Critical CTOD Research Articles

A Proposal for Two Characteristic Ratios of ASTM-CTOD to BS-CTOD

There have been two different CTOD calculations, geometric CTOD calculation in BS7448 and J-based conversion in ASTM E1290 since the ASTM International revised E1290 in 2002, and it is useful to transform one CTOD into the other CTOD. In this study, analytical and experimental investigations into the ratio of ASTM E1290-CTOD to BS7448-CTOD were carried out, and two ratios of ASTM-CTOD to BS-CTOD, R1 and R2, were characterised in terms of cleavage cracking. These two characteristic CTOD ratios are helpful in simply transforming one critical CTOD to the other.

溶接残留応力場中のき裂からの脆性破壊発生評価におけるワイブル応力仮説による塑性拘束補正方法

This paper studies the method for estimating the welding residual stress effect on brittle fracture of structural steel based on the Weibull stress criterion. Using wide plate (WP) specimens with and without welding residual stress, the brittle fracture tests have been conducted at -75°C. The material used is a high strength steel of 780MPa class. Welding residual stress significantly decreases the fracture force and the critical CTOD of the wide plate with welding residual stress at the onset of brittle fracture initiation. And it has been shown that the critical CTOD of the WP specimens can be predicted from fracture test results of the 3-point bend specimens without residual stress based on the Weibull stress criterion. The present paper proposes the assessment method of constraint loss effects on CTOD of wide plate with residual stress based on the equivalent CTOD ratio, βr under residual stress field. The equivalent CTOD ratio, βr is defined as the ratio, δ/δrWP,whereδandδrWPare CTODs of the standard fracture toughness specimen and wide plate with welding residual stress, respectively, at the same level of the Weibull stress.Fracture assessment results using βr are shown within the framework of failure assessment diagram (FAD). An excessive conservatism observed in the conventional procedure is reasonably reduced by applying the equivalent CTOD ratio, βr.

ASTM CTOD試験法改訂に伴う限界CTOD評価値の変化:日本溶接協会FTS委員会での検討結果報告

ASTM CTOD試験法改訂に伴う限界CTOD評価値の変化:日本溶接協会FTS委員会での検討結果報告

Fracture Assessment of Wide Plate Component Subjected to Biaxial Loading

This study investigated the effect of biaxial loading on the fracture performance of a center through cracked panel and a center surface cracked panel of a structural steel of 25mm thick. The experiment revealed that the critical CTOD of the biaxially loaded specimen was smaller than that of the uniaxially loaded specimen, due to the constraint effect of the biaxial loading. This constraint effect was remarkable in the case of the through thickness notched specimen. The stress distributions near the cracks and crack tip deformation behaviors were simulated by FE-analyses, in order to discuss brittle fracture initiation conditions of the cracked wide plate specimens. All fracture initiation points were located in the large CTOD part of the crack front, as well as in the widely extended high opening stress region. However, the initiation points did not always correspond to the highest opening stress point along the crack front. These results indicated that the existence of a extensive high stress region accompanied by a large CTOD, is significant in the brittle fracture initiation condition. These results also suggested that fracture assessment based on the Weibull stress criterion is reasonable, because the Weibull stress takes the volume effect of the high stress area into account.

ワンビード溶接法によるHAZ靱性評価法の考察

The engineering method for fracture toughness evaluation of beam-to-column weld HAZ using one-bead welds, called one-bead weld method, has been examined in this study. Materials used are 26 structural steels of 400 to 490 MPa strength class produced by different milling conditions. One-bead welds are made by CO2 arc welding with single bevel groove under heat inputs of 4.0 to 5.0 kJ/mm. From these welded joints, the Charpy specimens and 3-point bend CTOD specimens with the same thickness of 10mm are extracted. The notch is located at the weld HAZ on a straight bond side perpendicular to the plate surface. Charpy tests and CTOD tests are carried out at 0°C and -40°C, respectively, where the CTOD test temperature is determined on the basis of WES2808 correlation formula. It has been found that the Charpy absorbed energy is affected by the chemical composition of the base steel, such as C, Mn, P, S, N, and Ti. Based on similar test results, an engineering parameter, fHAZ, is proposed by the Building Center of Japan for estimating the HAZ toughness. However, the present study indicates that the total amount of N is not a proper measure, but the free N controls the HAZ toughness. From the fracture mechanical point of view, the correlation between the Charpy energy and the critical CTOD is useful. It is noted that the CTOD test results are sensitive to the length of CGHAZ along the crack front. The Charpy-CTOD correlation with respect to the HAZ toughness should be made with attention to the base-metal chemical compositions as well as the CGHAZ size along the crack front.

