Measurement of local creep properties in stainless steel welds

A high temperature measurement system for creep deformation based on the digital image correlation (DIC) technique is described. The new system is applied to study the behaviour of a multi-pass welded joint in a high temperature tensile test and a load controlled creep test at 545°C. Spatially resolved tensile properties and time dependent creep deformation properties across a thick section type 316 stainless steel multi-pass welded joint are presented and discussed. Significantly lower creep strain rates are observed in the HAZ than in the parent material which is attributed to the introduction of substantial plastic strain in the parent material on initial loading. The weld metal shows the fastest creep rates and a variation that appear to correlate with individual weld passes. The visual information provides not only the local creep strain distribution but also the reduction of area and true stress distribution based on strains measured in the transverse direction. The results demonstrate the capability of the DIC technique for full field measurement of displacement and strain at high temperature long term creep tests

Effect of prior cold work on the mechanical properties of weldments

Heat exchanger units used in steam raising power plant are often manufactured using many metres of austenitic stainless steel tubes that have been plastically formed (bent and swaged) and welded into complex shapes. The amount of plastic deformation (pre-straining) before welding varies greatly. This has a significant effect on the mechanical properties of the welded tubes and on the final residual stress state after welding. The aim of the present work was to measure and understand the combined effects of pre-straining and welding on the properties and residual stress levels in stainless steel tubing weldments. Effects of plastic deformation were simulated by plastically straining three identical stainless steel tubes to different strain levels (0%, 10% and 20%). Then each tube was cut into two halves and welding back together. The variation in mechanical properties across weldments was measured using digital image correlation (DIC) and a series of strain gauges (SG). Residual stresses were measured on the 0% (undeformed) and 20% prestrained and welded tubes by neutron diffraction. It was found that the welding process had a marked effect on the tensile properties of parent material within 25mm of the weld centre-line. Evidence of cyclic strain hardening was observed in the tube that had not been pre-strained, and evidence of softening seen in the 10% and 20% pre-strained tubes. Macroscopic residual stresses were measured to be near zero at distances greater than 25 mm from the weld centre-line, but measurements in the 20% pre-strained tube revealed the presence of micro residual stresses having a magnitude of up to 50 MPa

Visual Sensing and Defect Detection of Gas Tungsten Arc Welding

Weld imperfections or defects such as incomplete penetration and lack of fusion are critical issues that affect the integration of welding components. The molten weld pool geometry is the major source of information related to the formation of these defects. In this dissertation, a new visual sensing system has been designed and set up to obtain weld pool images during GTAW. The weld pool dynamical behavior can be monitored using both active and passive vision method with the interference of arc light in the image significantly reduced through the narrow band pass filter and laser based auxiliary light source.Computer vision algorithms based on passive vision images were developed to measure the 3D weld pool surface geometry in real time. Specifically, a new method based on the reversed electrode image (REI) was developed to calculate weld pool surface height in real time. Meanwhile, the 2D weld pool boundary was extracted with landmarks detection algorithms. The method was verified with bead-on-plate and butt-joint welding experiments.Supervised machine learning was used to develop the capability to predict, in real-time, the incomplete penetration on thin SS304 plate with the key features extracted from weld pool images. An integrated self-adaptive close loop control system consisting the non-contact visual sensor, machine learning based defect predictor, and welding power source was developed for real-time welding penetration control for bead on plate welding. Moreover, the data driven methods were first applied to detect incomplete penetration and LOF in multi-pass U groove welding. New features extracted from reversed electrode image played the most important role to predict these defects. Finally, real time welding experiments were conducted to verify the feasibility of the developed models

Strain Prediction Using Deep Learning during Solidification Crack Initiation and Growth in Laser Beam Welding of Thin Metal Sheets

The strain field can reflect the initiation time of solidification cracks during the welding process. The traditional strain measurement is to first obtain the displacement field through digital image correlation (DIC) or optical flow and then calculate the strain field. The main disadvantage is that the calculation takes a long time, limiting its suitability to real-time applications. Recently, convolutional neural networks (CNNs) have made impressive achievements in computer vision. To build a good prediction model, the network structure and dataset are two key factors. In this paper, we first create the training and test sets containing welding cracks using the controlled tensile weldability (CTW) test and obtain the real strain fields through the Lucas–Kanade algorithm. Then, two new networks using ResNet and DenseNet as encoders are developed for strain prediction, called StrainNetR and StrainNetD. The results show that the average endpoint error (AEE) of the two networks on our test set is about 0.04, close to the real strain value. The computation time could be reduced to the millisecond level, which would greatly improve efficiency

