Abstract:
A Z-axis test coupon structure and method for additive manufacturing process are disclosed. An example method of fabricating Z-axis test coupons for additive manufacturing processes, includes fabricating tensile specimens, the fabricating of the tensile specimens including providing a web between adjacent ones of the tensile specimens, and the fabricating of the tensile specimens including using an additive manufacturing process, removing the web from between the adjacent ones of the tensile specimens, and testing a tensile strength of one of the tensile specimens.

Description:
This patent arises from a divisional of U.S. patent application Ser. No. 12/772,619, filed May 3, 2010, and claims the benefit of U.S. Provisional Patent Application No. 61/266,301, filed Dec. 3, 2009. The entireties of U.S. Provisional Patent Application No. 61/266,301 and U.S. patent application Ser. No. 12/772,619 are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to additive manufacturing processes. More particularly, the disclosure relates to a Z-axis test coupon structure and a quick and repeatable method for producing Z-axis (vertical) test coupons in additive manufacturing processes that require support structure, with related methods of use. 
     BACKGROUND 
     Additive Manufacturing (AM) encompasses a variety of forms to cover an entire range of quick-response direct fabrication, typically in a layered format, of end-use articles. These end use articles generally exhibit high geometric customization, such as hearing aids, dental implants and customized applications. In addition, AM may also exhibit relatively low production volume for applications residing in formula car racing, aerospace and medical industries, for example and without limitation. 
     AM technologies have proven their capabilities for mass-customized production in different domains. The robustness of these technologies has not completely been demonstrated in a mass production context. In aerospace applications, mechanical property performance is important. The end-use parts fabricated using AM must meet the requirements of the design engineer. A common type of additive manufacturing process is known as fused Deposition Modeling (FDM). 
     Fused Deposition Modeling (FDM) is an extrusion-based process that feeds material in solid wire form and then melts it into a shape and forms a solid. FDM is a nonlaser filament extrusion process that may utilize engineering thermoplastics, which may be heated from filament form and extruded in very fine layers to build each model from the bottom up. The models may be made from acrylonitrile butadiene styrene (ABS), polycarbonate, polyphenylsulfone (PPSF), and various versions of these materials. Furthermore, in many cases, the models may be tough enough to perform functional tests. The material used is fed into an extrusion head in solid wire form and then liquefied in the extrusion head and deposited through a nozzle in liquid form. The extrusion head is able to move in the X-Y plane and is controlled to deposit very thin beads of molten material onto the build platform to form the first layer. 
     Aerospace and other applications may demand that rigorous testing and certification be carried out preparatory to using materials and processes for the manufacture of components. Moreover, specific material requirements may be associated with the part candidates for aircraft and other structures. In order for additive manufacturing to become fully adopted as a credible manufacturing process, it may be necessary to organize a path that provides a roadmap to production for candidate parts that are inclined to become AM candidate parts. 
     For FDM and other additive manufacturing processes such as SLS (Selective Laser Sintering), Z axis orientation of parts may be generally weaker than both X and Y directions of the parts. This Z axis limiting effect may be due primarily to the additive nature inherent in most all RM processes. Due to the Z axis limitation, design engineers may limit the technology to the weakest anisotropic plane and place emphasis on Z axis testing. This emphasis on Z axis testing may be relevant for many testing types. Due to budget constraints, industry may focus on one or two types of testing techniques that simulate physical conditions of applications and a large population of samples. This constraint has lead to an overall decision to test for both uniaxial tensile and flexural tests. 
     The additive manufacturing industry does not currently have a standard for building Z-axis (vertical) tensile bar coupons for processes that require support material. Therefore, a Z-axis test coupon structure and a quick and repeatable method for producing Z-axis (vertical) test coupons in additive manufacturing processes that require support structure are needed. 
     SUMMARY 
