Structural Integrity Assessment of a Unit Cell in a Laboratory-Scale Printed Circuit Heat Exchanger for Molten Salt Reactors With Supercritical CO2 Power Cycle

The Printed Circuit Heat Exchanger (PCHE) is considered promising as an intermediate heat exchanger for Molten Salt Reactors (MSRs) due to its highly compact construction, high heat transfer effectiveness, and capability of withstanding high pressures. In this study, thermal-mechanical simulations were performed using a two-channel unit-cell model with the attempt to investigate the structural integrity of a laboratory-scale PCHE that was designed for molten salt-to-supercritical carbon dioxide heat transfer, with the temperature field obtained from Computational Fluid Dynamics (CFD) simulations. It is shown that the fillets on the semi-circular channel walls are stress concentration regions and that the stress intensity decreases quickly as the distance from the fillets increases. A quick drop in the maximum stress intensity is observed with the increase of the fillet radius. There is no significant increase in the stress intensity for locations around the zigzag sharp corners. With a lower bulk temperature and a higher stress intensity, the region close to the outlet of the PCHE hot channels is more vulnerable to potential failures than the inlet region of the hot channels. In addition, the choice of channel models has a weak impact on the maximum stress intensity around the cold channel fillets.

Mechanical Integrity Assessment of Two-Side Etched Type Printed Circuit Heat Exchanger with Additional Elliptical Channel

Printed circuit heat exchangers (PCHEs) are often subject to high pressure and temperature difference between the hot and cold channels which may cause a mechanical integrity problem. A conventional plate heat exchanger where the channel geometries are semi-circular and etched at one side of the stacked plate is a common design in the market. However, the sharp edge tip channel may cause high stress intensity. Double-faced type PCHE appears with the promising ability to reduce the stress intensity and stress concentration factor. Finite element analysis simulation has been conducted to observe the mechanical integrity of double-etched printed circuit heat exchanger design. The application of an additional ellipse upper channel helps the stress intensity decrease in the proposed PCHE channel. Five different cases were simulated in this study. The simulation shows that the stress intensity was reduced up to 24% with the increase in additional elliptical channel radius. Besides that, the horizontal offset channels configuration was also investigated in this study. Simulation results show that the maximum stress intensity of 2.5 mm offset configuration is 9% lower compared to the maximum stress intensity of 0 mm offset. This work proposed an additional elliptical upper channel with a 2.5 mm offset configuration as an optimum design.

Investigation Into Crack Closure Effects for Fatigue Crack Growth Under Negative R Conditions

Fatigue crack growth laws are typically dependent on the ratio between minimum and maximum Stress Intensity Factor (SIF), referred to as the load ratio (R). When part of the SIF range is compressive (and hence R is negative) the amount of growth for a given SIF range is reduced due to crack closure effects. Methods for capturing the effect of crack closure were presented in a previous PVP paper [1]. These methods are based around defining a scaling factor (q0) which is dependent on R and applied to the SIF range before calculated growth. Equations were provided for both best fit and bounding q0 factors. This paper presents a comparison between these methods and results of testing. The specimens used were square cross-section bars and were made from Type 304L stainless steel with an initial corner crack. A range of load ranges and R ratios (including some positive R values) were used and the testing was undertaken at 250°C in both air and a simulated PWR environment. The growth rate observed in the tests was used to derive the effective q0 factor observed in each stage of the testing. These values were then compared with the q0 methods that are used in actual defect tolerance calculations. The results agreed very closely with the derived best estimate q0 curves, with no discernible difference between the air and water results.

Failure Analysis of Spur Gears Used in Transmission System Applied on a Hand Tractor

Damage or failure frequently occurs on gearbox gears. If this occurs in the gearbox of tractors, this can be severe. This study aimed to determine the cause of the spur gear fracture through empirical and simulation studies. The hardness test was undertaken employing the Rockwell method where the fracture surface was observed using a scanning electron microscope (SEM) in order to identify crack initiation and the type of fracture. The stress intensity factor was next analysed using the finite element method (FEM). The results of the chemical composition testing indicated that the material used was according to the AISI 8620 standard containing an element of Carbon (C) of about 0.142 %.The hardness value of the gear was 109 HRB. The observation of the fracture surface showed a brittle fracture surface, suggesting that an impact load had occurred. The simulation results using the FEM also showed that the maximum stress intensity factor and KI value occurred at the centre of the tooth. The value of KI was shown to be larger than the fracture toughness and (KIC). Therefore, this result indicates that a crack will continue to propagate until final failure.

