C.M. Branco

Influence of weld and plate geometry on the fatigue strength of cruciform joints

Abstract Results of fatigue life prediction in cruciform joints are reported. Using Fracture Mechanics methods, fatigue life was computed as a function of initial flaw size, plate length and thickness, weld geometry and loading mode. The Bueckner weight function method was used for computing the stress intensity factor using a 2-D finite element program. The results have shown that plate length has little or negligible influence on fatigue strength provided that the plate length to thickness ratio is below 20 to 50 depending on the plate thickness. The attachment thickness to main plate thickness ratio also has little or no effect on fatigue strength. The plate thickness itself and the loading type (tension and bending), however, do affect the fatigue. Their effects are quantified and introduced into the SN-curve for design.

Fatigue performance of tungsten inert gas (TIG) and plasma welds in thin sections

Abstract The greatest potential for the use of TIG and plasma welding is in the joining of thin sections, less than 10 mm thickness. This may introduce an additional benefit from the fatigue viewpoint, since fatigue strength is expected to increase with a decrease in plate thickness. Superior fatigue performance was confirmed for TIG and plasma transverse butt, cruciform and non-load carrying fillet welds of a carbon-manganese steel, all failing from the weld toe, justifying a one class increase in Eurocode 3. Weld details, which failed by fatigue cracking in the weld throat, showed no influence of welding process. Extensive measurements of the weld toe geometries confirmed that TIG and plasma welds had more favourable profiles than MMA welds, with lower weld toe angles and larger weld toe radii. Fracture mechanics modelling confirmed that the differences in weld toe geometry were consistent with the differences in fatigue life actually observed. Weld toe radius appeared to be more significant than weld toe angle.

Influence of fillet weld joint geometry on fatigue crack growth

Abstract Investigated in this work are the effects of fillet weld joint geometry on fatigue crack growth. The main plate of the weldment is subjected to three-point bending while the attachment experiences either tension or bending. A two-dimensional elastic-plastic analysis is performed to find the local stress as a function of the initial defect size. Numerical results are obtained for different curvature ϱ of the weld toe and ratios of T/B where T and B stand, respectively, for the thickness of the attachment and main plate. Based on the open literature data for mild steel, two different locations are assumed for calculating crack growth. Constant fatigue life data are obtained for cracks with an initial length of 0.15 mm in the attanchment or main plate. Results are presented for different sizes of the fillet weld joints.

High temperature fatigue crack growth in Inconel 718

Abstract The nickel base superalloys are extensively used in high temperature applications, so it is important to know their behaviour under conditions of high-temperature fatigue. This paper studies the influence of ΔK, loading frequency, stress ratio and temperature on the high temperature fatigue crack growth rate of nickel base superalloys. This study is based on fatigue tests carried out in corner crack specimens of Inconel 718 at 600°C and at room temperature. Three stress ratios (R = 0.05, 0.5 and 0.8) and loading frequencies ranging from 0.0017 to 15 Hz were considered in the tests. For frequencies below 0.25 Hz, the load wave shape was trapezoidal with different dwell times at maximum load. At relatively high frequencies the propagation is cycle dependent, while for lower frequencies it is time dependent. At intermediate frequencies a mixed crack growth occurs. The transition frequencies from cycle dependent to mixed regime and from mixed to time dependent regime were obtained for each R. The increase of R increases the transition frequencies, i.e., extends the time dependent crack growth to higher frequencies. The increase of R also produces an increase of cyclic crack growth rate for all regimes of crack growth. In the time dependent regime, a higher variation is observed, that can be explained by an acceleration of oxidation damage promoted by the increase of maximum stress. An approach for modelling the high-temperature fatigue crack growth in nickel base superalloys is presented. A good agreement was observed between time dependent fatigue results and mathematical models based on static load results.

A failure analysis study of cast steel railway couplings used for coal transportation

Abstract This paper presents the results obtained in a failure analysis study carried out in cast steel railway couplings of the trains of one coal transportation line. A significant number of failures have occurred in these components which led to great losses and disruption of services. The type of failure was mainly fatigue cracking. This paper presents the results from detailed microstructural and fractographic analysis. Strain gauge data were analyzed and from this data fatigue cycles were derived. Cumulative frequency diagrams of stress range and mean stress were obtained and fatigue damage values were calculated. Damage factors were obtained in several zones of the coupling and for the different stages of the journey and loading conditions. Results of a 3D stress analysis obtained with a FE program are presented. The values of the principal and equivalent stresses were obtained in the critical areas of the coupling. Stress concentration factors were calculated from the computed stress distribution and for the critical areas of the coupling.

