Geometrical Defects Research Articles

Investigation on a novel variant-dimension vibration-assisted drilling system for CFRP: locus model, control strategy, and machining experiments

Ultrasonic vibration-assisted drilling, which can obtain the minimization of geometrical defects, provides a solution for the drilling of carbon fiber-reinforced plastic (CFRP) by adding a displacement of a micro-scale amplitude with a frequency to the tool tip. However, single vibration mode in drilling of CFRP cannot satisfy the different requirements of various drilling stages in CFRP and limits the applications of current ultrasonic vibration-assisted drilling of CFRP. To overcome the limitation and improve the performance of vibration-assisted drilling process in CFRP components, a novel variant-dimension vibration-assisted drilling system (VD-VADS) is proposed. The VD-VADS is specially designed to be capable of delivering variant-dimension vibrations at the tool tip for drilling holes in CFRP by producing one-dimension longitude vibration, two-dimension elliptic vibration, and three-dimension composite vibration, which can be transferred to adapt to various drilling stages in CFRP. An analytical model of the VD-VADS’s locus will be formulated to facilitate the prediction of the loci of the tooltip and design and realization of the prototype device. The control strategy of variant-dimension vibration mode of the proposed VD-VADS is established and realized for various drilling stages in CFRP. To validate the machining performance of the VD-VADS, five groups of drilling experiments under the conventional drilling and various vibration-assisted drilling processes were performed. The performance tests and comparison results of the obtained holes between the conventional drilling and various vibration-assisted drilling processes indicate that the proposed VD-VADS is capable of generating holes with better overall quality than the current single vibration-assisted drilling processes.

Analysis of a forging die wear by 3D reverse scanning combined with SEM and hardness tests

The work concerns the application of non-contact measurement 3D scanning techniques in conjunction with a study of the microstructure of a forging die (made of W360 steel) for the production of an engine valve (made of NCF 3015 steel) in a hot die forging process in order to analyse the changes in the working surface of the tools and identify the destructive mechanisms. The detailed analysis presented in this paper examines the possibility of using 3D reverse engineering techniques for a direct quality control and examination of the changes in the surface layer geometry of the forging dies, based on the measurement of the geometry changes for cyclically collected forgings. The selected area of the valve forgings cyclically retrieved from the forging process was scanned with the use of an intermediate scanning method - reverse 3D scanning. On this basis, an analysis of the progressive material growth on the selected surface of the forgings was made, which also meant a loss of material on the tools. The performed analyses showed a good agreement of the geometrical properties of the surfaces (of the selected forgings representing the proceeding wear of the tool) and the geometrical defect of the working impression of the tool, based on the direct measurements during the production process. The reverse 3D scanning method developed by the authors has been repeatedly verified by them, which is confirmed by numerous studies and applications. The obtained results combined with SEM analyses and microhardness measurements enable a fast analysis of the forging tool life with respect to the quality and quantity (of material defect), which, in consequence, leads to significant economical savings.

Geometric/microstructural imperfection sensitivity in the vibration characteristics of geometrically non-uniform functionally graded plates with mixed boundary conditions

In the present article, the vibration response of geometrically imperfect functionally graded material (FGM) plates with geometric discontinuities and microstructural defects have been investigated. The structural kinematics used in the present study is based on logarithmic shear-strain function. A refined generalized porosity model has been developed to encompass even and uneven types of porosity models available in the literature. The geometric discontinuities have been assimilated in terms of a circular cutout of different sizes at the center of the plate. To incorporate the geometric imperfection, a generic function has been used. A C0 continuous iso-parametric finite element formulation has been used to attain the results whereas the accuracy of the present solution has been confirmed by comparing the results with the available literature. The analysis reveals that after a specific cutout radius, the FGM plates exhibit the nonlinear vibration characteristics. It has been observed that the influence of boundary conditions, cutout dimensions, geometric imperfections, and porosity inclusions on frequency parameters are significant. Most of the results presented in this paper are novel and may be treated as benchmark results for future reference.

