H.E.M. Sallam

Evaluation of Steel I-Beams Strengthened by Various Plating Methods

Flexural behavior of up-graded steel I-beams, IPE 160, with bonded/welded steel plates and bonded carbon fiber reinforced polymer (CFRP) plates/sheets were evaluated in this work. Different types of end anchorage (belted CFRP sheet, steel clamped, and end welded) were applied to enhance the efficiency of the bonded element. The present experimental results showed that, plate end anchorage increased markedly the efficiency of bonded steel and CFRP plates. CFRP sheet showed higher resistance to debond than CFRP plate. Two separate CFRP strips bonded below the lower flange of the I-beam is the best method for bonded CFRP plate to I-beams. Bonded/welded hybrid joint technique for attachment of the steel plate in tension side is the most efficient technique. All strengthened beams and control beam failed due to distortional buckling after the yield occurrence.

Significance of crack tip plasticity to early notch fatigue crack growth

Abstract This work demonstrates the capability of the extents of both monotonic and cyclic crack tip plasticity in correlating experimental fatigue crack growth (FCG) rates from notches. Experimental results previously published by the authors on early mode I stage II FCG rates from different single edge U-shaped notches in low carbon steel plates were utilised. The plates had different stress concentration factors and had been tested near their fatigue limits at a zero stress ratio and at constant amplitude uniaxial stresses. Eleven tests were numerically simulated with a previously developed in-house two-dimensional cyclic elastic–plastic finite element programme. The cyclic plastic deformation accommodated at the tip of a physical short through-thickness crack artificially advancing from the root of each tested notch was analysed. When the tip of a crack was advancing within the affected zone of the simulated notch, transition behaviour from the notch-affected physical short crack regime to the relatively long crack regime was recognised. The extents of crack tip opening displacements and plastically deformed zones were the subject of that behaviour. Equivalent edge crack lengths were proposed on the assumptions of equal stress intensity factors and monotonic and cyclic crack tip plastically deformed zones. A length parameter devised from the behaviour of short crack tip plasticity showed its capability to correlate early FCG rates from notches. The proposed parameter started its behaviour with a relatively high extent at zero crack length, decreased to a minimum value when the tip of the propagating crack approached the cyclic elastic–plastic boundary before it increased to match the relatively long crack behaviour. Comparison with experimentally observed early FCG in the simulated tests showed an agreement.

Mode II stress intensity factors for central slant cracks with frictional surfaces in uniaxially compressed plates

Abstract The effect of crack surface friction on mode II stress intensity factor (SIF) of a central slant crack in a plate uniformly loaded in uniaxial compression is quantified. A previously developed two-dimensional finite element analysis was utilised after its modification to accommodate the friction between the crack surfaces. The plane strain state was assumed. A new numerical technique was devised to avoid the iteration procedures, which had to be employed due to the existence of frictional forces. The crack inclination angle varied between zero and 75° measured from the horizontal direction. The coefficient of friction of the crack surfaces changed from zero to 1. In case of relatively sliding crack surfaces, mode II SIF existed. As is well known, the resulting mode II SIF decreased with increasing the coefficient of friction of the crack surfaces. Further, mode II SIF increased with increasing crack line inclination angle and then decreased after reaching a maximum value. The angle corresponding to that maximum SIF increased as the coefficient of friction of the crack surfaces increased.

Mode I notch fatigue crack growth behaviour under constant amplitude loading and due to the application of a single tensile overload

Abstract Low carbon steel plates having different single edge U-shaped notches with different geometries were tested near their fatigue limits at different stress ratios and constant amplitude axial stresses. The present work demonstrates experimentally obtained early mode I fatigue crack growth rates from such notches. The experimental fatigue crack growth behaviour clearly showed a minimum rate when the propagating crack reached a length approximately equal to the extent of the notch cyclic plastic zone. Further experiments investigated the effect on the notch fatigue crack growth behaviour of a single tensile overload applied within constant amplitude base load cycles. The tested parameters included the notch geometry, the maximum applied stress and the stress ratio of the base load, the overload ratio and the location of the crack tip at which the overload was applied. In case of an overload cycle applied after having an initiated fatigue crack propagating from the root of a notch, the effects of both overload and stress ratios on the proceeding fatigue crack growth rates were similar to those commonly observed in un-notched plates. When the overload cycle was applied before fatigue testing, more resistance to fatigue crack initiation and propagation was observed. In tests with an overload cycle applied after having a short fatigue crack with its tip located outside the notch root cyclic plastic zone, two minimum rates were observed. One minimum rate was observed if the overload cycle was applied when the initiated crack tip was propagating inside the notch root cyclic plastic zone.

