Incremental Centre Hole Drilling Research Articles

Development of microstructure and residual stress in electron beam welds in low alloy pressure vessel steels

Reduced-pressure electron beam (EB) plate butt welds were manufactured in two low-alloy pressure-vessel steels, SA508 Gr 3 Cl 1 and SA508 Gr 2, at two thicknesses in both steels, 30 mm and 130 mm. Transient temperatures during welding were recorded using thermocouple arrays. Residual stresses in the as-welded condition and after post-weld heat treatment were measured using diverse methods: neutron diffraction and the contour method at 30 mm thickness; and deep hole drilling and the contour method at 130 mm. Incremental centre hole drilling measurements were performed at 130 mm thickness to better understand near-surface stresses. Weld and heat-affected zone microstructures and microconstituents were evaluated using a combination of hardness mapping, optical microscopy and electron microscopy. The as-welded residual stresses exhibit the characteristic M-shaped distribution for hardenable steels, reaching 500–600 MPa in tension in both steels at both thicknesses. However, the modest changes to the chemical composition and the change in plate thickness both significantly influenced microstructures, mechanical properties and residual stress distributions. These sensitivities underline the need for physically faithful models. This extensive characterisation study enables the development and validation of models that predict the development of microstructures and residual stresses in EB welds in low alloy pressure vessel steels.

Acoustic signature of laser shock peening for a qualitative evaluation of residual stresses

Acoustic signals recorded from laser shock peening (LSP) of aluminium, titanium and stainless steel alloys were analysed for different laser pulse widths, energy and confinement conditions. In this paper, we report that different materials and conditions registered unique acoustic signatures. Further, the acoustic signatures in the time domain were transformed to the frequency domain and were fitted to a lognormal distribution, yielding a unique set of parameters for each material and set of conditions. These parameters were benchmarked using the measurements of residual stress by the incremental centre hole-drilling (ICHD) method. Detailed analysis of the acoustic signal reveals that this method once calibrated with few ICHD data, can identify successful peening of materials.

Shot peening effects on residual stresses redistribution of offshore wind monopile multi-pass weldments

In some industrial applications, welding is the only alternative to join different parts. However, the major problem in welded structures is the tensile residual stresses that are inevitably produced during welding process. Surface tensile stresses can threaten the performance of the weldments as they act as an accelerant in fatigue crack initiation and failure. Shot peening is a well-known method which can enhance weldments' fatigue performance and longevity by inducing surface compressive residual stresses in order to eliminate or limit tensile residual stresses.In this study, the effect of shot peening on redistribution of multi-pass welding residual stresses was numerically and experimentally investigated on very large components typical of welded joints used in offshore wind turbine monopiles. In experimental part of the study, the residual stresses were measured using the Incremental Centre Hole Drilling (ICHD) method before and after shot peening. Finite element studies were carried out using 3D welding models and random shot peening analyses. Moreover, extensive finite element analyses were conducted to study the effect of model dimensions and the number of passes on prediction of welding residual stresses. Interesting set of results obtained from both the numerical studies and the ICHD measurements were in good agreements and showed that shot peening can be advantageous even for large components with multi-pass welded joints. Additionally, reducing the number of weld passes in finite element models could considerably lower the computational time without affecting the accuracy of results at surface regions of the models.

Directional and oscillating residual stress on the mesoscale in additively manufactured Ti-6Al-4V

In additive manufacturing of metals as compared to conventional processing, the directional track-by-track and layer-by-layer nature of the fabrication process can lead to residual stresses that locally are both directionally and spatially heterogeneous. Much of the existing literature has focused either on the macroscale residual stress inside the entire part, or on the microscale residual stresses that are created around a scan vector, thereby neglecting the intermediate length-scale of the different layers. The objective of this research is to investigate such mesoscale residual stress distributions across several layers in Ti-6Al-4V components produced by laser metal deposition process. The incremental centre hole drilling and incremental slitting methods provide measurements with excellent spatial resolution within and across the layer length scale. In this work, the two methods complemented each other to quantify both mesoscale and directional variations of residual stress that correlate with the deposition pattern. Our findings also provide strong evidence that an oscillatory residual stress variation persists even after thermal cycling that occurs during deposition of subsequent layers.

