Experimental characterization of interlaminar shear failure in sandwich structures under three-point bending

We applied an improved six-step phase-shifting method in digital photoelasticity to an adhesively bonded aluminum/epoxy/aluminum sandwich structure in order to study interlaminar shear failure behavior. Before and after three-point bending, a self-balanced thermal residual shear stress appeared on the interface because of the difference in thermal expansion coefficients between aluminum faces and epoxy core interlayer. At the beginning of loading, the shear stress in the core layer distributes continuously and forms shear bands tilting at a 45° direction. It then joins with the upper and bottom aluminum faces in order to realize the shear load transfer. As the bending load increases, the maximum interface shear stress occurs near the supports and a partially debonded region appears at the interface. The interfacial shear stress in the partially debonded region decreases rapidly until a shear failure occurs. A load–flexibility curve of the vibration-damping–type sandwich structure agrees well with the theoretical prediction of a laminated beam.     

Interlaminar shear behaviors of 2D needled C/SiC composites under compressive and tensile loading

The interlaminar shear strength of 2D needled C/SiC composites was measured using the double-notch shear test method. Interlaminar shear tests were performed under compressive and tensile loading. Shear stress–strain response and shear strain field evolution were studied using the digital image correlation (DIC) technique. The results show that the interlaminar shear strength of the specimen using the compressive loading method is 15% higher than that of the tensile loading method. Severe shear strain concentration was observed near the upper notch of the tensile loading specimen. Acoustic emission (AE) was utilized to monitor the damage during the tests. Typical damage mechanisms were categorized according to AE signal characteristics. The statistical results show that more matrix cracks were produced in the tensile loading specimen and no separate fiber/matrix debonding signal was detected in both specimens.     

Dynamic shear properties of particle mixture by rotational shear test

The effects of particle size and composition of binary mixtures on the dynamic angle of internal friction were investigated by rotational shear test. The internal angle of friction was found to increase for the particles of the larger size and to increase initially and then to decrease slightly with increasing fraction of the larger particles owing to the increase in the roughness of shear surface and to the decrease in the voidage of particle mixture.     

Digital image correlation strain measurement of thick adherend shear test specimen joined with an epoxy film adhesive

Structural bonding and bonded repairs of composite materials become more and more important. Understanding the strain within the bondline leads to suitable bonding design. For new design approaches the strain distribution within the bondline has to be analyzed. Thus, often finite element analysis (FE) are used. However, a huge challenge is the availability of reliable material properties for the adhesives and their validation. Previous work has shown that it is possible to measure the small displacements resulting within thin epoxy film adhesives using high resolution digital image correlation (DIC). In this work a 2D DIC setup with a high resolution consumer camera is used to visualize the strain distribution within the bondline over the length of the joint as well as over the adhesive thickness. Therefore, single lap joints with thick aluminum adherends according to ASTM D 5656 are manufactured and tested. Local 2D DIC strain measurements are performed and analyzed. Two different camera setups are used and compared. The evaluation provides reliable material data and enables a look insight the bondline. The results of the full field strain data measured with DIC are compared with numerical simulations. Thus, material models as well as chosen parameters for the adhesive are validated. Compared to extensometers, giving only point-wise information for fixed measuring points, the DIC allows a virtual point-wise inspection along the complete bondline. Furthermore, it allows measuring close to the bondline to reduce the influence of adherend deformation.     

Measurement of adhesive shear properties by short beam shear test based on higher order beam theory

This paper refers to the measurement of the shear properties of adhesive bonding by a new beam theory using the short beam shear (SBS) test configuration. A novel higher-order sandwich beam theory has been developed to analyze the adhesive bonded beam that consists of two adhered laminates and a single layer of adhesive in between. The closed form analytical solution for the SBS test model of the adhesively bonded beam is obtained in terms of deflection and stress distribution. The present theory has been used for calculating the adhesive shear modulus from the structural compliance. The initiation of stiffness degradation for the short beam shear test model was used as the critical load value for deriving the adhesive shear strength. A finite element model is built for validating the present model, and to evaluate its suitability for measuring adhesive shear properties. The present theory shows better accuracy for measuring the shear modulus than existing theories for both thin and thick adhesive layers. The measured strength values are more accurate than those obtained from the single lap joint shear test model. This theory can be used for adhesive materials with linear elastic deformation behavior.     

