Numerical study on the chip removal and surface quality of CFRP/Ti6Al4V stacks

369-378Machining of multilayer stacks constituted by carbon fibre reinforced polymer (CFRP) composites and titanium (Ti) alloys has been a hot topic receiving extensive attention in both academia and industry due to their superior mechanical properties and widespread applications in the modern aircraft. Compared with the wide availability of experimental studies dealing with the cutting of CFRP/Ti6Al4V stacks, the present work aims to utilize a finite element (FE) method to address fundamentally the machining characteristics of this metallic-composite material. In this paper, a two-dimensional numerical model of orthogonal cutting configuration has been established to improve the mechanism investigations of the bi-material cutting process. The CFRP/Ti6Al4V model has been constructed by establishing three different physical constituents including the Ti phase, the interface and the CFRP phase. Disparate constitutive laws and damage criteria have been implemented to build the anisotropic machinability of the sandwich material. The key cutting phenomena including the chip removal process, machined surface quality and parametric effects on the stack cutting response have been addressed with a particular focus on the quantification of the machining-induced composite damage. The numerical analysis sheds light on several implicit mechanisms dominating the stack machining and offers a better CFRP/Ti6Al4V cutting understanding

Channel Pruning Guided by Classification Loss and Feature Importance

In this work, we propose a new layer-by-layer channel pruning method called Channel Pruning guided by classification Loss and feature Importance (CPLI). In contrast to the existing layer-by-layer channel pruning approaches that only consider how to reconstruct the features from the next layer, our approach additionally take the classification loss into account in the channel pruning process. We also observe that some reconstructed features will be removed at the next pruning stage. So it is unnecessary to reconstruct these features. To this end, we propose a new strategy to suppress the influence of unimportant features (i.e., the features will be removed at the next pruning stage). Our comprehensive experiments on three benchmark datasets, i.e., CIFAR-10, ImageNet, and UCF-101, demonstrate the effectiveness of our CPLI method.Comment: AAAI202

Measuring Urban Spatial Activity Structures: A Comparative Analysis

Abstract: Human activity recognition has been of interest in the field of urban planning. This paper established a general framework by which expected human activity intensity (HAI) measured by the built environment and factual HAI measured by the Baidu thermal chart were estimated and comparatively analyzed so as to identify abnormal human activities in Hanghzou, China. Three elements of the built environment (i.e., residential density, road connectivity, and land-use mixing degree) from multi-source data with high precision are selected to assess the expected HAI. Results indicate Hangzhou has evolved into a polycentric city with three urban clusters. In addition, a significant positive correlation exists between the two types of HAIs. However, there are areas with spatial mismatches, particularly in the “urban village” and new towns, suggesting human activities are not equally distributed all over the city. Research implications, limitations, and future research needs are discussed

Planning Emergency Shelters for Urban Disasters: A Multi-Level Location–Allocation Modeling Approach

In recent years, cities are threatened by various natural hazards. Planning emergency shelters in advance is an effective approach to reducing the damage caused by disasters and ensuring the safety of residents. Thus, providing the optimal layout of urban emergency shelters is an important stage of disaster management and an act of humanitarian logistics. In order to study the optimal layout of emergency shelters in small mountain cities, this paper constructs multi-level location models for different grades of emergency shelters so as to minimize the travel and construction costs and maximize the coverage rate. Specifically, the actual service of emergency shelters is determined using Geographic Information System (GIS) software and Weighted Voronoi Diagram (WVD) models under the limitation of site capacity, and the space layout is adjusted through combining the actual urban land with the construction position. In this paper, the Jianchuan county seat at Yunnan Province, China, was considered as a case study to illustrate the models of emergency shelters in which the feasibility of the presented models is verified. The proposed research methods and models have provided theoretical basis and a benchmark for the optimal layout of emergency shelters in other small mountain cities

