5.1 Certification

System-level Certification

As the highest level of certification, system certification is the farthest removed from the manufacture of an AM part. As mentioned above, a large part of certification has to do with system tests (flight tests in the case of an aircraft, crash tests for automobiles), full-scale structural tests, subsystem tests, simulations, and software testing. When the AM part is either carrying load or necessary to operate, a test of the system or subsystem is necessary for system level certification. However, many parts in a system may not be fully loaded in a certification test.

Let’s use an example: A bracket that attaches the overhead bins in an aircraft. The brackets are required to not fail and drop luggage on the passengers if the aircraft undergoes a hard landing. Because of the difficulty and risk entailed to correctly perform a test that fully simulates a hard landing, that part of the certification is done by analysis. That is to say, the calculations related to the design, loads, and material properties are analyzed to show compliance with the requirements.

In the case of an AM part used in the overhead bins, we are concerned with analyzing two aspects: 1) the design of the part itself and how it attaches to the surrounding structure, and 2) the strength of the AM material comprising the part. Because one of the benefits of AM is the ability to design parts of new, and more efficient geometries, additional structural analysis and verification may be necessary. This may require subsystem tests in a lab on the part of the overhead bin attachment hardware that includes the AM part. The part would be tested to a load greater than would be encountered in such a crash landing. The other way in which the use of AM is involved in certification is the determination of the strength of the actual material of which the AM part is made, and how consistent those properties are. This will be discussed in a later section.

Part Certification

Unlike system certification, that may only happen a few times in the lifecycle of a product family (airplane type in the example above), part certifications happen hundreds, or even thousands of times, in a single year for a product. This certification, which is common across many industries and manufacturing processes, entails the manufacturer of a part or a subsystem certifying that the part or subsystem being delivered meets its technical requirements. In the case of an AM part shipped from the part maker to the system builder, a document known as a Certificate of Conformance is either provided with the hardware or delivered electronically to the system builder. This Certificate of Conformance states that the manufacture is not only certifying that the part meets dimensional requirements, but that it was manufactured in accordance with the appropriate process, using the appropriate feedstock, and has passed all other inspections, such as metallurgical, mechanical property, nondestructive testing, hardness, and any other requirements.

Operator Certification

Another type of certification that takes place in AM is the certification of the operators of equipment. This is also not unique to AM, as many critical processes, such as welding, heat treatment, and composite part fabrication require certified operators. Operator certification usually requires a certain amount of classroom and on-the-job training, along with demonstrations of proficiency, such as written and practical (on-machine) examinations. Once an operator has demonstrated the necessary level of proficiency, they are then certified as operators, and allowed to produce production hardware on their own. These certifications are often tracked in a company’s manufacturing resource planning (MRP) systems, that will only allow a certified operator to either initiate or electronically sign off a manufacturing operation.

Material Certification

The final level of certification relevant to AM is Material Certification. This has to do with ensuring that the feedstocks used to make the parts (liquids, powders, wires, foils) conform to the relevant specifications. Like part certifications, the Certificate of Conformance, which is a document certified by a competent authority that the supplied good or service meets the specifications, may be provided with the feedstock or delivered electronically. In some cases, they may just list the specification or specifications the feedstock conforms to, and in other cases, contain the actual test values and comparison with the specification values. While material is often certified to a public specification, such as an ASTM, SAE Aerospace Material Specification (AMS), or Military Specification, in the case of AM, many Original Equipment Manufacturers (OEMs, e.g. General Electric, Airbus, Toyota, Wabtec, Stryker) have their own proprietary specifications. In this case, a single lot of material may be certified to multiple specifications

Development of Design Values

Key to certifying a system made from engineered materials is knowing the properties of those materials. If the stiffnesses of the materials are not known, then one cannot be confident of the load paths or the stresses in the parts. If the strength properties of the materials are not known, one cannot be confident that the parts will be able to withstand the loads or stresses they are subjected to. If the thermal properties of the materials are not known, one cannot be confident of the operating temperature of that part and the surrounding parts, and their ability to withstand the temperatures without static, fatigue, creep, or excessive oxidation failure. Thus, in order to certify a system, it is necessary to know with confidence the properties of the materials that the parts are made from.

This knowledge of properties includes the following:

1. Knowing the strength that a significant amount of the population of a material will have
2. Knowing the physical properties of a material
3. Understanding the relationships between material composition, processing, and strength
4. Knowing the impact of environmental conditions on the material properties

The first of these often requires the development of what might be called “minimum” properties. They are minimum properties because it is a statistical characterization describing what can be expected to be achieve a very high percentage of the time, so therefore the process should always be able to deliver higher properties ensuring the part’s success. One can imagine that if a design used median properties, it could be expected that as much as half of the parts used would have insufficient strength to meet service needs. In most industries, that would be considered a deficient design, and the system would not be certified. Therefore, many industries use minimum material properties in determining a design. Because knowing the true minimum properties of a material would require testing all of it, industries have developed methodologies for determining the minimum properties used for design.

