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 and analysis techniques that enable characterization of material structure and fatigue and fracture behavior of additively manufacturing metals to underpin a rapid qualification framework
• Determine and quantify physical effects that link processing parameters (including post-build processing) and structure (e.g., internal defects, surface defects, residual stress, crystallographic microstructure, and chemistry) to fatigue and fracture behavior of additively 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 areas of this project.  Current project focus areas are listed below, and more detail on the workshop can be found in the workshop findings report (https://doi.org/10.6028/NIST.AMS.100-4).   

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.

Processing-Structure-Property-Performance Relationships

Active Staff: Hrabe, Quinn, Lucon, White, Rentz, Benzing, Derimow

During the workshop, specific variables within processing-structure-properties-performance (PSPP) were identified and in some cases prioritized.    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 (K_Ic), elastic-plastic fracture toughness (J-int), fatigue crack growth rate (FCGR), and impact toughness (Charpy).

Small-Scale Mechanical Testing

Active Staff: Liew, Hrabe, Lucon, Johnson, Rentz, Benzing, Derimow

This work includes development of sub-size specimen mechanical test techniques (and complementary acoustic and microstructure-characterization techniques) to understand the influence of microstructural heterogeneities on mechanical behavior at appropriate length scales.  Additionally, these techniques are used to understand mechanical behavior to failure for the small feature sizes commonly observed in near-net shape AM components.

Acoustic NonDestructive Evaluation (NDE)

Active Staff: Johnson, Hrabe

This work aims to develop accurate, sensitive, and rapid NDE techniques for AM alloys.  Innovative resonant acoustic NDE techniques for measuring nonlinear and anelastic effects are the current focus of this research, since they work well on parts with complex near-net-shape geometry and are highly sensitive to the presence of nanoscale and microscale defects (e.g., dislocations) that affect bulk mechanical properties. Once physical mechanisms of observed acoustic material characteristics and correlations with bulk fatigue and fracture behavior are established, these techniques may find broad and impactful use in post-process qualification of as-manufactured parts, enabling effective prediction of in-service mechanical performance.     

Computational Methods for Fatigue and Fracture Prediction

Active Staff: Garboczi, Moser, Kafka

Due to the unique processing aspects of AM, the fatigue properties of AM metal components are different than those commonly associated with wrought materials. Therefore, high-fidelity models are being created to offer novel insight into why these differences in fatigue behavior exist. Through these new models, this work aims to accurately predict fatigue and fracture behavior at both the microstructural and component level for AM metals. The computational topics involved with this work include finite element analysis, elastic-plastic modeling, continuum damage mechanics, crystal plasticity, and multiscale methods. Furthermore, advanced experimental techniques are being pursued to create new calibration methods for these computational models, such as X-ray computed tomography.

Additional Potential Focus Areas

Additional needs identified during the workshop may provide areas for future expansion of the scope of the current metal AM F&F project.  If you are interested in collaboration, please contact project leader Nik Hrabe (nik.hrabe [at] nist.gov, 303-497-3424).  If you are interested in joining NIST, NRC post-doctoral opportunities are usually available (see below).

PUBLICATIONS

See individual staff webpages for current list of publications from this project.

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

This is a competitive, proposal-based 2-year post-doctoral fellowship program, and it is the most common pathway for PhD-level researchers to work at NIST.  NIST participates in two of the application cycles (Feb 1 and August 1) each year.  Applicants should plan to work closely with NIST staff to develop their research proposals in order to ensure programmatic fit and suitable resources for the proposed work.  The following opportunities are active within this project:

1. Fatigue and Fracture of Metallic Materials Processed via Additive Manufacturing (http://nrc58.nas.edu/RAPLab10/Opportunity/Opportunity.aspx?LabCode=50&ROPCD=506472&RONum=B7092)
2. Nondestructive Evaluation of Additively Manufactured Alloys (http://nrc58.nas.edu/RAPLab10/Opportunity/Opportunity.aspx?LabCode=50&ROPCD=506472&RONum=C0160)