Multiaxial Tests Research Articles

Seismic fragility assessment of non-structural drywall partitions with screw and adhesive fixing through multi-axis cyclic testing

Drywall partitions are the most common type of non-structural walls used in modern multi-story buildings. Damage investigation of these walls during the past earthquakes has shown their vulnerability under small to moderate inter-story drifts leading to significant economic losses. To propose innovative solutions to this problem, accurate damage assessment must be conducted using the data obtained from realistic experiments. In this study, the classic performance-based earthquake engineering framework is used as a method for performing a probabilistic estimation of damage progression in the drywall partitions in a limited-ductility reinforced-concrete building. The Multi-Axis Substructure Testing system at Swinburne University of Technology is used to conduct large-scale three-dimensional cyclic tests on two assemblies of drywall partitions. The assemblies included two types of fixings, namely, screw fixing as a common construction method, and adhesive fixing recently developed for mitigating the creaking noise under serviceability wind loads. The results of the experiments are used to define three levels of damage states to produce fragility curves of drywall systems given the building inter-story drifts. Vulnerability curves are then developed to account for the uncertainties in the building response due to record-to-record variability, and the factors controlling the progression of damage in partition walls such as the variability in material and construction quality.

Fatigue failure mechanisms for AlSi10Mg manufactured by L-PBF under axial and torsional loads: The role of defects and residual stresses

Additive Manufacturing (AM) is with no doubt the most revolutionary manufacturing process developed in the last two decades. Despite the indisputable advantages of this technology, the poor surface quality of net-shape components, the presence of internal defects and the development of process induced residual stresses still represent the main problems for the fatigue strength of critical stressed components. In previous works investigating the same alloy, the uniaxial fatigue strength of both machined and net-shape specimens was correlated with the defect size through a Kitagawa diagram, allowing to describe the problem from the threshold perspective. The aim of this work is to extend this approach by investigating the failure mechanisms under torsion in presence of manufacturing defects, both volumetric and superficial anomalies. Specimens manufactured with laser powder bed fusion (L-PBF) technique out of AlSi10Mg, featuring both machined and net-shape surface state, were tested and analysed. The two experimental campaigns allow to investigate the competition between internal defects and superficial features and their effect on the fatigue performances. Tests were performed under two loading conditions, namely fully reverse torsion (RT=−1) and positive torque ratio (RT=0.1). It was found that for the net-shape specimens manufacturing residual stresses have a key role in influencing fatigue strength of this material, making the fatigue limit in torsion of the two considered loading conditions comparable. All the tested specimens failed onto a maximum principal stress plane, which is in line with multiaxial tests performed on a cast A356-T6 aluminium alloy. In some relatively high shear stresses there is a competition between Mode I and Mode II crack propagation, whose threshold condition is controlled by ΔKth,I.

Autologous Fibrin Sealants Have Comparable Graft Fixation to an Allogeneic Sealant in a Biomechanical Cadaveric Model of Chondral Defect Repair

PurposeThe purpose of this study is to assess the integrity of chondral defect repairs filled with a cartilage allograft and sealed with either allogeneic fibrin sealant or autologous fibrin sealants created with platelet-rich plasma (PRP) or platelet-poor plasma (PPP) in a cadaver model.MethodsTwenty-millimeter medial femoral condyle (MFC) chondral defects were created in five human cadaveric knees. The defects were filled with particulated cartilage allograft hydrated with PRP from human donors until slightly recessed. Sealants were applied until flush with the articular surface using PRP and autologous thrombin serum, PPP and autologous thrombin serum, or commercial allogeneic sealant. The MFC defects were cycled using a multiaxial testing system to simulate continuous passive motion undergone during rehabilitation. After testing, the repairs were assessed for integrity by quantitatively comparing defect exposure and qualitatively assessing sealant delamination.ResultsThe mean defect exposures were 4.20% ± 5.02% for the PRP group, 4.60% ± 5.18% for the PPP group, and 1.80% ± 2.95% for the allogeneic sealant group. No significant differences were observed between groups (P = .227), and each group had significantly less defect exposure when compared to the critical clinically relevant value assigned to be 30% (P = <.001 for all). No complete sealant delamination was observed, although the allogeneic sealant delaminated with a higher magnitude than did the autologous sealants.ConclusionsThe PRP and PPP sealants were comparable to the allogeneic sealant for graft fixation when used in conjunction with an underlying PRP-hydrated particulated cartilage allograft. The autologous sealants had better delamination resistance than the allogeneic sealant.Clinical RelevanceThe time-zero model is critical in elucidating the retention properties of fibrin and allogenic sealants after cartilage repair and before healing processes help stabilize the repair.

