Structures 2 Flashcards
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
Tensile Fracture (Ductile)
*Classic “Cup-Cone” fracture
*Equalized dimples are a classic example
*Points back to the center where the fracture started
Chevron Marks
Point back to the origin/initiation site (both ductile and brittle)
Beachmarks vs Striations
Beachmarks - can be seen by naked eye; point where the crack was arrested after propagating (towards initiation)
Striations - microscopic; crack growth
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)
Compression Failure (Ductile)
*GPD - cross section area increases - specimen mashes down/squashes/swells up
*Slight trace marks around perimeter
Thin walled structures will BUCKLE
Shear Failure (Brittle)
Material will fail on one of two 45 degrees
Shear Failure (Ductile)
Appreciable GPD - shear deformation (not bending)
Fracture surface will be smooth, almost machined looking
Failures in the vertical direction
Torsional Stress
Primary stress is from shear
Secondary stress is tension and compression normal stress at 45 degrees to shear stress
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
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
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
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
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
What is required for buckling to occur?
Compressive Normal Stress
Buckling Boundary Conditions
*Pinned-Pinned - rotating ends (L)
*Fixed-Fixed - non-rotating ends (1/2L)
*Fixed-Free - Flagpole, single locked (2L)
What kind of stress do aircraft panel carry?
Shear stress (generally)
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
- There must be something to corrode (anodic medal)
- There must be a cause for corrosion (cathodic medal)
- There must be a continuous liquid path (water, salt water, etc)
- 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
