Fracture and load-carrying capacity of 3D-printed cracked components

Highlights

• Fabrication of U-notched 3D-printed components.

• Evaluation of 3D-printed parts via experimental tensile test.

• Combination of J-integral failure criterion and the equivalent material concept.

• Numerical simulation on fracture and load carrying capacity of 3D-printed cracked parts.

Abstract

Regrading to the numerous potentials of additive manufacturing in producing components, three-dimensional (3D)-printed parts are becoming more prevalent in various industries and research associations. In this paper, fracture of U-notched 3D-printed parts under mode I and mixed mode I/II are experimentally investigated. To this aim, specimens are 3D-printed by polycarbonate (PC) and Nylon filaments using fused deposition modeling (FDM) 3D printing. In the fabrication, U-notched rectangular specimens are produced. A series of experimental practices are performed to determine load-carrying capacity of U-notched 3D-printed parts. In the current study, a combination of J-integral failure criterion and the equivalent material concept (EMC) was implemented to investigate failure of the specimens. Since the tested material has shown elastic–plastic behavior, EMC was utilized to avoid computationally inefficient non-linear failure analyses. By the obtained results, it is concluded that combination of EMC and J-integral criterion is able to predict the experimental results for the different 3D-printed materials. Parallel to the experimental investigations, numerical simulations are conducted and a very good agreement between simulation finding and reported experimental results is shown.

Introduction

Advances in additive manufacturing (AM) leads to producing numerous engineering parts from a variety of materials by approach. Reducing material waste, creating optimized geometries, and reducing lead time can be considered as advantages of AM technique compared to traditional manufacturing processes. Currently, AM is employed to produce prototypes from computer-aided design (CAD) software mainly through common methods such as selective laser melting (SLM), fused deposition modeling (FDM), or stereolithography (SLA) [1], [2], [3], [4], [5]. Also, there is possibility to produce complex shapes with a degree of design freedom unachievable with traditional manufacturing techniques, so AM is being explored in wide engineering concepts.

Mechanical reliability is a critical issue in various engineering applications. Since, engineering components must present high performance during their service, behavior and fracture analyses of the components are essential which are studied over the years [6]. In an edge-notched ductile specimen the crack can grow abruptly or slowly. The ultimate point in the stress–strain curve is so critical. Due to the stable crack grow in the most of the cases, there is possibility on detection of crack in the notched ductile components before final fracture [7], [8], [9]. It should be noted that because of deformations around the notch before the crack onset, failure analysis of notched parts made by ductile material is more complex compared to notched brittle parts. Experimental practice on evaluation of notch fracture resistance is one of the best methods to check the safety of notched ductile parts. Concerning wide applications of ductile materials, various researches have been performed to evaluate mechanical fracture of the notched ductile components [10], [11], [12]. For instance, in [12] ductile fracture of bainitic functionally graded steel is experimentally investigated. To this aim, the researchers fabricated functionally graded samples and conducted three-point bending tests. They studied different conditions (mode I and mixed mode), and calculated fracture properties of examined specimens at each point by employing the concept of the instability point. Moreover, numerical simulation considering constitutive modeling of ductile material was performed. In [13], ductile fracture in notched bulk glass was investigated. In this regard, metal-like ductile fracture in bulk metallic glasses was demonstrated under tensile loading. More in deep, quasi-static tensile tests were performed on the cylindrical bar specimens. Triaxial stress state was achieved by introducing a circumferential notch in the specimens. The researchers used an extensometer to measure the elongation, and fracture morphology was examined by a scanning electron microscope. Consequently, it was reported that size effect of notch on the transition from brittle to ductile fracture can be negligible.

In [14], fracture behavior of 3D-printed components was experimentally studied. In this regard, the researchers fabricated a lattice model and predicted crack pattern under tensile load. By a series of tests the load-displace- ment curves and formation of crack were obtained. Additionally, they presented numerical simulation where two types of fabricated parts were considered in order to cover effect of typical layered structure of printed lattice material. The fracture pattern and interfacial behavior of printed carbon fiber composites were also examined in [15]. The researchers printed continuous carbon fiber, and determined effect of printing process and sizing on fracture pattern of the specimens. As the interfacial performance of composites is the most important issue in evaluation of mechanical properties, it has a crucial role in fracture patterns and crack growth. In detail, the researchers constructed specimens and applied the forming pressure in order to evaluate interfacial performance. Also, the sizing treatment is applied which improved interfacial performance of 3D-printed components with low-complexity at low cost. Moreover, they proposed interface optimization strategy for 3D-printed parts which can speed up the progress that is beneficial for industrial applications. Recently, in [16] ductile fracture of specimens with notches under biaxial loading was investigated by numerical simulations. In this respect, the researchers considered axisymmetric specimens with circumferential notches and under biaxial monotonic tensile and torsional loads conditions. As force–elongation curve and shape of fracture surface depends to the loading, these issues were discussed. By the obtained results, the researchers reported that fracture initiation can be occurred at the notch root, near the notch or on the symmetry axis, depends on shape of the notch and the loading conditions. The researchers concluded that utilized criterion is easy compared to other methods, and a low number of material parameters is needed.

Considering fracture mechanics points of view, crack initiation and propagation are fundamentally different for brittle and ductile materials [17]. For analysis of mechanical fracture in the cracked components which shows elastic–plastic deformation, usually the elastic–plastic fracture mechanics is employed and such this analysis is time consuming and complex. The Equivalent Material Concept (EMC) was introduced where the elastic–plastic fracture mechanics analyses are avoided in prediction of failure on pre-notched parts. In EMC method, a virtual brittle material is equated with the real ductile material. Later, in [18] is conformed that EMC can be combined with different brittle fracture criteria (e.g., average strain energy density (ASED), mean stress, and maximum tangential stress) to predict ductile fracture of pre-notched parts.

In the present research, we employed EMC to provide a new failure model in failure prediction of 3D-printed specimens. In detail, we used FDM technique and fabricated 3D-printed specimens made from two different materials weakened by U-notches. As the J-integral failure criterion is a prevalent brittle failure concept which commonly used in study of notched components, here we attempted to use EMC if it can be combined with J-integral failure criterion to predict fracture of U-notched 3D-printed specimens subjected to tensile loading. Indeed, the present research evaluate failure of different 3D-printed components under mode I and mixed mode I/II loading regimes in the frame of the mentioned technique. To this aim, polycarbonate (PC) and nylon filaments are utilized for fabrication of 3D-printed specimens. The produced components are weakened by a central crack. A series of tensile tests under static loading regime is performed and experimental results are documented. Then, EMC and J-integral criterion are combined to obtain the numerical results by this method. Finally, the load-carrying capacities of the examined specimens are measured. We have organized the rest of this paper in the following way. In Section 2, fabrication of specimens is described in detail. In Section 3, experimental investigations are comprehensively explained and the obtained results are presented. In Section 4, EMC, the J-integral for U-notches and their combination are explained. In Section 5, numerical simulations is discussed and obtained results are depicted. Finally, Section 6 concludes this paper.