Short Beam Specimens Research Articles

Titanium/FRP hybrid sandwich: in-plane flexural behaviour of short beam specimens

Adopting novel sandwich structures with FRP (Fibre Reinforced Polymer) skins and a metallic lattice core, both of which have high specific strength and stiffness, is one way to achieve better mechanical performance while remaining lightweight. Flexural stress is a load pattern that frequently occurs in the structural frame components of automobiles; nonetheless, while the in-plane load scheme has scarcely been examined, the out-of-plane load one has. As a result, the former configuration received consideration in this work. Moreover, short beam specimens were taken into account. The mechanical response of specimens with three different kinds of composite materials as skin material was analysed. The skins were made of CFRP (Carbon Fiber Reinforced Polymer), with two different weaving styles, and AFRP (Aramid Fiber Reinforced Polymer). All-titanium specimens were studied, too. Similar maximum loads and maximum displacement at break were recorded for both CFRP and AFRP specimens, while the all-titanium one resulted stronger. In terms of the load-displacement curves, the first section featured an initial linear phase, followed by a minor load drop, likely attributed to the breakage of fibres. The CFRP specimens showed a sharp fracture of the skin fibres, while for the AFRP, a fraying was observed.

Application of modified fracture criteria incorporating T-stress for various cracked specimens under mixed mode I-II loading

Accurate assessment of fracture behavior and life prediction of cracked materials based on the mixed mode fracture criteria is of utmost importance in fracture mechanics. In this study, the modified fracture criteria incorporating T-stress for mixed mode I-II cracks were comprehensively reviewed. Additionally, a comparative analysis was conducted between the experimental results obtained from five different cracked configurations and the theoretical predictions according to these modified criteria. Moreover, a brief discussion was held regarding the effect of T-stress on both crack initiation angle and fracture toughness, accompanied by providing some suggestions. The results revealed that the predictive accuracy of modified criteria exhibited noticeable variations across different cracked configurations due to the significant disparities in both the sign and magnitude of T-stress around the crack tip. However, it was observed that the predicted curves based on the modified fracture criteria for the semi-circular bend and edge-crack triangular specimens had remarkable similarities owing to their similar biaxial stress ratio B. When the mode II loading dominated, the generalized maximum tangential stress, generalized minimum strain energy density and generalized averaged strain energy density criteria were suitable for the semi-circular bend, edge-crack triangular, central crack rectangular and short bend beam specimens with positive T-stresses, whereas the maximum tangential strain, generalized maximum tangential stress and generalized maximum tangential strain energy density criteria could provide better theoretical predictions for central cracked Brazilian disk specimens having a negative T-stress.

Scalable, roll-to-roll manufacturing of multiscale nanoparticle/fiber composites using electrophoretic deposition: Novel multifunctional in situ sensing applications

Multiscale composites, where traditional fiber reinforcements are combined with nanoscale reinforcements, have emerged as multifunctional materials and have found potential for in situ strain and damage sensing, and energy harvesting/storage applications. Critical to the advancement of future applications is the development of manufacturing techniques that are industrially scalable. Electrophoretic deposition (EPD) can uniformly coat functionalized nanoparticles, such as carbon nanotubes (CNT), on conductive and non-conductive fiber substrates, producing multiscale composites with controlled microstructures and functional properties. In this research, a pilot-scale roll-to-roll EPD system is developed to continuously manufacture CNT-integrated fabrics for in situ sensing applications. Based on fundamental knowledge of CNT deposition mechanisms, key experiments are conducted to determine electrode configurations to optimize deposition yield in the roll-to-roll process. Functionalized CNTs are then deposited on 4–6 m long continuous rolls of randomly oriented non-woven glass fiber veil with different areal weights, and the CNTs form a continuous, conductive fiber-level coating. The thin and open fabric microstructure allows embedded sensors that are minimally invasive to the composite structure and allow ultraviolet (UV) light transmittance for use with UV-curable resins. The electrically conductive CNT network created on the fabric allows for the integration of sensing functionality. Short beam specimens with the CNT sensors embedded at the midplane showed no deterioration in strength. Multiple functionalities, including strain sensing and flow/UV cure monitoring during vacuum infusion are demonstrated. Sensors tested under flexural loading demonstrated sensitivity in tension and compression, and during resin infusion, the sensors could track resin flow and cure progression.

