CO;2-K [ 5 ] Gu, H. Materials and Design, 30, Composites: Part A, 41, H, Reddy Rasad, D. Kim, M. Composites: Part B, 43, Composites Science and Technology, 58, Composites Science and Technology, 51, Micromechanical Modelling. Polymer, 42, Composites, 25, Composites: Part A, 32, Phase Interaction in Composite Materials, Composites Science and Technology, 57, Standard Practice for Conditioning and Testing Textiles. American Society for Testing and Materials, Pennsylvania. Journal of Polymers and the Environment, 15, Composites Science and Technology, 61, Surface Review and Letter, 14, International Journal of Fracture, 37, Fracture Mechanics.
Mixed-Mode Crack Behavior
Recommend to Peers. Recommend to Library. Contact Us. This procedure cuts out part of the crack front and leaves a straighter crack front. In many cases, no single procedure overcomes the problem; therefore, it is necessary to use a mixture of them. Local compression over the lateral sides of the crack path is recommended to modify the residual stress state in fracture toughness specimens [ 7 ]; however, the fracture toughness could be reduced by using this method [ 57 ].
However, this is the best option available to modify the residual stresses [ 12 ]. Meith et al. The applied load depends of the chosen device, thickness and the geometry of the specimen, and yield stress of the material. Figure 1 shows the SE B specimens geometry and dimensions from a API X80 steel two-passes FSW joint; more details about the welding procedures and fracture toughness results could be consulted elsewhere [ 59 ].
The notches were located at the stirred zone within the FSW joint. Figure 1a shows the effect of residual stresses of a frictions stir welded joint FSW during the precracking procedure, where the crack front did not comply the standard. The test is performed using a monotonic load with a constant displacement rate, commonly 1. When the test is done, for the conventional clip gauges the curve of force vs. CMOD is generated, which is then used to determine the respective fracture toughness parameter. From the complete analysis, the onset of the stable and unstable crack growth can be determined.
If the purpose of the test is to determine the size of the crack for each load and fracture toughness parameter value, as well as toughness in each stage, the best option is creating a crack growth resistance curve R curve. However, materials showing high plastic behavior cannot be used to reach a complete R-curve because the crack will not grow. This problem is exacerbated when testing thin pipeline [ 30 , 60 , 61 ]. A complete experimental description of the R-curve is reported in chapter 7 of [ 12 ].
During the fracture toughness test it is recommended to use the closest temperature possible to the real application. With decreasing temperature, most of the metals increase the constraint at the crack tip because of the suppression of the plastic deformation mechanism under low-temperature conditions; hence, in such materials the likelihood of cleavage increases with decreasing temperature. The fracture toughness depends of thickness at the ductile-brittle transition region, and thick specimens presented lower toughness than thin specimens; therefore it is not safe to predict fracture toughness in that region by using thin specimens [ 62 ].
Another technique is to carry out the test in an environmental chamber with a chilled nitrogen gas atmosphere. It is highly recommend measuring the temperature directly on the test specimen. The test parameter definitions are well explained in [ 12 ]. Regarding CTOD tests showing tunneling, both plane strain and plane stress coexist in different parts of the crack front, which means that the plastic region in front of the crack tip is bigger than that presented in only plane strain, and its size changed according with each position of the crack front.
Therefore, toughness tests undergoing those conditions presented overestimate toughness results, and did not fulfill the standards consideration of straight crack, where only a plane strain state is accepted. Thus, the real toughness behavior of each selected notch within the welded joints is hidden by the low-triaxiality effect. On the other hand, fatigue crack fronts with curve shape provide also valid fracture toughness in terms of whether the tested notches were still sensible of the microstructure ahead the crack tip.
Other authors assessing fracture toughness of FSW welded joints of steels have reported fatigue crack fronts with curve shape [ 33 , 34 ] while others have not [ 63 , 64 ]; for both cases, differences in toughness of the different evaluated regions are appreciable. Those results that do not present a crack tip in a plane strain state are no able to be compared quantitatively, but are able to be compared qualitatively; unless, the effect of the curve fatigue crack front in the CTOD results are determined.
