Dog-bone Test Research Articles

Analytical modelling of the orientation dependency of 3D printed SHCC at increasing levels of scale

3D printable strain hardening cementitious composites (3DP-SHCC) stand out for their high ductility and increase in tensile strength after initial cracking. Due to the printing process, which includes the pumping and extruding of the material, the fibres are not uniformly orientated in the material but have an orientation dependency due to the specific printing process applied. Three different cases for the fibre orientation distribution functions are presented with increasing levels of relation to the printing process. These three cases have been used in a micro-mechanical model to verify with experimental stress-crack opening results (single crack). Furthermore, a spring model is used to give insight into the potential to develop pseudo strain hardening behaviour (multiple cracking) in various printing directions. The method is also used to model the experimentally obtained spread in uni-axial tensile dogbone test results. From these experimental dogbone tests, average stress-strain relations are determined in two directions. Finally, a multi-layer method is used on the component level to validate the use of these average stress-strain relations for modelling the bending behaviour of printed elements in multiple directions.

Mesoscopic discrete modeling of compression and fracture behavior of concrete: Effects of aggregate size distribution and interface transition zone

This paper presents a mesoscopic discrete modeling approach for studying compression and fracture behavior of concrete. A modified piecewise linear sieve curve was proposed to generate irregularly shaped aggregates following specific particle size distributions, and then a random particle model and the parallel bond model were implemented within a 3D framework of discrete element model (DEM). To characterize the interface transition zone (ITZ) which is generally the weak region in concrete, a reduction factor was employed to adjust the strength and consider the effect of ITZ on the mechanical response of concrete. The numerical modeling was calibrated and validated by comparing simulation results with experimental data from compression, splitting tensile, and dog-bone tests. The results demonstrate that the random aggregate model provides accurate representations of the concrete mesostructure, and the discrete model can effectively capture the effects of aggregate size distribution and ITZ on mechanical properties and crack propagation of concrete.

Experimental study on the fretting fatigue of Inconel 718 superalloy

Fretting fatigue is a surface damage phenomenon appearing in mechanical assemblies subjected to time-varying contact forces. This type of failure is a very frequent problem in many engineering components, with special relevance in the blade-disc connections in aircraft gas turbines. In this study, different plain and fretting fatigue tests were done using different load levels and also under two different temperatures, at room temperature and high temperature (650 °C), in order to obtain the fretting fatigue behaviour in both conditions and compare them with that of plain fatigue test. The contact pair used to carry out these tests is the frequently used cylindrical pad pressed against a dog-bone test specimen with a rectangular cross-section. The material used to manufacture both elements is the often used Nickel alloy Inconel 718, which is widely used in the manufacture of gas turbine parts. Moreover, and because the material was received in annealed condition, this material was heat treated to improve its mechanical properties. Results of tests under different conditions are analysed and compared. Subsequently, the fracture surfaces for different specimens are analysed and fatigue crack growth rate under different testing conditions are presented and compared with data from the literature.

Real-time structural validation for material extrusion additive manufacturing

Material extrusion additive manufacturing is a technology that produces a part by controlling the melting and extrusion of a thermoplastic filament. A void defect in a specific position can significantly impact the whole product’s structural quality and mechanical properties. Different defect detection systems have been built and have achieved good detection accuracy. However, a challenge remains in determining if the structural performance of a part with defects will fail to meet design requirements. This research develops a real-time product structural quality validation system using a multi-dimensional accumulation-threshold-based decision-making approach. The proposed system is validated on a consumer-grade 3D printer with an optical camera. The layer-wise damage information is output from a trained convolutional neural network. The developed structural quality validation system links the obtained defect information with the decision boundary developed by the accumulation-threshold-based decision-making approach to evaluate the effect of the defect. The accumulation-threshold-based decision-making approach is trained on a novel component health index that links the location and size of defects across layers. Experimental results show that the decision boundary obtained from the accumulation-threshold-based decision-making approach performs well on test data with 96% recall and a 91% F1-score, which means the decision boundary can provide good overall results while being biased to limit false negative results. The proposed real-time structural quality validation system is validated online for a dog-bone test specimen. Results show that the computational time of the structural quality validation system requires at most 688 ms, while a dog-bone layer takes 75 s to print, thus demonstrating that the proposed system can meet required real-time constraints. The capability to run online enables users to cancel a print mid-process for specimens that will not meet structural loading requirements. The developed system is demonstrated through a video, which is provided in the supplemental materials. The dataset for this work is published as a public repository containing 450 samples with 221 failure classes.

