03 Material Characterization and Mechanical Properties

Which kind of testing methods are there?

• Destructive testing: characterization of material properties and validation of component parameters in test & design phases, control

• Non-destructive testing: quality assurance, inspection

Examples of destructive testing (6)

• Tension test

• Compression test

• Bending test

• Hardness test

• Creep test

• Fatigue test

Examples of non-destructive testing (3)

• Ultrasonic inspection

• X-ray

• Acoustic emission

Usual loads for material characterization (5)

• Tensile

• Compressive

• Shear

• Torsional

• Bending

Are testing methods standarized?

• Yes, normally by DIN or ISO

What are isotropic materials?

• Materials with the same properties in every direction

Whar are orthotropic properties?

• Properties that vary as a function of the direction

What is the setup for a tensile test? (6)

1. Electro-mechanical test frame with moveable traverse and gauge to determine position s

2. Material specimen

3. Gripper

4. Mechanical extensomenter/strain gauge/optical extensometer to measure ΔL

5. Load cell to determine F

6. Data acquisition device over time t

Which parameters can be read from the Stress-Strain-Diagram? (7)

• Modulus of elasticity (E)

• Proportional limt

• Yield strength

• Ultimate tensile strength

• Elastic strain (strain in elastic (linear) region)

• Plastic strain

• Elongation to failure

What is the elastic region?

• Region in which deformation is reversible (no permanent deformation after loading)

What is the plastic region?

• Region in which deformation is permanent

What is the ultimate tensile strength?

• Point in which σ is max.

What is the breaking strength?

• σ close to failure of specimen

What is the elongation to failure?

• ε at breaking strength

What is the yield strength?

• σ at the elastic limit (σ withstand by the material without undergoing plastic deformation)

How does brittle fracture look like?

• High E and abrupt break at (yield point, elastic limit)

How does a ductile fracture without yield point look like?

• Continuous graph without oscillations and eventual break in plastic region

How does a ductile fracture without yield point look like?

• Continuous graph with oscillations aroud yield point (between lower and upper yield strength) and eventual break in plastic region

Ductile vs. Brittle fracture

Where does the neck appear in the σ-ε-Diagram?

• After the ultimate strength

What are the regions of the σ-ε-Diagram?

How are aerospace materials ordered in a σ-ε-Diagram? (4 by E)

• Magnesium Alloy < Aluminium Alloy < Titanium Alloy < Carbon-Carbon (brittle) < Carbon-epoxy composite (brittle) < High-strength Steel

What is the relation between temperature and σ-ε-Diagram?

• The higher the temperature, the more ductile the material

• The lower the temperature, the more brittle the material

How is hardness defined?

• Resistance of a material to penetration by another materials

Which methods are used to test hardness? (5)

• Brinell (HB)

• Vickers (HV)

• Rockwell

• Knoop (HK)

• Shore

What are general characteristics of hardness tests?

• Only slightly destructive (small indentation)

• Inexpensive

• Used for quality assurance in production lines (deviations in composition/production is detectable in the grade of hardness)

What is the indenter of every main hardness test?

• Brinell: sphere

• Vickers: pyramid

• Knoop: pyramid

• Rockwell: cone (double sphere)

Pros and cons of Brinell:

• Pros:

  • Measure medium hardness for heterogenous materials

• Cons:

  • Not for thin layer materials

  • For hard materials the load is too large

Pros and cons of Vickers:

• Pros:

  • Wide range of materials

  • Thin plates

  • More precise than Brinell

Pros and cons of Rockwell:

• Pros:

  • Quick and automatable

• Cons:

  • Small indenter, sensitive local effects

  • Reduced accuracy at high hardness

  • Various scales

What is creep?

• Tendency of a solid material to slowly and permanently deform under the influence of constant mechanical stresses over extended periods, often at elevated temperatures.

How is creep displayed normally?

• With a creep curve (elongation as a function of time (logarithmic) for a given temperature)

• With the creep strength (withstood stress as a function of time (log) for a given temperature)

What is fatigue?

• Progressive and localized structural damage that occurs when a material is subjected to cyclic or fluctuating stresses or strains

• Like creep but due to cyclic loads and not to constant loads

• Ex: bending repeatedly a plastic fork until it breaks

Through which load cases is fatigue tested? (7)

• Pulsating compression load (σ_max < 0; R > 1)

• Pure pulsating compression load (σ_max = 0; R > -infinite)

• Alternating load (σ_m < 0; -infinite < R < -1)

• Pure alternating load (σ_m = 0; R = -1)

• Alternating load (σ_m > 0; -1 < R < 0)

• Pulsating tension load (σ_min = 0; R = 0)

• Pure pulsating tension load (σ_min > 0; 0 < R < 1)

What is described by the Wöhler Curve?

• How the stress withstood by an specimen decreases as a function of the number of cycles of the applied load for a given load case

How does an impact bending test works?

• A standarized specimen is stricken with a pendulum hammer

• Suitable for comparative assessment only

• High impact → ductile fracture

• Low impact energy → brittle fracture

SQ: What information do you get out of the uniform plastic elongation zone which you get out of a tension test?

• Yield point (initial point)

• Ultimate strength (final point)

SQ: Is it possible to use the Brinell hardness with thin sheet metals? If yes, under which circumstances?

• In general, no. Only if s >= 8h (thickness of sheet bigger or equal to 8 times the indentation depth)

SQ: How does a yield strength behave with temperature?

• Is lower for higher temperatures

SQ: Which types of test methods are available?

• Destructive

• Non-destructive

SQ: How does a typical test method work?

• Setup

• Initial, standarized, known conditions

• Physical relations

• Measurements

• Calculations

• Output