【Special Topic Review】Overview of Testing Methods and International Standards for Polymer Additive Manufacturing

Abstract: Mechanical property testing of materials and components produced via additive manufacturing (AM) is crucial for ensuring their structural integrity and functionality. Currently, two major international standardization organizations, ASTM and ISO, have established a series of testing standards applicable to traditional materials and composites. However, the applicability of these standards in the field of additive manufacturing still requires further validation. Due to the unique characteristics of AM processes (e.g., interlayer bonding, anisotropy, internal defects), directly applying traditional testing methods may not accurately characterize the mechanical behavior of AM materials.

Keywords: Additive Manufacturing, Mechanical Properties, Standards, Anisotropy

Currently, two international standardization organizations are dedicated to research in the field of additive manufacturing. The ASTM Committee F42 oversees the technical domain of additive manufacturing, with its F42.05 subcommittee specifically handling standards related to materials and processes. The ISO Technical Committee TC 261 is responsible for formulating international standards in the field of additive manufacturing. Both organizations currently address the mechanical property testing of AM materials and components by referencing existing standards. The following analysis elaborates on the applicability of current standards in the mechanical testing of polymer additive manufacturing materials and components.

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I. Tensile Testing

Relevant standards primarily target plastics (ASTM D638, ISO 527-2) and composites (ASTM D3039, ISO 527-4). These standards utilize dumbbell-shaped specimens or specimens with end tabs, with geometries based on sample thickness or composite type. Tensile testing yields parameters such as Young's modulus, Poisson's ratio, yield stress, ultimate strength, and elongation at break. Composite standards address fiber orientation issues, but their applicability to AM materials has not been fully elucidated in the literature. Ahn found that the geometry of ASTM D638 Type I specimens can lead to premature failure due to stress concentration in the radius region near the gauge length of the dumbbell-shaped specimen. This region contains fiber ends, causing excessive shear. Researchers switched to the ASTM D3039 specimen geometry to address this issue, but this is the only relevant case found in the literature. ISO 458 (not discussed in this text) specifies standards for stiffness testing under torsional loading.

II. Flexural Testing

ASTM D790 and ISO 178 are equivalent standards that employ the three-point bending method to measure flexural modulus, flexural strength, flexural stress, and fracture strain within a 5% strain limit. If the strain limit cannot be met, the four-point bending method of ASTM D6272 is used to increase the likelihood of obtaining failure measurements. This test reduces stress concentration associated with the center roller in three-point testing. These standards are applicable to both unreinforced and reinforced materials. For composites containing high-modulus fibers, ASTM D7264 should be used. However, the standards do not address the unique challenges faced by AM materials, which may exhibit anisotropic properties.

III. Compression Testing

Standards applicable to compression testing include ASTM D695 and ISO 604. ASTM D3410 and ISO 14126 specifically address in-plane compression property testing of fiber-reinforced composites. These standards can measure compression modulus, compression yield stress, ultimate compression strength, and ultimate compression strain, with geometric dimensional restrictions on specimen diameter and height.

IV. Shear Testing

Various standard test methods exist for measuring the shear modulus and strength of materials. Standards for fiber-reinforced composites (ISO 14129, ISO 14130, ASTM D2344, and ASTM D3518) are not directly applicable to additive manufacturing. These methods are developed for high-strength fiber or fabric-reinforced polymers with specific orientations, requiring the determination of specific interlaminar failure modes between oriented fibers—specimens that are typically not obtainable through additive manufacturing. ASTM D7078 and ASTM D3846 are notched specimen standards for measuring shear properties, utilizing specimens with specific notch geometries and oriented fiber reinforcement. These testing methods may not be directly applicable to AM materials for two reasons: first, AM parts do not exhibit the high modulus and failure strength ratios in different directions observed in fiber composites, leading to differences in load distribution and crack propagation mechanisms; second, composite laminates can be manufactured with sharp, pre-defined initial cracks in the interlaminar matrix to improve testing accuracy, but current AM heat treatment processes struggle to achieve such well-defined initial cracks, affecting the characterization of failure behavior. Although methods for introducing sharp cracks exist (e.g., fatigue methods used for metals), they are not suitable for polymer materials, and using sharp-edged tools like razors may not produce appropriate cracks in AM parts.

Currently, only two shear standards are directly applicable to additive manufacturing: ASTM D4255 and ISO 15310, which are used to determine the shear modulus of plastics and fiber-reinforced materials, respectively. These standards allow testing of isotropic materials but do not provide guidance specifically for AM materials.

V. Creep Testing

Creep measurement standards provide methods for measuring dimensional changes in samples under various exposure environments (e.g., temperature, aqueous solutions, or surfactant solutions). Test load environments include various forms such as tension, compression, bending, and solution immersion. ASTM D2990-09 references ASTM D543 Standard Practices for Evaluating the Resistance of Plastics to Chemical Reagents to specify solution compositions under environmental conditions, with the corresponding ISO standard being ISO 899. The standards impose strict requirements on the aspect ratio of specimens and recommend selecting at least two different test temperatures within the material's service range to evaluate temperature effects. Long-term performance data can be obtained through creep rupture tests lasting up to 3000 hours at seven stress levels. Design creep data are obtained through multiple stress level tests that produce 1% strain in the material within 1000 hours, but the standards do not provide guidance for anisotropic samples (e.g., fiber composites).

