3D CAFE modelling of ductile fracture in gas pipeline steel

This thesis describes a series of experimental and computational studies carried out on Xl00 pipeline steel with the objective to characterise the tearing resistance of the material. A Cellular Automata - Finite Element(CAFE) technique was used in this work to develop a 3D numerical model to provide a more realistic description of the ductile damage mechanisms of the pipeline steel. In this model, the Rousselier micro-mechanisms damage theory and an appropriate cell size in a CA array represent the material behaviour. The experimental work consisted of laboratory tensile specimens in four different orientations of the material to determine the properties of the pipeline steel. Two novel designs were conducted to measure the deformation behaviour when loaded in the through wall direction. Compact C(I) and tear specimens were also tested to capture the crack growth, and the flat and shear fracture characteristics. The experimental data of laboratory samples were used to calibrate the continuum damage models. SEM (Scanning Electron Microscope) micrograph observations were carried out in tensile tests, standard C(f), slant notch C(f) and tear specimens. These observations revealed that spacing between large dimples of flat fracture are of the order of five times larger than shear fracture. It is important since the transfer of the material model parameters is made by modifying the cell size according to the average spacing between large voids in the material, d. Therefore 3D CAFE models for flat and shear fracture were created according to the microstructural characteristics to interpret the experimental findings. The main aim of the research reported here is to investigate transferability of the damage model parameters to gas pipelines steels from laboratory scale samples, and then to predict the fracture response of real structures. The CAFE technique has been shown to be a powerful tool in reducing simulation time whilst maintaining good predictions of shear damage and material resistance in terms of CTOA criterion. This was not achieved by classical FE methods where a very fine mesh is required to represent the characteristic dimension of ductile fracture. Similar reasonable results were obtained when anisotropic flat fracture was predicted but transferability of the damage parameters to CT specimens needs still further investigation

2D FE Prediction of Surface Alteration of Inconel 718 under Machining Condition

Abstract Nickel-based super alloys such as Inconel 718 are widely employed in extremely hostile applications owing to their superior thermo-mechanical properties. On the contrary, these latter lead the industries to adopt conservative process parameters (e.g. low cutting speed) resulting in lower production rates. The possibility to increase the cutting parameters could lead to higher material removal rates and drastic reduction of the machining time of the process. The aim of this study is to investigate the effects of extreme cutting parameters on the surface and subsurface alterations such as grain size and hardness changes by developing a finite element (FE) numerical model. The Zener-Hollomon and Hall-Petch equation were implemented to predict the grain size and micro hardness variations due to the cutting process. In addition, the depth of the affected layer due to machining was predicted using the critical strain equation. The obtained results proved the accuracy and reliability of the proposed FE model showing a good agreement between the simulated and the experimental results

Hybrid-modelling of compact tension energy in high strength pipeline steel using a Gaussian Mixture Model based error compensation

In material science studies, it is often desired to know in advance the fracture toughness of a material, which is related to the released energy during its compact tension (CT) test to prevent catastrophic failure. In this paper, two frameworks are proposed for automatic model elicitation from experimental data to predict the fracture energy released during the CT test of X100 pipeline steel. The two models including an adaptive rule-based fuzzy modelling approach and a double-loop based neural network model, relate the load, crack mouth opening displacement (CMOD) and crack length to the released energies during this test. The relationship between how fracture is propagated and the fracture energy is further investigated in greater detail. To improve the performances of the models, a Gaussian Mixture Model (GMM)-based error compensation strategy which enables one monitor the error distributions of the predicted result is integrated in the model validation stage. This can help isolate the error distribution pattern and to establish the correlations with the predictions from the deterministic models. This is the first time a data-driven approach has been used in this fashion on an application that has conventionally been handled using finite element methods or physical models

Evaluation of Workpiece Temperature during Drilling of GLARE Fiber Metal Laminates Using Infrared Techniques: Effect of Cutting Parameters, Fiber Orientation and Spray Mist Application

The rise in cutting temperatures during the machining process can influence the final quality of the machined part. The impact of cutting temperatures is more critical when machining composite-metal stacks and fiber metal laminates due to the stacking nature of those hybrids which subjects the composite to heat from direct contact with metallic part of the stack and the evacuated hot chips. In this paper, the workpiece surface temperature of two grades of fiber metal laminates commercially know as GLARE is investigated. An experimental study was carried out using thermocouples and infrared thermography to determine the emissivity of the upper, lower and side surfaces of GLARE laminates. In addition, infrared thermography was used to determine the maximum temperature of the bottom surface of machined holes during drilling GLARE under dry and minimum quantity lubrication (MQL) cooling conditions under different cutting parameters. The results showed that during the machining process, the workpiece surface temperature increased with the increase in feed rate and fiber orientation influenced the developed temperature in the laminate

