Test Specimen Design Research Articles

Seeking the most informative design of test specimens for learning constitutive models

Accurate calibration of constitutive models is vital for predicting the mechanical behavior of engineering materials under various loading conditions. Traditionally, the calibration process involves a series of experiments on specimens with simple geometries to capture the complexities in the constitutive models. Each single test conveys a small amount of information that a well-trained human brain can handle, resulting in a large number of experiments needed for a complete calibration. Therefore, traditional calibration approaches are usually costly and time-consuming. With recent advancements in computational techniques, there is an emerging opportunity to leverage geometrically complex specimens in experiments to obtain a larger amount of information for computers to learn and calibrate the model. Despite some initial success, the most important question remains unsettled: How much information does a mechanical test convey? In this work, we answer this question by incorporating information entropy as a quantitative measure in the design of mechanical test specimens. We demonstrate the viability of the proposed approach by comparing the performance of selected test specimens for learning the plasticity model of sheet metal, e.g., the Hill48 anisotropic elastic-plastic model in this case. An optimal entropy criterion is proposed for selecting the appropriate heterogeneous test specimen for inverse calibration, depending on the cardinality of the stress state space considered in the model. Finally, Bayesian optimization is applied to uniaxial and biaxial tension specimens, using the stress state entropy as an objective function, to investigate the general principles of designing specimens with maximum information for learning constitutive models.

Design of dynamic test specimens and fixtures for large structural components of civil aircraft

In the course of designing and certifying civil aircraft, a significant number of dynamic tests are necessary. If the test specimen is too large in volume or weight, it will place higher demands on the loading capacity of the test equipment. The main focus in the design process of dynamic test specimens is to ensure consistency in the dynamic characteristics compared to the original structure, specifically with regard to natural frequency and mode shapes. Factors such as stiffness, damping, and boundary conditions are taken into consideration. In order to facilitate test implementation and reduce manufacturing costs, a combined approach involving finite element analysis and experimental verification is employed to provide guidance for the design of dynamic test specimens and fixtures for civil aircraft.

Design of an Environmental Barrier Coating Bond Strength Test Coupon

Abstract The bond strength of an environmental barrier coating (EBC) to a substrate such as a silicon carbide fiber/silicon carbide matrix (SiC/SiC) ceramic matrix composite (CMC) is a function of coating application parameters, matrix surface roughness, and environmental exposure, which results in the formation of a thermally grown oxide layer (TGO) layer. Current estimates of the EBC bond strength to a substrate are made assuming that the applied force divided by pull tab area is a representative metric. Although this is an expedient method for rapid strength estimates, the process of bonding the EBC to a pull-tab creates thermo-elastic residual stresses that are superimposed with the applied tensile load. Minimization of these stresses via test specimen design should give the most realistic estimate of EBC bond strength. We examine the residual stresses imposed on the EBC by cooling of an adhesive layer bonded to various metal and ceramic tabs. The results are extended to examine the stress distribution in a TGO layer formed on a Plasma Spray-Physical Vapor Deposited (PS-PVD) Yb2Si2O7 (ytterbium disilicate) EBC. An EBC/substrate coupon that overhangs a titanium or Kovar® pull tab is recommended in order to minimize thermal stresses, avoid edge effects, and provide an acreage strength measurement. The test method is sensitive to changes in substrate surface finish and oxidation time.

Experimental study on bond performance between UHPC and steel bars

This study presents the pullout experiments investigating the bond performance between steel bars and UHPC, which is becoming more significant due to the increasing utilization of UHPC as connection in precast concrete structures. Various parameters are considered in the experiments, including the bond length (l), steel diameter (d), and volume fraction of steel fibers in UHPC. Particularly, the current tests also consider the twin-steel bar configuration, which is commonly adopted in practice but scarcely incorporated in previous studies. The design of test specimens is introduced, followed by the test setup, measurement, and loading protocol. The observed damaged and measured responses are then presented, based on which the influence of each parameter is investigated. The results show that (1) the relative bond length of steel bars (n = l/d) is most substantial for the bond performance, with greater n leading to lower bond strength τu; (2) larger d causes reduced τu as well, due to lower ratios between ribs and diameter, as well as perimeter and area, of rebars; (3) the volume of steel fibers and steel configuration are not that significant factors on τu. Based on these results and reliability theory, the minimum bond length is finally suggested as 4d by referring to current design codes.

