Houichi Kitano

A study for high accuracy measurement of residual stress by deep hole drilling technique

The deep hole drilling technique (DHD) received much attention in recent years as a method for measuring through-thickness residual stresses. However, some accuracy problems occur when residual stress evaluation is performed by the DHD technique. One of the reasons is that the traditional DHD evaluation formula applies to the plane stress condition. The second is that the effects of the plastic deformation produced in the drilling process and the deformation produced in the trepanning process are ignored. In this study, a modified evaluation formula, which is applied to the plane strain condition, is proposed. In addition, a new procedure is proposed which can consider the effects of the deformation produced in the DHD process by investigating the effects in detail by finite element (FE) analysis. Then, the evaluation results obtained by the new procedure are compared with that obtained by traditional DHD procedure by FE analysis. As a result, the new procedure evaluates the residual stress fields better than the traditional DHD procedure when the measuring object is thick enough that the stress condition can be assumed as the plane strain condition as in the model used in this study.

Improvement of Residual Stress Determination by Deep Hole Drilling Considering Plastic Deformation

The deep hole drilling technique (DHD) has received much attention in recent years as a method for measuring through-thickness residual stresses. However, the accuracy of the measurements of residual stresses by the DHD technique is affected by the deformation produced in the DHD process. In this study, the effects of the deformation are investigated in detail by FE analysis. Then, a new procedure for high accuracy evaluation of residual stresses by the DHD technique is discussed. In addition, the procedure are applied to the evaluation of some inner stress fields.Copyright

Deformation State Evaluation Around Deep Drilled Hole After Three-Dimensional Stress Release

Deep hole drilling technique (DHD) has received much attention as a method for measurements of thorough thickness residual stresses in recent years. In the past, however, the measurements of residual stresses by DHD technique have been limited to evaluation for in-plane stress and stress in through thickness direction. In the present study, the effect of shear stresses in through thickness direction on deformation states around the through thickness hole after stress release is discussed by theoretical approaches. In addition, the method for evaluating shear stresses in cutting direction is proposed from the result obtained by the previous theoretical investigation. Then, an unequal three-dimensional residual stress field is evaluated by DHD (or iDHD) technique and the proposed method using measurement changes obtained by numerical analysis. Finally, the reasons why the differences between the evaluation results and the stress distributions on the center line of the trepanned core are exits are clarified for high-accuracy evaluation.Copyright

Evaluation of the effect of strength mismatch in undermatched joints on the static tensile strength of welded joints by considering microstructure

High-tensile strength 950 MPa steel, HT950, which is steel used in penstocks, was developed to provide two vital properties: fracture arrest to stop brittle fracture and high weldability. This steel has been already used for penstocks in some hydropower plants in Japan. To widely apply high-tensile strength steels in penstocks in the future, fewer restrictions against their welding conditions such as pre-heat and post-heat temperature controls are required. One proposed solution is to undermatch the strength of the filler metal to that of the base material. This allows less pre-heating, or no pre-heating at all, and the use of conventional rod and process management is easier. Previous studies have shown that there are softening conditions under which the strength of the joint can be considered as that of the base material. However, the shape and distribution of the soft region are assumed to be ideal. In this study, the method for calculating the change of the strength in heat-affected zone (HAZ) during the welding process is discussed. Then, the influence of the strength distribution of HAZ and welded zone to the strength of the joint is investigated by a wide plate test in both experiment and elastic–plastic analysis. Applicability of undermatched joints to penstock fabrication is considered by these discussions. As a result, it is concluded that the Vickers hardness distribution in the HAZ can be estimated by the method which is proposed in this study and the strength of the under-matched joints is high enough in both experiment and analysis in which the Vickers hardness distribution is considered. From these conclusions, the applicability of undermatched joints, of which the weld metal is the HT570 class, to penstock fabrication is conformed.

Effect of Softening on Tensile Strength Limit in Girth-Welded Pipe Joints

This paper discusses the ultimate tensile strength of girth-welded pipe joints with one or more soft interlayers, as determined by theoretical approaches and FE analysis. In FE analysis, the soft interlayer is assigned to be either the weld metal or heat-affected zone (HAZ). Based on the results of the FE analysis, an evaluation formula is proposed for the ultimate tensile strength of a welded joint including the soft interlayer.Copyright