Pultrusion Direction Research Articles

Effects of off‐axis load direction on tensile properties of pultruded fiber‐reinforced polymer bolted joints with multi‐directional fiber lay‐ups

AbstractThe limited joint performance of unidirectional (UD) pultruded fiber‐reinforced polymer (FRP) composites poses a critical issue that constrains their application in structural engineering. Moreover, UD pultruded FRP joints display a notable reduction in strength and stiffness as the load angle diverges from the fiber pultrusion direction. This paper presents a numerical study of bolted pultruded FRP joints with different off‐axis loading directions. Two types of fiber lay‐ups, including UD and multi‐directional (MD), were investigated. Static tensile tests were conducted with the loading direction parallel to the pultrusion. A finite element model was validated by the test results. Studied geometric parameters includes the end‐to‐diameter (e/d) ratio, width‐to‐diameter (w/d) ratio, and bolt arrangement. Compared to UD joints, MD joints exhibited significant advantages in strength and stiffness under off‐axis loading with an increasing of 78.3% at a loading angle of 90°. Compared to the end distance, plate width has a more significant impact on the off‐axis behaviors of MD joints, especially between 30° and 75°. Multi‐bolted joints were more susceptible to net‐tension failure than single‐bolted joints. Furthermore, classical laminate theory and Hashin failure criterion were employed to predict the resistance of single‐bolted FRP joints with different loading angles, achieving high accuracy.Highlights Under increased off‐axis loading angles, compared with unidirectional (UD) joints, multi‐directional (MD) joints display a less reduction in both strength and stiffness. At a 90° angle UD joints displayed only 21.7% of the joint resistance of MD joints. Compared to the end distance, plate width has a more significant impact on the off‐axis behaviors of MD joints, especially between 30° and 75°. Classical Laminate Theory, Hashin failure criterion were effectively utilized to predict the ultimate strength of pultruded joints under various tensile loading angles, yielding high accuracy. The error between theoretical and simulated values is within 20%.

Failure analysis of curved interface in pultruded fiber-reinforced polymer laminations using phase-field approach

Pultruded fiber-reinforced polymer (FRP) composites frequently encounter fabric misalignment perpendicular to the pultrusion direction, resulting in the formation of a curved interface. For predicting delamination in pultruded fabric with curved interfaces, the phase field model is employed in the commercial software ABAQUS® through the UMAT and HETVAL subroutines. In this study, the Mode-I, Mode-II, and mixed-Mode delamination behavior are successfully simulated to validate the efficiency of the proposed phase field method. A parametric study is conducted to investigate the impact of mesh size and phase field length on the simulation results. The recommended mesh size and interface length of the phase-field model are given. Finally, the validated phase-field model is used to predict the delamination behavior exposed to Mode-I, Mode-II, and mixed-Mode loading for the curved interface. The results indicate that specimens with a curved interface exhibit greater bearing capacity compared to those with a planar interface.

Tensile strength and modulus of FRP pultruded composites at varying load angles relative to the pultrusion direction

Tensile strength and modulus are key material properties for the design of pultruded fiber reinforced polymer (FRP) composite members and connections. Pultruded FRP composites are orthotropic materials and thus both the tensile strength and modulus vary with the angle between the pultrusion direction and the applied force. In current design standards, the variation of material properties with load direction is addressed by prescribing the use of longitudinal properties for angles between zero and five degrees, and the use of transversal properties for angles greater than five degrees. To derive a more precise and representative equational relationship between load angle and tensile material properties, 228 coupons extracted from pultruded FRP profiles were tested in tension in accordance with ASTM D638-22 to determine the resulting average tensile strength and tensile modulus values. Coupons were taken from different profiles from different manufacturers, different section locations (e.g., web versus flange), as well as varying angles between the pultrusion direction and the applied force. The characteristic design value of the tensile strength and tensile modulus at each tested angle was determined in accordance with ASTM D7290-22. Using this dataset, a predictive relationship based on a generalized form of Hankinson’s equation was developed for both average and characteristic tensile strength and both average and characteristic tensile modulus as a function of three factors: 1) the differential angle between the pultrusion direction and the applied force, 2) the longitudinal tensile property (i.e., strength or modulus), and 3) the transverse tensile property (i.e., strength or modulus). The average relative difference between tested and predicted values using the developed equations varied between 4.4% and 8.3%, depending on the considered mechanical property.

