Single-lap Specimens Research Articles

Investigation on mechanical property and failure mechanism of the carbon/glass hybrid laminate bolted joints using acoustic emission

In this study, tensile tests and acoustic emission (AE) monitoring were performed on single lap specimens of carbon/glass hybrid laminates with different carbon fiber layer angles. The results indicated that the carbon fiber layer angle not only affects the bearing strength and bearing stiffness of hybrid laminates, but also influences the macroscopic extension direction of damage. The study also found that carbon fiber and glass fiber damage exhibit different centroid frequencies. Building on these findings, the analysis identified the primary fiber type involved in fiber damage to different hybrid fiber laminates. Additionally, it was noted that the friction generated between the bolt and the laminate under load triggers AE signals with a high centroid frequency.

Improving performance of GFRP joints using bolted/mortise-tenon hybrid method

Abstract In this study, the effects of different connection methods such as adhesives and bolts on the performance of GFRP joints were compared. A hybrid method based on bolted/mortise-tenon was proposed, and the static strength of joints was studied through experiments and numerical simulations. Static trength testing using glass fiber reinforced polyurethane composites. The results showed that for adhesive-bonded joints, double-lap shear specimens have the highest shear strength. For bolted joints, double-lap specimens also have higher connection strength than single-lap specimens. And the strength of joints using bolts is much greater than that using adhesives. In addition, the bolted/mortise-tenon hybrid joint has a self-locking structure of dovetail and tenon connections, which converts shear strength into structural strength, effectively improves the load-bearing capacity, increases the reliability of the connection, and contributes to higher load-bearing capacity.

Mechanical characterisation and high temperature analysis of hyperelastic adhesives – Modelling and experimental validation

Adhesive bonds are subject to multiple environmental conditions that can affect their mechanical performance during service. Therefore, it is important to evaluate the influence of temperature on adhesive strength, as it can impact joint safety and should be considered during the design phase. This study presents an analysis of the effect of high temperatures on the mechanical behaviour of joints made with highly elastic adhesives, specifically a polyurethane and a silicon-modified polymer. Shear and tensile tests were conducted at temperatures of 23, 50, and 80 °C using dumbbell specimens for tensile tests and single lap specimens (SLJ) for shear tests. The tests were performed with two different substrates, aluminium (Al) and glass fibre reinforced polyester panel (GRP), and with varying adhesive thicknesses. These tests aim to assess the impact of temperature conditions on the mechanical properties of the adhesive and the behaviour of the joints. The analysis of the experimental results reveals that the adhesive degrades when exposed to high temperatures, resulting in reduced strength and stiffness, and less linear behaviour.Furthermore, this work involves the determination of a model able to reproduce the mechanical behaviour of hyperelastic adhesive at high temperatures, considering diverse constitutive modelling approaches. To achieve this, a testing protocol was conducted on basic uniaxial and planar specimens. The results indicate that the Ogden N=2 model is the most suitable for representing the non-linear behaviour of the hyperelastic adhesive at high temperatures. In contrast, the Mooney Rivlin model is more suitable to represent the material behaviour under ambient conditions. To conclude this work, the law has been satisfactorily validated by comparing the results of tests carried out on SLJ specimens with different adhesive thicknesses.

Strengthening of glass fiber sheets for multi-bolted pultruded GFRP connections: Effects of connection type and bolt-tightening force

This study investigated the effects of bolt-tightening forces on the connection strengths of non-strengthened and glass fiber sheet (GFS)-strengthened pultruded glass fiber reinforced polymer (PGFRP) connections through a series of experiments using double-lap and single-lap specimens. A total of 96 specimens (3 specimens for each parameter), half of which were non-strengthened, were tested under tensile loads and four bolt-tightening forces: 0, 5 N.m, 10 N.m, and 20 N.m. The (0°/90°)/chopped strand mat/(0°/90°) lamination was bonded to both PGFRP plates’ outside surfaces and improved the connection strengths. The results indicated that maximum failure loads increased with an increase in bolt-tightening forces. However, there were different trends in the maximum failure load changes, depending on the specimen type (non-strengthened or GFS-strengthened specimens; double-lap or single-lap specimens) and bolt-tightening forces. In addition, GFSs could effectively improve PGFRP’s connection strengths and ductility performances.