One Forward is Enough for Neural Network Training via Likelihood Ratio Method

While backpropagation (BP) is the mainstream approach for gradient computation in neural network training, its heavy reliance on the chain rule of differentiation constrains the designing flexibility of network architecture and training pipelines. We avoid the recursive computation in BP and develop a unified likelihood ratio (ULR) method for gradient estimation with just one forward propagation. Not only can ULR be extended to train a wide variety of neural network architectures, but the computation flow in BP can also be rearranged by ULR for better device adaptation. Moreover, we propose several variance reduction techniques to further accelerate the training process. Our experiments offer numerical results across diverse aspects, including various neural network training scenarios, computation flow rearrangement, and fine-tuning of pre-trained models. All findings demonstrate that ULR effectively enhances the flexibility of neural network training by permitting localized module training without compromising the global objective and significantly boosts the network robustness

Experimental study on drilling mechanisms and strategies of hybrid CFRP/Ti stacks

Mechanical drilling has been frequently used for hole making of hybrid CFRP/Ti stacks in order to ensure excellent fastening assembly. Owing to their inhomogeneous behavior and poor machinability, drilling CFRP/Ti stacks in one-shot time has brought great challenges to the modern manufacturing community. Compared to the previous studies on drilling CFRP/Ti, this paper aims to highlight the following aspects: (i) the features of tool-work interaction and machinability classification in the bi-material drilling, (ii) the influences of different cutting sequences on CFRP/Ti drilling responses, and (iii) the effects of different tool geometries/materials on CFRP/Ti drilling performance. The experimental results have shown that the drill geometrical features, which ensure the cutting contacts of the stack combination, have a more significant effect on CFRP/Ti drilling output than tool material composition. The Ti?CFRP drilling strategy promotes higher quality of the machined hole surfaces (e.g., more consistent hole diameters and much better surface finish) with lower Ti burr extents, while the CFRP?Ti drilling strategy reduces only the induced delamination. The experiments discussed in this paper allow besides several recommendations for the cutting sequence selection and drill geometrical design when drilling hybrid CFRP/Ti stacks

Numerical studies of frictional responses when cutting hybrid CFRP/Ti composite

In manufacturing sectors, machining hybrid CFRP/Ti is usually an extremely challenging task due to the disparate natures of each stacked constituent involved and their respectively poor machinability. The current research focus of hybrid CFRP/Ti cutting was primarily made via the experimental studies, which exhibited high cost and time consuming. In this paper, a new contribution was provided to study the key frictional responses dominating the bi-material machining via the numerical approach. To this aim, a multi-physical model was developed by implementing different constitutive laws and damage criteria to construct the anisotropic machinability of the stacked composite. The interrelated effects of the multi-toolwork frictional behavior on hybrid CFRP/Ti cutting were precisely investigated with respect to the specific cutting energy consumption, machined surface morphology, and affected subsurface damage. A special focus was made to clarify the cutting sequence’s influences on the hybrid cutting operation. The numerical results highlighted the reasonable CFRP→Ti cutting sequence for hybrid composite machining and the pivotal role of multi-tool-work interaction in affecting the frictional responses induced by cutting

