In-situ SEM study of transverse cracking and delamination in laminated composite materials

Abstract Transverse microcrack growth and delamination are two key damage mechanisms in laminated composite materials, and while often treated separately in damage prediction studies, they are, in fact, highly coupled. Essentially, transverse cracks initiate around fibres, coalesce and grow until they extend to ply boundaries, at which point they initiate micro-delaminations. Under increasing load these micro-delaminations eventually coalesce to form macroscopic delaminations, which severely reduce material stiffness and lead to catastrophic failure of the composite structure. This paper presents an investigation into how altering transverse crack densities can influence the growth of delaminations. Novel in-situ SEM micromechanical testing and acoustic damage detection techniques were coupled and used to determine transverse crack initiation loads, transverse crack density, and local micro-delamination lengths for a number of cross-ply laminates. The laminates were loaded in a four-point bending mode to induce crack opening direct stresses on the tension side. To examine the effect of combined direct and shear stresses, the laminates were also loaded in a three-point bending mode, and suitable comparisons between both bending modes allowed for the influence of the shear stress to be isolated. The main variable under investigation is the thickness of the transverse ply block, and it is shown that increasing the number of transverse plies (i.e. thickness) can significantly increase the load carrying capacity of the laminate by reducing the transverse crack density. It was found that the lower transverse crack densities meant that the micro-delaminations which initiated at the ply boundary required significantly greater stress to fully coalesce as the distance between transverse cracks was greater. Once micro-delamination had initiated, its length was found to be linearly related to the load applied. For all layups investigated, the average micro-delamination length seen immediately prior to catastrophic failure was approximately 1.2 times the thickness of the tensile 90° ply portion of the laminate.

[1]  B. Cox,et al.  The evolution of a transverse intra-ply crack coupled to delamination cracks , 2010 .

[2]  A.S.D. Wang,et al.  Initiation and Growth of Transverse Cracks and Edge Delamination in Composite Laminates Part 1. An Energy Method , 1980 .

[3]  Shoufeng Hu,et al.  The formation and effect of outer-ply microcracks in cross-ply laminates: A variational approach , 1992 .

[4]  Peter Gudmundson,et al.  Thermoelastic properties in combined bending and extension of thin composite laminates with transverse matrix cracks , 1997 .

[5]  Sheng Liu,et al.  Delamination and matrix cracking of cross-ply laminates due to a spherical indenter , 1993 .

[6]  G. Lubineau,et al.  Understanding the mechanisms that change the conductivity of damaged ITO-coated polymeric films: A micro-mechanical investigation , 2014 .

[7]  P. Ladevèze,et al.  Pont entre les « micro » et « méso » mécaniques des composites stratifiés , 2003 .

[8]  Leif A. Carlsson,et al.  Experimental characterization of advanced composite materials , 1987 .

[9]  Conor T. McCarthy,et al.  A highly efficient user-defined finite element for load distribution analysis of large-scale bolted composite structures , 2011 .

[10]  John A. Nairn,et al.  Fracture mechanics analysis of coating/substrate systems Part I: Analysis of tensile and bending experiments , 2000 .

[11]  Pedro P. Camanho,et al.  Prediction of in situ strengths and matrix cracking in composites under transverse tension and in-plane shear , 2006 .

[12]  Gilles Lubineau,et al.  Estimation of residual stresses in laminated composites using field measurements on a cracked sample , 2008 .

[13]  Masaki Hojo,et al.  Evaluation of interfacial strength in CF/epoxies using FEM and in-situ experiments , 2006 .

[14]  Pierre Ladevèze,et al.  On a damage mesomodel for laminates: micro–meso relationships, possibilities and limits , 2001 .

[15]  John A. Nairn,et al.  2.12 – Matrix Microcracking in Composites , 2000 .

[16]  J. Rebiere,et al.  Strain Energy Release Rate Analyse of Matrix Micro Cracking in Composite Cross-Ply Laminates , 2011 .

[17]  H. Mcmanus,et al.  On Microcracking in Composite Laminates under Thermal and Mechanical Loading , 1995, Engineering Plastics.

[18]  Costas Soutis,et al.  Analysis of multiple matrix cracking in [±θm/90n]s composite laminates. Part 2: Development of transverse ply cracks , 1992 .

[19]  J. Tong,et al.  On matrix crack growth in quasi-isotropic laminates—I. Experimental investigation , 1997 .

[20]  Pierre Ladevèze,et al.  An enhanced mesomodel for laminates based on micromechanics , 2002 .

[21]  E. Gamstedt,et al.  Micromechanisms in tension-compression fatigue of composite laminates containing transverse plies , 1999 .

[22]  J. E. Bailey,et al.  On multiple transverse cracking in glass fibre epoxy cross-ply laminates , 1978 .

[23]  Yan-qing Yang,et al.  SEM in situ study on the mechanical behaviour of SiCf/Ti composite subjected to axial tensile load , 2011 .

[24]  P. T. Curtis,et al.  On the transverse cracking and longitudinal splitting behaviour of glass and carbon fibre reinforced epoxy cross ply laminates and the effect of Poisson and thermally generated strain , 1979, Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences.

[25]  Joseph G. Sikora,et al.  Detection of Micro-Leaks through Complex Geometries under Mechanical Load and at Cryogenic Temperature , 2001 .

[26]  M. Rigamonti,et al.  Tension fatigue analysis and life prediction for composite laminates , 1989 .

[27]  Pierre Ladevèze,et al.  Towards a bridge between the micro- and mesomechanics of delamination for laminated composites , 2006 .

[28]  Yves Chemisky,et al.  In situ damage mechanisms investigation of PA66/GF30 composite: Effect of relative humidity , 2014 .