Dynamic test and fatigue life evaluation of compressor blades

High-cycle fatigue (HCF) has been identified as one of the primary causes of gas turbine engine failure. To verify the reliability of the high cycle fatigue fracture of the 5 MW gas turbine engine blade being developed by Doosan Heavy Industries & Construction Co., Ltd., dynamic tests were conducted using real size compressor rigs according to previous studies. The dynamic safety margin of the 5MW gas turbine engine blade was calculated on the basis of the ratio between the dynamic stress and endurance limit stress respectively determined through the compressor rig and fatigue tests. The HCF characteristics and the fatigue life stability of the DGT-5 compressor blades were verified through these processes. A fatigue life design procedure for the gas turbine compressor blade was established on the basis of the design, analysis, and test processes implemented in a previous study. In sum, the 5 MW class gas turbine compressor blades were found to be well designed in terms of resonance stability and fatigue life.     

Evaporation heat transfer and pressure drop characteristics of r-134a in the oblong shell and plate heat exchanger

The evaporation heat transfer coefficienthr and frictional pressure drop δp_f of refrigerant R-134a flowing in the oblong shell and plate heat exchanger were investigated experimentally in this study. Four vertical counterflow channels were formed in the oblong shell and plate heat exchanger by four plates of geometry with a corrugated sinusoid shape of a 45° chevron angle. Upflow of refrigerant R-134a boils in two channels receiving heat from downflow of hot water in other channels. The effects of the refrigerant mass flux, average heat flux, refrigerant saturation temperature and vapor quality of R- 134a were explored in detail. Similar to the case of a plate heat exchanger, even at a very low Reynolds number, the flow in the oblong shell and plate heat exchanger remains turbulent. The results indicate that the evaporation heat transfer coefficienthr and pressure drop Δp_f increase with the vapor quality. A rise in the refrigerant mass flux causes an increase in theh _r and Δp_f. But the effect of the average heat flux does not show significant effect on the h_r and Δp_f. Finally, at a higher saturation temperature, both theh _r and Δp_f are found to be lower. The empirical correlations are also provided for the measured heat transfer coefficient and pressure drop in terms of the Nusselt number and friction factor.     

Modal characteristics and fatigue strength of compressor blades

High-cycle fatigue (HCF) has been identified as one of the primary causes of gas turbine engine failure. The modal characteristics and endurance strength of a 5 MW gas turbine engine blade developed by Doosan Heavy Industries & Construction Co., Ltd. in HCF fracture were verified through analysis and tests to determine the reliability of the compressor blade. A compressor blade design procedure that considers HCF life was performed in the following order: airfoil and blade profile design, modal analysis, stress distribution test, stress endurance limit test, and fatigue life verification. This study analyzed the Campbell diagram and estimated resonance risk on the basis of the natural frequency analysis and modal test of the compressor blade to guarantee safe and operational reliability. In addition, the maximum stress point of the compressor blade was determined through stress distribution analysis and test. The bonding point of the strain gage was determined by using fatigue test. Stress endurance limit test was performed based on the results of these tests. This research compared and verified the modal characteristics and endurance strengths of the compressor blades to prevent HCF fracture, which is among the major causes of gas turbine engine damage. A fatigue life design procedure of compressor blades was established. The 5 MW class gas turbine compressor blade is well designed in terms of resonance stability and fatigue endurance limit.     

Formation of variable-spatial frequency interference patterns with the use of birefringent crystal prisms for laser Fourier spectroscopy

Splitting of orthogonally polarized radiation components at the exit of birefringent crystal prisms (BCPs) having a special structure and fabricated from uniaxial Iceland spar crystals is studied by the vector analysis method. For these prisms, the angle of splitting between the ordinary (o) and extraordinary (e) beams at the BCP exit depends linearly on the angle of incidence of a collimated laser beam onto the BCP entrance face. This dependence has the maximum slope only for two fixed orientations of the optical axis vectors a ₁ and a ₂ in both crystal wedges (BCP components). As a result of interference of polarized o- and e-waves, a straight-line interference pattern with a controlled and smoothly changed spatial frequency in the range from 0 to 125 periods/mm can be formed directly at the BCP exit. The BCP ability to form a sinusoidal interference pattern with the modulation depth not lower than 30% and with a varied spatial frequency at the aperture up to 30 mm makes it possible to use the BCP as a basis for creating a compact static Fourier spectrometer with the ultimate theoretical resolution λ/Δλ up to ∼2000.     

