1、NASA TN D-157 TECHNICAL NOTE D-1574 FATIGUE BEHAVIOR OF MATERIALS UNDER STRAIN CYCLIN 1 IN LOW AND INTERMEDIATE LIFE RANGE By Robert W. Smith, Marvin H. Hirschberg, and S. S. Manson Lewis Research Center Cleveland, Ohio NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON April 1963 Provided by
2、IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NATIONAG eERONAWICS AND SPACE ADMINISTRATION TECHNICAL NOTE D-1574 FATIGUE BEHAVIOR OF MATERIALS UNDER STRAIN CYCLING IN LOW AND INTERMEDIATE LIFE RANGE By Robert W. Smith, Marvin H. Hirschberg, and S. S. Manson SU
3、MMARY A series of constant strain range tests was made for a wide variety of mate- rials producing fatigue lives varying from a few cycles to about one million cy- cles. The specimens were subjected to axial, compression-tension, low-frequency fatigue about a zero mean strain. Load range was measure
4、d periodically through- out each test, enabling an analysis of fatigue results in terms of elastic, plas- tic, and total strains. AISI 4340 (annealed and hard), AISI 52100, AISI 304 ELC (annealed and hard), AIS1 310 (annealed), AM 350 (annealed and hard), Inconel X, titanium (6Al-4V), 2014-T6, 5456-
5、H311, and U.00 aluminum, and beryllium. Materials tested were AISI 4130 (soft and hard), During strain cycling, load range generally changes during the very early part of the test and then settles down to a fairly constant value for most of the fatigue life. Cyclic strain hardening or softening caus
6、es the observed load change and produces cyclic stress-strain relations that often differ substan- tially from the virgin tensile flow curve. of the test materials. These comparisons are made for each Fatigue-life relations between elastic, plastic, and total strain components were established. For
7、metallurgically stable materials, straight-line fits of the logarithmic elastic strain-life and plastic strain-life data produce a rela- tion that agrees well with the total strain-life data. The strain-life data are also used to explain changes in susceptibility to stress concentrations over a larg
8、e life span and to rate correctly the relative notch sensitivities of the four test materials that experienced nontest-section failures in comparison with the other materials. Relative performance of the test materials is illustrated on the basis of both strain range and stress range over a life spa
9、n ranging from a few cycles to about one million cycles. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-INTRODUCTION In recent years there has been an appreciable effort to incorporate low- cycle fatigue data obtained at various laboratories into fa
10、tigue design proce- dures. Manson (ref. 1) and Coffin (ref. 2) independently suggested that low- cycle fatigue life for a specific material is directly proportional to a power of the cyclic plastic strain (a straight line on a against cycles to failure). criterion to design after extensive testing o
11、f type 347 stainless steel in both constrained thermal cycling and constant-temperature strain cycling. experimental data were developed at four laboratories to establish parameters governing pressure-vessel design with respect to the plastic fatigue characteris- tics of the material (ref. 4). In th
12、is analysis use was made of total (elastic plus plastic) strain range test data. Manson (ref. 5) has related fatigue life to the elastic as well as the plastic strain range components of the total me- chanical strain range, thereby producing one relation suitable for cyclic lives of approximately 10
13、 to lo6 cycles. It has also been pointed out (ref. 6) that in elastic-plastic stress analysis of fatigue problems there is a definite need for knowledge of the relation between stress range (or amplitude) and strain range (or amplitude) during strain cycling. scribed a pressure-vessel design procedu
14、re using a stress amplitude-life equation based on two factors: an empirical relation between plastic strain and tensile ductility, and the endurance limit. log-log plot of plastic strain Coffin (ref. 3) discussed the application of this Correlated Most recently, Langer (ref. 7) has de- In order to
15、evaluate present design procedures, to develop new methods if necessary, and to increase the understanding of the stress-strain - life rela- tions during fatigue, it was believed desirable to obtain detailed fatigue test data for a wide variety of dactile materials using axial, compression-tension,
16、low-frequency fatigue machines in which both load and deformation were measured periodically throughout the test. The first phase of such a program, reported herein, provides the basic information obtained from room-temperature constant diametral strain range tests with zero mean strain. information
17、 was obtained : The following desired test (1) Behavior of load range during cycling at constant strain amplitude (2) Fatigue behavior for span of fatigue life ranging from a few cycles to about one million cycles (3) Data for a selection of materials in which there is a wide variation of chemical a
