Multi-scale Characterization of Hyperplasticity and Failure in Dual Phase Steels Subject to Electrohydraulic Forming

In this research, three commercial dual phase steel sheets, i.e. DP500, DP780 and DP980, were formed under quasi-static and high strain rate conditions using the Nakazima test and Electrohydraulic Forming (EHF), respectively. In EHF, as a result of a high-voltage electrical discharge between two electrodes in a water chamber, a shock wave was produced which travelled through the water and formed the sheet into the final shape. When a 34° conical die was used in EHF, significant formability improvement, known as hyperplasticity, was achieved in the specimens compared to the specimens formed in the Nakazima test. In this research, hyperplasticity as well as failure in the specimens were characterized at different scales of observation. Quantitative metallography showed relative deformation improvement of around 20% in ferrite and 100% in martensite when formed under EHF. Dislocations in ferrite and deformation twinning in martensite were found to be responsible for the significant improvements of deformation in the constituents under EHF. As a mechanism of failure, voids were found to nucleate in the ferrite/martensite interface due to decohesion. However, under EHF, the significant deformation improvement of martensite enhanced the plastic compatibility between ferrite and martensite. Consequently, the strain gradient across the ferrite/martensite interface, i.e. decohesion, was reduced and nucleation and growth of the interfacial voids was suppressed. Furthermore, quantitative analysis of the voids showed that void growth in the specimens formed under EHF was slower than in the specimens formed in the Nakazima test. The reason was attributed to impact of the sheet against the die that generates significant compressive and shear stresses which act against void growth. Therefore, under EHF, coalescence of the voids to form micro-cracks was postponed to higher levels of strains which resulted in suppression of failure. Fractography of the specimens showed ductile fracture as the dominant type of fracture under both quasi-static and high strain rate forming conditions. In addition, limited quasi-cleavage fracture was observed in DP780 and DP980 specimens. Shear fracture was also observed in the specimens formed under EHF

A hybrid additively manufactured martensitic-maraging stainless steel with superior strength and corrosion resistance for plastic injection molding dies

The customization of plastic injection molding dies is technologically and economically limited by conventional manufacturing processes. Recent advances in hybrid additive manufacturing (HAM) have provided more geometrical freedom for the manufacturing of parts with desired properties. In this paper, we report manufacturing of a hybrid 420/Corrax stainless steel with a reliable interface that can be applied in the manufacturing of next-generation geometrically complex plastic injection molding dies with enhanced strength and corrosion resistance. AISI 420 martensitic stainless steel is used as a cost-effective substrate, and a maraging stainless steel grade, known as Corrax, is printed on top of it using laser powder bed fusion (LPBF). A hybrid heat treatment cycle is applied to improve metallurgical properties and to enhance mechanical compatibility between the martensitic and the maraging stainless steels. Tensile tests coupled with scanning electron microscopy are carried out for analysis of failure, which show the development of shear bands in the microstructure of the 420 stainless steel substrate while a limited amount of deformation occurs in the interface region and Corrax microstructure. Void nucleation, growth, and coalescence are found at the 420/Corrax interface due to mechanical incompatibility and decohesion; however, microstructural instability mainly occurs along the shear bands on the 420 side and leads to fracture, which is quantified using high-resolution X-ray computed tomography. Nanoindentation tests show that the maximum level of hardness occurs at the interface due to the existence of sub-micron grains and the formation of AlN nanoparticles. Also, the formation of β-NiAl precipitates enhances the Corrax strength after heat treatment. In addition to a high strength, elevated corrosion resistance of the cooling channels is essential to extend the service life of plastic injection molding dies. Potentiodynamic corrosion testing at the interface shows that Corrax has remarkable corrosion resistance compared to 420. Therefore, additive manufacturing of the critical die areas such as the cooling channels using Corrax increases the service life of the mold

Exploring hospitals’ functional preparedness effective factors in response to disasters: a qualitative study in a lower middle-income country

Abstract Background Medical services are among the most urgent needs of the disaster-affected population. Consequently, hospital preparedness -as the main health services provider- is one of the vital factors in effective response to disasters. The present study aims to explore the perspectives of study participants about the influential factors of hospital functional preparedness in a lower middle-income country. Methods In this qualitative study, data were collected through 17 semi-structured interviews with disaster management authorities selected by purposive sampling. Content-Analysis was used to analyze the data. Results 138 codes were developed and categorized into ten categories and 34 subcategories. The main categories were: 1- leadership, command, and coordination (4 subcategories); 2- risk assessment (3 subcategories); 3- legislating and developing protocols, guidelines, and programs (3 subcategories); 4- estimating and storing the necessary supplies and equipment (3 subcategories); 5- human resource management (4 subcategories); 6- education, training, and development of staff (6 subcategories); 7- vital routes and facilities (3 subcategories); 8- communication (3 subcategories); 9- security, safety and locating of safe zones (3 subcategories); 10- underlying disaster risk factors (2 subcategories). Conclusion According to the participants of this study, ten categories of factors can affect hospitals’ functional preparedness; hospital managers and decision-makers can consider these factors to ensure the proper provision of medical services during disasters

Additional file 1 of Exploring hospitals’ functional preparedness effective factors in response to disasters: a qualitative study in a lower middle-income country

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