Experimental Studies on the Cutting Characteristics of Hybrid CFRP/Ti Stacks

Owing to their enhanced mechanical properties and improved structural functions, the use of hybrid CFRP/Ti stacks (a sandwich of one CFRP laminate and one Ti alloy) has experienced an increasing trend in modern aerospace industry. The emergence of such composite-to-metal alliances, however, poses a series of new challenges to the manufacturing sectors for high-quality finishing of the material-made components. The key machining problems usually arise from the disparate natures of the stacked constituents (CFRP laminate and Ti alloy) and their respective poor machinability. To study the fundamental cutting characteristics of the bi-material assembly, this paper presents an experimental study concerning the machinability evaluation of the hybrid CFRP/Ti stacks. An orthogonal cutting configuration (OCC) derived from the real manufacturing operation was adopted to finalize the CFRP/Ti cutting comprehension by using the polycrystalline diamond (PCD) tipped tools. The cutting trials were performed under the reasonable cutting sequence strategy of CFRP -> Ti as pointed out by most experimental studies. The key cutting responses including cutting forces, machined surface quality and tool wear mechanisms were precisely addressed versus the utilized cutting conditions. The experimental results highlight that a parametric combination of high cutting speeds and low feed rates often facilitates the reduction of cutting forces and induced damage extents. The basic damage modes promoted on the machined CFRP/Ti surfaces are observed to be fiber pullout, resin loss, surface cavity, deformation of feed marks and re-deposited materials. Moreover, the key wear mechanisms governing the PCD tool cutting are confirmed to be crater wear and flank wear, while the tool failure mode is edge chipping. To ensure the excellent machined surface quality, a stringent control of tool wear should be implemented when cutting hybrid CFRP/Ti stacks

Wear characteristics of polycrystalline diamond tools in orthogonal cutting of CFRP/Ti stacks

CFRP/Ti stacks have become a viable alternative to conventional composite laminates and metal alloys in various aerospace applications because of their enhanced mechanical properties and improved structural functions. Machining these multilayer stacks with acceptable quality still remains a great challenge for the modern manufacturing sectors due to the ineffective management of the tool wear performance resulting from the varying properties of the stacked constituents. This paper describes, on a fundamental basis, the wear characteristics of polycrystalline diamond (PCD) tipped tools during the orthogonal cutting of CFRP/Ti stacks. Two cutting sequence strategies, i.e., cutting from CFRP to Ti and cutting from Ti to CFRP, were implemented throughout the cutting tests. The experimental results confirm the predominance of the cutting sequence strategy in affecting the CFRP/Ti chip separation and the machined surface quality. The chip adhesion is a most influential factor leading to the disparate influences of the two cutting sequence strategies on the CFRP/Ti cutting responses. The PCD tool suffers severe crater wear due to the Ti phase cutting, while it undergoes minor flank wear owing to its superior wear resistance against abrasive carbon fibers. The wear signatures on the tool-chip contact zone via the scanning electron microscope (SEM) observation reveal that the Ti phase cutting dominates the formation of the tool crater wear land. The edge chipping occurs mainly due to the sudden force variation in the CFRP/Ti interface machining and the inherent brittleness of the PCD material. The fiber orientation seems to have a great impact on the wear behavior of utilized PCD tools by altering the tool-chip contact length

On the analysis of temperatures, surface morphologies and tool wear in drilling CFRP/Ti6Al4V stacks under different cutting sequence strategies

In drilling CFRP/Ti6Al4V stacks, the cutting sequence strategy, which determines the coupling effects of each phase machining, affects significantly the machinability of the sandwiched material as well as the tool wear characteristics. The present paper contributes to a scientific understanding of the effects of different cutting sequence strategies on the drilling performance of multilayer CFRP/Ti6Al4V stacks when using uncoated tungsten carbide and diamond-coated drills. Experimental quantification of the in-situ temperatures during the stack drilling was conducted using the method of infrared thermography camera and the instrumentation of drill bits by embedded thermocouples. Drilling forces, exit burr heights of the titanium holes, surface morphologies of the composite holes and tool wear signatures were analyzed. The results indicate that drilling from titanium to CFRP leads to higher magnitudes of the composite cutting temperatures while it benefits the reduction of the stack thrust forces, the improvement of the composite surface morphologies as well as the decrease of the exit titanium burr heights. Additionally, the coupling effects of drilling temperatures and chip adhesion are the influential factors leading to the disparate effects of the cutting sequence strategy on the drill wear progression. Drilling from titanium to CFRP reduces the drill adhesion and flank wear extents owing to the brushing effects of the composite drilling. The diamond-coated drills are confirmed superior to the uncoated ones in terms of lower drilling temperatures, lower drilling forces, minimal hole surface damage, less tool wear while machining the CFRP/Ti6Al4V stacks.publishe