# Accumulative Roll Bonding--A Review
## 4 Properties
### Fatigue  
The response of materials under cyclic loading is one of the main materials selection criteria in both engineering and biomedical applications. Investigation of fatigue response of ARBed materials is, therefore, critical for validating their reliable use in various applications. In general, the fatigue limit has been recognized to be half of the tensile strength in smooth specimens ([PERSON] and [PERSON], 1991; [PERSON] and [PERSON], 1992; [PERSON] and [PERSON], 1993). Fatigue limit and fatigue strength usually become greater when the tensile strength improves. On the other hand, the dependence of the fatigue limit on the tensile strength is not necessarily linear and monotonic, so one has to prudently check the fatigue limit and strength when the tensile strength jumps markedly. As discussed in the previous section, ARB successfully enhances the tensile behavior of metallic materials, nonetheless, the fatigue crack resistance of the ARBed materials has rarely been investigated. [PERSON] and [PERSON] (1993) investigated the cyclic deformation behavior of UFG Cu processed by ARB and found that the conventional equiaxial grains are able to accommodate a fraction of the applied cyclic plastic strain through the development of slip bands within them due to the composite nature of the as-received ARBed Cu. Furthermore, grain coarsening was observed when the samples were subjected to cyclic loading both in load controlled and strain controlled tests. It was assumed that grain coarsening ultimately cause cyclic creep and cyclic softening behaviors observed via the increase in volume fraction of large-sized grains with a capability of developing slip bands within them. Figure 21 shows a S-N diagram of the ARBed AA1050A/AA5005 composites with different ARB cycles and monolithic reference materials ([PERSON] and [PERSON], 1993) wherein the fatigue life of the N4 composite (i.e., composite with 4 ARBed cycles) is very similar to the N8 composite at higher strain amplitudes. At smaller strain amplitudes, however, the N8 sample has significantly higher fatigue lives. Fatigue lives and damage mechanisms of both the N4 and N8 samples depend on the number and thickness of layers and thus the number of cycles, particularly at the lower amplitudes, where a noticeable grain coarsening of the AA1050A occurs. At higher amplitudes, the N8 sample is slightly shorter than the N4 sample whereas the fatigue life of the N8 one is considerably higher than the N4 samples at lower amplitudes. This can be attributed to the different crack growth behavior of the composite at different amplitudes. The fatigue life of the N12 composite is significantly improved compared to the N4 and N8. The samples fail at an equal number of fatigue cycles as the UFG AA5005 monomaterial sheet. Their evaluations revealed that the microstructure and the hardness of N12 composite is rather similar to that of the AA5005 monomaterial; thus, it is not surprising that the fatigue lives are rather similar as well. At higher total strain, the maximum stress in the AA1050A layers increase to magnitudes more than the yield strength of AA1050A; therefore, AA1050A experiences plastic deformation whereas the AA5005 layers does not. Besides the fatigue strength, [PERSON] et al. [138] emphasized on the fatigue crack initiation and growth in ARBed IF steel. As the hardness increment has been empirically understood to enhance fatigue strength, they expected a positive effect of UFG microstructures on the fatigue crack resistance in terms of crack closure. Plasticity-induced crack closure (PICC), roughness-induced crack closure (RICC), transformation-induced crack closure, oxidation-induced crack closure, etc are some of the mechanisms of the fatigue crack closure; however, the degrees of PICC and RICC changes with UFG refinement. PICC occurs due to the formation of compressive residual stresses in the plastic wake while RICC is due to the deflection of crack paths and is related to the morphology of microstructures and grain sizes.