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Tensile properties of short-glass-fiber- and short-carbon-fiber-reinforcedpolypropylene compositesS.-Y. Fua,*, B. Laukeb, E. Ma ¨derb, C.-Y. Yuea, X. HuaaSchool ofApplied Science, Advanced Materials Research Centre, Nanyang Technological University, Nanyang Avenue, Singapore 639798bInstitut fuer Polymerforschung Dresden e. V., Hohe Strasse 6, D-01069 Dresden, GermanyReceived 4 November 1999; revised 11 February 2000; accepted 22 March 2000AbstractComposites of polypropylene (PP) reinforced with short glass fibers (SGF) and short carbon fibers (SCF) were prepared with extrusioncompounding and injection molding techniques. The tensile properties ofthese composites were investigated. It was noted that an increase infiber volume fraction led to a decrease in mean fiber length as observed previously. The relationship between mean fiber length and fibervolume fraction was described by a proper exponential function with an offset. The tensile strength and modulus of SGF/PP and SCF/PPcomposites were studied taking into account the combined effect of fiber volume fraction and mean fiber length. The results about thecomposite strength and modulus were interpreted using the modified rule of mixtures equations by introducing two fiber efficiency factors,respectively, for the composite strength and modulus. It was found that for both types of composites the fiber efficiency factors decreasedwith increasing fiber volume fraction and the more brittle fiber namely carbon fiber corresponded to the lower fiber efficiency factors thanglass fiber. Meanwhile, it was noted that the fiber efficiency factor for the composite modulus was much higher than that for the compositestrength. Moreover, it was observed that the tensile failure strain of the composites decreased with the increase offiber volume fraction. Anempirical but good relationship of the composite failure strain with fiber volume fraction, fiber length and fiber radius was established. 2000 Elsevier Science Ltd. All rights reserved.Keyword: Short glass fibers1. IntroductionShort-fiber reinforced polymer (SFRP) composites arevery attractive because oftheir ease offabrication, economyand superior mechanical properties. Extrusion compound-ing and injection moldingemployed to make SFRP composites [1–27]. In general, ahigh fiber content is required in order to achieve a highperformance SFRP composite. Therefore, the effect offiber content on the mechanical properties of SFRP compo-sites is of particular interest and significance. It is oftenobserved that the increase in fiber content leads to theincrease in the strength and modulus [2–6,13,14,17–19,22,25] and also in the toughness if the matrix has a lowtoughness [7]. However, for injection molded SFRP compo-sites, fiber breakage takes place during processing. Fiberbreakage results from fiber–polymer interaction, fiber–fiber interaction, and fiber contact with surfaces of proces-sing equipment [27,28]. Due to the increased fiber–fiberprocessesarefrequentlyinteraction and fiber-equipment wall contact, fiber lengthdecreases with increasing fiber content [2,5,14,19,21,22,27–30] and this reduction in fiber length then reducesfiber reinforcing efficiency. This is at least partially thereason why the addition of short fibers to a polymer matrixdoes not lead to a significant increase or brings about adecrease in composite strength [5,6,25,26] and modulus[3–6,13,16,18,19,22,26] and toughness [7]. It has beenshown that the mechanical properties such as strength,modulus and toughness increase generally with increasingfiber length [27,29,31–36]. In other words, in general thesemechanical properties decrease with decreasing fiber length.Therefore, the effect of fiber content on the mechanicalproperties of injection molded SFRP composites must becombined with the effect offiber length and the two compet-ing effects would determine the final mechanical propertiesof SFRP composites. Moreover, in order to present a quan-titative discussion or prediction ofthe effect offiber content(fiber volume fraction or weight fraction) on the mechanicalproperties of SFRP composites, it is necessary to give aproper function to describe the relationship between fiberlength and fiber content. In principle, the constants in such aComposites: Part A 31 (2000) 1117–11251359-835X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.PII: S1359-835X(00)00068-3www.elsevier.com/locate/compositesa* Corresponding author. Tel.: 65-790-6343; fax: 65-792-6559.E-mail address: assyfu@ntu.edu.sg (S.-Y. Fu).