Specimen Boundary Research Articles

High strain rate tensile testing of sheet materials using three Hopkinson pressure bars

In an attempt to circumvent the inherent problems associated with Split Hopkinson Tensile Bar (SHTB) systems, a new experimental technique is proposed for the high strain rate tensile testing of sheet specimens in Split Hopkinson Pressure Bar (SHPB) systems. Existing solutions employ a multiple gage section hat-shaped specimen to transform the incoming pulse from compression into tension at the specimen level. However, multiple gage section specimens are not suitable for studying the post-necking behavior of materials as the required symmetry of the mechanical system is no longer guaranteed in the post-necking range. Here, we present a new load-inversion device that is used in conjunction with three Hopkinson pressure bars for the tensile loading of single gage section sheet specimen. The device is designed to transform the compression load applied at its boundaries into a tensile loading of the specimen. Two output bars are used to measure the total force applied on the specimen boundaries, while Digital Image Correlation (DIC) is used to determine the strain histories on the specimen surface based on photographs acquired at a frequency of about 100 kHz. In addition to uniaxial tensile experiments on TRIP780 steel sheet specimens at strain rates ranging from 200/s to 1000/s, results are presented on the dynamic fracture testing of notched tensile specimens.

Application of the DSC model for nonlinear analysis of reinforced concrete frames

A nonlinear finite element method with eight-noded isoparametric quadrilateral elements for concrete and two-noded elements for reinforcement was used to predict the behavior of reinforced concrete frame structures. The disturbed state concept (DSC) and the hierarchical single surface (HISS) plasticity model with the associated modified flow rule were used to characterize the constitutive behavior of concrete in both compression and tension. The DSC model allows for the characterization of non-associative behavior through the use of disturbance, and it computes microcracking during deformation, which eventually leads to fracture and failure. In the DSC model, the critical disturbance, D c , identifies fracture and failure. Elastic perfectly plastic behavior is assumed in models of steel reinforcement. The DSC model was validated at two levels: (1) specimen and (2) practical boundary value problem. At the specimen level, predictions are obtained by integrating the incremental constitutive relations. The finite element procedure and the DSC model are used to obtain predictions for practical boundary value problems. Based on comparisons between DSC predictions and test data, the model is capable of providing highly satisfactory predictions.

Optimized butterfly specimen for the fracture testing of sheet materials under combined normal and shear loading

The present paper is concerned with multi-axial ductile fracture experiments on sheet metals. Different stress-states are achieved within a flat specimen by applying different combinations of normal and transverse loads to the specimen boundaries. The specimen geometry is optimized such that fracture initiates remote from the free specimen boundaries. Fracture experiments are carried out on TRIP780 steel for four different loading conditions, varying from pure shear to transverse plane strain tension. Hybrid experimental–numerical analyses are performed to determine the stress and strain fields within the specimen gage section. The results show that strain localization cannot be avoided prior to the onset of fracture. Through-thickness necking prevails under tension-dominated loading while the deformation localizes along a band crossing the entire gage section under shear-dominated loading. Both experimental and simulation results demonstrate that the proposed fracture testing method is very sensitive to imperfections in the specimen machining. The loading paths to fracture are determined in terms of stress triaxiality, Lode angle parameter and equivalent plastic strain. The experimental data indicates that the relationship between the stress triaxiality and the equivalent plastic strain at the onset of ductile fracture is not unique.

Reliability Evaluation of Glass Curtain Wall via Vibration Detection

Developing efficient onsite damage detection and safety evaluation methods for glass curtain wall is still a challenging task. In this work, a simple approach to predict the failure and risk of falling down of curtain wall glass was developed through the relationship between the natural frequency changes and the declined reliability of the structures. Natural frequencies of the curtain wall glass under various clamping conditions were determined using dynamic tests. It was discovered that, with the decrease of the clamping force at the boundary of the glass specimen, the measured natural frequencies decreased linearly. Thus, the in-service capacity in edge clamping (e.g. boundary is loosed or failure of the supporting structure) of the curtain wall glass can be identified by its natural frequency changes. The expected lowest safety natural frequency of a curtain wall glass (given elastic parameters, shape and size) is given. The implementation of this detection method is described, and the practicability and feasibility validated.