Strategy to shape, on a half-meter scale, a geopolymer composite structure by additive manufacturing

This work aims at proposing a strategy for 3D-printing geopolymer composite structures at a half-meter scale, without using organic additives. An original printing device based on cartridges is developed and adapted to a 6-axis robot. The yield stress, working time and apparent Young modulus of the extruded material are measured. A devoted software, procedure and printing path are set up, leading to the fabrication of a structure without height limitation, without major geometrical defects or instabilities. The working time ensures the consolidation of the material during printing and good adhesion between layers. As an example, four successive cartridges have been successfully used to elaborate a hollow cylinder (Φ ​= ​35 ​cm, H ​= ​45 ​cm).

The Interplay of Geometric Defects and Porosity on the Mechanical Behavior of Additively Manufactured Components

Additive manufacturing (AM) is becoming increasingly popular, but standards for critical defect sizes and porosity levels have not been fully established. The wide range of flexibility offered by AM is unable to be exploited by designers if they do not have confidence in their components’ mechanical behavior. The goal of this study was to understand how the factors of specimen geometry, geometric defects, porosity, and base material ductility impact the performance of realistic AM exemplar components. Geometric defects, including quarter cracks, internal voids, and through holes, were intentionally manufactured into thin-walled, tapered SS 316L and AlSi10Mg AM components. Two levels of porosity were introduced by reducing the laser power in the AlSi10Mg specimens to observe interactions of porosity and the larger-scale geometric defects. Many such components were manufactured, and each component was loaded to failure. Intentionally manufactured defects decreased the failure load and displacement-to-failure for both materials. Geometric defects had a greater effect in the less ductile materials. Increasing the porosity in the AlSi10Mg above a threshold led to density-dominated component behavior, with little impact of intentional large-scale geometric defects. This work demonstrated the interactions between the flaw types and the dependence of the AM component behavior based on inherent material ductility, porosity, and presence of geometric defects. The results from this study provide valuable insight for establishing a critical porosity-defect relationship for AM metals. It also demonstrates that predicting component behavior in AM metals requires an understanding of flaws types and material properties.

Effect of fiber-to-matrix bond on the performance of inorganic matrix composites

The strengthening of masonry structures is nowadays performed by means of high-strength fibers embedded in inorganic matrix (FRCM) where lime or cement-based matrix is used instead of epoxy adhesive to reduce debonding issues between substrate and matrix. However, some sliding phenomena and cohesive failures between fibers and the matrix mortar can occur. The paper examines the effect on the FRCM efficiency of the mechanical properties of fiber and matrix and potential geometrical defects, which are possible in real field applications or in qualification tests. The model application to simulate bond tests on typical PBO-FRCM and Glass-FRCM allowed to analyse slips as well as normal and shear stresses both in the bundle and in the matrix constituting the FRCM, for different defects due to application issues. The result of numerical simulations seems to interpret well the results of the qualification tests with a multi-bundle effect that justifies their scatter.The approach can be applied by varying main mechanical properties of the materials (e.g. elastic modulus, fiber cross section, bond properties) to consider their intrinsic variability in the assessment of the performance of the FRCM system or by changing the type of materials (i.e. mortar and fibre) to optimize the FRCM system.

Forming the inner flange of a rib by pulsed-magnetic field pressure

Electromagnetic flanging is a hole-protrusion of parts, a potential technology for forming sheet aluminum metal, provides rigidity, allows positioning and fixing. This paper describes the behavior of a workpiece formed by an electromagnetic flanging, in which the initial blank of annealed aluminum alloy D16AM with a circular precut hole is formed into the first part of the inner flanging (as the first operation of manufacturing a part of the rib). The purpose of this article is to assess the ability of electromagnetic shaping to produce flanges for aircraft components and to study the forming process using numerical simulation. Design of electromagnetic flanges with some elementary geometric shapes, the purpose of which is to clarify a few geometric defects encountered and to propose some solutions. The numerical model of this part was developed in the LS-DYNA computing environment. An example of the numerical simulation of electromagnetic forming of the inner flange of a rib with the obtained shape of the workpiece mesh using a clamp is given. To avoid corrugation at the corner of the workpiece, the magnetic force was increased by reducing the area of the coil adjacent to the corner.