It is highly recommended that new studies concerning fracture toughness assessment of FSW welded joints in steels use relief stresses methods, as describe in [ 7 , 8 ]. In order to analyze the fracture surface and to measure the crack size, it has to be exposed without damage or distortion. Figure 4 shows an SE B broken specimen by impact after being submerged in a liquid nitrogen, that region was marked as cleavage CL. Alternatively, fatigue cracking could be used to mark or break the remaining ligament without affecting the crack test surface. For brittle materials FCG-F is recommended, because this procedure allows identifying the size of the stable crack growth region [ 65 ].
When post-test cleavage or fatigue cracking are not suitable, it is recommend to use alternative methods, e. The higher the temperature the better the contrast, however, this procedure could change the microstructure completely, hindering microstructural analysis. Another option is to mark the specimens by using penetrant liquid as it is used for non-destructive examination; the photos of the fracture surface must be taken as soon as the specimen is broken open, because the penetrant can spread on the fracture.
A normal industrial ink might reproduce the same result of the former procedure; moreover this product presents a fast drying time and a permanent mark. In order to maintain the fracture surface in a conserved state it is recommended to use a protective film after the fractography analysis, nevertheless particles inside the fracture surface might be taken away when cleaning the surface.
CTOD: 0. In some cases, the origins of cleavage over the fracture surface could be identified by using electron microscopy [ 66 ] or even optical microscopy [ 67 ] which, in the heat affected zone of steel cleavage, starts mostly in brittle particles [ 68 ]. However, the complete characterization of those brittle particles needs to be done by using auxiliary techniques, such as Energy dispersive spectroscopy EDS [ 68 , 69 ] and transmission electron microscopy TEM.
In modern scanning electrons microscopes, there is an available technique named focus ion beam milling which allows to cut out specific region of the study and transform it into a TEM sample [ 70 ], therefore it could be very useful in order to isolate and analyze cleavage initiator particles [ 71 ].
Transversal cuts are recommended in order to determine the crack path over the microstructure; however, care must be taken when choosing the studied area. This procedure is important to confirm whether the crack grows through a specific microstructure, for example in a heat affected zone assessment [ 72 ]. Regarding the sectioning technique in weldments samples, consult the following references [ 12 , 67 , 72 , 75 , 76 ].
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Additional microstructural technique such as electron backscattered diffraction EBSD analysis could be used to determine the orientation of the grains where the crack grew [ 74 , 77 - 79 ], in addition, whether other deformation mechanism was activate such as twinning [ 80 ]. It is recommended to report the direction, sense, depth and location of the notches, specimen geometry, precracking and test parameters, size and shape of the crack, and whether the residual stress were modified.
Chemical content and microstructural features might be useful for other readers to compare results. It is important to carefully avoid using wrong information on materials properties, because this could lead to ignoring the mismatch of properties between the weld joint and base material. The fracture toughness in the pipeline industry has been assessed the most by using CTOD tests. API [ 84 ] considers the lower limit to be between 0. According to the pipeline application there is a trend to consider a range of critical CTOD values between 0.
Fairchild et al. Regarding the critical transition temperature T criterion in CTOD tests, it is suggested to choose the temperature T 0. Pop in: When the load vs. CMOD curve shows a sudden drop, the curve is considered to exhibit a pop-in, that is, the sudden load drop is considered to be a consequence of a huge increment of crack growth, or possibly a large delamination of the material.
ASTM-E standard [ 6 ] provides mathematical tools that allow users to determine the magnitude of the pop-in, and whether the pop-in must be considered or may be ignored. On the other hand, pipeline steel often presents pop-in because of delaminations. However, in most of the delamination event cases the crack continues the growth in stable fashion, which could be permitted in some engineering cases [ 88 ]. Delaminations: Delaminations are also known as slips; those are common in HSLA steels and are located in the mid thickness.
They could be associated with the impurities segregation line of the plates and the high stress concentration at the crack tip, where the material at the center could suddenly suffer a crack growth in a brittle fashion. Delamination could diminish the ductile-brittle temperature and lead to high toughness results [ 89 ].
Other causes of delaminations in pipeline steels are crystallography texture [ 90 ], segregation of phosphor and sulfur, microstructure anisotropy, microstructural banding, non-metallic particles and inclusion alignment [ 78 ], atomic impurities and carbides at the tip of long grains [ 89 ]. Lerech [ 91 ] argues that delaminations due to microconstituent martensite-austenite M-A is a statistical issue, since it depends more on from where the specimen is taken. Most of the HSLA steels presents banded microstructure due the lamination process [ 90 ]; the bands are composed mainly of polygonal ferrite and pearlite, bainite and M-A, therefore an anisotropy is included in the material and this impede the optimal performance of the steel in pipeline applications.
