Fracture toughness of ABS additively manufactured by FDM process

Purpose: The purpose on this article is to study the failure of FDM printed ABS by exhibiting an exhaustive crack growth analysis mainly based on raster angle parameter. Design/methodology/approach: Two approaches have been developed in this study; On one hand, mechanical experiments were carried out to determine the critical stress intensity factor KIC. On the other hand, numerical analysis was used to predict the paths within the part as well as the crack propagation. Findings: This work has clearly shown the effect of raster angle on the damage mechanism of the ABS printed by FDM. Indeed, for the combination 1 (0°/90°), the structure presents an important stiffness and a high degree of stress distribution symmetry with respect to the notch. Moreover, the crack propagation is regular and straight, and the damage surfaces are on the same plane. However, for the combination 2 (-45°/45°), the structure is less resistant with an asymmetrical stress distribution according to two different planes. Research limitations/implications: In order to present an exhaustive study, we focused on the effect of two raster angles (including 0°/90°, -45°/45°) on the ABS crack propagation, additively manufactured. This study is still in progress for other raster angles, and will be developed from a design of experiments (DoE) design that incorporates all relevant factors. To highlight more the cracking mechanisms, microscopic observations will be developed in more depth. Practical implications: Our analysis can be used as a decision aid in the design of FDM parts. Indeed, we can choose the raster angle that would ensure the desired crack propagation resistance for a functional part. Originality/value: In this article, we have analyzed the mechanism of damage and crack propagation. This topic represents a new orientation for many research papers. For our study, we accompanied our experimental approach with an original numerical approach. In this numerical approach, we were able to mesh distinctly raster by raster for all layers.

Simultaneous Improvement in Mechanical Properties and Fatigue Crack Propagation Resistance of Low Carbon Offshore Structural Steel EH36 by Cu–Cr Microalloying

Improving the mechanical performance of low-carbon offshore steel is of great significance in shipbuilding applications. In this paper, a new Cu-Cr microalloyed offshore structural steel (FH36) was developed based on EH36. The microstructure, mechanical properties, and fatigue crack propagation properties of rolled plates of FH36, EH36, and normalizing rolled EH36 plates (EH36N) manufactured by a thermo-mechanical control process (TMCP) were analyzed and compared (to simplify, the two rolled specimens are signified by FH36T and EH36T, respectively). FH36T showed an obvious advantage in elongation with the value of 29%, 52.2% higher than the EH36T plates. The normalizing process led to a relatively lower yield stress (338 MPa), but substantially increased the elongation (33%) and lessened the yield ratio from 0.77 to 0.67. Electron back-scattered diffraction (EBSD) analysis showed that SFs of the deformation texture of FH36T and EH36N along the transverse direction (TD) and normal direction (ND) were much higher than those of the EH36T plate, which enhanced the lateral movement ability in the width and thickness direction, enhancing the ductility. Moreover, FH36 plates showed a better fatigue crack propagation resistance than rolled EH36 plates. The formation of the jagged shape grain boundaries is believed to induce a decrease of effective stress intensity factor during the fatigue crack propagation process.

Mechanical Properties of Thin-Ply Composites Based on Acoustic Emission Technology

Compared with standard-ply composites, thin-ply composites exhibit a superior mechanical performance under various operating conditions due to their positive size effects. Thin-ply laminate failure modes, including matrix initial damage (MID), matrix failure (MF), and fiber failure (FF), have been distinguished through a systematic acoustic emission (AE) signals analysis combined with scanning electron microscopy (SEM). First, the characteristic frequencies of various failure modes are identified based on unidirectional laminates ([90] 68 and [0] 68). Then, according to the identified frequencies corresponding to distinctive damage modes, four lay-up sequences (02[[90m/0m]ns]02, m = 1, 2, 4, 8, n × m = 16) with a constant total thickness are designed, and the effects of the number of identical plies in the laminate thickness on the damage evolution characteristics and the damage process under uniaxial tension loads are dynamically monitored. The obtained results indicate that the characteristic frequency ranges for MID, MF, and FF are identified as 0–85 kHz, 165–260 kHz, and 261–304 kHz, respectively. The thickness of identical plies has a significant effect on onset damage. With the decrease of the number of identical plies (i.e., m in the stacking sequences), the thin-ply laminates exhibit the initiation of damage suppression effects and crack propagation resistance.

Improvement of the Crack Propagation Resistance in an α + β Titanium Alloy with a Trimodal Microstructure

The roles of microstructure in plastic deformation and crack growth mechanisms of a titanium alloy with a trimodal microstructure have been systematically investigated. The results show that thick intragranular α lath and a small number of equiaxed α phases avoid the nucleation of cracks at the grain boundary, resulting in branching and fluctuation of cracks. Based on electron back-scattered diffraction, the strain partition and plastic deformation ahead of the crack tip were observed and analyzed in detail. Due to the toughening effect of the softer equiaxed α phase at the grain boundary, crack arresting and blunting are prevalent, improving the crack growth resistance and generating a relatively superior fracture toughness performance. These results indicate that a small amount of large globular α phases is beneficial to increase the crack propagation resistance and, thus, a good combination of mechanical property is obtained in the trimodal microstructure.

Mechanism of Precipitate Microstructure Affecting Fatigue Behavior of 7020 Aluminum Alloy

The effect of different precipitate microstructures obtained by different heat treatments on fatigue behavior of 7020 aluminum alloy was investigated. The fine Guinier Preston I (GPI) zones in the under-aged alloy can be repeatedly sheared by dislocations produced in cyclic loading, making the fatigue crack initiate difficultly and fatigue crack path propagate tortuously. Fatigue strength and fatigue crack propagation resistance of the alloy with shearable precipitates are much higher than those of the alloy with unshearable precipitates. The peak-aged alloy with continuous grain boundary precipitate (GBP) and narrow precipitate free zone (PFZ) is prone to initiate fatigue cracks and reduce fatigue strength. With the growth of unshearable precipitates, the fatigue strength of the alloy firstly increases and then decreases. Precipitates with moderate size in the over-aged alloy improve the roughness-induced crack closure (RICC) effect. Soft matrix with appropriate width between the precipitates can promote the slip reversibility and relax the crack tip stress. The fatigue strength of the moderately over-aged alloy reaches to 122.1 MPa at 107 cycles of loading, and the fatigue crack growth rate (FCGR) is 35.6% slower than that of the peak-aged alloy at ΔK of 10 MPa·m1/2.

Enlarged Zn, Mg Contents with a same Zn/Mg Ratio Improve Fatigue Crack Propagation Resistance of Al-Zn-Mg-Cu Alloys with T7651 State

The fatigue crack propagation of Al-Zn-Mg-Cu alloys could be influenced by the content of main alloying element. In the present work, two Al-Zn-Mg-Cu alloys with a same Zn/Mg ratio were treated by two stage over-aging aging treatment and typical T7651 states were extracted via mechanical properties. Fatigue crack propagation of the two alloys were tested and the related precipitation characteristics and fracture morphology were observed. The results showed that the alloy with higher Zn, Mg contents possessed a better fatigue crack propagation resistance compared with the alloy with lower Zn, Mg contents. The corresponding fracture morphology also showed the difference of fatigue striation, which provided an additional support. The precipitation observation demonstrated that the both alloys possessed GPII zone, η' phase and η phase while the alloy with higher Zn, Mg contents had a larger average precipitate size and a larger proportion of large size precipitates compared with the alloy with lower Zn, Mg contents. Cut and bypass mechanisms of dislocation-precipitate interactions were used to explain the difference of fatigue crack propagation between the two alloys.