Influence of material extrusion parameters on fracture mechanisms of polylactic acid under three-point bending

In this work, a thorough fracture assessment of additive manufactured specimens made by 3D printed polylactic acid (PLA) under three-point bending is presented. This polymer is widely used in additive manufacturing for numerous applications, gaining prominence in fields such as biomechanics. Prismatic fully dense specimens and lattice structure specimens with different raster orientations and densities were fabricated using the material extrusion-based additive manufacturing technique. The objective is to analyse the effect of relevant process parameters on the fracture mechanism of short beam specimens tested under three-point bending. Digital image correlation and computed tomography have been used to measure the full field displacement and internal defects, respectively. Furthermore, the theory of critical distances was applied to assess the mechanical strength of lattice structure specimens. The results have shown the strong influence of raster orientation on the mechanical strength and fracture patterns even for negative air gap values. The transition from brittle to ductile failure was observed when changing density and raster orientation. Lastly, neck-shaped defects were detected in knots of specimens manufactured with a grid pattern. Defects in grid pattern specimens appear due to the prescribed path followed by the extruder during the building process. They were the main source causing a decrease in the mechanical strength and fracture trajectories. The objective of this research is to enhance the knowledge on the mechanical response of 3D printed components and to give an in-depth failure assessment.

Shear performances of reinforced ultra-high performance concrete short beams

This study aims to understand the shear performance of reinforced ultra-high performance concrete (R-UHPC) short beams and to propose a calculation method for their shear capacity. A total of 11 R-UHPC short beam specimens were tested to investigate the effects of key parameters on the shear capacity, including steel fiber volume fraction, longitudinal reinforcement ratio, stirrup ratio, and shear span-to-depth ratio. Experimental results indicated that the shear capacity increased with the increase of steel fiber volume fraction, longitudinal reinforcement ratio, and stirrup ratio, while it decreased with the increase of shear span-to-depth ratio. Results also showed that for R-UHPC beams, the effect of shear span-to-depth ratio can only be ignored for a ratio less than 1.2 rather than 1.5 that is required for reinforced concrete (RC) beams by the Chinese design code. Moreover, it was found that the calculation methods for the shear capacity of R-UHPC beams in the current codes underestimated the shear capacity of the specimens by 30 % ∼ 50 %, as these methods did not consider the aforementioned significant parameters. A calculation method was proposed based on the modified compression field theory (MCFT) to comprehensively consider all the aforementioned parameters. The proposed method exhibited a higher accuracy and less dispersion as compared to the methods specified by current codes.

Development and Experimental Assessment of Friction-Type Shear Connectors for FRP Bridge Girders with Composite Concrete Decks.

This paper details the development and experimental assessment of a friction-type connector, designed to transfer shear flow between the top flange of a fiber-reinforced polymer (FRP) tub girder and a composite concrete deck for bridge applications. In contrast with previously used bearing-type connectors, this system relies on a deformed FRP surface to transfer shear via direct interlock with the concrete deck. The connector is materially efficient, simple to fabricate, can be used with lower-grade structural or stainless-steel fasteners, and provides a high degree of interface stiffness. Six compression-shear specimens were tested to assess the connector fatigue resistance and ultimate connection strength. Additionally, two short beam specimens were tested in three-point bending, one of which was subjected to fatigue loading. Based on the compression-shear tests and short beam tests, the connection exhibited strength exceeding that predicted by AASHTO for frictional concrete-concrete connections. The connection strengths were significantly greater than the factored demand required by AASHTO for a typical model FRP bridge girder. The cyclic loading of the connection in both compression-shear and beam bending showed that connection stiffness and strength do not significantly degrade, due to the application of 1 × 106 to 6 × 106 cycles of traffic-induced factored fatigue load.

Relationship between low-temperature KIc and KIIc values of bitumen with different performance grades and comparison with naturally solid materials

As a visco-elastic material, asphalt binder acts as a brittle material under sub-zero temperature. The brittleness of this material at low-temperature conditions may result in sudden failure of asphalt pavements by passing the traffic loads. Utilizing the edge notched short bend beam (ENSBB) specimen, critical modes I and II stress intensity factors (KIc and KIIc values) of three bitumen materials (with performance grades 76–16, 64–22 and 82–28) are determined experimentally in this research at two low temperature (-12 and −24 °C) conditions. It was shown that KIIc is noticeably less than KIc for the whole fractured asphalt binders and testing temperatures. The ratio of fracture resistance (KIIc/KIc) that varies from 0.45 and 0.65 for the tested binder materials was estimated using minimum strain energy density, maximum hoop stress and maximum hoop strain failure theories. The low temperature fracture behavior of tested binders (including ratio of KIIc/KIc and cracking path) was in agreement with the behavior of natural solid materials such as Perspex, marble rock and gypsum.

