By Carolyn Krause

When metal alloys, ceramics, and composites are shaped into objects such as components for automotive and diesel engines, they go through "manufacturing boot camp." These materials may be subjected to bending, rolling, twisting, pulling, pressing, heating, cooling, casting, forging, and joining to produce desired shapes. And that's not all. The objects are then finished through machining, grinding, and coating. As with other manufactured objects put into "operational combat," when the car or truck accelerates, its components are subjected to rapid changes in temperature and other forces that may cause deformation.  As a result of these manufacturing steps and numerous forces, "residual" internal stresses will develop in the structural materials. Residual stresses are defined as "stresses that remain in the absence of any applied external forces." These residual stresses must balance over the part--negative (compressive) in one region and positive (tensile) in another.  A residual stress can be a problem waiting to happen. When you bend a metallic part, it may not break the first time. However, residual stresses in the part may have developed. The next time it is bent, more stresses may develop or a tiny crack may form. The residual stress combined with the applied stress of bending initiates a larger crack. The next time it is bent, it may tear apart. Catastrophic failure is one outcome of the buildup of residual stress.  In some cases, residual stresses can enhance mechanical performance (e.g., "compressive" residual stresses can make a material less susceptible to cracking). However, many tensile residual stresses eventually lead to degraded mechanical performance, stress corrosion cracking, shortened lifetime, and even catastrophic failure.

One key to predicting failure in metal and ceramic
objects is measurement of residual stresses.

To prevent failure of an object, it helps to know when failure is likely to occur. Then you can pull the object out of service and treat, repair, or replace it before it fails. One key to predicting failure in metal and ceramic objects is measurement of residual stresses.

Cam Hubbard at an X-ray diffraction instrument at ORNL's Residual Stress User Center.

Such measurements are now being done at ORNL's Residual Stress User Center through nondestructive methods using neutrons and X rays. These measurements are important for various reasons. They help industry determine (1)  which manufacturing processes minimize the generation of residual stresses, (2) whether an object can withstand service in demanding applications, and (3)  whether heat treatment (annealing) of an object that contained residual stresses successfully eliminated or reduced them. These measurements help developers of computer models construct and improve their models so that they can more accurately predict when residual stresses of various sizes combined with externally applied stresses of various sizes would cause cracking and failure.

The purpose of the center is to measure residual stresses in materials and components using X rays and neutrons.

The Residual Stress User Center was established in ORNL's High Temperature Materials Laboratory (HTML) in 1993. (Other HTML user centers focus on machining and inspection, material analysis, mechanical characterization and analysis, physical properties, and X-ray diffraction.) Principal investigators for the Residual Stress User Center are Cam Hubbard, Xun-Li Wang, Kris Kozaczek, and Tom Watkins, all of the Metals and Ceramics Division, and Steve Spooner of the Solid State Division. The center staff interacts with researchers in the automobile, paper (see How ORNL Helps the Paper Industry), nuclear power, and aerospace industries as well as in universities. The purpose of the center is to measure residual stresses in materials and components using X rays provided by two custom-made instruments at HTML and neutrons provided by ORNL's High Flux Isotope Reactor Facility (HFIR), which offers the highest neutron flux in the world for materials analysis. The User Program is sponsored by DOE, Office of Energy Efficiency and Renewable Energy, Office of Transportation Technologies.The HFIR is supported by DOE, Office of Energy Research, Office of Basic Energy Sciences.

David Schenk (a student from Tennessee Technological University), Steve Spooner, and Xun-Li Wang prepare for a residual stress experiment using neutron diffraction.

To understand residual stress, consider arrays of atoms in parallel planes in a crystalline material. In the stress-free area of the material, the distance between the planes is constant and measurable. Assume that this material contains the two main types of residual stresses compressive and tensile. These forces on the crystalline material cause lattice strains. Compressive stresses cause the distances between the planes to decrease, and tensile stresses cause the distances to increase in comparison with lattice spacings in a stress-free area.

A residual stress is calculated by measuring strains in three orthogonal directions length, width, and height in each of the material's "volume elements" (which are a few cubic millimeters in size). Strains in bulk materials can be measured by neutron scattering because neutrons penetrate several millimeters into a material. These neutrons scatter at different angles governed by the varying distances between the atomic planes. By relating scattering angles to interplanar distances at all locations, strains can be mapped, and residual stresses in bulk materials can be calculated.

Cam Hubbard, Stan David, and Steve Spooner use neutrons at the High Flux Isotope Reactor to make the first map of residual stresses in a complex multipass weld.