     The disclosure is generally directed to a Z-axis test coupon structure. An illustrative embodiment of the Z-axis coupon structure includes a plurality of circularly-arranged tensile specimens oriented along a Z-axis and at least one web connecting adjacent ones of the plurality of tensile specimens. 
     The disclosure is further generally directed to a method of fabricating standardized Z-axis test coupons for additive manufacturing processes. An illustrative embodiment of the method includes initiating fabrication of tensile specimens, providing at least one web between the tensile specimens, completing fabrication of the tensile specimens, removing the at least one web from between the tensile specimens and testing a tensile strength of the tensile specimens. 
     The disclosure is further generally directed to a method of developing consistent material allowables for additive manufacturing. An illustrative embodiment of the method includes providing a Z-axis coupon structure having a circular arrangement of tensile specimens; providing a set of material deposition control paths for the Z-axis coupon structure; assessing signal factors, noise factors and control factors of the Z-axis coupon structure; optimizing material laydown for maximum tensile strength of a production part based on the signal factors, the noise factors and the control factors; and developing a statistical allowables database for the production part by testing the tensile specimens. 
     The disclosure is also generally directed to a method to use the test coupon mechanical property evaluation data as a unique additive modeling Part Candidate selection method. These parts are generally highly complex, low quantity geometries that may be polymer based. They are typically difficult to construct using conventional manufacturing solutions. In addition, assembly integration, or the ability to combine several parts into a single part is taken into account during part selection. The unique elements of the FDM process compared to features of functional part candidates will indicate advantages of using one build method over another for each element trait. For an FDM candidate, each element shares a common theme that leverages a need for complex geometry that may be difficult or impossible to construct using conventional manufacturing technology. 
     In some embodiments, the Z-axis test coupon structure may include fourteen circularly-arranged tensile specimens oriented along a Z-axis and oriented at various angles with respect to at least one tool path direction. Each of the tensile specimens may include a generally elongated middle section and a lower grip and an upper grip provided at respective ends of the middle section. Orientation text may be provided on at least one of the lower grip and the upper grip. A lower web may connect adjacent ones of the plurality of tensile specimens and have a generally flat upper edge and a generally notched lower edge. An upper web may connect adjacent ones of the plurality of tensile specimens and have a generally flat upper edge and a generally notched lower edge. 
     In some embodiments, the method of fabricating standardized Z-axis test coupons for additive manufacturing processes may include initiating fabrication of tensile specimens each having a generally elongated middle section, a lower grip and an upper grip provided at respective ends of the middle section and orientation text provided on at least one of the lower grip and the upper grip; providing an upper web and a lower web each having a generally flat upper edge and a generally notched lower edge between the tensile specimens; completing fabrication of the tensile specimens; fabricating a production part simultaneously with initiating fabrication of the tensile specimens; defining at least one tool path orientation in the tensile specimens; removing the at least one web from between the tensile specimens; and testing a tensile strength of the tensile specimens. 
     In some embodiments, the method of developing consistent material allowables for additive manufacturing may include providing a Z-axis coupon structure having a circular arrangement of tensile specimens each having a generally elongated middle section and a lower grip and an upper grip at respective ends of the middle section and an upper web and a lower web between the tensile specimens and each having a generally flat upper surface and a generally notched lower surface; providing a set of material deposition control paths for the Z-axis coupon structure; assessing signal factors, noise factors and control factors of the Z-axis coupon structure; optimizing material laydown for maximum tensile strength of a production part based on the signal factors, the noise factors and the control factors; and developing a statistical allowables database for the production part by testing the tensile specimens. This database provides and supports a methodology for selecting candidate FDM parts 
    
