Towards understanding the cutting and fracture mechanism in ceramic matrix composites

Ceramic Matrix Composites (CMCs) are increasingly used for the manufacture of high-value parts for several industries such as the aerospace, nuclear and automotive. Nevertheless, their heterogenic, anisotropic and brittle nature make difficult to characterise the machining process and therefore, an in-depth understanding of the cutting mechanics is needed. In this regard, this paper aims to understand the different behaviours of CMCs while employing orthogonal cutting. The first part of this article proposes a novel theoretical approach to explain the different types of cutting behaviours (fracture and shear cutting) based on the inelastic and orthotropic properties of the CMC's by using a high imaging system and measuring the cutting forces. The second part aims to understand the cutting and fracture mechanism by developing for the first time a specific analytical model for each of the three main orthotropic orientations, defined by the three main relative fibre orientations respect to the feed direction, which are found in cutting of CMCs. This is approached by the calculation of the specific cutting energy needed to fracture the CMC's during cutting (energy release rate, Gc) using fracture mechanics and cutting theories. This analytical model has been successfully validated for a Carbon/Carbon composite with the experimental data obtained for the brittle cutting and by introducing the concept of a rising R-curve in cutting models. Moreover, comparing the results obtained for the energy release rate for the brittle and semi-ductile mode, it is observed that the material experiences an important change in the energy release rate according to the brittle-to-semi-ductile transition occurring while reducing the depth of cut. Finally, a novel monitoring method based on the vibrations of the sample has been found successful to understand the type of crack formation appearing while cutting CMCs

Modeling and Experimental Validation of a Compliant Underactuated Parallel Kinematic Manipulator

© 1996-2012 IEEE. Parallel kinematic manipulators (PKMs) are increasingly used in a wide range of industrial applications due to the characteristics of high accuracy and compact structure. However, most of the existing PKMs are structured with heavy actuators and high stiffness. In this respect, this article proses a simple, yet effective, parallel manipulator that distinguishes itself through the following basis. First, underactuation: it employs only a single motor and a driving cable to actuate its three legs. Second, novel foot location: it uses a smart shape memory alloy clutch-based driving system (SCBDS), which catches/releases the driving cable, thus, making possible the robot underactuation. Finally, adjustable compliance: its double compliant joints on each limb with a stiffness-adjustable section, which renders a safe human-robotic interaction. To support and predict the performance of this underactuated compliant manipulator, a novel kinetostatic model was developed by considering the generalized internal loads (i.e., force and moment) in three compliant limbs and the external loads on the upper platform. Finally, based on the physical prototype, a set of experiments were conducted to validate the model proposed in this article. It was found that the proposed kinetostatic model can be validated with the average deviations of 1.8% in position and 2.8% in orientation, respectively. Furthermore, the workspace of the system (e.g., discrete and continuous workspace) was studied when different actuating strategies were employed, thus, emphasizing the advantages and the limitations of this novel system

On-the-fly laser machining: a case study for in situ balancing of rotative parts

On-the-fly laser machining is defined as a process that aims to generate pockets/patches on target components that are rotated or moved at a constant velocity. Since it is a nonintegrated process (i.e., linear/rotary stage system moving the part is independent of that of the laser), it can be deployed to/into large industrial installations to perform in situ machining, i.e., without the need of disassembly. This allows a high degree of flexibility in its applications (e.g., balancing) and can result in significant cost savings for the user (e.g., no dis(assembly) cost). This paper introduces the concept of on-the-fly laser machining encompassing models for generating user-defined ablated features as well as error budgeting to understand the sources of errors on this highly dynamic process. Additionally, the paper presents laser pulse placement strategies aimed at increasing the surface finish of the targeted component by reducing the area surface roughness that are possible for on-the-fly laser machining. The overall concept was validated by balancing a rotor system through ablation of different pocket shapes by the use of a Yb:YAG pulsed fiber laser. In this respect, first, two different laser pulse placement strategies (square and hexagonal) were introduced in this research and have been validated on Inconel 718 target material; thus, it was concluded that hexagonal pulse placement reduces surface roughness by up to 17% compared to the traditional square laser pulse placement. The concept of on-the-fly laser machining has been validated by ablating two different features (4 × 60 mm and 12 × 4 mm) on a rotative target part at constant speed (100 rpm and 86 rpm) with the scope of being balanced. The mass removal of the ablated features to enable online balancing has been achieved within < 4 mg of the predicted value. Additionally, the error modeling revealed that most of the uncertainties in the dimensions of the feature/pocket originate from the stability of the rotor speed, which led to the conclusion that for the same mass of material to be removed it is advisable to ablate features (pockets) with longer circumferential dimensions, i.e., stretched and shallower pockets rather than compact and deep

