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For all materials studied in the present paper , fill densities , which seem to have significant impact on fatigue strength of 3D printed parts , have been selected based on the maximum fatigue strength of each part .
 materials, impact, fatigue strength, 3D printed parts, fatigue strength
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
Answer:
 materials, impact, fatigue strength, 3D printed parts, fatigue strength