Automated fiber placement (AFP) is widely used to manufacture composite lightweight structures in different industries, including aerospace. Prepreg tackiness plays an important role in the AFP ...process. This study aims to measure the prepreg tackiness at different AFP processing conditions and predict the optimum parameters for high tackiness levels. Two in-house setups were developed for layup and to measure tackiness. The first setup was designed to simulate the AFP process and precisely control layup speed, compaction force, and temperature. It is used to layup the prepreg under different conditions and with different rollers. The second setup performs a peel-off test to measure the sample’s tackiness. The Taguchi method is applied to optimize the layup process parameters and find the optimal combination for high resultant tackiness. It is also applied to study the effect of the placing roller. Analysis of results shows that prepreg tack is affected by the interaction among the process parameters rather than the individual effect of these parameters. The study shows that the Taguchi method is suitable to solve the stated problem with a minimum number of trials as its results are experimentally validated.
In Al/steel resistance spot welding, increasing welding current helped to reduce generation of dendritic grains and shrinkage, and increasing welding time tended to reduce shrinkage but promoted the ...transition of grains from preferable columnar to dendritic. Iron phase, existing between the Fe2Al5 fingers of a thin IMC layer facing the steel side, was observed to serve as crack propagation inhibitor and improve load carrying capacity of Al/steel interface. The amount of iron phase decreased as the IMC thickness grew, so did the load bearing capacity. Three fracture modes were observed in the cross tension test: interfacial fracture in the IMC layer, partial thickness fracture within the Al nugget next to the IMC layer, and partial button pullout fracture. All fractured coach peel test samples displayed consistent partial button pullout fracture mode following large deformation of the Al sheet. The crack in the cross tension test always initiated in the IMC layer while the crack in the coach peel test often started in the Al fusion zone. Two characteristic peak points were observed in the load-displacement curves of the coach peel test. The key point for improving mechanical performance under coach peel loading was to increase the first peak point by reducing Al thinning, increasing the Al nugget size, and limiting the IMC layer growth.
A novel cassette-like peeling system is developed to address the limitations of current peeling standards when evaluating bonding quality of soft-to-rigid assemblies. The system transforms the ...translation of a specimen in the conventional peeling configuration to rotation via a cassette-like spool clamping the specimen. The peeled film is loaded by tension to drive the winding of the spool, thus achieving self-similar crack propagation and a stationary peeling front unrelated to the stiffness of the film. These features enable the system’s compatibility with most universal testers and in situ observation of crack tip morphology with optical instruments. Analysis to derive the intrinsic fracture energy when peeling a soft film is conducted based on Griffith energy balance, making use of which, a parametric study is performed to clarify the related mechanisms. We carry out a comprehensive validation of the cassette-like peeling system by performing a series of peeling tests using our in-house prototype and by comparing the results with those from the conventional system. Owing to its universality and ease-of-use, the proposed cassette-like peeling system can potentially be applied to the development of the next generation of peel test standards.
The broad versatility of the T-peel test, widely used for characterizing adhesion across a plethora of adhesives, adherends, and geometries, results in a range of responses that may complicate ...meaningful interpretation. This research effort, involving several specific specimen types, was undertaken to investigate concerns that commonly used configurations may not always result in plateaus in the force-displacement response. An experimental and numerical study is conducted on debonding of T-peel specimens having 75 mm bond length and 0.81 mm thick adherends made of either 6061 aluminum (Al) or one of the three steels (G70 70U hot dip galvanized, E60 elctrogalvanized (EGZ), 1010 cold-rolled steel) bonded with either LORD® 406 or Maxlok™ acrylic adhesive. For the EGZ and the Al adherends, specimens with a bond length of 250 mm and adherend thickness of 1.60 mm are also examined. Effects of adherend materials and thicknesses, bond lengths, and adhesives on test results are examined using three metrics to interpret the T-peel bond performance. A limited correlation is found between the commonly used “T-peel strength” and the energy dissipated per unit debond area. For these two metrics, the relative performances of the CRS and the Al specimens are quite different. Quasi-static plane strain deformations of the test specimens are analyzed by the finite element method (FEM) and a cohesive zone model using the commercial software, ABAQUS, to help interpret the test data. Numerical results provided energies required to elastically and plastically deform the adherends, and help determine the transition from non-steady-state to steady-state debonding. The FE simulations also facilitate determination of the fraction of the crosshead displacement at which steady-state debonding occurs. Results reported herein could help engineers select appropriate peel specimen dimensions for extracting meaningful data for the adhesive performance and comparisons.