A study of surface swelling caused by graphitisation during pulsed laser ablation of carbon allotrope with high content of sp ³ bounds

Experiments and theory are employed to investigate the laser ablation of boron doped diamond and tetrahedral amorphous carbon using nanosecond pulses. For a single pulse at low values of fluence, the laser induces a swelling of the surface due to graphitisation, whilst a high level of fluence leads to recession of the surface due to vaporization. To understand and investigate the underlying phenomena during the diamond-laser interaction, a model has been developed to reliably and quickly predict the behaviour of the surface and the thickness of the heat affected zone. The model is based on conservation of heat and mass during the laser-workpiece interaction. It consists of a one-dimensional system of non-linear equations that models the material heating, evaporation, graphitisation and plasma shielding. There is excellent agreement between numerical and experimental results for the position of the interfaces up to a high laser fluence. This model is the first to investigate the ablation of diamond that is able to capture surface swelling due to the graphitisation of the diamond layer, the graphite thickness and the amount of ablated material within a single framework. Furthermore, the model provides a novel methodology to investigate the thermal stability of diamond-like carbon films. The activation energy for tetrahedral amorphous carbon is obtained using the model with an accuracy of 3.15+1.0−0.22 eV

Stochastic simplified modelling of abrasive waterjet footprints

Abrasive micro-waterjet processing is a non-conventional machining method that can be used to manufacture complex shapes in difficult-to-cut materials. Predicting the effect of the jet on the surface for a given set of machine parameters is a key element of controlling the process. However, the noise of the process is significant, making it difficult to design reliable jet-path strategies that produce good quality parts via controlled-depth milling. The process is highly unstable and has a strong random component that can affect the quality of the workpiece, especially in the case of controlled-depth milling. This study describes a method to predict the variability of the jet footprint for different jet feed speeds. A stochastic partial differential equation is used to describe the etched surface as the jet is moved over it, assuming that the erosion process can be divided into two main components: a deterministic part that corresponds to the average erosion of the jet, and a stochastic part that accounts for the noise generated at different stages of the process. The model predicts the variability of the trench profiles to within<8%. These advances could enable abrasive micro-waterjet technology as a suitable technology for controlled-depth milling

A concept for actuating and controlling a leg of a novel walking parallel kinematic machine tool

The scope of this paper is to present a novel method of actuating the legs of a walking parallel kinematic machine tool (WalkingHex) such that the upper spherical joint can be actively driven while walking and remain a free, passive joint while performing machining operations. Different concepts for the number of Degrees of Freedom (DoF) and methods for actuating the chosen concept are presented, leading to a description of a three-wire actuated spherical joint arrangement. The inverse kinematics for the actuation mechanism is defined and a control methodology that accounts for the redundantly actuated nature of the mechanism is explored. It is demonstrated that a prototype of the system is capable of achieving a motion position accuracy within 5.64% RMS. Utilising the concept presented in this paper, it is possible to develop a walking robot that is capable of manoeuvring into location and performing precision machining or inspection operations

Transient three-dimensional geometrical/thermal modelling of thermal spray: normal-impinging jet and single straight deposits

