Development of reliable modeling methodologies for fan blade out containment analysis – Part I: Experimental studies

High strength woven fabrics are ideal candidate materials for use in structural systems where high energy absorption is required. One of the more widely used applications for woven fabrics is in propulsion engine containment systems. In this first part of a two-part paper, details of the experiments to characterize the behavior of dry fabrics including Kevlar^® and Zylon^® are presented. The experimental program to characterize the behavior of 1420 Denier Kevlar^® 49 17 × 17, 500 Denier Zylon^® AS 35 × 35, and 1500 Denier Zylon^® 17 × 17 are discussed. The primary objective is to use the experimental results in the development of a constitutive model that can be used in an explicit finite element analysis program. These include Tension Tests in both the warp and fill directions of the fabric, Trellising Shear Tests and Friction Tests between fabric layers. The results from these tests provide the basis for development of the constitutive model – relating stresses to strains, characterizing failure and interaction between fabric layers. In addition to these basic material tests, tests on systems built with fabric wraps were also conducted. Ballistic tests of containment wraps subjected to a high velocity projectile were carried out at NASA-Glenn Research Center. While these tests provide a comparison between the energy absorbing characteristics of the three fabrics, they also provide benchmark results to validate the developed finite element methodology discussed in the second part of this paper.     

Non-contacting strain measurement for cement-based composites in dynamic tensile testing

This paper presents the development of a test procedure and application of non-contacting strain measurement for cement-based composites under moderately high strain rate tensile tests. The strain time histories of test specimens measured by a laser extensometer in high speed mode were derived by a phase-shift technique based on zero-crossing method. The accuracy of the linear variable differential transformer (LVDT) of the actuator in a servo-hydraulic high rate testing machine was verified by image analysis using sisal fiber reinforced cement composite at a strain rate of 25 s⁻¹. The same procedure was then applied to Alkaline Resistant (AR) glass fabric reinforced cement composite tested at an average strain rate of 17 s⁻¹. Comparison between the strain values measured by the laser extensometer and the LVDT shows a good agreement between these two measurement techniques. The test results show that the Young’s modulus, tensile strength, maximum strain, and toughness of the AR-glass fabric-cement composite increase with increasing strain rate. However, under both static and dynamic loadings the composite has similar behavior: multi-crack development and one dominant crack leading to final failure. In order to ensure the accuracy of dynamic tensile test procedures, non-contacting devices and techniques should be used as an independent means of verification of test results. The accuracy required in quantifying relative improvements in mechanical properties necessitates the various methods of measuring the displacement and strain rate properties.     

Characterization of Interfacial Properties in Fiber-Reinforced Cementitious Composites

For calculating interface properties from pullout tests, a simple theoretical model is proposed. The model enables calculating of the following material parameters: the parameter of shear stiffness of the fiber–matrix boundary layer, the shear bond strength, the frictional bond strength and the specific interfacial fracture energy. These parameters can be determined from the slope of the load-slip curve, the maximum pullout load and the corresponding slip value. Slip-controlled, multiple-fiber pullout tests were conducted in a closed-loop test system. The effects of embedment length of fibers on the model-predicted material parameters were examined. The model predictions were satisfactorily compared with some previously published test data.     

Properties of Fabric–Cement Composites made by Pultrusion

Practical manufacturing and use of thin cement-based elements composites require an industrial cost-effective production process in addition to proper reinforcement materials to improve the tensile and flexural performance. Reinforcement by means of fabric materials is an alternative to the use of short fibers. The objective of this study was to investigate use of pultrusion technique as a cost-effective method for the production of thin-sheet fabric-reinforced cement composites. Woven fabrics made from low modulus polypropylene (PP) and glass meshes were used to produce the pultruded cement composites. The influence of fabric type, PP and glass, processing method, pultrusion vs. cast and cement-based matrix modification were examined. Tensile and pullout tests as well as Scanning Electron Microscopy (SEM) observations were used to examine the mechanical, bonding and microstructure properties of the different composites. The rheology of the mix was correlated with the mechanical behavior of the pultruded composites. The tensile behavior of the pultruded fabric–cement components exhibited strain hardening behavior. The best performance was achieved for the PP pultruded composites.     

