Compressive damage mechanism of GFRP composites under off-axisloading: Experimental and numerical investigations

H.W. Zhou a,⇑, H.Y. Yi a, L.L. Gui a, G.M. Dai b, R.D. Peng a, H.W. Wang c, Leon Mishnaevsky Jr. b,⇑a State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing 100083, ChinabDepartment of Wind Energy, Risø Campus, Technical University of Denmark, DK-4000 Roskilde, Denmarkc School of Mechanical Engineering, Tianjin University of Commerce, Tianjin 300134, China

a r t i c l e i n f o

Article history:Received 2 May 2013Received in revised form 5 June 2013Accepted 9 June 2013Available online 19 June 2013

Keywords:A. Polymer–matrix composites (PMCs)B. StrengthC. Finite element analysis (FEA)C. Computational modelingD. Electron microscopy

a b s t r a c t

Experimental and computational studies of the microscale mechanisms of damage formation and evolu-tion in unidirectional glass fiber reinforced polymer composites (GFRP) under axial and off-axis compres-sive loading are carried out. A series of compressive testing of the composites with different anglesbetween the loading vector and fiber direction were carried out under scanning electron microscopy(SEM) in situ observation. The damage mechanisms as well as stress strain curves were obtained inthe experiments. It was shown that the compressive strength of composites drastically reduces whenthe angle between the fiber direction and the loading vector goes from 0� to 45� (by 2.3–2.6 times),and then slightly increases (when the angle approaches 80–90�). At the low angles between the fiberand the loading vector, fiber buckling and kinking are the main mechanisms of fiber failure. With increas-ing the angle between the fiber and applied loading, failure of glass fibers is mainly controlled by shearcracking. For the computational analysis of the damage mechanisms, 3D multifiber unit cell models ofGFRP composites and X-FEM approach to the fracture modeling were used. The computational resultscorrespond well to the experimental observations.

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1. Introduction

Glass fiber reinforced polymer (GFRP) composites are character-ized by the high ratio of strength to weight and has been widelyapplied in wind turbine blades, among others [1]. Under serviceconditions, wind turbine blades are subject to the external loading,which includes the flapwise and edgewise bending loads, gravita-tional loads, inertia forces, loads due to pitch acceleration, as wellas torsional loading. The flapwise and edgewise bending loadscause high longitudinal, tensile and compressive stresses in thematerial. The upwind side of the blades is subject to tensile stres-ses, while the downwind side is subject to compression.

Thus, the analysis of the effect of the strength and behavior ofwind blade composites under off-axis and compressive loading isof great importance for their practical applications, and can beused as a basis for improving the wind turbine reliability.

The microscale mechanisms of the composite damage and fail-ure under compressive off-axis loadings have been studied in anumber of works. Lee and Waas [3] observed that glass fiber com-posites ‘‘demonstrate a splitting failure mode for a range of low

fiber volume fractions and a simultaneous splitting/kink bandingfailure mode for high fiber volume fractions’’. Hahn et al. [4] testeda fiber bundle embedded in epoxy casting and tested in compres-sion. They observed that the failure of Kevlar 49 and P-75 graphiteis controlled by kinking of fibrils, while other fibers – (T300 andT700 graphite and E-glass) failed by localized microbuckling. Hahnand Williams [5] summarized compression failure mechanisms inUD composites, and concluded that the predominant microscopicscale failure mode is being shear crippling (in the form of bucklingof fibers in a soft resin, and as the formation of a kink band in stiffresins).

Piggott and Harris [6] studied composites reinforced with car-bon, glass and aromatic polyamide fibers under compression. Theyobserved the ‘‘Rule of Mixtures’’ behavior in glass-polyester com-posites up to the volume fractions (Vf) of 0.31 for strength and0.46 for elastic modulus. At higher volume fraction, the compositestrength is proportional to the matrix yield strength. For the samematrix yield strength a composite with an epoxy resin matrix wasstronger than polyester based composites. At Vf = 0.30, carbon fibercomposites are only slightly less stiff and weaker in compressionthan in tension. Gonzalez and Llorca [7] studied experimentallyand numerically the micromechanisms of damage in UD carbonor glass under transverse compression. They observed dominantdamage mechanisms, interface decohesion and matrix plastic

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⇑ Corresponding authors. Tel.: +86 10 62331286; fax: +86 10 62331490 (H.W.Zhou), tel.: +45 46775729; fax: +45 46775758 (L. Mishnaevsky).

E-mail addresses: zhw@cumtb.edu.cn (H.W. Zhou), lemi@dtu.dk (L. MishnaevskyJr.).

Composites: Part B 55 (2013) 119–127

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