Residual stresses are also classified according to their range. Microstresses are short-range stresses that vary from grain to grain in crystalline material. For example, because of thermal expansion, when a composite material such as zirconia in an alumina matrix cools from the forming temperature to room temperature, the zirconia particles may shrink more than the alumina matrix, creating a mismatch. These microstresses are compressive for one phase of the material while the other is placed under tension.

Microstresses occur across distances no larger than a few millionths of a meter, or a micrometer. Such microstresses can lead to failure of parts during manufacture (cracking on cooling) as well as under static or dynamic loads.

Macrostresses, on the other hand, extend through a part over longer distances typically a few thousandths of a meter, or millimeters. Macrostresses can be spatially resolved, or mapped. They are present in weldments and joining of dissimilar materials, and they arise from nonuniform cooling. These stresses are measured at the Neutron Residual Stress Facility at the HFIR. This facility consists of a neutron monochromator that provides a narrow beam of neutrons, all of the same wavelength. The neutrons diffracted from the specimen are recorded by a position-sensitive detector. The system is highly automated to provide measurements round the clock.

Stresses at or within a few micrometers of the surface of a material are better measured by studying the material using X rays. One new technique for profiling subsurface residual stress is called grazing incidence X-ray diffraction (GIXD). The X rays penetrate the material slightly and then backscatter out of the material, like a thrown pebble glancing off water in a pond. This technique is particularly useful for ground ceramics and photovoltaic coatings, and it is popular with users. The center's two X-ray diffraction instruments are equipped with high-precision goniometers that make possible highly automated data collection and analysis. The same instruments can also characterize texture or nonrandom orientation of crystallites, which occurs frequently when coating, forging, and hot pressing materials.

Neutron Studies

Ford Motor Company is looking for a way to improve disc brakes on its cars. Under severe use conditions, its disc brakes distort after overheating, leading to annoying vibrations during braking. Ford's goal is to reduce the cost of servicing brakes--to the company and the owner--over the lifetime of the vehicle. So Bill Donlon of Ford Research Laboratories and G.Vyletel of Ford Motor Company came to ORNL's Residual Stress User Center.

Bill Donlon of Ford Motor Company studies residual stresses in a disc brake rotor using neutrons at ORNL's Residual Stress User Center.

They wanted to better understand the effect of residual stresses on disc brake rotors. These discs spin when the car is moving until the brakes are applied, hydraulically pressing pads with friction linings against the discs to slow or stop them--and the car. It is believed that when the disc brake rotors overheat, changes in residual stresses in the discs cause them to distort, leading to vibrations during braking. The researchers wanted to test this hypothesis and to learn whether heat treatment of rotors would reduce distortions, making the rotors less likely to vibrate.

The Ford researchers brought to ORNL some standard rotors and some rotors that had been heat-treated to relieve stresses. Neutron-mapping studies at HFIR showed that standard rotors do have significant strains and that these can be reduced by heat treatment.

Neutron mapping studies at HFIR showed that strains
in brake rotors can be reduced by heat treatment.

Residual stresses in joints between metals and ceramics are also being studied at the ORNL center to identify the process that makes the most reliable joints. In research involving DOE's Idaho National Engineering Laboratory (INEL), neutron measurements were made on nickel joined to alumina because of past disagreement between computer model predictions and experimental measurements of residual stresses. In the samples studied, joints made of nickel and alumina vary in their composition and structure from end to end. The mixtures of nickel and alumina changed in steps (with the alumina concentration being low near the nickel piece and high near the alumina piece). The residual stress measurements made by INEL's Barry Rabin at ORNL agreed with model predictions of residual stress sizes for different joint types. The information could lead to selection and modification of processes for making improved metal-to-ceramic joints.

Microstresses in silicon nitride were studied at the ORNL center by I. M. Peterson and T. Y. Tien, both at the University of Michigan. They were interested in the relationship between microstresses and fracture toughness, a material's resistance to cracking. The silicon nitride they studied has an aluminosilicate in its grain boundaries, microscopic areas between the crystalline silicon nitride grains. As the silicon nitride is heated with sintering aids containing magnesium, calcium, or barium oxide along with alumina and silicon dioxide, these grain boundary constituents become glasslike and serve as a glue that holds the ceramic particles together. It was found that microresidual stresses increased with the fraction of the volume that is occupied by the glassy-grain boundary material. The reason: while compressive stresses develop in the silicon nitride grains, tensile stresses develop in the grain boundary phase. It is believed that not only does glassy material bridge the silicon nitride grains but also that tensile stresses may assist crack deflection, increasing fracture toughness and minimizing the ceramic's susceptibility to cracking and failure.