Fracture toughness of unidirectional fiber–metal laminates: crack orientation effect

Fiber–metal laminates (FMLs) are structural composites developed for aeronautical applications. The application of FMLs to structures demands a deep knowledge of a wide set of properties, including fracture toughness. The objective of this work was to evaluate the effect of crack orientation on the fracture toughness (critical J-integral and CTOD δ 5) of unidirectional FMLs. Small C(T) and SE(B) specimens with notches parallel and perpendicular to the fibers direction were tested. A study of the relation and equivalence between J C and δ 5C, which heavily depend on the yield strength and on the stress state, was performed motivated by apparently contradictory experimental results. These results can be explained by the direction-dependent yielding properties of unidirectional FMLs. The best overall equivalence between J C and δ 5C was obtained considering plane stress state and using the effective yield strength, both for unidirectional FMLs notched parallel and perpendicular to the fibers direction.

The effect of specimen thickness on the experimental and finite element characterization of CTOD in extra deep drawn steel sheets

Recent experimental results by us have indicated that the load-drop technique can serve as a valid fracture criterion for predicting elastic-plastic fracture in extra deep drawn (EDD) steel sheets or in predominantly plane stress conditions. The purpose of this investigation is to examine the validity of aJ-integral as a fracture parameter and theJ-CTOD relation for the determination of critical CTOD in predominantly plane stress fracture (CTOD-crack tip opening displacement). Fracture tests were performed and experimental results were generated on fracture behaviour of EDD (0·06%C) steel sheets with CT specimens and using ‘load-drop’ as a fracture criterion. Critical CTOD was determined by using theJ-CTOD relation in addition to several existing techniques. A full 3-D finite element model was formulated to verify the critical load, critical CTOD and plastic-zone size. The critical CTOD was shown to increase with increasing specimen thickness and appeared to be approaching a higher limiting value. The characteristic features of predominantly plane stress fracture or general yielding fracture mechanics are summarized in conclusion

Fracture toughness evaluation of unidirectional fibre metal laminates using traditional CTOD ( δ) and Schwalbe ( δ5) methodologies

Fibre metal laminates (FMLs) were developed for the aeronautical industry, which requires thin sheets with high resistance to fatigue crack growth, high damage tolerance and high specific strength. Considering all these requirements, FMLs are an advantageous choice when compared to metal alloys currently used. In order to employ FMLs in aircraft structures, designers must have a deep knowledge of a wide set of properties including fracture toughness. The aim of this work was to evaluate the available methodologies for critical CTOD measurement of unidirectional FMLs. To achieve this, tests were performed to obtain traditional (BSI/ASTM) and Schwalbe’s CTODs by using experimental procedures especially adapted to these laminates. Results achieved point out that there are differences between both CTOD parameters, that Schwalbe method proved more appropriate, and also that the standard plastic-hinge model does not work properly in FMLs.

超微細粒高強度鋼レーザ溶接継手の機械的性質

Mechanical properties of laser welded ultra-fine grained high strength steel were examined using a 20 kW CO2 laser facility. The welded joints showed good tensile strength as all pieces were broken at the base metal. In CTOD test the critical CTOD value was the lowest at the weld metal. To prevent the fracture path deviation (FPD) in Charpy impact test, we used three-weld test assembly. The results confirmed that reduction in carbon quantity improved the weld metal toughness.