Influence of Edge Restraint on Clip Fastener Loads of Standing Seam Metal Roof Panels

Abstract This study investigates the effect of boundary conditions on the structural load paths within standing seam metal roof (SSMR) panel specimens subjected to uplift wind pressures. The objective is to determine the effect of boundary conditions (restrained or free) on observed load distributions among clip fasteners installed within a SSMR test specimen. Two roof specimens with instrumented clip fasteners were installed in a steel pressure chamber and subjected to static uplift pressures, while measuring panel deflection and clip fastener load distributions. The test specimen boundary conditions were modified in turn to represent field panel location (all free boundaries), gable panel (one restrained boundary, long dimension), eave panel (one restrained boundary, short dimension), and a corner panel (having two adjacent restrained boundaries). The load-displacement characteristics of the SSMR system were determined in a third specimen. The study found statistically significant differences in clip fastener loads, among the first three fastener rows away from the crosswise panel restraint, i.e., eave boundary condition. The gable boundary appeared to have less influence on load on the nearest fasteners. For the eave boundary condition, the measured load on the three exterior fasteners was about 40 % lower than the total fastener load on interior clips located just 3 m (10 ft) away. The results suggest why clip failures seldom occur in corner locations during uplift pressure tests. An empirical design method is proposed for predicting loads on clip fasteners installed near roof boundaries. More importantly, the design of SSMR roofing can be made more efficient, since it is shown that eave clip fasteners carry less than half the load of the interior fasteners, if other failure mechanisms (i.e., sheet tearing or boundary anchor failure) do not control the roof edge failures of installed SSMR.

Boundary effect on the softening curve of concrete

The apparent fracture energy of concrete experimentally determined on the basis of the work of fracture in bending or wedge splitting tests becomes larger with increasing specimen dimensions. This experimental observation may be attributed to the varying local fracture energy along the crack path. When the crack tip approaches the specimen boundary, the size of the fracture process zone will be reduced and, consequently, only a portion of the fracture energy is activated; i.e., the local fracture energy is getting smaller. The influence of this boundary effect diminishes with increasing specimen size resulting in the size dependence of the apparent fracture energy determined by the work-of-fracture method as an average value in the ligament. With varying local fracture energy, the local softening curve will also show variations. The latter are subject of the present study. Wedge splitting tests with different specimen sizes as well as inverse analyses of these experiments were carried out. For the inverse analyses, the cohesive crack model was adopted and an evolutionary optimization algorithm has been used. The boundary effect on the local fracture properties was taken into account and, as a result, the variation of the softening curve along the crack path could be determined. It was found that the tail of the softening curve is shortened and lowered due to the boundary effect whereas the initial slope of this curve appears to be not affected.

Development of a Correlation Test Specimen for Highly Deformable Adhesive Systems

A new adhesive test specimen design was developed which embodied several key attributes, the most important of which was inclusion of tension, mixed-mode, and shear deformation mechanisms. The specimen design was easy to manufacture, required only simple tension testing equipment to test, and provided simple specimen geometry and boundary conditions for use in finite element model correlation exercises. By design, the individual failure points for each of the deformation mechanisms within the test specimen were separated and could be related directly to characteristics of the force versus displacement information from the experimental test. Finite element predictions using a Gurson constitutive model with element deletion at failure were generated and compared with experimental test results. It was found that while the order of failure modes within the prediction was the same as that found within the test, the element deletion technique yielded an abrupt change between failure modes not seen in the experimental test, and that a more complex numerical representation of failure would be a valuable development. Overall, the test specimen development was successful in providing a simple and repeatable method by which complex cohesive failure modes in a bonded system could be evaluated and correlated back to predictive tests.

