 2. There must be a cause for corrosion (cathodic medal) 3. There must be a continuous liquid path (water, salt water, etc) 4. There must be a conductor to carry the electrons from the anode to the cathode (structure, metal to metal connection)

Striations lie between beachmarks for what type of failure?

Fatigue failure

Knife-edges cause what types of crack?

Fatigue crack Fill with rivet, at least 0.01 gap