Recent advances in the manufacturing field have led to the development of additive manufacturing technologies. This can be attributed to the high demand of part accuracy and simple techniques for manufacturing more complex geometries. Presently, several methods such as laser beam melting have been developed for additive manufacturing of metals. Unfortunately, due to the involved complex thermal cycles, postprocessing like heating and surface treatments are necessary to achieve the desired surface finish and properties. This normally results in material defects that may adversely affect structural performance. Consequently, structural durability is an important design consideration in enhancing the performance of structural components.
Effective structural fatigue and defects analysis are deemed as promising solution in improving the properties and performance of additive manufactured components. Therefore, studies involving defects formation mechanisms will be of great significance in predicting the fatigue strength in component structures. Alternatively, recent studies showed that fatigue strength is highly influenced by the defect properties such as size and lack of fusion geometries. Unfortunately, axial fatigue strength, especially in additively manufacture maraging steels, have not been fully explored.
To this note, Researchers at the University of Padova: Professor Giovanni Meneghetti, Dr. Daniele Rigon and Dr. Claudio Gennari from the Department of Industrial Engineering performed defect analysis in maraging steel materials. Fundamentally, they used selective laser melting based additive manufacturing method to fabricate the maraging steels samples. They also investigated the effects of the defects on the fatigue behavior. Their research work is currently published in International Journal of Fatigue.
In brief, the authors commenced their research work by exploring the defects and their influence on the performance and properties of additively manufactured components. Next, they performed stress-controlled fatigue analysis. Additionally, more measures were taken into account to reduce the steel specimen distortions that may have resulted in additional bending stresses. This included turning the steel samples end and polishing the gauging sections. The tested specimens were positioned both parallel and perpendicular to the building direction for testing. Furthermore, the authors took into consideration the short crack effects so as to enhance the accuracy of the axial fatigue strength estimations.
The authors observed that the parallel steel samples exhibited almost the same fatigue strength as the perpendicular ones. However, parallel specimens further exhibited incremental fatigue strength at longer fatigue lives due to the different inherent behaviors of the material. Additionally, low fatigue strength was recorded for the additivity manufactured samples as compared to the initially tested vacuum melted specimens under the same conditions.
In summary, Professor Giovanni Meneghetti and colleagues successfully analyzed the influence of defects on the fatigue behaviors of additively manufactured steel samples. To actualize their study, they adopted the El-Haddad-Smith-Topper model to determine stress intensity factor, the length parameter of the model being included in the model to account for the short crack effect. This simplified approach involves the use of only one input parameter-Vickers hardness. Therefore, the study will improve the investigation of the influence of defects in additively manufactured materials thus further enhancing their design and performance.
Meneghetti, G., Rigon, D., & Gennari, C. (2019). An analysis of defects influence on axial fatigue strength of maraging steel specimens produced by additive manufacturing. International Journal of Fatigue, 118, 54-64.Go To International Journal of Fatigue