Influences of foliation orientation and lateral stress difference on the deformation and fracturing of a thin-layered rock around underground excavations: insight from multi-axial loading tests

Influences of foliation orientation and lateral stress difference on the deformation and fracturing of a thin-layered rock around underground excavations: insight from multi-axial loading tests

Numerical investigation of the failure mechanism of concrete specimens under tri-axial dynamic loads

The dynamic properties of concrete-like materials under true tri-axial loads were seldom studied because of the lack of proper testing facilities for conducting multi-axial impact tests. Although a three-dimensional Split Hopkinson pressure bar (3D-SHPB) device has developed in Tianjin University which could provide synchronized tri-axial dynamic loads with the same amplitude, conducting such tests is expensive and time consuming. As a supplement to physical tests, high fidelity numerical modelling is becoming more and more common to study the material and structural performances under different loading conditions. In this study, a mesoscale concrete model with the consideration of ITZ (interfacial transition zone) is developed and verified by the testing data of the 3D-SHPB tests. The dynamic properties of concrete material under different true tri-axial loading paths which the 3D-SHPB testing system could not provide are numerically investigated using the verified model. The results show that the strain rate effect on concrete compressive strength becomes less significant under tri-axial loading than that under uni-axial loading owing to the tri-axial loads acting on the specimen which limits the distribution and propagation of cracks. The change of the failure modes with strain rate under tri-axial loading is also less prominent compared to that under uni-axial loading. Based on the numerical results, an empirical relation is proposed to modify the DIF obtained from uni-axial impact loads to that under tri-axial dynamic loads for concrete material. The proposed empirical formulas can be used to more accurately model the dynamic strength increment of concrete material under tri-axial compressive loads.

Multiaxial fatigue behavior and life prediction of 2198-T8 Al-Li alloy notched tubular specimen based on DIC technique

2198-T8 aluminum-lithium alloy is an advanced aeronautical engineering material. The multiaxial fatigue tests of notched specimens are conducted with the variables of notch diameter, notch number, and equivalent stress amplitude, respectively. The strain field and fatigue life are characterized and calculated by digital image correlation (DIC) technique. The results show that: the fatigue life decreases with the increase of notch diameter, number of notches, and the equivalent stress amplitude. The strain curves calculated by VIC-3D are similar to those obtained by the testing. The normal and shear strain concentration becomes obvious at 80% to 90% fatigue life, and the surface cracks appear at 95% fatigue life, typical multiaxial fatigue morphologies including tire patterns and wear fatigue striations are observed on the fracture surface. The feasibility of the DIC technique to characterize multiaxial fatigue behavior is verified. The modified SWT model provides fine life prediction results with all predicted data within a factor of two.

Experimental Research on Manson-Coffin Curves for the Frame Material of an Unconventional Vehicle.