VI. Fatigue Testing

ASTM D7774 is a standard for uniaxial loading fatigue testing (with no corresponding ISO standard), with a test frequency range of 1-25 Hz (recommended below 5 Hz to avoid sample heating). This method establishes stress/strain-cycle number relationship curves, with specimen failure or reaching 107 cycles as the fatigue limit. The choice of 107 cycles is to control test duration, but specific applications may require adjustments. The load range is defined by the R ratio (minimum to maximum stress/strain ratio), and testing is conducted within the material's elastic limit, with tensile or compressive loads applied.

ASTM D7791 and ISO 13003 are methods for plastic flexural fatigue testing, with technical differences: ASTM employs three-point or four-point loading with positive-negative bidirectional cycling and an R ratio of -1, not exceeding the proportional limit; ISO determines the loading rate by calculating the ultimate tensile/flexural strength, with specimen stiffness reduction of 20% as the termination condition. Both standards do not address the material anisotropy issues caused by AM processes.

ISO 15850 and ASTM D6115 involve fatigue delamination/crack propagation testing, specifically measuring the interlaminar fracture energy of fiber composites. Similar to other composite-specific standards, it is uncertain whether AM materials conform to the fracture mechanics assumptions underlying these standards.

VII. Fracture Toughness Testing

Fracture toughness testing is used to determine the energy required for pre-crack propagation in materials or composites, with values used in component design and material development. These standards typically require pre-fabricating sharp cracks in the material and calculating fracture energy (GiC) and fracture toughness (KiC) through linear elastic fracture mechanics analysis (the subscript i denotes Mode I, II, or III loading, see Figure 6). Composite testing is used to determine interlaminar fracture toughness in materials containing high-modulus fibers/fabrics, while polymer testing provides material parameters for engineering design. Implementing these standards may pose challenges due to the size limitations of AM processes, which require pre-fabricating sharp cracks.

ISO 15024 and ASTM D5528 are designed for fiber-reinforced composites to generate crack resistance curves (R-curves) for evaluating delamination resistance, but they are not directly applicable to AM due to the lack of continuous fibers. ISO 29221 measures plane strain arrest toughness using compact tension specimens, requiring pre-fabricated sharp cracks and grooves to limit crack propagation paths, but the effects of AM's dimensional accuracy and build direction on crack propagation are unclear. ISO 13586 is applicable to rigid/semi-rigid thermoplastics and discontinuous fiber composites, with its amendment ISO 13586 providing guidance for longitudinal/transverse testing of injection-molded composites, which may serve as a starting point for evaluating the applicability of this standard to AM. ASTM D6068 studies cohesive zone model parameters for crack propagation in plastic materials through J-R curves, requiring crack processing on specimens formed by powder bed fusion of transparent thermoplastics or material jetting of photosensitive resins.

VIII. Impact Testing

ISO 179 and ASTM D6110 specify Charpy impact test methods, while ISO 180 and ASTM D256 specify Izod impact test methods. Although many data sheets for AM polymers mention impact testing, they do not specify material preparation and orientation details. The main differences between the two tests lie in specimen placement and notch direction: in Izod testing, the specimen is placed vertically with the notch facing the impact hammer; in Charpy testing, the specimen is placed horizontally with the notch facing away from the impact hammer (the notch can be V-shaped or U-shaped). Similar to fracture toughness testing, it is currently uncertain whether notches should be directly formed during AM or machined afterward.

IX. Bearing Strength and Open-Hole Compression Testing

These tests evaluate the functional strength of composite bolt joints and can study the impact of damage zones on performance. Specific standards include ASTM D953-10, ASTM D5961, ASTM D6484, as well as ISO 12815 and ISO 12817. Although there is currently no documented demand for these standards in the AM literature, as AM parts (e.g., human implants) are increasingly integrated with other structures, the industry needs to understand the impact of component design on load-bearing capacity and long-term deformation behavior. Current standards should be directly applicable, but additional explanations regarding material anisotropy are needed.

Summary

In summary, existing mechanical testing standards provide a basic framework for performance evaluation of AM materials and components. However, in practical applications, unique challenges posed by AM processes, such as anisotropy, internal porosity, and interlayer bonding strength, must be considered. Some standards (e.g., shear, fracture toughness testing) may not be directly applicable to AM materials due to their reliance on specific specimen preparation methods (e.g., pre-fabricating sharp cracks) or fiber-reinforced structures. In the future, it will be necessary to develop or revise testing standards tailored to the characteristics of AM processes, focusing on specimen preparation methods, the relationship between loading direction and build direction, and the influence of microstructure on mechanical behavior. Additionally, as AM continues to penetrate various fields (e.g., aerospace, biomedical), the improvement of functional testing standards (e.g., dynamic loading, environmental aging) will become an important research direction.