A new time-dependent approach to the geometrical and thermal modelling of the deposit footprint in thermal spray processes is proposed. Based upon a three-dimensional finite-difference numerical technique, the model is composed of two integrated sections: a geometrical analysis, accounting for deposit geometry analytical prediction, and a thermal analysis that computes the system temperature history. Primary process factors for the simulation, i.e. plume distribution parameters, jet heat transfer properties and temperature-dependent deposition efficiency are determined in a preliminary stage of model application. Through computation of the simulated impact surface temperature at each instant during the simulation, the deposition efficiency-mediated growth of the deposit is accurately predicted at arbitrary values of torch feed speed. The model is flexible as it only relies on an initial calibration stage performed at a specific set of process parameters to be able to predict deposition geometries at arbitrary conditions, thus avoiding the need of complex simulations and/or knowledge of single splats impact properties. Moreover, this modelling approach has the potential to be extended to several thermal spray processes at arbitrary values of process parameters (e.g. torch design, materials, etc.), opening the way for spray automation in difficult-to-spray geometries and/or repair applications. The proposed modelling framework has been validated on Combustion Flame Spray (CFS) deposition of CoNiCrAlY alloy on stainless steel substrates, yielding low errors (< 5% on average) in predicting the deposit footprint at various torch feed speeds

On the inverse design of discontinuous abrasive surface to lower friction-induced temperature in grinding: an example of engineered abrasive tools

In order to lower temperature, abrasive tools with passive-grinding, e.g. textured, areas (PGA) have been suggested. However, most of the reported PGA geometries (e.g. slots, holes) have been determined based on the engineering intuition (i.e. trial and error) rather than in-depth phenomenological analysis. To fill this gap, this paper proposes a method to design the PGA geometry according to the desired temperature, i.e. the inverse design method. In the method, the analytical model of grinding temperature for tools with PGA is established and treated as the primary constraint in the inverse problem, while the models of the ground surface roughness and grinding continuity as the subsidiary constraints. The method accuracy is validated by conducting grinding trials with tools with the calculated PGA geometries and comparing their performances (temperature, roughness and force fluctuation) to the required ones. In comparison with conventional tools, our tools designed by the method have been found effective to reduce harmful, or even destructive, thermal effects on the ground surfaces. This work might lay foundation for designing discontinuous abrasive tools, and future work can be probably extended to the tools or the workpiece with more complex shapes (e.g. ball end/cup tools, and free-form workpiece)

The new challenges of machining Ceramic Matrix Composites (CMCs): review of surface integrity

Ceramic Matrix Composites (CMCs) are currently an increasing material choice for several high value and safety-critical components, fact that has recently originated the need of understanding the effect of several machining processes. Due to the complex nature of CMCs - i.e. heterogeneous structure, anisotropic thermal and mechanical behaviour and generally the hard nature of at least one of the constituents (e.g. fibre or matrix) - machining become extremely challenging as the process can yield high mechanical and thermal loads. Furthermore, the orthotropic, brittle and heterogeneous nature of CMCs result in different material removal mechanisms which lead to unique surface defects. Hence, this review paper attempts to provide an informative literature survey of the research done in the field of conventional and non-conventional machining of CMCs with a main focus on critically evaluate how different machining techniques affect the machined surfaces. This is achieved by exploring and recollecting the different material characterisation techniques currently used to observe and quantify the mechanical and thermal surface and subsurface damages and highlight their governing removal mechanisms

On modelling of cutting force and temperature in bone milling

© 2018 Elsevier B.V. Cutting force and temperature are the key factors to be controlled during the orthopaedic surgery which could result in mechanical damage and necrosis of the bone tissue. Mechanistic modelling of the bone cutting process is expected to be an efficient method to understand and control these process challenges. However, due to the special structure and properties of the bone tissue (consist of osteon fibres and interstitial lamellae matrix), the conventional metal cutting models are not applicable in bone cutting process. This paper presents a novel cutting force and temperature mechanistic models for milling of bone. A cutting stress model of bone material was developed which takes into account its anisotropic characteristics based on the orthogonal cutting data. The cutting force coefficients are predicted incorporating the osteon orientation, tool geometry and edge effect with unified mechanics of cutting approach. Furthermore, a model of the induced cutting temperature based on heat flux developed during the process was proposed to predict the temperature distribution on bone cut surface. The experimental results showed a better consistency with the proposed model compared with the conventional Johnson-Cook model under different cutting conditions. A necrosis (potential cell injury from thermal effect) penetration depth was also proposed to evaluate the extent of thermal damage of bone tissue by the developed models. The proposed model can be used to assist the robotic surgery, to optimize the cutting parameters as well as to guide the orthopaedic tool design