Influence of workpiece constituents and cutting speed on the cutting forces developed in the conventional drilling of CFRP composites

This work investigates the influence of cutting speed and workpiece constituents on the thrust force and torque developed in the conventional dry drilling of woven carbon fibre reinforced polymer (CFRP) composites using uncoated WC-Co tools, by applying experimental techniques and statistical test methods. The type of thermosetting matrix showed significant impact on both the maximum thrust force and torque developed, whilst the type of carbon fibre fabric and cutting speed showed negligible effects on the maximum thrust force. Cutting speed exhibited a strong influence on the maximum torque developed; and high modulus CFRP composites showed increased sensitivity to cutting speed and strain rate compared with intermediate modulus composites. In the characteristic helical machining and feed directions in drilling, the strength and failure behaviour of the composite is dominated by the mechanical properties and failure mechanisms of the matrix, which explains the significant impact of resin on the cutting forces. On the other hand, the impact of cutting speed on torque is justified by the negative impact of strain rate on the ability of the matrix to transfer the load to the reinforcement, thus explaining the decreasing the maximum torque with the increasing cutting speed

3D Finite Element Modelling of Cutting Forces in Drilling Fibre Metal Laminates and Experimental Hole Quality Analysis

Machining Glass fibre aluminium reinforced epoxy (GLARE) is cumbersome due to distinctively different mechanical and thermal properties of its constituents, which makes it challenging to achieve damage-free holes with the acceptable surface quality. The proposed work focuses on the study of the machinability of thin (~2.5 mm) GLARE laminate. Drilling trials were conducted to analyse the effect of feed rate and spindle speed on the cutting forces and hole quality. The resulting hole quality metrics (surface roughness, hole size, circularity error, burr formation and delamination) were assessed using surface profilometry and optical scanning techniques. A three dimensional (3D) finite-element (FE) model of drilling GLARE laminate was also developed using ABAQUS/Explicit to help understand the mechanism of drilling GLARE. The homogenised ply-level response of GLARE laminate was considered in the FE model to predict cutting forces in the drilling process

An Investigation of burrs, chip formation, hole size, circularity and delamination during drilling operation of GLARE using ANOVA

While there has been a significant amount of research on drilling composite-metal stacks, limited work has been carried out on the machinability of fibre metal laminates (FMLs) used in aerospace structures. Challenges faced in drilling FMLs include those which exist in metals and composites such as delamination, chip and burr formations. The aim of this work is to extend the knowledge of machining fibre metal laminates through the assessment of twist drilling operations in order to improve workpiece quality. The current work presents an experimental study to analyse the effects of drilling parameters (spindle speed and feed rate) on hole quality in two grades of GLARE (2B & 3). The evaluation includes inspecting the hole size, circularity error, entry and exit burrs, chip formations and damage described at the macro level (delamination area) using computerised tomography CT scan, and at the micro level (fibre matrix debonding, chipping, adhesions, cracks) using scanning electron microscopy (SEM). In addition, the results are statistically analysed using analysis of variance (ANOVA) to determine the contribution of cutting parameters on investigated hole quality parameters

Assessment of cutting forces and hole quality in drilling Al2024 aluminium alloy: experimental and finite element study

Machining experiments were conducted to evaluate the impact of cutting parameters on the hole quality and cutting forces in drilling Al2024-T3 aerospace alloy. Al2024-T3 specimen were drilled using Φ6-mm TiAlN-coated carbide twist drills under dry cutting conditions. The hole quality was inspected in terms of its surface roughness, burr and chip formations, hole size, circularity error and post-machining microhardness of the subsurface of the holes. An analysis of variance (ANOVA) was carried out to determine the percentage contribution of cutting parameters on cutting forces and the inspected hole quality parameters. A three-dimensional (3D) finite element (FE) model of drilling Al2024-T3 is developed using Abaqus/Explicit to predict thrust force and torque. The FE model was validated using experimental results and found to be in good agreement. The results of the study showed that the cutting parameters have a significant impact on cutting forces and inspected hole quality parameters. Drilling at feed rates of 100 and 300 mm/min and spindle speeds of 1000, 3000, and 6000 rpm are recommended for producing holes with smaller surface roughness, deviation from nominal hole size, circularity error and burrs