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
       The disclosure will now be made, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of an illustrative embodiment of the Z-axis test coupon structure. 
         FIG. 2  is a top view of an illustrative embodiment of the Z-axis test coupon structure, more particularly illustrating breakaway bracing of tensile specimens in the Z-axis test coupon structure. 
         FIG. 3  is a top view, partially in section, of an illustrative embodiment of the Z-axis test coupon structure, more particularly illustrating angular orientation of the tensile specimens with respect to a tool path direction. 
         FIG. 4  is a callout diagram illustrating dimensions and structure of a tensile specimen of an illustrative embodiment of the Z-axis test coupon. 
         FIG. 4A  is an end view of the tensile specimen of  FIG. 4 . 
         FIG. 5  is a fiber placement diagram illustrating placement of fibers in a single layer of a fused deposition modulus (FDM) structure. 
         FIG. 6  is a diagram illustrating a multi-directionally loaded raster orientation of tensile specimens. 
         FIG. 7  is a diagram illustrating a uni-directionally loaded raster orientation of tensile specimens. 
         FIG. 8  is a diagram illustrating fused deposition modeling bead width. 
         FIG. 9  is diagram illustrating a 0° raster pattern. 
         FIG. 10  is a diagram illustrating a 90° raster pattern. 
         FIG. 11  is a diagram illustrating a 45° raster pattern. 
         FIG. 12  is a diagram illustrating a perimeter to raster air gap. 
         FIG. 13  is a diagram illustrating a raster to raster air gap. 
         FIG. 14  is a sample P-diagram for fused deposition modeling. 
         FIG. 15  is a perspective view of a fused deposition modeling tensile specimen nested within build volume. 
         FIG. 16  is a block diagram illustrating elements of part candidate screening for fused deposition modeling. 
         FIG. 17  is a block diagram illustrating elements of part screening for selective laser sintering. 
         FIG. 17A  is a top view of an alternative illustrative embodiment of the Z-axis test coupon structure. 
         FIG. 18  is a flow diagram of an illustrative method of fabricating standardized Z-axis test coupons for additive manufacturing processes. 
         FIG. 19  is a flow diagram of an alternative illustrative method of fabricating standardized Z-axis test coupons for additive manufacturing processes. 
         FIG. 20  is a flow diagram of an illustrative method of developing consistent material allowables for additive manufacturing using Design of Experiments (DOE). 
         FIG. 21  is a flow diagram of an aircraft production and service methodology. 
         FIG. 22  is a block diagram of an aircraft. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the described embodiments or the application and uses of the described embodiments. As used herein, the word “exemplary” or “illustrative” means “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” or “illustrative” is not necessarily to be construed as preferred or advantageous over other implementations. All of the implementations described below are exemplary implementations provided to enable persons skilled in the art to practice the disclosure and are not intended to limit the scope of the appended claims. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     Referring to  FIGS. 1-4 , an illustrative embodiment of the Z-axis test coupon structure is generally indicated by reference numeral  1  in  FIG. 1 . The Z-axis test coupon structure  1  may include a grouping of multiple ASTM D638 Type I test coupons or tensile specimens  2  oriented in the Z direction  14 . In some embodiments, the Z-axis coupon structure  1  may include at least five tensile specimens  2 . In some embodiments, the Z-axis coupon structure  1  may include fourteen tensile specimens  2 , as shown. Each tensile specimen  2  may be constructed from any fused deposition modeling (FDM) material. As shown in  FIG. 4 , in some embodiments, each tensile specimen  2  may have a generally elongated middle section  3  and a lower grip  4  and an upper grip  5  at respective ends of the middle section  3 . In some embodiments, the middle section  3  may be narrower than the lower grip  4  and the upper grip  5 . As further shown in  FIG. 4 , the middle section  3  of each tensile specimen  2  may have a gage length  6  of about 50 mm. The length  7  of the middle section  3  may be about 57 mm. The distance  8  between the lower grip  4  and the upper grip  5  may be about 115 mm. The overall length  9  of the tensile specimen  2  may be about 165 mm. The fillet radius  10  of each tensile specimen  2  may be about 76 mm. As shown in  FIG. 4A , the overall width  11  of each of each tensile specimen  2  may be about 19 mm. The overall thickness  12  of each tensile specimen  2  may be less than or equal to about 7 mm. 