Influence of mesoscale friction interface geometry on the nonlinear dynamic response of large assembled structures

Friction interfaces are unavoidable components of large engineering assemblies since they enable complex designs, ensure alignment, and enable the transfer of mechanical loads between the components. Unfortunately, they are also a major source of nonlinearities and uncertainty in the static and dynamic response of the assembly, due to the complex frictional physics occurring at the interface. One major contributor to the nonlinear dynamic behavior of the interface is the mesoscale geometry of a friction interface. Currently, the effects of the interface geometry on the nonlinear dynamic response is often ignored in the analysis due to the high computational cost of discretizing the interface to such fine levels for classical finite element analysis. In this paper, the influence of mesoscale frictional interface geometries on the nonlinear dynamic response is investigated through an efficient multi-scale modeling framework based on the boundary element method. A highly integrated refined contact analysis, static analysis, and nonlinear modal analysis approach are presented to solve a multi-scale problem where mesoscale frictional interfaces are embedded into the macroscale finite element model. The efficiency of the framework is demonstrated and validated against an existing dovetail dogbone test rig. Finally, the effects of different mesoscale interface geometries such as surface waviness and edge radius, are numerically investigated, further highlighting the influence of mesoscale interface geometries on the nonlinear dynamics of jointed structures and opening a new research direction for the design of friction interfaces in friction involved mechanical systems.

Determination of the fatigue behavior of mechanical components through infrared thermography

The determination of the fatigue behavior at a component level usually requires dedicated test rigs and an expressive amount of time. The hours spent on such machinery are expensive; therefore, solutions to reduce experimentation time are most welcomed. In this context, this investigation aims at developing a procedure for rapid determination of the fatigue strength of crankshafts by means of a thermographic methodology. The use of infrared cameras for fatigue strength analysis was first assessed in standard dog-bone specimens. Crankshafts were then tested in an in-house fatigue test rig using the conventional staircase method and the thermographic method. Sample batches with different manufacturing parameters were produced and tested to assess the robustness of the proposed alternative technique. Results of the dog-bone test campaign revealed a good correlation between fatigue strength estimates obtained with the conventional Wöhler curve and the thermographic methodology. Finally, the thermographic technique also delivered results in close agreement with the staircase method for all crankshaft batches. The proposed procedure was found to be a viable, rapid alternative to conventional fatigue test programs, with potential application for complex structural components such as crankshafts, among others.

MECHANICAL PROPERTIES OF 3D-PRINTED BLOOD VESSELS

The demand for vascular substitutes in clinical practice has increased, and 3D-printed blood vessels could be advantageous alternatives. Determining the mechanical properties of 3D-printed blood vessels is important to further improve the technology and clinical application. Here, dogbone test samples with and without cells were generated by 3D printing with combined UV curing and hydrogel ion curing. Tensile tests were performed to measure the mechanical parameters of the dogbones and build two kinds of fluid–solid interaction models. In accordance with hydrodynamics and momentum theorems, the velocities in accelerated ejection period, ejection peak, reduced ejection period, and early diastolic period were selected as inlet velocities. Velocity distribution on the symmetry surface of the fluid and the stress and strain of the solid tube under different velocities were analyzed. Results reveal that the velocity decreases gradually from the fluid center to the wall. At a high inlet velocity, the fluid velocity is high, while the stress and strain of the wall increase. During cardiac ejection peak, the stress on the wall reaches the maximum value of 2018 Pa, which is much lower than the ultimate stress. In addition, the strength and stiffness decrease for the vessels added with cells. This work provides a feasible method for measuring the mechanical properties of 3D-printed blood vessels. Keywords: 3D printing, mechanical properties, fluid–solid interaction

The Use of Surface Topography for the Identification of Discontinuous Displacements Due to Cracks

