Two-dimensional crack growth in FRP laminates and sandwich panels

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.

Load history effects in fibre-reinforced polymer composite materials

Abdolvahid Movahedirad

Today, it is widely accepted that fatigue is one of the most common failure mechanisms in structural components, and pure static failure is rarely observed. Engineering structures are subjected to complex mechanical fatigue loading histories, which cause damage that eventually leads to functional and structural deterioration. One reason for this complexity is that the cyclic loading is accompanied by time-dependent deformations including creep and recovery. Fiber-reinforced polymer (FRP) composite materials are susceptible to time-dependent deformation, due to the viscoelastic nature of their matrix. Therefore, fatigue-creep-recovery interactions can play a role in the overall deformation behavior of FRP composite materials. The limited experimental investigations of FRP composite materials showed considerable effects of time-dependent phenomena on the fatigue behavior. However, no systematic analysis to clearly determine the effect of creep and recovery on developed damage mechanisms of FRP composites has yet been conducted. The aim of this research is to understand the fatigue behavior of glass fiber-reinforced composite (GFRP) materials and establish a reliable methodology to analytically/numerically model the fatigue behavior of FRP composite materials. The fatigue behavior of (±45)2S angle-ply glass/epoxy laminated composite at different stress levels has been investigated in this thesis. It was observed that cyclic-dependent parameters (cyclic creep, fatigue stiffness, and hysteresis loop area) as well as damage development were a function of applied stress levels. In tension-tension continuous fatigue, by decreasing the stress ratio, the damage became more severe, fatigue stiffness exhibited greater changes, the hysteresis loop area became larger and a higher and more uneven self-generated temperature was recorded. The effect of loading interruption on fatigue behavior was studied by subjecting the specimens to tension-tension constant amplitude fatigue loading interrupted at ¿max by creep intervals. At high stress levels, the fatigue life was improved when hold time was short as a result of the fiber realignment caused by the creep strain, while longer hold time caused considerable creep damage and therefore premature fatigue failure of the specimens. The cyclic loading was also interrupted at zero stress level. The stiffness degradation rates and hysteresis loop area decreased during the cyclic loading phase, while specimen recovery occurred after each stress removal. It was observed that specimens loaded under interrupted fatigue exhibited longer fatigue lives than those continuously loaded at high stress levels as a result of partial fatigue stiffness restoration and crack blunting. It was concluded that fatigue design allowables based on continuous fatigue could be conservative. A model based on the theory of viscoelasticity was developed to simulate specimen recovery in viscoelastic materials and predict their cyclic-dependent mechanical properties. The developed model predicted the cyclic-dependent parameters well - the fatigue stiffness, hysteresis loop area, cyclic creep, storage and loss moduli as well as tan(¿).

EPFL2019

Mixed-Mode Static and Fatigue Failure Criteria for Adhesively-Bonded FRP Joints

Moslem Shahverdi

Fiber-reinforced polymer (FRP) composites are increasingly used in many load-bearing structures. Joints are the most critical structural elements as they often become the weak links in engineering structures. Bonding and bolting are two of the main joining techniques used for composite elements but, although bolted and bonded joints have their advantages and disadvantages, adhesively-bonded joints are often preferred. This is thanks to their better performance in terms of cost-effective structures with uniformly distributed stresses without the need for cutting fastener holes that result in high stress concentrations in the FRP adherends at the edges of the holes. In addition, due to the absence of stress concentrations, adhesively-bonded joints exhibit better fatigue behavior. However, the efficient and reliable use of adhesively-bonded joints requires a thorough understanding of their failure mechanisms under mixed-mode quasi-static and fatigue loading conditions. In real structural applications, failure of adhesively-bonded joints occurs due to crack initiation and propagation under varying mixed-mode ratios. Since it is difficult to describe this progressive fracture in a structural joint, fracture joints with constant mode-mixity ratios are used to determine the strain energy release rates for crack initiation and propagation. Subsequently, mixed-mode failure criteria based on the determined strain energy release rates can be established and used to model/predict the fatigue and fracture behaviors of structural joints. The main objectives of this thesis were to understand the fracture and fatigue behaviors of adhesively-bonded FRP fracture joints under Mode I, Mode II, and mixed-Mode I/II loading conditions and, consequently, develop mixed-mode static and fatigue failure criteria applicable to structural joints. To fulfill these objectives a combined experimental-analytical-numerical study was undertaken. Double cantilever beam (DCB-Mode I), end-load split (ELS-Mode II), and mixed-mode bending (MMB-Mode I/II) specimens were examined under quasi-static and constant amplitude fatigue loading. Analytical approaches were used to determine the strain energy release rates under quasi-static and fatigue loading, whereas phenomenological models were developed to characterize the fatigue and fracture behaviors of the joints. A new approach designated the “extended global method” was established and applied for the analysis of the quasi-static and fatigue experimental data and the fracture mode partitioning in the mixed-mode experiments. In parallel, non-linear finite element models were developed to simulate the quasi-static fracture behavior and investigate the effects of both the existing asymmetry and the observed fiber bridging on the fracture behavior of the examined joints. The bridging zone was modeled by zero-thickness cohesive elements. An exponential traction-separation description of the cohesive zone model was shown to appropriately model the fiber bridging and calculate its contribution to the fracture energy under quasi-static loading. The FE models were successfully employed for the separation of the fracture parameters, i.e. strain energy released at the crack tip (Gtip) and the amount of energy released along the crack-bridging zone (Gbr), in the quasi-static mixed-mode failure criterion for crack propagation. In addition to the studies on the quasi-static fracture behavior of the joints, a new phenomenological fatigue crack growth formulation for the modeling and prediction of Rratio effects on the Mode I fatigue behavior of adhesively-bonded joints was also derived. The model was used for prediction of the fatigue behavior of the examined joints under different R-ratios. The results proved that the model could accurately simulate the fatigue behavior of the examined joints and was also capable of predicting behavior exhibited under unseen loading conditions, i.e. the R-ratios used for estimating the model parameters were different from those used for validating its prediction capability. The experimental results and numerical analyses combined with the phenomenological models were used to establish mixed-mode static and fatigue failure criteria for crack initiation and crack propagation for the adhesively-bonded joints. The derived mixed-mode failure criteria can be used for simulating/predicting the crack initiation and progressive crack propagation in structural joints comprising the same adhesive and adherends.