     At least one web  16 ,  22  may connect the tensile specimens  2  to each other in the Z-axis test coupon structure  1 . In some embodiments, an upper web  16  and a lower web  22  may connect the tensile specimens  2  to each other. The upper web  16  and the lower web  22  may be made of the parent material from which the tensile specimens  2  are fabricated. Each of the upper web  16  and the lower web  22  may generally have a circular ring shape and may be removable from the tensile specimens  2  upon completion of fabrication of the Z-axis test coupon structure  1  at web breaks  19 , as shown in  FIG. 2 . 
     As shown in  FIG. 1 , in some embodiments, the upper web  16  of each Z-axis test coupon  1  may have a generally flat upper edge  17  and a generally arched or notched lower edge  18 . Likewise, the lower web  22  may have a generally flat upper edge  23  and a generally arched or notched lower edge  24 . As shown in  FIG. 3 , the circularly-arranged tensile specimens  2  in the Z-axis test coupon structure  1  may be oriented at various degrees with respect to at least one tool path direction or geometry in the Z-axis test coupon structure  1 . In the example shown in  FIG. 3 , the tensile specimen  2  is oriented at a 0 degree angle with respect to a tool path. The tensile specimens  2   a ,  2   b  and  2   c  are oriented at 30 degrees, 45 degrees and 60 degrees, respectively, with respect to the tool path direction. One or multiple tool path geometries may be explored within a single build of the Z-axis test coupon structure  1 . 
     The geometry of the Z-axis test coupon structure  1  may serve several functions. One function may be to replicate the layered pattern of any part built concurrently in the FDM process. The arched or notched geometry of the upper web  16  and/or the lower web  22  connecting tensile specimens  2  may act as a self-supporting feature to eliminate the need for support material to be constructed for the grouping of the tensile specimens  2 . Without the arched or notched feature, the requirement for support material may otherwise increase the estimated build time. In addition, by eliminating the need for support material, each tensile specimen  2  may require 45% less material than may otherwise be required. 
     The connecting arched geometry of the upper web and/or the lower web  22  of the Z-axis test coupon structure  1  may also act as a rigid body for the construction of the tensile specimens  2  to prevent a tensile specimen  2  from toppling over during building due to the vibrations which may be induced during the additive manufacturing process. In addition, this rigid body may also prevent the vibration inherent within the FDM process from manifesting itself as rough surfaces within the neck of each tensile specimen  2  during construction. As shown in  FIG. 4 , orientation text  13  may be provided on the lower grip  4  and/or the upper grip  5  of each tensile specimen  2 . The orientation text  13  may define the relative position of each tensile specimen during construction and may assist in accurate recording of where the tensile specimen  2  is located within the grouping of tensile specimens  2 . 
     By minimizing the use of support structure, the Z-axis test coupon structure  1  may significantly improve the build time of the tensile specimens  2 . Moreover, by reducing vibration during fabrication, repeatability between multiple builds of tensile specimens  2  may be increased. 
     In FDM, each deposited layer may be a bonded, close-packed fibrous lamina, similar to fiber reinforced composites. Using the FDM bead width as a similarity to fiber orientation of classic laminate theory, the material properties may be defined along the fiber, or, x-direction and perpendicular to the fiber, or, y-direction. Each layer of a tensile specimen  2  may be similar to an individual ply in laminate theory. 
     Expounding on these observations and recognition of classic composite laminate theory, material properties may be listed relative to on-axis coordinates. Consider that the properties of an off-axis ply, anything other than 0 degrees, can be calculated by transforming the properties of the 0-degree ply. Let 0 be the x-axis, and note that the angle θ is measured from the x-axis to the 1-axis and is positive in the counterclockwise direction; the y-axis is perpendicular to the x-axis and in the plane of the ply, as in  FIG. 5 . All subsequent calculations may be made using the x-y, or laminate axes; therefore, it may be necessary to transform the stress-strain law from the material axes to the laminate axes. If the stresses in the laminate axes are denoted by σ x , σ y , and τ xy , then these may be related to the stresses referred to the material axes by the usual transformation equations, 
                     [           σ   x               σ   y               τ   xy           ]     =       [           c   2           s   2             -   2     ⁢   cs               s   2           c   2           2   ⁢   cs             cs         -   cs             c   2     -     s   2             ]     ⁡     [           σ   1               σ   2               τ   12           ]               (   1   )               
where c denotes cos θ and s denotes sin θ. Also, the strains in the material axes may be related to those in the laminate axes, namely, ε x , ε y , and γx xy , by what is essentially the strain transformation:
 