Experimental study on interface failure behavior of 3D printed continuous fiber reinforced composites

Additive manufacturing (or 3D printing) for continuous fiber composite exhibits great potential for fabrication of next-generation lightweight sophisticated structural components. Nevertheless, understanding of interface characteristics of printed parts has remained an open research question for broader applications of this new technology in engineering practice. This study aimed to evaluate the interfacial quality of 3D-printed carbon fiber reinforced plastic (CFRP) composites from two aspects: interlaminar fracture toughness in a pure mode and interfacial failure mechanism in a mixed mode. Prior to the experimental tests, the microstructural quality of the 3D printed specimens was characterized, which showed that the void content and shape of different materials are different. There is a clear boundary line where the short fiber matrix layer and the continuous fiber reinforced layer are in contact, while the interface formed between the same material layers has no evident connection traces. The standard specimens were printed for testing pure mode I and mode II interlaminar fracture toughness, through which the G Ι C and G IIC of interface between continuous carbon fiber layers was characterized. It was found that the fiber bridging phenomenon presented in the double cantilever beam (DCB) experiments, resulting in a higher initial mode I fracture toughness. The end notched flexure (ENF) tests found that the fracture toughness of initial mode II in between the 0° continuous carbon fiber layers was relatively low. The scanning electron microscope (SEM) analysis revealed that the separated surface had only a small area of the matrix being sheared. The single lap specimen printed directly was proposed as a simplified model for simulating complex structural parts subjected to mixed stress of peel and shear. The mechanical responses and failure characteristics of the interfaces composed of different materials and different fiber angles under mixed stress were studied. It is found that each specimen presented some mixed failure modes, and the interface characteristics of different materials and different fiber orientations were completely different. The results gained in-depth understanding on the interfacial properties of 3D printing fiber reinforced structures, thereby providing key data and knowledge for practical applications. • Different layering methods with different interlayer morphology and void shape. • The critical strain energy release rates of type I and typeⅡwere obtained. • Continuous fiber bridging enhances type I fracture toughness. • The shear resistance of the interface between continuous fibers and matrix is very weak. • Reinforcement materials and fiber angles affect the interfacial bond strength and failure.

Experimental and numerical investigation on the corrosion effects on the bonding behavior between CFRP and steel

This paper intends to investigate the corrosion effects on the bonding behavior between CFRP and steel. The corrosion parameters such as the pit density Pd, the average pit depth Dave and width Wave were acquired by using an immersion simulation algorithm. By conducting the shear tests of the CFRP/steel single lap specimens, the bonding parameters such as the ultimate load Fu, the shear strength τmax and the fracture energy G were obtained and their relationships with corrosion parameters were established. In addition, finite element models considering the corrosion topography were proposed to simulate the bonding behavior. Results showed that the Fu and the G increased first and then stabilized as the Pd increased, and increased slightly then decreased as the Dave and the Wave increased, indicating that the corroded surfaces distributed with a large number of pits of appropriate sizes may maximize the bonding strength and the fracture energy; the τmax increased with the Pd, and decreased as the Dave and the Wave increased, indicating that a larger pit number and a smaller pit sizes would lead to a higher shear strength; the Fu and the interfacial shear stress distribution in finite element analysis were similar to the experimental results.

Bonding Characteristics of Single- and Joggled-Lap CFRP Specimens: Mechanical and Acoustic Investigations

Two different configurations of adhesive-bonded carbon fiber-reinforced plastic (CFRP) specimens, joggled lap-joint specimens and single lap-joint specimens, are mechanically tested. The mechanical tests show that the joggled lap specimens have lower strength than the single lap specimens. The damage modes in both the specimens are analysed by the Acoustic Emission descriptors recorded during the mechanical tests. The acoustic data as cumulative counts and cumulative energy show the critical points of failure in both the specimen groups under loading. Moreover, they also show that the damage modes in both the specimens are dissimilar. Finally, the data provided by acoustic emission descriptors are verified by fractographic analysis on the failed surface.