The determination of three components of displacements at material surfaces is possible using surface topography information of undeformed (reference) and deformed states. The height digital image correlation (hDIC) technique was developed and demonstrated to achieve micro-level in-plane resolution and nanoscale out-of-plane precision. However, in the original formulation hDIC and other topography-based correlation techniques perform well in the determination of continuous displacements. In the present study of material deformation up to cracking and filan failure, the ability to identify discontinuous triaxial displacements at emerging discontinuities is important. For this purpose, a new method reported herein was developed based on the hDIC technique. The hDIC solution procedure comprises two stages, namely, integer-pixel level correlation and sub-pixel level correlation. In order to predict the displacement and height changes in discontinuous regions, a smoothing stage was inserted between the two main stages. The proposed method determines accurately the discontinuous edges, and the out-of-plane displacements become sharply resolved without any further intervention in the algorithm function. High computational demand required to determine discontinuous displacements using high density topography data was tackled by employing the graphics processing unit (GPU) parallel computing capability with the paging approach. The hDIC technique with GPU parallel computing implementation was applied for the identification of discontinuous edges in an aluminium alloy dog bone test specimen subjected to tensile testing up to failure.

Influences of temperature and opening rate of substrate cracks on the mechanical behaviour, crack–bridging ability and deformation mechanisms of one–component, cementitious, flexible waterproofing membranes

One–component, cementitious, flexible waterproofing membranes bridge cracks because their polymer-dominated matrix takes up the deformation. The sensitivity of the deformation of polymers to temperature and strain rate raises interest to the influence of these two parameters on the crack–bridging ability of the waterproofing membranes. Temperature–controlled crack–bridging and dog-bone experiments with in–situ monitoring of the sample's surfaces show a decrease in the crack–bridging ability (CBA) of the waterproofing membranes and a decrease in plastic deformation at testing temperatures above the glass transition temperature (Tg) of the used polymer. At temperatures below the Tg, the membranes deform in a brittle manner. Rate–controlled crack–bridging and dog-bone tests with in–situ monitoring of the sample's surfaces reveal a decrease in crack–bridging ability and plastic deformation with decreasing displacement rate. Hence, temperature and displacement rate are crucial parameters governing the crack-bridging performance of waterproofing membranes.

Properties of Injection Molded Biocomposites Reinforced with Wood Particles of Short-Rotation Aspen and Willow.

Injection molded biocomposite specimens were prepared by using four different weight percentages, i.e., 10%, 20%, 30%, and 40% of aspen (Populus tremula L.) and willow (Salix caprea L.) wood particles in a biopolymeric matrix. Dog-bone test specimens were used for testing the physical, mechanical, and thermal properties, and microstructure of biocomposites. The tensile and bending strength changed with the change in weight percentages of wood particles and the bending stiffness increased with the increasing weight percentage of wood. In Brinell hardness, similar changes as a function of wood particle weight percentage were shown, and a relationship between hardness and tensile strength with wood content was also investigated. The prepared biocomposites could be an alternative for plastic-based materials and encourage the use of fast growing (aspen and willow) wood from short-rotation forests in biocomposites.

3D immersive visualization of micro-computed tomography and XRD texture datasets

Advancements in computer technology have enabled three-dimensional (3D) reconstruction, data-stitching, and manipulation of 3D data obtained on X-ray imaging systems such as micro-computed tomography (μ-CT). Likewise, intuitive evaluation of these 3D datasets can be enhanced by recent advances in virtual reality (VR) hardware and software. Additionally, the generation, viewing, and manipulation of 3D X-ray diffraction datasets, such as pole figures employed for texture analysis, can also benefit from these advanced visualization techniques. We present newly-developed protocols for porting 3D data (as TIFF-stacks) into a Unity gaming software platform so that data may be toured, manipulated, and evaluated within a more-intuitive VR environment through the use of game-like controls and 3D headsets. We demonstrate this capability by rendering μ-CT data of a polymer dogbone test bar at various stages of in situ mechanical strain. An additional experiment is presented showing 3D XRD data collected on an aluminum test block with vias. These 3D XRD data for texture analysis (χ, ϕ, 2θ dimensions) enables the viewer to visually inspect 3D pole figures and detect the presence or absence of in-plane residual macrostrain. These two examples serve to illustrate the benefits of this new methodology for multidimensional analysis.

