PSI - Issue 35

Mohammad Reza Khosravani et al. / Procedia Structural Integrity 35 (2022) 59–65 M.R. Khosravani and T. Reinicke / Structural Integrity Procedia 00 (2021) 000–000

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A first glance at the results of tests reveals that fiber reinforcement has significant e ff ect on the mechanical behavior of examined parts. The linear part of the stress-strain curve can be considered to determine the Young’s modulus of un reinforced and reinforced specimens. In Table 3, maximum fracture load, ultimate strength, maximum displacement, and Young’s modulus of examined unreinforced and reinforced specimens are presented. As expected, the experi mental results confirmed that reinforced specimens showed higher sti ff ness compared to unreinforced samples. The improvement in the mechanical behavior of reinforced parts was expected, because utilized fibers have greater elastic modulus than nylon. In the experimental tests, fiber pull-out and fiber breakage were observed when the specimens experienced tensile load. The fiber pull-out might be result of insu ffi cient adhesion between the matrix and fiber. The crack propagation in fibers can be started from the skin towards the core of the utilized fiber. If core of the fiber bears, most of the loading is applied to the fiber, thus there is crack initiation within the fiber. The bonding between fibers and matrix is one of the main factors for the mechanical performance of additively manufactured composite struc tural elements. It should be noted that although using fibers prevent crack propagation and improve strength of the reinforced parts, the increase of fiber content can increase porosity (air voids). The porosity can reduce the maximum strength of 3D-printed fiber reinforced composites. In this context, a compaction after the deposition of the filament would be beneficial for porosity reduction. Since defected regions are susceptible to fracture, defects reduction (e.g., overlap, o ff set, and gaps) is a necessity. Defects and discontinuities might be existed on the matrix and within or on the surface of the utilized fibers. During the mechanical fracture, crack initiates at the largest defect and propagates into small defects where final rupture occurs.

Table 3. Obtained results based on the experimental tests. Mechanical properties

Unreinforced

Reinforced

Maximum fracture load (N) Maximum displacement (mm)

1418.3

8831.7

6.2

4.1

Ultimate strength (MPa) Young’s modulus (MPa)

37.5 14.8

155.3

27.6

It should be emphasized that, type of fiber reinforcement, fiber volume content, and build orientation play crucial roles in sti ff ness and maximum strength of 3D-printed fiber reinforced composites. The conducted tests and docu mented results provided a proper understanding of the mechanical performance of the unreinforced and reinforced parts which can facilitate the design of 3D-printed composites.

6. Conclusions

Since applications of 3D printing have been increased in fabrication of functional structures, the study of the me chanical behavior of 3D-printed composites is a necessity. In the current study, fracture behavior of unreinforced and reinforced 3D-printed composites is investigated. In this respect, 3D-printed specimens were prepared based on the embedding the continuous fiber directly in the part with a dual extrusion method. In detail, nylon and fiberglass were used as matrix and reinforcement materials, respectively. In this work, printing parameters were kept constant and the specimens were tested under the same conditions. The 3D-printed dog-bone shaped specimens experienced tensile tests under static loading conditions. The results have shown that mechanical performance of 3D-printed fiber reinforced composites is significantly improved compared to the unreinforced 3D-printed components. Particularly, the average of fracture loads in unreinforced specimen was 1474.5 N which was increased to 8780.2 N in the re inforced specimens. Moreover, comparison of stress-strain curves of the examined parts confirmed that strength of 3D-printed composites was increased due to the fiber reinforcement. Although utilizing fibers prevent crack propaga tion in 3D-printed parts, increase of fiber content can increase porosity and reduce strength and sti ff ness of reinforced components. Therefore, fiber volume content plays a crucial role on the mechanical behavior of parts. In this ex perimental investigation, fiber pull-out and fiber breakage were observed. The fiber pull-out indicated poor interface bonding between matrix and fibers in the examined parts. Since 3D printing of continuous fiber-reinforced composites is a promising technology in the fabrication of structural element with complex geometries, more applications of this