Evaluation of fracture strength and material degradation for weldment of high temperature service steel using advanced small punch test

This paper presents an effective and reliable evaluation method for fracture strength and material degradation of the micro-structure of high temperature service steel weldment using advanced small punch (ASP) test developed from conventional small punch (CSP) test. For the purpose of the ASP test, a lower die with a minimized Φ1.5 mm diameter loading ball and an optimized deformation guide hole of Φ3 mm diameter were designed. The behaviors of fracture energy (E_sp), ductile-brittle transition temperature (DBTT) and material degradation from the ASP test showed a definite dependency on the micro-structure of weldment. Results obtained from ASP test were compared and reviewed with results from CSP test, Charpy impact test, and hardness test. The utility and reliability of the proposed ASP test were verified by investigating fracture strength, behavior of DBTT, and fracture location of each micro-structure of steel weldment for test specimen in ASP test. It was observed that the fracture toughness in the micro-structure of FL + CGHAZ and ICHAZ decreased remarkably with increasing aging time. From studies of all micro-structures, it was observed that FGH AZ microstructure has the most excellent fracture toughness, and it showed absence of material degradation.     

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.     

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.     

Failure analysis of turbine blade in atomic power plant

The turbine blade in an atomic power plant may be fractured by fatigue, stress corrosion cracking and bad fitting. Especially, fatigue fracture is caused by low stress amplitude below the yielding stress. SEM fractography does not have striation, but AFM fractography does on the fatigue fractured surface of 12% Cr steel used for the turbine blade. Surface roughness R _q measured by AFM is linearly related to the stress intensity factor range, ΔK, and is increased linearly according to the load range ΔP. Therefore, in this study, the loading condition applied to a turbine blade is predicted by the relation between the intersection of the ΔK-R _q relation and load range ΔP.     

Application of metal magnetic memory test in failure analysis and safety evaluation of vessels

Metal magnetic memory test (MMMT), which is a new subject in the field of nondestructive examination, can determine regions of stress concentration by testing the distribution of the magnetic field of metal structures so as to effectively diagnose premature defects. MMMT and other test methods are applied in the study to put a propylene purifier of a temperature-jump accident and a leaked ammonia vessel through safety evaluation. Results are as follows: The margin of safety declines after the purifier is overburnt; several stress concentrations are observed within the overburnt area and the level of stress concentration rises after one-month operation; and overpressure operation of the purifier must be strictly avoided and carefully monitored during later operation. Cracks are observed on the ammonia vessel after one year’s service. Extremely high residual stress is the primary cause of cracks. After four years in service, the residual stresses existing in the area of the base metal and weld zone are still greater than 0.5 σ _S, which results in numerous cracks due to stress corrosion. From the MMMT result of the ammonia vessel’s defects, it can be seen that the derivative of magnetic density (dH _p /dx) is an important reference variable. Within the 31 leakage points, the one whose values of dH _p /dx are above 10 occupy 67.7%, and the ones above 8 share 96.8%.     

A study on fatigue damage modeling using neural networks

Fatigue crack growth and life have been estimated based on established empirical equations. In this paper, an alternative method using artificial neural network (ANN) -based model developed to predict fatigue damages simultaneously. To learn and generalize the ANN, fatigue crack growth rate and life data were built up using in-plane bending fatigue test results. Single fracture mechanical parameter or nondestructive parameter can’t predict fatigue damage accurately but multiple fracture mechanical parameters or nondestructive parameters can. Existing fatigue damage modeling used this merit but limited real-time damage monitoring. Therefore, this study shows fatigue damage model using backpropagation neural networks on the basis of X-ray half breadth ratioB/B ₀ , fractal dimensionD _f and fracture mechanical parameters can estimate fatigue crack growth rateda/dN and cycle ratioN/N _f at the same time withinengineering limit error (5%).     

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.     