Experimental and finite element analysis of fracture criterion in general yielding fracture mechanics

Efforts made over the last three decades to understand the fracture behaviour of structural materials in elastic and elasto-plastic fracture mechanics are numerous, whereas investigations related to fracture behaviour of materials in thin sheets or general yielding fracture regimes are limited in number. Engineering simulative tests are being used to characterize formability and drawability of sheet metals. However, these tests do not assure consistency in quality of sheet metal products. The prevention of failure in stressed structural components currently requires fracture mechanics based design parameters like critical load, critical crack-tip opening displacement or fracture toughness. The present attempt would aim to fulfill this gap and generate more information thereby increased understanding on fracture behaviour of sheet metals. In the present investigation, using a recently developed technique for determining fracture criteria in sheet metals, results are generated on critical CTOD and fracture toughness. Finite element analysis was performed to support the results on various fracture parameters. The differences are within 1 to 4%. At the end it is concluded that magnitude of critical CTOD and/or critical load can be used as a fracture criterion for thin sheets.

Brittle Fracture Property of Structural Steel after Large Crack Growth by Cyclic Loading

This study is focused on the brittle fracture properties after crack extension under large scale cyclic loading for structural steel. Cyclic fracture tests are conducted with 4-point bend specimens with fatigue pre-crack or machined notch. These specimens are differ in stress/strain concentration. It is shown that the brittle fracture resistance under cyclic loading can be lower than that in monotonic loading at a lower temperature. The FE-analysis implies that the accumulation of large plastic strain, that acts as a prestrain, near the crack tip by cyclic loading is responsible for the lower fracture resistance. The deterioration of critical CTOD values under cyclic loading, which appear at more extensive temperature in machined notched specimens, can be quantitatively estimated with the translation of ductile/brittle transition temperature for monotonic loaded specimens with fatigue pre-crack to higher temperature. It seems to be found that this translation approximately correspond to the saturated temperature translation as increasing of uniform prestraining in steel used.

Evaluating Hydrogen Stress Cracking of Line Pipe Steels under Cathodic Protection Using Crack Tip Opening Displacement Tests.

Crack tip opening displacement (CTOD, δ) tests were carried out for line pipe steels in buffer solutions, sand, and clay to evaluate initiation of hydrogen stress cracking (HSC) at surface defects in buried pipelines under cathodic protection. Four series of line pipe steels and two series of seam welds showed a similar tendency in cathodic current density (i) versus the critical CTOD (δc) curves, irrespective of types, pH and water content of the soils; δc showed a minimum (δHSC) when i>ith (ith=1mA/cm2) in all the testing conditions. δHSC increased with the increasing fracture toughness of the steel. Fluctuation of cathodic current density influenced δc when the maximum value of cathodic current density (imax) was larger than ith. HSC could be initiated at surface defects in pipelines only when imax>ith and δ≥δHSC.

Analysis of the geometry dependence of fracture toughness at cracking initiation by comparison of circumferentially cracked round bars and SENB tests on Copper

In the first part of the paper, the use of circumferentially cracked round bars (CRB geometry) for characterizing fracture toughness of a ductile material, namely copper, is assessed experimentally through a comparison with the single edge notched bend (SENB) geometry. The J(R) curve method with multiple-specimens was applied, but, as unstable cracking appeared very early in the CRB specimen, an engineering definition of fracture toughness was not pertinent. Unloaded specimens were analyzed metallographically to determine the CTOD at physical cracking initiation. The fracture toughness measured using the CRB geometry was 50% larger than using the SENB geometry. The second part of the paper aims at justifying this difference of fracture toughness at cracking initiation. Finite element simulations revealed a slightly higher constraint in the SENB specimens. The main difference between the two specimen geometries lies in a 50% larger extension of the finite strain zone with respect to the CTOD in the case of the SENB specimens. Based on the observation that, in the studied material, the critical CTOD is one order of magnitude larger than the void spacing, we conclude that the geometry dependence of the fracture toughness is caused by the difference in the finite strain zone extension rather than by a stress triaxiality effect.

