implementation of corrective actions.] 

See section 3.2 of this NASA Technical Standard for definitions of rework and repair. 

See section 8 in this NASA Technical Standard for additional information regarding the QPP. 

4.11 Witness Testing for Statistical Process Control 

Use of witness specimens provides evidence of process control throughout the build cycle. This 
concept is based on the assumption that process development is complete and that a fixed 
"steady state" process has been reached. The implementation of witness sampling will vary from 
part to part depending on part class, the build layout, and specific part requirements. The PPP is 
used to document the approach to witness specimen testing.  

The test direction of witness specimens is almost always in the Z-direction defined by the build 
height since the properties in that direction are typically most sensitive to degradation in the 
build process. Testing should be performed during process development to determine the 
appropriate test direction or directions for a particular AM process, machine, or material. 

The direction, diameter, length, and shape of the witness specimens should be the same as the 
QMP specimens. Geometric differences in specimens can cause differences in the measured 
strength and ductility, which can prevent statistical combination or comparison of qualification 


and production witness data. All samples should have consistent surface finish between witness 
and QMP specimens; with the exception of surface-finish-dependent-testing (e.g., fatigue 
testing), it is recommended that all samples be fully machined. The locations of the witness 
specimens in the build volume or on the build plate should be specified and kept consistent for 
all builds.  

Because acceptable witness specimen test results are a prerequisite to part acceptance, 
significant programmatic risks occur when witness specimen testing lags continued part 
processing and additional part production. Witness specimen test results should be generated as 
soon as possible to reveal potential systemic changes to the AM process so rapid intervention 
can reduce the impact to production. 

Witness specimens are used to identify systemic changes in process control. By their nature, 
build witness specimens represent a small sample of the spatial (location in build volume) and 
time aspects of the build; they cannot necessarily indicate local, transient, or intermittent loss of 
process control during a build. 

This NASA Technical Standard requires SPC to monitor the results of witness specimen testing. 
Witness specimens are intended to provide a final evaluation of the complete AM process 
following any necessary post-build material processes (e.g., heat treating or curing). The 
application of SPC to operational aspects of the AM machine is highly encouraged; not every 
control factor with process influence can be monitored using witness specimen testing. 

The remainder of this section provides requirements and associated guidance for witness 
specimen evaluations for either builds evaluated independently per section 4.11.1 or builds 
evaluated in the framework of continuous production per section 4.11.2 of this NASA Technical 
Standard. See the definitions of “Independent Build” and “Continuous Production Build” in 
section 3.2 of this NASA Technical Standard. 

See section 7.4.1 of this NASA Technical Standard for control and documentation requirements 
and any deviations for using custom witness specimen test approaches that are part specific.  

The PCRD is a simple statistical model of witness specimen test results. PCRDs whose 
characteristics are determined to be acceptable (e.g., variance, skew) are used to set acceptance 
limits for the witness specimen tests. Witness specimen test results that fail to meet the 
acceptance limits or that negatively affect the PCRD's characteristics are used as indicators of a 
degradation in or loss of process stability that may influence a production part's material 
properties. See section 6.9 in this NASA Technical Standard.  

There are two primary methods of SPC available to the user depending on the chosen witness 
specimen testing methodology:  “continuous production” SPC and “independent build” SPC. 
Continuous production SPC is based on the classic methodology of tracking the process over 
time using control charts to identify changes or trends in process performance. The control chart 
acceptance criteria are based on the PCRD definition of the nominal process performance, or 


the control charts may provide the definition of nominal performance and acceptance metrics 
based on standard control chart procedures (e.g., ASTM E2587, Standard Practice for Use of 
Control Charts in Statistical Process Control). In the latter case, the control charts and their 
acceptance criteria directly serve the purpose of the PCRD. With the benefit of a continuous 
performance history, the quantity of witness specimen tests required per build operation is 
reduced compared with the independent build scenario, but there are operational requirements 
levied on the AM machine to maintain the status of being in continuous production (see section 
4.11.3 of this NASA Technical Standard). 

