Nesar Merah

Experimental Investigation of Heat Exchanger Tubesheet Hole Enlargement

An experimental study has been conducted to evaluate the effect of roller expansion of heat exchanger tubes in enlarged tubesheet holes on joint strength, tube wall reduction (i.e., thinning), and strain hardening of the tubes and surrounding tubesheet ligaments. Different levels of tube-to-tubesheet hole clearance, or overtolerance (OT), were investigated to simulate tubesheet in-service, and during-maintenance, hole enlargement process. The experimental results have shown that, in spite of increasing overtolerance level by up to seven times the maximum value allowed by the Tubular Exchanger Manufacturer Association (TEMA) standard, the average strength of the test joints has not declined below the strength of joints in the allowed TEMA OT range. Percent wall reduction (WR) of the expanded tube was found to increase linearly with increasing overtolerance value; although its maximum value remained well below the wall reduction limit for which a joint is rejected in industry. An observed increase in hardness of the expanded tube material is evidence of the high degree of strain hardening to which the expanded tubes are subjected compared with the surrounding ligaments. Pull-out forces, calculated using existing analytical methods, were found to compare well with the experimental results.

Design and Instrumentation of Force Feedback in Telerobotics

The design and instrumentation of force feedback (FF) is presented for a networked telerobotic system that consists of a master arm client station (MACS) and a slave arm server station (SASS). A motion coordination system maps the operator hand at the MACS to a user-defined floating tool frame at the SASS. Variational analysis for a wrist force sensor (FS) allows evaluation of the force at a floating point of the tool frame. Force is streamed from the SASS to the MACS, where it is displayed on the operator hand. Performance evaluation of contact with the environment is presented. First, a user-controlled teleoperation with FF is described. Contact instabilities are observed in the pre- and postcontact phases. Second, a programmed compliance loop is implemented at the SASS by selectively converting sensed forces into corrective motion, which allows minimization of contact forces. Third, a supervisory mechanism based on a user-controlled active compliance (AC) is presented. High FF gain improves operator sensitivity but may cause instability in case of contact with a stiff environment. Motion mapping minimizes the number of trials to set up the tool configuration. Light and stiff arms are highly recommended to reduce the degradation in telerobotic synchronization caused by elasticity in linkage transmission and by the network delays. The AC at the slave arm improved contact stability and provided an effective supervisory control.

Creep–fatigue crack growth in notched SS-304 plates at 600°C

Abstract This paper deals with crack propagation from circular holes in SS 304 plates under fatigue and fatigue with hold-time at 600°C. In pure fatigue loading, short crack propagation in terms of ΔK is strongly affected by crack closure; this effect is non-existent when the J-integral, ΔJf, is used. Neither fatigue nor creep parameters (ΔJf, ΔJc and J ∗ ) alone gave adequate representation of crack growth for the three frequencies, 1, 1.6×10−2 and 3.3×10−3 Hz. However, the sum of ΔJf and ΔJc resulted in a good correlation of crack growth for all frequencies. Fractographic analyses are used to explain the different conclusions concerning the choice of fracture mechanics parameters.

Calibration of DC Potential Technique Using an Optical Image Processing System in LCF Testing

The direct current electrical potential drop (DCPD) technique is applied to monitor fatigue crack growth in plate specimens with central circular holes at room and high temperatures. The application of the calibration curves obtained from replica specimens to both room and high-temperature fatigue testing conditions is studied. The actual fatigue crack length is measured using a video-camera with image processing to assess the reliability of the DCPD technique. The use of two pairs of DCPD probes across the hole and the introduction of a special reference voltage have eliminated the dependence of the potential drop on the hole size and shape as well as the material resistivity and the notch plastic zone size prior to crack initiation. At high temperature, however, the calibration curve should be corrected to take into account the shifting in the potential ratio due to the increases in crack tip plastic zone and crack opening displacement. The effect of the temperature difference between the different potential probes is also examined. By analyzing the results given by the DCPD in connection with the image processing technique, a criterion is suggested to define the interface between the crack initiation and propagation periods.

Effects of weathering on failure pressure of filament-wound GFRP thermoset pipes:

Burst tests have been performed on the glass fiber-reinforced (GFR) thermoset pipes before and after exposure to natural and accelerated environmental conditions. The main objective of this study was to explore the effects of long-term natural outdoor and artificial accelerated environmental conditions on the burst resistance of the vinyl-based and epoxy-based GFR filament wound thermoset pipes which are targeted for use in crude oil transportation. The pipes were exposed to natural outdoor and accelerated dry heat conditions. The results show that these environments did not produce any noticeable degradation in the hydrostatic burst resistance of the pipes up to 24 months of natural exposure and up to 10,000 h of dry heat exposure.

Impact behavior and finite element prediction of the compression after impact strength of foam/vinylester-glass composite sandwiches

This paper presents the impact and compression after impact behavior of PVC foam cored /E-glass reinforced/vinylester sandwiches used in a four-seat amphibian aircraft. The impact damage was sorted in three categories: barely visible impact damage; visible impact damage; and clearly visible impact damage. It was observed that when increasing impact energy, the extension of the damage from barely visible impact damage to visible impact damage corresponds to a significantly high rate of energy absorption as depicted by the absorbed energy/impact energy ratio. Sandwich coupons were modeled with the finite element analysis ANSYS software to predict the critical failure load in presence of damage zones equivalent to those observed experimentally. The finite element model predicts consistently the compression after impact strength of undamaged coupons and this result confirms the model used to represent the woven fabric by an equivalent cross-ply laminate model. However, the finite element overestimates the compression after impact strength of impacted ones. It is suggested that the induced out-of-plane displacements generate stress concentrations in the tip of cracks located at the borders of the damaged zone.