Cold Compression of 7075 and Factors Influencing Stress Relief

Abstract The residual stresses in heat-treated 7075 aluminum alloy blocks have been characterized using neutron diffraction, X-ray diffraction, and incremental center-hole drilling. Specimens were quenched to induce high-magnitude residual stresses, which were then stress relieved by the controlled application of plastic deformation using a cold compression technique. The experimental variables investigated were the quench water temperature and the postquench delay. This delay is considered to influence the final residual stress magnitudes because of hardening precipitation occurring by natural aging during the period between quenching and the application of plastic deformation. Cold compression significantly lowered the residual stresses in all specimens. Neutron diffraction measurements demonstrated a benefit to applying plastic deformation as soon as possible after quenching. However, this beneficial effect was not evident when characterizing surface residual stresses using hole drilling or X-ray diffraction.

Effects of Laser Shock Peening over Minimally Detectable Partial Through-Thickness Surface Cracks

Abstract Laser shock peening (LSP) has evolved as a viable alternative to other surface treatments (shot peening, burnishing, etc.) that induce beneficial residual stress in structural components. While fatigue life improvements have been recognized by the aerospace industry, in-service components may have surface flaws with sizes below current inspection limits. A concern is that the application of LSP over an existing crack may cause unintended detrimental consequences. To address this concern, two different LSP processes were applied over a 0.25 in. (6.35 mm) partial through-thickness surface fatigue crack in 7075-T651 aluminum. The resulting residual stresses were measured using combinations of included X-ray diffraction with layer removal, incremental center-hole drilling, and neutron diffraction. Both LSP processes resulted in a through-thickness residual stress profile in which compressive stresses at either surface were balanced by tensile stresses in a band centered in the mid-thickness of the specimen. Peened and unpeened (baseline) specimens were then tested under constant-amplitude cyclic loading to assess the fatigue response. The average fatigue life of the baseline specimens was about 49,000 cycles. The peened specimens (both treatments) survived to runout, exhibiting no crack growth when examined with an optical microscope even when the crack tip was either very close to or inside the region of tensile residual stress.

Application of multiple analysis methods in optimising complex residual stress characterisation

An accurate characterisation of residual stress plays an important role in the structural integrity assessment of an engineering component. Several techniques and tools are available for measuring and predicting residual stresses. For example, neutron diffraction (ND) and X-ray diffraction (XRD) are non-destructive techniques used for measuring through-thickness and surface residual stresses respectively, while the deep-hole drilling (DHD) and the incremental centre-hole drilling (ICHD) are semi-destructive techniques and measure through-thickness and sub-surface residual stress respectively. In most open literature, a more favoured method is traditionally used over others, with some degree of validation using finite element analysis (FEA) predictive tool. In this paper it will be shown that the different methods and tools available are not contradicting or more superior to the others, but rather, the use of more than one available technique complementary to each other can improve the quality and the confidence in the characterisation of the residual stress state in an engineering component. In particular, the accurate knowledge of the residual stress field for a safety critical component plays a vital role for subsequent structural integrity assessment.

Measurement of residual stresses induced by sequential weld buttering and cladding operations involving a 2.25Cr-1Mo substrate material

Dissimilar metal welds are necessary in high-pressure subsea systems and in cases where forged components must be welded to pipelines. F22 (2.25Cr-1Mo) steel is often used in such forged steel components and, since this steel cannot enter service without undergoing post-weld heat treatment (PWHT), the components are usually prepared for field welds through the application of a buttering layer. Furthermore, a weld overlay is deposited for the purpose of mitigating corrosion. This combination of multiple welding tasks and dissimilar materials leads to the possibility of developing substantial residual stresses. This study aims to provide insights to the evolution of residual stresses at each stage of the welding operation. The assessment has been undertaken on laboratory-scale weld mock-ups using the contour method for residual stress measurement, and incremental centre hole drilling. It was found that both buttering and cladding introduce near-yield levels of tensile residual stresses, but that these stresses are successfully relieved upon PWHT.