Thermo-stamping co-curing process for CFRP/steel hybrid sheets and its interface strength improvement

The co-curing interface between CFRP and metal has a significant influence on the comprehensive mechanical properties of the composite structure. In this paper, a series of single lap specimens were fabricated by a thermo-stamping process with fast curing prepreg and dual phase steel DP980. The thermo-stamping co-curing experiments were conducted under different forming conditions based on a grit-blasted metal surface to study the effect of process parameters on the interface strength. In addition, two types of interface strength improvement methods were tested: cutting mechanical grooves on the metal surface and smearing different carbon nanotube or graphene coatings on the metal and prepreg surfaces. The interface strength improvement mechanism was further explained by microscopic characterization. Finally, a finite element model combining cohesive element damage and three-dimensional Hashin failure was established to predict the failure mode of the co-curing joint. The results show that the initial pressure, heating curve and laying up methods have a certain degree of influence on the interface strength, and the best interface enhancement effect could be obtained by coating a carbon nanotube/epoxy resin and a curing agent on the metal and prepreg surfaces, respectively.

The effect of nanospike structures on direct bonding strength properties between aluminum and carbon fiber reinforced thermoplastics

With recent design developments in the automotive industry, it has become necessary to join dissimilar materials such as aluminum and carbon fiber reinforced thermoplastics (CFRTPs). In this study, a nanospike structure is fabricated on the surfaces of aluminum plates and directly bonded to CFRTP laminates. The effect of the nanospike structure on the adhesion strength is evaluated by single-lap joint tests. It is found that the nanospike structure improves the adhesion strength. Furthermore, combining the nanospike structure with a silane coupling treatment results in failure in the aluminum part of the single-lap specimens with an overlap length of 12.5 mm, rather than in the joined region. The average adhesion strength of the single lap joint specimens with an overlap length of 5.0 mm is found to be 24.9 MPa. Scanning electron microscopy observations of the fracture surfaces of the joined region only showed cohesive failure. On the fracture surface of the CFRTP laminate, the matrix exhibits a hairy structure due to the presence of the nanospike structure in some regions and in other regions carbon fibers are exposed due to adherend failure. Thus, in addition to an improved joint strength, the results suggest that the nanostructure will also improve the fracture toughness by causing ductile failure of the matrix.

Experimental and numerical investigation on the bond strength of self-sensing composite joints

Laboratory experiments demonstrate that a novel carbon nanotube (CNT)-based sensing layer embedded in the bondline of an adhesively bonded structural joint can detect and monitor deformation and damage progression of the adhesive layer. In this study, experimental and numerical investigations were performed to identify any effect of an embedded CNT-based sensing layer on the bond strength of that joint. To evaluate the mechanical behavior of such a bondline configuration, two sets of single-lap specimens, with and without sensing layer, were prepared and tested to determine the bond strengths of the respective types. Two-dimensional digital image correlation (2D DIC) was utilized to estimate the load-displacement response of the test specimens. Three-dimensional cohesive surface finite element models of the test specimens, with and without the sensing layer, were created and validated using the experimental measurements. It is shown that the embedded CNT-based sensing layer does not influence the bond strength of the single-lap joint.

Strength and bonding characteristics of adhesive joints with surface-treated titanium-alloy substrates

This paper presents a study on the effect of surface treatments on the mechanical behavior of adhesively bonded titanium alloy joints. Several different treatments were selected for the preparation of Ti-6Al-4V alloy faying surfaces, and bonded joints were fabricated using surface-treated titanium alloy substrates and a film adhesive. Tensile tests were performed on single-lap specimens to evaluate the joint strength and to assess the failure mode, i.e. cohesive failure, adhesive (interfacial) failure or a mix of both. Contact angle measurements were also carried out, and the surface free energies of titanium alloys and the thermodynamic works of adhesion for the adhesive/titanium alloy interfaces were obtained. A three-dimensional finite element analysis was used to predict the strength of the specimens exhibiting cohesive failure. In addition, an expression of the relationship between the joint strength corresponding to interfacial failure and the thermodynamic work of adhesion was introduced based on the cohesive zone model (CZM) concept. It is shown that two surface treatments, Itro treatment and Laseridge, lead to cohesive failure and a significant increase in the joint strength, and the numerically predicted strength values are fairly close to the experimental values. These surface treatments are possible replacements for the traditional surface treatment processes. For degreasing, emery paper abrasion, atmospheric plasma treatment, sulfuric acid anodizing, nano adhesion technology and high-power lasershot, the specimens fail at the adhesive/substrate interface and the joint strength increases linearly with the thermodynamic work of adhesion as expected from our CZM-based expression.