Experimental investigation of pultruded GFRP connections

At present, design and construction of all-FRP structures with the use of GFRP are developing. All-FRP frame structures use bolted connections to form nodes between the elements, particularly steel bolts. This research focuses on the use of FRP pultruded plates instead of chords and webs. Frame construction nodes are formed by adjoining frame elements at different angles to the chords through gusset plates. In accordance with the literature analysis, a small number of tests have been carried out to investigate connections at angles to the pultrusion direction the pultrusion direction. Both experimental and numerical studies made with pultruded plates and L-shape FRP profiles have demonstrated that the plates have considerable influence on the strength and deformation of joints so that the joints failure mode involves mainly the bolted plate. These results were then compared against numerical predictions obtained using Ansys for finite-element analysis.

Bearing capacity of pultrusion fiberglass gusset sheets in frame structures

Introduction. At present, design and construction of all-composite structures with the use of pultrusion fiberglass profiles (PFP) are developing. The bearing capacity of all-composite structures is often limited by their node connections. Over the last two decades, many studies have been devoted to the operation of fiber-reinforced polymer (FRP) element nodes connected at right angles (or in the direction of pultrusion and across it). Frame construction nodes are formed by adjoining frame elements at different angles to the bands through gusset sheets. In accordance with the literature analysis, a small number of tests have been carried out to investigate connections at angles to the pultrusion direction. Existing design solutions of gusset sheets in FRP frame structures are mainly made of steel or composite material produced using other technologies (compaction method, pressure treatment method). This study focuses on the implementation of a node connection on unidirectional gusset sheets in which the fibers are positioned at the angle of 0°. Taking into account the specific features of the material, the gusset sheet design in frame structures has been adapted to the properties of pultrusion fiberglass. The purpose of this study is to increase the efficiency of PFP gusset sheet use in frame structures based on the specifications of this material.
 Materials and methods. The adaptive method was used when designing the node connection design on the frame structure gusset sheets. The material of the trapezoidal steel truss is changed to the composite material while retaining the original design solution. Subsequently, the structure is upgraded to take into account the properties and features of the composite material.
 Results. The main results of the study involve determining the factor of safety of pultrusion fiberglass at different angles to the direction of force. In order to increase the bearing capacity of the element node connection, various gusset plate design solutions are provided, which take into account different forces in the frame structure elements.
 Conclusions. When designing PFP constructions, features of the material must be taken into account. Replacing traditional materials with composite without adjusting the design layout and upgrading the structure leads to increased material intensity.

A viscoelastic model for time-dependent behavior of pultruded GFRP

Abstract Several works in the literature have investigated creep of pultruded glass-fiber reinforced polymer (pGFRP), but mainly to characterize the material in longitudinal direction. Moreover, the empirical Findley Power Law has been widely used to describe the creep response, but this approach is particular for each case and may result in little predictive capacity for different loading conditions, resin types and fiber architecture. In this work, a linear viscoelastic model combining elastic behavior of fiber and viscoelastic nature of matrix is proposed to represent the long-term behavior of pGFRP material. To validate the model, flexural creep and recovery experimental tests were carried out for vinyl ester (VE) and orthopolyester (OPE)-based composites for specimens oriented parallel and perpendicular to pultrusion direction. To reduce uncertainties with respect to the degree of curing of resin during fabrication, tests were conducted for two different conditions: as supplied and post-cured. Dynamic mechanical analyses (DMA) were performed to evaluate qualitatively the degree of cross-linking and the fiber architecture was studied by stereomicroscopy. Tests revealed greater compliances for OPE resulting from lower viscoelastic properties of this resin, as well as by lower fiber content and less effective fiber architecture. For transverse direction, matrix dominates the behavior and compliances were more pronounced. The thermal treatment adopted for post-curing resulted in damage formation or propagation, increasing the compliances. Satisfactory agreement was achieved between proposed model and experiments.

Ultimate limit design of composite beams with modular GFRP deck and steel girder

This paper analytically formulates the ultimate capacities for various failure modes of composite beams consisting of a modular fiber reinforced polymer (FRP) web-flange sandwich deck supported by a steel girder. Pultruded FRP box sections are incorporated between two flat panels to form the web-flange sandwich deck, in which the pultrusion direction of box sections can be either parallel or perpendicular to the span direction. Such a deck can then be assembled via adhesive bonding or blind bolt connections to a supporting steel girder to form a composite beam system. On the basis of theoretical analysis of critical sections for such composite beams at ultimate states, potential failure modes are identified, including flexural failure of the steel girder yielding in tension and GFRP deck failure in compression, vertical shear failure, and horizontal shear failure at adhesive bonding at bolted connections and at webs of FRP box sections. For each failure mode, the corresponding ultimate capacity is analytically formulated; thereby, the critical failure mode and ultimate load capacity can be determined. The developed ultimate limit design approaches are validated by experimental results from the literature for such FRP-steel composite beams.

