Experimental Investigation of Strength–Ductility Trade-Offs and Failure Mechanisms in SLS-Fabricated Nylon 12 and Glass-Fibre-Reinforced Nylon

Technical Summary: Experimental Investigation of Strength–Ductility Trade-Offs and Failure Mechanisms in SLS-Fabricated Nylon 12 and Glass-Fibre-Reinforced Nylon

Problem Statement
The rapid adoption of Selective Laser Sintering (SLS) in additive manufacturing (AM) has created a demand for lightweight, high-performance polymer composites, particularly for automotive and aerospace applications. While Nylon 12 is a standard material for SLS due to its balance of strength and processability, there is a critical need to understand how reinforcing it with glass fibers affects its mechanical behavior under identical processing conditions. Existing literature often lacks systematic, side-by-side comparisons of pure Nylon 12 and glass-fiber-reinforced Nylon fabricated via SLS. Furthermore, inconsistencies in reported mechanical properties arise from variations in processing parameters and testing methodologies. A significant gap exists in understanding the specific trade-offs between strength, ductility, and hardness in SLS-fabricated composites, as well as the failure mechanisms—such as interfacial debonding and porosity—that may negate the theoretical benefits of fiber reinforcement.

Methodology
This study employed a systematic experimental approach to compare pure Nylon 12 and a 70:30 (by weight) Nylon 12/glass fiber composite. Both materials were fabricated using Selective Laser Sintering under identical, optimized process parameters to ensure a valid comparison. Key SLS parameters included a laser power of 20–30 W, scan speeds of 2500–3500 mm/s, layer thicknesses of 0.10–0.12 mm, and a consistent horizontal build orientation.

Standardized specimens were prepared according to ASTM guidelines (D638 for tensile, D790 for flexural, D256 for impact, and D785 for hardness) to minimize experimental variability. The mechanical characterization included:

• Tensile and Flexural Testing: To determine ultimate strength, yield strength, and stiffness.
• Impact Resistance: Evaluated using the Izod method to assess energy absorption.
• Hardness Testing: Conducted using both Rockwell (HRL) and Shore D scales.
• Microstructural Analysis: Scanning Electron Microscopy (SEM) was utilized to examine fracture surfaces, fiber distribution, and interfacial bonding.
• Uncertainty Analysis: A comprehensive error propagation analysis was performed, estimating uncertainties within ±2–3% for strength measurements and ±1–2% for impact and hardness.

Key Results
The experimental data revealed a distinct trade-off between ductility and surface hardness, with unexpected reductions in bulk strength for the composite:

• Tensile and Yield Strength: Contrary to the general expectation that reinforcement increases strength, the glass-fiber composite exhibited significantly lower performance than pure Nylon. Pure Nylon achieved a mean tensile strength of 36.58 MPa and yield strength of 35.02 MPa, whereas the composite dropped to 23.84 MPa (tensile) and 22.47 MPa (yield). This represents a reduction of approximately 34.8% in tensile strength and 35.8% in yield strength.
• Ductility (Elongation): Pure Nylon demonstrated superior ductility with an elongation at break of 30.74%. The composite showed a marked reduction to 16.30%, indicating a transition toward semi-brittle behavior.
• Flexural Strength: Similar to tensile results, the composite showed lower flexural strength (31.03 MPa) compared to pure Nylon (34.76 MPa).
• Hardness and Impact: The composite outperformed pure Nylon in surface resistance and energy absorption. Shore D hardness increased from 65.3 (Nylon) to 69.5 (Composite). Impact resistance also improved, rising from 0.63 J for Nylon to 0.85 J for the composite, though both remained lower than standalone glass fiber (1.42 J).
• Microstructural Findings: SEM analysis identified the root causes of the strength reduction. The composite exhibited non-uniform fiber distribution, fiber agglomeration, and significant interfacial debonding (fiber pull-out). Additionally, the presence of fibers disrupted the sintering process, leading to increased porosity and micro-voids that acted as stress concentrators.

Significance and Contributions
The primary contribution of this study is the direct, multi-property comparison of SLS-fabricated pure Nylon 12 and its glass-fiber-reinforced counterpart under controlled conditions. The paper challenges the assumption that fiber reinforcement invariably enhances mechanical strength in SLS processes. Instead, it demonstrates that without optimized interfacial bonding and fiber dispersion, reinforcement can lead to a degradation of tensile and flexural properties.

The study provides a framework for distinguishing between "notional" reinforcing advantages and the "actual" performance achieved through SLS processing. It highlights that while glass fiber reinforcement improves surface hardness and impact resistance, it compromises the ductility and bulk strength of the material due to processing-induced defects. These findings offer valuable insights for material selection, suggesting that pure Nylon is preferable for applications requiring high deformation and toughness, while the composite is better suited for applications prioritizing surface durability and moderate rigidity. The authors conclude that future improvements in SLS composite performance rely heavily on enhancing fiber-matrix interaction and controlling processing parameters to mitigate porosity and ensure uniform fiber distribution.