A Method for Determining the Workability Diagram by Varying Friction Conditions in the Upsetting of a Cylinder between Flat Dies

This paper aims to develop a method for determining the workability diagram by varying frictional conditions in the cylinder upsetting test. The method is based on a known theoretical relationship between the average stress triaxiality ratio and in-surface strains if the initiation of fracture occurs at a traction-free surface. This relationship is valid for any rigid/plastic strain hardening material obeying the Mises-type yield criterion and its associated flow rule, which shows the wide applicability of the method. The experimental input to the method is the strain path at the site of fracture initiation. Neither experimental nor numerical determination of stress components is required at this site, though the general ductile fracture criterion involves the linear and quadratic invariants of the stress tensor. The friction law’s formulation is neither required, though the friction stress is the agent for varying the state of stress and strain at the site of ductile fracture initiation. The upsetting tests are carried out on normalized medium-carbon steel C45E, for which the workability diagram is available from the literature. Comparison of the latter and the diagram found using the new method shows that the new method is reliable for determining a certain portion of the workability diagram.

An Approach for Predicting the Initiation of Ductile Fracture in Plane Strain Rolling

The paper extends Orowan’s method to the prediction of ductile fracture in plane strain rolling. In general, any uncoupled ductile fracture criterion can be used in conjunction with the solution found. However, the present paper focuses on a ductile fracture criterion based on the workability diagram. It is assumed that the initiation of fracture occurs at the axis of symmetry.

Reducibility of Stress-Based Workability Diagram to Strain-Based Workability Diagram

The strain-based and stress-based workability diagrams are often used to predict the initiation of ductile fracture in metal forming. The strain-based workability diagram is restricted to free surface fracture and postulates that the initiation of fracture is independent of the strain path. It is shown in the present paper that under these conditions the strain-based workability diagram is identical to the stress-based workability diagram. Using an available stress-based workability diagram the strain-based workability diagram is found in a much larger domain in the space of two in-surface principal strains as compared to the typical domain covered by standard tests used to determine strain-based workability diagrams. Two feasible tests are designed to determine the complete strain-based workability diagram.

A Study on Material Workability by Upsetting of Non-Axisymmetric Specimens by Flat Dies

Upsetting is a typical test for determining the workability diagram. In most cases axisymmetric samples are used for such tests. However, the shape of samples may have a significant effect on the ductile fracture initiation. Therefore, a greater variety of sample geometry should lead to a more accurate shape of the workability diagram. A difficulty here is that a theoretical treatment of samples in which three-dimensional flow occurs is more difficult and time consuming as compared to axisymmetric samples under axisymmetric loading. This difficulty can be overcome in the case of the ductile fracture criterion based on the workability diagram and the average value of the triaxiality factor. In particular, if fracture occurs at free surfaces then it is sufficient to determine experimentally in-surface strains after several stages of the upsetting process, up to the initiation of ductile fracture. After that, the corresponding point of the workability diagram can be found by means of a simple analytical procedure and numerical integration. This approach is used in the present paper to correct the workability diagram using non-axisymmetric upsetting of five different samples made of steel. Some previous results are combined with the new results to obtain the workability diagram over a wide range of the triaxiality factor.

‘Wet Handshake’: Workability Study of an Offshore Thruster Exchange Operation

Thruster exchange operations are performed when an azimuth thruster on a DP operated vessel needs to be replaced for repair or maintenance purposes. At present these operations are performed either during a dry-dock call or in sheltered waters with the assistance of a shearleg or heavy lift vessel moored alongside the DP operated vessel. In order to reduce downtime of the DP operated vessel a clear trend is observed of operations being carried out more offshore and in deeper waters by means of a wet handshake between a heavy lift vessel (HLV) and a DP operated vessel. To get insight into the workability of such offshore thruster exchange operations it is important to study the dynamic interactions between the bodies involved. This paper describes the development of a methodology which accurately determines the workability of an offshore thruster exchange operation. The methodology is developed by BigLift Shipping, which has performed several thruster exchange operations in sheltered waters in the past few years. Furthermore the results of a workability analysis of a thruster exchange operation offshore Ghana are presented. In order to determine workability, first the hydrodynamic behavior including interaction effects of the coupled configuration of the DP operated vessel, the heavy lift vessel and the azimuthing thruster is calculated for a range of seastates, which results in the response spectra of the vessels. Then for each seastate the response is compared with the defined operational criteria to determine whether this seastate is workable, critical or non-workable, which results in a workability diagram. The workability can be quantified by combining this workability diagram with a wave-scatter diagram of the location of interest. In general this is a thoroughly studied topic. In order to achieve a higher level of accuracy of the workability prediction the focus of the methodology described in this paper has been pointed on two aspects that can be of significant influence. • First the persistency of a certain seastate is investigated. In the approach described above any variation of the environmental conditions over the duration of the operation is not considered. The effect of changing weather in time can be of significant influence on the overall workability. In order to take into account persistency information a numerical dataset of 3 hours statistics over a period of 10 years is used. Combining this information with the duration of all consecutive steps of the operation, results in a more realistic workability prediction. Similar approaches in seakeeping of ships are shown by Dallinga et al. (2004) [1], Naito et al. (2006) [2]. • Secondly the influence of a frequency domain approach compared to a time domain approach is analyzed. The nonlinear effects that occur in the coupled configuration of multiple bodies can not be taken into account in the frequency domain approach. Therefore the motion analysis is performed in the time domain. As an example case a thruster exchange operation offshore Ghana has been studied. The results of the study show that the workability prediction based on scatter diagram metocean data is influenced substantially when persistency information of metocean data is taken into account. The effect of the duration and criteria of independent operational steps on the workability are clearly visible in the persistency approach. This enables a to-the-point approach in improving the workability. Although time domain analysis is taking into account nonlinear effects, the difference between time and frequency domain analysis can be neglected for the presently studied configuration.