design values. 

Feedstock lot variability requirements are recommended but not required for Class C parts. 


6.6 Used Feedstock Lot Controls 

[AMR-66]  When printing Class A and B parts using AM processes capable of feedstock reuse, 
limiting metrics for feedstock reuse shall be established and implemented to ensure the following: 

a. The effects of reuse on material performance are either demonstrated as negligible or 
material property data representing the limiting reuse state are incorporated directly into the MPS 
population. 

b. Parts are not built with feedstock exceeding the reuse limits. 

c. The methodology for incorporating the influence of feedstock reuse into the design 
values of each MPS is described as part of the MUA substantiating the methodology of the MPS 
development. 

[Rationale:  For AM processes where feedstock reuse is feasible (e.g., PBF), the AM process has 
the potential to degrade the feedstock with continued reuse. Degradation mechanisms vary by 
material and AM process; an understanding of feedstock reuse effects is required, and the AM 
process is to be controlled to preclude feedstock reuse from affecting part quality.] 

Examples of feedstock degradation by reuse include introducing contaminants (e.g., oxygen or 
combustion by-products), or changing a powder feedstock’s morphology and rheology 
characteristics. The effects of feedstock reuse are material specific; the intent is to evaluate 
feedstock reuse effects on material properties using only material procured to the feedstock 
specification identified by the QMP. 

The CEO, in collaboration with the AM part producer, is responsible for ensuring feedstock reuse 
metrics are defined and implemented in accordance with this requirement. The expectation is that 
the efficacy of the reuse metrics is substantiated through a dedicated test plan and subsequent data 
that characterize the influence of feedstock reuse on material performance or demonstrate that 
powder at the limiting reuse metric has a negligible influence on material performance. 

6.7 Influence Factors 

This NASA Technical Standard uses the term “influence factor” to describe any factor that has the 
capacity to alter the baseline performance of the material (i.e., the material allowable). These 
factors include those commonly evaluated for other material product forms, such as environment 
(e.g., temperature, humidity) or surface condition. For AM materials, influence factors may be 
fundamentally unique to the AM process, such as the unique impact of down-facing surfaces in the 
L-PBF process, local feature-driven anisotropy, or even part-specific geometric features whose 
performance can only be captured through part-analog specimens using a “building block” 
approach to design values.  


 Identification and Characterization of Influence Factors 

[AMR-67]  For Class A and B parts, the CEO shall provide for the systematic identification and 
characterization of any known factor having influence on the bulk or local performance of the AM 
material for the purpose of establishing design values. 

[Rationale:  As with all materials, the performance of AM materials is influenced by multiple 
factors that have to be taken into account for successful part design; all such factors with potential 
to alter material performance have to be identified and characterized as part of MPS development.] 

Examples of influence factor evaluations expected in the MPS include, but are not limited to: 

(1) Effects of surface texture, as-built or improved, on material performance as a 
function of build orientation, particularly in fatigue. 

(2) Effects of as-built surface texture and microstructure on thin-section performance, 
including geometric effects of realized cross section and mechanical property debits. 

(3) Effects of the build process pause and restart allowance in the QMP demonstrating 
that effects are negligible on material performance. 

(4) Effects of test specimen geometry for non-standard configurations, the evaluation of 
part-specific features through building-block tests, or evaluation of material taken 
from parts (e.g., in preproduction article evaluation).  

(5) Effects of feature-specific deposition tool paths, including starts, stops, changes in 
direction, and intersections of adjacent or crossing deposition paths on alignment of 
material features. 

(6) Effects of feature-specific anisotropy within a part. 

For comparison, the following are examples of variability that are not typically tracked as influence 
factors but are incorporated into the material allowables: 

(1) Feedstock lots, heat treating, and machine-to-machine and build-to-build variability.  

(2) Allowable variation in key process variables. 

(3) Effects of nominally occurring anisotropy in material physical and mechanical 
properties. 

(4) Effects of microstructure differences occurring spatially throughout a build due to 
AM build history (e.g., thermal, light curing processes, scan strategies, and interpass 
time or interpass temperature influences). 


 Influence Factor Effect on Material Allowables 

[AMR-68]  For Class A and B parts, when required for part assessment, design values appropriate to 
each identified influence factor shall be developed and incorporated into the MPS. 

[Rationale:  Influence factors alter the AM material performance and have to be incorporated 
rigorously into the design framework to establish reliable AM part design and production.] 

In this methodology, each known and applicable influence factor is evaluated for its effect on the 
material allowable and is used to create a design value applicable to the AM part’s use condition. 
Any design value developed through the characterization of influence factors have to maintain or 
bound the level of statistical significance required of its associated material allowable. In some 
cases, the variability incorporated into the material allowable (e.g., feedstock lots, builds, machines, 
thermal history) may be satisfactorily imparted to a design value through an experimentally 
substantiated multiplicative scale factor. In other cases where the influence factor presents a new 
and significant source of variability (e.g., surface finish effects in fatigue), the design value is 
independently substantiated, including all sources of variability required of its associated material 
allowable.  

