Dynamic Strain Research Articles

Low‐Cycle‐Fatigue Performance of Stress‐Aged EN AW‐7075 Alloy

The effect of a novel heat treatment, that is, aging under superimposed external stress, on the fatigue performance and microstructural evolution of a high‐strength aluminum alloy (EN AW‐7075) is presented. Stress aging, a combination of heat treatment and superimposed external stress, can enhance the mechanical properties of EN AW‐7075 under monotonic loading due to the acceleration of precipitation kinetics. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) reveal that a longer aging time and the presence of superimposed stress both promote the formation and growth of precipitates, that is, the precipitation of strengthening η´ precipitates. This is confirmed by differential scanning calorimetry (DSC) heating experiments of stressless and stress‐aged states. Furthermore, stress aging leads to a reduction of dimensions of precipitate‐free zones near grain boundaries. Cyclic deformation responses (CDRs) and half‐life hysteresis loops are evaluated focusing on the low‐cycle fatigue (LCF) performance of different conditions. A noticeable cyclic hardening seen in case of the specimens aged for a short time indicates the occurrence of dynamic strain aging (DSA). Eventually, stress aging allows for an enhancement of the monotonic mechanical properties of EN AW‐7075 without degrading the cyclic performance in the LCF regime.

Dynamic behavior of EPS amended lightweight soil under cyclic loading

The dynamic behavior of expanded polystyrene (EPS) particle amended lightweight soil was explored using consolidated undrained dynamic and monotonic triaxial tests conducted on samples of lightweight soil with different mixing ratios. The dynamic stress, dynamic strain, and dynamic pore pressure time history curves, and shear strength criterion were studied. As a kind of structural soil, the dynamic strain time history curves of lightweight soil could be divided into three stages as the vibration compaction, vibration deformation and vibration failure stages according to the cumulative deformation rate of dynamic strain, but the vibration compaction stage was not obvious. The dynamic strain accumulation for lightweight soil was mainly single amplitude in compressive region. The deformation rate during the vibration failure stage for the lightweight soil was larger than that of the remolded soil owing to its increased stiffness and brittleness with added cementation. Unlike that of the remolded soil, dynamic pore pressures fluctuated markedly in lightweight soil tests. The dynamic pore pressure development of lightweight soil was found to relate to the mixing ratios of its components and the confining pressures. For lightweight soil samples with high EPS particle mixing ratio, the dynamic pore pressures remained in the positive range, increasing unsteadily with some fluctuation. However, for lower EPS content samples in different confining pressures, the dynamic pore pressure showed decreasing trends into the negative range, again changing unsteadily with fluctuation, showing a trend of suction and soil skeletal dilation. Based on analysis of results, it was determined that, general pore pressure, limit equilibrium and yield criterias were not suitable to describe the dynamic behavior of lightweight soil, while the dynamic strain criterion was suitable. Excluding the plastic deformation range, the single amplitude dynamic compression strain of 5% was recommended as the dynamic strain criterion to determine dynamic strength of lightweight soil.

Measurement of Brain Strains in a Goat Head Under Impact Loading

Abstract Measurements of brain deformations under dynamic loading are required to understand the biomechanics of traumatic brain injury (TBI). In this work, we have experimentally measured two-dimensional (2D) brain deformations in a sacrificed goat brain under injurious impact loading. To facilitate imaging, the goat head was dissected along the longitudinal midline. Goat head response was studied for sagittal plane rotation. Full-field, 2D deformations in the midsagittal plane of a goat brain were obtained at spatiotemporal resolutions of ∼1 mm and 0.4 ms, respectively. Results elucidate the dynamic strain evolution. The goat brain underwent large deformation. The strain pattern was heterogeneous. Peak strains in various brain regions were established within ∼20 ms, followed by negligible strain development due to the considerable viscous dissipation. The Cerebellum region experienced the highest strain, followed by cortical and subcortical regions. Strain concentration in the goat brain near the stiff membrane of the tentorium was observed. The strains in a brain simulant of a head surrogate (obtained previously) were also compared against the goat brain response. A response in the brain simulant was comparable to the goat brain in terms of strain pattern, peak strains in various substructures, and strain concentration near the membrane. However, the brain simulant was less dissipative than the goat brain. These results enhance the current understanding of the biomechanics of the brain under dynamic loading.

