Multiaxial Tests Research Articles

Analyses of drives power reduction techniques for multi-axis random vibration control tests

Abstract In multi-axis vibration control testing, the power required by the excitation system for replicating the user defined test specifications is a limiting factor which cannot be overlooked. Excessive power, on top of over-stressing the often expensive test equipment, could cause data acquisition overloads which inevitably interrupt the test even before the full level run. An accurate definition of the Multi-Input Multi-Output (MIMO) control target allows to perform the control test minimising the overall power required by the shakers. In this sense, in the recent years advanced procedures have been developed in this research direction. This paper analyses the available drives power reduction techniques, offering a detailed overview of the current state-of-the-art. Furthermore, this paper provides a novel solution to manage the cases where most of the power is required by a single drive of the multiple inputs excitation system. In order to point out the pros and cons of each procedure and to show the capabilities of the novel technique, the MIMO target generation algorithms are firstly theoretically explained and then experimentally compared by using a three-axial electrodynamic shaker.

Multiscale modeling of copper and copper/nickel nanofoams under compression

Pure metal nanofoams in the form of interconnected ligament networks have shown strong potential over the last few years in areas such as catalysts, batteries, and optics. However, they are often fragile and therefore difficult to integrate into engineering applications. For these reasons, a new class of materials, composite metallic nanofoams made of ligaments coated with thin metallic layers, have been proposed to solve these issue. The mechanical properties of these nanofoams depend on their relative density and their internal geometrical structure. In this work, we combine molecular dynamics (MD) and finite element method (FEM) to investigate the compressive behavior of nanofoams made of copper ligaments coated with nickel. We also investigate how this behavior relates to their microstructures. For that purpose, we built different types of representative cell structures, and atomistic simulations of multiaxial compression tests were performed to produce yield surfaces. Then the generated yield surfaces were curve fitted into a normalized model to obtain the shape parameters. Last, a plasticity model was introduced to study and compare their mechanical behavior under compression. The results suggest that the coating of the ligaments can improve the mechanical behavior of the nanofoams. They also reveal the importance and the limitations of the microstructural geometry on the strengthening of these structures. These findings can be used for the design of metallic nanofoams with tailored mechanical properties.

Uncemented megaprosthesis stem fixation using “Scratch Fit” to achieve improved implant fixation

Background: Despite a proposed lower incidence of aseptic loosening, uncemented distal femoral implants present challenges with the intra-operative assessment of the adequacy of stem fixation within the femoral canal. Our biomechanical study was intended to assess the adequacy of oncologic implant press-fit stem fixation within the reamed canal of the distal femur for the Stryker GMRS oncologic distal femoral implant. We hoped to answer the following: #1 Does the initial stem placement in the femoral canal using a standardized force (50 N) (i.e., “Scratch Fit”) predict the (adequacy)stability of the final stem placement (and implant fixation) within the femoral canal? #2 Is there a difference in the uncemented stem fixation and stability within the femur for different stem (Stryker GMRS) diameters (13, 14, and 15 mm) for the Stryker GMRS uncemented press-fit stems/(Stryker Global Orthopaedics) Methods: Femoral cadaveric specimens were thawed and cut at the distal end of the femur, at 13 cm from the distal joint line, to e represent a distal femoral tumor resection. Stryker GMRS uncemented stems were placed, after femoral reaming, into the distal femoral canal with firm, hand pressure applied via a customized, spring-based insertion tool positioned over the standard Stryker insertion tool and calibrated to apply a standard stem insertion force of 50 N (11.2 lbs). Initial stem placement, utilizing this method, resulted in a stem that was only partially implanted into the femur with a recorded distance (defined as “Scratch Fit”) between the stem collar and the cut surface of the femoral shaft. After completing final stem impaction into the femur, stem torsional testing was performed on a multi-axis biomechanical test frame with a 3-D Vicon motion-capture system with axial torsion applied to the stems with the proximal femur fixed to a potted base. Kinematics of both the implant and the distal femur were captured using the Vicon system which tracked reflective infrared targets at a 60 Hz sampling rate. The peak torsional moment at failure was compared to “Scratch Fit” metrics for each implant diameter to address the proposed research questions. Results: Scratch fit distances ranged from 7–46 mm with a mean of 29.1±12.7 mm. Peak torques ranged from 11.5 to 57.5 Nm with a mean of 33.6±17.0 Nm. Figure shows peak (max.) torque plotted against scratch fit for all stems/specimens with good correlation (r 2 =0.6404). When separated by stem diameters, figure shows strong correlations between peak torque and scratch fit. Conclusions: while there may be multiple metrics that affect uncemented stem implant placement and stability (i.e., femoral canal size, femoral reaming, and implant type/size (diameter), there appears to be some correlation between initial stem placement (i.e., “Scratch Fit”) after femoral reaming to implant torsional (rotational) stability; this correlation is stronger when controlled for stem diameter. This suggests that a greater initial “Scratch Fit” distance may provide improved press-fit stem fixation and may provide a better operative standard for making decisions regarding the fixation or stability of these implants.

