A comparative study for beams on elastic foundation models to analysis of mode-I delamination in DCB specimens

Shokrieh, Mahmood Mehrdad,Heidari-Rarani, Mohammad Techno-Press 2011 Structural Engineering and Mechanics, An Int'l Jou Vol.37 No.2

The aim of this research is a comprehensive review and evaluation of beam theories resting on elastic foundations that used to model mode-I delamination in multidirectional laminated composite by DCB specimen. A compliance based approach is used to calculate critical strain energy release rate (SERR). Two well-known beam theories, i.e. Euler-Bernoulli (EB) and Timoshenko beams (TB), on Winkler and Pasternak elastic foundations (WEF and PEF) are considered. In each case, a closed-form solution is presented for compliance versus crack length, effective material properties and geometrical dimensions. Effective flexural modulus ($E_{fx}$) and out-of-plane extensional stiffness ($E_z$) are used in all models instead of transversely isotropic assumption in composite laminates. Eventually, the analytical solutions are compared with experimental results available in the literature for unidirectional ($[0^{\circ}]_6$) and antisymmetric angle-ply ($[{\pm}30^{\circ}]_5$, and $[{\pm}45^{\circ}]_5$) lay-ups. TB on WEF is a simple model that predicts more accurate results for compliance and SERR in unidirectional laminates in comparison to other models. TB on PEF, in accordance with Williams (1989) assumptions, is too stiff for unidirectional DCB specimens, whereas in angle-ply DCB specimens it gives more reliable results. That it shows the effects of transverse shear deformation and root rotation on SERR value in composite DCB specimens.

Experimental and analytical studies on one-way concrete slabs reinforced with GFRP molded gratings

Shokrieh Mahmood Mehrdad,Heidari-Rarani Mohammad 국제구조공학회 2009 Steel and Composite Structures, An International J Vol.9 No.6

Corrosion of steel rebars in bridge decks which are faced to harsh conditions, is a common problem in construction industries due to the porosity of concrete. In this research, the behavior of one-way concrete slabs reinforced with Glass fiber reinforced polymer (GFRP) molded grating is investigated both theoretically and experimentally. In the analytical method, a closed-form solution for load-deflection behavior of a slab under four-point bending condition is developed by considering a concrete slab as an orthotropic plate and defining stiffness coefficients in principal directions. The available formulation for concrete reinforced with steel is expanded for concrete reinforced with GFRP molded grating to predict ultimate failure load. In finite element modeling, an exact nonlinear behavior of concrete along with a 3-D failure criterion for cracking and crushing are considered in order to estimate the ultimate failure load and the initial cracking load. Eight concrete slabs reinforced with steel and GFRP grating in various thicknesses are also tested to verify the results. The obtained results from the models and experiments are relatively satisfactory.

Fatigue modeling of chopped strand mat/epoxy composites

Shokrieh, M.M.,Esmkhania, M.,Taheri-Behrooz, F. Techno-Press 2014 Structural Engineering and Mechanics, An Int'l Jou Vol.50 No.2

In the present research, fatigue behavior of chopped strand mat/epoxy composites has been studied with two different techniques. First, the normalized stiffness degradation approach as a well-known model for unidirectional and laminated composites was utilized to predict the fatigue behavior of chopped strand mat/epoxy composites. Then, the capability of the fatigue damage accumulation model for chopped strand mat/epoxy composites was investigated. A series of tests has been performed at different stress levels to evaluate both models with the obtained results. The results of evaluation indicate a better correlation of the normalized stiffness degradation technique with experimental results in comparison with the fatigue damage accumulation model.

Flexural fatigue modeling of short fibers/epoxy composites

M.M. Shokrieh,A.R. Haghighatkhah,M. Esmkhani 국제구조공학회 2017 Structural Engineering and Mechanics, An Int'l Jou Vol.64 No.3

In the present research, an available flexural stiffness degradation model was modified and a new comprehensive model called "X-NFSD" was developed. The X-NFSD model is capable of predicting the flexural stiffness degradation of composite specimen at different states of stresses and at room temperature. The model was verified by means of different experimental data for chopped strand mat/epoxy composites under displacement controlled bending loading condition at different displacements and states of stresses. The obtained results provided by the present model are impressively in very good agreement with the experimental data and the mean value of error of 5.4% was achieved.