Discrete modeling of plastic cement paste subjected to drying

Prior to initial setting, cement paste may be regarded as a suspension consisting of solid particles and water. Drying leads to a curved water surface in the spaces between the solid particles at the specimen boundary. A negative capillary pressure is built up which results in shrinkage strain and, possibly, cracking. The concept of discrete modeling has been employed for simulating these processes. In 2D models consisting of circular solid particles and a liquid phase, the drying induced capillary pressure build-up and the corresponding particle displacements are simulated by using the Distinct Element Method. Under certain conditions, crack initiation due to capillary forces may be observed. The simulations allow the demonstration of different influences on the risk of plastic shrinkage cracking.

Effect of grain boundary microstructure on fatigue crack propagation in austenitic stainless steel

The effect of grain boundary microstructure on fatigue crack propagation in austenitic stainless steel was investigated in order to control fatigue crack propagation. The fraction of low-Σ coincidence boundaries in specimens was controlled by thermomechanical processing. The specimen with the higher fraction of low-Σ boundaries (73%) showed the lower propagation rate of fatigue crack than the specimen with the lower fraction of low-Σ boundaries (53%). The ratio of intergranular fracture segments to the total crack length was lower for the specimen with the higher fraction of low-Σ boundaries. Moreover, the roles of grain boundaries in the fatigue crack propagation were investigated in connection with grain boundary microstructure, i.e., the character distribution and geometrical configuration of grain boundaries. It is evidenced that the approach to grain boundary engineering is applicable to controlling fatigue crack propagation in austenitic stainless steel.

Frequency–wavenumber domain analysis of guided wavefields

Full wavefield measurements obtained with either an air-coupled transducer mounted on a scanning stage or a scanning laser vibrometer can be combined with effective signal and imaging processing algorithms to support characterization of guided waves as well as detection, localization and quantification of structural damage. These wavefield images contain a wealth of information that clearly shows details of guided waves as they propagate outward from the source, reflect from specimen boundaries, and scatter from discontinuities within the structure. The analysis of weaker scattered waves is facilitated by the removal of source waves and the separation of wave modes, which is effectively achieved via frequency–wavenumber domain filtering in conjunction with the subsequent analysis of the resulting residual signals. Incident wave removal highlights the presence and the location of weak scatterers, while the separation of individual guided wave modes allows the characterization of their separate contribution to the scattered field and the evaluation of mode conversion phenomena. The effectiveness of these methods is demonstrated through their application to detection of a delamination in a composite plate and detection of a crack emanating from a hole.

The Effect of Vanadium and Chromium on the Precipitation of Carbide in Si-Mo Ferritic Ductile Cast Iron

Carbide precipitation behavior in Si-Mo ferritic ductile cast iron was investigated as functions of vanadium and chromium contents. Vanadium addition promoted the precipitation of carbides both in ferrite grains and at its grain boundaries. Pearlite was found to form near the cell boundary next to the eutectic carbides in the as-cast chromium alloyed specimens, and was fully decomposed by an annealing heat treatment. Vanadium addition led to the formation of fine precipitates, prohibiting the ferrite growth, resulting in smaller ferrite grains. The precipitate phase at grain boundaries in vanadium alloyed specimens was identified as vanadium carbide (VC1-x) and the stoichiometry of the eutectic carbide was almost same as that in Si-Mo ferritic ductile cast iron except for a higher vanadium content. However, in the case of chromium alloyed specimen, the ratio of iron in the composition of eutectic carbide was higher than those of Si-Mo and Si-Mo-0.5V cast irons.