Prediction methods of fatigue critical point for notched components under multiaxial fatigue loading

AbstractTwo methods based on local stress responses are proposed to locate fatigue critical point of metallic notched components under non‐proportional loading. The points on the notch edge maintain a state of uniaxial stress even when the far‐field fatigue loading is multiaxial. The point bearing the maximum stress amplitude is recognized as fatigue critical point under the condition of non‐mean stress; otherwise, the Goodman's empirical formula is adopted to amend mean stress effect prior to the determination of fatigue critical point. Furthermore, the uniaxial stress state can be treated as a special multiaxial stress state. The Susmel's fatigue damage parameter is employed to evaluate the fatigue damage of these points on the notch edge. Multiaxial fatigue tests on thin‐walled round tube notched specimens made of GH4169 nickel‐base alloy and 2297 aluminium‐lithium alloy are carried out to verify the two methods. The prediction results show that both the stress amplitude method and the Susmel's parameter method can accurately locate the fatigue critical point of metallic notched components under multiaxial fatigue loading.

Development of scanning electron microscope-compatible multiaxial miniature testing system

Knowledge of deformation and failure mechanisms at micro- to nano-length scales is important for the prediction of material behavior as well as the development of new materials with desired properties. In situ multiaxial testing with scanning electron microscopes (SEM) can reveal physical deformation mechanisms under realistic multiaxial loading conditions. Although in situ SEM testing has gained traction in recent years, there is currently no multiaxial in situ SEM testing stage available with axial-torsional loading capabilities which can generally be used in any SEM. In this study, we report the development of a multiaxial miniature testing system (MMTS) with a unique capability for performing axial-torsional testing of a tubular specimen with a 1–2 mm outer diameter, inside most SEMs. The different challenges of developing a multiaxial in situ SEM testing stage, such as small load frame size, appropriate specimen position, high vacuum compatibility of MMTS load frame components, as well as the development of installation accessories, were addressed. A custom SEM stage door was developed for the MMTS load frame. Verification tests have confirmed the successful development of the MMTS for in situ SEM testing. In addition, digital image correlation was used with recorded SEM images during the test to determine the surface strain.

Development of new critical plane model for assessment of fatigue life under multi-axial loading conditions

The present work is aimed at development of new critical plane model based on envelope of strain energy density as fatigue damage parameter. This model eliminates the use of hypotheses for evaluation of resultant shear. The fatigue life has been predicted and compared with test life for a large set of multi-axial test data available on SA 333 Gr. 6, 16MnR, S460N and 7075-T651 Al materials. Predicted fatigue life is in good agreement with test life. The predicted crack initiation orientations are also comparable with the measured angles.The shortcomings of existing models have been brought out.

