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 to improve the likelihood that material allowables will 
accommodate the variability and differences in strength properties between qualification/witness 
samples and flight parts.] 

If the CoV of the data set is equal to or greater than the minimum, no change is required. If the CoV 
of the data set is less than the specified minimum, then the data set is transformed, keeping a 
constant mean, to have a CoV equal to or greater than the required minimum. Methods for 


transforming a data set to a minimum CoV can be found in the CMH-17 and in DOT/FAA/AR-
03/19, “Material Qualification and Equivalency for Polymer Matrix Composite Material Systems: 
Updated Procedure.” 

When well controlled, the AM process has the ability to produce very repeatable results in standard 
specimen geometries, particularly for a given platform and within a single feedstock lot. This makes 
tracking of process performance by control charts of witness testing attractive, but it also means 
that it is feasible, or even likely, to generate data sets that do not adequately represent the 
variability that is induced when building parts due to various geometry features and thermal history 
during the build process. The analogy in AM is similar to that of generating composite material 
allowables on well-controlled sub-size panels for application to parts with unique fabrication 
procedures. In addition to using the building block approach, the CMH-17 philosophy is to 
maintain an assumed minimum variability to help protect against non-conservative material 
allowables. 

If the material data set can be shown to incorporate specimen-to-part variability sufficient to 
establish material allowables that effectively include this source of variability, then the 
requirements for the minimum CoV may be tailored out on a case-by-case basis by MUA. 

Table 21—Required Minimum Coefficient of Variation in Metallic Strength Material 
Allowables 
Quantity of Specimens Required Minimum CoV, 
Metals 
Required Minimum CoV, 
Polymers 
≤30 5% 7% 
31 to 60 4.5% 6.5% 
61 to 99 4% 6% 
100 to 299 3.5% 5% 
≥300 3% 4% 

 Methods for Establishing Material Allowables and Design Values for Metals 

 Physical and Constitutive Properties 

Physical and constitutive properties are presented as typical basis (mean value) and are defined as 
a function of temperature. These values are intended to be generated on the AM material product 
form. These values are generated as described by the MMPDS. 


 Material Allowables for Tension Properties 

[AMR-77]  For Class A and B parts, statistical assessment of metallic AM material test data to 
derive material allowables for ultimate strength, yield strength, and elongation shall be governed by 
the following: 

a. Test specimens are machined to represent bulk AM material and are tested according to 
ASTM E8/E8M or an equivalent standard.  

b. Material allowables are bounded by the 99% probability at a 95% confidence one-sided 
tolerance limit estimated for the population using a properly fit statistical distribution.  

c. A minimum of 100 specimens, distributed according to the lot requirements found in 
section 6.5 of this NASA Technical Standard, are required to initially establish material allowables. 

d. Material allowables incorporate the minimum CoV requirements of section 6.11.3 of this 
NASA Technical Standard. 

e. The tensile property database is maintained by the CEO and updated on a periodic basis 
as additional data become available from process control-related activities, including witness 
sampling, preproduction article evaluations, QMP development, and machine qualification. 

f. Data analysis methodologies, except as noted in this requirement, follow the intent of the 
MMPDS guidelines for static tensile property development. 

[Rationale:  These requirements for material allowables development are designed to accommodate 
the unique, process-sensitive scenario of AM materials, while achieving the level of reliability 
associated with traditional metallic material allowables.]  

The intent of the static strength property requirements of NASA-STD-6016 are satisfied when all 
material characterizations and process controls of this NASA Technical Standard are fully 
implemented. The submittal of an MUA describing the substantiation of the MPS satisfies the NASA-
STD-6016 material property control requirements. This MPS documentation also satisfies material 
property requirements levied by other structural requirements documents, such as NASA-STD-5012 
or JSC 65828. 

When established material allowables decrease, the programmatic costs are significant. It is 
strongly recommended that initial material allowables incorporate a design value margin sufficient 
to guard against such a decrease as the supporting data population of the MPS grows and periodic 
reassessments of material allowables occur. As AM processes and the MPS data population reach 
maturity, conservatism can be reduced by reducing the design value margin, if the data support. 