     
       
         
           
             
               
                 
                   
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     Consider a similar approach using FDM whereas the tensile specimen highlighted in  FIG. 1  is shown in top view in  FIG. 6 . Raster orientations may be additively constructed in the FDM process. Therefore, each ply in composite laminate theory may be thought of as a raster pattern layer in the FDM process and by specifying each layers orientation to 0° each ply, or layer, the specimen may be defined to serve as a function of loading for multi-directionally loaded parts. 
     Each tensile specimen  2  may offer unique orientations to correlate to the laminate theory fundamental principles listed above. By tailoring several orientations within the same tensile specimen  2  in a circular pattern, the tensile specimen shown in  FIG. 6  represents a multi-directionally loaded part. From a testing perspective, each multi-directionally loaded tensile specimen  2  may yield the following quantity of tensile bars or specimens listed in Table I below. By gaining raster orientation direction mechanical property data relative to the Z axis, a thorough knowledge capture may be attained for the FDM process. 
     
       
         
               
             
               
               
             
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Test Specimen for Multi-Directional Loaded Geometry 
               
             
          
           
               
                   
                 Quantity of 
               
               
                   
                 Tensile Bars 
               
               
                   
                 Per Specimen 
               
               
                   
                   
               
             
          
           
               
                   
                  0° 
                 2 
               
               
                   
                 90° 
                 2 
               
               
                   
                 30° 
                 4 
               
               
                   
                 45° 
                 4 
               
               
                   
                 60° 
                 4 
               
               
                   
                   
               
             
          
         
       
     
     It may also be considered that if a design engineer designed a part to only take loads solely in a unidirectional format, an alternative tensile specimen configuration may be required. In such a situation, a raster pattern layout with all rasters constructed at 0° shown in  FIG. 7  may be appropriate to simulate the functional unidirectional loading requirement. Using a unidirectional design requirement may yield a different configuration and quantity of tensile specimens as shown below in Table II. 
     
       
         
               
             
               
               
             
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 Test Specimen for Uni-Directional Loaded Geometry 
               
             
          
           
               
                   
                 Quantity of 
               
               
                   
                 Tensile Bars 
               
               
                   
                 Per Specimen 
               
               
                   
                   
               
             
          
           
               
                   
                  0° 
                 16 
               
               
                   
                 90° 
                 0 
               
               
                   
                 30° 
                 0 
               
               
                   
                 45° 
                 0 
               
               
                   
                 60° 
                 0 
               
               
                   
                   
               
             
          
         
       