Geometrical imperfection sensitivity on in-plane compressive modulus and strength of honeycomb core and deviations from isotropy

Honeycomb core sandwich may suffer from face/core debonding. Candidate debond fracture test specimens such as End-Notch Flexure (ENF) and Mixed Mode Bending (MMB) contain unbonded regions of the sandwich where the core is required to carry in-plane com-pressive load. In-plane modulus and compressive strength of honeycomb core are not pro-vided by core producers and must be estimated from unit cell models assuming perfect hexagonal cells. In this study it is shown that geometrical imperfections, such as deviations in the angle between cell walls and curved cell walls have a strong influence on modulus and strength because of deviations from transverse isotropy. A range of Nomex honeycomb cores were tested under in-plane compression. Modulus and strength ratios ( L/W) were found to vary from 0.3 to 3.24 and 0.96 to 1.8, respectively. Analysis based on unit cell models verified that deviations of the inclined cell wall angle, θ, from 30 ◦ is a major cause for orthotropy. Rounded corners and curved walls were analyzed using finite element analysis. Both principal elastic moduli of the core ( E L and E W) were reduced by the curvature while the Poisson’s ratios remained largely unaffected. Implications of the results for design of MMB and ENF fracture test specimens are discussed.

Mitigation of Concrete Cover Separation in Concrete Beams Strengthened with Fiber-reinforced Polymer Composites

Strengthening of reinforced concrete (RC) beams using fiber reinforced polymers (FRP) is an effective method that is gaining more acceptance worldwide. However, failure of the strengthened beams at the plate-end region by concrete cover separation (CCS) compromises the effectiveness of such a strengthening system. Among available anchorage systems to remedy such problem, the use of FRP U-wraps at the end region of the externally bonded FRP has the potential to prevent CCS failure. Most studies in the literature suffer from poor design of test specimens and the use of random amounts of FRP anchorage. This resulted in nonconclusive findings and contradicting results on the real effectiveness of FRP U-wraps as an anchorage to prevent plate-end debonding failure. That in turn has been reflected in the lack of reliable design methods for the design of FRP U-wrap anchorage. In this paper, the effect of FRP U-wraps on the mitigation of CCS failure in FRP-strengthened RC beams was experimentally examined and quantified. The study included full-scale RC beams strengthened in flexure using carbon FRP (CFRP) sheets and anchored at the plate-end region using vertical CFRP U-wraps. The test variable was the amount of CFRP U-wrap that is required to achieve complete mitigation of CCS. The test results indicated the effectiveness of FRP U-wraps on mitigating CCS failure once the required area of CFRP U-wrap was placed. Relevant design models were presented and assessed against the experimental results of the current study. The comparisons indicated inaccurate predictions for the area of FRP U-wrap that is required to prevent CCS. This sheds the light on the need for developing a more reliable and accurate method for the design of an end anchorage to prevent CCS failure.

Mechanoluminescent study for optimization of joint design on cross tension test

ABSTRACT To develop cross tension (CT) tests for adhesive dissimilar material joints, we investigated the design of CT test specimens (particularly the shape of the bonding area) through mechanoluminescence. To clarify the factors that cause high scattering of cross tension strength (CTS) values in the case of conventional CT specimens with square adhesive regions, we visualized the mechanical behavior during the CT test (particularly the fracture progress behavior inside the adhesive layer) by using mechanoluminescence. The mechanoluminescence revealed inconsistent patterns in each CT specimen with a square bonding area. This indicates that the different stress concentration behaviors and crack growth/peeling behavior during the CT test are factors that cause high scattering of CTS values. Meanwhile, in the case of CT specimens with circular adhesive regions, the scattering of CTS values was reduced significantly, and the mechanoluminescent patterns were consistent as circles of almost identical size in all the specimens. It can be considered that mechanoluminescence has the potential to effectively visualize mechanical behavior for optimizing test designs.

