F. R. Biglari

Physical and numerical modelling of ram extrusion of paste materials: conical die entry case

Abstract A ram extrusion process with a conical die entry is analysed using a physically based analysis and a numerical modelling procedure involving the finite element method. The aim of the study is to test the applicability of the elasto-viscoplastic constitutive model combined with an established boundary condition criterion for describing this forming operation. “Plasticine”, a commercial modelling clay which exhibits an elasto-viscoplastic flow response, is used to study the nature of the material deformation. For the numerical modelling, an elasto-viscoplastic finite element program has been implemented. Satisfactory agreement, between experiment and simulation, is obtained for the force–time data and the material displacement fields which indicates that the bulk and interfacial constitutive relationships adopted, along with the associated numerical parameters, are an appropriate description of the flow behaviour of the system. The evolution of the deformation, within the material during the extrusion process, is interpreted. The study indicates that the procedures described may ultimately provide a numerical rheometric tool from which the effects of various process boundary conditions, on the evolution of paste material deformation, can be examined and quantified for the ram extrusion process with conical die entries.

Optimum design of forging dies using fuzzy logic in conjunction with the backward deformation method

A novel shape optimization method is presented for the design of preform die shapes in multistage forging processes using a combination of the backward deformation method and a fuzzy decision making algorithm. In the backward deformation method, the final component shape is taken as the starting point, and the die is moved in the reverse direction with boundary nodes being released as the die is raised. The optimum die shape is thereby determined by taking the optimum reverse path. A fuzzy decision making approach is developed to specify new boundary conditions for each backward time increment based on geometrical features and the plastic deformation of the workpiece. In order to demonstrate this approach, a design analysis for an axisymmetric disk forging is presented in this paper.

Computational and Experimental Studies of High Temperature Crack Initiation in the Presence of Residual Stress

Residual stresses have been introduced into a notched compact tension specimen of a 347 weld material by mechanical compression. The required level of compressive load has previously been determined from finite-element studies. The residual stress in the vicinity of the notch root has been measured using neutron diffraction and the results compared with those obtained from finite-element analysis. The effect of stress redistribution due to creep has been examined and it is found that a significant reduction in stress is measured after 1000 h at 650°C. The implications of these results with regard to the development of damage in the specimen due to creep relaxation are examined.

Determination of fracture mechanics parameters J and C* by finite element and reference stress methods for a semi-elliptical flaw in a plate

The fracture mechanics parameters J and C p used, respectively, to describe ductile fracture and creep crack growth can be determined either by finite element methods or reference stress techniques. In this paper solutions for a partially penetrating semi-elliptical flaw in a plate subjected to tension and bending loading are considered. Estimates of J and C p are obtained from finite element calculations for a range of work-hardening plasticity and power law creep behaviours and from reference stresses derived from ‘global’ collapse of the entire cracked cross-section. Comparisons are made with solutions taken from the literature for a range of loading conditions, plate geometries and crack sizes and shapes. Generally it is found that although there are significant variations between the different finite element solutions, satisfactory estimates of J and C p that are mostly conservative are obtained when the reference stress procedure is adopted.

CREEP CRACK INITIATION IN A WELD STEEL: EFFECTS OF RESIDUAL STRESS

In this paper, the effect of residual stress on the initiation of a crack at high temperature in a Type 347 austenitic steel weld is examined using the finite element method. Both two and three dimensional analyses have been carried out. Residual stresses have been introduced by prior mechanical deformation, using a previously developed notched compact tension specimen. It has been found that for the 347 weld material, peak stresses in the vicinity of the notch are approximately three times the yield strength at room temperature and the level of stress triaxiality (ratio between hydrostatic and equivalent stress) is approximately 1 (considerably higher than that for a uniaxial test). The finite element analysis includes the effects of stress redistribution and damage accumulation under creep conditions. For the case examined the analysis predicts that crack initiation will occur under conditions of stress relaxation if the uniaxial creep ductility of the material is less than 2.5%. Furthermore, the predicted life of the component under constant load (creep conditions) is significantly reduced due to the presence of the residual stress field.

Stress Intensity Factors Due to Residual Stresses in T-Plate Welds

Residual stress distributions in ferritic steel T-plate weldments have been obtained using the neutron diffraction method. It is shown that the transverse residual stress distribution for two plates of different yield strength are of similar shape and magnitude when normalized appropriately and peak stresses are on the order of the material yield strength. The resultant linear elastic stress intensity factors for these stress distributions have been obtained using the finite element method. It has been shown that the use of the recommended residual stress distributions in UK structural integrity procedures leads to a conservative assessment. The stress intensity factors for the welded T-plate have been shown to be very similar to those obtained using a smooth edge cracked plate subjected to the same local stress field.

