CompositesWorld
Issue link: https://cw.epubxp.com/i/442058
JANUARY 2015 10 CompositesWorld DESIGN & TESTING » Among the most common properties reported for fber-rein- forced polymers are those obtained from tensile testing of a unidi- rectional (UD) composite in the fber direction. Tese properties include modulus of elasticity, Poisson's ratio, tensile strength, and ultimate tensile strain. Te tensile test that measures these proper- ties is, on the face of it, very simple: A thin strip of a UD composite is placed into the wedge grips of a mechanical testing machine and loaded slowly in tension. To determine the modulus of elas- ticity and Poisson's ratio, strains are measured during the initial stage of the test, using strain gages or extensometers. Loading continues to ultimate failure, the point at which tensile strength and ultimate tensile strain are determined. In practice, however, obtaining the desired results can be rather difcult. Measuring the modulus of elasticity and Poisson's ratio is not the problem. Tese properties are measured at load levels well below the point of failure, typically corresponding to strain levels between 0.1% and 0.3%. Te tensile strength and ultimate tensile strain values are the challenges, and they become more difcult to obtain as the specimen's tensile strength increases. Why the difculty? For starters, it requires a relatively large load to produce specimen failure, and it's no easy task to cause failure in the test section before failure occurs elsewhere. Te specimen, therefore, must be designed in such a way that stress during the test will be greater in the central test section than in the load-introduction regions on each end. To do so, the cross- sectional area in the central section must be smaller than the area at each end. For metals and unreinforced plastics, the width of the specimen is gradually reduced leading into the central test section (Fig. 1). Although width tapering, or "dog-boning," appears to be an attractive option, in the case of UD composites, axial splitting of the specimen often occurs well before ultimate failure, efectively producing an untapered specimen. Tese axial cracks result from the relatively low shear strength of the UD composite material coupled with relatively high shear stresses in these regions. Although width tapering isn't an option, producing a specimen with greater thickness at each end is an attractive alternative. Rather than reduce the thickness of the composite specimen in the test section, the thickness at the ends is increased with bonded tabs (Fig. 2). Glass fabric/epoxy printed circuit board material is commonly used for tabbing for several reasons, including its commercial availably at low cost, its relatively low stifness and its high strength. Further, the bonded tabbing strips also can be machined in the same manner as the tested composite material when individual specimens are cut from a test panel. Bonded tabs not only prevent axial cracking but also protect the surface of the composite specimen from direct contact with the serrated grip faces, which could cut or tear it during loading. When a tabbed tensile specimen is tested, the load is introduced through shear at the gripped tab surfaces. Te tabbing material and the adhesive bond must have shear strength adequate to transfer the load into the composite to produce failure. However, it's not enough to simply produce specimen failure. A "good" tensile test is one that measures the fullest extent of the composite material's tensile strength during the test. Tat requires a uniform distribution of axial stress throughout the composite test, or gage, section between the tabs. Toward that end, a tabbing confguration must be designed to minimize the stress concentrations produced in the tab termi- nation region. Although there are several geometric and material parameters that can be used to design a tabbed tensile specimen that minimizes stress concentrations 1 , the following four parameters are, perhaps, the most important. TAB TAPER ANGLE: An efective means of minimizing the stress concentration is to taper the tab thickness, efectively "feathering" the load into the specimen gage section. Although it's tempting to use a very small tab taper angle to minimize stress concentrations, there are practical limits. Out-of-plane peel stresses produced at the tip of the tapered tab increase as the taper angle decreases, leading to failure of the adhesive bond. Tis design consideration is often over- looked, and it is not uncommon for the tips of the tabs to debond from the tensile specimen prior to ultimate failure — a highly undesir- able outcome because it efectively eliminates the benefts of the tab taper. Tab debonding can be detected as audible "pings" and, some- times, observed under low magnifcation during testing. Te choice of a tab taper angle involves a compromise between minimizing stress concentrations and preventing tab disbonding. ASTM D 3039 2 ... tensile strength and ulti- mate tensile strain values are the challenges. Tensile testing of composites: Simple in concept, difcult in practice Adhesive Thickness Tab Termination Region Tab Region Gage Section Tab Length Tab Taper Angle Specimen Thickness FIG. 1 Width-tapered tensile specimens with axial cracks. FIG. 2 Typical tabbed composite tensile specimen. Source (both illustrations) / Dan Adams