Design and validation of simple bend beam specimen for covering the full range of I+II fracture modes

Using short beam specimen containing an edge crack (perpendicular to the beam axis) and subjected to asymmetric three-point bend loading a new test configuration (called ASBB) was suggested for investigating the mixed mode I/II fracture. Despite the conventional beam specimen with high aspect ratio of length to width that is not able to produce mode II or dominantly mode II conditions, complete mode mixities were obtained using the proposed three-point bend asymmetric short beam specimen. The fracture parameters including modes I and II stress intensity factors and T-stress were determined numerically for this test specimen with different crack lengths and bottom rollers distances relative to the crack. The practical ability of the ASBB specimen was examined experimentally by conducting several mixed-mode I/II fracture tests on PMMA material. The critical stress intensity factors, the direction of fracture kinking and the path of fracture trajectory were investigated both experimentally and theoretically using MTS and GMTS criteria. The fracture toughness of PMMA tested with the ASBB specimen was increased by increasing the contribution of mode II component. The effects of beam length aspect ratio, crack type and loading type on the mixed mode fracture of cracked beam samples subjected to three-point bend loading were also examined.

Mixed Mode I/II Fracture Analysis of Bi-Material Adhesive Bonded Joints Using a Novel Short Beam Specimen

Until now, some test specimens with different shapes and loading mechanisms have been utilized for investigating the cracking behavior of adhesive bounded joints. In this research, using a novel test configuration called adhesive short bend beam specimen containing an inclined crack and loaded by three-point bending, mixed mode I/II fracture parameters of a crack initiated in the adhesive part is studied. Compared to other test methods, the specimen used in this research needs a lesser amount of material and the fracture test can be performed easily. A large number of finite element models of this specimen were analyzed using ABAQUS code to study the effect of adhesive and adherent type, and also the crack length and loading span distance on KI, KII, T-stress and fracture initiation direction under different mode mixities. The results showed that the fracture parameters (and in particular the shear mode component) are sensitive to the type and location of adherent in the bounded joint; however, the shape and size of fracture plastic zone is not affected noticeably by the type of adhesive-adherent materials. It was also shown that the complete mode mixities ranging from pure mode I to pure mode II can be introduced for adhesive bounded joints using the proposed test specimen and therefore the specimen is a good candidate test configuration for investigating the mixed mode I/II fracture behavior of adhesive bounded joints.

Experimental demonstration of the in-situ effect under transverse shear

Failure of composite laminates, in load cases where transverse shear prevail, involves shear cracks and delaminations, and yet it is unclear which damage mechanism triggers the other or how it depends on ply-thickness. Combining interrupted interlaminar shear strength tests with X-ray tomography inspections, we compared the damage sequence of [45°/0°/-45°/90°]ns short-beam specimens manufactured with standard- (n=2) or thin-ply (n=4) non-crimp fabrics (fibre areal weights of 134 and 67 gsm per ply). Failure manifested as a load drop in the force-displacement curve. In both materials, we associated the onset of instability to a shear crack tunnelling across the central 902° ply-cluster. The intersection of this crack with the ones developing in the adjacent layers induced a delamination which, in turn, activated the load drop. A 3D elasto-plastic model predicted that, for the same applied load, the ply-cluster of both laminates would display a similar maximum principal stress distribution. However, the thin-ply samples improved the interlaminar shear strength by 34%, evidencing the so called in-situ effect: the resistance to matrix cracking under transverse shear (τ23 in the local coordinates of the 90° plies) increases when ply-thickness is reduced.

Edge treatment of short beam shear tests for improved assessment of structural strength

The certification and quality control of large aerostructures relies on testing of small-scale coupons, which may be unrepresentative of the final product. Edge effects, which are either non-existent or less significant in large aerospace components, cause coupon failure and often lead to considerable strength underpredictions. In this work, a resin treatment is applied to the edges of short beam specimens, resulting in a net strength improvement of 16% and 36% in laminates using two carbon-fibre reinforced plastic material systems. The effects of the treatment are analysed in detail using experiments, computed tomography, high-fidelity linear and non-linear finite element models allowing for damage propagation. The treatment produces interlaminar shear strength predictions which are more representative of those found away from edges in large aerostrucures by reducing interlaminar stresses at the free edges and delaying delamination onset. The procedure can be used to improve testing and weight/cost efficiency of aerospace components.

