Experimental method for determining through thickness residual hoop stresses in thin walled pipes and tubes without inside access

The determination of through thickness residual stresses in pipes and pressure vessels is of growing interest because of emphasis placed on life prediction, design, and failure analysis of piping systems. Most of the through thickness residual stress measurement techniques require the placement of gauges on the outside and inside of the pipe. These methods are severely hampered when gauges cannot be placed on the inside of the pipe. This constraint could arise for small diameter pipes, long pipes or for pipes that have been used in a service condition causing corrosion or fouling of the inner surface.
This paper focuses on the first step of a three step procedure for determining residual hoop stresses in thin walled pipes and tubes. The method described is designed for cases where it is impossible to place gauges on the inside of the pipe. The method yses biaxial strain gauges on the outside of the pipe and involves a through thickness axial cut of the pipe. Based on the change in strain on the outside of the pipe, changes in the hoop residual stress distribution due to the axial cut are obtained with the method presented here. The method provides a means to evaluate changes in stresses on both the outside surface and the inside surface of the pipe as well as an evaluation of the change in through thickness hoop stress distribution at any location in the pipe cross section. This paper further demonstrates that the problem of shortening long pipes to enable placement of gauges on the inside of the pipe can result in the loss of significant residual stress information.     

Distortion and residual stresses in structures reinforced with titanium straps for improved damage tolerance

Distortion and residual stresses induced during the manufacturing process of bonded crack retarders have been investigated. Titanium alloy straps were adhesively bonded to an aluminium alloy SENT specimen to promote fatigue crack growth retardation. The effect of three different strap dimensions was investigated. The spring-back of a component when released from the autoclave and the residual stresses are important factors to take into account when designing a selective reinforcement, as this may alter the local aerodynamic characteristics and reduce the crack bridging effect of the strap. The principal problem with residual stresses is that the tensile nature of the residual stresses in the primary aluminium structure has a negative impact on the crack initiation and crack propagation behaviour in the aluminium. The residual stresses were measured with neutron diffraction and the distortion of the specimens was measured with a contour measurement machine. The bonding process was simulated with a three-dimensional FE model. The residual stresses were found to be tensile close to the strap and slightly compressive on the un-bonded side. Both the distortion and the residual stresses increased with the thickness and the width of the strap. Very good agreement between the measured stresses and the measured distortion and the FE simulation was found.     

Effects of oxide thickness, Al2O3 interlayer and interface asperity on residual stresses in thermal barrier coatings

During high temperature operation, the thermally grown oxide (TGO) usually forms along the bondcoat/topcoat interface in thermal barrier coating (TBC) and was characterized as a driving force for the failure of the coating system. The effects of TGO thickness and Al₂O₃ interlayer applied as an oxygen barrier layer between the bondcoat and topcoat on the magnitude of residual stresses in TBC during cooling process were interpreted using concentric-circle model. The results were coupled with finite element method. The influences of interface asperity and interface topography on the distribution of residual stresses normal to interfaces in TBC were also discussed.     

Finite element based model for predicting induced residual stresses and cutting forces in AISI 1020 steel alloy

The application of finite element models is a promising method for ensuring part quality during machining to accurately predict induced residual stresses and cutting forces. The present study applied Analysis System software to formulate a 3D model to predict induced residual stress and forces for AISI 1020 alloy. Taguchi method was applied in the design of the experiment with three levels and three factors selected: Cutting speed, feed rate and depth of cut. For validation, stresses are measured using an x-ray diffractometer from the surface to a depth of 0.6 mm in steps of 0.2 mm. The cutting forces are determined using a force dynamometer. Simulation results showed that cutting speed, feed rate and depth of cut contributed 94.76 %, 0.048 %, and 0.11 % respectively. The predictive model equations were statistically significant with a p-value of <0.005. The average induced residual stress on the superficial layer from the experiment and simulation were −367.7 MPa and −365.6 MPa respectively. The average residual stresses obtained at depths of 0.2 mm, 0.4 mm, and 0.6 mm were −260 MPa, −233 MPa, and −211 MPa, respectively. The proposed model offers a potential solution to reducing the costs of experimental methods.