     
     In summary, for FDM, the orientation of the raster pattern may be tailored to the specific design function needs required by the design engineer. These tensile specimens  2  may then be constructed concurrently with the production part and tested to ensure that product performance has been established. However, before any part is produced for production, the process may be optimized for structural integrity. 
     The second stage in the development of performance of FDM may include a thorough optimization effort to maximize the mechanical property performance of the Z direction. In parameter design, the engineer may select an effective characteristic to measure as data. Quality problems may take place because of variability in the energy transformation. Using FDM as an example, the process may be broken down to include an investigation of several parameters that influence mechanical property performance in the Z axis direction. 
     Mechanical property variability for the Fused Deposition Modeling process has been researched extensively. Several energy transfer parameters inherent to the FDM process may include raster orientation, raster air gap, bead width, and build temperature. Build temperature may become unavailable to users of newer FDM technology. Build temperature may be automatically controlled via a microchip material canister. Despite this restriction, bead width, raster orientation, and raster air gap may remain available to be adjusted by the end user. 
     The bead width may be dictated by the FDM user to provide a width to the toolpaths taken by the process. Two types of bead width are contour and raster. The contour is the outside wall of an X-Y sliced plane. The raster is the filled pattern inside the contour.  FIG. 8  illustrates the bead width cross section  34  of a raster pattern  35 .  FIG. 9  is diagram  36  illustrating a 0° raster pattern  37 . 
     Raster angle may be defined as the relative angle placement of contours in the Z direction. If a cross section were taken of a tensile specimen  2  in the Z direction  FIG. 11  would highlight a 0°, 90°, 45° raster  41  orientation pattern  40  respectively. 
     The next parameter allowed for adjustment may be the perimeter to raster air gap. This feature may be defined as the amount of bead overlap between the interior fill material and the outside contour of a planar cross section. By lowering the airgap value, the user may create more overlap and reduce the voids between the raster pattern and the outside contour.  FIG. 10  illustrates a build plane cross-sectional cut  38  of a piece of geometry reflecting perimeter to raster air gap. 
     A third parameter allowed for adjustment may include raster to raster air gap. This factor may be defined as the amount of interstitial bonding among horizontal fill rasters. By reducing the raster to raster airgap, more overlap may create fewer voids in the process. If the airgap is reduced too much, then backpressure may occur in the deposition process and result in clogged deposition tips.  FIG. 12  illustrates raster to raster airgap  44 .  FIG. 13  illustrates a no raster to raster airgap configuration  46  juxtaposed with a raster to raster airgap configuration  47 . 
     It has been noted in industry that if several FDM machines are used for production, each may need parameter adjustments to reduce the amount of variation that exists within the process to consistently produce direct parts that exhibit similar mechanical properties. Using the known parameters, a planned Design of Experiment (DOE) may be conducted to determine the appropriate settings for maximum mechanical properties in the Z direction. An expansive list of research has been completed regarding the use of Designed Experimentation for FDM, specifically the popular Taguchi method. The objective of this research is to simply introduce the audience to steps involved for appropriate technology optimization, for more detailed information regarding Designed Experiments consult Fowlkes and Creveling (1995). 
     The first step of a planned DOE shall be the development of a P-Diagram.  FIG. 14  offers a P-Diagram example  57  that may be used as a model for FDM testing. The P-diagram of Z-bar testing  57  may include noise factors  54  (different machines, different cans of material), signal factors  55  (contour width, part raster width) and control factors  56  (raster angle, perimeter to raster airgap, raster to raster airgap) which are used to determine a quality characteristic (maximum ultimate tensile strength  58 . From this point, a partial orthogonal array may be used to develop an effective control factor effect plot of each of the control factors to determine their levels of influence to the overall system. Based on applied research, Taguchi method of Robust Design may include development of a P-Diagram  57 , choose an orthogonal array to represent the FDM tensile bar layout, run a static controlled experiment and conduct an analysis of means (ANOM) to determine the appropriate combination of parameters that results in the highest tensile strength  58 . 
     Next, in order to understand variation within the system, another partial arrayed experiment and tensile test may be conducted to capture Signal to Noise ratio as the metric for system variation. Because the tensile properties are to be maximized, the larger-the-better S/N ratio may be used and illustrated below: 
     
       
         
           
             
