Numerical modeling of two-dimensional delamination growth in composite laminates with in-plane isotropy

The two-dimensional (2D) delamination growth in fiber-reinforced polymer (FRP) laminates with in-plane isotropy under Mode I loading condition was numerically investigated using finite element analyses. Two sizes of plate models were developed, focusing on different fracture stages. Cohesive elements were employed to simulate the fracture behavior in the presence of large-scale bridging (LSB). The influences of the pre-crack shape/area, loading zone shape/area and fracture resistance were parametrically studied. It was found that either a flatter pre-crack shape or a flatter loading zone shape could result in higher initial structural stiffness and less uniform distribution of the strain energy release rate (SERR) along the pre-crack perimeter during crack initiation and early propagation. However, they had only a minor effect on the stiffness after full fiber bridging development in all directions. The plates finally achieved constant stiffness, which increased linearly with the fracture resistance. The final crack shape was dependent on the loading zone shape and area, but the effects were relatively weak.

Specimen thickness dependence of large scale fiber bridging in mode I interlaminar fracture of carbon epoxy composite

John Botsis, Joël Cugnoni, Ebrahim Farmand Ashtiani

Large scale bridging is an important toughening mechanism in composite laminates that depends on the specimen geometry as well as the constituent materials. In this work, an iterative approach is applied to identify the effect of specimen thickness on traction-separation behavior in the bridging zone. Double cantilever beam specimens (DCB) with embedded arrays of wavelength-multiplexed fiber Bragg grating (FBG) sensors are subjected to monotonic mode I fracture loading. As processed specimens with thicknesses h = 2, 4, 8 and 10 mm as well as specimens of h = 4 mm milled down from the 8 and 10 mm are tested. Non-homogeneous strain distributions in the vicinity of the interlaminar crack plane are locally monitored by means of embedded FBGs. The measured strain data are used to quantify the bridging tractions associated with each specimen thickness and consequently the energy release rate (ERR) due to the bridging. The results show that the bridging zone length and the ERR at the plateau level increase with specimen thickness while the results for all specimens with h = 4 are the same thus excluding any processing effects. Moreover, the maximum stress of traction-opening in the bridging zone and crack opening displacement at the end of the zone are independent of thickness. In contrast, the rate of tractions decay depends on the specimen thickness in that it decreases with thickness scaling. Thus the so-called bridging law is not a material parameter. The identified traction-separation relations are employed in a cohesive zone model to predict fracture of DCB specimens with different thicknesses. (C) 2014 Elsevier Ltd. All rights reserved.

Elsevier2015

Delamination in cross-ply laminates: Identification of traction-separation relations and cohesive zone modeling

David Enrique Alanis Rojas, John Botsis, Joël Cugnoni, Ebrahim Farmand Ashtiani

Identification of traction separation relations is at the forefront of damage and fracture mechanics of composite laminates. Such relations are of particular interest in fracture of laminates consisting of layers of different fiber orientations. In this study, an iterative method based on internal strain measurements and parametric numerical modeling is employed to identify a traction-separation relation in quasi-static mode I dominated delamination of a cross-ply carbon fiber epoxy laminate. The results demonstrate that crack propagation is accompanied by a large bridging zone which is smaller than the zone in a uniaxial composite specimen of the same materials and linear dimensions. While the initiation value of the energy release rate (ERR) is the same, the ERR at steady state propagation and the maximum stress in the bridging zone are respectively 1.7 and 3.1 times larger than the corresponding values of the unidirectional specimen. The obtained traction-separation relation is employed in a cohesive zone model to predict the loading response and crack growth. The adopted approach is a step towards a better understanding of delamination in cross-ply composites. (C) 2015 Elsevier Ltd. All rights reserved.

Elsevier2015

Two-dimensional crack growth in FRP laminates and sandwich panels

Aida Cameselle Molares

Fiber-reinforced polymer (FRP) composite materials are currently being selected for the design of lightweight and efficient structural members in a wide number of engineering applications. Sandwich panels with glass fiber-reinforced polymer (GFRP) face sheets and low-density cores are notably one of the most common applications of FRP materials in the civil engineering field. The load-bearing capacity of FRP structures can be significantly reduced by delamination and debonding damage which, in actual structural members, may extend all around its perimeter, thus constantly changing the size of the crack front. However, most research efforts concerning the fracture characterization of delamination and debonding damage have focused on one dimensional (1D) fracture specimens where cracks propagate longitudinally with an approximately constant crack width, thus resulting in fracture properties that may lead to inaccurate predictions of fracture behavior in real structures. With the aim of establishing a more realistic fracture approach to delamination and debonding damage in FRP structures and thus improving their fracture characterization, the two dimensional (2D) crack growth in GFRP laminates and GFRP/balsa sandwich panels is investigated in this research. The 2D quasi-static delamination behavior in GFRP plates with embedded defects subjected to out-of-plane tensile loads was experimentally investigated. Increasing loads were obtained due to the disproportionate increase of the crack area throughout the propagation of the 2D crack. Analysis of the load-bearing and compliance responses indicated that a stiffening of the opened part of the plates occurred as a result of in plane stretching stresses that developed due to the boundary conditions associated with an embedded 2D crack. To investigate the effect of the stress stiffening and associated fracture mechanisms, the 2D delamination behavior was numerically studied. Results confirmed a 50% increase, compared to 1D fracture, in the total strain energy release rate (SERR) required to propagate the crack. This was correlated to the increase of the fiber-bridging area as a result of the increase in the flexural stiffness (from beam to plate) and the stress-stiffening effect. A numerically-based method, suitable for determining the mean total SERR involved in the Mode I-dominated 2D delamination of FRP laminates was further developed and validated. This method permits a reduction of experimental and computational efforts. The 2D fracture investigation was extended to GFRP/balsa sandwich panels whose 2D debonding behavior was experimentally studied under quasi-static and fatigue out of-plane tensile loads. Circular embedded disbonds were introduced at the face sheet/core interface. Stretching strains again developed, thereby activating shear fracture modes which, depending on the face sheet layup, triggered different crack propagation behaviors. The fatigue load-displacement hysteresis loops exhibited an increase in the cyclic stiffness due to the stiffening caused by the stretching stresses. The introduction of plies prone to develop fiber-bridging at the face sheet/core interface resulted in enhanced quasi-static debonding resistance, while under fatigue their fracture efficiency was highly dependent on the fatigue amplitude. High R-ratios improved fatigue performance due to a reduction in fiber-bridging crushing.

EPFL2019

Two-dimensional crack growth in FRP laminates and sandwich panels

Aida Cameselle Molares

EPFL2019