 Explicit Evaluation of Anisotropy 

[AMR-69]  Each MPS for Class A and B parts shall explicitly include supporting data necessary to 
evaluate the effects of anisotropy present in the AM material produced to a QMP such that 
anisotropy is either properly incorporated into the material allowables and associated design values, 
or rationale for a bounding isotropic assumption is established. 

[Rationale:  The AM process creates material using a directional process where the likelihood of 
the process yielding anisotropic material is high unless controls are in place to minimize this effect; 
quantifying the effects of anisotropy and rendering appropriate material properties is important to 
valid structural integrity assessment.] 

To make anisotropy evaluations feasible, the build orientation should always be identified and 
maintained for all material property development activities. 

Material allowables and design values developed from the supporting data in the MPS are not 
required to be orientation-specific if the values of the bounding orientation are used and anisotropy 
is demonstrated as negligible, typically accepted as a less than a 5% difference in properties by 
orientation. If the anisotropy is not negligible, then orientation-specific properties are required; and 
complexity in process qualification and part qualification will follow. 


6.8 Criteria for the Use of External Data in the MPS 

[AMR-70]  For Class A and B parts, material property data generated outside the jurisdiction of this 
NASA Technical Standard (e.g., prior industry or government data), shall meet each of the 
following criteria prior to incorporation into or establishment of an MPS: 

a. Properties are generated from material produced by a documented AM process 
fundamentally the same as those already registered to the MPS. 

b. Authenticating records of traceability are available for the feedstock chemistry and post-
AM operations (e.g., heat treatment). 

c. Properties are generated from material tested in a material condition (e.g., heat treatment 
and microstructure) equivalent to that defined by QMPs registered to the MPS. 

d. Authenticating records of traceability are available that illustrate the material internal 
quality and final microstructure. 

e. The size, geometry, build orientation, and surface condition of test specimens are 
defined. 

f. The specifications governing the material test methods are defined. 

g. The external data are provided in the form of actual test results to allow material 
allowables and PCRD criteria to be established or independently verified. 

h. Demonstration that active QMP(s) produce(s) materials equivalent in structure and 
mechanical properties, based on the registration process in section 5.6 of this NASA Technical 
Standard. 

[Rationale:  The incorporation of prior databases for AM material properties into an MPS will 
become standard practice as the technology matures. These criteria ensure the database contains 
sufficient information to follow the process controls required by this NASA Technical Standard.] 


6.9 Process Control Reference Distribution (PCRD) 

[AMR-71]  For Class A and B parts, a PCRD shall be established as part of the MPS for each 
material property identified in Tables 5 and 6 requiring a PCRD for witness specimen test 
acceptance. 

[Rationale:  Maintaining consistency of material performance in AM parts is essential to structural 
integrity. To ensure this control is sustained throughout all AM operations requires a reference 
definition of expected material performance by which process performance can be evaluated. The 
PCRD provides this reference.] 

As per the definition, the PCRD is a simple statistical model used to create acceptance criteria for 
evaluation of witness specimens used to monitor the AM process. The goal of the PCRD is to employ 
SPC to alert the manufacturer when the process control has failed and the printed items’ material 
properties are no longer in the same statistical population as those in the MPS. Control charts 
provide a visualization and tracking tool for upper and lower acceptance limits and the process’s 
results. Many times it is important to define multiple PCRDs per QMP. For example, in metallic AM 
builds, four PCRDs are defined for each MPS:  UTS, yield strength, elongation, and fatigue life at a 
fixed cyclic stress condition. The choice of defined PCRDs may change with AM process and 
material, depending on which properties are determined to be most useful for monitoring process 
control.  

The PCRDs and their associated acceptance criteria are best documented in a manner that allows 
for occasional revision and sustained use as an acceptance criteria for witness specimen testing for 
part acceptance. 

The type of statistical distribution or other metrics used for the PCRD is not dictated by this NASA 
Technical Standard. The distribution model should be chosen based on the quality of the 
distribution’s fit to the data or the ability of the metrics to describe the nominal material 
performance. Any appropriate distribution and associated characteristic parameters may be used to 
define the PCRD. 

For additional commentary for PCRDs, see Appendix C of MSFC-STD-3716. 


6.10 PCRD Maintenance 

[AMR-72]  For Class A and B parts, PCRDs specific to each MPS shall be reevaluated regularly 
following the incorporation of new witness data and updated as required. 

[Rationale:  The PCRDs associated with an MPS need to represent the current, evolving state of the 
data within the MPS, reflecting process variability as data are added; PCRDs have to be evaluated 
to ensure they accurately represent the witness specimen population of the associated MPS and 
updated as necessary because they are used to define acceptance criteria.]  

Prior to meeting the criteria for lot-maturity, PCRD updates are considered an integral part of 
verifying QMP stability with new sources of data coming from ongoing witness testing. 
Maintenance of the PCRD is necessary to preserve the functional process control relationships 
among the PCRD, observed process performance, and the material allowables of the MPS. If the 
distribution of the PCRD changes, it is important to evaluate that the witness specimen acceptance 
criteria are still valid and the material allowables are protected. Because it is disruptive to alter 
process controls, acceptance criteria, and material allowables, these updates should be carefully 
considered but implemented when necessary. 

