Mechanical Properties 1a Strain

Buffl

Engineering Properties of Materials

by Niklas V.

Name Techniques to measure strain and displacement.

Potentiometers (or variable electrical resistance transducers)

Linear (or Rotary) Variable Differential Transformer (LVDT or RVDT

Strain-gages and extensometers:

• resistance (bonded) strain gages (metals or semiconductor)

• resistance extensometers

• optical extensometers

Optical field techniques:

• photoelasticity

• Moirè interferometry

Describe two variants of a Potentiometer.

What challenges faces the resulution of potentiometers?

The resolution is limited by the number of wire turns per unit distance

On which principle relies the Linear Variable Differential Transformer (LVDT) to measure displacement?

(give the formula)

Electro magnetic induction

the change in magnetic flow over time:

Describe the voltage output of the Linear Variable Differential Transformer (LVDT).

Name the typical Voltage and Input frequency of a Linear Variable Differential Transformer (LVDT)

1-10V

50Hz-25kHz

Name the advantages and disadvantages of a Linear Variable Differential Transformer (LVDT)

pro:

very robust
limited possibility to overload
unsensitive to temperature
no amplification for output needed
reuseable
cheap

contra:

low resolution (compared to strain gauges)
moving mass can produce errors due to inertia

Which material classes are there for strain gauges?

metallic strain gauges
semiconductor strain gauge

How does the Principle of a Strain Gauge work? (basic)

The wire is deformed by the stress and due to the deforming of the geometry the resistivity changes. From this change one can directly measure the change in strain.

What is the relationship between strain and resistivity in a strain gauge?

R as the resistivity

rho as the resistivity (=spez. Wiederstand)

epsilon x as the strain

How do you call an array of strain gauges in different directions and what are the common orientations?

Strain rosette

0, 45, 45 degree

30, 60, 60 degree

What are advantages and disadvantages of semiconductor strain gauges compared to metal strain gauges?

pros:

miniaturized
higher fatigue resitance
lower hysteresis

cons:

non-linear output
lower strain range
temperature dependancy

What is the temperature dependancy of a semi conductor strain gauge?

In which kind of circuit are strain gauges normally wired?

Wheatstone bridge (or half bridge)

How does the output voltage calculate from the input voltage in the wheatstone bridge of a strain gauge?

E for Voltage Ei=input voltage, E0=output voltage

By what optical phenomenom can strain be optically measured in plastics?

by the change of polarization

What is the stress optic or brewster law? Name the components.

delta=relative retardation between two lightbeams

n_x,y=indices of refraction

K=strain optical coefficient

epsilon=strain

How is the experimental arangement for optical strain measurement called?

Plane Polariscope

How can strain directions be determined optically throughout the body?

The variations in strain produce a pattern of visible fringes, which are called isoclines. They are used to determine the stress states.

The strain at the fringes is 0

How can optical stress analysis be conducted for non opitcal conductive bodies?

constructing a model from a translucent material
coating in a photoelastic material

What can be analyzed with optical strain analysis in fibre commposite materials?

fiber buckling can be determined
fiber fragmentation can be determined

What is the parameter of a moire grating?

the pitch, which gives the lines per milimeter

How can strain be measured by a moire grating?

Two gratings are applied to a body. One unconstrained on top of the other. The change in strain is deforming the lower grating. Fringes can be seen at integer multiples of the original dimension. S is the distance between two fringes, p the original dimension, p’ the straine dimension and N an integer.