 Secondary Properties 

[AMR-78]  For Class A and B parts, statistical assessment of the secondary properties of metallic 
AM materials to derive material allowables shall be governed by the following: 

a. Test specimens are machined to represent bulk AM material and are tested according to 
industry standard procedures. 

b. Material allowables are bounded by the 99% probability at 95% confidence one-sided 
tolerance limit estimated for the population using a properly fit statistical distribution.  

c. A minimum of 20 specimens, distributed according to the lot requirements found in 
section 6.5 of this NASA Technical Standard, are required to establish material allowables for 
secondary properties. 

d. Material allowables incorporate the minimum CoV requirements of section 6.11.3 of this 
NASA Technical Standard.  

e. The secondary property database is maintained by the CEO and considered for updating 
if there is a change in properties.  

f. Test and data analysis methodologies, except as noted in this requirement, should follow 
the intent of the MMPDS guidelines. 

[Rationale:  These requirements for secondary properties development are designed to 
accommodate the unique, process-sensitive scenario of AM materials, while achieving the level of 
reliability associated with traditional metallic material allowables.]  

The most common secondary properties are compression yield, shear ultimate, and bearing 
strength. In cases of minor anisotropy, tension properties in orientations other than the primary one 
can usually be treated as secondary properties. 

If secondary properties are not used in the design, then they generally do not have to be derived. If 
the design has high margins for secondary properties, then a statistically derived secondary 
property may not be necessary either; but limited testing should be performed to demonstrate the 
margin and substantiated in an MUA. 


 Fatigue Material Allowables 

[AMR-79]  For Class A and B parts, as required for structural assessment or at customer discretion, 
the MPS for any AM material shall include material allowables for fatigue developed in accordance 
with the following policies: 

a. Fatigue initiation life properties are developed in the form of stress-life or strain-life 
curves through testing according to recognized industry standards (e.g., ASTM E466, Standard 
Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic 
Materials, or ASTM E606/606M, Standard Test Method for Strain-Controlled Fatigue Testing).  

b. Specimens are machined to represent the bulk AM material and meet the surface finish 
requirements of fatigue test standards. 

c. Ten or more tests are used to define a fatigue curve for a given condition and, for HCF, a 
minimum of four tests are within 10% of the stress defined as the fatigue limit (see the definition of 
fatigue limit for this NASA Technical Standard). 

d. Fatigue properties are subject to the lot requirements of section 6.5 of this NASA 
Technical Standard. 

e. The process for developing material allowable fatigue curves from the test data is 
described in the documentation of MPS development per section 6.3 of this NASA Technical 
Standard. 

f. All fatigue curves are labeled with their basis (e.g., typical or bounding). 

g. If the MPS fatigue material allowables are applied to Class A and B parts with cycle 
counts ≥108, then fatigue test data are required to substantiate the fatigue curve in this regime, 
except for Class B parts, where a methodology for conservatively estimating such fatigue limits 
based on fatigue data at cycle counts <108 may be employed when properly documented. 

 [Rationale:  Fatigue properties are critical to AM part integrity. The baseline properties of the bulk 
material in material allowables are important to understand the relative effects of influence factors 
used to develop fatigue design values.] 

It is important that the fatigue curve basis be consistent with the analytical methodology prescribed 
by governing structural requirements. 

 Fatigue Design Values 

If a fatigue assessment is required and involves influence factors not represented in the fatigue 
allowables (see section 6.11.4.4 of this NASA technical standard), the development of fatigue design 
curves is required by section 6.11.2 of this NASA Technical Standard. 


Most factors that influence fatigue also include the propensity for increased variability. Unlike 
tensile design values, it is uncommon to sufficiently represent the fatigue influence factor through a 
simple multiplicative factor applied to the entire fatigue allowable curve. As such, fatigue design 
curves are developed from additional testing and largely independent of the baseline fatigue 
allowable curves. In such cases, it is important that sample size and lot (e.g., feedstock, heat, build) 
be considered when independently developing fatigue design curves. Documentation of these curves 
would be expected in the MUA or the PPP. The following aspects are important to the development 
of fatigue design values: 

a. Effects of surface textures rendered by the AM process are important influence factors 
that are expected to be evaluated. 

b. Surface treatment methods (e.g., honing or polishing) that do not ensure complete, 
uniform removal of all as-built surface remnants from all treated surfaces are expected to be 
evaluated as fatigue influence factors. 

c. With confirmation tests, fully machined AM surfaces may use standard surface finish 
influence factors applied to the neutral-surface fatigue material allowable curves. 

d. Surface treatments (e.g., peening) that improve fatigue life by altering the near-surface 
stress state without actually removing the surface are expected to be directly evaluated as fatigue 
influence factors. 

e. Surface treatments used to improve fatigue capability are intended to be controlled by a 
process specification and placed under process control according to section 7.4.3 of this NASA 
Technical Standard, with the application of such processes addressed in the PPP and evaluated in 
the preproduction article.  

