Engineering Applications of Residual Stress, Volume 8

This paper describes three different optical systems designed to be used outside the lab for pipeline inspections. The first one is a robust and portable ESPI based hole-drilling unit with radial sensitivity used for residual stresses measurement. The device has a special diffractive optical element that produces an achromatic interferometer. The displacement component around the hole drilled is measured by ESPI with radial in-plane sensitivity and is fitted by least square methods to evaluate residual stresses. An infield application for analyzing the integrity of a gas pipeline is presented as an application example. The second system is a conical laser triangulation device to measure the geometry of the inner surface of pipes. A laser beam is deflected by the tip of a 45° conical mirror and produces a radial light plane that intercepts the inner surface of the pipe producing a bright ring all way around 360°. The image of the light ring is used to compute the radius of about 1400 points in each section while the device is moved along the pipe axis. Finally, the third system uses active photogrammetry to measure in cylindrical coordinates the details of the inner geometry of pipe junctions and welding seams. It was designed to inspect welded joints, to check weld seam quality as well as to identify transverse and angular misalignment between adjacent sections.

The traditional contour method measures a cross-sectional map of residual stress by cutting a body carefully in two and measuring the surface contour. This talk will present two new advances, both motivated by the measurement of a single challenging part. The first advance is a two-step process for measuring hoop stresses in cylinders. In the first step, a cut is made to split the cylinder (from an “o” cross-section to a “c”). That cut releases a bending moment which would otherwise causes errors in the contour measurement. The amount the cylinder springs open or closed is measured and used to determine the bending moment stresses. In the second step, the traditional contour method is applied: a cut is made to measure the remaining hoop stresses on a cross section normal to the hoop direction. The total residual stresses are given by superimposing the bending stresses and the remaining stresses. In this paper, the two-step process is applied to measuring the stresses in a circumferential welded cylinder of depleted uranium and is compared to neutron diffraction results. The welded cylinder also contains a further measurement complication. The weld was only partial penetration, leaving part of the joint unwelded. The measured surface contour therefore had a discontinuity across the joint. Proper handling of the surface discontinuity is presented.

Residual stresses are known to play a significant role in many material failure processes (e.g., fatigue, fracture, and stress corrosion cracking). For example, pressure vessels typically contain welded joints. In many cases these welded joints are large (e.g., traveling around the entire circumference), contain significant amounts of residual stress, have reduced material properties, and contain defects. For these and other reasons, the welded joints tend to be critical locations in terms of design and performance of pressure vessels. In the aerospace industry, where large integral components are often machined from a single piece of material, bulk residual stresses can lead to significant distortion of the machined part. Thus, the ability to accurately quantify residual stresses through measurement is an important engineering tool.

Magnesium alloys are unique materials because tensile twinning and de-twinning can be easily activated, which leads to rapid texture and hardening evolution [1]. Here we present a study of the mechanical response of a magnesium alloy (AZ31) when deformed under twinning dominated conditions, i.e. uniaxial compression along prior extrusion axis. In-situ neutron diffraction measurements were carried out using the SMARTS (Spectrometer for Materials Research at Temperature and Stress) instrument at LANSCE (Los Alamos Neutron Science Center) [2].

The tailoring of strain distributions within semiconductor features represents a key method to enhance performance in current and future generations of complementary metal-oxide semiconductor (CMOS) devices. Although the impact of strain on carrier mobility in semiconductor materials was first investigated over 50 years ago [1,2], its implementation within the inversion layer of the channels in CMOS device channels has only occurred within the past decade. This includes the deposition of liner materials that possess significant values of residual stress [3]. Eigenstrained structures, deposited epitaxially within recesses on either side of the Si channel, can be used to induce either compressive strain in the channel region, by using materials that possess a larger lattice parameter than Si (e.g., SiGe)[4], or tensile strain, by using materials with a smaller lattice parameter (e.g., SiC). Because these methods generate heterogeneous strain distributions within the composite structure, it is critical to experimentally determine the distribution of strain across the current-carrying paths of the device and the surrounding environment.