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     Whereas MSD is the mean squared deviation of data, n is the number of samples and y i  are the individually measured response values squared to increase or bias the impact of any large y values. 
     Upon determining the S/N, the next step may be to perform an analysis of variation (ANOVA) to determine overall system robustness. ANOVA may use a mathematical technique known as the sum of squares to quantitatively examine the deviation of the control factor effect response averages from the experimental mean response. Using ANOVA, the process may be understood to gain knowledge into which control factors are of higher criticality and which are of lesser importance. 
     To begin the ANOVA process, one may calculate the grand total sum of squares (GTSS). This may be calculated by the following: 
     
       
         
           
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     Once computed, an error variance estimating may be conducted and F-ratio testing may be analyzed to test for significance of factor effects. Upon completion of the ANOVA test, data may be analyzed to determine which factors contribute to the overall variance of the process. This information may be helpful to develop an optimization plan for the process. 
     Once the FDM system is optimized for Z axis properties, a database of tensile properties may exist to aid design engineers in making decisions regarding performance of FDM. Known in industry as design allowables, these data may offer substantiation for candidate parts to perform in relative operational conditions. The larger the database, the more credibility may be added for AM to act as a legitimate manufacturing process. Finite Element Analysis packages may generally allow for isotropic property input for modulus; therefore, the design engineers may use Z axis modulus data to determine a worst-case condition for operating usage environments. 
     Next, candidate parts may be selected to exploit the FDM process. The tensile specimen  2  may be placed within the build volume as shown in  FIG. 15 . Each tensile specimen  2  may offer a unique build pattern to match closely to a part fabricated in the FDM process. This build pattern may allow for part geometry to be constructed with most feature definition simulated via tensile specimen grouping with the test geometry shown in  FIG. 15 . 
     Once a representative part is constructed within a build volume as depicted in  FIG. 15 , the tensile specimen  2  may be removed from the build chamber, broken into individual tensile specimens  2  and tensile tested per ASTM D638 conditioning. Data may be analyzed to provide a clear understanding of the range of raster orientations contribution to Z axis tensile strength. As shown in  FIG. 2 , the tensile specimens  2  may be separated from each other by forming web breaks  19  in the upper web  16  and/or the lower web  22 . 
     Part candidate selection may go hand-in-hand with mechanical property evaluation. These parts may be generally highly complex, low quantity geometries that may be polymer-based. These parts may be difficult to construct using conventional manufacturing solutions. In addition, assembly integration, or the ability to combine several parts into a single part, may be taken into account during part selection. 
     Once parts are selected, correct identification of appropriate parts that can/could be manufactured may be necessary. There may be parts that are suitable for AM, parts that are competitive with conventional techniques and some parts that can only be manufactured additively—it is these last two categories that may be pursued, but a robust methodology for identifying these applications may be investigated. 
     A single part may not be compared among technologies without first leveraging the unique characteristics of the emerging manufacturing process. The main emergent technology trait of AM may be part design flexibility. Using FDM as element of AM,  FIG. 16  shows the unique elements  61  of the FDM process  60  and what features of functional part candidates  62  may be an attractive trait for each element. For FDM, each element may share a common theme that leverages a need for complex geometry that is difficult or impossible to construct using conventional manufacturing technology. The target application, in the case of FDM, tooling and fixtures, may be generated from blending the elements aligned to the FDM technology. 
     Because SLS (Selective Laser Sintering) may be a separate AM technology than FDM, its fundamental process elements may be different than FDM; therefore, a different model may be constructed for SLS. SLS may be more aligned to encapsulated geometry; however, its material choices may be much more limited. Also, SLS may not be as accurate as FDM. However, SLS may offer an advantage in that its support mechanism for part construction may be self-supporting powder. Therefore, a snapshot of how SLS screening  64  with the various SLS components  65  may look is illustrated in  FIG. 17 . Due to its ability to build encapsulated geometry easily, SLS may offer the most design flexibility of comparative AM technologies and position itself for part integration opportunities. 