The effects of matrix and fiber properties on the mechanical behavior and acoustic emission in continuous fiber reinforced metal matrix composites

Abstract This paper examines and compares the mechanical properties and acoustic emission activity of four metal matrix composites (MMCs). The composites were manufactured with either 6061 Al or a high purity aluminum (HPAL) matrix. The two fibers used were the Nextel 440 and 610 fibers made by the 3M Corporation. The two fibers differed in stiffness, strength, composition and geometry. The strengths of the composites were found to increase from the 610/6061 composite to the 440/6061 and 440/HPAL composites to the 610/HPAL composite. The high strength and low ductility of the 6061 matrix led to a rapid propagation of failure in these materials. The increased ductility of the HPAL matrix slowed crack propagation and more fibers broke prior to failure of these composites. The increased stiffness of the 610 fibers led to an increase in residual stresses and earlier yielding of the matrix. A flat fracture surface was observed for all the composites with little fiber pullout indicating a strong fiber/matrix bond. The acoustic emission (AE) events could be separated into two regions for all the composites. Events in region one were attributed to dislocation motion during yielding of the matrix. The events in this region were predominantly low amplitude. Events in region two were attributed to continued dislocation motion and fiber or inclusion breaking (cracking). The events were predominantly lower amplitude with some higher amplitude events occurring. The cracking was more extensive in the 440 fiber composites due to the increased number of fibers breaking prior to failure as indicated by the increase in AE activity of the 440 composites compared with the 610 fiber reinforced composites.

Simulation of mixed mode I/II cyclic deformation at the tip of a short kinked inclined crack with frictional surfaces

Abstract The paper presents a model for near tip-displacements of a short kinked slant crack in a two-dimensional plate subjected to uniaxial cyclic loading. Possible regimes of sticking and sliding contact and separation along the two frictional surfaces of the main crack and the kink were identified. Both stationary and artificially advancing central cracks were analysed with an inhouse finite element package and compared in terms of plastically deformed zones and crack tip relative displacements. An artificial crack tip advance equivalent to one element was allowed along the direction of the assumed kink. Three stress ratios of 0.2, 0 and −1 were assumed. The main crack angle was 45° with a crack length to plate width ratio of 0.3 and the coefficient of friction varied from 0 to 1. The kink angle varied between 60 and −80° measured in a counter-clockwise direction from the main crack line. In the case of a negative stress ratio, extents of both monotonic and cyclic crack tip deformation decreased with increasing the coefficient of friction between the crack surfaces. For positive stress ratios, such an effect was not obvious. The extent of the cyclic crack tip opening displacement appeared as an appropriate candidate to predict the crack initiation angle in the case of mixed mode I/II loading. The present results did not support the use of a maximum effective stress range based on crack tip closure for such prediction. The maximum extent of the crack tip opening displacement corresponded to a kink angle which was nearly the same as that measure obtained from an elastic analysis. The extent of the crack tip sliding displacement at that kink angle was negligible.

Stress intensity factors of a shortly kinked slant central crack with frictional surfaces in uniaxially loaded plates

