HISTORY and PURPOSE of the Surface Integrity Institute
Surface Integrity Institute maintains a database with thousands of publications and research reports, documenting over 70 years of continuing surface integrity and surface enhancement research. This institution and database are dedicated to preserving and disseminating this valuable resource to benefit current and future researchers.
Defining Surface Integrity and Surface Enhancement
Surface Integrity refers to the influence of a component's surface properties on its useful service life and performance. The importance of surface condition is as old as forging, heat treatment, grinding, and machining.
The strength and safe operating life of components subject to fatigue failure or stress corrosion cracking have long been known to be dependent upon the surface condition.
Surface integrity generally pertains to all surface properties, including roughness of finish, friction, phases present, residual stress, corrosion properties, etc. These may either enhance or degrade strength or service life, properties of primary importance in engineering. The pronounced effect of surface grinding practice on the fatigue strength of high strength steel is show in Figure 1.
Over the last half of the 20th century, industrial research into manufacturing methods ranging from machining and grinding to electrical discharge machining (EDM) and electrochemical machining (ECM) has focused on optimizing surface integrity in manufacturing.
Surface Enhancement, a related term, refers to any treatment or process performed to improve the surface integrity of a component.
Surface enhancement also dates from the origins of metalworking. Hammer peening by blacksmiths to eliminate tensile residual stresses from forming and welding, and the complex forging, heat treating, and quenching of steel tools and weapons were developed long before the metallurgy needed to understand the processes.
Modern surface enhancement, including shot peening, gravity peening, glass bead peening, deep rolling, laser shocking, and low plasticity burnishing, can dramatically improve component performance by creating a surface layer of compressive residual stress. Figure 2 shows the air blast shot peening of a welded joint to eliminate surface tensile stress produced by weld shrinkage in the fusion and heat affected zones.
Figure 1:
Air Blast Shot Peening of a Welded Joint to Eliminate Surface Tension after Welding
Figure 2:
Fatigue Characteristics of Surface Ground 4340 Steel, Q&T, 50 HRC.
Surface enhancement can be applied to a component to improve surface integrity as a manufacturing operation or during overhaul to restore performance and mitigate damage occurring in service.
Fatigue and Stress Corrosion Failures Initiate at the Surface
The integrity of the surface is of critical importance because failure under stress virtually always initiates at the surface. Therefore, surface integrity relates to the external surface and the shallow layer, nominally 0.25 mm (0.010 inch) deep below the surface.
• Fatigue failure initiates as persistent shear slip bands in favorably aligned grains, and then propagates as normal stress cracks. The initiating shear stress is always highest at the free surface.
• Stress corrosion cracking (SCC) can only initiate at the exposed corroding surface once the surface stress exceeds the tensile SCC threshold. Figures 3 shows suppression of SCC cracks in an austenitic weld by surface enhancement to eliminate tension on the right side.
Even internal failures initiate at discontinuities, like inclusions or voids, that form an internal "surface" at the boundary of the material discontinuity. But internally initiated failures are the subject of material purity and "cleanliness", and are not relevant to surface integrity.
Figure 3:
Austenitic Stainless Steel Weld SCC Test Sample with Extensive SCC on Untreated (left) Half and SCC Eliminated by Compression LPB Treated (right) Half
The History of Surface Integrity Research
1800s
Figure 4:
A Wöhler’s depiction of the residual stress distribution after unloading a plastically bent rectangular beam. {After Almen and Black, Ref 2)
The origins of surface integrity can be traced back to the first modern study of fatigue by A. Wöhler around 1870. To improve railroad axle fatigue life, Wöhler designed, built, and used the first accurate modern fatigue testing machines to establish the fundamental principle that fatigue was a function of accumulated cycles of alternating stress. 1
Wöhler is also credited with the first observation and explanation of improving surface integrity. He showed that a surface layer of residual compression could be introduced by bending a bar to produce tensile plastic strain in the surface. He further recognized that compressive layer produced added to the applied mean stress and improved fatigue life. This is fundamental to surface enhancement.
The 1900s
In the 20th century, the automotive and aircraft industries followed the railroads to become primary areas of interest in improving surface integrity. The need to improve mechanical performance, avoid fatigue failures, and reduce weight led to technical advances and the development of engineering disciplines related to the surface integrity of mechanical components. A few specific applications and historical developments in each industry are noteworthy.
