Littérature scientifique sur le sujet « Anti-Buckling Fixture »

The currently available anti-buckling fixtures for uniaxial tension-compression testing of sheet metal could not fulfill the requirements and standards. In this study, a unique system of the anti-buckling fixtures to enable getting accurate and repeatable isotropic and kinematic behavior of sheet metal was designed. The front surface of the specimen was fully supported during the initial position, tension, and compression and free from any buckling. Because using acrylic blocks, has a greater level of optical clarity, there was the possibility to measure the deformation on the front surface of the specimen using a digital image correlation system. The special pusher vice with compressed spring was attached to the fixture to avoid misalignment and calculating supplementary forces related to uniaxial direction. Furthermore, the new fixture was simple, flexible, and suitable for any universal testing machine.

Despite superior specific mechanical characteristics of carbon-fiber-reinforced polymers (CFRPs), a lack of understanding of their fracture mechanisms under different impact conditions has limited the application of CFRP energy-absorbing structures. To avoid complex and expensive tests on the final structure, it is more convenient to test flat elements. To prevent catastrophic crushing due to the global buckling, flat specimens must be supported by a specific fixture. Previously developed fixtures had shortcomings like tearing of the specimen, jamming of the fixture, short crushable length, or they were specifically designed only for one failure mode. This newly designed fixture overcomes the limitations of previously published solutions. The final configuration includes cylindrical anti-buckling columns 10 mm in diameter and spaced 65 mm apart with adjustable heights. The fixture is designed for rectangular specimens with dimensions of 150 × 100 mm and different thicknesses up to 16 mm, like the ones mandated by the ASTM D7137 standard test method for compression after impact analysis. Other features of this new fixture are the possibility to study the effects of different defects on the crashworthiness of composites, higher crushing area, and integration with Instron drop tower and hydraulic testing machines.

In this paper, the in-plane compression behaviour of open-hole carbon fibre composite specimens adhesively bonded with the external carbon fibre composite patches on the single- and double side are studied. Uniaxial compression tests are conducted on MTS machine using ASTM anti-buckling fixture. A 3D progressive damage model is developed to predict the damage initiation and failure in both unrepaired open cutout and repaired carbon fibre composite specimens under compressive load. Stress-based 3D-Hashin's failure criteria are used for predicting the fibre and matrix damage in carbon fibre composite. The cohesive zone model element is used for modelling the interlaminar delamination in carbon fibre composite specimen and also the adhesive layer between patch and specimen. Initial stiffness, damage initiation load and ultimate load of the specimen are obtained using progressive damage model based on finite element analysis, and they are compared against the experimental values. The load–deflection curve and the damage progression obtained from finite element analysis using progressive damage model is found to be in good coherence with the experimental predictions. In case of patch bonded carbon fibre composite specimens, failure mechanism starts with partial patch debonding followed by complete specimen failure.

The diffusion of composite materials in aeronautical and aerospace applications is attributable to the high specific mechanical properties they offer. In particular, the recent use of Carbon Fiber Reinforced Polymer (CFRP) materials is highly increased. The main disadvantage in using this kind of material is related to the possibility of including damages or defects not visible on the surface that compromise their behavior and make their use extremely unsafe if not properly supervised. The most conventional nondestructive techniques allow the detection of damages when they already compromise the life of these materials. The use of the same techniques makes it harder to monitor in-situ of the progress of damages, especially if they occur inside the materials. The implementation of the innovative strain analysis method, like those based on full-field measurements, could provide additional information about the damage mechanisms by supplying the complete strain distribution of the surface of the sample. The present paper examines the mechanical behavior of two different CFRP specimens, with and without damage, subjected to compressive load in an anti-buckling fixture by using the Digital Image Correlation (DIC). The purpose is to measure the out-of-plane displacements, characteristics of the compression tests, in all the points of the ROI (Region of Interest), using a full-field and noncontact technique. The innovative aspect of this work is therefore to solve this problem through an experimental approach with DIC 3D technique.

A novel experimental method was proposed for characterizing the energy absorbing capability of composite materials during the progressive crushing process under impact loading. A split Hopkinson pressure bars system was employed to carry out the progressive crushing tests under impact loading. The stress wave control technique was used to avoid the inhomogeneity of dynamic stress field in the specimen. The progressive crushing behavior was successfully achieved by using a coupon specimen and anti-buckling fixtures. With increasing strain rate, the absorbed energy during the crushing process slightly decreased, whereas the volume of the damaged part clearly increased regardless of material type. Consequently, the energy absorbing capability decreased with increasing loading rate. The effects of material composition, such as fiber type, matrix type and fabric pattern, on energy absorbing capability were also investigated by using the proposed method.

