In this study, various types of drop-weight tear test (DWTT) were conducted on a high-toughness line-pipe steel in order to analyze abnormal fracture appearance occurring in the region impacted by a hammer. A pressed notch or a chevron notch was introduced into DWTT specimens, some of which had a back slot designed to decrease the inverse fracture area. After the DWTT, percent shear area, area of inverse fracture surface, and hardness and work hardening exponent of the hammer-impacted region were measured. The results indicated that the shear area of the pressed-notch DWTT specimens was larger than that of chevron-notch DWTT specimens. In the hammer-impacted region of all the DWTT specimens, abnormal inverse fracture having a cleavage fracture mode appeared, and its fracture area was correlated well with hardness and work hardening exponent measured by an indentation test. The formation of this inverse cleavage fracture raised the fracture propagation transition temperature (FPTT) by decreasing the shear area measured in accordance of the API 5L3 specification, which was confirmed by relating to the Charpy impact test data of pre-strained steel specimens. Thus, it was desirable to exclude the inverse fracture formed in the hammer-impacted region when calculating percent shear area of DWTT because compressive pre-strain induced by work hardening did not exist in actual applications of line-pipe steels.

Analysis of abnormal fracture occurring during drop-weight tear test of high-toughness line-pipe steel

The effects of microstructure on inverse fracture occurring in the hammer-impacted region were analyzed after conducting a drop-weight tear test (DWTT) on high-toughness pipeline steels. Three kinds of steels were fabricated by varying the alloying elements, and their microstructures were varied by the rolling conditions. The pressed-notch (PN) or chevron-notch (CN) DWTT and Charpy V-notch (CVN) impact tests were conducted on the rolled steel specimens, and the results were discussed in comparison with the data obtained from CVN tests of prestrained specimens. In the hammer-impacted region of the DWTT specimens, abnormal inverse fracture having a cleavage fracture mode appeared, and the inverse fracture area correlated well with the upper-shelf energy (USE) obtained from the CVN test and with the grain size. The steel specimens having a higher USE or having coarse polygonal ferrite tended to have a larger inverse fracture area than those having a lower USE or having fine acicular ferrite. This was because steels having a higher impact absorption energy required higher energy for fracture initiation and propagation during the DWTT. These results were confirmed by the CVN data of prestrained steel specimens.

Effects of Microstructure on Inverse Fracture Occurring during Drop-Weight Tear Testing of High-Toughness X70 Pipeline Steels

Experimental analysis of the practical LBZ effects on the brittle fracture performance of cryogenic steel HAZs with respect to crack arrest toughness near fusion line

Structural life assessment periodically evaluates the state and condition of a structural system and provides recommendations for possible maintenance actions or the end of structural service life. It is a diversified field and relies on the theories of fracture mechanics, fatigue damage process, probability of failure, and reliability. With reference to naval ship structures, their life assessment is not only governed by the theory of fracture mechanics and fatigue damage process, but by other factors such as corrosion, grounding, and sudden collision. The purpose of this series of review articles is to provide different issues pertaining to structural life assessment of ships and ocean structures. Part I deals with the basic ingredients of the theory of fracture mechanics, which is classified into linear elastic fracture mechanics and elasto-plastic fracture mechanics. The amount of energy available for fracture is usually governed by the stress field around the crack, which is measured by the stress intensity factor. The value of the stress intensity factor, which depends on the loading mode, is evaluated by different methods developed by many researchers. The applications of the theory of fracture mechanics to metallic and composite structures are presented with an emphasis to those used in marine structures. When the inertia of relatively large pieces of a structure is large enough that the correct balancing of the energy of fracture requires the inclusion of kinetic energy, then the dynamic nature of fracture dominates the analysis. For a crack that is already propagating, the inertial effects are important when the crack tip speed is small compared with the stress wave velocities. This fact has been realized in the theory of fracture mechanics under the name of dynamic fracture and peridynamic. In essence, peridynamic replaces the partial differential equations of classic continuum theories with integro-differential equations as a tool to avoid singularities arising from the fact that partial derivatives do not exist on crack surfaces and other singularities. A brief overview of fracture dynamics and peridynamics together with damage mechanisms in composite structures is presented. The limitations of fracture mechanics criteria are also discussed. Life assessment of ship structures depends on the failure modes and the probabilistic description of failure, which are addressed in Part II. Life assessment of ship structures depends on the failure modes and the probabilistic description of failure. In view of structural parameter uncertainties, probabilistic analysis requires the use of reliability methods for assessing fatigue life by considering the crack propagation process and the first passage problem, which measures the probability of the exit time from a safe operating regime. The main results reported in the literature pertaining to ship structural damage assessments resulting from to slamming loads, liquid sloshing impact loads of liquefied natural gas in ship tankers, and ship grounding accidents, and collision with solid bodies are discussed in Part III. Under such extreme loadings, structural reliability will be the major issue in the design stage of ocean structures. The treatment of extreme loading on ship structures significantly differs from those approaches developed by dynamicists. Environmental effects on ship structures play a major factor in the life assessment of ocean systems. In particular, these effects include corrosion and hydrogen embrittlement. Part IV is devoted to a ship's life assessment resulting from corrosion and hydrogen embrittlement. Because structural components made from aluminum and its alloys are vital to the ship and aerospace industries, the influence of environment on aluminum structures and the means of corrosion control and monitoring in both aluminum and nonaluminum metals are presented. Hybrid ships consist of a stainless steel advanced double-hull center section, to which a composite material bow and/or stern is attached. Such structures require strong joints between the composite and the steel parts. Some of the difficulties with joining composites and metal are related to the large difference in mechanical properties such as stiffness, coefficient of thermal expansion, etc., between the adherents and the large anisotropy of composites. Such differences generally lead to large stress concentrations and weak joints. Fatigue crack growth, stress concentrations resulting from details, joints, and fasteners are addressed in Part V. Fatigue improvement in welded joints is considered one the major tasks of this part. Brittle fracture of hull structures causes serious structural damage and this motivated the ship structure community to develop some means to prevent brittle cracks from occurring. The basic principle behind the use of a crack arrester is to reduce the crack-driving force below the resisting force that must be overcome to extend a crack. The crack arrestor can be as simple as a thickened region of metal or may be constructed of a laminated or woven material that can withstand deformation without failure. Part VI provides different approaches of passive crack control in the form of crack arresters to stop crack propagation before it spreads over a structure component. Crack arresters used in ship structures and pipelines are described for both metal and composite materials. This six-part review article is by no means exhaustive and is based on over 1800 references. It does not address the structural health monitoring, which constitutes a major task in the structural diagnostic process.

