D. Ouinas

Numerical modelling of the interaction macro–multimicrocracks in a plate under tensile stress

Abstract In this work, the evaluation of the stress intensity factor of a macrocrack in interaction with one or several microcracks in a material containing a geometrical defect was investigated. Several configurations were considered in order to apprehend the mechanisms induced by the interaction effect and in particular the effects of reduction and/or amplification of the stress field between macro and single or multiple microcracks. The obtained results show that, macro–microcrack spacing is an important parameter if the microscopic cracks is relatively close to the macrocrack tip. The macrocrack has the tendency to accelerate as it propagates towards the microcrack. When the relative distance characterizing this spacing is higher than 0.2, the interaction effect can be neglected and the stress intensity factor remains unchanged for both defect types. When this ratio is lower than 0.2, the interaction between the two defects becomes significant and the stress intensity factor at the macrocrack tip strongly increases. For an inclined microcrack configuration, the interaction becomes significant and cannot be neglected.

SIF for Double- and Single-sided Composite Repair in Mode I and Mixed Mode

In this study, the finite element is used to analyze and compare the performances of the double- and single-sided composite repair in aircraft structures in mode I and mixed mode. The stress intensity factors in modes I and II at the crack tip are computed to perform the objective of the study. The effects of the different parameters of repair such as adhesive and patch properties on the SIF variation were highlighted. The obtained results show that the adhesive properties must be optimized in order to increase the advantage of the double patch and to avoid adhesive failure. The patch properties have a significant effect on the beneficial effects of the double symmetric patch.

Effect of Disbonding Between a Composite Patch and a Cracked Aluminum Plate on the Stress Intensity Factor

Bonded composite patch repairs to metallic structures is receiving increased attention in the recent years. It offers various advantages over rivetted doubler, particularly for airframe repairs. This article presents the crack growth behavior of an edge cracked aluminum plate repaired using boron/epoxy composite patch. The effect of the size and location of this disbond on the tensile behavior of the repaired plate is investigated. An analysis procedure, involving finite element modeling of the cracked plate, adhesive, and composite patch, is conducted to compute the stress intensity factor. The results indicate that the crack growth rate is dominated by the stress intensity factor and the size of the pre-existing disbonds. Cracking of the plate and propagation of the disbond results in an increase in the patch deformation.

Erosive Wear Modeling of Polymeric Composite Materials

To study the behavior of composite materials to erosive wear, we have carried out bench tests simulating the erosive wear according to ASTM D 673 standard. The principle consists in impacting composite specimens by an abrasive water jet containing sand particles, taking into account several parameters affecting the erosive wear. The first material consisting of thermoplastic matrix reinforced with E-glass fiber and the second with 8H Satin carbon fiber where as the thermoset matrix material was reinforced with 2/2 Twill glass fiber. Influential parameters such as composite hardness, abrasive particle size, speed, and impact angle were taken into account. During the experimental study, only the most influential parameters have been taken into account such as: hardness of the composite materials, abrasive size, abrasive particle speed, and angle of impact. Applying Design of experiment (DOE) strategy with experimental planning, a mathematical model was obtained describing the wear of these materials as a function of the most influential parameters led to the following results: Wear is maximum when the angle of incidence of the jet is normal to the specimen surfaces; wear is minimum when the angle of incidence of the jet is (α = 30°); mass loss is proportional to abrasive particle size and speed and inversely proportional to composite hardness.

The optimization thickness of single/double composite patch on the stress intensity factor reduction

The repairing process of structures using bonding of composite materials is an efficient and economic way of increasing service life of damaged structures. The finite element method is developed and applied to assist in the design, assessment and optimization of the proposed plans before they are implemented, therefore reducing cost and time. In this study, the finite element method is used to analyze the behavior of a repaired crack using single and double circular composite patches by determining the stress intensity factor reduction. The effects of geometrical and mechanical properties of the patch on the fracture parameters are highlighted. The results show that the thickness gain and the reduction of the asymptotic value of the SIF in the case of the double patch is 7% when compared to the single one. Thus, the gain in the thickness decreases when the thickness of the bonded patch increases up to a value eP = 0.8 mm and beyond this value, it starts increasing. However, an inverse behavior occurs for the mixed mode. In addition, whatever is the size of the strengthened crack, the gain in thickness exceeds 16% in mode I and 20% in mixed mode.

Bonded Composite Repairs Analysis in Pipes Under Internal Pressure Using Finite Element Technics

The use of composite systems as a repair methodology in the pipeline industry has grown in recent years. However, there are still no widely accepted standards governing the design and installation of such systems but the potential cost savings can be significant. Whilst composite repairs are gaining acceptance and approvals they do not as yet have full regulatory approval and this should be considered during the repair assessment process. In this study, the analysis of the behaviour of circumferential through cracks in repaired pipe with bonded composite patch subjected to internal pressure is performed using three dimensional finite element methods. The stress intensity factor is used as a fracture criteria which describes the elastic behavior of the structure. The obtained results show that the presence of the bonded composite patch reduces significantly the stress intensity factor, what can improve the lifespan of the pipe.

Effect of notch and crack initiating from notch on the behavior of adhesively bonded composite laminated plate

Reinforcing structures using bonded composites is an effective and economic method to increase the service life of damaged structures. In this study, we use the finite element method to analyze the behavior of a laminated structure by computing the stress concentration factor and the stress intensity factor at the crack tip in mode I. The effects of the mechanical properties of the laminate layers and the adhesive layer on the behavior of the assembly are highlighted. The results show that the factors of concentration and stress intensity are affected by the orientation of fibers and the presence of geometrical defects. The magnitude of the latter affects and amplifies the stress concentrations at the crack tip.