Observation of Crack Growth Behavior of Various Cracks in 4.762mm Diameter Silicon Nitride Balls under Cyclic Compressive Load

In order to investigate the effect of pre-crack lengths on strength of silicon nitride balls under cyclic pressure loads, growth behavior of 600~700μm pre-cracks were compared to those of 200μm~300μm and 400~500μm pre-cracks. Furthermore, the change in initial threshold limit of the maximum stress intensity factor was discussed. It was found that the increasing ratio of stress intensity factor during N=0 and N=1000 distinguished the failure and non-failure, and pre-crack length had strong effect on the threshold limits of the increasing ratio.

Consideration on Fatigue Crack Growth Thresholds Under Negative Stress Ratio

Fatigue crack growth thresholds ΔKth are provided by several fitness-for-service (FFS) codes. When evaluating cracked components subjected to cyclic loading, maximum stress intensity factor Kmax and/or minimum stress intensity factor Kmin are required. However, the definitions of the thresholds ΔKth under negative stress ratio R are not clearly written, except for BS (British Standards) 7910. In addition, the ΔKth are given by constant values under negative R. Fatigue crack growth rates under negative stress ratio is recommended to use maximum stress intensity factor Kmax by ASTM (American Society of Testing and Materials) E 647, because of the Kmax being close to crack driving force. Therefore, it deems that the ΔKth under negative R seems to be Kmax. This paper shows that the Kmax converted by the ΔKth are not constant values under negative R based on the survey of experimental data. The Kmax decreases with decreasing the stress ratio R. Therefore, the ΔKth for the FFS codes are less conservative. As experimental data under negative stress ratio R were taken by Kmax – Kmin, the definition of the threshold ΔKth is benefit to use Kmax – Kmin, instead of Kmax.

Observation of Crack Growth Behavior of 400μm Cracks in 4.762mm Silicon Nitride Balls under Cyclic Compressive Load

In order to investigate the effect of pre-crack lengths on silicon nitride balls under cyclic pressure loads, the pre-crack lengths ranging from 400μm to 500μm were observed. Their growth behavior was compared to that of 200μm to 300μm pre-cracks. Furthermore, the initial threshold limits of their maximum stress intensity factors were measured.

Fatigue Crack Growth Thresholds at Negative Stress Ratio for Ferritic Steels in ASME Code Section XI

Thresholds of fatigue crack growth rates are important characteristics for fatigue crack growth assessment for the integrity of structural components. ASME Code Section XI provides fatigue crack growth thresholds for ferritic steels in air and water environments. The threshold is given as a constant value under a negative stress ratio. However, the thresholds are not clearly defined in the range of negative stress ratios. The definition seems to be maximum stress intensity factors. Besides, the thresholds expressed by the maximum stress intensity factors decrease with decreasing stress ratios. This means that the thresholds under negative stress ratios become unconservative assessments. The objective of this paper is to discuss the definition of fatigue crack growth threshold and to propose the threshold equation for the ASME Code Section XI, based on experimental data obtained from a literature survey.

Dynamic Analysis and Evaluation of Control Rod Device Mechanism Missile Impact on Shielding Plate

The flying of missile will severely jeopardize the structural integrity in control rod ejection accident. In order to analyze the strength of a new type of shielding plate under control rod drive mechanism (CRDM) missile impact, this article develops the simulation model and conducts the response analysis of the missile under 4 cases. In addition, the strain analysis and evaluation of protection shielding plate at the most dangerous case are performed. The motion analysis of CRDM missile indicates that the fracture at trapezoid thread place as well as the shielding plate rim under impact is most dangerous because the maximum kinetic energy of the impact can be obtained. So only this case should be examined when performing the evaluation of the shielding plate. Stress analysis shows the maximum stress intensity of the shielding plate will exceed the yielding stress and thereby local plasticity will occur. Strain analysis shows that compared with the extension ratio at structural failure, the computed strain still has margin to ensure the shielding plate will not be penetrated. Meanwhile the strain analysis of bolts which fix shielding plate are calculated. The strain level of two bolts are exceed limit and others is relatively low. The shield plate can be firmly fixed. Hence, this new type of the protection shielding plate is capable to prevent the damage of other components by the flying of CRDM missile.