Advanced experimental methods for the characterization of welded structures

Welding is one of the most prevalent techniques for mechanical fastening of metals. Recent developments in welding technology have led to welding techniques being more readily employed in safety-critical engineering structures. Since the existence of residual stresses and material property variation around welds affects the mechanical performance and thereby structural integrity, it is essential to improve our knowledge in understanding and modelling the mechanical response of the welded structures. The present work focuses on mechanical characterizations of such structures. This work can be broadly classified into two parts; the first part investigates the residual stress distribution in welded specimens of different metals and the second part presents investigations of mechanical properties in welded specimen using full field optical techniques. A newly invented destructive technique for residual stress measurement, the contour method, was used for the investigations of the residual stress in welded joints in this study. The principle of the contour method is based on a variation of Bueckner's superposition theory. By means of a single straight cut, the 2D residual stress component normal to the region of interest can be determined. In this work, first the numerical simulations of the contour method using two and three dimensional bodies have been demonstrated. The contour method was then applied to one-pass and three-pass groove weld specimens and the results obtained from the contour method were compared to those obtained by the neutron diffraction strain measurement technique. The capability of the contour method to measure multiple residual stress components was also investigated in this project. A recent development of the contour method of stress measurement, the multi-axial contour method, permits measurement of the 3D residual stress distribution in a body, based on the assumption that the residual stresses are due to an inelastic misfit strain (eigenstrain) that does not change when a sample containing residual stresses is sectioned. The eigenstrain is derived from measured displacements due to residual stress relaxation when the specimen is sectioned. By carrying out multiple cuts, the full residual stress tensor in a continuously processed body can then be determined. In this study, finite element simulations of the technique were carried out to verify the method numerically. The method was then used to determine the residual stresses in a VPP A-welded sample, and the results were validated by neutron diffraction measurements. As part of the characterization of the welded structures, a study was undertaken to develop a method of extracting local mechanical properties from weld metal by strain mapping using the digital image correlation (DIC) technique. The feasibility of determining local stress-strain behaviour in the weld zone of a 316H stainless steel pipe with a girth weld was investigated by tensile tests on miniature and standard tensile test specimens. In addition, electron speckle pattern interferometry (ESPI) was utilized to obtain the full-field strain maps of a standard tensile specimen during loading and compared to those obtained in the same specimen by digital image correlation in order to verify the DIC measurements

Index to 1984 NASA Tech Briefs, volume 9, numbers 1-4

Short announcements of new technology derived from the R&D activities of NASA are presented. These briefs emphasize information considered likely to be transferrable across industrial, regional, or disciplinary lines and are issued to encourage commercial application. This index for 1984 Tech B Briefs contains abstracts and four indexes: subject, personal author, originating center, and Tech Brief Number. The following areas are covered: electronic components and circuits, electronic systems, physical sciences, materials, life sciences, mechanics, machinery, fabrication technology, and mathematics and information sciences

In situ monitoring of additive manufacturing using digital image correlation: A review

UIDB/00667/2020 POCI-01-0145-FEDER-016414This paper is a critical review of in situ full-field measurements provided by digital image correlation (DIC) for inspecting and enhancing additive manufacturing (AM) processes. The principle of DIC is firstly recalled and its applicability during different AM processes systematically addressed. Relevant customisations of DIC in AM processes are highlighted regarding optical system, lighting and speckled pattern procedures. A perspective is given in view of the impact of in situ monitoring regarding AM processes based on target subjects concerning defect characterisation, evaluation of residual stresses, geometric distortions, strain measurements, numerical modelling validation and material characterisation. Finally, a case study on in situ measurements with DIC for wire and arc additive manufacturing (WAAM) is presented emphasizing opportunities, challenges and solutions.publishersversionpublishe

Digital Image Correlation for Measuring Full-Field Residual Stresses in Wire and Arc Additive Manufactured Components

This study aims to demonstrate the capability of the digital image correlation (DIC) technique for evaluating full-field residual stresses in wire and arc-additive-manufactured (WAAM) components. Investigations were carried out on WAAM steel parts (wall deposited on a substrate) with two different wall heights: 24 mm and 48 mm. Mild steel solid wire AWS ER70S-6 was used to print WAAM walls on substrates that were rigidly clamped to H-profiles. DIC was used to monitor the bending deformation of WAAM parts during unclamping from the H-profiles, and residual stresses were calculated from the strain field captured during unclamping. Residual stresses determined from the proposed DIC-based method were verified with an analytical model and validated by the results from established residual stress measurement techniques, i.e., the contour method and X-ray diffraction.</jats:p

Effects of plastic strain history on the properties of stainless steel boiler tube welds