•A failure parameter prediction model was newly provided for RSW CP samples.•The heterogeneous microstructures and hardness distributions of B1500HS RSWs were characterized using EBSD and ...microhardness tests.•RSW minimum hardness, Hmin, significantly affected RSW mechanical properties. The relationship between the Hmin and LSMP σb values was newly revealed.•For the steels whose σb < 800 MPa, no obvious HAZ softened regions can be observed.
Based on mechanical tests, microstructure characterizations and finite element simulations, the mechanical properties and failure mechanisms of resistance spot welds (RSWs) were investigated in the study. Different types (B1500HS, 340/590DP, 420/780DP and 820/1180DP) and thicknesses (1.2–1.8 mm) of steel plates were welded as coach-peel (CP) samples. To quantify the effect of plate thicknesses on the failure bending moment, MCP, of CP samples, a new parameter, standardized failure bending moment, MSCP = MCP /T1 /T2, was introduced. Based on the new revealed relationship between the MSCP and low strength metal plate tensile strengths (LSMP σb), a MCP prediction model was provided. Moreover, the lap-shear (LS) test data (standardized shear strength, σSS) published in a previous work were also analyzed. With the increment of σb values (<800 MPa), σSS and MSCP parameters increased linearly. When the σb values > 800 MPa, the slopes of σSS and MSCP fitting lines were decreased. The similar variation trends of σSS and MSCP parameters implied similar RSW failure mechanisms. The new revealed relationship between the RSW minimum hardness values (Hmin, measured from base metals or heat affected zones) and LSMP σb values can explain the trends. Finally, by simulations analyses, the effect of different loading conditions on the mechanical performance of RSWs was also studied. CP loads induced the most severe stress concentration near the RSWs, which can severely weaken RSW mechanical performance.
Safety evaluation of resistance spot welds necessitates the accurate measurement of local constitutive properties. This study employed miniature mechanical tests to investigate the deformation and ...failure behaviors of nugget, heat affected zone (HAZ), and corona bond of resistance spot welded JSC980YL steel. A novel mini-peel test was developed to enable local fracture in HAZ for numerical inverse calibration of constitutive parameters. The fracture constants of weld zones calibrated using Cockcroft-Latham ductile failure criterion were incorporated in finite element models to predict the failure modes of spot welds in tensile-shear and cross-tension coupon tests. The result indicates that the ultimate tensile strengths of the nugget and the corona bond were 37.6% higher and 5.8% lower, respectively, than that of the base material. The nugget and HAZ exhibited ductile fracture, whereas the corona bond was brittle fracture with only 1.2% elongation. In the coupon tests, the increase of nugget diameter slowed down the damage accumulation rate in the nugget and accelerated that in the HAZ, resulting in the transition of failure mode from interfacial to pullout. The failure load of corona bond in coupon tests increased with the increase of nugget diameter while its contribution to the peak load decreased.
Display omitted
•Mini-peel test is a novel way to enable local fracture in heat-affected zone for numerical calibration of fracture parameter.•The increase of nugget size accelerates damage accumulation in heat-affected zone, causing interfacial-to-pullout transition.•The nugget exhibits 74.9% higher yield strength and 37.6% higher ultimate tensile strength than the JSC980YL base material.•The corona bond has yield strength and ultimate tensile strength similar to those of base material, but only 1.2% elongation.
Lithium-sulfur batteries have high cathode theoretical energy density, but the poor conductivity of sulfur and polysulfide shuttling result in serious polarization and low sulfur utilization. ...Moreover, the addition of insulating binder in the electrode increases the internal resistance, reducing specific capacity and rate performance. Herein, we develop a composite binder with higher electronic conductivity, superior mechanical property and strong adsorption of polysulfides that imparts it some electrocatalytic activity. The reduced graphene oxide- polyacrylic acid (GOPAA) binder is prepared via a simple solution process. At constant loading fraction of 10wt%, using GOPAA binder induces a 30% enhancement in the cathode capacity, better cycle life and rate capability compared to using PAA binder, reducing both the local charge-transfer resistance and the global electronic resistance before and after cycling. These are attributed to the enhanced binding strength and synergistic effect of reduced graphene oxide and PAA forming well-dispersed conductive bridges to promote rapid electron transfer. Additionally, GOPAA provides active sites for adsorption of lithium polysulfides and electrocatalytic activity, shifting redox peaks in cyclic voltammetry and improving roundtrip efficiency.