EPFL2013

Fracture and Fatigue of Adhesively-Bonded Fiber-Reinforced Polymer Structural Joints

Ye Zhang

Being good structural replacement for other conventional material, the pultruded glass fiber reinforced polymer (GFRP) profiles are being increasingly used in civil engineering structures. The connection between components is considered the most suspect area for failure initiation. The adhesive bonding is preferred for FRP composite structures, rather than the mechanical fastening, due to the brittle failure nature of composite materials. During past decades, many efforts have been made by researchers to better understand the mechanism of adhesive bonding, to analyze the stress distributions and to improve the strength of composite structural joints. However there is still no commonly accepted design code/standard existing for adhesively-bonded joints in civil engineering infrastructures since several important knowledge gaps are to be filled. Besides the joint strength at failure, the characterization and modeling of the progressive failure process, in particular involving the so-called crack initiation and propagation phases, is also an important concern. By employing the strain energy release rate (SERR) as the fracture parameter, the linear-elastic fracture mechanics (LEFM) approach is considered an efficient method to model the fracture behavior of structural joints. However, due to the uncontrollable crack initiation and the complex geometric configurations, the crack measurement techniques and the calculation method for the SERR are to be validated. In fracture mechanics, the fracture of a material or component can be described by a single mode or the combinations of the following three basic modes: opening mode (Mode I), shearing mode (Mode II), and tearing mode (Mode III). During the fracture of a structural joint, crack initiation and propagation are driven by combined through-thickness tensile (peeling), and shear stresses, thus resulting in a mixed mode fracture. In order to use the fracture results of structural joints to form the mixed fracture criterion for a specific composite material, a feasible analytical or numerical method are to be developed to determine the Mode I and II components of the SERR during fracture. Although many efforts have been made to better understand the short-term behavior of structural joint under quasi-static loading, the long-term performance under fatigue loading and different environmental conditions is a more demanding task when adhesively-bonded joints are applied in a real structure. Most of structural failures occur due to mechanisms that are driven by fatigue loading and for composite structures, the fatigue produced by the repeated application of live load is more critical due to its lighter self-weight, in other words the lower dead load. Besides the fatigue loading, a structure in practice may also experience the combined environmental effects of two basic factors: temperature and humidity. These environmental conditions may directly affect properties of structural joints, including the failure mechanism, the stiffness and strength, the crack initiation and propagation and etc.. Thus, the missing knowledge and confidences in the long-term behavior under cyclic loading and the durability under different environmental conditions are the main obstacles to the further development of FRP composites in civil engineering infrastructures. In this research, the mechanical and fracture behavior of adhesively-bonded double-lap and stepped-lap joints (DLJs and SLJs) composed of pultruded GFRP laminates and an epoxy adhesive were experimentally and numerically investigated under both quasi-static and fatigue loadings. The crack measurement techniques and the calculation methods for the SERR were validated for DLJs and SLJs. The LEFM approach was successfully applied to characterize and model the progressive failure process of structural joints. The Mode I and II components of the SERR of DLJs and SLJs were determined using the Virtual Crack Closure Technique in finite element analysis. Combining with the results of pure Mode I and II experiments, a mixed mode fracture criterion for pultruded GFRP composite was formed. Under fatigue loading, the fatigue behavior of structural joints was successfully modeled by using the stiffness-based and fracture mechanics approaches, besides the F-N curves. Based on the stiffness degradation, a linear and a sigmoid non-linear model were established and the fatigue live corresponding to the failure and allowable stiffness degradation can be predicted. Concerning fracture mechanics approach, the Fatigue Crack Growth (FCG) curves were formed for DLJs and SLJs and the corresponding fracture parameters were obtained. Similarly to stiffness-based approach, fatigue lives corresponding to the failure and allowable crack length can be predicted. The environmental effects on both short- and long-term performances of structural joints were experimentally evaluated and numerically modeled based on experimental results. The temperature-dependent joint stiffness can be predicted using the finite element analysis based on the thermomechanical properties of constituent materials. A relationship between the equivalent quasi-static joint strength under different environmental conditions and the cyclic stresses and the fatigue life was established.

EPFL2010