Residual stress measurement round robin on an electron beam welded joint between austenitic stainless steel 316L(N) and ferritic steel P91

This paper is a research output of DMW-Creep project which is part of a national UK programme through the RCUK Energy programme and India's Department of Atomic Energy. The research is focussed on understanding the characteristics of welded joints between austenitic stainless steel and ferritic steel that are widely used in many nuclear power generating plants and petrochemical industries as well as conventional coal and gas-fired power systems. The members of the DMW-Creep project have undertaken parallel round robin activities measuring the residual stresses generated by a dissimilar metal weld (DMW) between AISI 316L(N) austenitic stainless steel and P91 ferritic-martensitic steel. Electron beam (EB) welding was employed to produce a single bead weld on a plate specimen and an additional smoothing pass (known cosmetic pass) was then introduced using a defocused beam. The welding residual stresses have been measured by five experimental methods including (I) neutron diffraction (ND), (II) X-Ray diffraction (XRD), (III) contour method (CM), (IV) incremental deep hole drilling (iDHD) and (V) incremental centre hole drilling (iCHD). The round robin measurements of weld residual stresses are compared in order to characterise surface and sub-surface residual stresses comprehensively.

Analysis of Residual Stresses in Laser-Shock-Peened and Shot-Peened Marine Steel Welds

Laser peening is now the preferred method of surface treatment in many applications. The magnitude and depth of the compressive residual stress induced by laser peening can be influenced strongly by the number of peen layers (the number of laser hits at each point) and by processing conditions including the use of a protective ablative layer. In this study, residual stresses have been characterized in laser and shot-peened marine butt welds with a particular focus at the fatigue crack initiation location at the weld toe. X-ray diffraction, synchrotron X-ray diffraction, incremental center-hole drilling, and the contour method were used for determination of residual stress. Results showed that the use of ablative tape increased the surface compressive stress, and the depth of compressive stress increased with an increase in number of peening layers. A key result is that variation of residual stress profile across laser peen spots was seen, and the residual stress magnitude varies between the center and edges of the spots.

Minimization and Mitigation of Wire EDM Cutting Errors in the Application of the Contour Method of Residual Stress Measurement

The contour method of residual stress measurement relies on the careful application of wire electro-discharge machining (WEDM) for the cutting stage. Changes in material removal rates during the cut lead to errors in the final calculated values of residual stress. In this study, WEDM cutting parameters have been explored to identify the optimum conditions for contour method residual stress measurements. The influence of machine parameters on the surface roughness and cutting artifacts in the contour cut is discussed. It has been identified that the critical parameter in improving the surface finish is the spark pulse duration. A typical cutting artifact and its impact on measured stress values have been identified and demonstrated for a contour cut in a welded marine steel. A procedure is presented to correct contour displacement data from the influence of WEDM cutting artifacts, and is demonstrated on the correction of a measured weld residual stress. The corrected contour method improved the residual stress magnitude up to 150 MPa. The corrected contour method results were validated by X-ray diffraction, incremental center hole drilling, and neutron diffraction.

A comprehensive experimental and numerical study on redistribution of residual stresses by shot peening

Shot peening is one of the most effective surface strengthening treatment technologies in which compressive residual stresses are induced beneath the specimen surface. Effects of various factors on the distribution of residual stress profile induced by shot peening have been investigated by many researchers. However, initial residual stresses are one of the important factors which affect the shot peening residual stress.This study is aimed to present comprehensive numerical and experimental study on the effect of initial residual stresses on the shot peened specimen. Initial residual stresses were induced using a four-point bending rig and grinding. Incremental center hole drilling (ICHD) technique was employed to measure residual stresses on bent, ground, shot peened, bent plus shot peened and ground plus shot peened specimens. Numerical analyses of these processes were performed to provide quantitative comparison of different combinations of residual stresses. The comparison with experimental results helped to have a better understanding on how shot peening residual stresses were redistributed. Furthermore, the surface hardness was measured for all specimens.