Assessment of debonding in sandwich CF/EP composite beams using A0 Lamb wave at low frequency

The aim of this study is to quantitatively assess debonding in sandwich CF/EP composite structures with a honeycomb core using acoustic waves activated and captured by surface-mounted PZT elements. For experimental investigation, debonding was introduced at different locations in sandwich CF/EP composite beams. The fundamental anti-symmetric A0 Lamb mode was excited at a low frequency. The transmitted and reflected wave signals in both surface panels were captured by PZT elements after interacting with the debonding damage and specimen boundaries. Aided by finite element analysis (FEA), the differences in wave propagation characteristics in sandwich composite beams and composite laminate (e.g. skin panel only) were investigated. The debonding location was assessed using the time-of-flight (ToF) of damage-reflected waves, and the severity of debonding was evaluated using both the magnitude of the reflected wave signal and the delay in the ToF of Lamb wave signals. Good correlation between the experimental and FEA simulation results was observed. The results demonstrate the effectiveness of Lamb waves activated and captured by surface-mounted PZT elements on either surface of sandwich composite structures in assessing debonding.

A first approach to the analysis of fatigue parameters by thermal variations in static tests on plastics

Following previous results showing that under static loads it is possible to detect the first plasticization of the specimens at the end of the thermoelastic phase, the authors have conducted experimental trials to verify that this effect can be pointed out in notched and unnotched polyvinyl chloride (PVC) specimens. The goal is to define the real elastic phase also for materials for which the elastic limit and the yield stress are not easily defined, different from the case of steel. The results show a variable thermal behaviour depending on the distance from the notch. The thermal behaviour, proportional to the stress in the totally elastic phase, accordingly with the thermoelastic effect, deviating from the linearity, points out the beginning of the local plasticization. The thermoelastic limit, moving from the notch edge to the specimen boundary, allows to follow the paths of plasticization. The results are also compared with those found by cyclic loading using the thermographic methodology already verified by the authors and other researchers.

Simulation of Acoustic Emission in Planar Carbon Fiber Reinforced Plastic Specimens

The simulation of acoustic emission waveforms resulting from failure during mechanical loading of carbon fiber reinforced plastic structures is investigated using a finite element simulation approach. For this investigation we focus on the dominant failure mechanisms in fiber reinforced structures consisting of matrix cracking, fiber breakage and fiber-matrix interface failure. To simulate the failure process accurately, we present a new acoustic emission source model that is based on the microscopic source geometry and micromechanical properties of fiber and resin. We demonstrate that based on this microscopic source model these failure mechanisms result in excitation of macroscopic plate waves. The propagation of these plate waves is described using a macroscopic three-dimensional model geometry which includes contributions of reflections from the specimen boundaries. We further present a model of the acoustic emission sensors used in experiments to simulate the influence of aperture effects. To enhance the understanding of correlation between macroscopically detectable acoustic emission signals and microscopic failure mechanisms we simulate the response to different source excitation times, crack surface displacements and displacement directions. The results obtained show good agreement with fundamental assumptions about the crack process reported by various other authors. The simulated acoustic emission signals obtained are compared to experimentally measured waveforms during four-point bending experiments of carbon fiber reinforced plastic structures. The simulated signals of fiber-breakage, matrix-cracking and fiber-matrix interface failure show systematic agreement with the respective experimental signals.

Identification of non-homogeneous stress fields in dynamic experiments with a non-parametric method

Stress and strain fields are generally assumed homogeneous in high strain rate experiments. Therefore, only mean stress and strain are measured. However, this assumption is not valid for soft materials and/or very high strain rates. For heterogeneous dynamic tests, inverse techniques are usually proposed to identify the stress and strain fields. In this paper, a non-parametric method to identify the stress field is presented. This method is based on a force measurement on the specimen boundary and a displacement (strain) field measurement carried out by a digital image correlation technique. It is applied to measure the strain rate sensitivity of a synthetic rubber (Criblex 80). This new technique estimates the stress field with good accuracy.