An evaluation of cast stainless steel (CF8M) fracture toughness caused by thermal aging at 430°C

Cast stainless steel may experience embrittlement when it is exposed approximately to 300°C for a long period. In the present investigation, the three classes of the thermally-aged CF8M specimen were prepared using an artificially-accelerated aging method. After the specimens were held for 300, 1800 and 3600hrs. at 430°C, respectively, the specimens were quenched in water which is at room temperature. Load versus load line displacement curves andJ- R curves were obtained using the unloading compliance method,J _IC values were obtained using the ASTM E 813–87 and ASTM E 813–81 methods. In addition to these methods,J _IC values were obtained using the SZW (stretch zone width) method described in JSME S 001–1981. The results of the unloading compliance method areJ _Q = 543.9kj/m² for virgin materials. The values ofJ _IC for the degraded materials at 300, 1800 and 3600hrs. are obtained 369.25kJ/m², 311.02kJ/², 276.7kJ/², respectively. The results obtained by the SZW method are compared with those obtained by the unloading compliance method. Both results are quite similar. Through the elastic-plastic fracture toughness test, it is found that the value ofJ _IC is decreased with an increase of the aging time.     

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.     

Static and dynamic fracture analysis for the interface crack of isotropic-orthotropic bimaterial

In the present study, interfacial cracks between an isotropic and orthotropic material, subjected to static far field tensile loading are analyzed using the technique of photoelasticity. The fracture parameters are extracted from the full-field isochromatic data and the same are compared with that obtained using boundary collocation method. Dynamic photoelasticity combined with high-speed digital photography is employed for capturing the isochromatics in the case of propagating interfacial cracks. The normalized stress intensity factors for static cracks are greater when α=90° (fibers perpendicular to the interface) than when α=0° (fibers parallel to the interface), and those when α=90° are similar to ones of isotropic material. The dynamic stress intensity factors for interfacial propagating cracks are greater when α=0° than α=90°. For the velocity ranges (0.1<c/c _s1 <0.7) observed in this study, the complex dynamic stress intensity factor |K _D |, I increases with crack speedc, however, the rate of increase of |K _D | with crack speed is not as drastic as that reported for homogeneous materials.     

Experimental study on helium-air exchange flow through small openings with vertical partition

The helium-air exchange flow may occur at the rupture accident of a standpipe in a high temperature engineering test reactor. A test vessel with three types of small opening is used for experiments. An estimation method of mass increment is applied to measure the exchange flow rate. Flow measurements are made with the single opening and partitioned, opening, for opening ratiosH ₁/D ₁ in the range 0.05 to 10, whereH ₁ andD ₁ are height and diameter of the opening, respectively. At lower opening ratios (H ₁/D ₁<0.75), the difference in the exchange flow rates between the opening systems is small. At higher opening ratios (H ₁/D ₁≥0.75), exchange flow rates of the partitioned opening system are higher than those of the single opening system because of separated (unidirectional) flows by partition. An effect of variation of diameters of the partitioned openings on the exchange flow rate is investigated. The exchange flow rate increases with the opening diameter. Finally, an experiment with two-opening is designed to investigate the effect of fluid interaction of the partitioned opening system. It is demonstrated that the exchange flow rate of the two-opening system is higher than that of the partitioned opening system because of the absence of the fluid interaction.     

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.     

A method for estimating the metal residual life of the weld pipes of gas main pipelines

A method was proposed for estimating residual life T _res with the use of the plastic strain ɛ_op accumulated by the metal of a pipe during its operation, the minimum critical plastic strain increment Δɛ_cr, upon whose accumulation during further after-inspection operation a mechanical property attains its maximally allowable regulatory value, and the known operating time T _op of an inspected gas pipeline section. The method makes it possible to increase the reliability of the estimation of the metal residual life of inspected pipes and to exclude the factor of randomness due to the fact that the strain ɛ_op accumulated by the metal of a pipe and the minimum plastic strain increment Δɛ_cr are determined on specimens that are cut from an inspected pipe instead of those cut from an emergency stock pipe or from a thermally treated inspected pipe.     

Environmental strength evaluation of welded steel joint in sea water

Corrosion fatigue crack growth behaviours were experimentally evaluated for the parent metal, as-welded and PWHT specimens of SM53B steel. Multi-pass welding was done by a submerged arc welder. Metallographic observations along the weld fusion boundary were made to investigate the variation of microstructures through the thickness direction. PWHT was carried out at 650°C with holding time of 1/4hr and 40hr. The corrosion fatigue test was conducted in 3.5% NaCl solution with the frequency of 3Hz. In all cases, crack growth in corrosive environment was faster than that of in air. Besides, at the low †K region, crack growth was greatly influenced by corrosive environment and the heat treatment condition.     