A Criterion Describing the Dynamic Yield Behavior of PMMA

Dynamic uniaxial and multiaxial tests were conducted to characterize the mechanical behavior of poly(methyl methacrylate) (PMMA). In this case, the multiaxial yield behavior of materials can be captured by the shear-compression samples (SCS). The experimental results were compared with different yield criteria, and which does not have a good agreement between theoretical prediction and experimental yield loci of PMMA undergoing combined shear-compression. Therefore, a phenomenological yield criterion is developed, and it notably involves hydrostatic stress sensitivity, lode angle, and one parameter. The parameter can be expressed by the asymmetry of yield strength between the tension and compression. The proposed criterion was verified by the utilization of differently combined shear-compression tests. The investigative results reveal that the proposed criterion can be successfully applied to describe the complex yield behavior of PMMA under dynamic loading conditions. The present study shows an efficient method to develop yield criterion of the reviewed materials.

Fracture Prediction for Mild Steel Sheet and High-Strength Steel Sheet Subjected to Draw Bending Using Forming Limit Stress Criterion

Abstract A fracture criterion for sheet metal subjected to draw bending is proposed based on the concept of the forming limit stress criterion. An experimental apparatus that is capable of draw bending sheet specimens with forming speeds (approximately 100 mm·s−1) comparable to those for commercial press forming machines was designed and built. The test materials were an ultralow-carbon steel sheet and a high-strength (590 MPa) steel sheet. Specimens underwent bending and unbending under tension when passing over a die. The magnitude of the true stress, σ D B , at which the stretch-drawn specimens fractured was precisely determined from the measured drawing force and the cross-sectional area of the specimen after fracture. Moreover, multiaxial tube expansion tests were performed to measure the forming limit stress, σ P T , for the test materials under plane-strain tension. It is found that σ D B is consistent with σ P T when the strain rate is properly taken into account in evaluating σ P T .

Collection of experimental data for multiaxial fatigue criteria verification

AbstractThe present paper aims to propose data from incisive multiaxial fatigue experimental tests which results from the analysis of several works within current literature. Approximately 200 experimental datasets have been collected and paired with the widest variety of materials and stress combinations. Furthermore, we carried out a critical analysis of the methods with which said datasets were obtained, also focusing on the kind of tests that were conducted, the type of specimens used, modes of data cataloging, and some other aspects which we will further analyze during the discussion. We will only refer to data which come from primary sources, avoiding data transcription errors. We thus created a unified data collection useful for all multiaxial reduction methods verification for ductile materials.

High-pressure compressibility and shear strength data for soils

Soil behaviour is often an important consideration in the design of protective systems for blast and impact threats, as the properties of a soil can greatly affect the impulse generated from buried explosive devices, or the ability of a soil-filled structure to resist ballistic threats. Numerical modelling of these events often relies on extrapolation from low-pressure experiments. To develop soil models that remain accurate at very high pressures there is a need for data on soil behaviour under these extreme conditions. This paper demonstrates the use of a high-pressure multi-axial test apparatus to provide compressibility and shear strength data for four dry sandy soils. One-dimensional compression experiments were performed to axial stresses of 800 MPa, where the effects of particle-size distribution were observed with respect to compressibility and bulk unloading modulus. Each soil followed a bilinear normal compression line (NCL): more uniform soils initially had higher compression indices, but all four NCLs began to converge at void ratios below e ≈ 0.3. The failure surface of a sand was characterized to mean effective stress [Formula: see text] > 400 MPa using reduced triaxial compression experiments, removing the need to rely on extrapolation from low-pressure data.