Plain and threaded bearing strengths for the design of bolted connections with pultruded FRP material

Presented are results from testing 28 batches of 5 or 10 nominally identical specimens to characterise the laterally unrestrained pin-bearing strength when bolting is with and without thread. For the test series flange material is taken from a 254x254x9.53 mm Pultex® SuperStructural 1525 series shape. Strengths are measured with the Fibre Reinforced Polymer (FRP) material oriented at either 0° or 90° to the direction of pultrusion. Four steel bolt sizes of M10, M12, M16 and M20 are used, and when threaded there are different standard teeth (pitch) geometries. To remove this variable in a comparison with plain pin strengths a unique test series of 12 batches was carried out with three non-standard thread profiles. The effect on pin-bearing strength of having a threaded bolt is evaluated using mean and characteristic strengths, the latter determined in accordance with EN 1990. A key finding is that the proposed reduction factor of 0.6 in a forthcoming American LRFD standard to calculate a thread characteristic strength from the plain value is acceptable. Other findings are important to the determination of pin-bearing strength, and to us having knowledge and understanding to prepare a universal design procedure for resistances in bolted connections when the mode of failure is bearing.

Improved bearing capacities of pultruded glass fibre reinforced polymer square hollow sections strengthened by thin-walled steel or CFRP

Pultruded glass fibre reinforced polymer (GFRP) square hollow sections (SHS) are emerging as an alternative construction material in civil engineering. Different from thin-walled steel SHS, GFRP SHS is an orthotropic material and exhibits reduced capacity when concentrated bearing load is applied in the transverse to pultrusion direction. This paper examined two approaches to enhance the bearing capacity of the GFRP SHS, by using carbon fibre reinforced polymer (CFRP) sheets or thin-walled steel channel sections. Experimental studies were conducted on the strengthened pultruded GFRP SHS with two different dimensions. Two loading conditions, end-two-flange (ETF) and interior-two-flange (ITF), were applied to investigate the effect of loading position on the bearing capacity of the GFRP SHS. Experimental results were discussed with relation to the failure modes and load–displacement responses. It was found that the bearing capacity of pultruded GFRP SHS was effectively improved by up to 70% after CFRP sheets strengthening, and by over 380% after steel strengthening. Finally, equations were proposed for calculation of the improved bearing capacities of pultruded GFRP SHS with either CFRP sheets strengthening or steel strengthening. A reasonable agreement was achieved between predicted capacities and the experimental results.

Web crippling behaviour of pultruded glass fibre reinforced polymer sections

We investigated the web crippling behaviour of pultruded GFRP sections under concentrated loading, employing four square hollow sections of different sizes. End-two-flange (ETF) and interior-two-flange (ITF) loading conditions were adopted, with specimens seated on a bearing plate. Specimens were also placed on the ground with end or interior bearing load to simulate the loading conditions of floor joist members. The observed failure initiated at web–flange junctions and was followed by buckling or crushing in the webs, and these modes were quite different from metallic sections. Correspondingly, two load limits were defined according to the progressive failure process. The effects of the loading positions (end loading or interior loading) as well as the supporting conditions (on a bearing plate or on the ground) on the web crippling behaviour are discussed. An effective web area in the pultrusion direction is identified to characterize the load transfer path within the web. Finally, a simple mechanism based design equation is proposed to estimate the strength of such pultruded GFRP sections subjected to web crippling.

Pin-bearing strengths for bolted connections in fibre-reinforced polymer structures

Presented in this paper are pin-bearing strengths for pultruded fibre-reinforced polymer materials that are required in order to check the bearing resistance when designing bolted connections. For steel pin diameters up to 16 mm, equivalent test results, at room temperature, using the European standard test method BS EN 13706-2 and another test method (such as ASTM D 5764) are shown not to be significantly different. As the latter method uses much smaller specimen blanks, new pin-bearing strengths can be determined for the web material in a 203 × 203 × 9·5 mm wide flange shape, with the connection force at 0°, 45° or 90° to the direction of pultrusion. An evaluation is made of the test results and recommendations are given on how pin-bearing strengths are to be determined so that they will match the geometries of bolted connections and connection forces found in practice.

