Modeling the interfacial failure and resulting mechanical properties of z-pinned additively manufactured composites

https://doi.org/10.1016/j.mtcomm.2023.105735Get rights and content

Highlights

  • Cohesive-zone based FE-model for z-pinned structures with experimental validation.

  • Enhanced stiffness and strength by 40% via tailoring z-pin parameters.

  • Structure-property maps for z-pinned composite structures.

Abstract

The mechanical properties of Fused Filament Fabrication (FFF) parts are limited in the build direction, albeit advantages such as design flexibility and in-house customization. In this paper, a novel z-pinning approach that enhances mechanical properties in the build direction by depositing material across multiple layers within the part was investigated through numerical simulations and validated experimentally. A finite element model for z-pinned composite structures was developed by assigning fiber-orientation dependent material properties obtained using a micromechanics approach to beads and pins. The bead-to-bead and pin-to-pin adhesions within the z-pinned structures were modeled using a cohesive traction separation law. The properties of cohesive elements for carbon fiber-reinforced polylactic acid (CF-PLA) z-pinned composites were calibrated using tensile experiments. The elastic modulus and tensile strength of CF-PLA z-pinned composites in the build direction were predicted with the developed numerical model. The numerical investigation on various geometrical parameters revealed that the largest pin volume increases the stiffness and tensile strength by 40% and thus, has the greatest influence on the mechanical properties of the z-pinned composites. The effect of z-pin geometrical parameters on the mechanical properties was summarized to aid in the design of z-pinned additively manufactured composite structures.

Introduction

Fused filament fabrication (FFF) is one of the earliest and most widely used additive manufacturing process. In FFF, molten polymer or polymer composites are extruded from the nozzle and deposited layer-by-layer on a heated bed to form three-dimensional parts by stacking up two-dimensional (x-y plane) patterns along the build direction (z-axis) [1]. Although FFF process has gained significant attention over the last decade due to its simplicity, design flexibility, and in-house customization, the mechanical properties in the build or z-direction are limited by the weak interlayer bonding [2], [3], [4], [5], [6], [7], [8]. The layer-by-layer approach in the FFF process allows tunability of in-plane mechanical properties through preferential print direction within each layer. However, the properties in the out-of-plane direction are significantly affected by this approach to the extent that z-directional strength reduces by 25–80% depending on the material being extruded and scale of the 3D printer [3], [4], [5], [6], [7]. This is caused by the weak interlayer bonding resulting from insufficient cross-linking of polymer chains between two successive layers. The temperature of the top layer during printing must be at least the glass transition temperature of the material to ensure good bonding and mechanical properties in the z-direction [8], [9], [10], [11].

Several techniques have been explored to heat the top layer during the deposition process to improve the interlayer bonding in z-direction. A laser-based system which locally heats up the existing layer near nozzle before depositing a new layer improved flexural strength of ABS (with 0.03% carbon black) by 50% [12]. A similar pre-heating approach developed for the big area additive manufacturing (BAAM) system by mounting infrared lamps on the deposition head doubled the fracture energy of 20% CF-ABS samples [13]. Post-processing techniques such as ionizing radiation for PLA and annealing for CF-PETG and CF-ABS were also found to enhance the strength in the build direction [2], [14]. The fracture strength of PLA coated with carbon nanotubes (CNTs) improved by 275% using localized microwave radiation [15]. Departing from the layer-by-layer approach, FFF systems were also developed to enable non-planar deposition for curved parts and aligning deposition of layers with predicted local stress tensors to enhance structural performance of the parts in out of plane directions [16], [17]. A deposition system with six degrees of freedom that can effectively deposit material across multiple planes on the surface of the part with improved interlayer strength was also developed recently [18], [19].

However, all the above-mentioned processes to enhance mechanical properties in the build direction involves either post-treatment of printed samples or hardware modifications to the existing printers. To this end, a novel “z-pinning” approach where material is deposited across multiple layers to improve the interlayer strength by only software modifications to the existing slicing algorithm was introduced [20], [21], [22]. The z-pinning approach demonstrated that strength and toughness can be increased by 20% and 100%, respectively for PLA specimens with 35% infill pattern [20]. The experiments on z-pinned PLA samples with largest pin volume exhibited isotropic mechanical properties [22]. Adopting this approach for carbon fiber-reinforced PLA (CF-PLA) exhibited improvement in strength and toughness by 3x and 8x respectively, depending on the volume of pins [23]. Subsequently, the penetration quality of the pins for PLA and CF-PLA specimens was analyzed for various hole depths, width, and pin volumes to investigate the effect of pin parameters [24], [25]. The tensile strength of CF-PLA specimens with 80% infill pattern from experiments for various z-pin configurations exhibited an improvement of 35% as compared to samples with no z-pins [26]. Implementing z-pinning approach for CF-ABS samples on a large-scale additive manufacturing system showed increase in bending strength by 9.75% for a solid infill pattern [27]. Despite the great potential shown by the z-pinning approach, an investigation determining the effect of pin parameters on the mechanical properties of z-pinned composites that could aid in their design is still lacking. Furthermore, the existing works rely only on experiments and thus necessitates the development of a numerical model to efficiently evaluate the mechanical properties of z-pinned composite structures.