X-Ray Studies

The GIXD capability at ORNL was developed in response to a user need. AlliedSignal came to the center for help in developing a quality assurance technique to check whether a vendor used the correct procedure in grinding silicon nitride parts for fuel metering in jet engines. If the incorrect grinding technique is used to finish this ceramic component, it can be damaged. Such damage could lead to unexpected failure or improper operation of the fuel system. So the ORNL center was used to measure the near-surface residual stresses to determine whether the ceramic was damage free. The researchers found that the conventional grinding technique produced stress gradients compressive at the surface and tensile within a few microns below the surface. Participants in the research included Rick Rateick, Philip Whalen, and Franz Reidinger from AlliedSignal. Subsquently, other users, including Ben Ballard and Paul Predecki from the University of Denver in Colorado and Warren Liao and Kun Li from Louisiana State University, have assisted with developing the technique and applying it to other materials such as photovoltaic coatings and other ground ceramic parts. Ceramics must be machined to exacting dimensions for use in engines. Development of proper grinding processes that do not adversely affect mechanical strength is being guided by ORNL's GIXD technique.

Julia Bjerke of Caterpillar, Inc., uses X rays to study stresses in plasma-sprayed zirconia coatings.

In the X-ray studies with Louisiana State University, ORNL researchers found that machined silicon nitride shows damage in the form of "steep stress gradients." They observed that the subsurface damage layer becomes deeper as the machining or grinding wheel speed increases.

In other X-ray studies at the Residual Stress User Center, GIXD provided the U. S. Navy with critical information about expensive machining. TRW and Vanderbilt University used residual stress data from the center to improve the design and performance of gas containers for air bags in automobiles. The University of Florida at Gainesville used the center to characterize diamond coatings.

X-Ray and Neutron Diffraction Studies

Using both X rays and neutrons for residual stress studies, scientists working at the ORNL center provided data on residual stresses in welds to help Lockheed Martin Manned Space Systems, Pennsylvania State University, and the University of Alabama refine knowledge of welds and weld repair procedures for a high-strength aluminum alloy called Weldalite. Because it is a strong, lightweight alloy, Weldalite may be used to build a lighter external fuel tank for the U.S. space shuttle so that it can handle a heavier payload.

George Rading of the University of Alabama and ORNL's Xun-Li Wang (left) use neutrons to study residual stresses around welds in aluminum for future space shuttle fuel tanks.

The center also has made contributions to the
improvement of gears for automobiles.

The center also has made contributions to the improvement of gears for automobiles. Studies at the center helped Penn State improve the quality of gears and aided DOE and industry in developing a computer model that predicts the ability of heat treatment to introduce beneficial residual stresses in gears being manufactured for automotive applications, including the first gear for General Motors' Saturn cars.

Andrew Hunt, a student at Georgia Institute of Technology, uses X-ray diffraction to assess the effectiveness of a novel thin film deposition technique. The screen image shows crystallite texture.

Anantha R. Sethuraman of Rodel, Inc., Newark, Delaware, uses X rays at ORNL to look for residual stresses in silicon wafers as part of a project to study the effects of polishing on stresses in metal interconnect structures.

Summary and Outlook

Since the Residual Stress User Center opened 3 years ago (the neutron faciliities have been available to users for 1 1/2 years), user activity has exceeded expectations. The center has been involved in more than 45 user research agreements as well as 3 DOE-sponsored programs and 4 proprietary projects for industry. To meet the surprising demand for the center's services, ORNL has developed office and instrument space at the HFIR; extended data collection and analysis methodologies; and established mechanisms for effective user support.

It is believed that tensile stresses may assist crack deflection, increasing fracture toughness in silicon nitride.

To speed neutron residual stress measurements, a seven-detector array was designed, assembled, tested, and installed in the fall of 1995. This detector array will permit ORNL to better characterize the strains in materials more completely and more rapidly. Plans have been made for the design and construction of a dedicated neutron strain mapping instrument involving better neutron monochromators and improved beam delivery. This instrument will be automated, remotely monitored and controlled, and capable of complete data analysis. Our goal is a tenfold enhancement of existing capabilities. Advances in the X-ray facilities are also planned to serve user and DOE program needs.

In short, because design engineers need knowledge of residual stresses in materials to produce reliable, safe, and efficient designs of components, the need for residual stress measurements grows. ORNL is making every effort to meet the need and fill the information gaps.

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