Effect of Hot-Wet Aging on the Pin-Bearing Strength of a Pultruded Material with Polyester Matrix

This paper presents test results to show the effect of hot-wet conditioning on the pin-bearing strength of a pultruded fiber reinforced polymer material. Knowledge of this strength property, taking account of any reduction over the service lives of structures, is required to reliably calculate bearing strength when designing bolted connections. Pin-bearing strength is determined using an in-house test method with batches of nominally identical specimens, cut from the web of a 203 x 203 x 9.53 mm wide flange shape. This shape is from the 1525 series of Creative Pultrusion Inc., having a polyester based matrix. Specimens were immersed prior to strength testing under water for 3000 hours at the constant temperature of 40° C. The paper discusses the accelerated aging protocol and its relation to service life, and an explanation is given to why the material was aged for an unknown number of service years. Variables in the test matrix are the direction of the bearing force (0°, 45° and 90° to the direction of pultrusion) and plain pin diameter (four sizes from 9.7 mm to 25.4 mm). Comparing aged pin-bearing strengths with equivalent strengths for non-aged material it is found that the average reduction in characteristic strength (calculated in accordance with Eurocode 0) of the 12 batches, is in the range of 18 to 31%. The extent of strength reduction is found to be independent of pin size, except when the diameter is 25.4 mm. For the 0o situation a comparison is made between the characteristic strengths for the four pin diameters determined using BS EN 1990:2002 and ASTM D7290 to show that the latter Weibull distribution values are lower, and by 4 to 18%.

Prediction of Net-Tension Strength for Multirow Bolted Connections of Pultruded Material Using the Hart-Smith Semiempirical Modeling Approach

Presented in this paper is a study to show that the Hart-Smith semiempirical modeling approach can be used to predict the net-tension strength of multirowed bolted connections of pultruded material. Using the original 1987 paper by Hart-Smith a strength equation is developed for the specific connection configuration of two rows with a centrally placed steel bolt. The reported equation can be directly used for the two orientations of material that have the tension load parallel or perpendicular to the direction of pultrusion. Using experimental measurements for material properties and single-bolted connections from Rosner's 1992 work and the open-hole tension strengths from Turvey and Wang's 2003 paper, representative values to the modeling parameters in the strength equation are established. For model verification a comparison is made between theoretical and experimental strengths, using 17 test results from Hassan et al.'s 1997a work. Only two of the 17 experimental-to-theory strength ratios are < 1.0, and only one of these two could be said to have predicted unsafe net-tension strengths. With none of the ratios exceeding 1.2, it is seen that the simple and versatile modeling approach gives very acceptable predictions. To determine the modeling parameters that will enable the Hart-Smith approach to be in a load resistance factor design standard there is a need for a comprehensive series of strength tests, for net-tension failure with filled- and open-holes, that covers the complete range of multirowed bolted connections that is to be permitted by the standard.

Mechanical properties of hybrid fabrics in pultruded cement composites

This work concerns the tensile properties of cement-based hybrid composites manufactured as: (i) sandwich composites that combine different layers of single fabric types; and (ii) hybrid composites, made from several yarn types within the same fabric. Hybrid combinations of low-modulus fabrics of polyethylene (PE) or polypropylene (PP) and high-modulus AR glass or aramid fabrics were prepared by the pultrusion process and tested in tension. Influence of pultrusion direction on the results was one of the parameters studied. It was found that hybrid composites made from PE and AR glass sustain strains better than 100% AR glass composites, and are stronger than a single PE fabric composite. A hybrid fabric composites made with combination of high strength–high cost aramid and low stiffness–low cost PP yarns performed better than a single aramid fabric composite relative to their reinforcing volume contents. Results show that making hybrid composites is an attractive option for cement-based elements. The performance of hybrid fabric composites is also influenced by the arrangement of fabric layers in the laminates. Composites with brittle and relatively strong fabrics (glass) at the mid-section and ductile fabrics (PE) near the surfaces of the composite performed better in tension than composites with the opposite arrangement.

