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Title: STRESS-STRAIN CHARACTERISTICS OF MATERIALS AT HIGH STRAIN RATES. PART I. DISPLACEMENT MEASUREMENTS DURING HIGH-VELOCITY IMPACTS

Abstract

These two reports were issued separately, but are cataloged as a unit. A photoelectric method for measuring displacements during high-velocity impacts is described. The theory of thc system is discussed in detail, and a prototype system which was buiit and tested is described. The performance of the prototype system is evaluated by comparing the results which it gives with results obtained by other methods of measurement. The system was found capable of a resolution of at least 0.01 inches. static and dynamic stress-strain characteristics of seven high polymers, polyethylene, teflon, nylon, tenite M, tenite H, polystyrene, and saran, plus three metals, lead, copper, and aluminum, are described and compared by means of stress-strain curves and photographs. Data are also presented which show qualitatively the effects produced on stress-strain characteristics by specimen configuration, temperature, and impact velocity. It is shown that there is a definite strain-rate effect for all these materials except polystyrene. The effect is one of an apparent stiffening of the material with increasing strain rate, which is similar to the effect producod by lowering the temperature. The stress-strain measurements are examined critically, inconsistencies are pointed out, and possible sources of error suggested. Values of yield stress, modulus ofmore » elasticity and energy absorption for all materials (except copper and aluminum), specimen configurations, temperatures, and impact velocities included in the investigation are tabulated. (auth)« less

Authors:
Publication Date:
Research Org.:
Texas Univ., Austin, TX (US). Structural Mechanics Research Lab.
Sponsoring Org.:
US Atomic Energy Commission (AEC)
OSTI Identifier:
4300455
Report Number(s):
SCDC-798
NSA Number:
NSA-13-004719
DOE Contract Number:
AT(29-2)-621
Resource Type:
Technical Report
Resource Relation:
Other Information: For Sandia Corp.; These two reports were issued separately, but are cataloged as a unit. Orig. Receipt Date: 31-DEC-59
Country of Publication:
United States
Language:
English
Subject:
METALLURGY AND CERAMICS; ABSORPTION; ALUMINUM; AMIDES; CONFIGURATION; COPPER; DEFORMATION; DIAGRAMS; ELASTICITY; ENERGY; ERRORS; IMPACT SHOCK; LABORATORY EQUIPMENT; LEAD; MEASURED VALUES; MOTION; NYLON; ORGANIC FLUORINE COMPOUNDS; PERFORMANCE; PHOTOELECTRIC EFFECT; PHOTOGRAPHY; POLYETHYLENES; POLYMERS; POLYSTYRENE; RESOLUTION; STANDARDS; STRESSES; TABLES; TEFLON; TEMPERATURE; THICKNESS; VELOCITY

Citation Formats

Fitzgibbon, D. P. STRESS-STRAIN CHARACTERISTICS OF MATERIALS AT HIGH STRAIN RATES. PART I. DISPLACEMENT MEASUREMENTS DURING HIGH-VELOCITY IMPACTS. United States: N. p., 1958. Web. doi:10.2172/4300455.
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Fitzgibbon, D. P. STRESS-STRAIN CHARACTERISTICS OF MATERIALS AT HIGH STRAIN RATES. PART I. DISPLACEMENT MEASUREMENTS DURING HIGH-VELOCITY IMPACTS. United States. doi:10.2172/4300455.
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Fitzgibbon, D. P. Tue . "STRESS-STRAIN CHARACTERISTICS OF MATERIALS AT HIGH STRAIN RATES. PART I. DISPLACEMENT MEASUREMENTS DURING HIGH-VELOCITY IMPACTS". United States. doi:10.2172/4300455. https://www.osti.gov/servlets/purl/4300455.
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@article{osti_4300455,
title = {STRESS-STRAIN CHARACTERISTICS OF MATERIALS AT HIGH STRAIN RATES. PART I. DISPLACEMENT MEASUREMENTS DURING HIGH-VELOCITY IMPACTS},
author = {Fitzgibbon, D. P.},
abstractNote = {These two reports were issued separately, but are cataloged as a unit. A photoelectric method for measuring displacements during high-velocity impacts is described. The theory of thc system is discussed in detail, and a prototype system which was buiit and tested is described. The performance of the prototype system is evaluated by comparing the results which it gives with results obtained by other methods of measurement. The system was found capable of a resolution of at least 0.01 inches. static and dynamic stress-strain characteristics of seven high polymers, polyethylene, teflon, nylon, tenite M, tenite H, polystyrene, and saran, plus three metals, lead, copper, and aluminum, are described and compared by means of stress-strain curves and photographs. Data are also presented which show qualitatively the effects produced on stress-strain characteristics by specimen configuration, temperature, and impact velocity. It is shown that there is a definite strain-rate effect for all these materials except polystyrene. The effect is one of an apparent stiffening of the material with increasing strain rate, which is similar to the effect producod by lowering the temperature. The stress-strain measurements are examined critically, inconsistencies are pointed out, and possible sources of error suggested. Values of yield stress, modulus of elasticity and energy absorption for all materials (except copper and aluminum), specimen configurations, temperatures, and impact velocities included in the investigation are tabulated. (auth)},
doi = {10.2172/4300455},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Tue Jul 15 00:00:00 EDT 1958},
month = {Tue Jul 15 00:00:00 EDT 1958}
}
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Technical Report:
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DOI:  10.2172/4300455
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  • The effects of strain rate on the stress-strain characteristics of copper and lead were studied by measuring both stress and strain as functions of time using short cylindrical specimens supported at one end on a modified Hopkinson pressure bar and impacted at the other end by a steel projectile. Corresponding stresses and strains were computed according to an elementary nonstrain-rate theory (sometimes referred to as the von Karman theory) in which the dynamic stress-strain curve is assumed to be the same as the static stress- strain curve. Stresses and strains were also computed according to an elementary strain-rate theory (sometimesmore » referred to as the Malvern theory) in which the dynamic stress may exceed the static stress for a given strain by an amount which depends upon the strain rate. It was found that the predictions of the nonstrain- rate theory agreed with measured values only for low impact velocities and for points at least two diameters from the impact end of the specimen. By proper choice of the flow or relaxation constant in the elementary strain-rate theory, measured and computed values of strain, or of stress, but not both simultaneously, could be brought into agreement. In the more general exponentialtype, strain-rate law, two independent parameters appear. Presumably with two constants to adjust, this theory could be made to correctly predict both stresses and strains for the conditions under which the tests were performed. If this procedure forces the theory to account for variations which are actually caused by lateral inertia and shear, erroneous conclusions regarding the properties of the material will be drawn. Further study of the effects of shear and lateral inentia is indicated. (auth)« less