An important aspect of these operational requirements is a periodic SPC evaluation build, 
potentially using the same sample set as a Sub-QMP, which evaluates a broader range of 
material and build performance. Note that “continuous production” implies the machine 
operates continuously under the same or equivalent QMPs. It does not mean a single part is in 
continuous production. A variety of parts can be produced on a machine in continuous 
production. Most AM machines that are fully qualified to the requirements of this NASA 
Technical Standard should be capable of continuous production status. 

Another available method of SPC is associated with the independent build. The application of 
SPC in this scenario differs from the classic construct of SPC in that it does not have the 
advantage of build history and trending, but rather evaluates systemic build quality based only 
on the available witness specimen test information from that build. A higher quantity of witness 
specimen tests is required for this approach to allow an improved engineering assessment of 
build performance. This method is considered a form of SPC because, through the use of the 
PCRD and its associated statistically based acceptance criteria, the process performance is 
monitored for consistency within the limits of pragmatic sample quantities. This insight into 
process control is not available from simplistic minimum value acceptance criteria. 

The independent build scenario is useful for accepting parts while developing the needed witness 
specimen performance history for continuous production SPC. It is also of value for smaller 
build runs that may take place on controlled AM machines that do not yet meet the prerequisites 
to qualify for continuous production. 

A witness sub-article is a chosen portion or representative geometry of a part that is produced 
concurrently as a witness article during the build. The use of witness sub-articles is not 
mandatory for every part. The witness sub-article allows for evaluation, inspection, or dissection 
of challenging representative geometry when direct evaluation of the feature on the actual part is 
not possible. For example, a small representative section of a complex, thin-walled heat 
exchanger may be built alongside the actual part. Under the assumption that the process 
performance is reasonably equivalent for both, the witness sub-article can be sectioned to look 
at the quality of the thin walls. Witness sub-articles may be used to provide sufficient process 
control evidence for part features that cannot otherwise be inspected or verified in the part 
directly and may be used for any appropriate evaluation (e.g., mechanical, metallurgical, 
dimensional, surface texture, calibration of nondestructive inspection tools, etc.). 


Witness articles for Class A1 parts are required for parts that have high structural demand and 
high AM risk based on a lack of complete inspection capability (first criterion in Table 2). One 
witness article is required to be evaluated for every six flight parts produced. The witness article 
is evaluated according to the preproduction article evaluation criteria specified by the PPP.  
Rationale for tailoring this requirement will be part- and situation-specific and is presented and 
approved through the PPP.  

See MSFC-STD-3716, Appendices B and C, for additional commentary on witness testing and 
SPC. 

 Witness Testing for Independent Builds 

[AMR-23]  All AM parts manufactured by an independent build shall include witness specimens 
integral to the build of the types and quantities required in Table 5, Metals Witness Testing 
Quantities and Acceptance Results for Independent Builds (metals), and Table 6, Polymer 
Witness Testing Quantities and Acceptance Results for Independent Builds (polymers), exposed 
to the same post-processing documented in the QMP, and evaluated per the acceptance 
methodologies in Tables 5 and 6, such that any witness test failing to meet the defined 
acceptance criteria is documented as a nonconformance and traceable via a records management 
system controlled by the QMS. 

[Rationale:  Witness sampling is required to provide evidence of process control throughout the 
build cycle for independent builds, the witness sampling requirements provide sufficient data to 
reasonably evaluate the build solely on its own content.] 

See Table 13, Minimum Mechanical Property Tests for Metal QMP Builds; Table 14, Minimum 
Mechanical Property Tests for Metal Sub-QMP and SPC Evaluation Builds; Table 15, Minimum 
Mechanical Property Tests for Polymeric QMP Builds; and Table 16, Minimum Mechanical 
Property Tests for Polymeric Sub-QMP and SPC Evaluation Builds in section 5.5.3.5 for 
recommended test specifications. 

