Infrared Evaluation of Heat Generation During the Cyclic Deformation of a Cellular Al Alloy
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Mat. Res. Soc. Symp. Proc. Vol. 591 02000 Materials Research Society
65x55x55mm. A few tests were conducted on narrower samples, 12mm thick. A small elliptical hole (12x4mm) was introduced into the center section by EDM. The minor axis of the hole was slightly larger than the largest cells [1]. Accordingly, the hole provides a well-defined strain concentrator that should localize the onset of fatigue degradation. For testing, both ends of the specimen were thermally insulated. This was achieved using inserts between the specimen and the loading platens. This insulating layer was needed to prevent heat generated by friction at the platens, as well as by the cyclic operation of the testing machine, from diffusing into the test specimen. Testing was performed in a compression-compression mode, using a servo-hydraulic machine subject to an R-ratio of 0.1 at a frequency, A =IOHz [1, 2]. The maximum stress was varied between 0.8 and 0.95 a,,, where r,, is the monotonic plateau stress [1, 7, 9]. Strains were measured using a linear variable displacement transducer (LVDT) and recorded digitally. Images obtained by optical and scanning electron microscopy (SEM) were taken during testing to characterize the deformation phenomena. Thermal imaging of one of the side surfaces was performed using a high resolution infra-red camera (Amber-Galileo) with a 256x256 array and a 30Rjm pixel size. A 50mm IR lens was used to provide a magnification of 0.215 mm/pixel. The surface of the specimen was covered with a black thin film to control its emissivity. In order to monitor small (< 0. 1C) variations in temperature, an integration time of 1.2ms was selected, with 15 images acquired at a rate of 60 frame/s, by employing a digital control system. The temperature was calibrated by replacing the specimen with a black body, which was gradually heated, and the actual temperature monitored by using thermocouples. Comparing the output of the camera with that of the thermocouples related the digital counts to the temperature. MEASUREMENTS AND OBSERVATIONS The cumulative strains, E,,ax(N), exhibited by the specimens with elliptical holes are summarized on fig. 1, using the net-section stress as the load reference. The strain is that remote from the hole. Note that the curves have a form similar to that found for un-notched
-(nt
U)
Z
a
t4
0.08
E E
=1.5 MPa
x 0.06o
.S
1.3 MPa
=1.25MPal®
0.04
or) Z E
.~0.02-
rr
0
Notched
NT
.0i 0
. - -Unnotched
10
102
103
10
4
Number Of Cycles, N Fig. 1 Cumulative remote strain measured at peak load as a function of the number of cycles for three different values of the maximum net section stress, compared with some results from unnotched materials at a similar maximum stress level.
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material [1, 2], including a quiescent phase, followed by rapid strain accumulation beyond a critical number of cycles, NT, and eventual saturation. It is difficult to compare .m.(N) curves for specimens with and without holes because the strains are measured on a different basis. This comparison
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