     Referring next to  FIG. 17A , an alternative illustrative embodiment of the Z-axis test coupon structure is generally indicated by reference numeral  1   a . The Z-axis test coupon structure  1   a  may be similar in design to the Z-axis test coupon structure  1  which was heretofore described with respect to  FIGS. 1-4 . In the Z-axis test coupon structure  1   a , tensile specimens  28  may be connected to each other in edge-to-edge relationship in a circular configuration via at least one web  30  which connects the specimen edges  29  of the adjacent tensile specimens  28  to each other. 
     Referring next to  FIG. 18 , a flow diagram  1800  of an illustrative method of fabricating standardized Z-axis test coupons for additive manufacturing processes is shown. In block  1802 , orientation of a raster pattern may be tailored to the design function needs of a part. In block  1804 , fabrication of tensile specimens may be initiated. In block  1804   a , the production part may be fabricated simultaneously. In block  1806 , at least one web may be provided between the tensile specimens during fabrication. In block  1808 , fabrication of the tensile specimens may be completed. In block  1810 , the web or webs may be removed from between the tensile specimens after fabrication. In block  1812 , the tensile strength of the tensile specimens may be tested to ensure performance of the production part. 
     Referring next to  FIG. 19 , a flow diagram  1900  of an alternative illustrative method of fabricating standardized Z-axis test coupons for additive manufacturing processes is shown. In block  1902 , orientation of a raster pattern may be tailored to the design function needs of a part. In block  1904 , fabrication of tensile specimens may be initiated. In block  1906 , at least one web may be provided between the tensile specimens during fabrication. In block  1908 , fabrication of the tensile specimens may be completed. In block  1910 , the web or webs may be removed from between the tensile specimens after fabrication. In block  1914 , the tensile strength of the tensile specimens may be tested to ensure performance of the production part. In block  1916 , the production part may be fabricated after optimization in block  1914 . 
     Referring next to  FIG. 20 , a flow diagram  2000  of an illustrative method of developing consistent material allowables for additive manufacturing using Design of Experiments (DOE) is shown. In block  2002 , a Z-axis test coupon structure having a circular arrangement of tensile specimens may be provided. In block  2004 , a set of material deposition control paths may be provided. In block  2006 , signal factors, noise factors and control factors of the Z-axis coupon structure may be assessed. In block  2008 , material laydown for maximal tensile strength of a production part may be optimized based on the signal factor, the noise factors and the control factors. In block  2010 , a statistical allowables database may be developed by testing the tensile specimens. 
     Referring next to  FIGS. 21 and 22 , embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method  78  as shown in  FIG. 21  and an aircraft  94  as shown in  FIG. 22 . During pre-production, exemplary method  78  may include specification and design  80  of the aircraft  94  and material procurement  82 . During production, component and subassembly manufacturing  84  and system integration of the aircraft  94  takes place. Thereafter, the aircraft  94  may go through certification and delivery  88  in order to be placed in service  90 . While in service by a customer, the aircraft  94  may be scheduled for routine maintenance and service  92  (which may also include modification, reconfiguration, refurbishment, and so on). 
     Each of the processes of method  78  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 22 , the aircraft  94  produced by exemplary method  78  may include an airframe  98  with a plurality of systems  96  and an interior  100 . Examples of high-level systems  96  include one or more of a propulsion system  102 , an electrical system  104 , a hydraulic system  106 , and an environmental system  108 . Any number of other systems may be included. Although an aerospace example is shown, the principles of this disclosure may be applied to other industries, such as the automotive industry. 
     The apparatus embodied herein may be employed during any one or more of the stages of the production and service method  78 . For example, components or subassemblies corresponding to production process  84  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  94  is in service. Also one or more apparatus embodiments may be utilized during the production stages  84  and  86 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  94 . Similarly, one or more apparatus embodiments may be utilized while the aircraft  94  is in service, for example and without limitation, to maintenance and service  92 . 
     Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.