Fatigue modeling of chopped strand mat/epoxy composites

M.M. Shokrieh,M. Esmkhani,F. Taheri-Behrooz 국제구조공학회 2014 Structural Engineering and Mechanics, An Int'l Jou Vol.50 No.2

In the present research, fatigue behavior of chopped strand mat/epoxy composites has beenstudied with two different techniques. First, the normalized stiffness degradation approach as a well-known model for unidirectional and laminated composites was utilized to predict the fatigue behavior of chopped strand mat/epoxy composites. Then, the capability of the fatigue damage accumulation model for chopped strand mat/epoxy composites was investigated. A series of tests has been performed at different stress levels to evaluate both models with the obtained results. The results of evaluation indicate a better correlation of the normalized stiffness degradation technique with experimental results in comparison with the fatigue damage accumulation model.

A comparative study for beams on elastic foundation models to analysis of mode-I delamination in DCB specimens

Mahmood Mehrdad Shokrieh,Mohammad Heidari-Rarani 국제구조공학회 2011 Structural Engineering and Mechanics, An Int'l Jou Vol.37 No.2

The aim of this research is a comprehensive review and evaluation of beam theories resting on elastic foundations that used to model mode-I delamination in multidirectional laminated composite by DCB specimen. A compliance based approach is used to calculate critical strain energy release rate (SERR). Two well-known beam theories, i.e. Euler-Bernoulli (EB) and Timoshenko beams (TB), on Winkler and Pasternak elastic foundations (WEF and PEF) are considered. In each case, a closed-form solution is presented for compliance versus crack length, effective material properties and geometrical dimensions. Effective flexural modulus (E_fx) and out-of-plane extensional stiffness (E_z) are used in all models instead of transversely isotropic assumption in composite laminates. Eventually, the analytical solutions are compared with experimental results available in the literature for unidirectional ([0˚]_6) and antisymmetric angle-ply ([±30˚]_5, and [±45˚]_5) lay-ups. TB on WEF is a simple model that predicts more accurate results for compliance and SERR in unidirectional laminates in comparison to other models. TB on PEF, in accordance with Williams (1989) assumptions, is too stiff for unidirectional DCB specimens, whereas in angle-ply DCB specimens it gives more reliable results. That it shows the effects of transverse shear deformation and root rotation on SERR value in composite DCB specimens.

A comparison of destructive behaviors of distilled water, salty water, sulfuric acid and heat on glass/vinyl ester composites

Asli, S.A.,Shokrieh, M.M.,Kamangar, M.A. Techno-Press 2021 Composite materials and engineering Vol.3 No.3

In the present paper, the destructive behavior of distilled water, salty water, sulfuric acid, and heat on glass/vinyl ester composites was investigated by experimental methods. Hetron 922 vinyl ester resin and two types of mat and woven glass fibers as the reinforcements were used to fabricate composite test samples. All samples were immersed in distilled water, salty water, and sulfuric acid with three different concentrations. The tests were performed at 20℃ and 70℃ for the exposure duration of 1, 2, 4, and 8 weeks. Bending tests were performed after aging for all composite samples to check the degradation of the bending modulus and strength. The results show that the effect of distilled water, in comparison with salty water, on the degradation of composite samples was significant. On the other hand, almost non-sensitivity of concentrations of salty water on the weight gain of specimens has been observed. In addition, it was also observed that the degradation of samples at 70℃ temperature is much more than that of at 20℃. Also, it was observed that the flexural modulus of virgin specimens exposed to salty water (2% concentration) has been recovered just after two weeks of immersion. Furthermore, in some cases, composite samples under the sulfuric acid solution have lost almost 80% of their mechanical properties.