Simulation of a Novel Joining Process for Fiber-Reinforced Thermoplastic Composites and Metallic Components

In this study, a new joining technology to produce hybrid structures with continuous-fiber-reinforced thermoplastics and metallic components is presented adapting the concept of classical clinching for thermoplastic composites. To demonstrate the capability of the thermoclinching process, prototypic joints were manufactured using an experimental joining installation developed. Nondestructive and destructive analyses of the thermoclinched joints showed that a relocation of the reinforcement into the neck and head area of the joining zone could be achieved. For a first estimation of the maximum load-carrying capacity of the joints, single-lap specimens with both reinforced and nonreinforced thermoplastics were manufactured and tested, revealing up to 50% higher failure loads of the reinforced joints. To understand the local material configuration and to achieve a defined and adjustable fabric structure in the head area of the joint, further analyses with regard to material- and tool-side conditions of the joining zone are necessary.

Atmospheric-pressure plasma treatment of polyamide 6 composites for bonding with polyurethane

An atmospheric-pressure plasma jet (APPJ)-based surface treatment process was investigated for the structural (τB > 15 MPa) adhesive bonding of polyamide 6 (PA6) composites. The treated surfaces were examined by contact angle measurement, X-ray photoelectron spectroscopy, and atomic force microscopy (AFM). Additionally, the shear strengths of single lap specimens were determined as a function of different plasma intensities and polyurethane adhesives. Our results show that APPJ leads to an increase of the surface free energy, oxygen concentration, and number of functional groups. Furthermore, the topography of the surface was significantly modified by exposure to APPJ. AFM measurements show that special attention has to be paid to the intensity of the plasma treatment to avoid melting and flattening of the PA6 surface on the nanometer scale. With optimized multiple APPJ treatments, lap shear strength of 20 MPa was achieved for the first time for this material system, allowing the material system to be employed in future automobile applications.

Dependency of dynamic interlaminar shear strength of composites on test technique used

Studies are presented on dependency of dynamic interlaminar shear (ILS) strength on the experimental technique used for a typical plain weave E-glass/epoxy composite. Dynamic ILS strength was determined based on two experimental techniques, namely torsional split Hopkinson bar (TSHB) apparatus using thin walled tubular specimens and compressive split Hopkinson pressure bar (SHPB) apparatus using single lap specimens. The results obtained from these techniques are compared. In general, it is observed that dynamic ILS strength for composites obtained by TSHB testing using thin walled tubular specimens is lower than the dynamic ILS strength obtained using single lap specimens in compressive SHPB. The issues involved in TSHB testing of thin walled tubular specimens made of composites are discussed and the reasons for reduced dynamic ILS strength using thin walled tubular specimens are highlighted. Finite element analysis (FEA) of thin walled tubular specimens made of composite and resin subjected to quasi-static torsional loading is presented. Using FEA results, the reasons for lower ILS strength of composite thin walled tubular specimens are substantiated.

OS0511-444 破壊形態を考慮したFe-Al異材接合継手の疲労寿命予測

Dissimilar material joint makes a machine light and efficient. Welding is widely used to connect dissimilar materials, but an intermetallic compound produced in the interface and stress specificity at interface end are concerned to decrease the joint strength. It is important to evaluate the strength of welded joint for designing machine and structure. In this study, single lap specimens with welding joint of Fe and Al plates were prepared for a tensile shear test and a fatigue test. Joint strength and interfacial strength were investigated by modifying shape of welding joint. Fracture analysis revealed the interface was brittleness destruction and the joint strength depends on the strength of the starting point of the destruction. Fatigue curve can be classified every destruction form by using the hotspot stress of the starting point of the destruction.

THE CHOICE OF THEORY FOR ADHESIVE LAYER EFFORT

In the paper, the problems of adhesive layer effort was considered. The experimental tests of adhesives bonds were conducted in order to determine breaking loads. A technique a finite element mesh of adhesive layer subjected to shearing was proposed. Next the numerical calculations were conducted in order to determine distribution of stresses in adhesive layers at breaking loads. The components of stress tensor were used for the analytical calculations of reduced stresses according to seven hypotheses. It was found out that the maximal principal positive stresses hypothesis, the maximum principal strain hypothesis and the strain energy hypothesis make possible to estimate the adhesive layer effort with similar accuracy. Shape and the dimensions of the steel (S355J2G3) single lap specimen, the compression curves for Araldite AW 136H, the maximal principal stress values in the most loaded element of the adhesive layer depending on the elements’ dimensions, modelling versions of the tree-layer adhesive layer’s edges , distribution of the maximal principal stresses along edge section, comparison of the maximal principal stress distribution along adhesive layer’s length, the stress-strain curve for Belzona 1111 adhesive and for Epidian57/Z1 adhesive, distribution of maximum principal stresses in adhesive layer of single lap bond subjected to shearing, are presented in the paper.