The submitted research paper describes the fundamental findings in terms of multiaxial fatigue of the basic material EN AW6063 and its welds for implementation in the frame design of an unconventional vehicle. It also includes a briefly-presented conceptual design of a technical solution for optimizing the functionality of a steering mechanism in a patented unconventional vehicle, designed by the authors to increase the cornering stability of a vehicle–tricycle. The most important part of this article is the description of the ongoing research and the results of multiaxial fatigue (bending–torsion combination) of the structural material for the construction of the vehicle frame. The research in this area is important due to the increased load on the frame during operation caused by the unconventional steering mechanism. The measured and constructed Manson–Coffin curves indicate that the use of material EN AW6063 is possible for this vehicle in terms of multiaxial stress. This also applies to the material affected by the technology in the frame production (TIG welding). A higher fatigue of the basic material was observed at a 90° phase shift. The difference between the 0° and 90° phases practically makes up approximately 10 to 15% of the difference in the cycle numbers with the same deformation amplitude. At the same time, the measured results show that the phase shift between loads will not play such an important role in welded joints of aluminum alloy EN AW6063. When comparing the dependences with a constant deformation amplitude in bending and in torsion, it can be said that the bending stress will react more to even a small change in the deformation amplitude. Bending has been proven to be the more dominant component of the total deformation amplitude in multiaxial tests. In terms of low-cycle multiaxial fatigue (up to 5 × 105 cycles), a higher fatigue of the basic material is found in comparison with the weld. At lower deformation amplitudes, a higher fatigue of the welded material is detected.

Effects of Temperature and Strain Rate on the Ductility of an API X65 Grade Steel

In the last few decades, great effort has been spent on advanced material testing and the development of damage models intended to estimate the ductility and fracture of ductile metals. While most studies focused on static testing are applied at room temperatures only, in this paper, multiaxial tests have been executed to investigate the effects of dynamic action and temperature on the mechanical and fracture behavior of an API X65 steel. To this end, a Split Hopkinson Bar (SHB) facility for dynamic tests, and a uniaxial testing machine equipped with a high-temperature furnace, were used. Numerical simulations of the experiments were setup for calibration and validation purposes. Based on the experimental results, the Johnson–Cook and Zerilli–Armstrong plasticity models were first tuned, resulting in a good experimental–numerical match. Secondly, the triaxiality and Lode angle dependent damage models proposed by Bai–Wierzbicki and Coppola–Cortese were also calibrated. The comparison of the fracture surfaces predicted by the damage models under different loading conditions showed, as expected, an overall significant increase in ductility with temperature; an appreciable increase in ductility was also observed with the increase in strain rate, in the range of low and moderate triaxialities.

Steel-reinforced resin for bolted shear connectors: Confined behaviour under quasi-static cyclic loading

Injections bolts were initially conceptualized to repair riveted connections in bridges with long service lifetime and now they are widely used in steel and composite structures. Injecting steel-reinforced epoxy resin in bolted connections with oversized holes is a novel approach to enable fast and easy assembly of composite structures while limiting slip between the components. The performance of these bolted shear connectors is mainly dependent on the injected material. In this paper, short-term mechanical properties of an epoxy-based resin mixed with steel shots (steel-reinforced resin) are evaluated to consider it for bolted shear connectors. Uniaxial static tests allowed to compare the stiffness and ductility of resin and steel-reinforced resin and to define a hardening law, while multiaxial tests were conducted under quasi-static cyclic loading in order to characterize the material behaviour under confined conditions. Steel-reinforced resin specimens showed a stiffness under confined conditions 2.6 times higher compared to the bare resin specimens. This increase of stiffness is fundamental to achieve slip resistant behaviour when resin-injected shear connectors are used in significantly oversized holes. The linear Drucker-Prager plastic model was used to define the material behaviour. The good agreement observed indicates that future investigations on numerical and experimental performance of bolted shear connectors can be implemented with the parameters proposed in this paper.

Design of Triaxial Tests with Polymer Matrix Composites.

