Online proceedings - EDA Publishing Association

7-9 October 2009, Leuven, Belgium
electromagnetic actuator (B) is connected to modulate driving
DC-Mean value (M)
force. The point of force application is variable, so that a scaling of
force is adjustable from 1 / 10 to 1 / 3.
The driving force can be measured with a load cell placed in the
load path and also by measuring the electrical current of actuator.
30 µm
To measure the strain amplitude directly at the sample a
1 mm
displacement transducer (C) is placed.. With this setup it is possible
to load the sample (A) mechanically and observe the specimen by
IR-camera (E) at a narrow working distance of about 20 mm. An
external PC controls and measures all important values. Both force
and displacement controlled experiments can be conducted.
120 µm
Dynamic loads (triangular, sinusoidal and pulsed) with frequencies
1 mm
up to 5 Hz and forces to 450 N on the sample can be realized.
So it is possible to build a cost burden system.
In a laser safety enclosure this design enables crack tracing by
means of laser pulse excitation.
150 µm
II. EXPERIMANEL RESULTS
Loading amplitude
1 mm
Phase image(P)
1 mm
1 mm
1 mm
In an initial investigation CT-specimen of PMMA were loaded
sinusoidal, with different amplitudes (30, 120, 150 and 250
microns) at a frequency of 0.5 Hz. In figure 4 the observed
(correlation of reference signal and image signal) uniform surface
cooling at the notch groove is shown. There are blue to white areas
of cooling and red to bright yellow areas of warming. The result is
similar in quality as the estimation given in figure 1 (right).
250 µm
1 mm
1 mm
Fig. 5 Phase images and DC-mean value image for different loadings
Figure 5 shows a summarized comparison of results of initial
investigation. The load amplitude was varied from top to bottom
(30 ... 250 microns). The respective left-hand images show the DC
average images. They show qualitatively similar results except of
the smallest load amplitude. Lower DC values are founded with
increasing load. This is expected
by increasing the stresses (σ 1 , σ
2 , σ 3 ) and consequently an improved cooling too. Much more
sensitive are the phase images shown on the right. They are from+
π to - π (3.12 ...- 3.13). Negativee values mean a lagging signal with
respect to the reference signal (here, the sinusoidal stress
amplitude). This behavior is by
equation (1) expected. In tensile
(positive reference signal is the sum of principal stress is positive
and thus results in a negative Δ T. This means a negative surface
signal. Both signals are phase-shifted by 180 ° (-π). This
relationship is clearly seen in the
diagram in Figure 6.
Fig. 4 CT-specimen of PMMA loaded with 30 µm
(without pre crack)
To increase the stress concentration a crack
was introduced with
an industrial razor blade. Taking into account the resulting pixel
resolution of approximately 5.2 microns / pixel of IR-images
a ~ 420 microns crack length can estimate.
surface signal
6450
6440
6430
6420
6410
6400
0,04
0,02
0,00
reference signal
6390
0 100 200
index
300 400 500
of image
Fig. 6 Phase shift between reference signal (displacement transducer)
and surface signal (250 µm load amplitude)
©EDA Publishing/THERMINIC 2009 93
ISBN: 978-2-35500-010-2