Automotive applications of surface integrity dating from the early 1900s addressed failures from grinder burn on cams and gears, stress concentrations in crank shafts, and surface roughness and flaws. The need to understand these failure mechanisms helped to develop modern methods of failure analysis. First mechanical and then electrical resistance strain gauges were developed to determine strain and thus the state of stress.
Surface enhancement methods were developed, including roller burnishing of crank and drive shafts, followed by shot peening. Ford and General Motors developed exceptional research centers that produced these developments and supporting technologies, including metallurgical, fatigue, and residual stress measurement methods.
Engineering societies formed committees focused on surface integrity.
• The Fatigue Design and Evaluation Committee (FD&E) of SAE International, previously known as the Society of Automotive Engineers, leads the development of x-ray diffraction and mechanical residual stress measurement methods. 3
• Previously known as the American Society for Testing and Materials, ASTM committee E9 has been the primary source of fatigue testing standards development for decades.
• The Society of Experimental Stress Analysis (SESA) was a significant contributor to developing optical and mechanical strain measurement methods.
• The Society of Manufacturing Engineers (SME) has played a major role in the integrity of surfaces produced by manufacturing methods such as machining, grinding, welding, EDM, and ECM.
Aircraft requirements of optimal performance with minimal weight have driven many developments related to surface integrity and surface enhancement in engines and structures. During World War II, the development of high-performance military aircraft extended the applications for shot peening with a variety of media, surface burnishing, and coining to introduce residual compression in fatigue critical parts.
Figure 5:
Major Structural Fatigue Failure of the Aluminum Alloy Fuselage
The 1954 catastrophic structural fatigue failures of the DeHavilland Comet (Figure 5, the first jet engine-powered commercial airliner, led to new design methods to eliminate stress concentrations. It changed the fundamental method of fatigue management to the periodic inspection and component lifing that is in use today. 4
The SAFE-LIFE design method that assumed the fatigue life of an undamaged surface would exceed design life was replaced by the FAIL-SAFE design. In modern FAIL-SAFE design, damage (typically Kf=3) is assumed to exist, components are life limited, and regular inspections are required to avoid failure.
The modern FAA was formed in 1958 for regulatory oversite of commercial aviation. The Aircraft Structural Integrity Program (ASIP) 5, 6 , and the Engine Structural Integrity Program (ENSIP) 7 followed, defining fatigue life design regulations, including approved surface enhancement methods, for military aircraft structures and engines.
Dr. Michael Field and Dr. M. Eugene Merchant at Cincinnati Milling Machine (now Milacron) developed methods of optimizing machinability and the physics of chip formation 8 that influence surface conditions, respectively.
Major aircraft and engine OEMs supported internal surface integrity research of alloy and application-specific manufacturing processes, and how they influence fatigue performance. At GE's jet engine plant in Evandale, Ohio, Guy Bellows 9, Frank W. Gorsler, and Herbert G. Popp were leaders of engineering groups researching the influence of surface condition on fatigue and methods of surface enhancement to improve fatigue performance.
With a long history of machine tool manufacturing and development, GE's primary jet engine research, engineering and manufacturing plant, and proximity to Wright Patterson AFB, Cincinnati became a center for surface integrity research.
Metcut Research Associates was formed in the late 1940s and became the leading independent machining research facility under Dr. Michael Field and Norman Zlatin, formerly with Cincinnati Milacron, and Prof. John Kahles of the University of Cincinnati.
By 1950 Metcut offered unique expertise, experience, and capabilities for machinability testing. Dr. Wm. Koster, a student of Dr. Kahles, joined Metcut and expanded the laboratory facilities to include metallurgical, residual stress, creep, surface wear, residual stress measurement and fatigue testing capabilities supporting surface integrity research.
With the support of the United States Air Force (USAF) and major aircraft OEMs, comprehensive surface integrity studies spanning the full range of manufacturing processes and aerospace alloys were undertaken for the first time. The USAF chose Metcut to establish a center for the study of machining and surface integrity for aircraft applications.