  • The existing theories pertaining to the propagation through rods or wires of stresses exceeding the proportional limit are not in agreement with some aspects of experimental results. In order to resolve these differences, accurate dynamic stress-strain diagrams, obtained under conditions of constant strain rate, are especially required in order to establish the effect of strain rate upon the dynamic stress-strain law. A brief survey is made of the experimental techniques which have been employod in the past for measuring transient loads and strains aad for deducing the strain-rate effects. Based on the assumptions made in these previous methods and themore » results obtained from them, a direct method of recording simultaneously the stress-time and strain-time curves was chosen and impact tests were conducted on short, hollow cylinders of aluminum and copper at strain rates from 40 to 1600 in./in./sec. The method did not permit the establishing of complete stress-strain diagrams, but the ultimate stresses and elastic moduli were obtained and have been presented as functions of strain rate. The results of the experimental investigation show that: (1) The ultimate stress for 606l-T6 aluminum alloy cylinders increases approximately uniformly with increase in strain rate at about 5.4 psi per in./in./sec. (2) The ultimate stress for the copper increases from the static test value of 49,400 psi to 60,000 psi at a strain rate of 200 in./in./ sec. From this point, the stress then increases uniformly at about 4.5 psi per in./in./sec. (3) Great care must be taken to insure planeness of impact when the ratio of the length to diameter of the specimen is small; otherwise test results may vary widely. (4) The basic method used does not give the complete stress-strain diagram because of difficulties of interpretation of the test records in the vicinity of the elastic limit stress. (5) A second method, used for the 1600 in./in./sec strain rates, offers more promise for further development. (6) The effective elastic modulus, sigma / epsilon , of copper decreases with increase in strain rate. (auth)« less

  • These two reports were issued separately, but are cataloged as a unit. A photoelectric method for measuring displacements during high-velocity impacts is described. The theory of the system is discussed in detail, and a prototype system which was built and tested is described. The performance of the prototype system is evaluated by comparing the results which it gives with results obtained by other methods of measurement. The system was found capable of a resolution of at least 0.01 inches. static and dynamic stress-strain characteristics of seven high polymers, polyethylene, teflon, nylon, tenite M, tenite H, polystyrene, and saran, plus threemore » metals, lead, copper, and aluminum, are described and compared by means of stress-strain curves and photographs. Data are also presented which show qualitatively the effects produced on stress-strain characteristics by specimen configuration, temperature, and impact velocity. It is shown that there is a definite strain-rate effect for all these materials except polystyrene. The effect is one of an apparent stiffening of the material with increasing strain rate, which is similar to the effect produced by lowering the temperature. The stress-strain measurements are examined critically, inconsistencies are pointed out, and possible sources of error suggested. Values of yield stress, modulus of elasticity and energy absorption for all materials (except copper and aluminum), specimen configurations, temperatures, and impact velocities included in the investigation are tabulated.« less

  • Definite conclusions regarding the effects of specimen configuration and related variables on the dynamic compressive yield stress of copper have not been forthcoming previously because of considerable variation in experimental results. The variations are attributed to data interpretation techniques, lateral restraint from radial friction, strain hardening caused by machining cold work, and impurities in the material. Investigation of these factors using pure copper indicates: radial friction on specimen faces definitely affects the dynamic yield stress for low lengthto-radius (L/R) ratios; the effects of machining cold work are negligible; and an averaging curve data-reduction technique is necessary to obtain reproducible results.more » Further measurements, using a suitable lubricant to reduce radial friction and the averaging curve technique for data interpretation, consistently indicate that dynamic yield stress increases for increase in specimen radius and decrease in specimen length. (auth)« less