Biaxial mechanics of 3D fiber deposited ply-laminate scaffolds for soft tissue engineering part I: Experimental evaluation

Tissue engineering strategies require the provision of a micromechanical state of stress that is conducive to the generation and maintenance of healthy mature tissue. Of particular interest, angle-ply biomimetic scaffolds augmented with cellular content have been proposed for annulus fibrosus (AF) engineering in order to repair the intervertebral disc. However, the influence of the inherent variability of fabricated constructs and physiological conditions on overall scaffold mechanics, micromechanical environment within the scaffold, and consequent cellular differentiation is relatively unknown. In this study, melt extrusion 3D fiber-deposition (3DF) was used to fabricate five different polycaprolactone angle-ply scaffold architectures which were subject to multiaxial tensile testing and linear elastic orthotropic constitutive fitting. All scaffold groups predicted stiffnesses similar to previously reported native AF moduli in biaxial and uniaxial tensile strain. However, no single scaffold group in this study simultaneously achieved all target AF mechanics in all loading regimes. In equibiaxial tension, the biaxial stiffness ratio of native AF (EEr = 0.55 to 0.62) was predicted between fiber angles of 30° and 35°, which is similar to the collagen orientation in native AF. In global equibiaxial loading, an apparent asymptote in the transverse moduli (EEx ranging −380 MPa to 700 MPa) was observed near the 40° fiber angle scaffolds in equibiaxial tensile strain, attributed to stiffening from the transverse loading. These results highlight that tissue engineering scaffold designs should target replication of physiologically-relevant native tissue mechanics and demonstrate the importance of designing constructs that are unaffected by anticipated variations in manufacturing and clinical application.

Investigation of fretting fatigue behavior and micro-structure evolution in LZ50 steel subjected to torsional load

In this paper, torsional fretting fatigue in LZ50 steel was investigated experimentally using a multiaxial fatigue testing machine. After subjecting the samples to fatigue, they were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The details of the fretting damage and the evolution of the samples’ microstructures in the contact zone were studied under different torsional loads. The results showed how the combined action of fretting and fatigue led to the failure of the specimen. Fretting wear in the contact zone produced a fretting scar and cracks in the specimens initiated in the fretting contact zone. These were caused by the local stress concentrations, and the cracks then propagated due to cyclical torsional fatigue stresses. The modes of microstructural evolution occurring during torsional fretting fatigue were dislocation angling and climbing, and these formed dislocation cells in the structure of the material. Within the LZ50 steel, this pattern of behavior began in the ferrite areas of the structure. For the harder phases, the evolution started later, and in the pearlite structures. The microstructural evolution in the pearlite was accompanied by bending, torsion, and even cracking of the cementite lamellae. The evolution of the microstructures showed that the micro-cracking nucleation mechanism was the deformation of dislocation cells.

Multiaxial failure surface of PVC foams and monitoring of deformation bands by three-dimensional digital image correlation

Closed-cell polyvinylchlorid foams are among the most used core materials for sandwich composites. Therefore, a deep knowledge of their multiaxial behaviour is fundamental. More precisely, a literature review proves the lack of investigations concerning combined tensile-torsional loadings. In the present work simple tests, namely tension, compression and torsion have been performed for determining failure loads. Afterwards, multiaxial tests have been executed by applying different percentages of the previously calculated failure loads as preload and subsequent superposition of torsion, keeping simultaneously the axial load constant. For each multiaxial test, failure data have been collected in order to construct the failure surface for the foam material. The whole specimen surface has been inspected by means of the three-dimensional digital image correlation, in order to detect the failure of the specimen, as well as the onset and the development of deformation bands. Monitoring of the specimen has been performed for each main loading case. Afterwards, the analysis of deformation bands has been coupled with stress-strain diagrams and linked with the arising of buckling or fracture and the instrinsic orthotropy of the foam.