Tension-compression (T-C) fatigue response is one of the important design criteria for carbon-fibre-reinforced polymer (CFRP) material, as well as stress concentration. Hence, the objective of the current study is to investigate and quantify the stress concentration in CFRP dog-bone specimens due to T-C quasi-static and fatigue loadings (with anti-buckling fixtures). Dog-bone specimens with a [(0/90),(45/−45)4]s layup were fabricated using woven CFRP prepregs and their low-cycle fatigue behaviour was studied at two stress ratios (−0.1 & −0.5) and two frequencies (3 Hz & 5 Hz). During testing, strain gauges were mounted at the centre and edge regions of the dog-bone specimens to obtain accurate, real-time strain measurements. The corresponding stresses were calculated using Young’s moduli. The stress concentration at the specimen edges, due to quasi-static tension, was significant compared to quasi-static compression loads. Furthermore, the stress concentration increased with the quasi-static loading within the elastic limit. Similarly, the stress concentration at the specimen edges, due to tensile fatigue loads, was more significant and consistent than due to compressive fatigue loads. Finally, the effects of the stress ratio and loading frequency on the stress concentration were noted to be negligible.

Abstract Understanding a magnesium alloy sheet's response to load reversals is important to accurately simulate and optimize a component's manufacturing process. Through this research, the room temperature compression-tension and tension-compression experiments with strains up to ∼12% are performed on AZ31B-H24 sheet specimens along the normal direction of a 6.35 mm-thick sheet. Miniature specimens machined through thickness are tested using a novel setup designed for large strain reverse loading data generation where specimen size is limited. The reliability of the devised setup is verified by finite element simulation and by reproducing in-plane curves obtained via an anti-buckling fixture. A shot peening process involving prevailing through-thickness deformation is modeled and numerical results indicate that employing only in-plane properties of magnesium sheets for simulating such processes can lead to inaccurate predictions.

Abstract Improved material models for engineered polymer and composite materials including both monotonic and fatigue characteristics are necessary for creating more accurate digital simulations for heavy duty trucks. Unlike steel and other alloys that are commonly included in truck designs, these advanced polymer materials do not have pre-existing fatigue characteristic data. Additionally, there are no individual standard test procedures that can be commonly cited and followed during a research program. These materials are found in hoods, dashboards, body panels and splash shields of trucks, and are subject to cyclic loading conditions at various amplitudes and durations throughout the entire use or “duty cycle” of the vehicle. The applied loads vary between truck models, as some trucks will be used for vocational purposes and others will remain on the highway. This paper describes the testing of isotropic non-reinforced, and anisotropic glass-fiber-reinforced polymers and the subsequent calculation of the monotonic and fatigue properties that are needed to describe their behavior under various loading conditions. Material characteristics are measured using a series of constant amplitude strain-controlled fatigue tests that follow standard practices from ASTM D638 (Standard Test Method for Tensile Properties of Plastics), ASTM E606 (Standard Practice for Strain-Controlled Fatigue Testing) methods, and SAE J1099 (Technical Report on Low Cycle Fatigue Properties of Ferrous and Non-Ferrous Materials). The ASTM D638 Type 1 coupon geometry is used for all materials, with a varied sample thickness and length. An axial extensometer is incorporated to measure strain data through the duration of all tests, and an anti-buckling fixture is installed during cyclic tests to eliminate any bending in the specimen during the compressive portion of the fully-reversed waveform. A transverse extensometer is also installed on the gauge length of the material coupons to measure instantaneous cross-sectional area as well as Poisson’s ratio during monotonic testing. The data collected through the monotonic testing procedure is used to calculate Young’s Modulus, Poisson’s ratio, ultimate tensile strength, elongation (% strain), yield strength and strain, and true fracture strength and strain. The fatigue testing procedure yields data that can be used to calculate the fatigue strength coefficient (σf′), fatigue strength exponent (b), fatigue ductility coefficient (εf′), and fatigue ductility exponent (c). These parameters provide accurate stress-strain, cyclic stress-strain, and strain-life curves for the materials in question. A method will also be suggested for calculating the stress-life fatigue parameters, stress range intercept and slope, from the strain-controlled data. Furthermore, mold-flow analysis is applied to predict general orientation of the reinforcement fibers induced by the direction of material flow as a part is injection-molded. The calculated monotonic and fatigue parameters in conjunction with mold-flow analysis can immediately be applied within digital s imulations, allowing improved accuracy in life-expectancy estimations for truck parts.