Measurement and Prediction of Residual Stresses in Quenched Stainless Steel Components

Austenitic stainless steel cylinders and rings are spray water quenched to create residual stresses at or greater than the yield strength. The residual stresses are measured using neutron diffraction, and two mechanical strain relaxation methods: deep hole drilling and incremental centre hole drilling. This paper compares the measurements with predictions of quenching using finite element analysis. Also finite element analysis is used to mimic deep hole and incremental centre hole drilling methods and to reconstruct residual stresses as if they have been measured. The measurements reveal similar trends to the predictions but there is only limited agreement between their magnitudes. However, there is better agreement between the reconstructed stresses and the measurements. Both the two mechanical strain relaxation methods reveal that large discrepancies occur between measurements and predictions arise because of plasticity. Irrespective of this and surprisingly there is good agreement between deep hole drilling and neutron diffraction measurements.

Validation of Mechanical Strain Relaxation Methods for Stress Measurement

Most validation studies of mechanical strain relaxation (MSR) methods for residual stress measurement rely on using the saw-tooth residual stress distribution resulting from four point bending and elastic–plastic deformation. Validation studies using simple applied stress profiles in rectangular steel beams are used in this work, together with beams subjected to elastic–plastic bending. Two MSR methods are explored, deep-hole drilling (DHD) and incremental centre hole drilling (ICHD). As well as a series of experiments, finite element analyses are conducted to determine the accuracy in the inversion of measured deformation to reconstruct stress. The validation tests demonstrated that apart from the applied stresses, the initial residual stresses also contribute even when samples are expected to be stress free. The uncertainty in measurement for the two MSR methods is determined, with the uncertainty in near surface measurement found to be significantly larger than uncertainty for interior measurement. In simple loading cases (and simple stress profiles) the uncertainty in measurement and hence the degree of validation is shown to be within about ±50 MPa for steel for “known” stress up to about 140 MPa. However, if the residual stress distribution is more complex there arises increased uncertainty in the predicted residual stress and lack of confidence between measurements methods.

Measurement, analysis and reconstruction of residual stresses

Residual stresses, created in a steel beam by elastic–plastic bending, are predicted using an approximate analysis and the finite element method. The predictions are compared to experimental measurements obtained from the application of incremental centre hole drilling, deep hole drilling and neutron diffraction methods. Finite element simulations of the incremental centre hole drilling and deep hole drilling methods applied to the predicted residual stresses permitted an assessment of their ability to reconstruct the stresses. An analytical reconstruction analysis using an Airy stress function together with boundary and equilibrium conditions is developed and applied to the predictions and measurements. The reconstruction of the predictions enabled a reference case to be developed and used to assess the application of the reconstruction process to experimental data. It is shown that satisfactory through depth residual stress profiles can be obtained, providing the errors are about the same of those found in the measurement techniques.

Measurement of Residual Stresses within a PWR Swaged Heater Tube

Frequent failures of the pressuriser heater tubes used in Pressurised Water Reactors (PWRs) have been found. Axial cracks initiating from the tube outer diameter have been detected in some tubes as well as the resulting electrical problems. Replacement of the heater tubes requires an undesirably prolonged plant shutdown. In order to better understand these failures a series of residual stress measurements were carried out to obtain the near surface and through-thickness residual stress profiles in a stainless steel pressuriser heater tube. Three different residual stress measurement techniques were employed namely, Deep-Hole Drilling (DHD), Incremental Centre Hole Drilling (ICHD) and Sachs’ Boring (SB) to measure the through thickness residual stress distribution in the heater tubes. Results showed that the hoop stresses measured using all three techniques were predominantly tensile at all locations, while the axial stresses were found to be tensile at the surface and both tensile and compressive as they reduce to small magnitudes within the tube. The magnitude of the in-plane shear stresses was small at all measurement depths at all locations. The various measurement methods were found to complement each other well. All the measurements revealed a characteristic profile for the through-thickness residual stress distribution.

