Materials and Structures(2008)41: 879-890 Fig 1 (a) FRP products bars. lamellas. fabric and b)glass fiber fabric that as tested fibers depend on the volume ratio of fibers in the and dimension is based on the overall cross section of composite. The usual fiber content of fibers in the composI ite product(fiber matrix) lamellas is between 35 and 70 vol%o. The modulus of elasticity and tensile strength of final composites 3.2 Dry, one-way or two-way oriented can be calculated from the volume ratio of fibers and interwoven fibers matrix using Eqs. (1) and (2)[1] E=Eb·wib+Em·Vm (1) These products come to the market in the form of f=f·Vb+fm·v (2) without the matrix. In the process of strengthening where straps, or fabric should be applied on cleaned and even (leveled) element surface that has previously Er= modulus of elasticity of longitudinally ori ented FRP composites; been coated with a fresh synthetic glue of a specific quality. The straps or fabric should then be pressed Efib modulus of elasticity of fibers: Em=modulus of elasticity of matrix: toward treated surface, causing the glue to penetrate between fibers. Since the composite product, fibers Vtb= volume ratio of fibers in composit matrix,emerges only after the strap or fabric Vm= volume ratio of matrix in composite ously been treated with the glue, the volume proportion of the glue (matrix) significantly varies Vm volume of matrix in composite and it is very difficult to control thickness of such Vib= volume of fibers in composite composite. Therefore, the analysis of strengthenin fr= tensile strength of longitudinally oriented FRP that is oriented to the cross section area of the composite composite product is difficult to estimate. Most of the fib tensile strength of fibers; fm= tensile strength of matrix researchers recommend analysis that is based only on the properties of the fibers, i. e, their tensile strength, Equations (1)and(2) do not always match the which neglects the influence of the matrix. That kind experimental results, so it is necessary to determinate, of analysis underestimates the effectiveness of for special types of lamellas and rods, the real strengthening because the tests that were carried ou modulus of elasticity and the real tensile strength. So and the results of which will be shown later in this far,the standard for FRP product verification does not paper show that epoxy glue significantly increases the exist,but verification for plastic products is usually strength of composite product. It should be pointed made according to one of the two standards: either out that the tensile strength of fabric is smaller than ASTM D 3039/D 3039M(ASTM 1995)[3], or en the sum of strengths of all fibers In specimens, some ISO527-5(sO1997)[4 of the fibers were initially stretched more than others The real properties of the composites should be and thus they broke earler The fabric strength, fab, btained by experiment and those values will later be which could be expressed as the first part of the eq used for further analysis. The analysis of resistance (2), was tested according to the standards [1,3, 4]fibers depend on the volume ratio of fibers in the composite. The usual fiber content of fibers in lamellas is between 35 and 70 vol%. The modulus of elasticity and tensile strength of final composites can be calculated from the volume ratio of fibers and matrix using Eqs. (1) and (2) [1]. Ef ¼ Efib
mfib þ Em
mm ð1Þ ff ¼ ffib
mfib þ fm
mm ð2Þ where: Ef = modulus of elasticity of longitudinally ori￾ented FRP composites; Efib = modulus of elasticity of fibers; Em = modulus of elasticity of matrix; vfib = volume ratio of fibers in composite: vfib ¼ Vfib VfibþVm ; vm = volume ratio of matrix in composite: vm ¼ Vm VfibþVm ; Vm = volume of matrix in composite; Vfib = volume of fibers in composite; ff = tensile strength of longitudinally oriented FRP composite; ffib = tensile strength of fibers; fm = tensile strength of matrix; Equations (1) and (2) do not always match the experimental results, so it is necessary to determinate, for special types of lamellas and rods, the real modulus of elasticity and the real tensile strength. So far, the standard for FRP product verification does not exist, but verification for plastic products is usually made according to one of the two standards: either ASTM D 3039/D 3039M (ASTM 1995) [3], or EN ISO 527-5 (ISO 1997) [4]. The real properties of the composites should be obtained by experiment and those values will later be used for further analysis. The analysis of resistance and dimension is based on the overall cross section of the composite product (fiber + matrix). 3.2 Dry, one-way or two-way oriented interwoven fibers These products come to the market in the form of fiber straps and fabrics. They consist of fibers only, without the matrix. In the process of strengthening, straps, or fabric should be applied on cleaned and even (leveled) element surface that has previously been coated with a fresh synthetic glue of a specific quality. The straps or fabric should then be pressed toward treated surface, causing the glue to penetrate between fibers. Since the composite product, fibers + matrix, emerges only after the strap or fabric is pressed toward the element surface that has previ￾ously been treated with the glue, the volume proportion of the glue (matrix) significantly varies and it is very difficult to control thickness of such composite. Therefore, the analysis of strengthening that is oriented to the cross section area of the composite product is difficult to estimate. Most of the researchers recommend analysis that is based only on the properties of the fibers, i.e., their tensile strength, which neglects the influence of the matrix. That kind of analysis underestimates the effectiveness of strengthening because the tests that were carried out and the results of which will be shown later in this paper show that epoxy glue significantly increases the strength of composite product. It should be pointed out that the tensile strength of fabric is smaller than the sum of strengths of all fibers. In specimens, some of the fibers were initially stretched more than others and thus they broke earlier. The fabric strength, ffab, which could be expressed as the first part of the Eq. (2), was tested according to the standards [1, 3, 4]. Fig. 1 (a) FRP products: bars, lamellas, fabric and (b) glass fiber fabric that was tested 882 Materials and Structures (2008) 41:879–890