Monitoring of Processing Conditions of an Ultrasonic Vibration-Assisted Ball-Burnishing Process.

Although numerous references present the beneficial effects on surface integrity of ultrasonic vibration-assisted ball burnishing (UVABB), nothing has been reported about the dynamic behavior of the UVABB tool, workpiece, and machine triad during the process. In this paper, a dynamic monitorization through a set of 5 accelerometers is tested to analyze the interactions between the tool-workpiece-machine mechanical assembly. A UVABB tool attached to a milling machine and equipped with a piezoelectric stack that is able to assist the process with a 40-kHz vibration is tested on a milled C45 steel surface. First, the natural frequencies of the mechanical system are obtained through hammer impact tests. Then, the vibratory signals transmitted during the execution of the process are monitored and compared to those: two feed velocities and two burnishing preloads, all with and without vibration-assistance. Results show that the proposed accelerometer set is valid to assess the behavior of a UVABB process. The system's natural frequencies are not varied by vibration-assistance and are not excited when the piezoelectric is functioning. It is confirmed that UVABB is safe for the machine and the tool, and there is no unexpected excited frequencies due to the piezoelectric excitation.

Investigating mechanical properties of 3D printed polylactic acid / poly-3-hydroxybutyrate composites. Compressive and fatigue performance.

The 3D printing technique known as Material Extrusion (MEX) was initially employed for prototyping, but it has evolved to fit applications in mechanical and biomedical industries. Polylactic acid (PLA) stands out as a commonly used polymer for manufacture pieces by MEX, due to its good properties and organic origins. Pursuing renewable and biodegradable thermoplastics has led to the development materials such as composite of PLA with wood fibers and blends with poly-3-hydroxybutyrate (PHB). This study aims to characterize the effect of the most relevant printing parameters on the mechanical properties of a PLA/PHB blend, motivated by the interest to facilitate the use of this type of materials in industrial applications. To achieve it, compressive and fatigue tests were carried out, comparing the results with those obtained in previous studies for pure PLA and PLA-wood composite. Results show that the compressive behavior of PLA/PHB is influenced by the layer height, nozzle diameter and fill density. Its fatigue behavior is mainly determined by the nozzle diameter and the fill density. Moreover, the mechanical performance of PLA/PHB (Young's Modulus of 1.67 GPa, yield Strength of 33.8 MPa and maximum fatigue life of 9711 cycles) is inferior compared to pure PLA and PLA-wood composite. Despite the increase in the biodegradability that PHB introduces into PLA, the findings of this study reveal that there is statistically evidence that it can also hinder the mechanical performance of the base material.

Analysis of Ultrasonic Vibration-Assisted Ball Burnishing Process on the Tribological Behavior of AISI 316L Cylindrical Specimens.

In this study, we analyzed the effects of vibration assistance, combined with a ball burnishing process, in terms of topology, residual stresses, and tribological properties on 316L shafts. The burnishing variables consisted of the variation of the input force, the number of passes, and the activation of the vibration assistance, which is based on a 40 kHz frequency and 8 µm of vibration amplitude, derived in a screening design of three factors. The results show that the medium-high level of burnishing force, high level of the number of passes, and the activation of the vibration assistance are the best options in order to improve the average roughness, the microstructure, the increase in the compressive residual stresses, and the wear enhancement, besides all variables being significant in the p-value analysis through ANOVA. Statistically, the vibration-assisted ball burnishing improved the average roughness by 2.9%, enlarged the von Mises stress on the surface by 11.5% and enhanced the wear resistance of a 316L shaft and WC-Co ball contact up to 7.3%.

Study of the Influence of the Manufacturing Parameters on Tensile Properties of Thermoplastic Elastomers.

Additive manufacturing (AM) has increased its field of application, not only for prototypes but also for final parts. Therefore, the need to study new materials is currently growing. This paper aims to study the effect of the printing parameters used in two different thermoplastic elastomers (PEBA 90A and TPU 98A) subjected to tensile tests, evaluating a competent alternative to the currently most used 3D printed materials. To achieve it, a full factorial design experiment is applied to analyze the influence on the tensile responses of two printing parameters: the layer height and the fill density. In addition, an analysis of variance (ANOVA) is used to describe the relations among the parameters and the mechanical responses obtained. Moreover, assessment of damping properties was done. Results show that each thermoplastic elastomer should be studied separately, although the proposed methodology can be used for each material independently of their nature. Finally, a correlation between the printing parameters and the mechanical behavior of TPU 98A and PEBA 90A was found: the layer height and the infill are statistically influential parameters for both materials.