Further discussion on the intent and implementation of witness testing can be found in MSFC-
STD-3716, Appendix B, section B6.2.2.1. 

The witness sample requirements for an independent build are scoped to generate sufficient 
evidence during a single build operation to conclude that material rendered by the machine and 
feedstock at the time of the build is consistent with past performance and supports the 
assumptions in the MPS. 

It is highly recommended that the results of independent builds be tracked whenever possible 
using process control charts and following the other policies of sections 6.9 and 6.10 of this 
NASA Technical Standard to establish or maintain process performance history. A build with 
witness sampling adequate for an independent build can also be a part of a continuous operation 
build because the sampling requirements are enveloped. 


Witness sample geometries may vary depending on the AM process being used. Witness 
specimens built alongside the part are capable of capturing a representative thermal history of 
the part. Some processes (e.g., DED) challenge this methodology. For these systems, material 
from tag ends, prolongations, or other features may be used as the material for witness 
specimens. 

Because they challenge the layer-by-layer methodology, these systems also require tensile and 
microstructure samples to confirm system performance has not changed since the build process 
began. 

For efficiency, witness specimens may serve multiple witness requirements, such as using 
specimen grip ends for metallurgical evaluations, so long as witness data are not compromised. 

For the purposes of this NASA Technical Standard, integral to the build means produced during 
a process development production run or during a part production run, and for the latter, 
simultaneously with the production part. 


 Metals Witness Testing Quantities and Acceptance Results for Independent Builds 
 a. Minimum Quantities of Witness Specimen Types by Part Class 
  Class          
  A1 A2 A3 A4 B1 B2 B3 B4 C  
 Tensile 6 6 6 6 6 6 6 6 2  
 FH Contingency* 1 1 1 1 1 1 - - -  
 Post-build Tensile** 6 6 6 6 6 6 6 6 2  
 Microstructure 2 2 1 1 1 1 - - -  
 Post-build Microstructure† 2 2 1 1 1 1 - - -  
 Chemistry 1 1 - - - - - - -  
 HCF 2 2 2 2 2 - - - -  
 Low Margin Point A/R A/R - - - - - - -  
 Witness Sub-article A/R - A/R - A/R - - - -  
 Witness Article 1 for 6 - - - - - - - -  
 Customized QMP A/R A/R A/R A/R A/R A/R A/R A/R -  
  Notes:          
  *FH contingency = Full-height contingency specimen for PBF only 
  **Post-build tensile = Post-build tensile specimens required for DED only 
  †Post-build microstructure = Post-build microstructure specimens required for DED only 
  A/R = As required when specified in the PPP/QPP  

Sample direction should be chosen from the most process-sensitive orientation (typically the Z-direction). 
See Tables 13 through 16 in section 5.5.3.5 in this NASA Technical Standard for recommended test specifications.  


b. Basis for Acceptance of Witness Specimen Results  
  Class          
  A1 A2 A3 A4 B1 B2 B3 B4 C  
 Tensile PCRD PCRD PCRD PCRD PCRD PCRD PCRD PCRD >85% typical  
 FH Contingency* A/N A/N A/N A/N A/N A/N - - -  
 Post-build Tensile** PCRD PCRD PCRD PCRD PCRD PCRD PCRD PCRD >85% typical  
 Microstructure Comp Comp Comp Comp Comp Comp - - -  
 Post-build Microstructure† Comp Comp Comp Comp Comp Comp - - -  
 Chemistry A/S A/S - - - - - - -  
 HCF PCRD PCRD PCRD PCRD PCRD - - - -  
 Low Margin Point DV Min DV Min - - - - - - -  
 Witness Sub-article Comp - Comp - Comp - - - -  
 Witness Article Comp - - - - - - - -  
 Customized QMP A/S A/S A/S A/S A/S A/S A/S A/S -  
  Notes:          
  PCRD = Process Control Reference Distribution defined acceptance criteria      
  A/N = As needed         
  >85% typical = Average to be within 15% of typical value 
  *PBF only 
  **Post-build tensile = Post-build tensile specimens required for DED only 
  †Post-build microstructure = Post-build microstructure specimens required for DED only 

