Residual Stress in Fatigue Analysis
The effect of RS on fatigue behavior of welded element could be analyzed based on experimental studies and/or computation (Figures 1 and 2). One of the examples of computation of the effect of RS on fatigue life of welded elements is presented in Figure 7. These data show how the redistribution of residual stresses will affect the fatigue performance of welded joint. The calculated fatigue curves for the transverse loaded butt weld at R=0 with different levels of the initial RS are shown. The fatigue curve of the welded element will be located between lines 1 and 2 in case of partial relieve of harmful tensile RS (line 3 and line 4). The decrease of the tensile RS from initial high level to 100 MPa causes, in this case, an increase of the limit stress range at N=2×106 cycles from 100 MPa to 126 MPa.
Figure 7. Fatigue curves of transverse loaded butt weld at R=0: 1 – with high tensile residual stresses; 2, 3, 4, 5 and 6 – with residual stresses equal to 0 MPa, 200 MPa, 100 MPa,-100 MPa and -200 MPa
The relieving of the RS in welded element to the level of 100 MPa could be achieved, for example, by heat treatment or overloading of this welded element at a level of external stresses equal to 0.52ay. As a result, this originally fatigue class FAT 100 welded element could be considered after relieve of RS as the fatigue class FAT 125 element in the multi-cycle region (N>106 cycles of loading) [4]. After modification of welding RS, the considered welded element will have an enhanced fatigue performance and, in principle, can be used instead of transverse loaded butt weld ground flush to plate (Structural Detail No. 211) or longitudinal butt weld (Structural Details No. 312 and 313) [4]. Introducing of the compressive RS in the weld toe zone can increase the fatigue strength of welded elements even to a larger extend (line 5 and line 6 in Figure 7).
The results of computation presented in Figure 8 show the effect of the redistribution of RS by the UP on the fatigue life of welded joints in steels of different strength. The data of fatigue testing of non-load-carrying fillet weld specimens in as-welded condition (with high tensile RS) were used as initial fatigue data for calculating the effect of the UP. The fatigue strength of certain welded element in steels of different strength in as-welded condition is represented by a universal fatigue curve [2,4].
Figure 8. Fatigue curves of non-load-carrying fillet welded joint: 1 – in as-welded condition for all types of steel; 3, 5, 7 and 9 – after application of the UP to Steel 1,Steel 2, Steel 3, and Steel 4
Four types of steels were considered for fatigue analysis: Steel 1 – (ay = 270 MPa, au = 410 MPa); Steel 2 – (ay = 370 MPa, au = 470 MPa); Steel 3 – (ay = 615 MPa, au = 747 MPa) and Steel 4 – (ay = 864 MPa, au = 897 MPa). Line 1 in Figure 8 is the universal fatigue curve of considered welded joint for all types of steel in as-welded condition, determined experimentally. Lines 3, 5, 7 and 9 are the calculated fatigue curves for the welded joint after application of the UP for Steel 1, Steel 2, Steel 3 and Steel 4, respectively.
As can be seen from Figure 8, the higher the mechanical properties of the material – the higher the fatigue strength of welded joints after application of the UP. The increase in the limit stress range at N=2×106 cycles under the influence of UP for welded joint in Steel 1 is 42%, for Steel 2 – 64%, for Steel 3 – 83% and for Steel 4 – 112%. These results show a strong tendency of increasing the fatigue strength of welded elements after application of UP with the increase in mechanical properties of the material used.
Residual Stress Modification. Ultrasonic Peening
One of the new and promising processes for effective redistribution of RS is Ultrasonic Peening (UP) [1, 6, 11-13]. During the different stages of its development the UP process was also known as ultrasonic impact treatment (UIT) [14-16] and ultrasonic impact peening (UIP) [17-18]. The beneficial effect of UP is achieved mainly by relieving of harmful tensile RS and introducing of compressive RS into surface layers of metals and alloys, decreasing of stress concentration in weld toe zones and enhancement of mechanical properties of the surface layers of the material. The fatigue testing of welded specimens showed that the UP is the most efficient improvement treatment as compared with traditional techniques such as grinding, TIG-dressing, heat treatment, hammer peening, shot peening etc. [2, 11-13].