Experimental determination of tensile strength and K_Ic of polymer concretes using semi-circular bend (SCB) specimens

Aliha, M.R.M.,Heidari-Rarani, M.,Shokrieh, M.M.,Ayatollahi, M.R. Techno-Press 2012 Structural Engineering and Mechanics, An Int'l Jou Vol.43 No.6

An experimental method was suggested for obtaining fracture toughness ($K_{Ic}$) and the tensile strength (${\sigma}_t$) of chopped strand glass fiber reinforced polymer concretes (PC). Semi-circular bend (SCB) specimens subjected to three-point bending were used for conducting the experiments on the PC material. While the edge cracked SCB specimen could be used to evaluate fracture toughness, the tensile strength was obtained from the un-cracked SCB specimen. The experiments showed the practical applicability of both cracked and un-cracked SCB specimens for using as suitable techniques for measuring $K_{Ic}$ and ${\sigma}_t$ in polymer concretes. In comparison with the conventional rectangular bend beam specimen, the suggested SCB samples need significantly less material due to its smaller size. Furthermore, the average values of ${\sigma}_t$ and $K_{Ic}$ of tested PC were approximately 3.5 to 4.5 times the corresponding values obtained for conventional concrete showing the improved strength properties of PC relative to the conventional concretes.

Low Velocity Impact Behavior of 3D Hollow Core Sandwich Composites Produced with Flat-Knitted Spacer Fabrics

Mohammad Azadian,Hossein Hasani,Mahmoud Mehrdad Shokrieh 한국섬유공학회 2018 Fibers and polymers Vol.19 No.12

Impact failure of newly-designed textile composites from three-dimensional integrated weft-knitted spacer fabrics (3D-IWK-SF) was aimed to be investigated under drop weight loading. Using a computerized flat knitting machine, three different 3D-IWK-SF as the composite reinforcements varied in their cross-sectional shapes, were produced from E-glass yarns. The produced fabrics were then impregnated with unsaturated polyester resin via the vacuum assisted resin transfer molding (VARTM) which eventually results in 3D integrated spacer weft-knitted sandwich composites (3D-IWK-SC). For comparing the results, an additional core/sheet sandwich composite composed of the same sheet and the polyurethane foam core was also fabricated. From the resulted values of contact force, it was concluded that 3D-IWK-SC of V-shaped crosssection had the highest resistance against low velocity impact load. On the other hand, the evaluated damaged area of both Vshaped 3D-IWK-SC and the foam core sandwich composites were lower than the other samples. Moreover, it was observed that matrix crack-formation in face-sheets of all samples as well as transversely propagated cracks at the connecting layers of 3D-IWK-SCs were of dominant failure modes under drop weight impact test, while no sign of face-core de-bonding were observed for all type of 3D-IWK-SCs.

A strain-rate-dependent analytical model for composite bolted joints

Alireza Shamaei-Kashani,Mahmood M. Shokrieh 국제구조공학회 2021 Steel and Composite Structures, An International J Vol.41 No.2

In the present research, a novel analytical approach was developed to predict the bearing chord stiffness and the damage initiation bearing-load of single-lap composite bolted joints under medium strain rate loading. First, the elastic moduli, Poisson’s ratio, and strength of a unidirectional composite ply at an arbitrary strain rate were predicted by available micromechanical equations. Then, the bearing chord stiffness of the joint at any arbitrary strain rate was predicted. For this purpose, the available spring-based model was modified. The strain-rate-dependent damage initiation bearing-load of the joint was predicted by using the moduli and the stress concentration factor of a pin-loaded unidirectional ply. Four types of single-lap joints with [-45/0/+45/90]s and [90/-452/+45]s layups with and without carbon nanofibers were tested at the strain rates of 0.0048 s-1, 0.36 s-1, and 0.89 s-1. The results of experiments showed that mechanical properties of single-lap composite bolted joints increased with increasing the strain rate. Also, employing carbon nanofibers has a significant effect on the mechanical properties of the joints. The predicted results in comparison with the conducted experimental data show good agreements.