The Airforce Machinability Data Center was created at Metcut in 1964. It collected and maintained a database of worldwide publications on machining and surface processing methods to support industrial technological advancement, especially in aerospace-related industries.
The data center edited and published the Machining Data Handbook that became the primary reference for optimizing virtually all machining methods from milling, turning, and grinding to ECM and EDM. 10
The realization of the impact of manufacturing methods on surface integrity and the resulting fatigue performance led to the expansion of surface integrity research.
Figure 6:
Subsurface Residual Stress Distributions Produced by Surface Grinding 4340 Steel, 50 HRC, with Gentle, Conventional and Abusive Grinding
Surface Integrity Studies of the 1960s and 1970s
The Air Force Research Laboratory granted Metcut Research a series of contracts in the 1960s and 1970s to undertake extensive detailed surface integrity studies of the range of machining and surface treatment methods used in aerospace manufacturing. As a result, surface properties, including roughness, residual stress depth profiles, phase transformations, metallography, and resulting fatigue performance, were determined for a range of aerospace alloys. Figure 6 shows the subsurface residual stress distributions producing the range of fatigue strength for ground 4340 steel, 50 HRC, shown in Figure 1.
Samples of numerous alloys, including Al 7075-T6, Ti 6Al-4V, Ni Alloy 718, and 50HRC 4340 steel, were prepared using a wide range of chip forming, grinding, and electrochemical methods, including EDM and ECM. For each method, samples were prepared by "Gentle," "Conventional," and "Abusive" practices by varying tool sharpness, depth of cut, feed rate, loss of coolant, etc that could occur in practice. Surface finishing, including vapor honing and tumbling, were included. Surface enhancement by shot peening over the machined surface to mitigate surface damage was also studied.
The surface integrity research programs spanned a decade employing dozens of researchers.
• The machinability machine shop prepared test samples for metallography, residual stress measurement, and fatigue testing.
• Residual stress was measured by initially mechanical layer removal and later by x-ray diffraction.
• High edge retention metallography was developed to characterize the near-surface grain deformation and phase transformations.
• Fatigue tests were developed in either cantilever or 4-point uniform stress bending using large area gauge section specimens to minimize fatigue data scatter and accurately assess the effect of surface properties on fatigue performance.
Volumes of unique data were developed during these surface integrity studies, and the amount, quality, and range of data produced are exceptional. Portions of the work have been published in the open literature 11, but distribution of the full technical reports was initially limited. Those full reports, volumes containing data comparable to Figures 1 and 6, now form the core for the Surface Integrity Institute archives.
Several commonly used terms were created during the decade of research.
• The now widely used term "Surface Integrity" was coined by Drs. Field and Kahles to describe the combination of properties influenced by machining an surface preparation. 12
• The term Low Stress Grinding or "LSG," which was developed in these programs, and is now a requirement (e.g., GE S400) when manufacturing fatigue or tensile test specimens to control the effect of residual stress induced during manufacture.
Figure 7:
Original Four-Point Bend Test System for Low Cycle Fatigue Surface Integrity Studies
Lambda Technologies Continuation of Surface Integrity Research
The author of this abbreviated history was fortunate enough to have participated for eleven years, first as a college student and then as an engineer, in both the fatigue testing and residual stress measurement phases of these surface integrity programs. The experience provided an incomparable education in materials, surface integrity, fatigue, and residual stress measurement.
The Surface Integrity Institute has been formed to preserve the original machinability database, containing over 40,000 publications and the surface integrity research reports in accessible, searchable PDF format. To this original work, we have added our own publications documenting nearly 50 years of continuing surface integrity and surface enhancement research at Lambda Technologies, including the bibliographies of related work by others that we have assembled.
With the advent of computer modeling to design optimized alloys, it has been said that further improvement of fatigue performance by adjusting elemental composition has been essentially exhausted. In contrast, surface enhancement can dramatically improve fatigue strength and damage tolerance of exiting components--without altering either the material or design. In addition, introducing designed residual compression at fatigue critical locations can increase fatigue life by orders of magnitude.
We hope that this Surface Integrity Institute database resource will preserve this valuable data so that it is available to benefit future research for generations.
Paul S. Prevéy, Lambda Technologies Group, 2021