Moreover, the hoop direction presents higher toughness than longitudinal direction [ 92 ]. In a API X70 it was shown that the thicker the specimen the more severe the delamination [ 88 ].
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However, the crack growth did not stop there; it maintained a constant growth until it reached an unstable crack growth. Test validation or qualification: The validation of the test results is guided by standards which review three aspects: 1 Geometry, dimensions and tolerances of the specimen and fixtures; 2 Fixture and specimen alignment, as well as the displacement rates, load, stress-intensity factor and temperatures during the precracking and testing; and 3 The crack front locus before the fracture toughness test should be plane, at least with the standard requirements, see section 2.
If all conditions listed are fulfilled the test result is valid, otherwise the result could not be considered. Equation 1 from reference [ 29 ] presents the recommended way to estimate the average of the crack size, where 9 measurements equally spaced are taken a 1 , …, a 9. In addition, the a 0 must satisfy 0. It is recommended to review the requirement for the crack size measurement from the particular standard being used [ 3 , 4 , 6 , 29 ].
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It could be especially useful for new researchers in this area. In addition, it contains an extensive literature review, which allows the reader to locate directly some important issues from the original sources. The authors would like to acknowledge the financial support of the Colciencias for granting the scholarship No. Engineering Fracture Mechanics. BS fracture mechanics toughness tests. London: BSI; ISO metallic materials: determination of plane-strain fracture toughness. Analytical and experimental study of fracture in bend specimens subjected to local compression. Fatigue and Fracture Mechanics.
State of the art for use of SENT specimens to test fracture properties in pipes for reeling operations. Trondheim: Norwegian University of Science and Technology; Elastic-plastic fracture mechanics where has it been? Where is it going? Fatigue Fract. Fracture mechanics: fundamentals and applications.
Deformation and fracture mechanics of engineering materials. Hoboken: Wiley; Mechanical behavior of materials.
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Upper Saddle River: Prentice Hall; Developments in the application of the CTOD approach to fracture assessment of welded construction. BS guide to methods for assessing the acceptability of flaws in metallic structures. Evaluation of fracture toughness of nuclear piping using real pipe and tensile compact pipe specimens.
Nuclear Engineering and Design.
Constraint dependence of the fracture toughness of reduced activation ferritic—martensitic Eurofer steel plates. Key Engineering Materials. BS method of test for determination of fracture toughness in metallic materials using single edge notched tension SENT specimens. Irving; Fracture testing of the heat affected zone from welded steel pipes using an in situ stage. Experimental Mechanics. Ductile tearing assessment of high-strength steel X-joints under in-plane bending. Engineering Failure Analysis. ISO metallic materials: unified method of test for the determination of quasistatic fracture toughness.
Subsea pipeline engineering. Tulsa: PennWell Publishing; Provo: Brigham Young University; [access 26 feb. Determination of microstructural criterion for cryogenic toughness variation in actual HAZs using microstructure-distribution maps. Materials Science and Engineering A. Fracture testing of welded single edge notch tensile specimens. Welding Journal. Fracture toughness of friction hydro-pillar processing welding in C-Mn steel. Campinas; Evaluation and interpretation of ductile crack extension in SENT specimens using unloading compliance technique.
A multi-tiered procedure for engineering critical assessment of strain-based pipelines.
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Determination of CTOD resistance curves in side-grooved single-edge notched tensile specimens using full field deformation measurements. Optics and Lasers in Engineering. Investigation of fracture in small-scale SENT tests of a welded X80 pipeline steel using Digital image correlation with node splitting. CTOD-R curve construction from surface displacement measurements. Journal of Testing and Evaluation. ASTM-Ee1: standard test method for measurement of fatigue crack growth rates. Towards a uniform precracking procedure for fracture toughness testing. International Journal of Fracture.
Friction stir welding of HSLA steel. Part II: the influence of weld speed and tool material on the residual stress distribution and tool wear. Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science. Using the instrumented indentation technique for stress characterization of friction stir-welded API X80 steel.