Effect of Temperature on Fatigue Crack Growth in CPVC

This study addresses the effect of temperature on fatigue crack growth (FCG) behavior of CPVC. FCG tests were conducted on CPVC SEN tensile specimens in the temperature range -10 to 70°C. These specimens were prepared from 4-in. injection-molded pipe fittings. Crack growth behavior was studied using LEFM concepts. The stress intensity factor was modified to include the crack closure and plastic zone effects. The effective stress intensity factor range ΔK eff gave satisfactory correlation of crack growth rate (da/dN) at all temperatures of Interest. The crack growth resistance was found to decrease with temperature increase. The effect of temperature on da/dN was investigated by considering the variation of mechanical properties with temperature. Master curves were developed by normalizing ΔK eff by fracture strain and yield stress. All the da/dN-ΔK curves at different temperatures were collapsed on a single curve. Crazing was found to be the dominant fatigue mechanism, especially at high temperature, while shear yielding was the dominant mechanism at low temperatures.

Notch and Temperature Effects on Crack Propagation in SS 304 Under LCF Conditions

Crack growth behavior at room temperature and at 600 C of SS 304 plates with a central circular hole is studied using linear elastic and elastic-plastic fracture mechanics concepts. Due to large local plastic zone at the hole edge, short fatigue crack propagation is strongly affected by closure phenomena; and the extension rate of a short crack could not be adequately expressed in terms of stress intensity factor {Delta}K. However, the parameter {Delta}K would be appropriate for investigating the behavior of long cracks. Crack acceleration due to temperature increase is discussed and a strain-based normalization factor for {Delta}K is suggested for long cracks. The J-integral obtained from a finite element analysis is successful in establishing the propagation rate of both short and long cracks; simple power relations exist between da/dN and cyclic J-integral ({Delta}J{sub f}) at room temperature and at 600 C. With the modified J-integral, using the actual flow strain, the temperature effect could be incorporated in the correlation between da/dN and {Delta}J{sub f}.

FEA of the Effects of Initial Tube-Tubesheet Clearance, Wall Reduction and Material Strain Hardening on Rolled Joint Strength

The tube-to-tubesheet joint strength is measured in terms of residual contact pressure between the tube’s outer surface and tubesheet hole surfaces. The joint integrity is affected by several design parameters, including the type of tube and-tubesheet materials, level of expansion and the initial radial clearance between the tube and tubesheet. In the present work, an axisymmetric finite element model based on the sleeve diameter and rigid roller concepts is developed. The model has been used to evaluate the combined effects of clearance, wall reduction level and the-strain hardening of tube and tubesheet materials on the interfacial pressure between tube and tubesheet. The FE results show that the initial clearance effect is dependent on the strain hardening capability of the tube material. For low strain hardening tube material the interfacial pressure remains almost constant well above the TEMA (Tubular Exchanger Manufacturing Association) maximum radial over tolerance of 0.0254 mm. These results are validated by the experimental data developed during the research program. As expected, a drastic reduction in joint strength is observed at high values of radial clearances. The cut-off clearance (clearance at which the interfacial pressure starts to drop) is found to vary linearly with the level tube material hardening level. The residual pressure is found to increase slightly for moderate strain hardening tube materials but shows lower cut-off clearances. Wall reductions ranging from 1% to 10% were utilized in calculating the contact pressure as a function of radial clearance. The results show that for low strain hardening materials the optimum value of residual contact stress is obtained for the industry recommended value of 5%. Finally, because of the absence plastic deformation in the ligament, the level of tubesheet material strain hardening does not have any noticeable effect to the joint strength.Copyright

Modeling the Effects of Initial Tube-Tubesheet Clearance, Wall Reduction and Material Strain Hardening on Rolled Joint Strength

between the tube’s outer surface and tubesheet hole surfaces. The joint integrity is affected by several design parameters, including the type of tube and tubesheet materials, the level of expansion, and the initial radial clearance between the tube and tubesheet. In the present work, an axisymmetric finite element model based on the sleeve diameter and rigid roller concepts is developed. The model has been used to evaluate the combined effects of clearance, wall reduction level, and strain hardening of tube and tubesheet materials on the interfacial pressure between tube and tubesheet. The finite element results show that the initial clearance effect is dependent on the strain hardening capability of the tube material. For low strain hardening tube materials, the interfacial pressure remains almost constant well above the Tubular Exchanger Manufacturing Association maximum radial over tolerance of 0.0254 mm 0.001 in.. These results are validated by the experimental data developed during the research program. As expected, a drastic reduction in joint strength is observed at high values of radial clearances. The cutoff clearance (clearance at which the interfacial pressure starts to drop) is found to vary linearly with tube material hardening level. The residual pressure is found to increase slightly for moderate strain hardening tube materials but shows lower cutoff clearances. Wall reductions ranging from 1% to 12% were utilized in calculating the contact pressure as a function of radial clearance. The results show that for low strain hardening materials the optimum value of residual contact stress is obtained for the industry recommended value of 5%. Finally, because of the absence of plastic deformation in the ligament, the level of tubesheet material strain hardening does not have any noticeable effect on the joint strength. DOI: 10.1115/1.2967809