FRACTURE MECHANICS CRITICAL ASSESSEMENT OF THE STEEL STRUCTURES JOINTS

Dorin Radu 1 Teofil Galațanu 2 UDK: 539.42:624.014.2 DOI:10.14415/konferencijaGFS2017.026 Summary: Elaboration of a methodology for determining the acceptability of detected cracks/flaws in a structure, has a major practical importance in the overall assessment and life integrity of a structure. The relation given by fracture mechanics links a parameter which describes the stress intensity at a crack tip to a material characteristic – fracture toughness. This relation provides the possibility of assessing the fracture conditions of the structural elements with defects (cracks) [1]. The present paper is proposing assessing the structural elements welded joints from the fracture mechanics point of view. The analysis is carried out in accordance with the British Standard procedure BS 7910 ('Guide to methods for assessing the acceptability of flaws in metallic structures')[5]


INTRODUCTION
Most welding fabrication codes specify maximum tolerable flaw sizes and minimum tolerable Charpy energy, based on good workmanship, i.e. what can reasonably be expected within normal working practices.These requirements tend to be somewhat arbitrary, and failure to achieve them does not necessarily mean that the structure is at risk of failure.An Engineering Critical Assessment (ECA) is an analysis, based on fracture mechanics principles, of whether or not a given flaw is safe from brittle fracture, fatigue, creep or plastic collapse under specified loading conditions.An ECA can be used: during design, to assist in the choice of welding procedure and/or inspection techniques; during fabrication, to assess the significance of: a) known defects which are unacceptable to a given code [2], or b) a failure to meet the toughness requirements of a fabrication code; during operation, to assess flaws found in service and to make decisions as to whether they can safely remain, or whether down-rating/repair are necessary.This type of assessing can be done if the following elements are known: material fracture toughness, geometry and size of the crack, resulted stresses from the applied forces.The fracture mechanics based methodologies are permitting the following types of assessments: 1 Assist.Eng.Dorin Radu, Phd., Transilvania University of Brașov, Faculty of Civil Engineering, Turnului 5, Brașov, Romania, tel: +40 268 548 228, email: dorin.radu@unitbv.ro 2 Lect.Eng.Teofil Florin Galatanu, PhD, Transilvania University of Brașov, Faculty of Civil Engineering, Turnului 5, Brașov, Romania, tel: +40 268 548 228, email: galatanu.teofil@unitbv.ro-Maximal crack dimension assessment to which the structural element will not fail, named also the admissible crack dimension; for this type of assessing is needed the maximal stresses values and the value of the material fracture toughness; -Maximal stress value assessment to which the structural element with a crack will not fail -Minimal fracture toughness value assessment to the structural element with a crack; this assessment needs knowing the maximal stress value and the admissible crack dimensions.Considering a simple casea steel plate under tension (figure 1), can be underlined the following types of fracture: Brittle fracturecontrolled by the value of the applied tension force, dimension of the crack, material fracture toughness and geometry of the element; Plastic fracturethe net section in which the yielding phenomena appears, controlled by the applied tension force value, the yielding limit and the element geometry; Rupture as a result of extended material yielding, controlled by the applied tension force, crack size, material fracture toughness and the element geometry.

Figure 1. Describing fractureplate under tension
The transition domain between these types of fracture is governed by the interaction between the brittle fracture and the plastic failure.This is expressed through a dependency relation of two parameters Kr and Sr.These parameters are defined based on the geometrical dimensions of the structural element, crack dimensions and geometry, stresses that appear in the cross section following the applied loads, taken into account the fracture toughness of the material Kmat.

FAILURE ASSESSEMENT DIAGRAMS -FAD 2
The Failure Assessment Diagram (FAD) describes the interaction between the brittle fracture and plastic failure through a Ff = f(Sr) function.Structures using reasonably tough materials (high KIc) and having only small cracks (low K) will lie in the strength-of-materials regime.Conversely, if the material is brittle (low KIc) and strong Sr (high yield strength), the presence of even a small crack is likely to trigger fracture.Thus, the fracture mechanics assessment is a crucial one.The special circumstances that would be called into play in the upper right corner of figure 2 in this regime, a cracked structure would experience large-scale plastic deformation prior to crack extension.The level 2 (FAD-2) assessment is the normal evaluation path for general application.The method is presenting an assessment line given by an equation of a curve and a cut-off line.
If the assessment point is in the interior of the surface limited by the assessment line, the flaw is acceptable and if the assessment point is at the outside area, the flaw is considered unacceptable.
The equations which are describing the assessment line are: The cut-of line is fixed in point where Lr = Lrmax where: σYthe yielding resistance of the material σuthe ultimate resistance of the material For the assessment on level 2 FAD is necessary to pass through the following phases [4]: • Determining the stressesfollowing a structural analysis.The assessments are considering the real distribution of the stresses in the proximity of the flaws -Pm, Pb, Qm and Qb.• The fracture ratio Kr must be determined in which ref σ is obtain according with a relation specific with the flaw type.The point/points of assessment are represented graphically in (Kr, Lr) coordinates on the FAD level 2 [5].
• The evaluation of the position of the point is done according with the specifications done.As a study case, there is presented an existing structurewind turbine column.Following visual inspection and NDT testing, there were discovered several flawscracklike type.An indepth study was needed in order to conclude upon the realibility and safety of the structure.

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Considering the position of the flaw regarding the stress direction and position in the assembly of the steel shell element, it resulted 10 types of assessed flaws as presented in table 1.
The flaws were considered as present in the steel shell elements.In a conservative manner, the W dimension at some part of the flaws types was considered 200mm, taken into account that the area of tension from the segment joint of the steel pillar, is about 200mm.
Increasing the W dimension will decrease the safety of the structure in the area of the joint.
Table 2 -FAD 2in case -flaws assed: geometry and results (with corresponding figures and flaw namefigure 3 and 4)

DETERMINING THE CRITICAL VALUE OF THE FLAW
Determining the critical value of the flaws is important because it serves to a limit value for fatigue further analysis based on fracture mechanics principles, needed for

Conclusions and discussions on the results
There were assessed ten types of flaws (thrugh thickness flaw and edge flaw) which were discovered in the wind turbine pillarin the welded joint of the segment connection,

Figure 4 .
Figure 4. FP-EF -Group of flawsassessmentWith the presented procedure, further assessment can be made on the joint and steel shell element taken into account different dimensions.Figure4and 5 shows the assessment of the flaws grouped.It can be seen that the FP-TTF 5through thickness flawcrack of 30mm in the flange of the segment joint is a critical flawthe assessment point is in the unsafe area of the diagram.Also the FP EF 5edge flaw in the flange of the segment joint is a critical flaw -t15mm flaw is putting under risk the joint and the structure.

Table 1 -
Flaw cases description

Table 3 .
-FAD 2critical dimension of the flaw (with corresponding figures and flaw name)