Overview of Structural Life Assessment and Reliability, Part I: Basic Ingredients of Fracture Mechanics

Two types of full thickness compact crack arrest (CCA) specimens have been used to characterize the crack arrest toughness of a 15.5-mm-thick ship plate steel in both the L-T and T-L orientations. One was a modified version of that used in ASTM Standard E 1221, and the other was based on the proposed procedure of Crosley and Ripling [1] for non-plane strain testing. The latter specimens employed a strain age embrittled chevron notch as the crack starter. The testing was performed over the temperature range from −40° (the nil-ductility transition temperature (NDTT) of the steel) up to −5°C. None of the results from the ASTM E 1221 type specimens met the validity requirements of the ASTM procedure. Even though they had the largest dimensions allowed in the test specification, the limited thickness of these specimens meant that they were too small for valid plane strain testing. In the non-plane strain test procedure, the in-plane dimensions are scaled to the expected crack arrest toughness at the test temperature, and the main qualifying requirement is for the crack to be arrested at a point in the specimen lying within certain defined limits. Although difficulties were encountered with variability in run/arrest behavior, this specimen gave some useful crack arrest toughness data for this steel, particularly with regard to the relatively high values obtained when testing near the top of the test temperature range. The effectiveness of the crack starter technique used in both type of specimens is also discussed, as well as possible modifications for future tests.

Measurement of Crack Arrest Fracture Toughness of a Ship Steel Plate

Full thickness fracture toughness of conventional ship plate grades, including modern high-strength steels, was determined at loading rates representing quasi-static, intermediate, and impact conditions. For testing at impact strain rates ({dot {epsilon}} = 5 s{sup {minus}1}), customized equipment was designed and developed for a drop tower. It was shown from the CTOD transition temperature curves that the transition temperature increase from quasi-static to intermediate rate (a three-order rate increase) is much greater than the increase from intermediate to impact (a two-order rate increase). Comparison of the dynamic CTOD, CVN, and NDTT indicated that the correlation between the CVN and 0.1-mm dynamic CTOD transition temperature does not hold for the TMCP steels, for which the CVN transition temperature is much lower. By contrast, the results display a very good correlation between the NDTT and 0.1-mm dynamic CTOD transition temperature for all of the steels tested in this program.

Strain rate effects on fracture toughness of ship plate steels

Effects of plastic deformation on fracture toughness of ship plate steels

The awareness of the presence of local brittle zones (LBZs) in the heat-affected zone (HAZ) of welds has led to the requirements for minimum initiation toughness for the HAZ for critical applications. A fracture control philosophy that is proposed to be an attractive alternative for heat-affected zones containing LBZs is the prevention of crack propagation rather than of crack initiation. Such an approach would be viable if it could be demonstrated that cracks initiated in the LBZs will be arrested without causing catastrophic failure, notwithstanding the low initiation (CTOD) toughness resulting from the presence of LBZs. Unstable propagation of a crack initiating from an LBZ requires the rupture of tougher microstructural regions surrounding the LBZ in HAZ, and therefore the CTOD value reflecting the presence of LBZ is unlikely to provide a true indication of the potential for fast fracture along the heat-affected zone. Base metal specifications usually ensure that small unstable cracks propagating from the weld zone into the base metal would be arrested. To investigate the likelihood of fast fracture within the HAZ, a test program has been carried out that involved performing compact plane strain and plane stress crack arrest tests on a heat-affected zone that containedmore » LBZs, and thus exhibited unacceptable low CTOD toughness for resistance to brittle fracture initiation. The results indicated that the crack arrest toughness was little influenced by the presence of local brittle zones. Instead, the superior toughness of the larger proportion of finer-grain HAZ surrounding the LBZ present along the crack path has a greater influence on the crack arrest toughness.« less

L. Malik

Papers

Strain rate effects on fracture toughness of ship plate steels

Effects of plastic deformation on fracture toughness of ship plate steels

Crack arrest toughness of a heat-affected zone containing local brittle zones