In thermo-mechanically coupled forming processes for industrial mass production residual stresses are an unavoidable consequence of the alternating inhomogeneous fields of temperature and mechanical stress developing in tools and components dependent on the process parameters applied. Hence, a considerable interest exists to get reliable information about origin and distribution of the relevant residual stress fields and to understand the basic principles of their formation. By way of example a metal forming process based on a predefined locally and temporally differential temperature profile is described, which leads to a characteristic materials property profile and geometrical shape. The development of residual stress in tools (steel AISI H11) used for the thermo-mechanical forming operation of cylindrical flange shafts (steel SAE 6150) is outlined. To this end residual stress analyses by X-ray as well as by neutron diffraction were carried out. The loading situation of the tool was simulated by isothermal, thermal and contact fatigue tests, providing information about cyclic stress and plastic deformation during the manufacturing process.

Residual Stresses are inherent in most materials and structures which have been exposed to a machining or manufacturing process. They are known to have both a beneficial as well as detrimental influence on the performance of manufactured components and yet often go by undetected.

This paper presents the results of a research project aimed at determining the use of Digital Shearography as a suitable method to identify inherent material and structural properties. In order to apply the technique, 3 samples were prepared, one in its fully annealed state and the other 2 with different levels of residual stresses introduced into one of the sample surfaces. This was repeated for three different materials. Investigations were then conducted to determine the samples deflection curvatures in response to an applied load. From these deflections the investigation attempted to determine the Young’s Modulus and magnitude of residual stresses present. The results are presented and compared with tensile specimen results for accuracy. From the results obtained it is apparent that Digital Shearography cannot necessarily be used to detect the presence of residual stresses, but can determine the material’s Young’s modulus.

Different methods exist to measure the residual stresses (RS). One of the advantages of ultrasonic techniques for RS measurement is that they are non-destructive. Using such techniques, one can measure the RS in the same points many times, studying, for instance, the changes of RS under the action of service loading or effectiveness of stress-relieving techniques. An Ultrasonic Computerized Complex (UCC) for non-destructive measurement of residual and applied stresses was developed recently. The UCC includes a measurement unit with transducers and basic supporting software and an advanced database and an Expert System, housed in a laptop, for analysis of the influence of RS on the fatigue life of welded elements. In general, the ultrasonic method allows one to measure the RS in both cases: averaged through thickness or in surface layers. The advanced ultrasonic method, the equipment and some examples of RS measurement in welded elements and structures are discussed in this paper.

Residual stress measurements using non-destructive techniques including neutron diffraction and X-ray diffraction are dependent on assumptions made in the analysis. For example, the different stress free reference sample and the presence of precipitates in a material can influence the measured results. Previous residual stress measurement using neutron diffraction technique in a quenched aluminium block revealed that the result can be influenced by the choice of the stress free reference sample. A benchmark method is therefore essential to validate the different measurement techniques. In this paper, the deep-hole drilling technique was used first as a benchmark technique to validate (1) the measured residual stresses in a water quenched 7449 aluminium alloy block using the neutron diffraction technique, and (2) residual stresses measured using the layer removal technique in a 7449 aluminium plate which was water quenched followed by stretching. Second, the incremental centre-hole drilling technique was used to validate the near surface residual stresses in an aluminium strip measured using the X-ray diffraction technique. The excellent correlations with the benchmark measurement methods validated the different results.

Residual stress measurements in a thick cylindrical steel electron beam (EB) welded sample are described. This is a particularly challenging problem because the residual stresses are distributed in a very narrow region, in and adjacent to the EB weld, when compared to more conventional welding methods. Several variations of the deep hole drilling (DHD) method were applied, including the original conventional method, an incremental approach and a newly developed over-coring method. It is demonstrated that the over-coring method can be used just as effectively as the more difficult incremental method.

The incremental core-drilling method (ICDM) is a nondestructive technique to assess in-situ stresses in concrete. In contrast to other available methods of in-situ stress measurement in concrete, the ICDM can quantify stresses that vary through the thickness of the concrete member under investigation, such as those due to bending or eccentric prestressing. In this method, a core is drilled into a concrete structure in discrete increments. The displacements which occur locally around the perimeter of the core at each increment are measured and related to the in-situ stresses by an elastic calculation process known as the influence function method. This paper presents the analytical and numerical techniques necessary for practical use of the ICDM, as well as results from experimental tests in which simple concrete beams were subjected to controlled loads and insitu stresses measured via the ICDM were compared to known stress distributions. The ability of the technique to accurately measure a variety of different stress distributions is demonstrated, and practical considerations for an ICDM investigation are discussed.