Fatigue crack growth behavior in ultrafine grained low carbon steel

Ultrafine grained (UFG) low carbon (0.15 wt.% C) steel produced by equal channel angular pressing (ECAP) was tested for investigating the effect of load ratio on the fatigue crack growth rate. Fatigue crack growth resistance and threshold of UFG steel were lower than that of as-received coarse grained steel. It was attributed to the less tortuous crack path. The UFG steel exhibited slightly higher crack growth rates and a lower ΔK_th with an increase of R ratio. The R ratio effect on crack growth rates and ΔK_th was basically indistinguishable at lower load ratio (R>0.3), compared to other alloys, which indicates that contribution of the crack closure vanishes. The crack growth rate curve for UFG steel exhibited a longer linear extension to the lower growth rate regime than that for the coarse grained as-received steel.     

Automatic discontinuous 2 transmission lines with sections of different productivity (<Emphasis Type="Italic">q</Emphasis><Subscript>1</Subscript> < <Emphasis Type="Italic">q</Emphasis><Subscript>2</Subscript>)

An analytical solution is derived for automatic discontinuous 2s’ transmission lines with sections of different productivity (q ₁ < q ₂). On that basis, a range of characteristics that depend on the reliability of the production sections and the bunker capacity may be determined. Accordingly, the solution provides the basis for improvements in the design of such lines.     

Fatigue design of various type spot welded lap joints using the maximum stress

Recently, a new issue in designing spot welded structures such as automobile and train car bodies is to predict an economical fatigue design criterion. One of the most typical and traditional methods is to use a ΔP—N_f curve. However, since the fatigue data on the ΔP—N_f curve vary according to the welding conditions, materials, geometry of joint and fatigue loading conditions, it is necessary to perform the additional fatigue tests for determining a new fatigue design criterion of spot-welded lap joint having specific dimension and geometry. In this study, the stress distributions around spot welds of various spot welded lap joints such as in-plane bending type (IB type), tension shear type (TS type) and cross tension type (CT type) were numerically analyzed. Using these results, the ΔP—N_f curves previously obtained from the fatigue tests for each type were rearranged into the Δσ -N _f relations with the maximum stresses at the nugget edge of the spot weld.     

The effect of bridging on fatigue crack growth behavior in Aramid Patched Aluminum Alloy(APAL)

A new hybrid composite (APAL: Aramid Patched Aluminum Alloy), consisting of a 2024-T3 aluminum alloy plate sandwiched between two aramid/epoxy laminate (HK 285/RS 1222), was developed. Fatigue crack growth behavior was examined at stress ratios of R=0.2, 0.5 using the aluminum alloy and two kinds of the APAL with different fiber orientation (0°/90° and 45° for crack direction). The APAL showed superior fatigue crack growth resistance, which may be attributed to the crack bridging effect imposed by the intact fibers in the crack wake. The magnitude of crack bridging was estimated quantitatively and determined by a new technique on basis of compliances of the 2024-T3 aluminum alloy and the APAL specimens. The crack growth rates of the APAL specimens were reduced significantly as comparison to the monolithic aluminum alloy and were not adequately correlated with the conventional stress intensity factor range(ΔK). It was found that the crack growth rate was successfully correlated with the effective stress intensity factor range (ΔK _eff =K _br -K _ct ) allowing for the crack closure and the crack bridging. The relation between da/dN and theΔK _eff was plotted within a narrow scatter band regardless of kind of stress ratio (R=0.2, 0.5) and material (2024-T3 aluminum alloy, APAL 0°/90° and APAL±45°). The result equation was as follow:da/dN=6.45×10⁻⁷(ΔK _eff )^2.4.     

增压器涡轮叶片疲劳蠕变寿命预测方法

根据径流式增压器涡轮的结构与工作特点,分析了涡轮叶片的载荷与应力空间分布特征;针对增压器涡轮由疲劳与蠕变交互作用引起的叶根断裂失效模式,研究了增压器涡轮叶片叶根的载荷与应力变化历程,建立了涡轮叶片叶根的载荷与应力描述方法;然后建立了增压器涡轮叶片叶根疲劳蠕变寿命预测方法及模型,并运用建立的模型对增压器涡轮叶片叶根进行了寿命评估。     

Reliability assessment and prediction of a fatigue design criterion for the gas-welded joints

