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Additive Manufacturing Fatigue and Fracture

Summary

This project aims to enable use of metal additive manufacturing (AM) in fatigue and fracture critical applications via two main thrusts:

  • Develop appropriate measurement science for fatigue and fracture behavior of additive manufacturing metals, to underpin a rapid qualification framework
  • Determine effect of processing (including post-processing) and structure (e.g. internal defects, external defects, residual stress, crystallographic microstructure, and chemistry) on fatigue and fracture properties of additive manufacturing metals

Description

A NIST co-organized workshop identified the current needs for metal AM fatigue and fracture (F&F), which helped create the focus of this project.  During the workshop, specific variables within processing-structure-properties-performance (PSPP) were identified and in some cases prioritized.  More detail can be found in the workshop findings report (https://doi.org/10.6028/NIST.AMS.100-4).  Processing includes not just machine settings (e.g. layer thickness) and melt parameters (e.g. energy beam power and scan speed) but also powder characteristics (e.g. particle size distribution, flowability, spreadability) and post-processing (e.g. heat treatment, machining).  Structure includes chemical composition, crystallographic microstructure (e.g. phase composition, grain size and shape, texture, dislocations), residual stress, internal defects (e.g. entrapped gas porosity, lack-of-fusion voids), and external defects (e.g. surface roughness from sintered powder and melt flow).  F&F properties of interest identified during the workshop include high-cycle fatigue (HCF), low-cycle fatigue (LCF), linear elastic fracture toughness (KIc), elastic-plastic fracture toughness (J-int), fatigue crack growth rate (FCGR), and impact toughness (Charpy).

In addition to the investigation of PSPP relationships in metal AM F&F, another need identified during the workshop that will be addressed in this project is evaluation of F&F mechanical test methods.  This work will start with determining applicability of standard-sized test specimens and existing F&F test methods for metal AM.  However, this work will include development of sub-size specimen F&F test methods to address the known anisotropy in metal AM.

Additional needs identified during the workshop may provide areas for future expansion of the scope of the current metal AM F&F project.  Participation in Integrated Computational Materials Engineering (ICME) efforts building toward rapid qualification is one potential area of future work for this project.  Another strong candidate is development of predictive design tools (e.g. exceedance curves informed by critical flaw size measurements).

Scanning electron microscope (SEM) images of AM titanium alloy (Ti-6Al-4V) high-cycle fatigue fracture surfaces showing fatigue crack initiation at lack-of-fusion (LOF) defect (white arrow).  Also shown on the same fracture surface are entrapped gas pores
Scanning electron microscope (SEM) images of AM titanium alloy (Ti-6Al-4V) high-cycle fatigue fracture surfaces showing fatigue crack initiation at lack-of-fusion (LOF) defect (white arrow).  Also shown on the same fracture surface are entrapped gas pores.

 

PUBLICATIONS

See web pages for individual staff (linked under the NIST STAFF section on the right) for current publications for this project.

NRC Post-Doc OPPORTUNITIES (you will leave NIST site when selecting these links)

Fatigue and Fracture of Metallic Materials Processed via Additive Manufacturing

Created February 7, 2017, Updated September 20, 2019