In the present study, a finite element (FE)-based numerical model was developed for the analysis of z-pinned additively manufactured composites. The influence of pin parameters on the tensile properties of z-pinned CF-PLA samples was evaluated using the developed FE model. Representative volume elements (RVEs) for each case with different pin volume, hole width, and hole depth were modeled by assigning composite material properties and fiber orientations. The bead-to-bead and pin-to-pin adhesion at multiple locations within the RVE was modeled using cohesive elements. The geometrical and material parameters for cohesive elements were calibrated using tensile strengths determined from experiments on z-pinning samples with different pin volumes [28]. The characteristic stress-strain responses from RVEs with different pin parameters were extracted from the numerical simulations to compute modulus, and tensile strength of z-pinned additively manufactured composite structures.

The paper is organized as follows: In Section 2, a description of the geometry of z-pinned structures for various pin parameters is provided. In Sections 3, 3D printing of z-pinned composites and experiments for validating the numerical model is included. Section 4 describes the FE model for z-pinned additively manufactured composites considering fiber orientations for composite material properties and adhesion between pins and beads. The validation of numerical model with experiments and the effect of various pin parameters on the mechanical properties of z-pinned composites is discussed in Section 5. The conclusion is presented in Section 6.

Section snippets

Geometry of z-pinned structures

The z-pinning process involves deposition of the material across multiple layers along the voids intentionally aligned in the z-axis, referred as “z-pins”. The weak points in the z-pinned structures known as seams are aligned in a staggered configuration such that their relative spacing with respect to neighboring pins can disrupt the path of crack propagating between layers. A detailed description of the z-pinning process is included in [22]. The schematic of a z-pinned composite structure is

Experiments

The tensile experiments performed on z-pinning samples in [28] were summarized in this section for the sake of numerical validation. Vertical walls that measured 12.7 cm × 1.27 cm× 12.7 cm were printed using 15% carbon fiber-reinforced PLA in a MakerGear M2 desktop printer as shown in Fig. 5(a). The walls with 35% infill were printed for pin volumes (Pv) of 80%, 100% and 120% with a hole width (hw) and hole depth (hd) of 2 mm. Five tensile test samples for each set of pin parameters were

Numerical methodology

The mechanical properties of CF-PLA z-pinned structures were investigated by developing a numerical model in Abaqus, a commercially available FE-based software. Abaqus (Explicit), a nonlinear explicit dynamics solver with mass scaling was adopted here for the analysis of various z-pinned structures. The details of the numerical model and calibration procedure for the experimental validation are elaborated in this section.

The properties of z-pinned structures made up of CF-PLA are highly

Results and discussion

In this section, the developed numerical model was first validated with the uniaxial tensile experiments on various z-pinned samples and subsequently used to study the influence of pin parameters on the mechanical performance of various z-pinned structures.

The experiments were performed on z-pinning tensile samples with pin volumes (Pv) of 80%, 100%, and 120% for a hole width (hw) and hole depth (hd) of 2 mm as elaborated in Section 3. The average ultimate tensile strengths of these samples

Conclusions

In this paper, a cohesive zone-based numerical model was developed for the novel z-pinning approach that enhances mechanical properties of 3D printed structures in the build direction. The bead-to-bead and pin-to-pin adhesions were modeled using cohesive regions for which the parameters were calibrated from experiments. Beads were assigned with fiber orientation along the bead/longitudinal direction while pins were assigned with random fiber orientations. The effect of pin volume, hole depth,

CRediT authorship contribution statement

Aslan Nasirov: Lead in script development, equal contribution on formal analysis, data curation, and writing – original draft. Deepak Kumar Pokkalla: Equal contribution in formal analysis, data curation, writing – original draft, lead on writing – review & editing. Brenin Bales: Support in experimental validation and data curation. Tyler Smith: Support in resources, software, and validation. Chad Duty: Lead in conceptualization, validation, and funding acquisition. Support in methodology and

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Portions of the research were supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Industrial Technologies Program, under contract DE-AC05–00OR22725 with UT-Battelle, LLC. The authors gratefully acknowledge support from U.S. Army Combat Capabilities Development Command Aviation & Missile Center (DEVCOM AvMC) (DISTRIBUTION A, approved for public release, distribution is unlimited, PR20221280). The authors express gratitude to Amiee Jackson for developing

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    Notice: This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05–00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the work for publication, acknowledges that the US government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the submitted manuscript version of this work, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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