Influence of carboxylic acids on mechanical properties of thermoplastic starch by spray drying

Biodegradable packaging is gaining much attention in food industry as the awareness on sustainability has increased. Thermoplastic starch is a possible alternative. This study evaluated the influence of malic acid (MA) and citric acid (CA), used as a plasticizer, on the mechanical properties of thermoplastic starch (TPS) obtained by spray drying. TPS powder was produced from solution spray drying. This powder was further compression molded to prepare TPS dog-bone test samples. X-ray Diffraction (XRD) results showed that both the spray dried TPS powder and dog-bone test samples were amorphous in nature irrespective of the amount of plasticizer added. Scanning electron microscope (SEM) was used to examine the morphology of solution spray dried TPS powder. No noticeable difference was observed in the morphology. Particles were spherical in shape with homogenous surface. The FT-IR analysis indicated the interaction of plasticizers with starch chains by hydrogen bonding. During TGA analysis, apart from moisture loss at 100 °C, samples were thermally stable up to 170 °C. Mechanical testing of TPS dog-bone revealed that sample containing malic acid as plasticizer exhibited a more elastic behavior as compared to citric acid plasticized formulations. It was revealed that the tensile strength of TPS dog-bone samples was inversely proportional to the quantity of plasticizer used.

Numerical analysis of projectile penetration and perforation of plain and fiber reinforced concrete slabs

SummaryThe research presented in this paper deals with the numerical analysis of projectile impact on regular strength concrete (RSC), high‐strength concrete (HSC), and engineered cementitious composites (ECC) using the Lattice Discrete Particle Model (LDPM). The LDPM is chosen in this study as it naturally captures the failure mechanisms at the length scale of coarse aggregate of concrete, and its capabilities include the accurate depiction of both intrinsic and apparent rate effects in concrete, as well as fiber reinforcement effects. The model is used to predict the experimental impact response performed by four independent testing laboratories, and for each data set the model parameters are calibrated and validated using a combination of uniaxial compression, triaxial compression, uniaxial strain compression, and dogbone tests. In the first study, perforation experiments on RSC and HSC for varied impact velocities are carried out, and the exit velocity is compared with the available experimental data. The second study focuses on ECC, where multiple impact of steel and plastic fiber reinforced concrete panels are explored. A third investigation is performed on four RSC panels with varied thicknesses and subjected to the same impact velocity. In this instance, the model is used to predict the penetration depths for the different cases. Finally, in the last study, the response of large‐thickness infinite panels of sizes ranging from 300 mm to 700 mm under projectile impact is considered. Copyright © 2016 John Wiley & Sons, Ltd.

Damage model calibration and application for S355 steel

Current demands for improved structural integrity and risk assessment involves the evaluation of alternative unloading paths, ensuring that a whole building (or a significant part of it) remains stable when subject to an unforeseen event: natural hazards (earthquakes, foundation failure, fire,…) or even terrorist attacks. The design under these conditions requires that the structural elements and, particularly in steel framed structures, the joints connecting elements are able to undergo elevated deformations without fracturing, thus providing means of energy dissipation.The finite element method (FEM) is nowadays a widespread practice assisting in the simulation of many physics phenomenon. Looking forward to an accurate finite element simulation of steel connections up to its fracture, the implementation of a failure criterion based on continuum damage mechanics is explored in this paper. It is done by comparing the results reached from an undamaged analysis and the ones obtained from a damaged analysis using a ductile failure with “element deletion” technique to simulate the fracture. The analyses are carried out using the finite element software ABAQUS. The establishment of the fracture strain dependency to the triaxial stress state is based on the experimental evidences reached from 12 tensile coupon (S355) tests, including both notched and unnotched flat dog-bone test specimens.

Effects of part build orientations on fatigue behaviour of FDM-processed PLA material

This paper investigates the fatigue behaviour of polylactic acid (PLA) parts processed by fused deposition modelling (FDM) additive manufacturing process. PLA is becoming a commonly used thermoplastic in open-source FDM machines for various engineering applications and it is essential that mechanical properties and performance of FDM-processed PLA parts must be properly understood. Very little data exist on the fatigue performance of PLA parts processed by FDM additive manufacturing. This study looks at the effect of part build orientations on the tensile fatigue properties of PLA material. A Cube 3D printer was used to print dog-bone test specimens in three (X, Y and 45°) different build orientations. These dog-bone parts were based on ASTM D638 standard and were cyclically tested at 80, 70, 60 and 50 % nominal values of the ultimate tensile stress by using a Zwick Z010 universal testing machine. Results show that in static loading, the FDM-processed PLA parts in X build orientation exhibit higher tensile stress, in the range of 60–64 % of that of injection moulded PLA material, compared to those built in Y and 45° orientations. But under tensile cyclic loading condition, the parts in 45° build orientation show higher fatigue life than the parts in X and Y build orientations for the same percentage of applied static loads. This paper adds knowledge to fill the gap on the fatigue characteristics of the PLA parts processed through FDM and would be useful in engineering design applications of such parts subjected to cyclic loading conditions.