Viscoelastic Characterization of a Thermoplastic Elastomer Processed through Material Extrusion.

Objective. We aim to characterize the viscoelastic behavior of Polyether-Block-Amide (PEBA 90A), provide reference values for the parameters of a constitutive model for the simulation of mechanical behaviors, and paying attention to the influence of the manufacturing conditions. Methods. Uniaxial relaxation tests of filaments of PEBA were used to determine the values of the parameters of a Prony series for a Quasi-Linear Visco-Elastic (QLVE) model. Additional, fast cyclic loading tests were used to corroborate the adequacy of the model under different test criteria in a second test situation. Results. The QLVE model predicts the results of the relaxation tests very accurately. In addition, the behavior inferred from this model fits very well with the measurements of fast cyclic loading tests. The viscoelastic behavior of PEBA under small strain polymer fits very well to a six-parameter QLVE model.

Ultrasonic Vibration-Assisted Ball Burnishing Tool for a Lathe Characterized by Acoustic Emission and Vibratory Measurements.

This paper focuses on a resonant system used to induce a low-amplitude movement and ultrasonic frequency to complement a ball burnishing process on a lathe. The system was characterized through the combination of different techniques. A full vibratory characterization of this process was undertaken with the purpose of demonstrating that the mechanical system-composed of the tool and the machine-does not present resonance phenomena during the execution of the operation that could lead to eventual failure. This dynamic analysis validates the adequateness of the tool when attached to an NC lathe, which is important to guarantee its future implementation in actual manufacturing contexts. A further aim was to confirm that the system succeeds in transmitting an oscillating signal throughout the material lattice. To this end, different static and dynamic techniques that measure different vibration ranges-including impact tests, acoustic emission measurement, and vibration measurement-were combined. An operational deflection shape model was also constructed. Results demonstrate that the only high frequency appearing in the process originated in the tool. The process was not affected by the presence of vibration assistance, nor by the burnishing preload or feed levels. Furthermore, the frequency of the assisting ultrasonic vibration was characterized and no signal due to possible damage in the material of the specimens was detected. These results demonstrate the suitability of the new tool in the vibration-assisted ball burnishing process.

Mechanical Properties of 3D-Printing Polylactic Acid Parts subjected to Bending Stress and Fatigue Testing.

This paper aims to analyse the mechanical properties response of polylactic acid (PLA) parts manufactured through fused filament fabrication. The influence of six manufacturing factors (layer height, filament width, fill density, layer orientation, printing velocity, and infill pattern) on the flexural resistance of PLA specimens is studied through an L27 Taguchi experimental array. Different geometries were tested on a four-point bending machine and on a rotating bending machine. From the first experimental phase, an optimal set of parameters deriving in the highest flexural resistance was determined. The results show that layer orientation is the most influential parameter, followed by layer height, filament width, and printing velocity, whereas the fill density and infill pattern show no significant influence. Finally, the fatigue fracture behaviour is evaluated and compared with that of previous studies' results, in order to present a comprehensive study of the mechanical properties of the material under different kind of solicitations.

Fatigue Performance of ABS Specimens Obtained by Fused Filament Fabrication.

In this paper, the fatigue response of fused filament fabrication (FFF) Acrylonitrile butadiene styrene (ABS) parts is studied. Different building parameters (layer height, nozzle diameter, infill density, and printing speed) were chosen to study their influence on the lifespan of cylindrical specimens according to a design of experiments (DOE) using the Taguchi methodology. The same DOE was applied on two different specimen sets using two different infill patterns-rectilinear and honeycomb. The results show that the infill density is the most important parameter for both of the studied patterns. The specimens manufactured with the honeycomb pattern show longer lifespans. The best parameter set associated to that infill was chosen for a second experimental phase, in which the specimens were tested under different maximum bending stresses so as to construct the Wöhler curve associated with this 3D printing configuration. The results of this study are useful to design and manufacture ABS end-use parts that are expected to work under oscillating periodic loads.