High-performance distributed dynamic strain sensing by synthesizing φ-OTDR and BOTDR.

We propose and demonstrate a high-performance distributed dynamic absolute strain sensing technique by synthesizing φ-OTDR and BOTDR. The technique synthesizes the relative strain obtained by the φ-OTDR part and the initial strain offset estimated by fitting the relative strain with the absolute strain signal from the BOTDR part. As a result, it provides not only the characteristics of high sensing accuracy and high sampling rate like φ-OTDR, but also the absolute strain measurement and the large sensing dynamic range like BOTDR. The experiment results indicate the proposed technique can realize the distributed dynamic absolute strain sensing with a sensing dynamic range of over 2500 µɛ, a peak-to-peak amplitude of 1165 µɛ, and a wide frequency response range from 0.1 to over 30 Hz over a sensing range of about 1 km.

Influence of infill density on the dynamic behavior of 3D-printed CF-PEKK composites using split Hopkinson’s pressure bars

The dynamic mechanical behavior of 3D-printed CF-PEKK composites has been studied using the split Hopkinson pressure bar (SHPB). Three sets of samples with different infill densities (20, 50, and 100%) were impacted at different impact pressures (1.4, 1.7, 2.0, 2.4 bar) to characterize their dynamic behavior in terms of strain rate, stress, and strain performance. Moreover, the samples’ dynamic response and failure behavior are cross-referenced with each infill density sample's microstructural behavior and physical parameters, which can give an excellent dynamic response under different strain rate ranges. The results showed that dynamic stress and strain behavior are proportional to the infill density with a maximum dynamic stress of approximately 90 MPa with 100% infill density. In addition, the behavior of the transmitted wave during the dynamic impact showed that a sample with less infill density could attenuate/absorb dynamic impact. These results obtained under this impact pressure indicate the high precision and repeatability of the SHPB approach for the tested 3D composites under dynamic loading and prove that the striker bar velocity significantly impacts the amplitudes of different recorded waves to understand the behavior in detail. In addition, high-speed camera images also showed that the 3D-printed composite sample showed that using a higher infill density enhanced both the damage performance and the impact resistance of the three-dimensionally printed composites at each impact pressure because of the minimized intensity and the amount of final macro damage. This study is useful and helpful for design engineers to study the influence of the process parameter of infill on the dynamic behavior of printed composite materials and confirms the growing interest in 3D printing of composite materials to be employed in different engineering applications which are not limited to static or quasi-static loading circumstances but mainly include dynamic loading conditions.

A robust phenomenological modeling framework based on cross-slip propensity factor for capturing the effect of dynamic strain aging on work hardening behavior of an Al-Mg alloy

In the present work, a new modeling framework for analyzing flow behavior of Al-Mg alloys is proposed by considering both direct and indirect effect of solutes on flow stress. While direct effect addresses the increased slip resistance due to dynamic strain aging (DSA), the indirect effect represents the role of solutes on cross-slip probability. This affects the dislocation annihilation propensity and consequently the work hardening behavior. The dislocation density evolution equation is suitably modified by including a factor based on activation volume and DSA-induced strengthening (σdsa) to account for the effect of solutes on dislocation annihilation propensity and develop a new phenomenological modeling framework for predicting flow curves spanning over a wide range of strain rate (0.00002–0.2 s−1). The proposed modeling framework is shown to be performing better than the existing models which consider only the direct effect of solutes on flow stress. Experimental validation of the proposed concept is shown via dislocation density measurements using X-ray line profile analysis, dislocation substructure analysis via transmission electron microscopy and fractography studies. The new modeling framework is also extended to predict flow curves of several Al-Mg alloys with varying Mg concentration to establish the robustness of the proposed concept.

