15 Detection of Early Stage Material Damage Using Thermophysical Properties 97 Fig. 15.2 Experimental setup showing the pulsed thermography. The test sample is a rectangular carbon-epoxy laminate with a centre (fastener) hole. Two thermocouples are attached at the front and back near the centre hole in the mode-I direction. Surface temperatures are measured after the application of the high intensity flash at the front of the specimen The fatigue test specimen is loaded on a hydraulic servo controlled uniaxial test frame and is subjected to a tension-tension sinusoid at a frequency of 5 Hz and load ratio¢min/¢max D0.1. The test specimen is [(˙45 o) 2/0 o]s symmetric laminate with a thickens of 0.128 in (3.25 mm). The peak load of the sinusoid Pmax D20kN. During data acquisition, the cyclic load is paused periodically at predefined cycle intervals starting from 100 cycles through end of test at 150 k cycles. At each pause, the specimen is allowed to cool down to room temperature so as to bring the specimen to the same initial energy state during each successive pause while allowing for the dissipation of viscoelastic heating before probing the thermal properties. After the specimen is cooled to room temperature ( 22 ıC), the front side is instantaneously heated with a high intensity flash followed by temperature measurements by two thermocouples attached at the front and back sides the rectangular specimen and IR imaging. 15.4 Results and Discussions Figure 15.3 shows the readings of the front and back thermocouples at after the application of a high intensity flash at the given fatigue cycle. Each curve denotes the evolution of (T-t) at specific number fatigue cycles. The plots are color mapped from blue (low cycle count) to red (high cycle count). Fig. 15.3a shows the temperature decay at the front of the specimen, Fig. 15.3b shows the transient temperature increase at the back of the specimen while Fig. 15.3c shows the relative changes in the surface temperature at the back and front of the specimen. Immediately after the application of flash, the temperature of the front surface increase by about 4-ıF. After a brief delay the temperature of at the back increases as the heat conducts through the thickness. The data shows that the (T-t) evolution is well correlated to the number of fatigue load cycles that the specimen has absorbed at any given time. Notice the clear trend of the color mapped lines with blue (low cycle) lines clearly separated from the red (high cycle) lines. As N increases, the specimen appears to lose the input energy, i.e. the external heat imparted by the flash, rather very quickly. The data from the back thermocouple shows that, the peak recorded temperature at the back of the specimen drops sharply as the fatigue cycles continue to increase. A possible explanation for this observation is as follows: the process of accumulation and development of fatigue damage is associated with crazing and microcracking, which is determined by the processes of initiation, motion, generation, and merging of point defects. The density of microcracks grows with increase of the loading cycles N. High density microcrack leads to loosening of material, primarily around the interface and in/around regions of high stress concentration in the matrix. This creates higher thermal resistance in the transverse direction where the heat is supposed to flow. The high transverse thermal resistance (created by the microcraks and loose material) forces the heat transfer possibly along the fiber direction (high thermal conductivity). A comprehensive explanation of the observed heat flux phenomena, however, is yet to be established.
RkJQdWJsaXNoZXIy MTMzNzEzMQ==