CTOD Test Research Articles

Reliability Consideration on Acceptance Criteria of CTOD Fracture Toughness Testing

This paper presents a method of applying the probabilistic fracture mechanics technique to the selection of better acceptance crieria for CTOD test, primarily in terms of the required fracture toughness and the number of test specimens. Particular reference was given to welded mists of marime structures subjdcted to sea wave induced stresses, such as ships and offshore structures. In carrying out the analysis, sea wave induced stress, crack size in weldments and fracture toughsess of materials were taken as random variables.It was proposed to decide the required fracture toughness in a probabilistic manner by estimating the distribution of fracture driving parameter which depends on mechanical environmental conditions such as boundary force and crack size and by specifying the risk level of required fracture toughness which corresponds to the importance of structural component to be evaluated.From the viewpoint of structural safety and reliablility, it is rational that the materials of fracture toughness distribution characteristics with low fracture probability should be given high probability of acceptance in an acceptance criterion. The superiority of acceptance criteria was considered in terms of fracture probability vs. probability of acceptance. The dicussions are made on the numerical results obtained for some specified values of the pamameters inuolved. It is found out that the most excellent acceptance criterion varies with the given conditions, which implies that a single criterion can not be adopted as the universally best one.

Development of High-Strength Steel Plates for Low-Temperature Use

This paper describes the metallurgical approaches for producing 415MPa and 460MPa yield strength offshore structural steel plates and the mechanical properties of the steel plates and their welded joints. A thermo-mechanical control process (TMCP) was adopted to manufacture YP415MPa and YP460MPa steel plates with weldability comparable to conventional YP355MPa steel plates. The Charpy impact and CTOD tests of the steel plates and their welded joints proved to be very good.

A method of estimating CTOD for a surface crack through measurement on the crack opening behavior

The conversion method to CTOD from measured clip gauge displacement for a through thickness center cracked specimen and a three point bend CTOD specimen has been already established, for the purpose of obtaining the fracture toughness of the material. However the method for a semielliptical surface flaw has not been established yet because of difficulties of considering the three dimensional effect for the COD.In this paper, we assumed that COD for a surface flaw on the cross sectional plane perpendicular to the plate surface, which is the plane with a single edge crack, can be approximately given by the superposition of CODs for the single edge cracked plane under bending loading and for the center cracked plane under tensile loading, of which the center line corresponds to the plate surface. The bending and the tensile load are determined by the measured mouth COD and crack angle at the surface of the cracked body with a surface flaw.Tensile and bending tests for specimens with various sized semi-elliptical surface notches were carried out to confirm the above idea. In the tests, mouth COD and crack angle were measured by using double clip gauge method and simultaneously, the COD inside the surface notch was also measured by using silicon gum casting method. As a result, the converting method to CTOD at the deepest point of a surface flaw is driven from the measured mouth COD and crack angle at the surface which are obtained by double clip gauge method.The testing result of wide plate estst with a semi-elliptical surface fatigue crack on a multipass welded joint, of which crack tip located at local brittle zone within HAZ, was also analyzed. It was obtained that the critical CTOD for a surface crack of which the tip locates local brittle zone corresponds to the lowest critical CTOD obtained from the three point bend standard CTOD tests of the joint with a through thickness fatigue pre-crack.

Jc DETERMINATION FOR SHALLOW NOTCH WELDED BEND SPECIMENS

Abstract— Methods are presented for the determination of J from 3 point bend specimens having 0.1 ≤a/W≤ 0.5 using the load versus crack mouth opening displacement trace. It is intended that the procedure be used to determine Jc, J at unstable fracture in the brittle to ductile transition, from specimens and test procedures which conform to the CTOD test standard BS5762. Elastic‐plastic finite element computations are used to demonstrate the adequacy of the proposed estimation procedures for J and to show that it is still possible to apply them to specimens containing welded joints. The problems of defining a centre of rotation for plastic deformation in shallow notched bend specimens are discussed.

Evaluation of Brittle Fracture Strength of Tubular T-Joints of Offshore Structures under Tensile Loading (1st Report)

As offshore structures are used in rougher and colder seas, the rigorous safety has been demanded. Nowadays, it becomes more common to conduct CTOD tests for weldments as a steel prequalification test and/or a welding procedure test. However, there have been arguments that the CTOD criterion give a much conservative assessment.In these circumstances, it is essential to establish the precise evaluation method for the structural integrity from fatigue crack growth to brittle fracture in offshore structures. For offshore structures, the most important structural members are tubular joints, where high stress concentration takes place.From this viewpoint, the strain distribution and the fatigue crack growth characteristics in the hot spot area have been investigated with tubular T-joints model specimens under tensile loading, and the brittle fracture strength of the model tests has been evaluated based on the CTOD criterion.The results obtained are summarized as follows.(1) The elastic stress distribution around the connection of the tubular joint is well evaluated by the FEM stress analysis, and the elastic stress concentration factor at the hot spot agrees with the predicted value.(2) The plastic strain concentration factor can be estimated using Eq. (1) as a function of the elastic stress concentration factor and the nominal applied stress. In case where residual stress exists, Eqs. (2) to (6) can be used to analyse the effect of residual stress.(3) The fatigue crack propagation behaviour at tubular joints can be evaluated reasonably using the linear elastic fracture mechanics.(4) The critical CTOD obtained from the tubular joint model test agrees well with that from 3-point CTOD test. Therefore, the CTOD criterion can be applied to the fracture of structural component with 3-dimensional stress concentration such as tubular joints.(5) The brittle fracture strength of the tubular joint models can be evaluated using the estimated hot spot strain and the proposed CTOD design curve by taking into account the welding residual stresses.

Significance of pop-in fracture toughness testing

O . • . C J~ r R-curve testlng. Figure l(a) shows the load vs cllp gauge ~isplaceIC ment trace from a CTOD test on a ferritic steel which is in the middle of its transition temperature range. Cleavage fracture is preceded by considerable plasticity and possibly some tearing. Figure l(b) shows a test record from a specimen exhibiting pop-in. A small, rapid drop in load is observed, which is subsequently recovered. There are many possible causes of this load drop [i], but the one of interest here is the initiation and arrest of a cleavage crack.

The effect of specimen thickness and local embrittle zone on critical CTOD at HAZ of As-weld joint

Side notch type CTOD tests with various specimen thicknesses including full thickness were carried out by using a narrow gap welded joint with plate thickness of 200 mm to ensure the effect of specimen thickness on critical CTOD for as-weld joint. All the specimens were intended to have a fatigue pre-cracked notch at HAZ. After completion of CTOD tests, notch location was checked for each specimen. At the same time, the peak temperature of welding thermal cycle at each point was analyzed from the macro structure of the welded joint and the part in the vicinity of the weld metal was classified approximately according to metallography due to estimated peak temperature, such as simple coarse grain region, coarse grain region with island martensite (C+M*), simple island martensite region, simple fine grain region etc., It was investigated what kinds of region were cut across by the front line of the fatigue pre-crack and their crossing length for each specimen.As a result, it becomes clear that brittle fracture initiated from the most locally embrittle zone which was suggested by thermal cycle calculation. Moreover critical CTOD's were converted to those for full thickness CTOD specimens by using equations which represents the mathematical effect of specimen thickness on critical CTOD and was confirmed by experiments using base metals with various strength levels. From the result of the relation between a maximum crossing length of a fatigue pre-crack front for the most local brittel zone and the converted critical CTOD, it becomes clear that the critical CTOD for full thickness CTOD specimen can be estimated by using small size CTOD specimen sampled adequately from the joint.