Gas metal arc welding is a very important and useful technology in the fabrication of railroad cars and commercial vehicle structures. However, since the fatigue strength of the joints welded by gas metal arc welding is considerably lower than that of the parent material due to stress concentration and metallurgical changes at the weld, the fatigue-strength assessment of welded joints is very important for the reliability and durability of railroad cars and the establishment of a criterion for long-life fatigue design. In this paper, in order to save time and cost for the fatigue design, an accelerated life-prediction method that is based on the theory of statistical reliability was investigated. Its usefulness was verified by comparing the (Δσ_a)_R-N_f relationship that was obtained from actual fatigue test results with the (Δσ_a)R-(N_f)_ALP relationship that was derived from accelerated life testing. And the reliability of the predicted life was evaluated. The reliability of the accelerated life-prediction on the base of actual test data was analyzed to be 80% for the plug-type gas-welded joints and 95% for the ring-type gas-welded joints.     

重型燃气轮机叶片离心载荷下应力特性的研究

针对重型燃气轮机叶片疲劳寿命研究的需要,通过设计和搭建的全尺寸叶片疲劳试验装置,模拟离心载荷工况下的低周疲劳试验,开展离心载荷下叶片应力分布和疲劳寿命预测的研究。该全尺寸叶片疲劳试验装置能够模拟低周疲劳中的等效离心载荷工况,为叶片试验提供必要的条件和手段。同时,对叶片进行网格划分和有限元仿真计算后,得到叶片Von mises应力分布结果,发现叶身应力最大处位于中部偏下边缘薄壁侧。然后将有限元仿真及应力试验相结合,提出叶片疲劳试验的离心载荷等效方法。在此基础上完成叶片的动应力试验,获得应力时间历程数据,并且给出离心载荷应力谱。结果表明,叶身中部边缘的应力水平最高,动应力谱幅值与频数概率的分布服从6阶麦克劳林拟合函数,用等效离心载荷疲劳寿命进行叶片寿命预测的结果是偏安全的,该结论可作为优化设计和试验研究的参考依据。     

Response of radiation driven transient burning of AP and HMX using flame modeling

The radiation driven response function (R _q ) for AP and HMX propellant was obtained and compared with experimental results by using a simple αβγ flame model rather than with detailed chemistry. For an AP propellant, the profile of heat release was assumed by the experimental data. The calculatedR _q shows a frequency shift of the peak amplitude to the higher frequency and a decrease in the maximum amplitude as radiation increases. In addition, it was found the increase in the total flux could enhance the mean burning rate[`(r)]_b\bar r_b while the phase differences between the radiation and resulting conduction could consequently reduce the fluctuation amplitude Δy _b . Fortunately, this is the qualitative duplication of the behavior recently observed in the experiments of RDX propellants. For HMX, the response functionR _q has been calculated and showed a quite good agreement with the experimental data. Even though the fairly good agreement ofR _q with experimental ones, the unsteady behavior of HMX was not reproduced as the radiation input increased. This is due to lack of the material properties of HMX or the physical understanding of HMX burning at high pressure.     

The role of casting porosity in fatigue properties of Al−Si 319 lost foam cast alloy

The tensile, fracture toughness and fatigue properties of Al−Si 319 lost-foam-cast alloy were determined at room temperature. The fatigue properties of this alloy were also determined at 150°C. Fatigue cracks were always initiated at the largest casting pore. Initial pore sizes were measured using a scanning electron microscope. Surface replication showed that majority of the fatigue life was spent in fatigue crack propagation and permitted the estimation of the constants in the Paris power law and the threshold stress intensity factor (ΔK _th ). The role of internal casting porosity was quantified using a linear elastic fracture mechanics (LEFM) model for fatigue crack growth. The predicted lives agreed with the measured values within a factor of two.     

Convergence characteristics of the Crank-Nicolson-Galerkin scheme for linear parabolic systems

This paper is concerned with the investigation on the stability and convergence characteristics of the Crank-Nicolson-Galerkin scheme that is widely being employed for the numerical approximation of parabolic-type partial differential equations. Here, we present the theoretical analysis on its consistency and convergence, and we carry out the numerical experiments to examine the effect of the time-step sizeΔ _t on theh- and ρ-convergence rates for various mesh sizesh and approximation ordersρ. We observed that the optimal convergence rates are achieved only when Δt,h andρ are chosen such that the total error is not affected by the oscillation behavior. In such case,Δ _t is in linear relation with DOF, and furthermore its size depends on the singularity intensity of problems.     

