Arrest Fracture Toughness Research Articles

Measurement of Crack Arrest Fracture Toughness of a Ship Steel Plate

Abstract 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.

The Crack Arrest Capability of 9-Percent Ni Steels (ASTM 553, Type 1) LNG Storage Tanks

Safety of structures can be assured, even if cracks initiate in localized regions of abnormally low toughness, and/or abnormally high stress (LT/HS), if the materials from which they are fabricated have a high enough crack arrest fracture toughness. When this requirement is met, fast-running cracks that initiate in LT/HS regions arrest when their tip encounters material having normal toughness and stresses. The work described in this paper was carried out to determine the crack arrest capability of LNG storage tanks by determining the longest LT/HS region in which a crack could initiate and still arrest when it leaves this region. The determination consisted of relating a fracture analysis with the measured full-thickness crack arrest fracture toughness of three 9-percent Ni plates which were reported in reference [1]. The calculations used a residual stress pattern, produced by welding, superimposed on a typical membrane stress. The residual stress was selected as an example of a localized high stress region. It was found that tanks built from the poorest of the three tested plates could arrest cracks about 3/4 m long, while tanks built from the two tougher plates could arrest cracks almost 2 m long.

Full-Thickness Crack Arrest Fracture Toughness of 9-Percent Ni (ASTM 553, Type 1) Steels

Measurements were made of the full-thickness crack arrest fracture toughness (FTCAT) of three 9-percent Ni steels having the range of thicknesses and Charpy V-notch toughnesses of plates now in LNG storage tank service. Unlike the plates used in large pressure vessels, these thinner plates do not crack under a condition of plane strain, so that test method ASTM E1221 “Standard Test Method for Determining Plane-Strain Crack-Arrest Fracture Toughness, Kla, of Ferritic Steels,” was modified for these tests. The toughness of all three steels was measured at liquid nitrogen temperature (−196C), but two of the three were too tough to be measured at LNG storage temperature (−162C). Even the least tough plate had a high enough toughness to arrest long cracks in LNG storage service.

A Quality Control Test for Selecting Materials to Arrest Fast-Running, Full-Thickness Cracks

Abstract A method for measuring full-thickness crack arrest fracture toughness, Ka, using crack-line wedge-loaded compact specimens is described. The unique feature of the test is that so long as the prescribed test method is followed, the data can be analyzed by a small-scale plasticity procedure, with all validity requirements being met. This simple test and analysis procedure, and the fact that every test gives useful data, makes the method suitable for quality control/quality assurance testing. The test consists of pre-selecting a value of Ka that must be measured, identified as Kv. The specimen size and allowable limits on mouth opening displacement are selected by using Kv and the material's yield strength. The test is then conducted by driving a wedge into the specimen to develop a run-arrest event, after which the specimen is broken open and the crack length at arrest, af, is measured. If af is between 0.4W and 0.8W, Ka is calculable; if the crack length at arrest 0.5W or 0.8W, Ka Kv or Ka Kv. Crack propagation in the plates on which the test is expected to be used occurs by tunneling. Hence Ka increases with crack extension, attaining its saturation value when the crack attains its equilibrium shape. Measuring saturation values is not needed for quality control/quality assurance testing, however. This allows the use of smaller specimens, but a K-resistance versus crack extension curve for the type of steel being tested is needed to set acceptance criteria.

Statistical analysis of fracture arrest toughness of various structural steel plates

A statistical investigation was made relating to the brittle fracture arrest properties of various steel plates for ship and bridge structures, low-temperature storage tanks and pressure vessels. Fracture arrest toughness ( K a ) data obtained by means of the temperature gradient type of ESSO tests on around 200 steel plates were collected and analyzed. It was found that K a transition curves in terms of temperature for a set of steel plates possessed a ‘pivot’ on the temperature versus K a diagram when the data were grouped according to their chemical composition and production methods. It was also found that this pivot is moved to lower temperatures or to higher K a directions following the application of production processes and selection of the chemical composition such as the following: normalizing heat treatment; quenching and tempering heat treatment; rolling at intercritical temperature; reduction in carbon content and increase in nickel content. It was also found that the new production method of Thermomechanically Controlled Process (TMCP) for plate rolling provides an effective measure to improve the fracture arrest properties of steel plates.

The restraining effect of ductile ligaments on plane strain crack propagation and arrest in ferritic steels

In the transition regime plain strain crack propagation in ferritic steels proceeds by a combination of cleavage and ductile rupture processes, the latter being confined to ligaments that are parallel to the direction of macroscopic crack propagation. These ligaments rupture as the crack propagates, and it is generally accepted that they have a restraining effect on crack propagation, and also provide a major contribution to the arrest fracture toughness K Ia . In modelling the propagation and arrest phenomena, and in quantifying this restraining effect, it is tempting to average the effect of the discrete ligaments over a region behind the crack tip. The results of this paper's theoretical analysis justify the use of the averaging procedure.