18、nd metallurgical composition (4) Data for a selection of materials in which there is a wide variation in elastic and mechanical properties (elastic modulus, yield strength, ul- timate strength, and ductility) Such information is then used to (1) Determine the cyclic stress-strain relations necessary
19、 to the stress analyst for fatigue analysis 2 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-(2) Determine the elastic strain (or stress), the plastic strain, and the total strain range as a function of fatigue life (3) Compare relative performance
20、of materials on a basis of strain range and stress range (4) Illustrate the use of strain-life relations to indicate relative notch sensitivity of the materials MATERIALS, APPAFUTUS, AND PROCEDURE Materials Tested The nominal chemical composition, processing condition, and hardness of each test mate
21、rial are tabulated in table I. The test materials were three ferritic alloy steels: AISI 4130, AISI 4340, and AISI 52100; three austenitic heat- resisting steels: AISI 304 (extra low carbon), AISI 310, and AM 350; one heat- resisting nickel-base alloy: Inconel X; three types of aluminum: 2014-T6, 54
22、56-H311, and 1100; a 6Al-4V titanium alloy; and structural grade gMv beryl- lium. The steels AISI 4130, 4340, and 304 ELC, and AM 350 were tested in both soft and hard conditions. were measured at room temperature. Mechanical properties for these materials (table 11) Specimen Configuration The fatig
23、ue test specimens (figs. l(a) and (b) were bars, circular in cross section having an hourglass-shaped test section with a minimum diameter of 0.25 inch, unless otherwise noted in table 111. ricated from 1/2-inch-diameter blanks into fatigue specimens as shown in fig- ure l(b). blanks into buttonhead
24、 fatigue specimens as shown in figure l(a) (or fig. l(c) for beryllium only). Sepasate buttonheads were screwed onto the threaded-head specimens so that the same style of grips could be used for all materials. It was necessary to use a modified specimen configuration for certain very short life test
25、s to reduce the buckling problem that developed in the more ductile materials at large diametral strain ranges. The most common modification in- volved the reduction of the hourglass radius to 1.0 inch and the overall specimen length to 2.25 inches. Inconel X the hourglass radius was further reduced
26、 to 0.5 inch. Another modifi- cation was necessary to prevent failures outside the test section for some mate- rials under certain test conditions. In this case the minimum test-section diam- eter was reduced to 0.21 or 0.18 inch as circumstances dictated. AISI 4130 and 52100 steels were fab- All th
27、e rest of the materials were machined out of 3/4-inch-diameter In a few tests of AISI 4130 (annealed and hard) and A cylindrical test section (fig. l(c) was used to make the longitudinal strain measurements necessary for elastic modulus determinations. Beryllium fa- tigue specimens were made of this
28、 configuration also to allow longitudinal strain control instead of diametral. This procedure was necessary because the extremely 3 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-low value of Poissons ratio (p = 0.024) for beryllium means that the e
29、lastic di- ametrical strains are very small and, therefore, more difficult to measure accu- rat ely . Test Apparatus Four low-frequency mechanical fatigue testing machines (fig. 2) designed and built at Lewis Research Center were used for this test program. Alternate push and pull forces on the spec
30、imen were supplied by a 6-inch-diameter hydraulic cyl- inder. The buttonhead specimens were attached rigidly to the loading rods with split-cone and wedge-type grips. A comercial die set was used to maintain alinement under the action of compressive loads. The lower loading rod was at- tached in ser
31、ies to the lower movable die-set platen, the commercial load cell, and the hydraulically operated piston rod. Strain range control was obtained by a 10-to-1 deflection lever attached to the moving platen. This lever actuated microswitches which energized relays that control a solenoid-operated four-
32、way hydraulic valve. high-pressure oil to the opposite side of the double-acting piston and vented the unpressurized side to the supply tank. as 2 or 3 cycles per minute to about 30 cycles per minute. The valve transferred Cycling rate could be varied from as low A strip-chart recorder was used to m
33、ake either continuous or periodic rec- ords of load amplitude. pose powered the load cell which, in turn, supplied the load signals to the re- corder for amplification. A special circuit built into the recorder for this pur- Strain measurements were made with Tuckerman optical strain gages for all t