Effects of welding parameters on the microstructure and mechanical properties of the AA6061 aluminium alloy joined by a Yb: YAG laser beam

In this study, the effects of Yb: YAG laser welding parameters on the microstructure and mechanical properties of AA6061-T4 joints were analysed. Samples without any welding defects such as porosity, melt pool collapse, and hot cracking were produced with different welding parameters. Energetic processing parameters had a significant influence on the fusion zone (FZ) and heat-affected zone (HAZ) microstructure and dimensions, in addition to local and global mechanical properties. The weld bead width increased with increasing power and energy densities and reduced the weld bead tensile properties. High welding travel speed produced a more elongated weld pool and ripples resulted in a ‘V’ shape. A significant quantity of axial heterogeneous nucleation was observed in the FZ centre of these weld beads because of the low-energy density. In contrast, low welding travel speed produced C-shaped ripples and a few axial grain nucleation sites in the FZ. The latter was evidence of a slower solidification rate. Dendrite secondary arm spacing measurements confirmed this hypothesis. It was observed that axial grains in the FZ centre improve the weld bead tensile properties. Compared to the base material (BM), the hardness in the FZ was reduced and identical in the HAZ. The FZ hardness depended on the welding parameters. Digital image correlation (DIC) strain measurements indicated higher deformation near the FZ, but when geometrical defects were removed, the FZ deformed more homogeneously.

Dynamic Disintegration of Explosively-Driven Metal Cylinder with Internal V-Grooves.

Machining V-shaped grooves to the internal surface of cylindrical shells is one of the most common technologies of controlled fragmentation for improving warhead lethality against targets. The fracture strain of grooved shells is a significant concern in warhead design. However, there is as yet no reasonable theory for predicting the fracture strain of a specific grooved shell; existing approaches are only able to predict this physical regularity of non-grooved shells. In this paper, through theoretical analysis and numerical simulations, a new model was established to study the fracture strain of explosively driven cylindrical shells with internal longitudinal V-grooves. The model was built based on an energy conservation equation in which the energy consumed to create a new fracture surface in non-grooved shells was provided by the elastic deformation energy stored in shells. We modified the energy approach so that it can be applicable to grooved shells by adding the elastic energy liberated for crack penetration and reducing the required fracture energy. Cylinders with different groove geometric parameters were explosively expanded to the point of disintegration to verify the proposed model. Theoretical predictions of fracture strain showed good agreement with experimental results, indicating that the model is suitable for predicting the fracture strain of explosively driven metal cylinders with internal V-grooves. In addition, this study provides an insight into the mechanism whereby geometric defects promote fracturing.

Characterization of L-PBF lattice structures geometric defects

An increasing interest in lattice structures is developing in various industrial sectors. These structures, composed of several cavities or voids, are used in a wide range of applications. They allow to lighten workpieces while maintaining good mechanical characteristics.Although these structures offer new design opportunities, they are difficult to manufacture because of their complex geometry and internal surfaces that are difficult to access. The special characteristics of lattice structures almost always require the use of additive manufacturing processes. By using lattice structures, new problems arise in additive manufacturing, such as lattice defects characterization and measurement issues.The objective of this study is to characterize the geometric defects of lattice structures obtained using L-PBF additive manufacturing process. Lattice structures exhibit not only common defects inherent to L-PBF process, but other geometric defects specific to their unit cell that require appropriate measurement systems and new processing techniques. This paper deals with a systematic approach to evaluate geometric defects obtained from Additive Manufacturing simulation software. First, lattice structures are generated using Rhino-Grasshoper visual programming capabilities. Then, virtual parts are built using Ansys Additive software. Registration methods are used thereafter to compare the simulated and the nominal models. The advantage of non-rigid methods such as CPD and BCPD for registration is highlighted. Finally, the obtained data can be used in the future to enhance the predictive modeling of geometric defects in L-PBF.