Comparison of the plane strain crack arrest fracture toughness of 4340, 4140, and 1340 steel

The plane strain crack arrest fracture toughness of 4140 and 1340, both in a fully martensite condition, was measured over a yield strength range of 965 to 1240 MPa, and a test temperature range of -54 to 74 °C. These results were compared with each other and with the data collected earlier on 4340. The 4340 was found to be far tougher than the other two alloys at all yield strengths and test temperatures. The 4140 was tougher than the 1340, although the difference appeared to decrease as the yield strength increased. The toughness of the fully hardened 4140 was also compared with the toughness measured earlier on a 25 pct thicker plate that did not through-harden. Although all data were plane strain, there was a sizable toughness loss for the plate that was not through-hardened. Crack arrest fracture toughness was found to be sensitive to steel composition, test temperature, yield strength, and microstructure.

Experimental Investigation on the Relationship between Brittle Fracture Initiation and Arrest Toughnesses of 9% Ni Steel

The objective of this paper is to investigate the relationship between fracture toughnesses of 9% Ni steel at brittle crack initiation and at brittle crack arrest. It is the authors' point of view that the difference between these fracture toughnesses is due to the difference in strain rates. Based on this assumption, the effect of strain rate on the fracture initiation toughness was investigated by three-point bend test, while the fracture arrest toughness was investigated by Compact Crack Arrest (CCA) test. On comparing fracture initiation toughness, Kc, with fracture arrest toughness, Kca, it was found that Kca was almost equal to or higher than Kc. The cause is considered to be the effect of ductile lips left below the plate surfaces. This led the authors to the conclusion that an initiated brittle crack can inevitably be arrested after a short extension even if external force does not fall.To verify the finding, a notched very wide plate specimen was tested. The result showed that only a pop-in type of fracture could occur, that is, an initiated brittle crack was arrested after the extension comparable to plate thickness. The notched wide plate test also verified that the reinitiation of brittle fracture from the arrested crack could not take place without the substantial increase in external load.

Studies of epoxy resin systems: Part D: Fracture toughness of an epoxy resin: A study of the effect of crosslinking and sub‐Tg aging

AbstractThe fracture toughness of an epoxy resin system, diglycidyl ether of butanediol, DGEB, cured with 4‐4′ diaminodiphenyl sulphone, DDS, has been studied by varying the crosslinking density and state of aging. A stable, but rough, crack propagation was observed with specimens that were 99 percent cured and quenched. When the extent of curing was less than 99 percent or the material was aged for more than 20 min at 62°C, crack propagation was of the unstable stick‐slip nature. Aging was found to decrease the initiation fracture toughness dramatically, but the arrest fracture toughness was almost unchanged. This result was associated with a change of relaxation strength of the primary, a, transition with aging. An increase of crosslinking density was found initially to reduce the fracture toughness of this epoxy resin, but the fracture toughness increased after 87 percent of curing. The initial decrease of the fracture toughness was attributed to a decrease of relaxation strength of the primary transition (i.e., the area under the α‐relaxation peak), while the increase of the fracture toughness after 87 percent curing was explained by the onset of the stablerough crack propagation, Micrographs taken by scanning electron microscopy‐showed possible existence of blunting during crack propagation and a decrease of blunting with the extent of aging.

Strain Rate-Temperature Effect on Cleavage Controlled Fracture Toughness (K<SUB>Ie</SUB> and K<SUB>Id</SUB>) and Its Correlation with Crack Arrest Fracture Toughness (K<SUB>1a</SUB>) of Reactor Pressure Vessel Steel (A533B)

Strain Rate-Temperature Effect on Cleavage Controlled Fracture Toughness (K<SUB>Ie</SUB> and K<SUB>Id</SUB>) and Its Correlation with Crack Arrest Fracture Toughness (K<SUB>1a</SUB>) of Reactor Pressure Vessel Steel (A533B)

Influence of material properties on dynamic fracture toughness of steels

Recent studies of plastic enclave formation at running brittle cracks were extended to account for the influence of crack tip boundary conditions on the temperature at which the enclaves start to develop. The En 2A and three other steels were used in the analysis. It was found that this temperature depends very strongly both on the magnitude and on the distribution of the stresses in the discrete crack tip zone. This suggests that the onset of enclave formation and the rate of their growth are governed by the balance of two sets of material characteristics. The first set consists of at least two parameters describing the microscopic fracture resistance which promotes enclave formation. The second set includes the macroscopic yield and flow properties which may make enclave formation more difficult in higher strength steels. These findings are related to the dynamic or crack arrest fracture toughness which is found to be derived from two different sources. One is connected with the microscopic plastic deformation of the fracturing metal in the crack tip zone and is present at all temperatures. The other is the result of enclave formation, it is present only at higher temperatures and is responsible for the energy transition. In contrast to the case of crack initiation, the dynamic fracture toughness depends not only on the microscopic fracture strength or strain but on the complete stress-displacement relationship of the weakened material which is governed by the microscopic fracture mechanism at the tip of a running crack. It is noted that the present results can be expected to be valid for all steels which fracture in the cleavage or quasi-cleavage modes.