Coupled Consolidation and Contaminant Transport in Compressible Porous Media

This paper presents an experimental and numerical investigation of coupled consolidation and contaminant transport in compressible porous media. Numerical simulations were performed using the CST2 computational model, in which a dual-Lagrangian framework is used to separately follow the motions of fluid and solid phases during consolidation. Diffusion and large strain consolidation-induced transport tests were conducted on composite specimens of kaolinite slurry consisting of an upper uncontaminated layer and a lower layer contaminated with potassium bromide. Assessment of the importance of the consolidation process on solute transport is based on measured and simulated solute breakthrough curves and final contaminant concentration profiles. CST2 simulations closely match the experimental data for three different loading conditions. Diffusion and consolidation-induced advection made important contributions to solute transport and mass outflow in this study. Additional simulations indicate that consolidation-induced contaminant transport may also be affected by specimen boundary drainage and initial concentration conditions.

Determination of V-notch SIFs in multi-material anisotropic wedges by digital correlation experiments

In this paper, theoretical formulations based on the Stroh’s complex function approach were used to find the displacement field and H-integral of a sharp V-notch formed from several anisotropic materials. Displacements from the image-correlation experiments are then substituted into the least-squares formulation to find V-notch stress intensity factors (SIFs) in multi-material anisotropic wedges. Validations using the H-integral indicate that the experimental SIFs evaluated from the proposed method of acceptable accuracy. The major advantage is that the proposed method only requires displacements inside the specimen, and displacements near the notch tip, specimen boundaries, or notch surfaces are not necessary.

Isogrid stiffened syntactic foam cored sandwich structure under low velocity impact

Abstract In this study, a new sandwich structure with a hybrid cellular core (periodic isogrid stiffened stochastic syntactic foam) was fabricated using the pin-guided dry-weaving technology, evaluated by low velocity impact and in-plane compression after impact (CAI) tests, visualized by C-scan and SEM, and modeled by finite element analysis. It is found that the stretch-dominating isogrid skeleton develops synergy with and provides confinement to the foam, leading to an CAI strength much higher than that of the laminated control plate. Subjected to concentrated impact loading, each cell responds in a boundary controlled quasi-static manner. Although the bay absorbs the highest amount of impact energy, the CAI strength is the highest when the impact is in the bay, followed by impact in the rib and impact in the node. Also, the CAI strength reduces as the bay area increases. The sandwich deviates slightly from in-plane quasi-isotropy primarily due to incomplete cells along the boundary of specimens.

Asymptotic thermal and residual stress distributions due to transient thermal loads

ABSTRACTThe paper deals with the development of thermal and residual stress distributions arising from the solidification of a fusion zone near a V‐notch tip. A set of numerical solutions of the problem was carried out under the hypothesis of generalized plane strain conditions by means of SYSWELD code. The intensity of the thermal and residual asymptotic stress fields at the sharp V‐notch tip was studied for a given V‐notch specimen geometry and a predefined fusion zone dimension after simulations on materials with different thermal, mechanical and phase transformation properties and after changing the clamping conditions at the specimen's boundary. The results were compared in terms of the elastic or elastic‐plastic notch stress intensity factors giving a contribution to the interpretation of the experimental behaviour of welded joint.

Visualization and Analysis of Microstructure in Three-Dimensional Discrete Numerical Models

A computer program has been developed to allow for the virtual slicing of irregularly spaced and irregularly shaped three-dimensional image data. The program was used to virtually slice three-dimensional particle assemblies from discrete element method (DEM) simulations, allowing, for the first time, direct comparison to two-dimensional slices extracted from solidified physical specimens. Based on slices obtained from the numerical specimens, it is possible to compare quantitatively numerical microstructure directly to its physical analog, which should lead to greatly improved calibrations of granular mechanics models, and could facilitate the calibration of models across all scales of interest rather than solely at specimen boundaries. Improved confidence in the ability of the DEM to realistically simulate the microstructure of granular assemblies (through improved multiscale calibration) should result in increased confidence in microstructural parameters measurable in numerical simulations but inaccessible in the laboratory. Algorithm development within the framework of the open-source Visualization Toolkit is described and performance of the algorithm is quantified for two platforms. Results from virtual slices of a test assembly with regular particle packing are verified against known analytical solutions. A slice of a more complex assembly comprised of nearly 40,000 spheres is quantified statistically and compared to an analogous slice from a physical specimen of uniform sand.