34、ests in which the diametral strain range fell below the maximum practical capa- bility ( = 0.034 in. /in. or less) of the 1-inch-gage-length extensometer. (Symbols are defined in appendix A. ) For larger strain ranges, a dial indicator type of diametral extensometer was used. Tuckerman optical strai
35、n gages were selected because of their reliability for a practically unlimited number of cy- cles at large as well as small strains, and because their excellent sensitivity is particularly desirable for diametral strain measurements. is possible to discern a strain of 8 microinches per inch over a 1
36、/4-inch gage length. fastened with piano-wire springs to a special diametral strain-gage fixture (figs. 3(a) and (b) . idly fastened to a U-shaped leaf spring which pressed the bearing edges against the test section. stabilized the strain-gage fixture against tipping during the fatigue test. Longitu
37、dinal strain measurements, used for the beryllium fatigue tests and for all elastic modulus measurements, were made by fastening two 1-inch-gage-length extensometers directly to specimens having cylindrical test sections. Au. opti- cal strain readings were made with the use of the Tuckerman autocoll
38、imators. With these gages it Diametral strain measurements were made using two 1-inch extensometers Two diametrically opposed aluminum bearing edges were rig- Long flexible loops of piano wire attached to the loading rod 4 Provided by IHSNot for ResaleNo reproduction or networking permitted without
39、license from IHS-,-,-Procedure Determination of mechanical properties. - The usual procedure involved the measurement of tensile and elastic properties before fatigue testing each test material. For tensile testing, standard hourglass fatigue specimens were in- stalled in a Rhiele universal testing
40、machine and were fitted with a dial-gage- type diametral extensometer. As the tensile load was increased, simultaneous readings of the dial gage and load indicator were recorded. With these data the true stress - true strain relation beyond the yield point was established. quantities measured and us
41、ed to calculate ultimate strength, fracture strength, and ductility were maximum load, fracture load, and the minimum test-section di- ameter after fracture. In all cases the reported tensile properties (table 11) represent the average of at least three tests. Cylindrical test-section speci- mens (f
42、ig. l(c) were used to determine elastic modulus and Poissons ratio. 1-inch-gage-length optical extensometers were attached in the axial direction to opposite sides of the test section for strain measurement. Simultaneous strain and load readings taken at regular increments of both increasing and dec
43、reasing load provided the information necessary to plot the stress-strain curve from which the elastic modulus was determined by averaging the slope of the straight- line portions of this curve. test section and replaced with the specially designed diametral strain-gage fix- ture for use with optica
44、l gages (fig. 3). A similar series of simultaneous load and diametral-strain readings was used to plot this relation. straight-line portions of this curve represents the ratio E/p, so that Poissons ratio p. can be established from this slope and the previously calculated elas- tic modulus E. The val
45、ues of E and p (table 11) were rounded off to two sig- nificant figures. Other TWO The optical strain gages were then removed from the The slope of the Fatigue testing. - The fatigue machine was operated manually during the first few cycles of each test to set the platen displacement limit switches
46、(fig. 2) to produce the desired diametral strain range A and, in order to maintain a constant strain range at the test sec- tion, it was necessary to change the displacement limits between the platens. For a strain-hardening material these limits had to be increased, and for a strain-softening mater
47、ial they had to be decreased. made whenever the diametral strain range deviated 40 to 80 microinches from the desired value. Acd. These adjust- Microswitch adjustments were Continuous load recordings were taken during the early part of every test. Periodic load measurements were taken thereafter thr
48、oughout the test in conjunc- tion with the strain measurements. The basic test information periodically re- corded throughout each test comprised total diametral strain range range A, and number of cycles N. Specimen life was defined as the number of cycles causing separation of the test section. Ac
49、d, load Special precautions, described in appendix B, were taken while testing be- ryllium to prevent distribution of toxic beryllium dust particles into the air. RESULTS AND DISCUSSION The basic test data, descriptive test information, and calculated stresses and strains are tabulated in table I11 for each fatigue test. Equations used to calculate these various quantities will be discussed later. Information from this table was used to
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