An Analysis of Dynamics of Retaining Wall Supported Embankments: Towards More Sustainable Railway Designs

Retaining walls are structures used to retain earth materials on a slope. Typically, they are designed for static loads, but for highway and railway infrastructures, vehicle-induced dynamic responses are also relevant. Therefore, retaining wall structures are often designed with a factor of safety that is higher than necessary, because it can be challenging to quantify the magnitude of expected dynamic stresses during the design stage. This unnecessary increase in material usage reduces the sustainability of the infrastructures. To improve railway retaining wall sustainability, this paper presents the results from a field monitoring campaign on a heavy-haul rail line with a retaining wall, studying the dynamics induced in response to 30-ton axle load trains running at speeds of between 5 km/h and 100 km/h. The site comprises an earth embankment supported by a gravity retaining wall, with accelerometers on the sleepers, roadbed surface, and retaining wall, velocity sensors on the roadbed, and strain gauges on the rail web to record wheel–rail forces. The vibration intensities collected from various locations are processed to explore the peak particle velocities, maximum transient vibration values, and one-third octave band spectrums. Two transfer functions define the vibration transmission characteristics and attenuation of vibration amplitude along the propagation path. The long-term dynamic stability of the track formation is studied using dynamic shear strain derived from the effective velocity. The peaks of observed contact forces and vibrations are statistically analyzed to assess the impact of train speed on the dynamic behavior of the infrastructure system. Next, a 3D numerical model expresses the maximum stress and displacements on the roadbed surface as a function of train speed. The model evaluates the earth pressures at rest and vehicle-induced additional earth pressures and horizontal wall movement. The investigation provides new insights into the behavior of railway track retaining walls under train loading, and the field data are freely available for other researchers to download. The findings could facilitate the design of more sustainable retaining walls in the future.

Experimental and numerical studies on pulse shaping techniques used in split Hopkinson pressure bar for testing concrete material

Split Hopkinson pressure bar (SHPB) is commonly used to characterise materials under high strain rates. However, conventional SHPB tests on brittle materials have encountered several experimental challenges for high strain rate loading. In relatively brittle materials like concrete, the deformation of the specimen is very small when subjected to impact loading; hence, it is very difficult to obtain the prerequisites of valid SHPB tests such as dynamic equilibrium and constant strain rate in the specimen. To overcome these issues, the current study presents the importance of the pulse shaper approach in SHPB application for the dynamic characterisation of concrete material. Selection of the appropriate dimension of pulse shaper assists in facilitating dynamic stress equilibrium and constant strain rate in the specimen. In the present study, copper pulse shapers are used for the evaluation of concrete under high strain rate loading using an SHPB set-up. Parameters such as the effect of dimensions (diameter and thickness) of pulse shapers on loading pulses, dynamic equilibrium, constant strain rate and material responses are studied. Experimental results reveal the prediction of suitable pulse shapers for 50–200/s strain rates. In addition, numerical simulation is also performed and results are validated with the experimental data.

Experimental study on compressive behavior of PE-ECC under impact load

Compared with polyvinyl alcohol-engineered cementitious composites (PVA-ECC), engineered cementitious composites containing polyethylene fibers (PE-ECC) has captured the attention of researchers because of lower cost. Nevertheless, there is a limited amount of research that focuses on their dynamic behavior. Thus, specimens with different water binder ratios were tested by split Hopkinson pressure bar (SHPB) to unveil the dynamic compressive behavior of PE-ECC. The dynamic behaviors were analyzed based on the tests, including dynamic failure modes, dynamic stress-strain curves, dynamic compressive strength, dynamic peak strain, and dynamic toughness. The result shows the PE-ECC has excellent compressive ductility, and a long post-peak descending part in the stress-strain relations. Furthermore, the sensitivity analysis reveals that the strain rate is the most crucial factor that directly affects the dynamic compressive strength, dynamic peak strain, and dynamic toughness in a monotonic fashion.

Dynamic large strain measurement under high-temperature environment using a modified FBG sensor and plasma surface treatment.

