Yasuhito Takashima

Fracture Toughness Evaluation of Laser Beam-Welded Joints of 780 MPa-Strength Class Steel

For fracture assessment of steel structures with laser beam-welded joints, it is significantly important to evaluate the fracture toughness of the weld metal. However, the fracture toughness of weld metal is often impossible to measure by standard Charpy impact testing in the brittle-to-ductile transition or upper-shelf temperature region, because of fracture path deviation (FPD) from the weld metal due to narrow weld bead and a high degree of overmatching in strength. In this work, evaluated fracture toughness of laser beam-welded joints of 780 MPa-strength class (HT780) steel is examined by Charpy impact testing and CTOD fracture toughness testing. The improved Charpy impact tests, using the specimen with three parallel welds or the side-groove specimen, are conducted in order to prevent FPD. The applicability and significance of the improved impact test methods to the laser beam-welded joints of high-strength steel are investigated by means of Weibull stress analysis. The Weibull stress analysis indicates that the 3-weld method could not necessarily prevent FPD in laser beam-welded joints of HT780 steel, since side beads are not as hard as main weld beads, due to reheating at the main weld. On the other hand, a side-groove specimen is effective for avoiding FPD, but intrinsic VE of the weld metal could not necessarily be obtained by using this specimen.

Evaluation method for Charpy impact toughness of laser welds based on lateral contraction analysis

An evaluation of Charpy impact toughness of the weld metal for laser beam welds is often impossible due to occurrence of fracture path deviation (FPD) from the weld metal into base steel. Therefore, the improved toughness evaluation method using side-grooved specimen instead of normal specimen has been suggested for preventing FPD. However, for a pre-qualification of the toughness of laser welds, the toughness value obtained with side-grooved specimen should be corrected to the toughness value of normal Charpy specimen. In this study, the method for correction of side-grooved Charpy energy for laser welds to normal Charpy impact toughness suggested by Hagihara et al. (Q J Jpn Weld Soc 25: 165–172, 2007) is advanced by means of FE analysis with focus on the deformation behaviors associated with strength mismatch. Furthermore, the distinctive strength mismatch of laser welds on lateral contraction of Charpy specimen has been analyzed by means of the Weibull stress approach. The applicability of the advanced engineering method is demonstrated by comparing experimental result, whereas the further verification should be accumulated.

Three-Dimensional Dynamic Explicit Finite Element Analysis of Charpy Impact Test

Dynamic explicit finite element (FE) analysis of the Charpy impact test was conducted in this study to investigate the inertial effect on the stress field ahead of the V-notch in a Charpy specimen. The deformation behavior of the Charpy specimen and the constraint effect on the stress field in the plastic zone near the V-notch were numerically simulated using three-dimensional FE analysis, while considering the contact of the specimen with the striker and anvil. The effect of the strain rate on the flow stress and the increase in temperature during impact loading were included in the dynamic analysis. This analysis shows that the impact load exhibits oscillation and the contact stiffness between the specimen and the striker affects the oscillation of the impact load. The analysis was validated by comparison with experimental results obtained using an instrumented Charpy impact testing machine, which measured the impact load and the load point displacement. The oscillation of the load–time curve was recorded. The magnitude and period of the peak inertia load obtained by the FE analysis were almost consistent with the experimental results. The contact stiffness between the specimen and the striker affected the stress field near the V-notch in the specimen. This indicates that the stress field in the Charpy specimen should be analyzed by the dynamic analysis procedure considering the contact stiffness based on the Hertzian contact theory.

Constraint-based assessment of fracture in welded components

A toughness correction method, developed as ISO 27306, specifies an equivalent crack tip opening displacement (CTOD) ratio, β, for the correction of CTOD toughness for constraint loss in structural components. Using β, the standard fracture toughness specimen and structural components are linked at the same level of the Weibull stress. This paper applies the IST method to the fracture assessment of welded components. Effects of the strength mismatch and residual stress in welds on β are implemented into the toughness correction. The strength mismatch does not have an influence on β, provided that the strength mismatch between the weld and base metals is in the range from 0.9 to 1.2. On the other hand, residual stress exerts a large influence at a low CTOD level below full yielding: β is elevated by the tensile residual stress. It is shown on the failure assessment diagram that the CTOD toughness correction with β eliminates the excessive conservatism often found in the conventional approach and leads to accurate fracture assessments of welded components.