Characterization of Powder Metallurgy (PM) Nickel Base Superalloys for Aeronautical Applications

During the last decade, some major improvements have been achieved concerning the evaluation of new types of materials suitable for aeronautical components exposed to severe operational conditions, such as turbine disks. Due to their outstanding mechanical properties, nickel base superalloys assumed a preferential position when compared with other conventional metallic alloys, benefiting from both their superior fatigue strength and high temperature behaviour. However, these alloys evince a high sensibility concerning possible defects that can arise due to certain types of loading, such as porosities and cavities associated with creep-fatigue at high temperatures. The present paper compiles some experimental results obtained for two types of recent nickel base superalloys. Some fatigue tests were performed using two configurations of these materials: a set of Udimet 720Li specimens (CT geometry) and a set of RR1000 specimens (CN geometry). A maximum temperature of 650°C was considered in both types of materials. The mechanical properties of these alloys were inferred via typical FCGR parameters, such as da/dN vs K curves, complemented with detailed analyses of the cracking mechanisms based on SEM observations. Finally, some metallographic characterization tests were carried out in order to determine the average grain size of these PM alloys and to confirm the presence of important microstructural constituents that can influence the overall fatigue performance of these materials.

A failure analysis study of wet liners in maritime diesel engines

Abstract This paper presents the results of a study regarding the failure mechanisms detected in alloyed cast iron liners of marine engines. The liners have shown cracks and extensive corrosion after about 1200 service hours, located in the adjustment flange between the liners in the cylinder block. This study includes the material characterisation using chemical, microstructural, hardness and fractographic analysis. The metallurgical results have shown that the material was acceptable concerning the composition and microstructure. The paper also presents the results obtained for the distribution of stresses and temperatures in the critical areas of the cylinder liner. A 2D finite element code was used for this study, using axisymmetric elements with reduced integration. The results have shown that the thermal stresses are significantly more important than the mechanical stresses, the latter being due exclusively to the effect of the combustion gases in the cylinder. For the two geometries which were analysed, the peak stresses were slightly above 100 MPa in tension. This value of stress is high for the cast iron of the liners, and leads to a great potential risk of initiation and propagation of the tiny surface corrosion defects found in these zones. The detailed geometric changes introduced in the second design of the liner gave a little improvement in the stress distribution in the critical areas, although this may not be enough to avoid cracking conditions in the critical areas.

A fracture mechanics based fatigue life prediction for welded joints of square tubes

Abstract Square tubes of mild and low alloy steels are extensively used in vehicle structures and offshore platforms. Despite this fact the fatigue behaviour of welded nodes of these tubes is not sufficiently known, especially in thin sections. The paper presents the results of fatigue life predictions based on fracture mechanics, in non-load carrying T and also load carrying cruciform joints. The fatigue life prediction models include crack initiation and crack propagation. The stress intensity factor formulations were obtained using the weight function solution for semi-elliptical surface cracks with the stresses computed using a three-dimensional finite element program. The prediction results are compared with experimental results and a good agreement was obtained. The tube material was mild steel St 37 (DIN 17100). The loading modes were in-plane three point bending and cantilever bending or tension.

Fatigue behaviour of a phenolic matrix composite

Abstract This paper presents results of a fatigue life investigations carried out in plate specimens of a fibre-glass-reinforced phenolic matrix composite. Tensile and Youngs modulus data were obtained at four different testing temperatures (room temperature, 100, 150 and 200 °C). The fatigue S−N data were obtained at room temperature only and for two stress ratio values (R = 0 and 0.4). Fatigue and tensile behaviour was assesesed in the composite with the fibres aligned in the longitudinal loading direction. The results were obtained for two values of volume fraction (0.28 and 0.42) and three different glass surface treatments. A detailed comparison of fatigue results is given taking into account several fatigue parameters and also the testing variables. Results of observations of SEM fracture surfaces are also presented.

The Effect of Test Conditions on the Fatigue Properties of a Phenolic Composite

The paper reports experimental work carried out on a composite with a phenolic matrix and reinforced with glass fiber. The fatigue results were obtained on both unidirectional fiber composites and crossed fiber composites. For unidirectional fiber composites the two glass fiber treatments were investigated. All the fatigue tests were carried out on a mechanical fatigue machine equipped with frequency control and a furnace. The unidirectional composites were tested in the frequency range from 1.5 to 25 Hz and in the temperature range from 20 to 200°C. The crossed composites were tested in the frequency range from 2 to 20 Hz at 20°C. The paper discusses and draws conclusions about the fatigue damage in terms of variation of Youngs modulus, the influence of the temperature and frequency on the fatigue strength and surface temperature variation during testing.