Hemodialysis membranes prepared from poly(vinyl alcohol): Effects of the preparation conditions on the morphology and performance

Poly(vinyl alcohol) was employed for the preparation of hemodialysis membranes with and without the addition of acetic acid and poly(ethylene glycol) with the phase‐inversion process. Aqueous solutions of sodium sulfate and sodium hydroxide were chosen as coagulant baths. The performances of the membranes were estimated by the measurement of the removal of uremic toxins (urea, uric acid, and creatinine) from human blood serum. The morphologies of the membranes were investigated and correlated to the membrane performance. Increasing the poly(ethylene glycol) concentration in the polymer solutions resulted in porous, spongelike structures because of the higher polarity of the polymer solutions and the enhancement of the diffusion rate of the nonsolvent (sodium sulfate and sodium hydroxide) into the polymer solutions. The porous structures of the membranes enhanced the removal of uremic toxins. The presence of acetic acid, with greater ionization strength, resulted in higher electrostatic interactions between positive and negative ions in the coagulation baths and polymer solutions. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2490–2497, 2007     

Cracking mechanisms in durable sisal fiber reinforced cement composites

Fiber reinforced cement composite laminates with long sisal fibers were manufactured using a cast hand lay up technique. A matrix with partial cement replacement by metakaolin and calcined waste crushed clay brick was used in order to improve the durability aspects. Mechanical response was measured under tension and bending tests while crack formation was investigated using a high resolution image capturing procedure. Crack spacing was measured using image analysis and correlated with the applied strain under both the tensile and bending response. Various stages of loading corresponding to initiation, propagation, distribution, opening, and localization of a crack system in the specimen are discussed. The effect of flexural cracking on the location of neutral axis during the bending tests was measured using strain-gages.     

Modeling of fiber toughening in cementitious materials using an R-curve approach

This paper presents the theoretical formulation describing the role of fibers in enhancing the fracture toughness of quasi-brittle cement based materials. The formulation is based on the well known R-curve approach which correlates the increase of the apparent fracture toughness of a material with the existence of a pre-critical stable crack growth region.By assuming that the critical crack length in plain matrix is a function of an initial crack length a ₀, a formulation for the R-curves has recently been derived and applied to predict the response of positive and negative geometry specimens of various sizes and materials. This approach is further applied to uniaxial tensile specimens containing various fiber types. Fiber reinforcement is modeled by means of applying closing pressure on crack surfaces resulting in closure of the crack faces and a decrease in the stress intensity factor at the tip of the propagating crack. Incorporation of these two factors in the energy balance equations for crack growth results in increases in both the slope and the plateau value of the R-curve representing matrix response. Enhancement in material response is shown to occur only if precritical crack growth exists, causing fibers to convert the stable cracking process into an increase in load carrying capacity of the material. Fracture response of fiber reinforced composites can be predicted up to the bend-over-point. The theoretical predictions are compared with the experimental results of cement-based composites containing unidirectional, continuous glass, steel or polypropylene fibers.     

Drying shrinkage of expansive cements

Drying shrinkage behaviour of expansive cement pastes were studied and compared with those of portland cements. Results indicate that the shrinkage behaviour of these two cements is significantly different from each other. Generally, expansive cements shrink more than portland cements, and especially more so if they have not been adequately cured, i.e. curing period of at least 7 days is necessary to ensure good performance against shrinkage. Internal damage caused by large amounts of expansion leads to a large magnitude of shrinkage. In that event curing does not seem to have any beneficial impact on shrinkage performance. Steel reinforcement also seems to decrease shrinkage magnitude, but it has no effect on the shrinkage characteristic. Much of the research on expansive cements has so far been focused on the expansion behaviour rather than on the shrinkage behaviour. More research on shrinkage is needed to improve its field performance.