The mixed-mode fracture behavior of epoxy by the compact tension shear test

In this study, pure mode I, pure mode II and mixed mode fracture behavior of an epoxy were investigated. Specifically, the mixed mode values of fracture toughness and critical strain energy release rate (CSERR) were measured. Specimens were subjected to mixed mode loading using compact tension shear (CTS) test. Some experimental modifications were found to be necessary to eliminate rotation and ensure crack propagation at the notch when testing epoxy specimens at high mode II loading. A failure criterion for the mixed mode loading of polymer is developed and its predictions are compared with the experimental results. The crack propagation direction in epoxy was investigated in this research as well. A detailed study of failure mechanisms on the fracture surface was performed. The results indicate that the increase in the value of toughness can be directly related to the fracture morphology.     

Crystallization of polyethylene under shear flow as studied by time resolved depolarized light scattering. Effects of shear rate and shear strain

We have studied structure formation during crystallization of polyethylene (PE) under shear flow using time resolved depolarized light scattering (DPLS) in order to elucidate the formation mechanism of the so-called shish-kebab structure. Two-dimensional (2D) DPLS pattern clearly showed streak-like scattering normal to the flow direction in the early stage during the crystallization after pulse shear, suggesting the formation of the shish-like structure in μm scale. In order to analyze the 2D DPLS pattern we defined measures for the acceleration of the crystallization rate and the degree of anisotropy and found that there are critical shear rates for both of the acceleration and the anisotropy at a given shear strain: the former is much smaller than the latter. We also determined the critical shear rate for the anisotropy as a function of the shear strain. Extrapolating to inverse of the infinite shear strain=0, we found the critical shear rate for the anisotropy at the infinite shear strain to be 1.5 s⁻¹. The results were discussed in relation to a competition between the relaxation rate of polymer chains and the orientation-induced crystallization rate.     

Silks cope with stress by tuning their mechanical properties under load

We compare the nonlinear mechanical properties of silks under load with the quasi-static and isothermal dynamic mechanical properties of nylon as well as human hair. For silk and nylon, the dynamic storage modulus increases with increasing static load, while the quasi-static modulus decreases considerably through yield. However, the modulus of hair decreases irreversibly for both loading conditions. For silk, the increase in storage modulus is only partially reversible after high-loading and is accompanied by a non-recoverable strain. For nylon, the dynamic modulus increase is largely reversible after increased static loading up to a second high stress yield point, where modulus then decreases. Taken together, our data suggest that the dynamic modulus increases with increasing order in the silk and nylon structures under static load, whereas the morphology of hair is gradually degraded under load.     

Novel hybrid block copolymer nanocarrier systems to load lipophilic drugs prepared by microphase inversion method

Nanoparticles based on block copolymers of oligosaccharides [β-cyclodextrin (βCyD) and maltoheptaose (Mal₇)] and poly(ε-caprolactone) (PCL) were prepared by microphase inversion method. Zeta-potential, particle size measurements and morphological analysis of drug-free and drug-loaded nanoparticles were performed by using, respectively, laser-doppler anemometry, dynamic and static light scattering and transmission electron microscopy. ρ-Ratio values were correlated with transmission electron microscopy observations. Both types of amphiphilic block copolymers, βCyD-b-PCL_5k and Mal₇-b-PCL_5k, self-assembled in water to form spherical vesicles, presented a hydrodynamic diameter of 72 and 34 nm, respectively. The incorporation of drugs into nanoparticles did not affect significantly the particle size for βCyD-b-PCL_5k-based nanoparticles with progesterone, unlike the other tested systems. On the other hand, all nanoparticles (with and without drug) were negatively charged. Both nanoparticulate systems showed high drug loading efficiency (higher than 95%), confirming their suitability as delivery system for lipophilic drugs.     