ARTIFICIAL SPECIMENS MANUFACTURING OF GLASS FIBRE REINFORCED COMPOSITE FOR SUB-SURFACE DEFECTS INSPECTION BY ACTIVE LONG-PULSE THERMOGRAPHY

The detection and analysis of sub-surface defects in composite materials specimens have been problems of interest in recent years in different industries. For instance, in the wind industry, such materials are used for manufacturing wind turbine blades. A variety of defects can occur inside composite materials, e.g., porosities, fibre rupture, and inclusions. This work presents the design and manufacturing of artificial test specimens of composite material, to which specific defects were induced in a non-destructive way. The used manufacturing process was VARTM. In addition, a method to localise and characterise induced defects is proposed. Such method works by applying active long-pulse thermography to the test specimens. Further, we show that both the proposed low-cost optical system and the long-pulse active thermography technique allow the non-destructive thermal analysis of parts to detect sub-surface defects in composite materials based on fiberglass reinforced epoxy resin

Full-scale shake table tests of P&B type of Japanese three-story wood dwellings for the collapse characterization

The 2009 test program of E-Defense aims to investigate the new design concept compliant with high-performance expectations and long-span service life for wood structures under the act of quality assurance. This study describes the full-scale shake table test program of three-story Post and Beam (P&B) Grade-1 and Grade-2 index wood structures in the Japanese building taxonomy. Input ground motions of the test program are generated to represent a frequent, rare, and very rare level of earthquake ground motion using a random process with an envelope function. Design and analysis of test specimens were carried out following the Japanese seismic design code, where the component tests of structural elements were performed before the shake table test program. A sequence of spectrally matched artificial ground motions was used in the test program to assess the progressive collapse mechanism of both test specimens. Wall to plan ratio is a critical indicator in the structural design of wood structures to determine the minimum required amount of wall and proper plan layout on each floor. Simplified N value method and allowable stress design approach are conducted for configuring and proportioning the structural elements during the design process. Special attention is given to connection details and foundation anchorages to ensure a continuous load transfer path. The first mode-dominated response characteristics of the structure are derived from the test data. Progressive damage mechanism has yielded to cyclic strength and stiffness deterioration. The global drift angle of the test structure has exceeded the 5% drift ratio. The capability of the versatile hysteretic model for wood structures is well-justified through the comparison of the numerical model and the physical test data for collapse characterization under gradually increased earthquake demands. Accumulation of damage at the last phase of the test program is led to the loss of dynamic stability in the Grade-1 building and total collapse of Grade-2 wood-dwelling, respectively. Test results have provided valuable data to overview and improve the response characteristics of code-conforming wood structures with an extended service life towards earthquake resilience of cities.

Mechanical design of ring tensile specimen via surrogate modelling for inverse material parameter identification

The mechanical characterization of anisotropic thin walled-tubes along hoop direction is not a trivial task. It is necessary to develop experimental techniques, numerical methods and design test samples, which enable to determine the real tube properties along hoop direction without any external influences. In this study, first we propose a surrogate based-model for the mechanical design of the ring hoop tensile test (RHTT) specimen, in order to obtain the effective homogeneous stress and strain distribution of the uniaxial tensile test along hoop direction. Second, the optimized sample is used to carry out RHTT and to obtain the actual flow stress curve and the anisotropy coefficients of AA6063-O extruded tube. However, the experimental curve measured from RHTT (force –displacement) is a degenerate response, since it suffers from intermixture effects of the effective material behaviour with the friction between the sample and the sample-holding tool. Hence, we developed an inverse parameter identification method, which uses design of experiments, finite element analysis and artificial neural network to separate out the tubular material parameters from the friction coefficient. The assessment of the developed method is achieved by comparing the predicted material parameters and the identified flow stress curve obtained by artificial neural network algorithm. The finite element simulation results corroborate the obtained findings.

Direct Tension Test for Characterization of Tensile Behavior of Ultra-High Performance Concrete

Abstract Ultra-high performance concrete (UHPC) is characterized by its superior strength, ductility, durability, and particularly its unique post-cracking performance in tension. Dog-bone–shaped specimens are widely used for determination of the tensile behavior of UHPC, but there is no standard test method or specimen design for the characterization of tensile behavior. In this study, an evolving strategy on designing a direct tension test (DTT) specimen is first conducted using numerical finite element analysis. Seven series of DTT specimens made of UHPC and with well-designed dimensions to avoid local stress concentration are then tested experimentally. Results indicate that the post-cracking localization within the gauge measurement region is guaranteed, and the DTT specimen is capable of fully capturing tensile stress–strain responses of UHPC. An idealized constitutive model with three linear phases is proposed to fit the experimental data and thus characterize the linear elastic, strain-hardening, and strain-softening behavior of UHPC in tension. Four tensile material parameters extracted from the experimental stress–strain curves are implemented in the idealized constitutive model from which the multi-phase responses of UHPC in tension are reconstructed. It is found that most tensile material parameters extracted from experimental stress–strain curves, including tensile strength, modulus of elasticity, and dissipated energy, increase with the increased volume fraction of steel fibers, curing age, and displacement loading rate, while the strain capacity at the first cracking remains nearly constant. The DTT specimen developed can be used effectively to characterize the tensile behavior of ductile fiber-reinforced cementitious materials.