The UP technique is based on the combined effect of high frequency impacts of special strikers and ultrasonic oscillations in treated material. The developed system for UP treatment (total weight ~11 kg) includes an ultrasonic transducer, a generator and a laptop (optional item) with software for UP optimum application – maximum possible increase in fatigue life of parts and welded elements with minimum cost, labor and power consumption. In general, the basic UP system shown in Figure 9 could be used for treatment of weld toe or welds and larger surface areas if necessary.
Figure 9. Basic UP-600 system for fatigue life improvement of parts and welded elements
Figure 10 shows the basic set of working heads for different applications of UP. The working head could be easily replaced, if necessary. Six different working heads are provided with the standard UP package:
- one four-pin working head with the pin’s diameter of 3 mm,
- one three-pin working head with the pin’s diameter of 4 mm,
- one "zig-zag" four-pin head with the pin’s diameter of 4 mm,
- one single-pin working head with the pin’s diameter of 4 mm,
- one seven-pin working head with the pin’s diameter 5 mm,
- one two-pin working head with the pin’s diameter of 3 mm.
The UP could be effectively applied for fatigue life improvement during manufacturing, rehabilitation and repair of welded elements and structures [11-13]. The results of fatigue testing of large-scale welded samples imitating the transverse non-load-carrying attachments where the UP was applied to specimens in as-welded condition and also after 50% of expected fatigue life are presented on Figure 11. The UP caused a significant increase in fatigue strength of the considered welded element for both series of UP treated samples. The increase in limit stress range (at N=2-106 cycles) of welded samples is 49% (from 119 MPa to 177 MPa) for UP treated samples before fatigue loading and is 66% (from 119 MPa to 197 MPa) for UP treated samples after fatigue loading, with the number of cycles corresponding to 50% of the expected fatigue life of the samples in as-welded condition. The higher increase of fatigue life of UP treated welded elements for fatigue curve #3 could be explained by a more beneficial redistribution of RS and/or "healing" of fatigue damaged material by UP in comparison with the fatigue curve #2.
Figure 10. Basic set of the changeable working heads
Figure 11. Fatigue curves of welded elements (transverse non-load-carrying attachment): 1 – in as welded condition, 2 – UP was applied before fatigue testing, 3 – UP was applied after fatigue loading with the number of cycles corresponding to 50% of expected fatigue life of samples in as-welded condition.
The developed computerized complex for UP was successfully applied in different applications for increasing of the fatigue life of welded elements, elimination of distortions caused by welding and other technological processes, residual stress relieving, increasing of the hardness of the surface of materials and surface nanocrystallization. The areas/industries where the UP was applied successfully include: Railway and Highway Bridges, Construction Equipment, Shipbuilding, Mining, Automotive and Aerospace.
Summary
1. Residual stresses play an important role in operating performance of materials, parts and welded elements. Their effect on the engineering properties of materials such as fatigue and fracture, corrosion resistance and dimensional stability can be considerable. The influence of residual stresses on the multi-cycle fatigue life of welded elements can be compared with the effects of stress concentration. The residual stresses, therefore, should be taken into account during design, fatigue assessment, manufacturing and repair of welded elements and structures.
2. Certain progress has been achieved during the past few years in improvement of traditional techniques and development of new methods for residual stress management. A number of new engineering tools for residual stress management such as ultrasonic computerized complex for residual stress measurement, technology and equipment for ultrasonic penning and software for analysis of the effect of residual stresses on the fatigue life of welded elements and structures were recently developed and verified for different industrial applications.
3. The beneficial redistribution of the residual stresses is one of the efficient ways of fatigue improvement of welded elements. The ultrasonic peening is the most effective and economical technique for increasing of the fatigue strength of welded elements in materials of different strength. The higher the mechanical properties of treated materials – the higher the efficiency of ultrasonic peening application. It allows using to a greater degree the advantages of the high strength material application in parts and welded elements, subjected to fatigue loading.
