Zhang Yi Department of Mechanical Engineering and Engineering Mechanics, Michigan Technological University, Houghton, MI 49931 Sikarskie David L. Department of Mechanical Engineering and Engineering Mechanics, Michigan Technological University, Houghton, MI 49931 06 1996 30 9 966 983 A technique is developed in this paper to utilize a four-point-bend test for piecewise construction of the stress-strain curve for the outer fibers (tension and compression) of the sample. The generated flexural stress-strain curves are compared to uniaxial tension and compression stress-strain curves. Initial agreement is excellent while in the latter stages the flexural curves are stiffer and exhibit significantly higher failure stresses. This technique fully allows for the material softening (nonlinear material behavior) due to the onset of damage well below ultimate fracture as well as different stress-strain behavior in tension and compression. The potential importance of this technique in providing corrected estimates of both tensile and compressive elastic moduli and failure stress, cf. ASTM D 790-91, is discussed. composite random mat stress-strain behavior bending uniaxial tension compression four-point-bend test uniaxial test failure sagemeta-type Journal Article search-text On a Technique for Generating Stress-Strain Curves from Flexural Data YI ZHANG AND DAVID L. SIKARSKIE* Department of Mechanical Engineering and Engineering Mechanics Michigan Technological University Houghton, MI 49931 (Received August 3, 1995) (Revised December 27, 1995) ABSTRACT: A technique is developed in this paper to utilize a four-point-bend test for piecewise construction of the stress-strain curve for the outer fibers (tension and compres- sion) of the sample. The generated flexural stress-strain curves are compared to uniaxial tension and compression stress-strain curves. Initial agreement is excellent while in the latter stages the flexural curves are stiffer and exhibit significantly higher failure stresses. This technique fully allows for the material softening (nonlinear material behavior) due to the onset of damage well below ultimate fracture as well as different stress-strain behavior in tension and compression. The potential importance of this technique in providing cor- rected estimates of both tensile and compressive elastic moduli and failure stress, cf. ASTM D 790-91, is discussed. KEY WORDS: composite, random mat, stress-strain behavior, bending, uniaxial, ten- sion, compression, four-point-bend test, uniaxial test, failure. INTRODUCTION TT IS WELL known that the composite failure stress from bending tests is always Higher than that from uniaxial tension tests [2,3,5]. In the present paper this same trend is shown to exist for SRIM (Structural Reaction Injection Molding) random mat composites in both tension and compression. The explanation and correlation of this difference is based on the two-parameter Weibull statistical strength theory [2,3,5]. Bullock [2] reported excellent correlation between theoretical and experimental tensile and maximum flexural stress at failure for unidirectional graphite-epoxy composites. Knight and Hahn [3] applied three dif- ferent Weibull models (volume, surface and edge models) to a randomly- distributed short fiber composite. It was found that the best fit models for three- and four-point flexural tests were not same. Whitney and Knight [5] obtained dif- ferent shape parameters from unidirectional graphite-epoxy composites between *Author to whom correspondence should be addressed. 966 Journal of COMPOSITE MATERIALS, Vol. 30, No. 9/1996 0021-9983/96/09 966-18 $10.00/0 1996 Technomic Publishing Co., Inc. On a Technique for Generating Stress-Strain Curves from Flexural Data tensile and flexure test data, suggesting that their failures were governed by two different flaw distributions indicating a departure from th;