Characterization of interlaminar shear strength of laminated woven E‐glass/epoxy composites by four point bend shear test

An experimental and numerical investigation was performed to study the interlaminar shear response of laminated woven E‐glass/epoxy composites. The interlaminar shear strength results obtained from four point bend shear tests were compared with the results obtained from American Society for Testing and Materials (ASTM) test standards D2344 (short beam strength). The test results reveal that the four point bend interlaminar shear test results at a span to thickness ratio of 8 is higher than the short beam shear test results at a span to thickness ratio of 4. Numerical simulations were performed with ANSYS® software. The experimental results and the corresponding numerical results are in good agreement. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

A new method to measure the adhesion capability of the metallic surface under shear loading using a modified Arcan test

The single lap shear test is considered by carmakers as a standard test to control the adhesion of a bonded assembly. In some special cases, when thin steel surfaces are bonded to a structural toughened epoxy-based adhesive, interfacial failure could occur using this mechanical test. Understanding the contribution of the steel surface on the failure mechanisms using the single lap shear test is complex: heterogeneous distribution of stresses, contributions of edge effects, plasticity of the sheet metal, and stiffness of the specimen. It is very difficult to determine failure criteria using the single lap shear test for the interfaces.

This work proposes a new method to measure the adhesion capability of the metallic surface under a well-controlled loading. The method is based on the modified Arcan test in shear loading which is adjusted to steel plates. Three different types of surfaces were characterised experimentally and numerically with different adhesion behaviours regarding their failure patterns. The new modified Arcan test is a suitable test to identify the properties of the interfaces in the bonded assemblies using strain and energy criteria.     

粘质粉土的加卸载试验研究

土是一种散粒体材料,其性质受多种因素影响,不同场地、不同环境下的土性质相差悬殊。针对工程中比较棘手的软弱土———粘质粉土性质的模糊性,专门进行各项室内试验,获得大量数据,在此基础上对数据进行归纳分析,总结出该类土在压缩过程中表现出的特殊性质,并加以详细的解释说明。     

The effective interfacial shear strength of carbon nanotube fibers in an epoxy matrix characterized by a microdroplet test

The tensile properties of continuous carbon nanotube (CNT) fibers spun from a CNT carpet consisting of mainly double- and triple-walled tubes, and their interfacial properties in an epoxy matrix, are investigated by single fiber tensile tests and microdroplet tests, respectively. The average CNT fiber strength, modulus and strain to failure are 1.2 ± 0.3 GPa, 43.3 ± 7.4 GPa and 2.7 ± 0.5%, respectively. A detailed study of strength distribution of CNT fiber has been carried out. Statistical analysis shows that the CNT fiber strength is less scattered than those of MWCNTs as well as commercial carbon and glass fibers without surface treatment. The effective CNT fiber/epoxy interfacial shear strength is 14.4 MPa. Unlike traditional fiber-reinforced composites, the interfacial shear sliding occurs along the interface between regions with and without resin infiltration in the CNT fiber. Guidelines for microdroplet experiments are established through probability analysis of variables basic to specimen design.     

Crystallization of nylon 6–clay hybrid by annealing under elevated pressure

Nylon 6–clay hybrid (NCH) is a molecular composite of nylon 6 and uniformly dispersed silicate layers of montmorillonite. We found that the phase with the high melting temperature (HMT phase) in the NCH annealed under elevated pressure. The melting temperature of the HMT phase was 240°C. Nylon 6 annealed under elevated pressure did not have the HMT phase. Thus, the presence of the HMT phase was characteristic of the NCH. The relative heat of fusion of the HMT phase (heat of fusion of HMT phase/heat of fusion in the pressure annealed NCH) increased with increase in pressure. High-pressure differential thermal analysis (DTA) measurement revealed that the temperature, at which the relative heat of fusion showed a maximum value, was below about 20°C of the melting temperature of the original NCH under elevated presssure. It was considered that the nylon 6 crystallite near the melting temperature and the molecular mobility under elevated pressure were necessary to the appearance to the HMT phase. © 1994 John Wiley & Sons, Inc.     

