Weld, defects fatigue

In the case of the ASME codes for nuclear pressurised components, the questions of fatigue design and of flaw evaluation are dealt with separately in ASME Section III and Section XI Appendix A, respectively. The design S-A curve for machined butt welds typical of thick section pressurised components is set at a factor of two on stress range or twenty on cyclic life, whichever is more conservative, below the mean of S-N data developed on smooth cylindrical specimens in air. (A somewhat similar design curve obtained by a different method from experimental S-A data for machined butt welds is given in British Standard 5500.) These safety factors are intended to encompass any adverse influence of minor weld defects, size effects, data scatter and environment. As far as environmental effects are... 

Lindh Pershak (Ref 5) describe a semi-empirical method foir predicting the effect of weld defects on static and fatigue properties... 

The report of the inquiry [111] criticised the design and fabrication of the alterations made to the original pontoon. The actual cause of the accident was the failure of some tie bars in the detail around the jacking points. The failure was due to brittle fracture which initiated from severe notches such as a small radius curve at the fillet between the spade end and the shank of the tie bar. Weld defects and fatigue cracks were also present in tie bars subsequently recovered from the sea bed. The tie bars had been flame cut to shape and had weld repairs visible to the eye. There had been no post welding heat treatment of the steel. The steel complied with the original specification but tests showed low Charpy V notch impact values. Photo elastic tests indicated a stress concentration factor of 7 at the fillet between the spade end and the shank. The fracture was initiated in the opinion of the inquiry tribunal by the low ambient temperature of around 3°C. 

The use of welding is not recommended, considering the possible occurrence of the phenomenon of fatigue due to any welding defects. 

As the name implies, corrosion fatigue is affected by both the severity of corrosive conditions and mechanical, cyclical stress factors. Stress raisers such as notches, holes, weld defects, or corrosion pits can initiate fatigue cracks and a corrosive environment can reduce crack initiation time. For many materials, the stress range required to cause fatigue failm-e diminishes progressively with increasing time and with the number of cycles of applied stress. 

Recuperator External Area Reliability scaled to external area. Failures related to weld or braze defects, fatigue or micrometeroid strikes. Leak to space. 9999 per HX (.9998 - 200 kW HX)... 

These cannot be considered real welding defects which cause Joint seal problems, but are to be repaired due to the possibility of the start of corrosion or fatigue failure (cuts) or to facilitate subsequent operations of coating and installation (excess weld material)... 

General description. Incomplete penetration describes the condition in which the weld fails to reach the bottom of the weld joint, resulting in a notch located at the root of the weld (Fig. 15.12). This critical defect can substantially reduce the intrinsic mechanical strength of the joint and can combine with environmental factors to produce corrosion fatigue (Chap. 10), stress-corrosion cracking (Chap. 9), or crevice corrosion (Chap. 2). 

Figure 15.18 Examples of crack patterns due to stress-corrosion cracking and corrosion fatigue in butt welds. (Reprinted with permission from Helmut Thielsch, Defects and Failures in Pressure Vessels and Piping, New York, Van Nostrand Reinhold, 1965.)...

The toughness of wood is important in design for exactly the same reasons that that of steel is it determines whether a structure (a frame building, a pit prop, the mast of a yacht) will fail suddenly and unexpectedly by the propagation of a fast crack. In a steel structure the initial crack is that of a defective weld, or is formed by corrosion or fatigue in a wooden structure the initial defect may be a knot, or a saw cut, or cell damage caused by severe mishandling. 

Often contain defects (hydrogen cracks, slag inclusions, stop-start marks). Help initiate fatigue cracks. Critical welds must be tested non-destructively and defects must be gouged out. 

Moulded plastics will also have crack initiation sites created by moulding defects such as weld lines, gates, etc and by filler particles such as pigments, stabilisers, etc. And, of course, stress concentrations caused by sharp geometrical discontinuities will be a major source of fatigue cracks. Fig. 2.72 shows a typical fatigue fracture in which the crack has propagated from a surface flaw. 

These results do not give true values of the number of cycles to fracture and, therefore, true values of fatigue life of welded joints. However, they allow draw a conclusion that performance of welded joints in alloy T110 under cyclic load Omax = 500 MPa does not depend upon the welding method, provided that welds contain no technology defects. 

Fatigue, Fracture Toughness, Fatigue Crack Growth Rate. Fatigue strength of friction stir welded 6082 T4, T6, and T4 -i- 185 °C for 5 h with defect-free welds has been established (Ref 61, 64). The number of cycles to fracture at different stress levels for the T6 and T4 + 185 °C for 5 h conditions is illustrated in Fig. 5.29 (Ref 64). For the postweld heat treated T4 specimens, the number of cycles to failure is slightly below that of T6. This was an unex-... 

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