 Fracture Mechanics 

[AMR-80]  For Class A and B parts, when a design assessment includes evaluation of crack-like 
defects by fracture mechanics, the MPS shall include fracture toughness and fatigue crack growth-
rate properties, in the worst-case material orientation, tested from AM material produced to an 
appropriate QMP, with lot requirements per section 6.5 of this NASA Technical Standard. 

[Rationale:  Fracture mechanics properties vary with material product form; characterization of 
these properties is required in the AM product form.] 

Fracture mechanics properties may be bookkept as material allowables or design values, as 
appropriate. 


 Stress Rupture and Creep Deformation 

[AMR-81]  For Class A and B parts, when required for part assessment, the MPS shall include 
material properties for stress rupture or creep mechanisms tested from AM material produced to an 
appropriate QMP, with lot requirements per section 6.5 of this NASA Technical Standard. 

[Rationale:  Stress rupture and creep properties vary with alloy product form; characterization of 
these properties is required in the AM product form.] 

Stress rupture and creep properties may be bookkept as material allowables or design values, as 
appropriate. 

 Joining (Welding/Brazing/Soldering) 

[AMR-82]  For Class A and B parts, material properties for joined AM components shall be 
developed through testing according to recognized industry standards (e.g., AWS B4.0) and 
incorporated into the applicable MPS with a statistical significance comparable to the base AM 
material properties using specimens from AM material manufactured from representative QMPs.  

[Rationale:  For structural reliability, joining properties require an equivalent reliability to that of 
the base materials. Because joint performance varies with alloy product form, characterization of 
joining properties is required to use the AM product form.]  

Joining properties may be developed as either material allowables or design values depending on 
how the tests are conducted and how the data are applied in the structural assessment. These 
properties are intended to meet the intent of specimen and lot requirements of the base AM 
materials and be assessed to similar statistical significance. The methodology for developing joining 
properties, in particular, the approach to lot variability, is expected to be included in the 
documentation of MPS development per section 6.3 of this NASA Technical Standard. 

 Methods for Establishing Material Allowables and Design Values for Polymers 

Test standards for a majority of these material characterizations are available from ASTM 
International under the jurisdiction of the ASTM D20 Committee on Plastics. 

There are numerous properties of polymeric materials not discussed in this section that may be 
highly influential part design and application. Requirements for many of these properties are driven 
by part performance or safety. When safety related, these characterization requirements are 
generally under the auspices of NASA-STD-6016. Examples of such properties outside the scope of 
this NASA Technical Standard include outgassing characteristics, dielectric properties, 
surface/volume electrical resistivity, and flammability. 


 Common Aspects Required of AM Polymer Material Characterization 

6.11.5.1.1 Importance of Anisotropy Characterization 

Due to limitations in the ability to affect microstructural evolution in polymeric AM materials, 
evaluation of the effects of anisotropy on mechanical properties may require more detailed 
evaluation than for metallic materials with fully recrystallized microstructures (see section 6.7.3 in 
this NASA Technical Standard.)  

6.11.5.1.2 Viscoelastic and Viscoplastic Response in Mechanical Properties 

[AMR-83]  For Class B parts, loading rate dependence of mechanical properties, commensurate 
with application environments, shall be accounted for during material characterization and 
derivation of material allowables and design values. 

[Rationale:  Mechanical response of polymeric materials may demonstrate significant load rate 
dependence. Understanding application loading rates and testing the material accordingly is 
required for accurate material characterization.]  

Awareness of strain rate in testing plastic materials is required by the common test standards (e.g., 
ASTM D638). These test methods typically target a standardized testing rate that leads to specimen 
failure in 0.5 to 5 minutes. If the service environment includes higher strain rates, then this 
requirement intends that material properties also be evaluated at a strain rate equal to or greater 
than that of the service environment. 

6.11.5.1.3 Notch Sensitivity 

[AMR-84]  For Class B parts, materials with plastic tensile strain capability less than 3% shall be 
evaluated for notch sensitivity, commensurate with the applicable design criteria and structural 
assessment methods. 

[Rationale:  The effects of stress concentrations on materials with limited ductility may be 
unpredictable for brittle anisotropic AM materials with potentially unique failure mechanisms. 
Testing is required to confirm assessment failure criteria assumptions.] 

Materials with limited ductility, defined here as materials with less than 3% tensile elongation 
(plastic strain), should be assessed using a “brittle material” design criteria (e.g., evaluation of the 
peak maximum principal stress at the surface of the stress concentration). Consult structural