Int. J. Pres. Ves. & Piping 55 (1993) 183-186

On-Line Fracture Mechanics Assessment of the Safety of Structures Based on Ultrasonic Testing

R. Moser , E. T. Till & F. K r e n m a y r

VOEST ALPINE STAHL LINZ GmbH, TurmstraBe 45, PO Box 3, A-4031 Linz, Austria

ABSTRACT

In our company a system has been developed which allows the evaluation of the safety and the remaining life-span of a component, directly from measured ultrasonic data. These data come from the diagnosis system COMSON, which determines the position, size and type of defects in or also outside of welds.

The COMSON system has already been described in several publica- tions, e.g. Ref. 1 contains fundamentals, Ref. 2 describes practical applications. On the PC integrated in the COMSON system a program is installed which uses the defect data of the ultrasonic testing and data about material and geometry of the component as well as loads (which must be specified by the user) to compute the following quantities:

(1) static safety against failure by reaching the load-carrying capacity;

(2) static safety against brittle fracture; (3) static safety according to the two-criteria-method (combined

evaluation of load-carrying capacity and brittle fracture with the 'failure assessment diagram');

(4) remaining life span at cyclic loading for components considered as statically safe.

For the computation of the static safety the maximum values of the cyclic loads are used.

183 Int. J. Pres. Ves. & Piping 0308-0161/93/$06"00 ~) 1993 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland

184 R. Moser, E. T. Till, F. Krenmayr

• G e o m e t r y of comp.

• Loading

cylinder

pressure

axial force axial bending moment hoop bending moment

• Material standard material types or direct specification of material parameters by the user

• D e f e c t t y p e inner, surface or penetrating defect in or outside of the weld

Fig. l. The present range of application of the program.

The program is essentially based on the fracture mechanics recom- mendations of the Osterre ich ischer S tah lbauverband . 3

Figure 1 gives a survey of the present range of application of the program. As can be seen, presently only components of cylindrical form can be evaluated. However, this form will cover a large port ion of all practical cases.

The different types of loads also may appear in combination and in several kinds of cycles (cycle period, maximum, minimum).

Figure 2 shows the possibilities of the input of material properties. If there are welds, the material of the weld may be specified additionally with the same possibilities.

Figure 3 illustrates the defect data from the COMSON system and the parameters for the component geometry.

Based on the COMSON data the program classifies the defect as inner, surface or as penetrating defect. The distinction between

Standard material types 1 - St-37-3 (structural steel) 2 - 42CrMo4V (tool steel) 3 - GS-35CrMoV10.4 (alloyed cast steel) 4 - AlZnMgCu0.5 (aluminum alloy)

Direct specification of material parameters:

lower yield strength tensile strength Young's modulus constant C in Paris-law constant m in Paris-law fracture toughness

Fig. 2. The possibilities of the input of material properties.

Assessment of the safety of structures 185

Fig. 3

Defect data I ~ I defect size in radial direction A _ IQ distance from inner surface C1 distance from outer surface C2 ZL defect size in hoop direction U defect size in axial direction AX distance from edge Q

Component geometry inner radius RI ~ - . ' ~ cylinder thickness T length of cylinder ZL

Defect data from the COMSON system and the parameters for the component geometry.

voluminous and planar defects is not made, always all three dimensions are considered (of course, one of them may be zero or very small). Conservatively, the maximum extent of the defect in each direction is used, that means, for example, an ellipse is considered as rectangle, an ellipsoid as block.

For the computat ion of the safety of the component the extent of the defects normal to the directions of the principal stresses are decisive. Therefore the stress state of the flawless component in the neighbor- hood of the defect would have to be known for a very exact computat ion. Normally this is not possible or at least very expensive. However, in a cylinder the directions of the principal stresses can be assumed to be identical with the radial, axial and circumferential direction in very good approximation. Therefore, defect dimensions in these directions are sufficient. The magnitudes of the stresses are computed from the load data. In addition, the residual stresses and the influence of the form of welded joints may be accounted for by approximation formulas.

Crack growth is computed applying Paris' law. Growth in any direction is treated separately and the change of form or type of defect after each cycle is taken into consideration. The failure criterion is reached when either the safety limit for one of the three above- ment ioned static safeties or a user-specified maximum defect size is exceeded.

The final results are the life span (in cycles and time units), the three safeties and the obtained geometry of the defect.

In this way the system is able to determine on line the actual safety of a component containing defects.

186 R. Moser, E. T. Till, F, Krenmayr

REFERENCES

1. Frielinghaus, R., Krenmayr, F., Spengler, F. & Hackl, M., Werkstoffe & Konstruktion, 3(3) (1989) 312-15.

2. Krenmayr, F., Ponschab, H., Spengler, F. & Frielinghaus, R., Non- Destructive Testing (Proc. 12th World Conference), ed. J. Boogaard & G. M. van Dijk. Amsterdam, The Netherlands, 1989.

3. Osterreichischer Stahlbauverband, Empfehlungen zur bruchmechanischen Bewertung yon Fehlern in Konstruktionen aus metallischen Werkstoffen. 1. Ausgabe, November 1990