Fracture Performance Evaluation of Laser Beam Welded Joints Based on the Weibull Stress Criterion

This paper examines the fracture performance of laser beam welded joints of a high strength steel with a notch in the weld metal (WM). The standard fracture toughness specimen (3-point bend specimen, 3PB) and tension panel (edge through-thickness crack panel, ETCP) are tested at a low temperature in the brittle fracture range. The focus lies on the difference between near crack-tip plastic constraints for the standard fracture toughness specimen and the tension, and on the very narrow and highly hardened WM shielding high strain in the weld metal from external loading. The ETCP has presented a higher critical CTOD at brittle fracture initiation than the 3PB specimen. The fracture performance of ECTP is predicted from CTOD fracture toughness test results by means of the Weibull stress criterion with the consideration of the shielding effect of the weld metal. The results show an advantage of the Weibull stress criterion for the fracture transferability analysis of toughness results. Furthermore, the influence of very narrow and highly hardened weld metal of laser beam welded joints on the toughness requirement has been analyzed by means of the Weibull stress criterion. The effect of the distinctive strength mismatch of laser beam welded joints on required CTOD fracture toughness of laser beam welded joints in 3PB necessary to meet design requirement of joint performance is discussed taking into account the shielding effect on crack opening behavior and toughness collection due to constraint loss between ETCP and 3PB specimen. The required CTOD in 3PB is a little bit lower than arc welded joints. Therefore, it should be noticed that the very narrow and highly hardened WM does not necessarily lead to poor fracture performance of the welded joints.Copyright

Analysis of Statistical Scatter in Charpy Impact Toughness

Charpy impact toughness values show large statistical scatter, particularly in the ductile-to-brittle transition temperature (DBTT) range. Although the statistical distribution of Charpy absorbed energy has not been clarified, critical values of the stress intensity factor, J-integral and crack-tip opening displacement (CTOD) at brittle fracture generally show the Weibull distribution with two or three parameters. This study proposes a brittle fracture model, based on the weakest link theory, for evaluating the scatter in Charpy absorbed energy KV. The numerical results show that the amplitude of the opening stress fields ahead of the V-notch at varying amounts of KV are uniquely characterized as the square of the applied load. With these numerical results, the Weibull shape parameter of the statistical distribution of KV is almost equal to 2. The proposed statistical model is verified through experimental results. It is found that the statistical distribution of KV is characterized by a two-parameter Weibull distribution with the shape parameter of 2 under the condition of pure brittle fracture.

Evaluation of Charpy impact toughness using side-grooved specimen for hybrid laser-arc welds of ultra-high-strength steel

This paper discusses the significance of a narrow hard weld bead, produced by hybrid laser-arc welding, on the Charpy absorbed energy. A parametric finite element analysis with various strength mismatching and bead widths has been conducted on the strain field of side-grooved and normal Charpy specimens. The narrow weld bead promotes the accumulation of plastic strain energy on the base metal side. This feature is more significant in the normal specimen than in the side-grooved specimen. The results show that the method based on the Weibull stress criterion for converting the absorbed energy of the side-grooved specimen to that of the normal specimen can be applied to hybrid laser-arc welds of ultra-high-strength steel. The converted energy is consistent with the absorbed energy measured in the normal specimen. This study indicated that the absorbed energy of side-grooved specimen corresponding to the absorbed energy of the normal V-notch specimen increases with increasing hard zone width. On the other hand, the influence of strength mismatch ratio on the difference of the absorbed energy is rather weak.

Revision of ISO 27306 for CTOD Toughness Correction for Constraint Loss

As the result of the international standardization work in Japanese IST project, ISO 27306 were published in 2009 for correction of CTOD fracture toughness for constraint loss in steel components. ISO 27306 employs an equivalent CTOD ratio based on the Weibull stress criterion, which leads to more accurate fracture assessment than the conventional fracture mechanics assessment. On the occasion of the 1st periodical review, the revision of ISO 27306 has been proposed from Japan. This paper describes the key contents of the new ISO 27306. A case study is included on the fracture assessment of a wide plate component according to FAD (failure assessment diagram) approach specified in BS 7910:2013.

Numerical analysis of strength mismatch effect on stress field in Charpy specimen

The influence of strength mismatch adjacent to the V-notch in Charpy specimen on stress field has been numerically analyzed with a 3D finite element method. It is found that the softer material adjacent to the V-notch provides a “shielding effect” from high-speed straining introduced by impact loading. As this leads to reduction in flow stress ahead of the V-notch, lower opening stress compared with homogeneous specimen is found in the case of the V-notch within the harder material in heterogeneous specimen. By contrast, in the case of the V-notch within the softer material, plastic strain is concentrated near the V-notch and the opening stress is elevated by constraint due to hard material adjacent to the V-notch. The Charpy impact test was conducted with the Charpy specimen extracted from the clad steel consisted of low- and high-strength steels in order to investigate the strength mismatch effect Charpy absorbed energy KV. It is found that KV value is affected by strength mismatch adjacent to the V-notch and the Weibull stress criterion can be applied to the quantitative evaluation of the strength mismatch effect on KV.

Damage Caused by Dynamic/Cyclic Loading of Detonation Chamber

The plastic damage caused by multiple dynamic loading and its influence on the ductility and toughness of 3.5%Ni-steel (SA203E) and C-steel (SA516M) were examined for the discussion of integrity design of detonation chambers. V-notched specimens and hourglass specimens were welded on 1/7-scale model of a 1-ton explosive class detonation chamber and subjected to 30 detonation shots. It has been indicated that 30 detonation shots caused pre-strain in the specimens. The ductility was reduced by the pre-strain. The Charpy impact toughness was affected as well in a lower temperature. It has been noted that the damage is developed with fatigue crack growth at notch root of the V-notched specimen.© 2011 ASME