An example interval for reevaluating the PCRDs is given in Table 20, Notional Example for 
Reevaluating PCRDs per Number of Builds, or with a review prompted whenever a witness 
specimen fails to meet the PCRD. Reevaluation intervals may also be based on time or maintenance 
activities. Initially, lot variability may be lacking in the PCRD data set, and adjustments may be 
expected. Careful review is warranted whenever a PCRD is adjusted. Witness specimen data that 
fail to meet the PCRD acceptance criteria need particular attention. As part of the review and 
disposition of the nonconformance associated with the witness specimen test failure, the failing 
witness data need to be marked for inclusion or exclusion from the PCRD update process. 

Table 20—Notional Example for Reevaluating PCRDs per Number of Builds 

Number of Builds Update Frequency 
1 to 10 Every 5 
11 to 50 Every 10 
51 to 100 Every 20 
101 to 500 Every 50 
501 to 1000 Every 100 
Greater than 1000 Every 250 


6.11 Development of Material Allowables and Design Values 

[AMR-73]  For Class A and B parts, material allowables and design values specific to each AM 
material and condition shall be developed or otherwise substantiated according to the requirements 
and guidance of section 6.11 of this NASA Technical Standard for all applicable properties and 
environments required for structural assessment. 

[Rationale:  Material allowables and design values will differ with AM process and material 
condition; properties specific to each AM material and condition have to be established to enable 
reliable structural assessment.] 

This section provides requirements and guidance related to the methodology of establishing 
material allowable and design values for AM materials accounting for unique aspects of the product 
form and function. To meet the intent of these requirements, the material testing for physical or 
mechanical properties is based on applicable testing standards (e.g., ASTM E8/E8M, Standard Test 
Method for Tension Testing of Metallic Materials for tensile testing; ASTM E466, Standard Practice 
for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials, for 
fatigue testing; or ASTM E1820, Standard Test Method for Measurement of Fracture Toughness, for 
fracture toughness testing); and the tests are executed by a test laboratory accredited through 
Nadcap™, the American Association of Laboratory Accreditation (A2LA), or another nationally 
accepted accreditation body, or by direct approval of the CEO.  

 Configuration Control of Material Allowables and Design Values 

[AMR-74]  For Class A and B parts, material allowables and design values shall be maintained 
under configuration control as an integral part of an MPS. 

[Rationale:  Material allowables and design values are the end-product of an MPS that is made 
available to the design and analysis community; a methodology to maintain configuration control of 
the disseminated content is necessary to ensure reliable use of such content in design assessment.] 

In this context, all derived materials properties used for AM part design assessment are included in 
this configuration control requirement. 

 Design Values Development  

[AMR-75]  For Class A and B parts, design values established to incorporate the effects of influence 
factors shall be developed on the AM material and have a statistical significance equivalent to the 
respective type of material allowable. 

[Rationale:  Design values require the same, or equivalent, reliability as material allowables.] 

The development of design values to incorporate the effects of identified influence factors may 
require a variety of techniques and methodologies. The performance of non-AM product forms (e.g., 


forging, bar, plate, etc.) is not intended to be used to infer the performance the AM product form 
despite the similarity of the basis bulk material and part geometry. 

Design values are to include statistical assessment of the same or equivalent sources of variability 
required of material allowables. If it can be justified that the influence factor does not add an 
additional source of variability, ratio methods anchored to the material allowables are generally 
appropriate, with proper precautions (see MMPDS). If the influence factor does add a source of 
variability, then a methodology to include this additional variability is required, or the design value 
is to be developed independent of the material allowable while incorporating all lot and specimen 
variability requirements.  

 Design Value Margin 

Although not required by this NASA Technical Standard, it is recommended that the conservatism of 
all design values and their respective material allowables be objectively evaluated for potential 
programmatic risks of redesign efforts if these values have to be lowered as a result of currently 
unknown sources of variability. The risk of uncovering unanticipated variability is not negligible 
due to the relative immaturity and continued rapid evolution of AM technologies. The requirements 
of this NASA Technical Standard make all reasonable efforts to preclude this pitfall, including the 
minimum variability requirements of section 6.11.3; users are recommended to maintain a design 
value margin applied judiciously to any material allowable that is not fully lot mature or that is set 
at or near the statistical tolerance limit requirement. In this context, the design value margin may be 
considered as an influence factor associated with material property risk that is applied as 
appropriate to material allowables when creating design values. If a design value has been 
developed independent of a material allowable, then the design value margin simply represents an 
additional factor lowering its value to hedge against this risk. A design value margin formulated as 
a function of the coefficient of variation (CoV) provides a design value margin that scales according 
to need.  

 Minimum Variability Requirements for Strength-Related Material Allowables 

[AMR-76]  Statistical assessment of material data for strength-related material allowables shall 
incorporate a minimum CoV according to Table 21, Required Minimum Coefficient of Variation in 
Metallic Strength Material Allowables, unless the data set specifically includes data representing 
variability attributable to differences between specimen and part performance, in addition to all 
other specified requirements for variability in section 6.5 of this NASA Technical Standard. 

[Rationale:  The minimum CoV is required