Multiaxial testing in composites may generate failure modes which are more representative of what occurs in a real structure submitted to complex loading conditions. However, some of its main handicaps include the need for special facilities, the correct design of the experiments, and the challenging interpretation of the results. The framework of this research is based on a triaxial testing machine with six actuators which is able to apply simultaneous and synchronized axial loads in the three space directions. Then, the aim was to design from a numerical point of view a triaxial experiment adapted to this equipment. The methodology proposed could allow for an adequate characterization of the triaxial response of a polymer-based composite with apparent isotropic behaviour in the testing directions. The finite element method (FEM) is applied in order to define the geometry of the triaxial specimen. The design pursues to achieve homogeneous stress and strain states in the triaxially loaded region, which should be accessible for direct measurement of the strains. Moreover, a fixing system is proposed for experimentally reproducing the desired boundary conditions imposed on the numerical simulations. The procedure to determine the full strain tensor in the triaxially loaded region is described analytically and with the help of FEM virtual testing. The hydrostatic component and the deviatoric part of the strain tensor are proposed for estimating the susceptibility of the polymer-based composite to fail due to the triaxial strain state imposed. Then, the loading scenarios that cause higher values of the deviatoric components in the triaxially loaded region are considered to be more prone to damage the region of interest. Nevertheless, the experimental failure is expected to be produced in the arms of the specimen which are uniaxially loaded, since in all of the loading cases the simulations show higher levels of stress concentration out of the triaxially loaded region. Thus, although the triaxial strength could not be accurately determined by the proposed tests, they can be utilized for observing the triaxial response before failure.

A comparative study on energy-based fatigue criteria for lifetime calculation of magnetorheological elastomers under multiaxial loading

Predicting fatigue life of magnetorheological elastomers (MREs) containing, different volumes of carbonyl iron particles (CIPs) with various external magnetic fields has been investigated. Unlike many studies on fatigue behavior of MREs, representing experimental models, a novel three-dimensional constitutive magneto-hyper-viscoelastic semi-coupling model based on strain energy density function (3DSED) is derived, analyzing the fatigue of structures enhanced by MRE experimentally and numerically. In this 3DSED model, to obtain the best predicting MREs’ fatigue life, three different hyper-elastic functions, including Neo-Hookean, Mooney-Rivlin, and Yeoh are utilized. Using some uniaxial compression, such as loading phase, relaxation, and loading/unloading phases tests, hyper-elastic, magneto-elastic, and viscoelastic constants are determined. In addition, multi-axial fatigue tests are performed to define the number of cycles to failure. These tests have been induced by the magnetic fields of 0, 0.27, and 0.4 T. The fatigue tests re-performed with the magnetic field of 0.3 T to compare the predicted results using the proposed fatigue models. The results show that the 3DSED model can be used for the prediction of MREs fatigue life when subjected to complex and repetitive loading. For all CIP contents, the Mooney-Rivlin fatigue predictor has more accurate results. The Yeoh fatigue predictor indicates better results for MREs with higher CIP contents, while the Neo-Hookean fatigue predictor doesn’t show acceptable results for any CIP contents.

Multiaxial fatigue behaviour and damage mechanisms of P92 steel under various strain amplitudes and strain ratios at high temperature

High temperature multiaxial fatigue tests at different strain amplitudes and strain ratios were performed at 600 °C. Dynamic strain ageing (DSA) analysis and microstructure characterization were used to analyze the cyclic deformation and damage mechanisms. Results reveal that the increases in strain amplitude and strain ratio accelerate the cyclic softening. Moreover, the DSA behaviour shows a distinct dependence on the strain amplitude, strain ratio and loading direction. Furthermore, the increase in strain amplitude promotes subgrains growth and dislocation structure transformation. Particularly, the increase in strain ratio promotes the occurrence of planar slip of dislocation.

In situ tomographic study of a 3D-woven SiC/SiC composite part subjected to severe thermo-mechanical loads