Damage analysis of a GLARE laminate subjected to interlaminar shear

GLARE (Glass Laminate Aluminum Reinforced Epoxy) is a hybrid material composed of aluminum sheets alternated to fibre glass composite material and it is used for some applications in the aeronautical field. In the present work, the mechanical performance of a GLARE laminate is compared to that one of a GFRP (Glass Fibre Reinforced Polymer) laminate. In particular, the ILSS (Interlaminar Shear Strength) of both laminates were determined through a three-point bending test carried out on short beam specimens. From the experimental results, a slightly higher strength was found for the GFRP laminate, even if the difference was similar to the experimental scattering. Moreover, the analysis of the whole shear stress trend as a function of the loading nose displacement highlighted that the loading nose displacement at the first breakage of the GLARE was higher than that of the GFRP, as well as the residual strength (after the first breakage) of the former was higher than that of the latter. Therefore, the GFRP has a brittle behavior, while the GLARE has a higher toughness.

Comparison between long and short beam flexure of a carbon fibre based FML

A Fibre metal laminate (FML) consists of a stack of metal sheets alternating to composite material layers: in such a manner, the best characteristics of both constituents are combined together. In this work, the flexural behaviour of different kinds of FMLs was investigated. In particular, both long beam and short beam specimens were produced and tested, since the length-to-thickness ratio influences the stress type arising in the material and, consequently, the failure mode. Moreover, the influence of both the composite/metal interface and the thickness of the layers was analysed. It was found that the structural adhesive was deleterious for the flexural strength of the long beam, while it improved the behaviour of the short one. As concerns the thickness and the distribution of the layers, this factor was unaffecting for the short beam specimen, while it was decisive for the long beam one. A micrographic analysis was carried out on the tested specimens, in order to characterize the failure mode. It was found a preponderance of fibre breakage in the long beam, while in the short one the failure of the metal/composite interface was prevailing.

Sub-sized short bend beam configuration for the study of mixed-mode fracture

In this paper an angled edge cracked short beam specimen subjected to symmetric three-point bend loading was designed and examined for conducting mixed-mode I + II fracture toughness experiments. This specimen is suitable for being used in those experimental crack growth studies in which a very limited amount of raw material is available for the preparation of the test specimen. By conducting several finite element analyses, it was shown that the short bend beam specimen is able to produce full modes I and II mixities if the affecting parameters such as the crack length ratio, loading span ratio and crack inclination angle are set in suitable ranges. The practical ability of the short bend beam specimen was investigated for obtaining the mixed-mode I + II fracture toughness of bovine bone and the corresponding values of KIc and KIIc were determined for different mode mixities. While a significant discrepancy was observed between the experimental results and predictions of the conventional singular based fracture theories, it was demonstrated that the two-parameter (K &T) based fracture models such as GMTS and EMTSN criteria can provide significantly improved predictions for the fracture behavior of tested short bend beam specimen made of bone material.

Interlaminar shear strength study on CFRP/Al hybrid laminates with different properties

FML (Fibre Metal Laminate) is a hybrid material that presents outstanding structural properties, such as resistance to cyclic and dynamic loads, together with low specific weight. This material consists of metal sheets alternating to composite material layers. In the present work, the ILSS (Interlaminar Shear Strength) was evaluated for different types of carbon fibre/aluminium FML, produced varying the layer thickness and the bonding solution of layers. In fact, FMLs consisting of one or two metal sheets (a parameter strictly connected to the layer thickness, as the metal/composite volume fraction was kept at constant value) and bonded with structural adhesive or prepreg resin were considered for this study. The ILSS was determined according to the three-point bending method with short beam specimens. The experimental tests evidenced an effect of the adhesion methodology on the ILSS value, while the layer thickness did not influence the interlaminar strength. The mechanical behaviour after the maximum load point was investigated too, evaluating the trend of the shear stress as a function of the loading nose displacement.

