Feras H. Darwish

Defective Repairs of Laminated Solid Composites

This paper presents an experimental evaluation of the defective patch repairs of solid laminated composites. The study uses both destructive and nondestructive tests on full-scale repaired panels to evaluate the repair performance under static tension loading conditions. While most reported studies use small coupon specimens, this paper presents test results of full-scale panels that allow strain field monitoring around the patch. The experimental results presented in this paper are the baseline for more advanced study to evaluate the performance of defective repairs in severe loading conditions (fatigue) that is underway by the authors. The nondestructive tests are performed by Iowa State University while the destructive tests are performed by North Carolina A&T State University. The testing program consists of 14 panels of six-ply (300 mm 675 mm) ((60/60/0)s) quasi-isotropic laminates: three pristine (undamaged parent material panels), three damaged panels without repair, three good repairs, three defective repairs due to inserted engineered flaw, and two repairs with mismatched fiber orientation. Two types of defective repairs are investigated: inserting 1-in. circular disbond between the sixth ply and the parent material forms the first type while the second type is formed by fiber mismatching between the patch and the parent material. Based on the evaluation performed in this study, the restored tensile strength of both defective and good repairs is within 80% of the strength of the pristine panels.

PERFORMANCE OF PATCH REPAIRED COMPOSITE PANELS - STATIC AND FATIGUE

This paper focuses on studying the performance of bonded patch-scarf repair of full scale laminated composite panels. The study considers both nondestructive testing to characterize the quality of repair and destructive testing to evaluate the performance of repaired panels under static and fatigue loading conditions. AS4/3501-6 Carbon/epoxy prepreg material was used to fabricate 12 x 27-in six-ply quasi-isotropic (-60/60/0)s laminates. Scarf-patch technique as recommended in the Boeing repair manual was used to repair panels with 1- inch centered hole. The patching kit consisted of an adhesive film, filler ply and six layers of the same material and stacking sequence as of that used for the parent material. A defective repair was also engineered by inserting 1-inch circular Teflon flaw between the fifth and sixth layers of the patch. In this study, a total of 24 panels were prepared and divided into four categories: (a) Three pristine panels, (b) Three panels with 1-inch centered-hole, (c) Nine good repairs, and (d) Nine defective repairs. Three panels of each category were tested under static tensile loading to measure the tensile strength. Fatigue tests were conducted on six good repairs and six defective repairs under different load levels. According to the test results, bonded patch-scarf repair of composite laminates is an effective technique through which repaired panels can restore up to 93% of their tensile strength. A defective repair with 1-inch diameter surface delamination experienced a premature failure under fatigue loading.

Experimental and Analytical Modeling of Scarf Repaired Composite Panels

Experiments and finite element analysis (FEA) were performed on scarf-patch repaired composite panels. Tensile static tests were performed on pristine and repaired panels to evaluate their tensile strength. The obtained results showed that the repaired panels restored tensile strength by 95% of the pristine value. Subsequently, a verification finite element (FE) model was established. Two additional models, one with homogenized mechanical properties of the laminate and one that considered that the ply-by-ply properties were built to simulate the experimental repairs. The predictions of the two models agreed reasonably well with the experimental results and the optimum scarf angle was found at 2.5°.

Transient Response of a Clamped Slab under Pressure and Thermal Loads

The transient response of clamped thin slab under pressure and thermal loads is investigated through analytical and finite difference approaches. Due to the linear-elastic nature of the analysis, the superposition principle is implemented to treat each load individually and to obtain its corresponding response. The analysis introduced a new parameter, the pressure-temperature coupling term, in the mechanical stress solution. It is found that both mechanical and thermal stresses are of the same order of magnitude for coupling term values between 1000 and 10000 Pa/°C, the thermal stresses dominate for coupling term less than 1000 Pa/°C, and the mechanical stresses dominate otherwise.

A modelling strategy and strain concentration analysis for a countersunk hole in an orthotropic plate

Strain and stress analyses are performed for countersunk holes in orthotropic laminates under uniaxial tension. ANSYS software is implemented to establish finite element (FE) modelling strategies and to run the analysis. It is found that the homogenisation of the material properties can be adopted in the analyses of orthotropic plates, and is reasonably accepted in the strain-based analysis of quasi-isotropic plates. However, ply-by-ply modelling is needed for stress-based analysis of quasi-isotropic laminates. It is also found that in the homogenised models the strain concentration factor is always less than the stress concentration factor with differences less than 4% for 98% of the results.