Crack Opening Displacement Tests Research Articles

Fracture behaviour of 8%Ni 980 MPa high strength steel at various temperatures Part 2 – COD tests

The fracture behaviour of 8%Ni 980 MPa grade high strength steel is investigated by combining experimental results of crack opening displacement (COD) tests at various temperatures with detailed microscopic observations of fracture surfaces and crack configurations in unloaded specimens. The results reveal that this high strength steel possesses high toughness with a transition temperature around −150°C. Even though at a very low temperature (−196°C), cleavage cracking dominates the fracture process and the crack does not propagate immediately through the entire ligament: a ‘pop-in’ extension is observed in macroscopic tests, and the microscopic fracture mode is quasi-cleavage. It is found that resistance to crack propagation is provided by three barriers: original austenite grain boundaries, bainite colony boundaries and interlayers between bainite laths. These barriers manifest themselves by tear ridges with dimples on the fracture surfaces. At higher temperatures, the fracture mechanism is dominated by fibrous rupture, associated with a ‘dimpled’ fracture surface and some individual quasi-cleavage facets.

Effect of a hard artificial asperity on the crack closure behavior in an annealed SAE 1015 steel

The load–crack opening displacement (COD) curves and deformation characteristics in the vicinity of a hard artificial asperity in an annealed SAE 1015 steel were studied. The artificial asperity was found to have a significant effect on the trend of the load–COD curves. The lower portion of the load–COD curves in the unloading phase exhibited a convex shape without the asperity, but a concave shape with the asperity. The concave shape, signifying the acceleration in the COD decrease, was further verified by varying the size of the asperity, conducting special compression tests and elastic–plastic load–COD tests. The plastic deformation in the vicinity of both asperity and crack tip was studied via microhardness tests, etching techniques, and finite element analysis. Based on the experimental observations, a modified crack closure process model was proposed, where three stages of the unloading curve was defined: (i) the asperity does not contact the upper crack face, (ii) a process where both the asperity and the specimen material deform elastically, and the elastic-wedge model is applicable, and (iii) the plastic deformation of the specimen material adjacent to the asperity occurs, thus resulting in the concavely shaped load–COD curves. An equation was proposed to estimate the COD values, in which the plastic deformation both at the crack tip and at the asperity was considered. The residual COD calculated from the proposed equation was found to be consistent with the experimental results.

The effect of ageing temperature on the fracture toughness of an 8090 Al-Li alloy

It is well established that during plastic deformation of AI-Li alloys, the coherent 6' precipitates are sheared by dislocations. The resultant planar slip associated with 6' shearing leads to heavy localized dislocation pile-ups at grain boundaries. Suresh et al. [1] suggested that a major contribution to the fracture process in peak-aged and overaged alloys arises from intergranular failure promoted by microvoids formed around grain boundary precipitates. This process in enhanced by slip planarity, and Duva et al. [2] proposed that slip becomes more localized as the size of the coherent ageing precipitate increases. The size and distribution of the 6' phase in the AI-Li alloy 8090 will depend on the ageing temperature selected, and in the present work the fracture toughness of this alloy was investigated in specimens aged to the same hardness level at progressively lower ageing temperatures. A 90 mm thick 8090 plate supplied by Messrs Alcan Plate Ltd had the composition shown in Table I. It had been solution-treated at 545 °C before being cold-water quenched and stretched 7% before ageing. Samples 10 mm × 15 mm x 15 mm were cut from the mid-section of the plate and ageing was carried out at 130 and 150 °C in an air furnace and at 170 and 190 °C in an oil bath. The average value of five hardness indentations was recorded, and the surface of each specimen was ground before testing in order to remove any lithium-depleted layer. The object was to examine the variation of toughness with ageing temperature in material of constant hardness. The hardness selected was that achieved in 32 h at 170 °C, which is commonly commercially employed. The ageing times to achieve this hardness were 320 h at 130 °C, 78 h at 150 °C, 32 h at 170 °C and 8.3 h at 190 °C. Both K~ and crack opening displacement (COD) tests were conducted, and toughness specimens were also aged for 8 and 24 h at 170 °C, so that the variation of toughness with ageing time at constant temperature could be examined. K,~ tests were conducted to American Society for Testing and Materials standards on compact tension (CT) specimens of 20 mm thickness, and COD tests on specimens of 6.35 mm thickness. All specimens were machined so that they were strained in the longitudinal direction of the original plate, with the notch in the transverse direction. The change of hardness with ageing time at the four temperatures is shown in Fig. la to d. As expected, peak hardness was achieved at increasingly longer ageing times for increasingly lower ageing temperatures. There was also the expected tendency for the peak hardness to be greater the lower the ageing temperature was. ~ though the duration of ageing at 130 °C was insufficient to achieve a peak value, it is noteworthy that plateaux are present in the curve, which may reflect the formation of precursor phases to the 6' precipitate. These data appear to be consistent with those reported by Firrao et aL [3] for a ternary A1-Li-Cu, who similarly reported a maximum hardness achieved after ageing at 150 °C in comparison with the other ageing temperatures studied. It is obvious, however, that ageing to peak hardness at 150 °C would not be commercially viable in view of the protracted time involved. However, ageing at a temperature between 150 and 170 °C may produce a superior combination of strength and toughness within an economically viable ageing time. The values of KIo or K o obtained are shown in Table II. The COD values obtained on specimens after the different heat treatments are shown in Fig. 2, and it is apparent that the specimens aged at lower temperatures and for shorter times have higher toughnesses. From both sets of data it is apparent that the greatest toughness change occurs between ageing temperatures of 150 and 170 °C, which again suggests that an ageing treatment between these two temperatures would produce optimum properties. The materials showing higher toughnesses correspond to ageing treatments that lead to smaller 6' precipitate sizes. This in turn will result in a less planar slip distribution [2], and hence greater plasticity. It may thus be concluded that, first, greater peak hardnesses are achieved at lower ageing temperatures. Secondly, the form of the ageing curve at 130 °C suggests that other metastable transition phases precede the formation of the 6' precipitate. Thirdly, as the ageing time is increased (at a given temperature) the toughness decreases. Fourthly, if specimens are aged to the same hardness at different ageing temperatures, then the toughness (as measured by both Kic and COD) will be higher at