Bonding Characteristics of Macro Polypropylene (Pp) Fibre in Pva/Macro Pp Blended Fibre-Reinforced Styrene Butadiene Latex-Modified Cement-Based Composites

The bonding properties of macro polypropylene (PP) fibres in blended fibre-reinforced polymer-modified cement-based composites (BFRPMCCs) containing styrene butadiene (SB) latex were evaluated. A blend of polyvinyl alcohol (PVA) and macro PP fibres was used in the study. The pullout behaviour was investigated using dog-bone test specimens compliant with JCI SF-8. The macro PP fibre was added at 0.45 kg/m3 and the PVA fibre was added at 12.6 kg/m3 to fabricate the BFRPMCCs, and SB latex was added at 0–25 wt% of the cement. The addition of SB latex improved the dispersion of the PVA fibres, increased the pullout resistance of the macro PP fibre embedded in the BFRPMCC, and induced strain hardening in the debonded zone. The maximum bond strength occurred at an SB latex content of 10%, and the maximum interface toughness occurred at an SB latex content of 15%. Microstructural analysis of the macro PP fibre surfaces was carried out following pullout of the fibres embedded in the SB-latex-containing BFRPMCC. The amount of scratches increased with increasing SB latex content until 15%, but then decreased at SB latex contents above 20%, indicating that the bonding of the composite was optimum at a latex content of 15%.

Application of Halpin–Tsai equation to microfibril reinforced polypropylene/poly(ethylene terephthalate) composites

While the main goal of the first part of this study was to check the applicability of the Tsai–Hill equation to a polymer-polymer microfibril reinforced composite (MFC), in which the reinforcing elements represented microfibrils with a diameter around 1–3 μm and aspect ratio of about 100, in the present paper a similar goal involves the Halpin–Tsai equation. In addition, using compatibilised blends an attempt is made to shed light on the mechanism of the microfibril formation during drawing of isotropic polymer blends. For this purpose, injection moulded dog-bone test samples of blends of polypropylene/poly(ethylene terephthalate) (PP/PET) (60/40 wt%) have been prepared starting from highly drawn bristles, also from blends containing 0–9 wt% compatibiliser (ethylene-glycidyl methacrylate). The MFC structure of the test sample is established by X-ray and scanning electron microscope (SEM) analyses. The tensile data are compared with those predicted according to the Halpin–Tsai equation. It is shown that the predicted values are slightly higher than the measured ones and this finding is explained by the presence of a compatibiliser resulting in much lower aspect ratios of the microfibrils. The suggested mechanism of the microfibril formation is based on the coalescence of the individual elongated spheres during drawing. The SEM observations also show that the compatibilised blends contain much shorter microfibrils because the compatibiliser prevents the coalescence process.

A method for measuring the creep behavior of pressurized polymer tubing

Polymer components have been proposed for use in domestic solar hot-water heating systems. A polymer heat exchanger is under development for such systems. For heat transfer considerations, the heat exchanger will comprise many thin-walled tubes. The heat exchanger must survive 10 years of service at high pressure (1.55 MPa) and high temperature (82°C). A novel method has been developed for evaluating the long-term performance (creep) of the polymer tubing. Traditional creep testing, performed with dog bone test specimens, cannot be applied because the thin-walled tubing has anisotropic material properties. Consequently, performance must be evaluated directly on the extruded tubing. The method entails wrapping a Constantan wire around the tube specimen to continuously record the hoop strain. For pressure loading of tubing, this method offers significant improvements over strain gage instrumentation. In this paper, the test method is described, an analysis of the strain transfer between the tubing and wire wrap is presented, and strain data for polypropylene tubing measured with a strain gage and wire wrap are compared. The data show that the wire measurement method can be successfully used for the characterization of long-term mechanical behavior of polymer tubes.