On the use of edge cracked short bend beam specimen for PMMA fracture toughness testing under mixed-mode I/II

In this paper an inclined edge cracked short beam specimen subjected to symmetric three-point bend loading was designed and examined for conducting mixed-mode I/II fracture toughness experiments. The aspect ratio (i.e. length to width ratio) and the loading span distance are considered much lower than the other conventional cracked bend beam samples. Crack tip parameters such as stress intensity factors and T-stress were computed numerically for this specimen by several finite element analyses and it was demonstrated that the specimen is able to produce full combinations of mode I and II including pure mode II. The practical capability of the short bend beam specimen was studied experimentally by conducting a set of mixed-mode fracture tests on PolymethylMethacrylate (PMMA) as a well-known model brittle material. The critical stress intensity factors, the direction of fracture kinking and the path of fracture trajectory were investigated both experimentally and theoretically using two stress and strain-based fracture criteria. The fracture toughness of tested PMMA was decreased by moving towards mode II case due to the effect of T-stress on the fracture mechanism of the short bend beam specimen.

Effectiveness of diagonal shear reinforcement on reinforced concrete short beams

In the study, an experimental and numerical study is performed to investigate the efficiency of diagonal shear reinforcement (DSR) on reinforced concrete (RC) short beams. For this purpose, 7 RC short beam specimens were tested under a 4-point loading, and a numerical study is conducted by using finite element method. Additionally, the efficiency of addition of DSR to specimens is observed in the experimental study together with the increase in stirrup spacing. Analysis results are compared in terms of load-displacement behavior and failure modes. As a result of the study, a significant improvement both in shear and displacement capacities of the RC short beams are achieved along with addition of DSR in short beams. Moreover, it is deduced from the numerical results that increasing both the diameter and yield strength of DSR makes a significant contribution to the shear capacity and ductility of shear critical RC members.

Structural Characteristics of Composite Mortars and their Evolution with PET Substitution Level for Several Specimens’ Ages

This present study aims to investigate the evolution of structural response of PET-Mortar composite test with a short-beam specimen in three-point bending tests, with composite mortar ages and volumetric polymer rate and this, based on compression strength tests. The ultimate PET-mortar composite structural responses are calculated at the mid span of the short-beam by the mean of mechanics-of-materials theory basis. According to this theory, the distribution of bending moments and shear forces at any point of the composite short-beam specimen doesn’t depend on material mechanical properties especially the young modulus of modified mortar composite; so, the structural response analysis has been limited to investigate the evolution of ultimate deflection with several volumetric PET rates and composite mortar ages. In the other hand, we present a comparative study between experimental test results of splitting tensile and compressive strengths with the ones predicted by codale previsions (ACI-363 and B.S) codes in terms of PET mortar ages and volumetric PET rates in order to recommend the most suitable design code for PET-mortar composite applications in construction industries.

Structural Characteristics of Composite Mortars and their Evolution with PET Substitution Level for Several Specimens’ Ages

This present study aims to investigate the evolution of structural response of PET-Mortar composite test with a short-beam specimen in three-point bending tests, with composite mortar ages and volumetric polymer rate and this, based on compression strength tests. The ultimate PET-mortar composite structural responses are calculated at the mid span of the short-beam by the mean of mechanics-of-materials theory basis. According to this theory, the distribution of bending moments and shear forces at any point of the composite short-beam specimen doesn’t depend on material mechanical properties especially the young modulus of modified mortar composite; so, the structural response analysis has been limited to investigate the evolution of ultimate deflection with several volumetric PET rates and composite mortar ages. In the other hand, we present a comparative study between experimental test results of splitting tensile and compressive strengths with the ones predicted by codale previsions (ACI-363 and B.S) codes in terms of PET mortar ages and volumetric PET rates in order to recommend the most suitable design code for PET-mortar composite applications in construction industries.

Timber Beam Bearing Reinforcement with GFRP Glued-In Plates: Strength and Hydrothermal Effects

AbstractThis paper presents a simplified method of analysis and design for the repair of deteriorated timber beam ends using glass fiber–reinforced product (GFRP) bonded rigid plates. It assesses the load-bearing capacity of the plate-timber connection and assumes two failure modes: shear stresses at the bonding interface and in the timber piece. An experimental method was developed to verify the theoretical model for bending moment and shear force transfer. Ten short span beam specimens of Pinus radiata D. Don were spliced with bonded-in plates in one end, and they were tested by bending with different anchorage lengths to establish a relationship with load-carrying capacity. Furthermore, the effect of temperature (−20 to 50°C) and moisture content (9–30%) changes were analyzed in two groups of six short beam specimens and in bonding shear test specimens using two different epoxy formulations. The theoretical model predicts conservative values of load-carrying capacity. Temperature and moisture changes c...