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Crack tracker

APPLICATIONS

  • For homogeneous and electricity-conductive materials.
  • Measure of the length of through cracks making their way in thin metal sheets.
  • Determination of the shape of the front end of a crack, inside thick specimens (using a three-dimensional measurement is possible, in many cases).

DESCRIPTION

Cracking condition of structures is an essential characteristic used in fracture mechanics and it has to be determined during tests.
The 'CRACK TRACKER' instrument has been specially designed for that purpose. It applies a non-destructive electrical method, related to the so-called potential method that relates the length of a crack to the potential difference V existing between two significant points placed on both sides of the crack, a current I following the specimen.

TECHNICAL FEATURES

The 'CRACK TRACKER' instrument ensures following functions:
  • Powering the specimen: through constant-amplitude pulses during about 20 ms.
  • Internal or external triggering to synchronize measures from the force signal originated by the testing machine.
  • Adjustable amplification, from 100 to 20,000, and measure of the voltage, from the specimen.
  • Memorization of two successive measurement signals (20 ms between them). The voltage V is then determined: V = V1 – V2. It is quasi-static interference-free and is the required voltage. These operations are performed for each pulse.
  • Noise attenuation: limitation of the bandwidth of the amplifier, to 100 hz.
  • Elimination of noises caused par the mains, because two memorizations are performed, every 20 ms, which is the mains period.
  • Low-pass filtration on output voltage V: used to get a sensibility of 0.1 V for a response time of 1 second.
Implementation
The experimentation proceeds as follows:
  • On time t0 (the force signal synchronizes the measure), an Imax current is applied to the specimen (electrically insulated from the machine).
  • On time t1 = t0 + t1 (t1 being compatible with the time constant of the amplifier, i.e. 20 ms) the amplified potential difference V(t1) is memorized, then the current is switched off.
  • The residual potential difference V(t2) is memorized on time t2 = t1 + t2 (t2 is generally considered as equal to 20 ms to free from any possible interference voltage caused by the mains at 50 Hz).
  • The useful potential difference V = V(t1) – V(t2) is then measured.
  • The calibration of the result V(a) relates voltage V to a track length to measure, using a conventional mean and according to given experimental conditions (current, gain, temperature, etc.).

The reproducibility of results makes possible, for any test performed in the same conditions, to deduce ‘a’, knowing ‘V’. About calibration, some simple specimen geometric shapes (rectangular-shape tensile panels or bending bars) lead to an analytic calculation which experience correctly validates. The applied electrical method is made accurate by using the original procedure that consists in a dual memorization of potential difference, synchronized with the tensile load, for selected levels, in order to completely open the track.
Practically, this procedure leads to:
  • An insensibility to thermo electrical effects, and to noises and amplification drifts.
  • A reduction of the effective current intensity that goes through the specimen, thus of its heating and of corrosion effects.
  • To make sure that the crack length measured is not affected by contact effects between both sides of the crack.
Main characteristics
Current intensity imposed in the specimenAdjustable from 0 to 20A
Current stability10-3
Measuring voltageFrom 0 to 10V
Available output current5mA
Response time of the measuring way0.2 to 1s
Gain100 to 20,000
Dimensions Length: 455.5 mm – Height: 132.5 mm – Depth: 430 mm

Applications
  • Cold bending test, thin metal sheets (150 x 25 x 2 mm) made from AU2GN and AU4G1-T3, sensibility: 0.001 mm.
  • Cold tensile test on light-alloy specimen, bifurcation tests on biaxial testing machine.
  • Hot bending test at 1,000°C, thin metal sheets (150 x 25 x 2 mm) made from IN100, sensibility: 0.05 mm.
  • 4-point cold bending on light-alloy specimen (450 x 45 x 45 mm) with corner cracks (3D case).