Large strain measurement under high-temperature environment has been a hot but difficult research issue in the fields of measurement and metrology. However, conventional resistive strain gauges are susceptible to electromagnetic interference at high temperature, and typical fiber sensors will be invalid under high-temperature environment or fall off under large strain conditions. In this paper, aiming to achieve effective and precision measurement of large strain under high-temperature environment, a systematic scheme combining a well-designed encapsulation of a fiber Bragg grating (FBG) sensor and a special surface treatment method using plasma is presented. The encapsulation protects the sensor from damage while achieving partial thermal isolation and avoiding shear stress and creep, resulting in higher accuracy. And the plasma surface treatment provides a new bonding solution which can greatly improve the bonding strength and coupling efficiency without damaging the surface structure of the object under test. Suitable adhesive and temperature compensation method are also carefully analyzed. Consequently, large strain measurement up to 1500 µɛ under high-temperature (1000°C) environment is experimentally achieved in a cost-effective way.

BJS-05 DIS CLASS A MESHES PERMIT BIOMECHANICALLY STRONG RECONSTRUCTIONS WHICH ARE DURABLE AFTER A 3 YEAR FOLLOW UP

Abstract Aim Hernia meshes slip easily upon cyclic load (dynamic intermittend strain, DIS). Meshes were classified and those with the best adhesiveness were named DIS class A. How durable are reconstructions with these meshes after 3 years? Material and methods The adhesiveness of the meshes was analyzed with cyclic load on a bench test. The best-gripping meshes were clinically applied. All patients were followed in yearly intervals up to three years. Results A total of 166 patients (85 males, 85 ASA III/IV, 179 risk factors, 46 recurrent) were included. Progrip® was used in 60 cases (P), Dahlhausen® Cicat in 106 cases (C). Smaller hernias were augmented with P needing less overlap due to the higher tackiness (median minimal overlap P = 4.5, C = 6 cm). Hernia sizes treated with C were significantly larger (median P = 20, median C = 143 cm2; u-test: p < 0.001). The mesh-defect area ratio was higher with P compared with C (13.2 vs 6.3). Fixation requirements were lower with 8 points for P and 81 points for C. GRIP was twice that minimally required. Larger hernia took longer OR times (median P: 103 vs. median C: 203 min; p < 0.001). Larger hernias coincided more postoperative minor complications (5–14%, 1 re-operation for seroma). In-hospital stay was comparable with 5–6 days. Pain levels were low in both groups after 3 years (median NAS = 0). No recurrences were detected. Conclusion Incisional hernia can be repaired durably based on biomechanical principles.

Dynamic Disorder, Strain, and Sublimation of Crystalline Caged Hydrocarbons from First Principles

Dynamic Disorder, Strain, and Sublimation of Crystalline Caged Hydrocarbons from First Principles

In situ analysis of the Portevin-Le Chatelier effect from low to high-entropy alloy in equal HfNbTaTiZr system

The HfNbTaTiZr system is designed from low-entropy to high-entropy by unary, binary, ternary, quaternary, and quinary equal-molar element alloys. At first, the effect of mixing entropy on elevated temperature tensile behaviors ranging from 400 °C to 800 °C and strain rate 1 × 10−4 s–1 in body-centered-cubic (BCC) alloys is systematically discussed, including dynamic strain aging, serration behavior and in-situ deformation behavior of Portevin-Le Chatelier (PLC) band analysis. The results show that in BCC high-entropy alloys (HEAs), with the increase of the alloy elements, the range of the transition temperature of PLC bands expands, and the degree of dynamic strain aging at high temperature also enhances. The theoretical model was verified with the critical strain of unary, binary, and ternary alloys, and it predicts the PLC effect under the specific temperature and strain rate. Apply Applied to quinary alloys, the difference in serration phenomena between face-centered-cubic (FCC) and BCC HEA are investigated, showing that the high melting point of Nb and Ta elements lead to the slower self-adjusting velocity in HfNbTaTiZr alloy. Digital image correlation analysis is in-situ to analyze different types of PLC bands at various strain rates from 1 × 10–2 to 1 × 10−3 s–1 and elevated temperature from 200 °C to 600 °C. The BCC single-phase HEA reveals the transmission properties of the PLC bands and it is consistent with the serration behavior in the tensile stress-strain curve.