Effects of strain and strain aging on fracture toughness of C–Mn weld metal

Specimens of both as-deposited and fine-grained, reheated, manual metal arc C–Mn weld metal have been subjected to various strain and static strain-aging treatments in an attempt to simulate the thermomechanical cycles imposed upon the root region of a multipass weld as subsequent passes are made. The toughness was then measured, as a function of severity of treatment, using a crack opening displacement test. The strain aging treatments are found to lower markedly the cleavage resistance of the as-deposited and reheated microstructures. Non-metallic inclusions within the crack-tip plastic zone are found to be active as cleavage initiation sites in both types of microstructure. While the general shift in toughness can be explained by considering the changes in flow properties brought about by the various treatments, the observed variations in size and distance from the crack tip of the initiating inclusions are thought to be responsible for the associated experimental scatter.MST/157

An Assessment of Strain-Aging Effect on Weldable Offshore Structural Steel

Summary During the fabrication of offshore structures, the steel plate condition changes from the as-received normalized state to a strain-aged state. This paper assesses the effect of 5% prestrain and aging at 250C [482F] for 1 hour on the mechanical properties of normalized low-alloy BS 4360–50D steel plates obtained from two different suppliers. For both plates, strain aging led to an increase in yield strength, ultimate strength, Charpy and crack opening displacement (COD) transition temperatures, and fatigue resistance. The results of fracture toughness tests show that the degree of sensitivity to strain aging may depend on the type of test used; therefore, a test that more closely simulates offshore loading conditions, such as a COD test, is most appropriate. The paper also shows that the impact of strain aging on the integrity of offshore structures is minimized by the competing effects of lower fracture toughness and slower crack-growth rates. Introduction The selection of steel for offshore structures is governed by several factors, including strength, notch toughness, and fatigue strength. These properties are, in general, established on the basis of the as-received, normalized steel condition. Since offshore structures are generally fabricated by forming steel plates into tubular members and connecting these members by welding, a change in the properties would be expected as a result of strain aging. The strain occurs during forming, and aging is accelerated by the temperature rise during welding. Strain aging manifests itself mainly by an increase in yield strength on aging after straining. Strain aging may also change other properties, such as ultimate tensile strength, ductility, ductile/brittle transition temperature, fatigue strength, and electrical and magnetic properties. Nevertheless, the increase in yield stress on unloading and aging is the most universal indication for the occurrence of strain aging. Therefore, strain aging can be detected simply by the tensile test (Fig. 1). The increase in yield stress after aging, is often used as a measure of the strain-aging susceptibility. This change in properties is attributed to the pinning of the free dislocations that are formed during straining by the carbon and nitrogen atoms that are dissolved in ferrite. It is now universally accepted that strain aging occurs in the presence of interstitial elements such as carbon, nitrogen, and boron. Since virtually all offshore structural steels contain carbon and nitrogen and do not contain boron, it is the first two elements that are of greatest interest. It is therefore suggested that the effect of strain aging can be restricted by lowering the concentration of carbon and nitrogen atoms that are dissolved interstitially in the ferrite. The effect of free nitrogen can be reduced adding nitride-forming elements, such as aluminum. To bind the free carbon atoms, elements that form stable carbonitrides and carbides, such as niobium, vanadium, molybdenum, or titanium, are added. Available evidence suggests that a nitrogen content of 0.0001% produces detectable aging, while 0.001 to 0.002% produces severe aging. Increases above this level probably have little further effect. This suggests that the uncombined nitrogen must be reduced to about 0.0001% or less to eliminate strain aging. Wilson and Russell suggested that in moderately deformed steels, the segregation of about 0.0005 to 0.002 wt% carbon or nitrogen solute (depending on the exact dislocation density) is sufficient to complete dislocation locking. Since the solubility of nitrogen in ferrite is about 100 times that of carbon, it is always suggested that strain aging at low temperatures (less than 100C [212F]) is attributed to nitrogen. At high aging temperature >100C [212F]), the solubility of carbon is increased, and strain aging is produced even when nitrogen has been completely removed from solution. Aging at higher temperatures >350C [662F]) enables substitutional solutes, such as phosphorus, niobium, nickel, silicon, and titanium, to cause strain aging. Strain aging is known to affect toughness adversely. Baird recently has shown that the increase in impact transition temperature after 10% prestrain and aging at 250C [482F] for 30 minutes is not very sensitive to composition, prior heat treatment, or type of impact specimen. The overall increase (caused by both straining and aging) lies in the range of 40 to 70C [104 to 158F] for nearly all steels. Recognizing the potential effect of strain aging on fracture toughness of offshore structural steel, some technical standards restrict the use of Charpy V-notch impact test results from the parent plate to the conditions where the forming strain does not exceed 5% and the material temperature does not exceed 650C [1,202F]. Otherwise, Charpy tests shall be done with specimens machined from fabricated plates. The present study is aimed at establishing the effect of strain aging on one class of steel plates, BS 4360–50D, which is used extensively in the fabrication of offshore structures. JPT P. 141^