Analysis of Boundary Layer Effects Due to Usual Boundary Conditions or Geometrical Defects in Elastic Plates Under Bending: An Improvement of the Love-Kirchhoff Model

We propose a model of flexural elastic plates accounting for boundary layer effects due to the most usual boundary conditions or to geometrical defects, constructed via matched asymptotic expansions. In particular, considering a rectangular plate clamped at two opposite edges while the other two are free, we derive the effective boundary conditions or effective transmission conditions that the two first terms of the outer expansion must satisfy. The new boundary value problems thus obtained are studied and compared with the classical Love-Kirchhoff plate model. Two examples of application illustrate the results.

A Machine Vision-based Cyber-Physical Production System for Energy Efficiency and Enhanced Teaching-Learning Using a Learning Factory

Machine vision (MV) can help in achieving real-time data analysis in a manufacturing environment. This can be implemented in any industry to achieve real-time monitoring of workpieces for geometric defects and material irregularities. Identification of defects, sorting of workpieces based on their physical parameters, and analysis of process abnormalities can be achieved by using the real-time data from simple and cost-effective raspberry pi with camera and open source machine learning platform TensorFlow to run convolutional neural network (CNN) model. The proposed cyber-physical production system enables to develop a MV based system for data acquisition integrating physical entities of learning factory (LF) with the cyber world. Nowadays, LFs are widely used to train the workforce for developing competencies for emerging technologies and challenges faced due to technological advancements in Industry 4.0. This paper demonstrates the application of a cost-effective MV system in a learning factory environment to achieve real-time data acquisition and energy efficiency. The proposed low-cost machine vision is found to detect geometric irregularities, colours and surface defects. The simple cost effective MV system has enhanced the energy efficiency and reduced the total carbon footprint by 18.37 % and 78.83 % depending upon the location of MV system along the flow. The teaching-learning experience is also enhanced through action-based learning strategies. This not only ensures less rework, better control, unbiased decisions, 100% quality assurance but also the need of workers/operators can be reduced.

Fatigue evaluation and CFRP strengthening of diaphragm cutouts in orthotropic steel decks

The cracking at the transverse diaphragm cutout is one of the most severe fatigue failures threatening orthotropic steel decks (OSDs), whose mechanisms and crack treatment techniques have not been fully studied. In this paper, full-scale experiments were first performed to investigate the fatigue performance of polished cutouts involving the effect of an artificial geometrical defect. Following this, comparative experimental testing for defective cutouts strengthened with carbon fiber-reinforced polymer (CFRP) was carried out. Numerical finite element analysis was also performed to verify and explain the experimental observations. Results show that the combinative effect of the wheel load and thermal residual stress constitutes the external driving force for the fatigue cracking of the cutout. Initial geometrical defects are confirmed as a critical factor affecting the fatigue cracking. The principal stress 6 mm away from the free edge of the cutout can be adopted as the nominal stress of the cutout during fatigue evaluation, and the fatigue resistance of polished cutouts is higher than Grade A in AASHTO specification. The bonded CFRP system is highly effective in extending the fatigue life of the defective cutouts. The present study provides some new insights into the fatigue evaluation and repair of OSDs.

Localized defect states based on defective multi-quantum wells

The aim of this paper is to present a theoretical study of the transmission coefficient for a periodic CdMnTe/CdTe system containing a geometric and material defect layer. The dynamic tuning of this defect layer is investigated using the transfer matrix for the theoretical calculations. It is shown that the thickness and the concentration of the defect layer have a strong impact on the electronic states (called also defect modes) localized in the band gaps. Moreover, the transmission coefficients of these defect states reach maximal values when the defect layer is placed in the middle of the structure; this is due to the constructive interferences caused by the symmetry of the system. Further, the defect states shift to lower energies with the increase of the concentration or the thickness of the defect layer as well as the decrease of the quality factor. These findings are useful to design a nano-energy filtering and sensoring devices.