The objective of this research is to define drill speeds that produce acceptable results when using the hole-drilling technique for measuring residual stress. For this study, three common engineering materials; alloy 6061-T651 aluminum, 304 stainless steel, and A36 carbon steel were used. This was achieved by performing ESPI/hole-drilling stress measurement of a known applied stress in discrete rpm intervals ranging from 2-40K rpm for each material. To produce a known state of stress specimens were bent elastically in a four-point bend fixture. Stress measurements were taken using single-axis electronic speckle-pattern interferometry (ESPI). It was found that for 6061-T6 aluminum, accurate and repeatable results can be achieved between speeds of 2-40K rpm. For 304 stainless steel, result accuracy diminishes when drill speeds go below 6K rpm. The A36 steel had a large as-received stress gradient across the longitudinal dimension and was therefore removed from this study.

Ti6Al4V alloy combines mechanical strength, deformability, excellent fatigue and corrosion resistance and high strength to weight ratio. Furthermore, the mechanical behavior remains excellent at high temperature. Such characteristics make this material attractive for numerous applications (structural, aerospace and naval) because of recent improvements in welding techniques (laser, hybrid laser/MIG) that allow realizing high quality titanium welded joints. However some problems related to the welding, as deteriorated material properties, residual stresses and distortions, need further investigations. This paper presents a detailed study on residual stresses of Ti6Al4V butt plates welded by laser or hybrid laser/MIG process. Residual stresses were measured by hole drilling method using electrical strain gage rosettes bonded at different position, in order to evaluate magnitude and distribution of residual stresses along the cord. Residual stresses curves obtained for each type of joint and each welding technique are presented and discussed in terms of transversal residual stresses. Residual stresses were also measured at surface by means of X-ray diffractometer. The aim of this work is to present experimental data related to welding processes in order to confirm the validity of currently procedures or improve them.

Residual stresses (RS) can significantly affect engineering properties of materials and structural components, notably fatigue life, distortion, dimensional stability, corrosion resistance. RS play an exceptionally significant role in fatigue of welded elements. The influence of RS on the multi-cycle fatigue life of butt and fillet welds can be compared with the effects of stress concentration. Even more significant are the effects of RS on the fatigue life of welded elements in the case of relieving harmful tensile RS and introducing beneficial compressive RS in the weld toe zones. The results of fatigue testing of welded specimens in as-welded condition and after application of ultrasonic peening showed that in case of non-load caring fillet welded joint in high strength steel, the redistribution of RS resulted in approximately two-fold increase in the limit stress range. A concept of residual stress management (RSM) and a number of engineering tools were proposed recently that address major aspects of RS in welds and welded structures. According to the concept, three major stages, i.e. RS determination, RS analysis and RS redistribution are considered and evaluated, either experimentally or theoretically to achieve the optimum performance of welded elements and structures. The main stages of RSM are considered in this paper. A number of new engineering tools such as ultrasonic computerized complex for RS measurement, software for analysis of the effect of RS on the fatigue life of welded elements as well as a new technology and, based on it, compact system for beneficial redistribution of RS by ultrasonic peening are introduced. Examples of industrial applications of the developed engineering tools for RS analysis and fatigue life improvement of welded elements and structures are discussed.

Bonded tendons have been used in reactor buildings of heavy water reactors and the light water reactors of some nuclear power plants operating in Korea. The assessment of prestressed forces on those bonded tendons is becoming an important issue in assuring their continuous operation beyond their design life. In order to assess the effective prestressed force on the bonded tendon, indirect assessment techniques have been applying to the test beams which were manufactured on construction time. Therefore, this research mainly forced to establish the assessment methodology to measure directly the effective prestressed force on the bonded tendon of containment buildings using System Identification (SI) technique. To accomplish this purpose, simple SI method was proposed and adapted three dimensional finite element analysis of the 1:4 scale prestressed concrete containment vessel (PCCV) tested by Sandia nation Laboratory in 2000.