Modeling and Measurement of Residual Stresses in a Steel Vessel Containing Glass

Residual stresses in a stainless steel vessel containing glass have been evaluated using measurements and numerical simulation. High-level nuclear wastes are often vitrified in glass cast in cylindrical stainless steel containers. Knowledge of the internal stresses generated in both the glass and container during this process is critical to structural integrity and public safety. In this research, residual stresses were measured near the surface of a High Level Waste container using an Incremental Center Hole Drilling technique. Residual stress magnitudes were found to be at or near to the yield stress in the container wall. A transient finite-element thermal-stress model has been developed to simulate temperature, distortion, and stress during casting and cooling in a simple slice domain of both the glass and the container. Contact thermal-stress elements were employed to prevent penetration at the glass–container interface. Roughness of these contact surfaces was modeled as an equivalent air gap with temperature-dependent conductivity in the thermal model. The stress model features elastic-viscoplastic constitutive equations developed based on the temperature-dependent viscosity of the glass and elastic-plastic constitutive equations for the stainless steel. The simulation was performed using the commercial ABAQUS program with a user material subroutine. The model predictions are consistent with the residual stress measurements, and the complete thermal–mechanical behavior of the system is evaluated.

Laser shock peening to improve the fatigue resistance of AA7050 components

PurposeLaser shock peening (LSP) is a process capable of introducing compressive residual stresses into a metallic component. The residual compressive stress field can extend deeper below the treated surface than that produced by conventional shot peening (SP). The effect of such deep compressive stress profile is expected to result in a significantly greater benefit in fatigue resistance after LSP compared to SP. The purpose of this paper is to examine this further.Design/methodology/approachResidual stress profiles have been determined by X‐ray diffraction and incremental centre hole drilling. They have been correlated with the respective LSP process parameters and the obtained fatigue behavior.FindingsA significant improvement of the fatigue life was found for an R ratio of 0.1. SP leads to a fatigue improvement of about 15 percent. For the same specimen geometry, a fatigue life improvement of about 25‐35 percent, depending on the load level, can be obtained after LSP. However, not only for the positive R ratio, where it is quite obvious, but also for the negative R ratios, R=−1 and −3, an increase of the fatigue life is generated by SP and LSP.Originality/valueA shown LSP has a high potential for extending the service life of metallic components at the design stage, but it may also be possible to apply this technique to in‐service aircraft to extend the service goals of existing structures.

Measurement of Residual Stresses in Surface Treated Stainless Steel Groove Welds

This paper examines the extent to which mechanical shot peening (MSP), ultrasonic impact treatment (UIT) and laser shock peening (LSP) can affect the tensile residual stresses in the fusion zone caused by welding for a 10mm multi-pass 'V' groove weld within a 20 mm thick 304L stainless steel plate. Stresses are measured by deep hole drilling, neutron diffraction and incremental center hole drilling. For the UIT and LSP treated samples, the tensile stresses present in the as-welded plate are reversed to compressive stresses to a depth in excess of 2-4mm. For MSP the affected depth is much less (~0.5mm). The depth of these compressive stresses is similar to those measured in 20 mm thick parent plate test coupons.

Spatial variation of residual stresses in a welded pipe for high temperature applications

Measurements of residual macro-stresses have been undertaken in a feature multipass circumferential single V butt-weld made from a P91 ferritic steel pipe over different spatial depths: (i) ≤10 μm by X-ray diffraction, (ii) ≤1 mm by incremental centre-hole drilling and (iii) through wall section using deep-hole drilling. The ability to make near-surface X-ray residual stress measurements on as-oxidised surfaces has been demonstrated and the implications for use in the evaluation of overall integrity are discussed. Each of the three measurement techniques provides complementary and consistent measurement of induced residual stresses for weld metal, heat affected zone and parent metal for the as-welded and the post-weld heat-treated conditions over the complete spatial range. The results are discussed with respect to the importance of the weld capping run in introducing near-surface compressive residual stresses, the through-wall profiles of the residual stresses measured at the weld metal position in hoop and axial directions and the presence of existing surface oxide.