Thickness dependence of cracking resistance in thin aluminium plates

The influence of thickness on the fracture toughness of aluminium 6082T0 thin plates of 1–6 mm thicknesses was investigated experimentally and numerically from tensile testing of cracked DENT specimens. The critical J-integral, J c, critical CTOD, δ CTODc, and essential work of fracture, w e, are found to increase with thickness and to constitute equivalent measures of fracture toughness at small thickness. For larger thickness, J c and δ CTODc increase non-linearly with thickness and reach a maximum for 5–6 mm thickness whereas w e keeps increasing linearly with thickness. This difference is related to a more progressive development of the necking zone in front of the crack tip when thickness increases: at large thickness, cracking initiates well before the neck has developed to its stationary value during propagation. w e is more directly related to the steady-state crack growth resistance. A linear regression on the fracture toughness/thickness curve allows further separation of the two contributions of the essential work of fracture: the necking work and the fracture work spent for damaging. The maximum of the stress triaxiality ratio is shown to constitute a pertinent parameter for characterising how constraint affects cracking initiation in the present context where out-of-plane constraint dominates in-plane constraint. It allows justifying the shape of the J c/thickness relationship and results in the proposal of a 3D J c/thickness/triaxiality fracture locus. As fracture profiles are macroscopically flat with microscopic dimples and with only very small shear lips along the edges, a local criterion based on the growth and coalescence of voids has been used in order to predict fracture initiation.

A CTOD Approach to Predicting Crack Extensions in Aluminum Alloys

Abstract A practical method is proposed to calculate crack extensions during stable and subsequent unstable crack growth conditions in aluminum alloy 7075-T651 specimens. Also for aluminum alloy 2024-T351 specimens, the stable crack growth is calculated up to the limit stress. A crack initiated at a critical CTOD extends stepwise in successive increments of CTOD, so that the ratio of a CTOD increment to a crack-extension increment is a critical value, independent of an initial CTOD. The value attained instantaneously at each step of crack extension is different from measurable total CTOA. Some CTOD functions for MT and CT specimens of various sizes and crack lengths are calculated up to their limit loads using finite element analysis. The functions when plotted against applied stress vary little with crack length over a wide range except near the limit stress. The proposed method is based on the calculated functions for a mechanics solution. There seem to be two types in the final phase of fracture: (1) limit load for the 2024-T351 alloy and (2) fracture instability for the 7075-T651 alloy. When the total of CTOD increments during stable crack growth reaches a critical value, fracture instability occurs that characterizes crack extension without further increment of CTOD. Fracture constants are determined so that the calculations may be fitted to available test data with respect to effective and physical crack lengths. The determined fracture constants are transferred to other specimens to predict the failure loads, in good agreement with test loads. Discussions are made on specimen size effects and crack length measurement in the presence of crack tip tunneling.

Local Approach to Brittle Fracture under Residual Stress Field

In this study, an effect of residual stresses on brittle fracture initiation for a structural steel is investigated on the basis of the Local Approach. Compressive residual stress is introduced by pre-loading at a room temperature and the subsequent fracture test is conducted at low temperatures (-75°C and -40°C) with a high strength steel of 780 MPa class. It is shown that the pre-loading apparently elevates the critical load and critical CTOD at the onset of brittle fracture initiation. This is explained in terms of the effect of a compressive stress field near the crack tip. This paper employs the Weibull stress fracture criterion to evaluate the brittle fracture resistance of the pre-loaded specimen. The assertion is that the critical Weibull stress at brittle fracture initiation is a material property independent of test conditions with and without pre-loading. Using the Weibull stress fracture criterion, the critical CTOD of the pre-loaded specimen can be predicted from fracture test results of the specimen without pre-loading.