Multiaxial fatigue of extruded ZK60 magnesium alloy

AbstractMultiaxial monotonic and cyclic behaviors of ZK60‐T5 magnesium extrusion are investigated. Strain‐controlled tests were performed at standard laboratory condition with fully reversed straining. Twinning‐detwinning deformation plays an important role in the cyclic axial behavior for tests that were performed under strain amplitudes higher than 0.4%. However, the hysteresis loop for the 0.4% was found symmetric and no sign of twinning‐detwinning deformation was observed. On the contrary, the cyclic shear behavior was found to be similar to conventional alloys and no significant asymmetric or twinning‐detwinning deformations were observed. The multiaxial fatigue tests suggest that multiaxiality and nonproportionality are not detrimental to fatigue life. Three multiaxial fatigue damage models were used: Smith‐Watson‐Topper, Fatemi‐Socie, and Jahed‐Varvani. While Fatemi‐Socie and Jahed‐Varvani models show comparable estimation, Smith‐Watson‐Topper overestimates shear and nonproportional lives.

A Planar Biaxial Experiment Platform for In Situ High-Energy Diffraction Studies

An experimental platform for multiscale studies of materials subjected to plane stress loads is presented. Coupling with far-field high-energy diffraction microscopy for grain-by-grain measurements of elastic strains and rotations provides an additional benefit; it enables the direct assessment of elastic vs. inelastic deformation of the gauge sections of cruciform specimens subjected to plane stress loadings, without any a priori assumptions of the form of constitutive relationships, resolving a long-outstanding challenge of multiaxial mechanical testing. Specifically, a planar biaxial mechanical load frame with four independent hydraulic actuators capable of applying arbitrary loading paths and ratios of tension and compression was designed and built for in situ diffraction experimentation. The load frame is integrated for use at the Argonne National Laboratory Advanced Photon Source (APS) synchrotron, Sector 1, 1-ID-E endstation and the Los Alamos Neutron Science Center (LANSCE) spallation neutron source, Spectrometer for Materials Research at Temperature and Stress (SMARTS) instrument. Cruciform specimen geometries were designed to experience loading ratios in the gauges commensurate with those applied at the grips, and to minimize interference with diffracted X-rays and neutrons. The finite element models used to design the cruciform specimen geometries were experimentally validated using stereo digital image correlation measurements. This complete planar biaxial in situ diffraction platform provides a new capability for studying multiaxial micromechanics of crystalline materials (e.g., elastic, slip, twinning, phase transformation) and their dependencies on grain size, location, texture, and neighborhood characteristics.

Generalized Stress\u2013Strain Curves for IBII Tests on Isotropic and Orthotropic Materials

This article presents a particular use of the Virtual Fields Method to exploit the results of Image-Based Inertial Impact (IBII) tests. This test consists on an edge-on impact of a free-standing thin flat rectangular coupon. The specimen response is recorded using an ultra-high speed camera filming the deformation of a grid pattern printed at its surface. From these images, displacement fields are derived, from which strain and acceleration can be obtained. The Virtual Fields Method makes use of the acceleration fields to derive stress information. Until now, a very simple ‘stress-gauge’ approach was used that could only provide relevant stress-strain information if the test was predominantly uniaxial. The alternative was to use the full inverse approach with the Virtual Fields Method but this would not allow the same degree of data understanding as the ‘stress-gauge’ approach. This article proposes an extension to this ‘stress-gauge’ approach for fully multiaxial tests. The equations are first derived and then validated using simulated and experimental IBII test data on isotropic and orthotropic materials.

Multi-axial test and failure criterion analysis on self-compacting lightweight aggregate concrete