Crack Propagation Behaviour in Thin Aluminium Alloy Sheet Repaired with Adhesively Bonded CFRP Patch

For reducing the crack tip stress intensity factor, adhesively bonded composite patches are employed for crack repair in thin structure. Experimental work was carried out to study environmental assisted cracking behaviour of crack repaired with single-sided bonded patch in aluminium alloy of 7xxx series in peak-aged condition. The observations were also made on crack in this alloy without any patch repair. The specimen developed for conducting the experiments simulated plane stress condition of thin structure. The patched and un-patched tensile test specimens were loaded simultaneously at constant load in the presence of 3.5% weight NaCl solution. The experimental set-up simulated the saline or corrosive environment. Aggressive effect of corrosion attack on crack due to anodic dissolution along with hydrogen embrittlement leading to faster crack growth in patched specimen was revealed compared to un-patched specimen. The present study is the improvement over similar researches carried out earlier in terms of design of test specimen under plane stress condition so as to obtain all the three regions of crack growth.

Characterizing damage and fracture of sheet metal materials using large scale test specimen

Several approaches to characterizing damage evolution and fracture behaviour of sheet metal materials are currently in existence. These are based on different testing methodologies and test specimen designs, some of which, when combined with higher sheet thicknesses used for aluminium alloys in the automotive field, can reach and exceed geometrical limits required for the plane stress assumption commonly applied to numerical modelling of sheet metal materials. Furthermore, if shell elements are to be used in modelling of such structures, the characteristic length of the element should be larger than the thickness of the modelled sheet material. In order to allow for element sizes accounting for the two considerations outlined above, new test specimen geometries covering a range of stress triaxialities have been developed with a deformation region significantly larger than that of other commonly used test specimen designs. The geometries have been manufactured and tested on an automotive grade AA6xxx series aluminium alloy with DIC strain measurement to determine failure strains. The results were then applied to finite element models using various element sizes and the behaviour of the numerical models compared to the test results.

Application of model-based compensation methods to real-time hybrid simulation benchmark

Abstract Real-time hybrid simulation (RTHS) is an experimental testing technique widely used for performance evaluation of structural systems such as large buildings and bridges subjected to earthquake loading. While RTHS testing has demonstrated over the last 20 years to be an efficient and cost-effective alternative to shaking table tests, especially for large structural systems with rate-dependent behavior, accurate and stable results from this methodology are highly dependent on the test specimen, loading equipment, and controller design for dynamic compensation. This paper presents a study on the accuracy and stability of model-based compensation (MBC) approaches for the implementation of a real-time hybrid simulation benchmark problem. The controller architecture is based on feedforward compensator, designed for reference tracking, while a feedback regulator provides improved robustness for undesired disturbance and sensor noise. The results provide evidence of the improved performance of MBC controllers compared to benchmark results. Moreover, the MBC controllers surpass the benchmark controller in terms of robustness, when multiple partitioning cases and control plant uncertainty are considered in the numerical simulations.

Optimization of the Specimen Geometry of Unidirectional Reinforced Composites with a Fibre Orientation of 90° for Tensile, Quasi-Static and Fatigue Tests

When testing unidirectional reinforced composites with a fiber orientation of 90° in tensile tests with rectangular specimens, the influence of the clamping often causes a failure in their vicinity and therefore the test cannot be regarded as valid. In this paper, a test specimen design was determined which is well suited for testing the material properties transverse to the fiber direction by calculating the influence of geometric details of shoulder bar specimens with the help of finite element simulation. The particularly critical clamping and shoulder areas were examined more closely to ensure failure mainly in the test field. In the clamping area the design of the glued-on tabs was investigated and in the shoulder area an optimization of the shoulder geometry was done. Based on the two optimized design proposals, test specimens were produced and evaluated by monotonous tensile tests. Subsequently, Wöhler tests were carried out at different R-ratios and load levels and compared with results of rectangular specimens.