A high-temperature multi-axial test is carried out to characterize the thermo-mechanical behaviour of a 3D-woven SiC/SiC composite aeronautical part under loads representative of operating conditions. The sample is L-shaped and cut out from the part. It is subjected to severe thermal gradients and a superimposed mechanical load that progressively increases up to the first damage. The sample shape and its associated microstructure, the heterogeneity of the stress field and the limited accessibility to regions susceptible to damage require non-contact imaging modalities. An in situ experiment, conducted with a dedicated testing machine at the SOLEIL synchrotron facility, provides the sample microstructure from computed micro-tomographic imaging and thermal loads from infrared thermography. Experimental constraints lead to non-ideal acquisition conditions for both measurement modalities. This article details the procedure of correcting artefacts to use the volumes for quantitative exploitation (i.e. full-field measurement, model validation and identification). After proper processing, despite its complexity, the in situ experiment provides high-quality data about a part under realistic operating conditions. The influence of the mesostructure on fracture phenomena can be inferred from the tomography in the damaged state. Experiments show that the localization of damage initiation is driven by the geometry, while the woven structure moderates the crack propagation. This study widens the scope of in situ thermo-mechanical experiments to more complex loading states, closer to in-service conditions.

Micromechanics-based modeling of plastic and ductile fracture of aluminum alloy 2024-O

In this study, the plastic deformation behavior at a large strain range, as well as the ductile damage and fracture of aluminum alloy 2024-O materials, were investigated under quasi-static conditions. The plasticity of the material, including a large deformation, was characterized by conducting uniaxial tension and balanced-biaxial bulging tension tests. This condition was reproduced using the Hershey–Hosford nonquadratic isotropic plastic yielding criterion and combined Hollomon/Voce strain-hardening model. The ductile damage and fracture were analyzed using notched plane strain and in-plane shear experiments by incorporating a wide range of stress states. Finally, multiaxial Nakajima tests with a round sphere punch were conducted to assess the material formability. Simultaneously, finite element predictions of the ductile fracture behavior were performed using micromechanics-based material models, such as modified Gurson–Tvergaard–Needleman (GTN) and Hosford − Coulomb models. The force–displacement characteristics and surface strain distributions measured using the experimental approach were compared with the simulated results. The results indicated that the modified GTN model provided better prediction of plasticity and ductile damage behavior of aluminum alloy 2024-O materials.

Environmental Fatigue Analysis of nuclear components within the framework of INCEFA-SCALE project

INCENFA-SCALE is a five-year project funded by the EC Horizon2020 programme, successor of the INCEFA-PLUS project. INCEFA-SCALE started in October 2020. The objective of this project is to improve the capacity to predict the lifetime of Nuclear Power Plant (NPP) components subjected to environmental assisted fatigue. The project starts off by analysing the existing data and then provides new environmentally assisted fatigue data which allow the laboratory test outcomes to be applied to components with real geometries and loads. So far, the data mining of different finished projects (INCEFA-PLUS, USNRC, EPRI, MHI and AdFaM) has been carried out, and test conditions for filling the knowledge gaps have been established. Moreover, the test matrix for 2022 has been defined. In this first phase, tests are focused on producing reference data, analysing complex waveforms (variable amplitude) and the effect of the surface finish. The next testing phases will focus on particular conditions: multi-axial tests, notches, stress/strain gradient effect and size effect. Furthermore, the microstructural analysis of common materials and a guideline for fatigue striation measurement on the fracture surface have been developed. This article provides an update on the project status and the advances made in data analysis, mechanical understanding and testing conditions.

Assessment of multiaxial fatigue of crankshafts subjected to both designed and theoretically critical loadings

Fatigue failures of motor crankshafts operating in thermoelectric power plants are recently being reported. Since crankshafts are subjected to non-trivial stress states throughout their operation, multiaxial fatigue theory is required. A previously carried-out finite element analysis (FEM) that determined the stress states of critical points within the crankshaft’s was used as input. Biaxial stress states were extracted from the FEM analysis and submitted to multiaxial fatigue testing. The fatigue behaviour of the given loading conditions was assessed by using seven multiaxial high-cycle fatigue criteria, namely Findley (F), Matake (M), McDiarmid (McD), Susmel & Lazzarin (S&L), Carpinteri & Spagnoli (C&S), Liu & Mahadevan (L&M) and Papadopoulos (P). An additional set of critical loading conditions was also considered and submitted to both fatigue testing and theoretical predictions. By using experimentally observed fatigue resistance limits, the predictions deliver a slight tendency to fatigue failure in their prediction, in contrast to experimental observations. The results were used to compare the prediction capability of the selected criteria, where all presented reasonably fair agreement. Within the present study’s context, Papadopoulos’ model was the one that presented the least spread in fatigue predictions while also presenting the closest to ideal average behaviour, with the convenient feature of being slightly conservative.