Fatigue strength assessment of spot-welded lap joint using strain energy density factor

One of the recent issues in design of the spot-welded structure such as the automobile body is to develop an economical prediction method of the fatigue design criterion without additional fatigue test. In this paper, as one of basic investigation for developing such methods, fracture mechanical approach was investigated. First, the Mode I, Mode II and Mode III, stress intensity factors were analyzed. Second, strain energy density factor (S) synthetically including them was calculated. And finally, in order to decide the systematic fatigue design criterion by using this strain energy density factor, fatigue data of the δP-N _f obtained on the various in-plane bending type spot-welded lap joints were systematically re-arranged in the °S-N _f relation. And its utility and reliability were verified by the theory of Weibull probability distribution function. The reliability of the proposed fatigue life prediction value at 10⁷ cycles by the strain energy density factor was estimated by 85%. Therefore, it is possible to decide the fatigue design criterion of spot-welded lap joint instead of the δP-N _f relation.     

A method of determining test load for full-scale wind turbine blade fatigue tests

Full-scale fatigue test is an effective method for validating the fatigue performance of wind turbine blade. Its primary problem is how to design the test load. The conventional approach to determine test load requires a complicated and time-consuming process. Thus, a simplified method for directly converting load spectrum of blade into test load is proposed in this paper. Firstly, beam theory is used to obtain the relationship between stress, strain and bending moment of blade cross section. Based on the assumption of local stress concentration and linear relationship between stress and strain, M-N curves (applied moment vs. allowable number of cycles to failure) is defined. Secondly, based on Miner's linear cumulative damage theory and constant life diagram, the equivalent fatigue cumulative damage of load spectrum which is equal to the damage of full-scale fatigue test is obtained. Then, in the case of the selected test load ratio and cycles, the mean and amplitude of test load can be solved. Finally, the validity of the proposed method is verified by an illustrative example. The result indicates that the error of the calculated results between this method and the traditional method is close to 5 %, and it can be used for fatigue test and improve the efficiency of test load design.     

Fracture strength evaluation of welded steel joint by means of small punch test

It has been well known that the ductile-brittle transition temperature (DBTT) of each weld structure or its shift (ΔDBTT) from parent material is one of the very useful measures of the fracture characteristics in steel weldment. In order to present an applicability of small punch (SP) test technique to weldments, in this study, a fracture strength of microstructure at any localized region of interest on HAZ, weld metal and parent material in two steels was evaluated by using DBTT or ΔDBTT obtained from the SP test in relation to the data obtained from the COD test. The empirical correlation, (ΔDBTT) _SP ⋟0.55 (ΔDBTT) _COD , was obtained from the SP and COD test. In addition, the effects of test materials, that is the microstructures of welded region and the orientations of specimens etc, did not appear at the empirical correlation.     

Crack opening behavior of penetrated crack under fatigue load

The leak-before-break (LBB) behaviors of a structural component under high and low fatigue loads are an important problem in nuclear power plants, liquid nitrogen gas tankers and chemical plants. This paper is an experimental study to evaluate the crack opening behavior after penetration for plate and pipe specimens. Crack opening displacement after penetration under low fatigue load could be satisfactorily determined at the center of the plate thickness regardless of the specimen size. In the case of high fatigue load, it is shown that the crack opening displacement at the center of a penetrated crack can be derived using the gross stress,σ _C , and the front surface crack length, a _s , together with the back surface crack length, a _b .     

Numerical determination of the elastic compliance of roller bearings using the hertz theory

Analytical determination of the elastic compliance of roller bearings using the Hertz theory is presented. It is revealed that this parameter depends on the external load q and is varied from 15 × 10⁻⁶ up to 35 × 10⁻⁶ mm²/N for the wide range of q variation.     

Fretting fatigue strength estimation considering the fretting wear process

In fretting fatigue process the wear of contact surfaces near contact edges occur in accordance with the reciprocal micro-slippages on these contact surfaces. These fretting wear change the contact pressure near the contact edges. To estimate the fretting fatigue strength and life it is indispensable to analyze the accurate contact pressure distributions near the contact edges in each fretting fatigue process.So, in this paper we present the estimation methods of fretting wear process and fretting fatigue life using this wear process. Firstly the fretting-wear process was estimated using contact pressure and relative slippage as follows:

W=K×P×S,