Display omitted
•A conductive composite binder with high electric conductivity and strong adhesion was obtained by a simple solution process.•The conductive composite binder could trap lithium polysulfides by a chemical absorption.•Lithium-sulfur batteries using the conductive composite binder exhibit excellent electrochemical performance.
Failure of resistance spot welds in computer-aided engineering models is based upon criteria that incorporate test data obtained in various loading conditions including different proportions of ...tensile, shear, and moment loads. The decomposition of the critical load into its respective shear, tensile, and bending moment components is influenced by the rigid body motion during their corresponding mechanical tests. Continuous tracking of the weld orientation and the deformed coupons is required for accurate determination of the load components at the onset of failure. A comprehensive experimental investigation was performed to characterize the critical failure load components in combined loading using various orientations of KS-II tests and a range of coach peel coupon geometries. Mechanical testing was coupled with digital image correlation (DIC) to systematically evaluate empirical force-based failure models for resistance spot welds of two third generation advanced high strength steels with optimal and suboptimal fusion zone diameters. New analysis methodologies using DIC were developed to account for rotation and deformation of the joint in the determination of the shear, normal, and bending moments acting on the spot-welded joints. The coach peel test results for both steels revealed a non-convex experimental fracture locus in bending-tension loading cases. The conventional assumption of a convex failure locus overestimated the critical bending moment strength between 7 and 66%. Results indicated that changes in the operative failure mechanism from pullout/partial-pullout to interfacial can expand the fracture loci within the shear-tensile loading mixities. Improved alternative functional forms for the weld failure models were proposed and contrasted with conventional models that assume convexity.
Fluoropolymer films were treated by atmospheric pressure nitrogen plasma in a roll-to-roll configuration. This treatment aimed to modify the surface properties of the fluoropolymer and assess its ...adhesion with commercial silicone, rubber, and acrylic adhesive tapes. Fluoropolymer films and adhesives were characterized using contact angle measurements, Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), X-ray Photoelectron Spectroscopy (XPS), and T-peel tests. The plasma treatment resulted in increased wettability of the films, defluorination of the fluoropolymer surface, the introduction of oxygen- and nitrogen-containing functional groups on the surface, and a reduction in surface roughness. Peel strength increased at different levels, depending on the treatment speed and adhesive employed. Silicone adhesives did not present a significant increase on plasma-treated fluoropolymer films; however, they presented the higher peel strength against the untreated substrate. Acrylic adhesives are sensitive to fluoropolymer surface chemistry and can be used to evaluate different plasma treatments, while the highest adhesion was obtained with rubber adhesives. The results presented herein provide information about the appropriate selection of the type of adhesive for a specific application, as well as the evaluation of surface modifications by T-peel test.
Background
Peel tests are frequently used to perform measurements of adhesive strength for pressure sensitive adhesive (PSA) tapes. Current lab methodologies for 90° peel tests translate the model ...substrate orthogonally to the peel direction in order to maintain the peel angle, precluding testing from immovable substrates.
Objective
It was our objective to develop a peel fixture capable of testing temporary pavement marking (TPM) tapes and other PSA tapes from immovable substrates such as roadways surfaces.
Methods
We present a modular peel fixture for conducting peel experiments directly on immovable substrates. The fixture was validated through a series of peel tests on consumer tapes to reproduce the linear width dependence and viscoelastic rate dependence found in traditional peeling setups. To test the capabilities of the fixture, a series of peel tests were conducted with various tapes on controlled surfaces, and a commercial tape on various immovable substrates.
Results
We demonstrate the ability of our fixture to reproduce results reported for traditional peel tests from literature. In addition, we were able to conduct peel tests directly on immovable substrates such as the benchtop.
Conclusions
This fixture shows potential for both traditional peeling tests, and for use in
in-situ
peel experiments from substrates relevant to the end application of the PSA tape.
PDF