Abstract An elastic two-dimensional finite element analysis was used to evaluate the modes II and I stress intensity factors of a shortly kinked slant central crack with frictional surfaces in uniaxially loaded plates. Four main crack angles measured from the load direction, i.e. 45, 60, 75, and 85°, with crack length to plate width of 0.1, 0.3, and 0.5 were considered. The coefficients of friction of the crack surfaces were 0, 0.25, 0.5, 0.75 and 1. The kink angle was measured from the main crack line in a counterclockwise direction and varied in increments of 5° between 0 and –120° for the plates loaded in tension and between 0 and +120° for the compressed plates. The ratio of the kink length and the main crack length was 0.0065. The arrangement and the size of the elements around the crack tip were the same for all the meshes generated for the present work. The variation of the computed modes II and I stress intensity factors was dependent on the type of the remotely applied axial load and both main crack and kinking angles. For a fully opened crack, there was a negative kink angle at which mode I stress intensity factor attained a maximum value. That corresponded to approximately zero mode II stress intensity factor. The maximum value of mode I stress intensity factor increased as the main crack angle increased, i.e. the mode I stress intensity factor reached a maximum when the main crack was at 90° to the loading direction. In axially compressed plates, relatively sliding crack surfaces in contact showed opened crack tips only at positive kink angles, which were larger than an angle dependent on the main crack angle. The resulting mode II and mode I stress intensity factors decreased as the coefficient of friction increased. Further, the mode I stress intensity factor increased with the increasing positive kink angle and reached a maximum value before it decreased. On the other hand, the absolute value of the mode II stress intensity factor decreased with the increasing positive kink angle and had a zero value before it increased again. In both types of loading, the measure of the kink angle for the maximum mode I stress intensity factor was independent of the main crack length. The present results showed that the kink angles corresponding to maximum mode I stress intensity factors agreed well with predicted and experimentally observed initial crack growth directions found in the literature.

Efficient 3D modeling of damage in composite materials

Due to the complexity of composite material, numerical methods are generally utilized in their analysis and design. Commercial finite element (FE) codes, such as ANSYS and ABAQUS, allow the implementation of user subroutines in the program, which provides the advantage of using high meshing and solving technologies besides the improvement of materials and/or elements models. Nonlinearities arise for many engineering problems, for example, the progressive damage of a composite element that contains sources of stress concentration or damage localization such as holes, bolts, and/or flaws causes nonlinear material behavior. In order to simulate this nonlinear behavior, especially in 3D, an accurate material constitutive model is required. Therefore, the objective of this paper was to simulate the 3D progressive damage model of composite materials by using simple numerical models. In this paper, ANSYS user subroutine (USERMAT) was used to simulate the progressive damage behavior of a composite plate containing holes using simple models. Three different material models were used: ply discount model (PDM), simple progressive damage model (SPDM) by adding an empirical progressive damage criteria to the PDM, and continuum damage mechanics model (CDMM). Good agreements were observed between SPDM, CDMM, and published experimental results. Furthermore, CDMM showed the least dependence on mesh size. Three different damage evolution laws, linear, quadratic, and degradation laws, were adjusted and tested. It was found that there was no significant difference in the predicted failure load between these selected laws.

Composite Patch Configuration and Prestraining Effect on Crack Tip Deformation and Plastic Zone for Inclined Cracks

AbstractBonding a composite patch to a cracked steel section is an efficient technique to reinforce cracked members or to delay fatigue crack growth in the structural elements. In this paper, a numerical study was performed to highlight the effect of bonded prestressed composite patches on the fracture parameters, such as crack tip opening displacement and the plastic zone. A three-dimensional finite element model of the double sided strengthened specimen is used to study the fracture behavior of an inclined edge crack under different combinations of modes I and II loading conditions. The influence of the pretension level, patch stiffness ratio, and fiber orientation on the crack tip opening displacement and plastic zone are also investigated. The introduction of a compressive stress by pretensioning of the composite patch prior to bonding produces a significant reduction in the crack tip opening displacement and plastic zone. This pretensioning reduces the crack tip driving force and subsequently reduces...

Mechanical Behavior of Welded and Un-Welded Polyethylene Pipe Materials

The primary objective of the present paper is to depict the mechanical behavior of high density polyethylene, HDPE, pipes to provide the designer with reliable design data relevant to practical applications. Therefore, it is necessary to study the effect of strain rate and specimen configuration on the mechanical behavior of welded and un-welded pipes made from HDPE. Tensile tests are conducted on specimens longitudinally cut from the pipe with thickness (10, and 30 mm), at different crosshead speeds (5–500 mm/min), and different gauge lengths (20, 25, and 50 mm) to investigate the mechanical properties of welded and un-welded specimens. Butt-fusion, BF, welding method is used to join the different parts of HDPE pipes. In the case of test specimens taken from un-welded pipe a necking phenomenon before failure appears at different locations along the gauge section. On the other hand, the fracture of welded specimens almost occurs at the fusion zone. At lower crosshead speeds the fracture of welded specimen occurs in all specimen configurations at the fusion zone. The present experimental work reveals that the crosshead speed has a significant effect on the mechanical behavior of both welded and un-welded specimens.Copyright