One of the most widely used methods is the aerospace industry standard known as Metallic Materials Properties Development and Standardization (MMPDS), currently in its 14th version. The methods used in this standard have been developed over several decades with participation by the global aerospace industry and its regulators. It contains a chapter of over 200 pages that lays out the methodology, requirements, and the means of presenting the data. The types of Design Mechanical and Physical properties published in this are listed below.

• A-Basis Values – This is the value that 99% of the material will be stronger than with 95% confidence
• B-Basis Values – This is the value that 90% of the material will be stronger than with 90% confidence
• S-Basis Values – This is the minimum value that a specification will allow but does not have the statistical basis of A-Basis or B-Basis values

The criticality of the part will often determine the need for A-Basis, B-Basis, or S-Basis values. Because there are no published design properties for AM metals in MMPDS, we will use examples from wrought metals to illustrate these properties. The figure below contains the table of properties for 2024-T351 aluminum alloy plate, made to the Society of Automotive Engineering (SAE) Aerospace Material Specification (AMS) 4050.

Figure 5.3 MMPDS Design Mechanical and Physical Properties for 2024-T351 Aluminum Alloy Plate

The MMPDS table contains values for the following:

• Tensile (Ftu)
• Yield Tensile (Fty)
• Compression Yield (Fcy)
• Shear Ultimate (Fsu)
• Bearing Ultimate (Fbu)
• Bearing Yield (Fby)

Note that the properties are different for different directions, illustrating that the specific material, like most engineered materials, is anisotropic. It also has S-basis values for elongation. Finally note that the physical properties (moduli, Poisson’s ratio, density, heat capacity, thermal conductivity, and thermal expansion) provided are not minimum properties, but averages. This is because unlike strength properties, where bigger is better, and having a statistically derived minimum value provides design confidence; higher or lower physical properties may not be any better, hence the use of average properties. Note that in some very specialized applications, it may be necessary to have statistically based physical properties.

There are different sets of columns for different thicknesses. In the case of wrought aluminum alloy plate made to industry standards, there is an inverse relationship between strength and thickness. Additionally, these properties are achieved in material that has received this specific thermo-mechanical treatment (solution annealed, quenched, stretched, and overaged). In the case of a developing technology like AM, the understanding of these variables is a work in progress.

While the over 200 pages in MMPDS lays out the methodology, requirements, and the means of presenting the data is well established for wrought materials, the methods for AM materials still being developed. One of the primary questions for AM is establishing the range of part geometries and processing parameters to include in a data set for determining the design values. Until such methods are standardized and validated, design authorities are independently developing methods. Depending on the circumstances, different approaches are being used, examples of which are below.

• Part Specific Design Values – This entails building a large number of the same part, excising test coupons from them, and deriving the properties based on the test results and applying the proper statistics.
• Part Family Design Values – This can be developed from the start or can be an extension of Part Specific Design Values, and entails building replicates of a number of parts that are closely related in terms of geometry and processing parameters, and deriving the properties based on the test results and statistics. If done as an extension of Part Specific Design Values, it would usually involve building and testing parts similar to the original one and verifying consistency of properties.
• Feature Based Design Values – This is very much like those shown earlier for 2024-T351 plate (thickness). The key challenge in developing these for AM is determining if there is any good relationship with features exist, and how to demonstrate that. Would the appropriate ones be feature thickness, overall cross-sectional area, layer thickness, or some much more arcane definition? Additionally, the appropriate feature for one material or process may not be appropriate for another one.

Recall the property list above. Item four, knowing the impact of environmental conditions on the material properties, means that it is also necessary to understand environmental effects on material properties. For example, temperature has an effect on materials, such as impacting the temperature on tensile strength. So, if an AM part is to be used in elevated temperature conditions, it will be necessary to have an understanding of these effects. MMPDS may also include data tables or graphs/curves pertaining to environmental impacts such as temperature.

Depending on the application, it may also be necessary to understand the performance of a material under cyclic loading (fatigue) conditions. S-N curves show fatigue values, and can be applied to a variety of material applications, such as a notched or unnotched item. In the case of fatigue (sometimes referred to as durability), the wide scatter often makes it difficult or impossible to determine a statistically based minimum value. In that case, an average curve may be developed and used for analysis, but the certification requirements may require that the part be designed to last many (2 or more) lifetimes than predicted. Depending on the fatigue environment encountered, it may be necessary to develop a unique curve for that ratio.

In some critical applications (safety-of-flight aircraft parts, pressure vessels, underwater oil valves), it is also necessary to know the damage tolerance of a material. This requires knowing how quickly a crack will grow, and how long it can be before catastrophic failure occurs. These are based on properties known as crack growth resistance and fracture toughness. These properties (usually a minimum for toughness, and average values for crack growth), along with the assumptions of an initial crack size, which is related to the NDT methods used and the service environment; need to be known with some confidence to certify a system that has damage tolerance requirements.

While most of the properties of interest are mechanical and physical properties, in some cases, product-specific properties are required. Examples of these are:

• Flame, smoke, and toxicity for materials used in aircraft cabins and passenger rail cars to prevent small fires from killing the passengers or preventing them from escaping.
• Chemical-specific corrosion properties in chemical plants.
• Ignition temperatures in automotive braking systems.