溶接金属のＣＯＤ特性と組織の関係について

Crack Opening Displacement (COD) tests were carried out in submerged arc weld metals. The fracture initiation properties of wled metals were related to the microstructures. The relationship, between them was investigated in the view point of unit crack path on the fracture surface of COD specimen.The results obtained are as follows:(1) The evaluation method of the fracture toughness of submerged arc weld metal was established by means of COD test.(2) The crack initiation properties were related to the microstructures of weld metal.(3) The growth of the uniform fine ferrite structure improves the low temperature toughness of weld metal.(4) The brittle fracture surfaces in COD specimens were closely related to the microstructure, and it was found that the crack initiation temperature at δc=0.2 mm has a good correlation with the unit crack path based on the direct observation of fracture surface.

Slow crack growth and acoustic emission characteristics in COD test

Brittle material fractures at a critical value of stress intensity factor or crack opening displacement ( COD) without any prior crack growth. In the ductile materials, however unstable fracture initiation may be preceded by a period of slow crack growth from the notch tip. Such slow crack growth may be an essential prelude to unstable fast fracture and thus COD at the start of slow crack growth can also be regarded as a critical value of the fracture characteristics. We have tried to apply the acoustic emission techniques to the COD test. Acoustic emission techniques can be used to detect the onset of slow crack growth and to obtain a relation between COD and AE characteristics in the COD test.

A simple experimental capacitance transducer for use in crack opening displacement tests at elevated temperatures

A simple capacitance transducer, suitable for monitoring crack opening displacements (CODS) at elevated temperatures is described. The operation range may be up to several millimetres and so far a stability of +or-1 mu m has been obtained in vacuum at 565 degrees C over periods up to 1000 h.

Predicting the Fracture Initiation Transition Temperature in High Toughness, Low Transition Temperature Line Pipe With the COD Test

This paper describes the use of the crack opening displacement (COD) test to predict the fracture initiation transition temperature of high toughness, low-transition temperature in line pipe. A series of COD tests using t × t and t × 2t specimens made from this line pipe material. The COD test was conducted over a range of temperatures and the point where the upper shelf COD values began to decrease with decreasing temperature was defined. To verify the full-scale significance of this temperature, a series of three experiments was conducted on 48-in. (1.22m) dia line pipe to bracket the transition temperature defined in the COD Test. The results suggest that the COD transition temperature can ve used to define the fracture initiation temperature for static loading in pipe. In addition, in the transition temperature region, the full-scale results, while limited in number, suggest that the COD values could possibly be used to predict the critical flaw sizes in the pipe material.

An Investigation of static COD and dynamic COD

It is generally accepted that KIC criterion is usefull for initiation of brittle fracture, but this method has a weak point that the linear elastic fracture mechanics approach must be applied only when a yield zone arround a crack tip is very small compared to crack length. Brittle fracture of a structure after large scale yielding, however, is observed sometimes at comparative high temperature.As an approach to this phenomenon, the crack opening displacement (COD) concept available in large scale yielding was proposed by A. A. Wells about ten years ago. After that, many papers about mathematical theory of COD concept or engineering criterion of fracture have been published. Nevertheless some problems to be solved have been left still. One of them is how to decide the critical COD for brittle fracture at comparative high temperature, since at such temperature a ductile crack mostly propergates before initiation of cleavage fracture.For this reason, in this paper the initiation of ductile crack was investigated precisely using single-notched and double-notched specimens of 60 kg/mm2 high strength steel in static and dynamic pure bending COD test. As a result, it was made clear that the critical COD for the initiation of ductile crack is independent of temperature and strain rate. Moreover, an influence of notch distance of double-notched specimen on critical COD and distorsion behavior of specimen was studied experimentally and numerically, since double-notched specimen have been used sometimes in COD test. And the most suitable notch distance was obtained, of which the double-notched specimen shows small interaction of notches and distorsion behavior close to single-notched specimen.

Measurements of Electrical Potential for detecting Crack Extension in COD Tests

THE crack opening displacement (COD) test is currently being used by a large number of research establishments throughout Britain and the USA to establish the fracture characteristics of high toughness–low strength steels.