Abstract In order to study the mechanical properties of self-compacting lightweight aggregate concrete under complex stress, experimental research on multi-axial stress behavior of self-compacting lightweight aggregate concrete were conducted by using a large general triaxial test machine. Through multi-axial tests, failure morphology and stress-strain curves of self-compacting lightweight aggregate concrete under different lateral stresses were obtained. By extracting the stress-strain curve eigenvalues (peak stress and peak strain) and analyzing the failure morphology, and citing the relevant literature to compare and analyze the conclusions of multi-axial research on ordinary concrete and lightweight aggregate concrete, the multi-axial stress characteristics and failure mechanism of self-compacting lightweight aggregate concrete were studied. The results show that when the uniaxial compression and lateral stress values of self-compacting lightweight aggregate concrete are small, the biaxial compression-pressure failure morphology mainly exhibits the shear failure morphology, which is different from that of ordinary concrete and lightweight aggregate concrete. At the same time, the influence of lateral stress on self-compacting lightweight aggregate concrete is obviously stronger than that of ordinary concrete and lightweight aggregate concrete. The failure criterion of self-compacting lightweight aggregate concrete based on Kupfer biaxial failure criterion and octahedral stress space has certain applicability. The content studied in this paper has theoretical significance for the engineering application development of self-compacting lightweight aggregate concrete.

The Dynamic Mechanics Behavior on Triaxial Compression of the Recycled Aggregate Concrete

The tests under triaxial compression states were conducted on recycled aggregate concrete (RAC) by different stress ratios (-0.1: -0.1: -1; -0.1: -0.25: -1; -0.1: -0.5: -1; -0.1: -0.75: -1; -0.1: -1: -1) at the strain rates of 10-5/s, 10-4/s, 10-3/s, and 10-2/s. This study was accomplished in the servo-hydraulic multi-axial testing system. The experimental phenomena and failure modes of recycled aggregate concrete under triaxial compression are described, the results of triaxial compressive strength of recycled aggregate concrete are presented, the effects of the strain rate and stress ratio on the triaxial compressive strength of recycled aggregate concrete are studied. This paper built a new dynamic strength criterion. The strength criterion describes the characteristics of structural RAC at different stress ratios and strain rates. The feature of the model is that it can consider the influence of strain rates.

Investigation of Vibration Characteristics of Titanium Wide-Chord Fan Blade

The vibration characteristics of a wide-chord fan blade affect the safety, reliability, and operability of high-bypass ratio turbofan engines. In this study, the vibration characteristics of a fan blade were investigated using numerical and experimental methods. First, the natural frequencies and mode shapes were obtained and verified. Then, the rotating speed and frequency margins were employed to analyze the stability of the blade. The critical crossings in the Campbell diagram were indicated and evaluated. Next, the strain energy density index and weaklink were used to evaluate the dangerous locations for the high-cycle fatigue characteristics of the blade. Finally, single- and multi-axis random-excitation tests were performed. Results indicated that low-order bending modes were dominant under circumferential- and axial-direction excitations, while the torsional mode response was higher under radial-direction excitation. The weaklinks for different excitation directions exhibited different characteristics. This work presents a comprehensive understanding of the fan-blade vibration and response characteristics.

Experimental and FEM numerical assessment of multiaxial fatigue failure criteria for a rolling Stock's seats structure

This paper deals with the experimental and numerical investigations of the structural behaviour of a rolling stock's seats system under static and fatigue loading conditions, aimed to assess the reliability of some multiaxial fatigue failure criteria implemented in the FE model. According to the NF F31–119 standard, a specific full-scale multiaxial testing machine has been used for the fatigue test. During the test, a visual inspection approach with a detection interval of 3·105 cycles has been adopted. After 1.2·106 cycles the test has been stopped and a Liquid Penetrant Inspection (LPI) has been carried out in order to detect accurately cracks nucleation and propagation. Moreover, numerical analyses, based on the Finite Elements (FE) Method, have been performed on the EN-AW 6060 T5 aluminium alloy structure of a rolling stock's seats to predict the most affected zones where the cracks nucleation may occur under multiaxial fatigue loads. Since the in-service applied loads belong to Multiaxial High-Cycle Fatigue (MHCF) load class, Sines, Crossland and Dang Van criteria have been used to post-process the predicted results achieved by the FE analyses. For validation purpose, numerical and experimental results have been compared. According to the numerical-experimental results comparison, Dang Van criterion provided the best level of accuracy.