Building a Scalable and Interpretable Bayesian Deep Learning Framework for Quality Control of Free Form Surfaces

Deep learning has demonstrated high accuracy for 3D object shape error modeling necessary to estimate dimensional and geometric quality defects in multi-station assembly systems (MAS). Increasingly, deep learning-driven Root Cause Analysis (RCA) is used for decision-making when planning corrective action of quality defects. However, given the current absence of scalability enabling models, training deep learning models for each individual MAS is exceedingly time-consuming as it requires large amounts of labelled data and multiple computational cycles. Additionally, understanding and interpreting how deep learning produces final predictions while quantifying various uncertainties also remains a fundamental challenge. In an effort to address these gaps, a novel closed-loop in-process (CLIP) diagnostic framework underpinned algorithm portfolio is proposed which simultaneously enhances scalability and interpretability of the current Bayesian deep learning approach, Object Shape Error Response (OSER), to isolate root cause(s) of quality defects in MAS. The OSER-MAS leverages a Bayesian 3D U-Net architecture integrated with Computer-Aided Engineering simulations to estimate root causes. The CLIP diagnostic framework shortens OSER-MAS model training time by developing: (i) closed-loop training to enable faster convergence for a single MAS by leveraging uncertainty estimates of the Bayesian 3D U-net model; and, (ii) transfer/continual learning-based scalability model to transmit meta-knowledge from the trained model to a new MAS resulting in convergence using comparatively less training samples. Additionally, CLIP increases the transparency for quality-related root cause predictions by developing interpretability model which is based on 3D Gradient-based Class Activation Maps (3D Grad-CAMs) and entails: (a) linking elements of MAS model with functional elements of the U-Net architecture; and, (b) relating features extracted by the architecture with elements of the MAS model and further with the object shape error patterns for root cause(s) that occur in MAS. Benchmarking studies are conducted using six automotive-MAS with varying complexities. Results highlight a reduction in training samples of up to 56% with a loss in performance of up to 2.1%.

Hybrid Manufacturing Processes: an experimental machinability investigation of DED produced parts

As metal AM technologies have realized significant development, hybrid manufacturing, which integrates both additive and subtractive processes in a single machine tool, has emerged in order to tackle the geometrical and surface defects that are present in AM parts. Due to their unique microstructure, subtractive process planning has to be re-evaluated. This work aims at investigating the machinability of AM-produced components in the scope of hybrid manufacturing. A preliminary experimental investigation of surface quality and tool wear has been made using BOHLER K890 samples manufactured by utilizing DED process. Core milling parameters have been evaluated regarding their effect on the final result.

Multi-channel filters with high performance based on the creation of a geometrical defect in 1D phononic star waveguides structure

In this paper, we investigate the existence of one or two defect modes in the phononic band structure and the transmission coefficient in a one-dimensional phononic star waveguides (SWGs) structure. This structure exhibits large band gaps and pass bands, which due to the periodicity of the system and the resonance states of the grafted lateral branches (resonators). The defect modes result from the presence of a defective segment in the star waveguides structure and may occur in these gaps. We show there is dual frequency acoustic filtering based on two defect modes in a wide gap by creating a defect at the segment level in a one-dimensional star waveguides structure when pumped by an acoustic wave under a normal incidence. The defect modes are sensitive to the defect length d01, the number N of cells, and the position J of the defect. The color map of the transmission rate (TR) is discussed as a function of the defect segment length d01 and the resonator length d2. The transmission spectrum and the band structure are theoretically presented using the Green functions approach based on the formalism of the interface response theory for acoustic waves propagating in star waveguides structure. This structure has potential applications as an acoustic guide, multi-channel filters and demultiplexers.