A direct methodology for the calibration of ductile damage models from a simple multiaxial test

In the present work a straightforward calibration procedure of ductile damage models is proposed. The direct methodology involves the use of a simple multiaxial specimen, to be tested with a universal testing machine, capable to reproduce different stress states in the material. The specimen geometry was the one proposed by Driemeier et al. [1]. In addition, a numerical-analytical procedure was devised for the identification of material strains to fracture and corresponding stress states, directly from experimental tests. This allowed to overcome the use of Finite Element Analysis and inverse methods usually adopted to retrieve the local parameters representative of the material ductility.

Multi-axial cyclic loading tests for RC shear walls of nuclear power plant structures

Reinforced concrete (RC) walls have been widely studied through experimental testing. A great number of these existing studies considered only the in-plane loading cases, ignoring the out-of-plane loading effect on the in-plane structural response, although earthquake ground motions have multi-axis loading components. Some recent studies addressed the bidirectional loading effect on RC walls, and reported that the out-of-plane loading lowers the displacement capacity of RC walls. However, these studies are for flexural response of RC walls, and little experimental data for shear response of RC walls under bidirectional lateral loading is available. In this study, a series of cyclic loading tests were conducted to investigate the effect of multi-axial loading on the in-plane shear response of RC walls. A prototype nuclear power plant structure was selected, and a part of the prototype RC wall was manufactured as a test specimen, which was designed to have shear failure. Out of the five wall specimens manufactured for this study, two specimens were subjected to the same unidirectional lateral loading with different axial forces to investigate the effect of axial force on the wall response. Remaining three specimens were subjected to bidirectional lateral loading with different loading protocols. It was found that both out-of-plane loading and axial force can substantially impact on the in-plane shear strength and displacement capacity of RC walls.

Multiaxial plain and notch fatigue strength of thick-walled ductile cast iron EN-GJS-600-3: Combining multiaxial fatigue criteria, theory of critical distances, and defect sensitivity

Multiaxial fatigue tests were carried out on plain and notched specimens extracted from EN-GJS-600-3 pearlitic DCI heavy-section castings. Two notches of different severities were combined for the determination of mode I and mode III critical distances, along with the related intrinsic fatigue strengths. SEM and CT clearly showed that fatigue cracks initiated from graphite nodules and shrinkage pores in notched and plain specimens, respectively. Different multiaxial criteria were compared, and best predictions were obtained with the Carpinteri et al. criterion with four parameters: axial and torsional intrinsic fatigue strengths, combined with the two different related critical distances.

Crack paths in multiaxial fatigue of C45 steel specimens and correlation of lifetime with the thermal energy dissipation

The work reports the observed fatigue damage of C45 steel specimens tested in a previous work under multiaxial loading conditions and its relationship with the thermal energy dissipation which has been used in the last decades to estimate the uniaxial fatigue behavior of metals. For this purpose, fatigue data relevant to thin-walled samples made of quenched and tempered C45 steel tested under completely reversed combined axial and torsional cyclic loadings with different biaxiality ratios and phase-shift angles have been analysed. The analyses of crack paths at the initiation point of failure were performed after a 50% stiffness loss that corresponded to a crack size ranging from 7 to 15 mm; afterwards, the characteristic crack paths of each loading condition were analysed by using a digital microscope to identify the orientation of the crack initiation plane. After having broken all fatigue tested specimens under static tensile loading, the fracture surfaces were inspected close to the crack initiation point using a digital microscope. Despite the stress states and fatigue damage mechanisms dependent on the load condition, the Q parameter applied to the present experimental results proved to correlate all multiaxial fatigue test results in a single fatigue scatter band.