Evaluation of block shear properties of selected extreme‐pH structural adhesives by short‐term exposure test

Nine structural adhesives with varying pH were selected to examine the effect of adhesive pH on wood–adhesive bond quality. The adhesives evaluated included four highly alkaline phenol–formaldehyde, one intermediate pH phenol–resorcinol–formaldehyde, two acidic melamine–urea–formaldehyde, and two acidic melamine–formaldehyde resins. Block shear specimens were prepared using Douglas‐fir and black spruce wood. The adhesive performance was evaluated by measuring the shear properties (strength and wood failure) of the specimens tested at the dry and vacuum–pressure–redry (VPD) conditions. Adhesive pH, test condition, and wood species showed significant effects on shear properties. The different adhesives performed differently at the dry and VPD conditions. The high‐pH adhesives (phenol–formaldehyde and phenol–resorcinol–formaldehyde) showed similar high wood failures at both test conditions and performed better than the low‐pH adhesives (melamine–formaldehyde and melamine–urea–formaldehyde), especially after the VPD conditioning. The low‐pH adhesives showed high wood failure at the dry condition, but wood failure decreased significantly after VPD conditioning for both species, indicating that the low‐pH adhesives were less durable than the high‐pH adhesives. High‐pH adhesives did not have a negative impact on the strength of the bonded specimens. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Structural development of PAN-based carbon fibers studied by in situ X-ray scattering at high temperatures under load

PAN-based carbon fiber bundles were simultaneously loaded and heated in an X-ray transparent testing device, which made it possible to follow the structural development in situ. The stress-strain curves are recorded together with structural parameters such as the inter-layer spacing, the crystallite size L_c and the orientation of the graphene planes, obtained from the scattering intensity of the d₀₀₂-reflection. Simultaneously, the size and orientation of pores were determined from small-angle X-ray scattering. Experiments were performed at temperatures up to 1700 °C and stresses up to 260 MPa. It was observed that the main part of the structural change occurs in the short time period of heating the fibers up to high temperatures. An increased orientation and growth of crystallites take place as a consequence of combined mechanical forces and high temperatures. This is followed by an additional but only smaller amount of structural change with time, which is due to dislocation movement, dislocation annealing and gliding of graphene planes.     

The relative influences of molecular structure on brittle fracture by fatigue and under constant load in polyethylenes

The resistance to failure under an oscillating load, fatigue, and to failure under constant load (CL), are presented for a large number of linear polyethylenes. The effects of fatigue and a CL on brittle fracture are compared with respect to the influence of the molecular structure such as branching and the molecular weight. Molecular weight has a relatively greater influence on fatigue, whereas branching is more important under CL.     

Synthesis of hybrid polyaniline/carbon nanotube nanocomposites by dynamic interfacial inverse emulsion polymerization under sonication

This article describes an ultrasonically assisted in situ interfacial dynamic inverse emulsion polymerization process of aniline in the presence of multiwalled carbon nanotubes (MWNT) in chloroform. During polymerization, MWNT are coated with polyaniline (PANI) forming a core‐shell structure of nanowires, as evidenced by cryogenic transmission electron microscopy. Thermogravimetric analysis curves and conversion measurements provided important knowledge regarding the unique polymerization method. Scanning electron microscopy images and surface resistivity imply that PANI/MWNTs are characterized by a structural synergistic effect. The PANI coating of MWNT leads to a remarkable improvement in separation and dispersion of MWNT in chloroform, which otherwise would rapidly coagulate and settle. The presented interfacial dynamic polymerization process is very fast, reaching 82% conversion within 5 min of sonication and produces stable clear dispersions of doped PANI in chloroform. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011