Comparison of Analytical, Numerical, and Experimental Methods in Deriving Fracture Toughness Properties of Adhesives Using Bonded Double Lap Joint Specimens

ABSTRACT Stress and fracture analysis of bonded double lap joint (DLJ) specimens have been investigated in this paper. Numerical and analytical methods have been used to obtain shear- and peel-stress distributions in the DLJ. The generalized analytical solution for the peel stress was calculated for various forms of the DLJ geometry and, by using crack closure integral (CCI) and by means of the J-integral approach, the analytical strain energy-release rate, G, was calculated. Experimental fracture tests have also been conducted to validate the results. The specimens were made of steel substrates bonded by an adhesive and loaded under tension. Specimens with cracks on both sides and at either end of the DLJ interface were tested to compare the fracture behavior for the two crack positions where tensile and compressive peel stresses exist. Tests confirmed that the substrates essentially behave elastically. Therefore, a linear elastic solution for the bonded region of the DLJ was developed. The fracture energy parameter, G, calculated from the elastic experimental compliance for different crack lengths, was compared with numerical and analytical calculations using the experimental fracture loads. The stresses from analytical analysis were also compared with those from the finite element results. The strain energy-release rate for fracture, G f , for the adhesive has been shown to have no R-curve resistance, was relatively independent of crack length, and compared well with those obtained from numerical and analytical solutions. However, it was found that fracture energy for the crack starter in the position where the peel stress was tensile was about 20% lower than where the crack was positioned at the side, where the peel stress was found to be compressive.

Comparison of methods for obtaining crack-tip stress distributions in an elastic-plastic material

Under linear elastic and elastic-plastic conditions the K field and the HRR (Hutchinson-Rice-Rosengren) field respectively are expected to provide an accurate representation of the stress field close to the crack tip in an elastic-plastic material. It has recently been proposed in French and UK defect assessment procedures that the Neuber method, originally developed for sharply curved notches, provides an alternative approach to estimate both notch and crack-tip stress fields, for use in conjunction with the sigma-d (σd) method to predict creep crack initiation times. In this work, the crack-tip stress fields under plane strain conditions, predicted from the Neuber approach, are compared with the HRR and K fields as well as those obtained from full-field finite element calculations. A compact tension and a single edge notched tension specimen have been examined; the material model used is the Ramberg-Osgood, power law plasticity model. As expected, the K field and HRR field have been found to provide a good representation of the near-tip fields at low and high loads respectively. Satisfactory solutions have also been obtained through the use of the reference stress to estimate the amplitude of the crack-tip stress in conjunction with the HRR field. The Neuber approach provides a good estimate of the equivalent (von Mises) stresses over the full range of load levels. However, but the use of the Neuber approach directly to predict the maximum principal stress in the plane of the crack provides a non-conservative prediction. A modified Neuber method, using an appropriate scaling function, has been proposed to determine the maximum principal stress in the plane of the crack from the equivalent (von Mises) stress predicted by the Neuber approach. Using the proposed method, a significantly improved estimate of the crack-tip stresses is obtained.

Multi-constrained optimization in ultrasonic-assisted turning of hardened steel by electromagnetism-like algorithm

Ultrasonic-assisted machining is a method used to improve the machining technologies and overcome some problems in recent decades. To achieve optimal machining performance during hard turning, proper selection of turning parameters is considered as an important issue. The process becomes more complicated when methods such as ultrasonic-assisted machining are applied and ultrasonic vibration variables are also extracted. In this research work, a methodology is proposed to optimize ultrasonic-assisted turning process during machining of hardened steel AISI 4140. Neural network was employed to model process outputs (surface roughness and cutting force). Then electromagnetism-like algorithm was coupled to neural network to maximize the material removal rate with regard to outputs of process as constraints to achieve the machined part requirements. Experimental design technique was also used to obtain the needed experimental data for training neural network to predict process outputs. Material removal rate constitutes the main function of the electromagnetism algorithm; cutting force and surface roughness were applied as the constraints of the electromagnetism-like algorithm function. The unconstrained objective function that was created using penalty method was then optimized by the electromagnetism-like algorithm and genetic algorithm codes. It was observed that electromagnetism-like algorithm has more accurate results in comparison to genetic algorithm. Finally, the obtained optimum variables were experimentally tested in order to examine the mentioned method. Good compatibilities are observed between the values of optimization method and the experimental measurements.

Effect of residual stress on high temperature deformation in a weld stainless steel

This paper considers the measurement of residual stresses induced by mechanical loading in a weld Type 347 stainless steel. The work is based in part on an ongoing Round Robin collaborative effort by the Versailles Agreement on Materials and Standards, Technical Working Area 31, (VAMAS TWA 31) working on ‘Crack Growth of Components Containing Residual Stresses’. The specific objective of the work at Imperial College London and HMI, Berlin is to examine how residual stresses and prior straining and subsequent relaxation at high temperature contribute to creep crack initiation and growth for steels relevant to power plant applications. Tensile residual stresses have been introduced in the weld by pre-compression and neutron diffraction measurements have been carried out before and after stress relaxation at 650 oC. Significant relaxation of the residual stresses has been observed, in agreement with earlier work on a stainless steel. Preliminary results suggest that the strains local to the crack drop by over 60% after 1000 h relaxation at 650 oC for the weld steel. The results have been compared with finite element studies of elastic-plastic pre-compression and stress relaxation due to creep.