The influence of the coordinate errors of setting sensors of a piezoelectric antenna on the accuracy of localizing sources of acoustic-emission signals

The main factors that affect the error of determining the coordinates of an acoustic-emission (AE) signal source are considered. The influence of errors of determining coordinates ±Δx _i and ±Δy _i of piezoelectric-antenna transducers on the AE-signal localization accuracy are analyzed. It is theoretically and experimentally proven that the errors in the coordinates of the transducers substantially influence the AE-signal localization accuracy, especially at a low sampling frequency of the analog-to-digital converter of the AE system’s measuring channel and when low-frequency AE transducers are used.     

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.     

Application of combined EBSD and 3D‐SEM technique on crystallographic facet analysis of steel at low temperature

Electron backscatter diffraction has been increasingly used to identify the crystallographic planes and orientation of cleavage facets with respect to the rolling direction in fracture surfaces. The crystallographic indices of cleavage planes can be determined either directly from the fracture surface or indirectly from metallographic sections perpendicular to the plane of the fracture surface. A combination of electron backscatter diffraction and 3D scanning electron microscopy imaging technique has been modified to determine crystallographic facet orientations. The main purpose of this work has been to identify the macroscopic crystallographic orientations of cleavage facets in the fracture surfaces of weld heat affected zones in a well‐known steel fractured at low temperatures. The material used for the work was an American Petroleum Institute (API) X80 grade steel developed for applications at low temperatures, and typical heat affected zone microstructures were obtained by carrying out weld thermal simulation. The fracture toughness was measured at different temperatures (0°C, ?30°C, ?60°C and ?90°C) by using Crack Tip Opening Displacement testing. Fracture surfaces and changes in microstructure were analyzed by scanning electron microscopy and light microscopy. Crystallographic orientations were identified by electron backscatter diffraction, indirectly from a polished section perpendicular to the major fracture surface of the samples. Computer assisted 3D imaging was used to measure the angles between the cleavage facets and the adjacent polished surface, and then these angles were combined with electron backscatter diffraction measurements to determine the macroscopic crystallographic planes of the facets. The crystallographic indices of the macroscopic cleavage facet planes were identified to be {100}, {110}, {211} and {310} at all temperatures.     

Quality monitoring of resistance spot welding based on electrode displacement characteristics analysis

A new method is developed to monitor joint quality based on the information collection and process in spot welding. First, twelve parameters related to weld quality are mined from electrode displacement signal on the basis of different phases of nugget formation marked by simultaneous dynamic resistance signal. Second, through correlation analysis of the parameters and taking tensile-shear strength of the spot-welded joint as evaluation target, different characteristic parameters are reasonably selected. At the same time, linear regression, nonlinear regression and radial basis function (RBF) neural network models are set up to evaluate weld quality between the selected parameters and tensile-shear strength. Finally, the validity of the proposed models is certified. Results show that all of the models can be used to monitor joint quality. For the RBF neural network model, which is more effective for monitoring weld quality than the others, the average error validated is 2.88% and the maximal error validated is under 10%. __________ Translated from Chinese Journal of Mechanical Engineering, 2006, 42(10): 176–181 [译自: 机械工程学报]     

CO2 laser welding of zinc-coated steel sheets

Welding of zinc-coated steel sheets for the automotive industry has been investigated experimentally and theoretically, using a continuous wave 2 kW CO₂ laser. The specimens of 0.8, 1.0 and 1.2 mm thickness were welded as butt joint and lap joint. Argon gas was shielded co-axially to reduce the plasma and to protect the molten, pool from atmosphere. The mechanical tests of specimens were carried out to investigate the ductility of welds in butt joint and lap joint, using the Erichsen test, ball punch test and tensile shear test. The value of transverse weld pattern is higher than others. The fatigue life of longitudinal weld is superior, but that of circular weld pattern is inferior due to the high tensile residual stresses in the weld. The maximum Erichsen value was obtained as 96% and the deformability of zinc coated steel butt-welded was found to be 80% in the ball punch test. The high pressure formed by vaporization of zinc with the low boiling temperature during laser lap-joint welding splattered the molten pool and created porosities in the weld. The optimum gap was calculated to be 0.1 mm in the lap joint welding of zinc-coated steel sheet which was a good agreement with the experimental result.     

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.