*3.4. Thermal Fatigue Tests*

Since samples were heated in a heating furnace, the surface temperature of the samples slowly increased, so there was almost no temperature gradient between the sample surface and interior during heating; however, the samples expanded during heating. When samples were subsequently rapidly cooled, the surface immediately shrank, while the internal temperature was still high. Therefore, the shrinkage of the surface was limited by the internal material, and the sample surface generated tensile thermal stresses, which caused the cracks to expand.

Figure 14 shows the W-N curve which is drawn with the number of cycles (N) taken as the horizontal coordinate and the width of the thermal crack on each sample surface (W) taken as the vertical coordinate.

**Figure 14.** The W-N curve (W is the width of the thermal crack; N is the number of thermal cycles).

It is obvious that the crack width in the untreated specimen after 2000 thermal cycles was 499.21 μm, which was larger than the main crack of the repaired specimens. This shows that the locking unit can effectively prevent crack propagation. An increase in the rate of crack expansion in repaired samples was indicated by the different slopes of their curves in Figure 14, which shows that the crack width increase rates followed the order: No. 5 < No. 4 < No. 3 < No. 2 < No. 1. Figure 14 also shows that the increase in the crack width of sample No. 5 was the smallest, with a value of 118.31 μm. The specimen No. 1 with the largest crack width increment of 412.34 μm is 248.53% larger than sample No. 5.

Specimens T1, T2, and T3 were subjected to go through 0 thermal cycles and 2000 thermal cycles respectively, as shown in Figure 15, which shows that the increase in the crack width increment was obviously different after 2000 thermal cycles and followed the order T1 (57.68 μm) < T2 (118.31 μm) < T3 (150.62 μm) as can be seen from Figure 16. Therefore, it can be concluded that as the distance between units increased, the crack width increased gradually, and the locking effect of units on cracks gradually decreased.

**Figure 15.** The change of cracks width of sample T1, T2, and T3. (**<sup>a</sup>**,**b**) Sample T1; (**<sup>c</sup>**,**d**) sample T2; (**<sup>e</sup>**,**f**) sample T3. (**<sup>a</sup>**,**c**,**<sup>e</sup>**) 0 thermal cycles; (**b**,**d**,**f**) 2000 thermal cycles.

**Figure 16.** The crack width of sample T1, T2, and T3 under different thermal cycles.
