STRESS RELAXATION TESTING A symposium sponsored by ASTM Committee E-28 on Mechanical Testing AMERICAN SOCIETY FOR TESTING AND MATERIALS Kansas City, Mo., 24, 25 May 1978 ASTM SPECIAL TECHNICAL PUBLICATION 676 Alfred Fox, Bell Telephone Laboratories, editor List price $23.75 04-676000-23 4b AMERICAN SOCIETY FOR TESTING AND MATERIALS 1916 Race Street, Philadelphia, Pa 19103 Copyright © by AMERICAN SOCIETY FOR TESTING AND MATERIALS Library of Congress Catalog Card Number: 78-74563 NOTE The Society is not responsible, as a body, for the statements and opinions advanced in this publication Printed in Baltimore, Md June 1979 1979 Foreword The symposium on Stress Relaxation Testing for Improved Material and Product Reliability was presented at Kansas City, Mo., 24, 25 May 1978 The symposium was sponsored by the American Society for Testing and Materials through its Committee E-28 on Mechanical Testing Alfred Fox, Bell Telephone Laboratories, presided as symposium chairman and editor of this publication Related ASTM Publications Reproducibility and Accuracy of Mechanical Tests, STP 626 (1977), $15.00, 04-626000-23 Recent Developments in Mechanical Testing, STP 608 (1976), $14.50, 04-608000-23 Cyclic Stress-Strain and Plastic Deformation Aspects of Fatigue Crack Growth, STP 637 (1977) $25.00, 04-637000-30 The Influence of State of Stress on Low-Cycle Fatigue of Structural Materials: A Literature Survey and Interpretive Report, STP 549 (1974), $5.25, 04-549000-30 Cyclic Stress-Strain Behavior—Analysis, Experimentation, and Failure Prediction, STP 519 (1973), $28.00, 04-519000-30 A Note of Appreciation to Reviewers This publication is made possible by the authors and, also, the unheralded efforts of the reviewers This body of technical experts whose dedication, sacrifice of time and effort, and collective wisdom in reviewing the papers must be acknowledged The quality level of ASTM publications is a direct function of their respected opinions On behalf of ASTM we acknowledge with appreciation their contribution ASTM Committee on Publications Editorial Staff Jane B Wheeler, Managing Editor Helen M Hoersch, Associate Editor Ellen J McGlinchey, Senior Assistant Editor Helen Mahy,Assistant Editor Contents Introduction CONSTITUTIVE RELATIONS Load Relaxation Testing and Material Constitutive Relations— E W HART Discussion 18 Metal Def onnation Modeling—Stress Relaxation of Aluminum— R W ROHDE AND J C SWEARGENGEN Discussion A Phenomenology of Room-Temperature Stress Relaxation in ColdRolled Copper Alloys—p PARIKH AND E SHAPIRO 21 34 36 Stress Relaxation of Steel Tendons Used in Prestressed Concrete Under Condiflons of Changing Applied Stress—R I GLODOWSKI AND G E HOFF 42 MATERIAL AND PRODUCT APPLICATION AND TEST METHODS Room-Temperature Stress Relaxation of EBgh-Strengtti Strip and Wire Spring Steels—Procedures and Data—s u v IDERMARK AND E R JOHANSSON Discussion Stress Relaxation in Bending of AISI 301 Type Corrosion Resistant Steel Strip—ALFRED FOX Discussion 61 77 78 88 Stress Relaxation in Beryllium Copper Strip—E W FILER AND c R.SCOREY Discussion 89 108 Report on Bending Stress Relaxation Round Robin—P PARIKH 112 Negative Stress Relaxation in Polyuretfiane Induced by Volume Shrinkage—A Y C LOU Discussion 126 139 HOLD-TIMES CYCLIC EFFECTS AND RESIDUAL STRESS Stress Relaxation of Residual Metalworking Stresses—F T GEYLING AND P L KEY 143 In-Reactor Stress Relaxation of Type 348 Stainless Steel in-Pfle Tabe—j, M BEESTON AND T K BURR 155 Crack Growth Retardation in Two Low-Stren^ Materials Under Displacement Controlled Cyclic Loading—i A KAPP, J H UNDERWOOD, AND I I ZALINKA 171 Cyclic Relaxation Response Under Creep-Fatigae Conditions— J H L A F L E N ANDC E JASKE 182 SUMMARY Sammary 209 Index 213 STP676-EB/Jun 1979 Introduction Stress relaxation1 is the time and temperature dependent decrease of stress in a solid due to the conversion of elastic into inelastic strain Stress relaxation data can be used to develop stress-relief heat treatments for reducing residual stresses and for the design of such mechanical elements as joints, gaskets, and springs Stress relaxation data are also an important tool for evaluating the constitutive relations governing a material's inelastic behavior Until approximately I960, stress relaxation was primarily of interest only to those concerned with the design and manufacture of steam and power generating equipment and, to a lesser extent those concerned with the design of gaskets, reinforced concrete, and electric motors Thus we find that a considerable amount of the early work in this field has been done by the ASTM-ASME Joint Committee on the Effect of Temperature on the Properties of Metals and by individuals associated with this committee Microminiaturization in the computer and electronics industries coupled with the high reliability of the missile and nuclear reactor industries created a need for standard testing techniques for measuring this mechanical property This led to the creation of ASTM Subcommittee E28.ll on Stress Relaxation of ASTM Committee E-28 on Mechanical Testing and to the development of an ASTM Standard Recommended Practice (E 328) The present symposium was organized, and this Standard Technical Publication was prepared primarily to permit those studying the phenomenon to share technical skills, procedures, and analytical tools We also hoped to direct the attention of those teaching materials engineering and machine design to the importance of this property in evaluating the time and temperature dependence of stresses and strains in components intended for long term operation In selecting subdivisions for this publication we arbitrarily followed the arrangement of the three symposium sessions, and the papers were arranged into three groups involving (1) constitutive relations and modeling, These terms are more precisely defined in ASTM Standard Definitions of Terms Relating to Methods of Mechanical Testing (E 6) and ASTM Standard Recommended Practices for Stress Relaxation Tests for Materials and Structures (E 328), ASTM Annual Book of Standards, Part 10 Copyright® 1979 b y A S T M International www.astm.org 202 STRESS RELAXATION TESTING Time, hours FIG 9—Comparison of the creep predictions for three segments of an experiment in 2'ACrIMo steel to an ORNL creep equation particular heat from which the relaxation data are obtained, but this cannot be verified directly Discussion The objective of this paper is to evaluate the feasibility of obtaining physically meaningful results from the analysis of relaxation data using simplified approaches As can be seen in Figs through 5, reasonably accurate predictions of creep data can be obtained with the approaches under evaluation However, easy interpretation of these results is somewhat confounded by the following intertwined factors: (a) material variability versus experimental error, {b) numerical considerations, (c) material history dependence, (d) material path dependence, and (e) the limitations of the assumed creep equations and hardening rules These factors will be discussed in the ensuing paragraphs One nagging concern is the lack of repeatability of the derived coefficients This was observed to different extents in the evaluation of all three materials One explanation is the material variability versus experimental error aspects as is shown through the discussion of Fig 10 The data in Fig 10 are from results obtained from identical testing of two LAFLEN AND JASKE ON CYCLIC RELAXATION 203 Type 304 Stainless Steel ot 593C (IIOOF) After 20 cycles at Ae=0,6% oi,SI73 MPa (25.1 ksi) O Specimen • Specimen 9A c'^ a