Compressive Properties of Pultruded Composites

The compressive properties of E-glass fiber/polyester resin matrix pultruded composites were measured using a short-block compression test fixture. Twenty specimens were tested in the pultrusion (longitudinal) direction and twenty more specimens in the transverse direction. Six specimens were tested at 30, 45, and 60° orientations to the pultrusion direction. Compressive properties such as the compressive modulus, compressive strength, Poisson ratio, and compressive failure strain were measured for both 6.3 and 12.7 mm thick pultruded sheet materials. The test specimens were rectangular with a length of 44.4 mm, width of 25.4 mm, and gage length of 25.4 mm. The test fixture was initially validated by measuring the axial strain distribution across the width of the specimen; from the beginning of loading up to the final failure, the distribution of strains across the specimen width was found to be very uniform. Back-to-back strain gages were also mounted on representative specimens. The end shortening was also measured on all specimens. The compressive strength data were analyzed both for the longitudinal and transverse directions using a two-parameter Weibull, lognormal, and normal distributions. The compressive properties are presented as functions of the specimen orientation angle for both thicknesses.

Single-sided ultrasonic technique to characterize thick FRP composites

This paper reports on the development of a single-sided, laser ultrasonic technique that is capable of measuring all of the engineering constants of a pultruded FRP composite structural member. The complex nature of these members (which are used in civil infrastructure applications) precludes the determination of all of the engineering constants using mechanical tests, while immersion ultrasonic techniques have certain drawbacks. The proposed technique, which uses a combination of contact piezoelectric and optical methodologies, does not call for cutting the specimen or placing it in an immersion tank. Optical generation and detection of ultrasound enable the direct measurement of the longitudinal and surface acoustic wave speeds in the pultrusion direction, without requiring access to the ends of the specimen, or relying on reflected quasilongitudinal waves. The measured engineering constants are compared to immersion technique results, and the advantages of the proposed technique are clearly demonstrated.

Single-bolt tension joint tests on pultruded GRP plate — effects of tension direction relative to pultrusion direction

The literature on single-bolt tension joints in structural grade pultruded GRP plate is reviewed and the differences between the main investigations are highlighted. Details are given of the test setup and the joint configurations used in a series 54 such tests on single-bolt joints in which the angle between the pultrusion and tension axes (the off-axis angle) and the joint geometry are varied. Ultimate strength, intial stiffness, initial bolt slip and bolt displacement at failure data are presented as functions of the joints' principal geometric ratios. The observed joint failure modes show that, for off-axis angles ≥ 30 °, bearing failure (a relatively benign failure mode) does not arise. Instead, tension mode failure predominates and cracks tend to propagate parallel to the rovings diagonally across the width of the joint. It is concluded that the rovings play a significant role in controlling the crack propagation and this has implications for joint layout and design in pultruded GRP plate.

Torsion tests on pultruded GRP sheet

Details of a series of simple torsion tests on rectangular cross-section strip specimens cut from 6·4 mm thick pultruded GRP sheet are described. Stiffness and strength data are presented for specimens with their torsion axes parallel with and transverse to the pultrusion direction. The differences in the development of failure in the two types of specimen are described. The stiffness data are analysed on the basis of simple torsion theory for orthotropic rectangular cross-section bars to determine both the in-plane and through-thickness shear moduli for the pultruded GRP sheet and it is shown that the average value of the former modulus is somewhat higher than (but nevertheless consistent with) values derived from shear tests.

Industrial research in the European Union

We investigated the web crippling behaviour of pultruded GFRP sections under concentrated loading, employing four square hollow sections of different sizes. End-two-flange (ETF) and interior-two-flange (ITF) loading conditions were adopted, with specimens seated on a bearing plate. Specimens were also placed on the ground with end or interior bearing load to simulate the loading conditions of floor joist members. The observed failure initiated at web–flange junctions and was followed by buckling or crushing in the webs, and these modes were quite different from metallic sections. Correspondingly, two load limits were defined according to the progressive failure process. The effects of the loading positions (end loading or interior loading) as well as the supporting conditions (on a bearing plate or on the ground) on the web crippling behaviour are discussed. An effective web area in the pultrusion direction is identified to characterize the load transfer path within the web. Finally, a simple mechanism based design equation is proposed to estimate the strength of such pultruded GFRP sections subjected to web crippling.

Tensile Behaviour of Pultruded FRP Sandwich Composite Bolted Joints

In this experimental study, the tensile behaviour of bolted joints of pultruded glass fibre reinforced polyester sandwich composites was investigated. The transverse specimens cut normal to the pultrusion direction failed by net-tension whereas the longitudinal specimens cut parallel to the pultrusion direction failed by a combination of bearing and shear-out modes. The joint strength was dependent on the specimen geometry in the case of transverse specimens whereas it was not in the case of longitudinal specimens.