Fracture behaviour of specimens with surface notch tip in the heat affected zone (HAZ) of strength mis-matched welded joints

The generally accepted conditions for the strength overmatched welded joints of high strength steel are not clearly defined. In this paper, the fracture mechanics analysis of specimens, with surface notch tips completely embedded in the heat affected zones was conducted. The results showed that the strength of mismatching of a welded joint caused a redirection of the crack propagation towards the low strength region of the welded joint. This redirection of the crack propagation affected the values of the critical CTOD. In the cases of the overmatched welded joints containing a soft root layer it is possible to achieve a comparable fracture behaviour related to the homogeneous overmatched welded joint if the impact toughness of the soft root layer is higher than the impact toughness of the overmatched weld metal. Such a type of welded joint is therefore preferable for the welding of high strength low alloy steels, because it enables the manufacturing of a welded joint without preheating.

A property distribution map to explain HAZ CTOD toughness: Study of microcrack initiation properties of a steel HAZ having microscopic strength inhomogeneities (2nd Report)

Summary To visualise the fracture initiation characteristics in the multipass weld heat affected zone of YS = 460 N/mm2 class 30 mm thick TMCP high‐strength steel, the yield strength distribution map and CTOD toughness distribution map are drafted by computer on the basis of a series of HAZ simulation results. A comparison between these computationally estimated property distribution maps and the actual HAZ CTOD test results reveals the following: Fracture of the HAZ CTOD specimens is initiated from the peripheral part of a local hard zone (LHZ) which does not always correspond to the most brittle point on the crack front. The critical CTOD value derived from full‐thickness specimens is adequately represented by the local critical CTOD level at the fracture initiation point. Fracture initiation is due not only to local brittleness, but also to strength inhomogeneity in the HAZ.

Evaluation of HAZ Fracture Toughness of Welded Joints with Strength Mis-Matching by the Local

In this study, the local approach was applied to evaluate the cleavage fracture resistance ofHAZ-notched specimens with strength mis-matching between the base and weld metals. The main purpose of this study is to demonstrate that the critical Weibull stress at fracture initiation of HAZ-notched specimens can be material property independent of strength mis-matching.For the fundamental study to discuss the effect of strength mis-matching near the notch on the fracture resistance by the local approach, homogeneous specimens and model specimens with only strength mismatching near the notch were used. By using the critical Weibull stress obtained for the homogeneous specimen, strength overmatch effect and undermatch effect on the critical CTOD value were predicted. Close agreement was observed between the critical CTOD value predicted and that obtained by the experiment. As a result, independence of strength mis-matching on the critical Weibull stress was verified.The Weibull stress for the HAZ-notched specimens with strength mis-matching (match and overmatch specimens) was evaluated in consideration of that almost all specimens have fracture initiation points in the CGHAZ (LBZ : Local Brittle Zone). It was demonstrated that the cleavage fracture resistance for the HAZ-notched specimens controlled by the CGHAZ can be uniformly treated by using the Weibull stress irrespective of the existence of strength mis-matching.

Local Approach to Strength Mis-Match Effect on Cleavage Fracture of Notched Material

This paper discusses the strength mis-match effect on cleavage fracture of notched materials based on the local approach. The aim of the study is to verify the independence of the critical Weibull stress at fracture on the strength mis-match condition. Diffusion bonded joints were made with two materials S and H different in strength level. Two types of 3-point bend specimen were extracted. One had a notch in the lower strength material S near the bonded interface, and another in the higher strength material H. Homogeneous specimens were also made for each material. The critical CTOD value at cleavage fracture for the overmatched specimen with a notch in the material S was smaller than that of the homogeneous specimen of the material S. By contrast, the undermatched specimen with a notch in the material H showed apparently larger critical CTOD value than the homogeneous specimen of the material H. The strength mis-match effect on the CTOD results was explained in terms of the constraint effect on the near-tip stress field. Namely, the near-tip stress is elevated by strength overmatching and relaxed by strength undermatching. The Weibull stresses for the homogeneous and mis-matched specimens were evaluated in the light of the near-tip stress fields. The critical Weibull stress at fracture did not depend on the strength mis-match condition near the notch. By using the critical Weibull stress obtained for the homogeneous specimen, the strength mis-match effect on the CTOD results can be predicted. Close agreement was observed between the critical CTOD value predicted and that obtained by the experiment.