The subject of this dissertation is the study of the effects of fabrication history (prestraining, welding and heat treatment) on the mechanical properties of austenitic stainless steel thin wall boiler tubes. These tubes are usually cold bent to shape, and sometimes swaged, prior to interconnection by welding. The bends require solution heat treatment before welding. In addition, subsequent to welding, the residual stresses should be relieved. It is sometimes not practically feasible to follow these constructional practices strictly especially when a whole boiler is constructed as a single unit and becomes too large and complex and contains different tubing materials. As a result of this fabrication history, the mechanical properties of boiler tube materials can be significantly altered. Sample tubes simulating the fabrication steps were supplied by British Energy for this project. The primary aim of the study was to determine spatially resolved room-temperature tensile properties using digital image correlation (DIC) by testing cross-weld specimens machined from the thin wall welded tubes (with plain or prestrained base metal) before and after the heat treatment. The experimental procedure which is used to retrieve the tensile properties from these integrated tests was validated through finite element simulation. Digital image correlation, which is a full-field strain measurement technique, was implemented in order to obtain the local stress-strain curves from regions less than a square millimetre in area and to extract the corresponding local tensile properties such as offset proof strength. The variation of the 0.2% offset proof strength was successfully obtained along these specimens. Evidence of strain hardening due to constraint and weld thermomechanical cycles was found in the plain base metal near the weld pool and evidence of softening was seen in prestrained base metal. On the other hand, after the heat treatment, the effect of prestraining and welding is cleaned out and the strength along the specimen was almost homogenized. However, aswelded cross-weld specimens with prestrained base metal have demonstrated unusual local stress-strain behavior in the weld-affected region. For a better understanding of this behavior, tension test of a cross-weld specimen with a high strength mismatch between the weld metal and base metal was simulated using the finite element method. It was found that the strength mismatch in the specimen, in combination with the experimental procedure, may cause some anomalies in the local stress-strain curves. It was also confirmed that these anomalies are not very detrimental for the determination of the proof strength on the specimens with strength mismatch. Material characterization of the welds and detailed hardness surveys on crossweld specimens were performed. Plastic strain is known to be detrimental for high temperature performance of austenitic stainless steel tubes, therefore, the degree of the plastic deformation should be known before these tubes enter service. DIe, hardness, electron back-scattered diffraction and neutron diffraction (peak width and anisotropy strain) were used to determine the amount of plastic strain in the as-welded tubes. It was observed that there is a good agreement between the predictions of plastic strain in 20% prestrained and welded tube

The development of an experimental technique to measure the influence of temperature on the mechanical properties of weldments

In large industries, such as in power stations, welds are widely employed to join different components together to meet various property requirements. The thermal gradient that develops during welding causes an inhomogeneous distribution of material properties, in areas adjacent to the weld, known as the Heat Affected Zones (HAZ). Welded joints subjected to elevated temperatures and loads during operations often experience a degradation of mechanical properties and performance of the joint. Studies have found that mechanical phenomena’s such as, fatigue and creep have compromised the structural integrity of weld zones. In essence a welded component acts as a composite material, for which it’s overall performance is dependent on its weakest material component. This study focuses on developing an experimental technique that is capable of measuring the influence of temperature on the mechanical and material properties across a weldment. The development of the experimental technique includes the design and optimisation of the hot zone of a welded tensile specimen, identification and characterisation of the different weld zones as well as, refining a strain recording strategy to detect the localised strains in each of the different weld zones. The application of the experimental technique is applied to welded components from turbine steam penetrations, which were extracted from a coal fired power station. The steam penetrations are a low Cr structural steel; (Cr 0.66, C 0.24 by wt. %) and have been in service for approximately 24 year (± 212 000 hrs). Two primary systems namely the Gleeble 3800 thermo-mechanical simulator and digital image correlation are used in this study. In order to accurately map the in-service evolution of material properties, each of the welds were mechanically loaded in tension and exposed to elevated operating temperatures. To induce mechanical loading at constant elevated temperatures, a Gleeble 3800 thermo-mechanical simulator with a tensile module was used to deform specimens at a strain rate of 50 µε.s1 . Experiments were conducted at various temperatures, ranging from room temperature (RT) to 535 o C. The evolution of material properties across the weldment was evaluated using Digital Image Correlation (DIC). DIC is a non-contact digital technique, capable of measuring localized strain during mechanical loading at elevated temperatures. In order to investigate the localized strain across the different weld zones, virtual strain gauges of one millimetre in length were simulated at intervals of one millimetre. It was found that there was a continuous accumulation of strain from the Fusion Line (FL) into the Parent Material (PM). This finding suggested that the HAZ nearest to the PM; which was the Fine Grained Heat Affected Zone (FGHAZ) was the weakest zone as it strained the most. The FL was found to be the least ductile region of the weld as most of the absorbed thermal energy provided during the welding process was used for strain hardening. At elevated temperatures, localised strain occurred at lower strain values than those at RT. This finding suggested that at elevated temperatures there was more thermal energy available for dislocation activation and mobilization. The influence of temperature on the local weld zones were evaluated by extending a specimen, containing just the parent material. A simulation of a virtual strain gauge across the monolithic specimen’s gauge length, revealed that necking occurred at the centre of the specimen which corresponded to the hot zone. In contrast, a simulation of virtual strain gauges across both welds revealed that necking occurred in the region between the HAZ and weld material. This finding inferred that the presence of a weld reduced the strength of the component, as the weld material was the weakest material. Furthermore, the in-service operating conditions was found to have significantly influenced the material behaviour of the welds. A weld that was exposed to a more elevated temperatures and loads, was found to have undergone a higher degree of material degradation, and strained to a larger extent when compared to a weld that was exposed to a more moderate operating environment