Residual life establishment of welded joints of construction steels.

Ulrich, Koloman ; Karvanska, Silvia ; Kovarikova, Ingrid 等

1. INTRODUCTION

Fitness-For-Service (FFS) assessments are quantitative engineering evaluations which are performed to demonstrate the structural integrity of an in-service component containing a flaw or damage. In the Fitness- For-Service assessment procedures include analytical procedures, material properties including environmental effects, Nondestructive testing and documentation requirements. Residual life can be estimated based on the quality of available information, assessment level, and appropriate assumptions to provide an adequate safety factor for operation until the next scheduled inspection(1).

2. FATIGUE CRACK GROWTH

Crack Growth by Fatigue--Crack growth by fatigue occurs when a component is subject to time varying loads which result in cyclic stresses. Each increment of crack extension correlates to a certain increment of stress cycles. Linear elastic fracture mechanics (LEFM) has been validated to relate the crack growth per cycle to the applied stress intensity range through a fatigue crack growth law. The simplest and most common form of fatigue crack growth law is the Paris-Erdogan Equation(2,3):

da /dN = C. [DELTA][K.sup.m] (1)

More advanced forms of fatigue crack growth laws which take explicit account of such factors as stress ratio, ranges of [DELTA]K (4), effects

of a threshold stress intensity factor, [DELTA][K.sup.th], and plasticity-induced crack closure are available for certain materials and environments. This law should be considered in an assessment based on the applied loading, crack configuration, and service environment. The variation of fatigue crack growth rate with cyclic stresses which produce a range of [DELTA]K and the associated fracture mechanisms is shown in Figure 1(5).

3. RESIDUAL LIFE EVALUATION

FFS assessment modules require in general, for the components in-service the following interdisciplinary inputs:

* Description/knowledge of damage

* Determination of operating conditions,

* Flaw characterization

* Material properties.

* For residual life calculation we need following data:

1. mechanical properties:

--fracture toughness [K.sub.IC], [J.sub.IC]

--da/dN

--C,m parameters

--[R.sub.e], [R.sub.m], etc.

2. Nondestructive testing--estimate tolerable defect size [a.sub.D]

3. Type of defect.

[FIGURE 1 OMITTED]

For residual life calculation was established following defect size (2):

[a.sub.D] - [c.sub.D] ... tolerable defect size

[a.sub.L] - [c.sub.L] ... limit defect size in term of repair

[a.sub.z] - [c.sub.z] ... defect size at the end of construction life

[a.sub.c] - [c.sub.C] ... critical defect size

[a.sub.D] [less than or equal to] [a.sub.L] [less than or equal to] [a.sub.z] < [a.sub.C].

FFS assessment results aim to provide information on material selection and hence the most suitable fabrication route for safe and economical performance. Analysis results can yield, for example, the required minimum fracture toughness for a given loading conditions and postulated defect size or can provide maximum tolerable defect size (e.g., weld imperfection) for a given material, loading conditions and fabrication route. Again an applicability of the FFS analysis in an efficient manner to support the fabrication route and quality assurance in addition to the conventional good workmanship principles will depend on the capability of the applied NDT technique and its probability of detection of a flaw.

Analysis results can yield, for example, the required minimum fracture toughness for a given loading conditions and postulated defect size or can provide maximum tolerable defect size (e.g., weld imperfection) for a given material, loading conditions and fabrication route. Again an applicability of the FFS analysis in an efficient manner to support the fabrication route and quality assurance in addition to the conventional good workmanship principles will depend on the capability of the applied NDT technique and its probability of detection of a flaw.

Analysis results can yield, for example, the required minimum fracture toughness for a given loading conditions and postulated defect size or can provide maximum tolerable defect size (e.g., weld imperfection) for a given material, loading conditions and fabrication route. Again an applicability of the FFS analysis in an efficient manner to support the fabrication route and quality assurance in addition to the conventional good workmanship principles will depend on the capability of the applied NDT technique and its probability of detection of a flaw.

Analysis results can yield, for example, the required minimum fracture toughness for a given loading conditions and postulated defect size or can provide maximum tolerable defect size (e.g., weld imperfection) for a given material, loading conditions and fabrication route. Again an applicability of the FFS analysis in an efficient manner to support the fabrication route and quality assurance in addition to the conventional good workmanship principles will depend on the capability of the applied NDT technique and its probability of detection of a flaw.

4. FATIGUE CRACK GROWTH MEASURING RESULTS OF LASER WELDED JOINS

Measuring was made by following construction steels: GL-B, GL-D, GL-A, Hardox 400, which are used for ship production.

[FIGURE 2 OMITTED]

5. CONCLUSION

For the application of this FFS procedure to optimize the design of a new construction, it is usually a postulated defect is used to assess the critical condition of a new construction for a given material, load/stress conditions and geometry of the components. In this context, a postulated initial defect size will be based on the non-destructive detection. Depending on the design philosophy in combination of the NDT technique is used as well as good description of the loading conditions of a new construction, FFS can provide information for re-selection of material, design and fabrication route and NDT technique.

FFS Procedure can be used to establish the size limits for defects in various metallic engineering structures can provide substantial cost savings in operating such structures. FFS methodology can provide an engineering analysis to predict the critical condition of a new component using either postulated larger defect or defect size defined by the NDT detection limit and hence can give opportunity to the designer for possible reselection of failure criteria (i.e., against crack initiation or allowance for crack growth with respective inspection intervals), design load, material type or fabrication route (3).

6. ACKNOWLEDGMENT

The contribution was prepared under the support of APVV-0057-07, VMSP-P-0008-07, APVT20-020904 and APVT-99-011004 projects.

7. REFERENCES

Kocak, M. et al. (2006). FITNET Fitness-for-Service Procedure. Final draft MK7. Prepared by European Fitness-for-Service Thematic Network--FITNET

Karvanska, S. (2007). Laser welded joints of construction steels evaluation according to "Fitness-For-Service", Dissertation, MtF STU Trnava

Karvanska, S., Ulrich, K., Polak, P. (2007). Fatigue crack growth in welded joints prepared by methods MMAW, SAW and GMAW (MAG). In Materials Science and Technology [online].--ISSN 1335-9053. Roc. 7, c.2 (2007)

Ulrich, K. (2002). Fatigue crack growth in steel welded joints. Zeross, Ostrava 2002. ISBN 80-85 771-82-9

*** API RP 579: Fitness-for-Service, Washington 2000, D.C. 20005-4070

Tab. 1. Parameters from fatigue crack growth testing for
laser welded joints of steels

 Type of steel Paramater C Parameter m

GL-B ZK 1,766E-10 4,0658
GL-A ZK 6,592E-13 5,370
GL-D ZK 1,297E-11 4,765
HARDOX 400 ZK 4,158E-09 2,927