Comp = Comparative assessment based on defined criteria in the QMP/QPP, or 
PPP in the case of Witness Articles or Witness Sub-articles     
  A/S = Acceptance as specified in the QMP/QPP       


Polymer Witness Testing Quantities and Acceptance Results for Independent Builds 
a. Minimum Quantities of Witness Specimen Types by Part Class 
 Class         
 A1 A2 A3 A4 B1 B2 B3 B4 C 
Tensile 
Not applicable per section 4.3.1.1.1 of 
this NASA Technical Standard 
6 6 6 6 2 
FH Contingency 1 1 - - - 
Microstructure 1 1 - - - 
Flexural A/R A/R - - - 
Compression A/R A/R - -   
Density 1 1 - - - 
Witness Sub-article A/R - - - - 
Sustained Load 1 1 - - - 
Customized QMP A/R A/R A/R A/R - 
 Notes:         
 FH contingency = Full-height contingency specimen 
 A/R = As required when specified in the PPP/QPP      

Sample direction should be chosen from the most process-sensitive orientation (typically the Z-direction). 
See Tables 13 through 16 in section 5.5.3.5 for recommended test specifications.   


b. Basis for Acceptance of Witness Specimen Results 
 Class         
 A1 A2 A3 A4 B1 B2 B3 B4 C 
Tensile 
Not applicable per section 4.3.1.1.1 of 
this NASA Technical Standard 
PCRD PCRD PCRD PCRD >85% typical 
FH Contingency A/N A/N - - - 
Microstructure Comp Comp Comp Comp - 
Flexural A/S A/S - - - 
Compression A/S A/S - -   
Density 1 1 - - - 
Witness Sub-article A/S - - - - 
Sustained Load 1 1 - - - 
Customized QMP A/S A/S A/S A/S - 
 Notes:         
 PCRD = Process Control Reference Distribution defined acceptance criteria     
 A/N = As needed        
 A/S = Acceptance as specified in the QMP/QPP      

Comp = Comparative assessment based on defined criteria in the QMP/QPP, 
or PPP in the case of Witness Articles or Witness Sub-articles    

>85% typical = Average to be within 15% of typical value 
See Tables 13 through 16 in section 5.5.3.5 for recommended test specifications. 


 Witness Testing for Continuous Production Builds  

[AMR-24]  All AM parts manufactured by a continuous production build shall include witness 
specimens integral to the build of the types and quantities required in Table 7, Metals Witness 
Testing Quantities and Acceptance Results for Continuous Builds, and Table 8, Polymer Witness 
Testing Quantities and Acceptance Results for Continuous Builds, exposed to the same post-
processing as documented in the QMP and evaluated per the acceptance methodologies of Tables 7 
and 8. 

[Rationale:  Witness sampling is required to provide evidence of systemic process control 
throughout the build cycle.] 

See Tables 13 through 16 in section 5.5.3.5 for recommended test specifications. 

The continuous production build process allows for a reduced quantity of witness specimens 
compared with independent builds. 

Witness specimen geometries may vary depending on the AM process being used. Witness 
specimens built alongside the part are capable of capturing a representative thermal history of the 
part. Some processes (e.g., DED) challenge this methodology. For these systems, material from tag 
ends, prolongations, or other features may be used as the material for witness specimens. 

For the purposes of this NASA Technical Standard, integral to the build means produced during a 
process development production run or during a part production run, and for the latter, 
simultaneously with the production part. 


Metals Witness Testing Quantities and Acceptance Results for Continuous Builds 
a. Minimum Quantities of Witness Specimen