###### Abstract
The different manufacturing conditions directly affect the surface state of the parts (surface texture, surface morphology, surface residual stress, etc.) and affect the final performance of the workpiece. In particular, with reference to AM, its benefits are offset to a certain extent by the poor surface finish and high residual stresses resulting from the printing process, which consequently compromise the mechanical properties of the parts, particularly their fatigue performance. Ultrasonic impact treatment (UIT) is a surface modification process which is often used to increase the fatigue life of welds in ship hulls and steel bridges. In Reference [2], the benefits of ultrasonic impact treatment (UIT) on the fatigue life of Ti-6 Al-4V, manufactured by direct metal laser sintering (DMLS), were illustrated. Results showed that UIT enhanced the fatigue life of DMLS Ti-6 Al-4V parts by suppressing the surface defects originating from the DMLS process and inducing compressive residual stresses at the surface. At the adopted UIT application parameters, the treatment improved the fatigue performance by 200%, significantly decreased surface porosity, reduced the surface roughness by 69%, and imposed a compressive hydrostatic stress of 1644 MPa at the surface. On the other hand, with reference to milling technology, which can process parts of different quality grades according to the processing conditions, it is of great significance to reveal the mapping relationship between working conditions, surface integrity, and part performance for the rational selection of cutting conditions. In Reference [3], the effects of cutting parameters such as cutting speed, feed speed, cutting depth, and tool wear on the machined surface integrity during milling were thoroughly reviewed. At the same time, the relationship between the machined surface integrity and the performance of parts was also revealed. Furthermore, problems that exist in the study of surface integrity and workpiece performance in the milling process were pointed out with the final suggestion that more research should be conducted in this area in the future. When considering dentistry applications of newly advanced materials, one issue is related to the proper adjustment of crown implant abutment during installation. In Reference [4], the fracture resistance and stress distribution of zirconia specimens were compared considering four occlusal surface areas of implant abutment. Four implant abutment designs with 15 zirconia prostheses over the molar area per group were prepared for cyclic loading with 5 Hz, 300 N in a servo-hydraulic testing machine until fracture or automatic stoppage after 30,000 counts. Four finite element models were simulated under vertical or oblique 10-degree loading to analyse the stress distribution and peak value of zirconia specimens. Data were statistically analysed, and fracture patterns were observed under a scanning electron microscope. Cyclic loading tests revealed that specimen breakage had moderately strong correlation with the abutment occlusal area (\(r=0.475\)). Specimen breakage differed significantly among the four groups (\(p=0.001\)). The lowest von Mises stress value was measured for the prosthesis with a smallest abutment occlusal surface area (SA25) and the thickest zirconia crown. Thicker zirconia specimens (SA25) had higher fracture resistance and lowest stress values under 300-N loading. The second part of this Special Issue is concerned with traditional materials but under complex fatigue conditions, like those generated in rails and wheels undergoing rolling contact fatigue with consequent crack initiations. Such cracks then develop under non-proportional mixed mode I/II/III loading, whose assessment represents a challenge for scientists involved in railway accident prevention.