Incremental bending of conic profiles on CNC hydraulic bending machines

Sheet metal forming processes for manufacturing truncated cones can be divided into methods which use shape related tooling to produce the required form, for example, spinning and deep drawing, and processes that utilise kinematics to produce the required geometry, for example, roller, air and swivel bending. Such kinematic forming methods can achieve high degrees of forming flexibility but require process models to avoid elaborate trial and error approaches. For the incremental manufacture of cones, CNC swivel bending as yet lacks such a process model. In addition, with the current ‘state of the art’ processes, the manufactured cones exhibit geometric defects with respect to concentricity and skewing. In this study, the operation of a CNC bending machine fitted with an upper beam that can be inclined was investigated. The proposed inclination of the beam corresponds to the desired conical shape of the product. The machine was provided with a radial sheet feed mechanism (back-gauge) to feed material in even segments between bent corners thus avoiding geometric imperfections. A plasto-mechanic process model was developed to deliver the process parameters for swivel bending of specific conical geometries. The process model was validated by numerical simulations and practical bending experiments. Coarsely segmented conical shapes can be manufactured using the analytically calculated process parameters. For conical geometries, the process modifications of sequential swivel bending showed satisfactory improvements in the initial bending defects, that is, lack of concentricity and skewing. Compared with the target geometries, a tendency towards larger diameters remained in the experiments and is explained by the boundary conditions of the sequential process and if a larger number of bending increments is carried out. The present study delivers a first approach to determine the process parameters for the sequential operation of a swivel bending machine in order to provide relevant process parameters for the industrial production of conical components.

Contact stiffness and dynamic behavior caused by surface defects of spiral bevel gear in mixed lubrication

Geometric defects may occur on the gear surfaces during manufacturing and service, such as pitting. However, the defect is rarely incorporated into gear mesh analysis. In this study, a transient mixed lubrication model and a two-dof-torsional dynamic model are proposed for spiral bevel gears, wherein the time-varying contact parameters and the defect on the meshing path are all considered. First, the time-varying contact parameters are calculated by Tooth-Contact-Analysis (TCA). After that, by simulating the gear meshing process, an engineering rough surface with a circular defect moves through the contact region. The transient mixed lubrication performances with the consideration of surface defect are obtained including the film thickness and pressure along the meshing path. Based on this, the normal stiffness of single tooth from engaged-in to engaged-out can be calculated through the normal contact deformation. Finally, the dynamic responses of gears are analyzed according to the normal stiffness through a two-dof-torsional model. Numerical results show that the surface defect has a significant effect on the meshing process of spiral bevel gears and the dynamic response also changes with the size, depth and location of the defect. This provides a theoretical reference for identifying early failures caused by defects based on dynamic signals.

Geometrical, microstructural, and mechanical properties of curved-surface AlSi10Mg parts fabricated by powder bed fusion additive manufacturing

Laser-based powder bed fusion (L-PBF) enables the fabrication of complex-structural AlSi10Mg parts with low material waste and high manufacturing resolution. Thus far, there have been comprehensive investigations to reveal the process-microstructure-property link of as-built AlSi10Mg planar-surface or lattice/honeycomb structural parts. However, little is known about the geometrical, microstructural, and mechanical properties of as-deposited curved-surface parts. In this paper, the effects of curvatures on the geometrical performance, defects, microstructure, and mechanical properties of as-built AlSi10Mg parts with curved surfaces were investigated. Specifically, the results of average cloud-to-cloud distances indicated that the largest curvature (the peak position) exhibited higher dimensional accuracy than the others of as-built curved-surface parts. In addition, the lowest porosity was achieved at the largest curvatures owing to the sufficient localized energy input. We also found that the grain size and the width of grain boundaries increased with an increase in the curvature. Moreover, the tensile property of as-built curved-surface specimens was assessed for the first time. The positions with the largest curvatures became the preferable failure positions due to the largest bending moment. This paper provided great insights into the curvature-microstructure-property relationship for the industrial adoption of PBF-built AlSi10Mg parts with curved surfaces.