Investigation on the improvement of mechanical properties and surface quality of the polylactic acid (PLA) fused filament fabrication parts manufactured without and with vibrations dependent utilization.

Fused Filament Fabrication (FFF), is one of the most widely used additive manufacturing technologies today, which has been used for a variety of applications. Due to the layer-by-layer manufacturing process, FFF parts are inferior to those fabricated by traditional methods in terms of tensile properties, which is one of the most significant defects that hinder the development of this technique. In this study, a vibration was utilized during the FFF process by piezoelectric ceramics electric plates to improve the mechanical properties of the built parts and surface quality of PLA FFF parts. Subsequently, an investigation of the tensile and the surface quality of PLA FFF specimens built-in X and Z-direction fabricated individually without and with vibrations utilized has been done. Furthermore, a theoretical model has been established to predict the tensile strength and plasticity of FFF parts fabricated without and with vibrations utilized based on classical laminated plate theory, with the anisotropic and laminated characteristics taken into consideration. Young's modulus model has been established based on the laminated plate theory and flexural vibration theoretical approaches of a plate for the PLA FFF parts manufactured without and with vibrations utilized respectively. Compared with the previous models this model provides the tensile strength and plasticity of FFF parts both manufactured without and with vibrations utilized. The results indicate that the predicted tensile strength and plasticity of the PLA FFF parts manufactured with vibrations utilized have a good consistency with the experimental ones, meanwhile, vibration utilization can significantly improve the surface quality of the PLA FFF samples manufactured in the Z-direction, and the scanning electron microscopy (SEM) analysis confirmed that vibration utilization can improve the forming quality of FFF manufactured parts.

Identification of mechanical parameters of fiber-reinforced composites by frequency response function approximation method.

This article proposes frequency response function approximation method to identify mechanical parameters of fiber-reinforced composites. First, a fiber-reinforced composite thin plate is taken as a research object, and its natural characteristic and vibration response under pulse excitation are solved based on the Ritz method and mode superposition method, so that the theoretical calculation of frequency response function of such composite plates can be realized. Then, the identification principle based on frequency response function approximation method is illustrated and its correctness is validated by comparing with other published literature in the verification example, and the specific identification procedure is also proposed. Finally, frequency response function approximation method is applied in a study case, where the elastic moduli, Poisson's ratios, and loss factors of the TC300 carbon/epoxy composite thin plate are identified, and the influences of boundary conditions, approximation points, total number of modes, and calculation step size on the identification accuracy and efficiency are discussed. It has been proved that the proposed method can identify mechanical parameters of fiber composite materials with high precision and efficiency.

Frobenius and nuclear hybrid norm penalized robust principal component analysis for transient impulsive feature detection of rolling bearings.

Transient impulsive feature detection is of vital importance in fault diagnosis of rolling bearing. However, the transient impulsive feature of rolling bearing is always heavily buried in the noise contaminated signal, which makes it difficult to be detected. Robust principal component analysis (PRCA) is an effective approach to exploit the underlying structure from the corrupted observation, where the decomposed low-rank matrix (LRM) and the sparse matrix can represent the useful diagnostic information and the unwanted background noise respectively. In this study, a Frobenius and nuclear hybrid norm penalized RPCA (FNHN-RPCA) is served as a specific RPCA solver on account of it has a great ability to approach to the rank of the LRM and make the execution procedure efficiently To make the recorded signal suitable for the input criterion of the RPCA solver, a fault information matrix (FIM) construction method is proposed to arrange the recorded signal into a matrix form. After the RPCA solver is conducted on it, a reversed recovery operation is also proposed to rearrange the two dimensional LRM into a one-dimensional signal form. To confirm all recorded data is processed by the RPCA solver, both the forward and backward FIMs are constructed and a synthesis of the recovered signals from both the forward and backward FIMs is served as the final transient impulsive feature enhanced signal. The diagnostic results on simulated and experimental case studies verify that the presented technique is suitable for transient impulsive feature detection of rolling bearings even when the test bearing works in a low speed operating condition or the operating environment is in the presence of random impact interference.

Material property sensitivity analysis on resonant frequency characteristics of the human spine.

The aim of this study is to investigate the effect of material property changes in the spinal components on the resonant frequency characteristics of the human spine. Several investigations have reported the material property sensitivity of human spine under static loading conditions, but less research has been devoted to the material property sensitivity of spinal biomechanical characteristics under a vibration environment. A detailed three-dimensional finite element model of the human spine, T12-pelvis, was built and used to predict the influence of material property variation on the resonant frequencies of the human spine. The simulation results reveal that material properties of spinal components have obvious influences on the dynamic characteristics of the spine. The annulus ground substance is the dominant component affecting the vertical resonant frequencies of the spine. The percentage change of the resonant frequency relative to the basic condition was more than 20% if Young's modulus of disc annulus is less than 1.5 MPa. The vertical resonant frequency may also decrease if Poisson's ratio of nucleus pulposus of intervertebral disc decreases.

Influence of anteroposterior shifting of trunk mass centroid on vibrational configuration of human spine.

This study attempts to determine the influence of anteroposterior (A-P) shifting of trunk mass from the upright sedentary posture on dynamic characteristics of the human lumbar spine. A three-dimensional finite element (FE) model comprising of the T12-Pelvis spine unit was used to mimic the human spine system. It is not clear how the A-P shifting of the upper part of human upper body affect on vibrational modality of the human lumbar spine under whole body vibration. Five trunk mass point locations were assumed by 2.0cm anterior, 1.0cm anterior, 1.0cm posterior and 2.0cm posterior to the upright sedentary posture including no shifting posture. FE modal analysis was used to extract the resonant frequencies and vibration modes of the human spine. The analytical results indicate that trunk mass centroid shifting onwards or rearwards may result in a reduction of vertical resonant frequency of the human spine. The human spine has the highest vertical resonant frequency at the normal upright sedentary posture with the trunk mass locating around 1.0cm anterior to the L3-L4 vertebral centroid. Larger A-P deformations and rotational deformations were also found at the spine motion segments L3-L4 and L4-L5, which imply higher compressive stress and shear stress at the disc annulus of those spinal motion segments. The findings in this study may explain why long-term whole body vibration might induce the degeneration of human spine at the relevant spinal motion segments.