Investigation on very high cycle fatigue of PA66-GF30 GFRP based on fiber orientation

The purpose of this study is to evaluate the effects of glass fiber (GF) of PA66-GF30 orientation on very high cycle fatigue (HVCF) life and also to develop an optimized test apparatus such as measurement of dynamic elastic modulus and design of test specimens. Dynamic elastic modulus, tensile strength and fatigue strength tests were conducted at room temperature based on GF orientation (0°, 45°, 90°). Conventional fatigue (3 Hz) and ultrasonic fatigue (20 kHz) tests were performed at the same stress ratio (R = −1). Fracture surface observation showed that GF 0° exhibited a higher fatigue strength in comparison with GF 45 and 90°. Various defects such as voids and microcracks to the load direction around the GF end or debonding in glass fiber reinforced plastic (GFRP) and the slope change in very long life regime of S–N diagram.

Fiber-reinforced polymer composites test specimen design for selected damage mechanisms

Fiber-reinforced polymer composites yield a range of microscopic damage mechanisms even under apparently “simple” load cases, e.g. quasi-static uniaxial tensile tests to failure. Even reducing the fiber-reinforced polymer composite to a single fiber embedded in a matrix will produce several different damage types under quasi-static tensile loading. These are fiber breaks and simultaneous fiber–matrix debonding as well as friction between fiber and debonded matrix, and sometimes, additional matrix cracks. Acoustic emission monitoring of mechanical load tests on material specimens or components is often used to identify the occurrence of damage and attempts at locating the source and identifying the damage mechanism or type in a wide range of materials. The complex damage behavior of fiber-reinforced polymer composites poses a challenge in that respect. Therefore, various approaches (e.g. pattern recognition or neural networks) have been developed to identify the different microscopic damage mechanisms from acoustic emission signals obtained from various load tests. For improved understanding of the damage mechanisms in fiber-reinforced polymer composites as well as for validating the procedures for identifying the types of mechanisms, fiber-reinforced polymer specimens showing a “single dominant damage mechanism”, i.e. occurring in significantly larger number than all others, at least in certain stages of damage accumulation or in localized volume elements, can be designed. The contribution discusses and classifies selected examples of single dominant damage mechanism for fiber-reinforced polymer composites and their application.

Short fiber-reinforced composite restorations: A review of the current literature.

A newly-recommended method for restoring large cavities is the biomimetic approach of using short fiber-reinforced composite (SFRC) as dentine-replacing material. The aim of the current review was to present an overview of SFRC and to give the clinician a detailed understanding of this new material and treatment strategy based on available-literature review. A thorough literature search was done up to December 2017. The range of relevant publications was surveyed using PubMed and Google Scholar. From the search results, articles related to our search terms were only considered. The search terms used were "short fiber-reinforced composite", "everX posterior", and "fiber-reinforced composite restorations". Of the assessed articles selected (N=70), most were laboratory-based research with various test specimen designs prepared according to the ISO standard or with extracted teeth; only four articles were clinical reports. A common finding was that by combining the SFRC as a bulk base with conventional composite, the load-bearing capacity and failure mode of the material combination were improved, as compared to plain conventional composite restoration. In the reviewed studies, the biomimetic restoration technique of using SFRC showed promising characteristics, and therefore, might be recommended as an alternative treatment option for large cavities.

Design of Fatigue Test Specimen under High Compressive Mean Stress

Design of Fatigue Test Specimen under High Compressive Mean Stress

Comparative evaluation between glass and polyethylene fiber reinforced composites: A review of the current literature.

BackgroundFiber reinforced composite (FRC) is a promising class of material that gives clinicians alternative treatment options. There are many FRC products available in the market based on either glass or polyethylene fiber type. The aim of this study was to present a comparison between glass and polyethylene fiber reinforced composites based on available literature review.Material and MethodsA thorough literature search, with no limitation, was done up to June 2017. The range of relevant publications was surveyed using PubMed and Google Scholar. From the search results, articles related to our search terms were only considered. An assessment of these articles was done by two individuals in order to include only articles directly compare between glass and polyethylene FRCs. The search terms used were “fiber reinforced dental composites” and “glass and polyethylene fibers in dentistry”.ResultsThe search provided 276 titles. Full-text analysis was performed for 29 articles that met the inclusion criteria. Most were laboratory-based research with various test specimen designs prepared according to ISO standard or with extracted teeth and only three articles were clinical studies. Most of studies (n=23) found superior characteristics of glass FRCs over polyethylene FRCs.ConclusionsSignificant reinforcement differences between commercial glass and polyethylene fiber reinforced composites were found. Key words:Fiber reinforced composite, glass fiber, polyethylene fiber.