| Steel structure is widely used in all walks of life with its unique advantages,and can be seen everywhere in the field of building engineering.Particularly,Q355 B steel is the most commonly used material for building structures.Under the action of fatigue load,the steel structure may suffer from fatigue damage due to material surface damage and internal defects.Fatigue damage is mainly caused by the formation and propagation of cracks in steel materials.Cold spraying technology is to use high-speed gas to drive metal particles to hit the metal surface to repair cracks and strengthen the local areas,which can effectively define the stress distribution and microstructures of metal damage location,so as to improve the resistance of crack growth and improve the fatigue life.At present,cold spraying technology hasn’t been widely applied in the field of steel structures.Research methods and means are also relatively limited and require considerable investment of resources.In this paper,cold spraying technology on theory of the fatigue crack propagation characteristics and its working mechanism,as well as the reasonable and effective cold spraying process parameters,were experimentally studied with Q355 B steel substrate.Fatigue growth test was conducted on the standard single-notched 3-point bending specimens to analyze the effects of cold spraying technology and coatings on the fatigue behaviors of substrate.Combined with the fracture mechanics and micro & macroscopic fracture morphology,the life extension mechanism of whole-process fatigue propagation was analyzed.The specific and primary research contents and results are as follows:(1)Based on the fundamental theories of linear elastic fracture mechanics and elastic-plastic fracture mechanics,the effects of surface grit-blasting and subsequent cold sprayed metal coatings on the fatigue crack propagation performance parameters of 3-point bending specimens was analyzed,including the stress intensity factor at crack tip K,the fatigue crack growth threshold Kth,crack growth rate da/d N and crack closure effect.According to the Paris formula,the fatigue crack growth life estimation formula of 3-point bending specimens under the combined action of fatigue external load and cold spray coatings after grit-blasting was derived.(2)The mechanical properties of Q355 B steel substrate were tested,and the cold spraying Al,A6061,Cu and 316 L powder particle sizes were analyzed to ensure that the test requirements were met.Polishing and grit-blasting were adopted to treat the substrate surfaces,and then the surface roughness and hardness were measured to provide parameter basis for cold spraying.Cold spraying test was carried out according to the selected parameters and the surface hardness of deposited coatings was measured.SEM was used to observe the microstructures and estimate the cross-section porosity of coatings and surficial combination quality between substrate and coatings.Meanwhile,EDX was conducted to analyze the coatings’ element distribution.Finally,the bonding strength of substrate to coatings was measured by tensile test.(3)Fatigue growth test was conducted on the 3-point bending fatigue specimens with prefabricated cracks.Crack length was read with microscope during the test.According to the fitting da/d N-ΔK relation curves,the fatigue growth performance and growth lifetime of specimens respectively treated by multi-size grit-blasting and cold-sprayed after grit-blasting were estimated.Meanwhile,the crack tip opening displacement CTOD was used as the evaluation parameter to analyze the experimental relationship curves of da/d N-CTOD and CTOD-N,and the same conclusion was obtained as the stress-intensity factor amplitude criterion.(4)SEM was used to observe the macroscopic and microscopic morphology of fatigue fracture.The macroscopic morphology observation areas include prefabricated crack propagation area,stable propagation area,stable propagation and transient fault boundary area,transient fault area,and the morphology indicates that grit-blasting or the subsequent cold spray coatings is beneficial in preventing fatigue crack growth.The microscopic morphology observation areas include prefabricated crack propagation area,stable propagation area and transient fault area,and the morphology of fatigue step,secondary crack,fatigue striation,dimples reveals the improvement mechanism of fatigue crack propagation by grit-blasting and cold spray coating. |
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