Fig. 1:
IrT setup according to [6]
This infrared radiation is detected by the camera, which is connected to
a processing unit that converts the radiation data into thermal images.
An optional thermal stimulation unit is needed for active IrT methods.
The method of passive IrT measures the temperature variations on the
object’s surface. Internal conversion processes for instance, that go
along with (local) temperature changes can be detected measuring the
attending changes in infrared radiation. Thereby, temperature hot spots
resulting from crack formation and crack growth processes for instance
can be detected. For more detailed information on IrT see [6].
Active IrT methods on the other hand use external excitation units for
the thermal stimulation of the specimen. Optical LT for instance uses an
external heat source, denoted as optional thermal stimulation in Fig. 1,
introduces a heat flow into the specimen. The energy flow moves through
the specimen and is partly reversed at material boundaries, such as
voids or similar. The reversed energy flow is detected by the IrT camera
system. Local differences in infrared radiation can thereby be
visualized. This way, thermal images can be used to find subsurface
irregularities.
2.2. Test Description
Two test series are performed, one under quasi-static and the other
under fatigue loading. For each test, three different configurations are
tested: specimens without patch, specimens with patch monitored from the
metallic side and specimens with patch monitored from the patched side.
The infrared camera used is of type ImageIR R 8380 S (InfraTec GmbH,
Dresden) in combination with a tele zoom lense (\(f=50\) mm). The test
equipment used is a tensile and compression machine of type Instron 5567
(Instron, Darmstadt) for the quasi-static tests and a servo-hydraulic
test machine of type Schenck POZ 160 with a static nominal load of
+/-160 kN. For the quasi-static tests, a test velocity of 9 mm/min and a
monitoring frame rate of \(f=200\ \)Hz is used. The fatigue tests are
performed with a stress ratio \(R=0.1\), mean load \(F_{m}=4000\) N,
load amplitude \(F_{a}=3272\) N and a monitoring frame rate\(f=100\) Hz. To reduce data, recording is done every 10000 load
cycles for about 5 seconds.
All specimens are made of aluminium 5083 with a thickness of\(h=3\) mm. The patch material used is a 0/90 epoxy-based carbon fibre
reinforced plastic prepreg (SGL Technologies GmbH of type
C W410-TW2/2-E323/42%/6k and a ply thickness of \(h=0.6\ \)mm).
Specimen geometries and dimensions are given in Fig. 2.