Development of a Monitoring System for Crack Growth in Bonded Single-Lap Joints Based on the Strain Field and Visualization by Augmented Reality

In this work, a method for monitoring fatigue crack growth in a metal to composite bonded joint based on the strain field is proposed and applied in the framework of a visualization tool based on Augmented Reality (AR). This tool superimposes some virtual objects, which are the data acquired by the sensors and the crack length, directly on top of the specimen under inspection and in real time. By finite element (FE) analyses, a good correlation between the crack tip position and the strain field in a single lap specimen is found and this feature is exploited to monitor the crack length during fatigue tests and to feed the AR system to virtually visualize the crack on the real specimen. An array of electrical resistance strain gauges is bonded to the surface of one adherend. A Matlab function collects values from the strain gauges mounted on the specimen under investigation analyses them on the basis of the FE analysis and finally feeds the AR system. The validation of this process is done by measuring the crack by optical microscope. This procedure is also tested with the use of Fiber Bragg Gratings (FBG) optical strain gauges.

OS0304 Fe-Al異材接合継手の強度評価

Welding is widely used to connect dissimilar materials, but an intermetallic compound produced in the interface is concerned to decrease the joint strength. It is important to evaluate the strength of welded joint for designing machine and structure. In this study, single lap specimens with welding joint of Fe and Al plates were prepared for a tensile shear test and a fatigue test. Influence of rotation caused by tension and distribution of interfacial strength were investigated by modifying specimen length, number of tabs, and shape of welding joint. It was found that the rotation did not have significant influence on interfacial strength, but fracture behavior varied and indicated uneven strength distribution along interface.

Combined joining technique for thermoplastic composites with embedded sensor networks

Lightweight components suitable for high-volume production are realizable by utilizing textile-reinforced thermoplastics. These composite structures can be equipped with additional functionalities by embedding sensor networks. With regard to assembly processes, material- and function-adapted joining techniques are required to connect such components efficiently. Therefore, the joint should enable both the transmission of mechanical loads and electrical signals. In order to fulfill these requirements, a combined joining technique based on blind riveting is analyzed in this article. Specimen plates made of glass-fiber-reinforced polypropylene were fabricated by compression molding. They contained adapted conductors embedded between the glass-fiber-reinforced polypropylene layers. In a next step, specimens were joined by blind riveting. The experimental analysis of several joint configurations revealed differences in interface quality and electrical performance. Finally, the reliability of the selected joining concept was verified by the experimental investigation of single-lap specimens under thermal and tensile loading.

Evaluation of fatigue life of adhesively bonded aluminum single-lap joints using interfacial parameters

This work experimentally studies the effect on adhesive dimensions on the fatigue strength of epoxy adhesively bonded aluminum single-lap joints. Various types of single-lap specimens with different overlap lengths and adhesive thicknesses were used in the experimental program to investigate the effect of bonding dimensions on fatigue strength. Experimental results indicate that under the fixed average shear stress condition, the larger adhesive thickness detrimentally affects fatigue strength. Similarly, the fatigue resistance decreases as the overlap increases except for the specimens with an adhesive thickness of 0.5 mm. The finite element method was adopted herein to obtain the local stress states at the interface between the adhesive and the adherend. Three selected parameters based on the simulated interfacial stresses were considered to correlate with the fatigue life data of all specimens with various adhesive dimensions. These parameters are maximum interfacial peeling stress, maximum interfacial shear stress and a linear combination of interfacial peeling stress and shear stress. These three interfacial parameters yield much better correlation results than the bulk average stress parameter. The evaluation results demonstrate that peeling stress and the linear combination of interfacial peeling stress and shear stress provide better correlation results than the interfacial shear parameters, revealing that the interfacial peeling stress is the main driving force of the fatigue failure of the single-lap joints.