Dual-Mode Stretchable Sensor Array with Integrated Capacitive and Mechanoluminescent Sensor Unit for Static and Dynamic Strain Mapping

Electronic skin (e-skin) has the potential to detect large-scale strain, which is typically achieved by integrating multiple strain sensors into an array. However, the latency and limited resolution of sensing have hindered its large-scale sensing applications. Here, we have developed a high-resolution detection sensing system capable of detecting static and dynamic strain with a simple fabrication process by combining capacitive and mechanoluminescent (ML) sensor units. An elastic polydimethylsiloxane (PDMS) composite film doped with ZnS:Cu and BaTiO3(BT) particles are fabricated as the functional film of the capacitive sensor. In contrast, the transparent electrode was fabricated on the surface of the as-prepared film. By incorporating BT nanoparticles into the elastic substrate, the ML intensity of the ZnS:Cu was improved up to 2.89 times that without BT addition, and the sensitivity of the capacitive sensor was increased as well. The capacitive part of the sensor presented a GF of 0.9 and good stability, while the ML part exhibited excellent performance, making it suitable for both static and dynamic sensing. Furthermore, the strain sensor integrated by 10 × 10 sensing units is demonstrated to detect large-scale strain with high resolution. Moreover, finger joint strain distribution tracking is achieved by attaching the strain sensor unit to the finger joint. With these characteristics, the e-skin may have great potential for bio-motion monitoring and human-computer interaction applications.

Transient Strain Monitoring of Weldments Using Distributed Fiber Optic System

The primary objective of this study was to evaluate the capability of a distributed fiber optic sensor to capture in situ dynamic transient strain formation during and post-weldment on the surface of a steel plate. The study involved a vertical manual weld of a bead on a plate on a 300 mm × 300 mm × 6.35 mm A36 steel plate (European equivalent S235J2; Chinese equivalent Q235B) clamped at the corners. A fiber optic distributed sensor was used to measure the surface total and thermal strains on the welded side of the plate adjacent to the weld path. Experimental results show a complex behavior of strain changes during the welding process and the residual strain formation post-welding. This study aims to document the use of distributed fiber optic sensing techniques in welding applications. Validations of the experimental data were performed using VrWeld, a commercial software framework for computational weld mechanics, and an iPhone FLIR One Pro. thermographic camera. The experimental results demonstrated that although distributed fiber optic sensing based on Rayleigh backscattering is an appropriate and useful technique for total strain measurements, the manufacturing and the materials used for the thermal sensors are critical in obtaining optimal results. Finally, this study highlights the challenges encountered in synchronizing large experimental data sets captured with different instruments with computational welding mechanic (CWM) models.

Experimental Study on the Effect of Limestone Powder Content on the Dynamic and Static Mechanical Properties of Seawater Coral Aggregate Concrete (SCAC).

The development of island construction concrete can serve as a basis for the development and utilization of island resources. Complying with the principle of using local materials to configure seawater coral aggregate concrete (SCAC) that is able to meet the requirements of island and reef engineering construction could effectively shorten the construction period and cost of island and reef engineering construction. In this paper, quasi-static mechanical experiments and dynamic mechanical experiments were carried out on SCAC with different limestone powder contents. High-speed photography technology and Digital Image Correlation (DIC) were used to monitor the dynamic failure process and strain field of SCAC, and the influence of limestone powder content on the dynamic and static mechanical properties of SCAC was investigated. The results showed that, when the limestone powder content was 20% and 16%, the quasi-static compressive strength and quasi-static tensile strength exhibited the best improvement. Additionally, with increasing limestone powder content, the dynamic tensile strength of SCAC first showed and increasing trend and then a decreasing trend, reaching its maximum value when the limestone powder content was 16%. Moreover, the maximum strain value of SCAC with the same limestone powder content increased with increasing strain rate grade, showing an obvious effect on strain rate.

Microscopic Origins of the Nonlinear Behavior of Particle-Filled Rubber Probed with Dynamic Strain XPCS.

The underlying microscopic response of filler networks in reinforced rubber to dynamic strain is not well understood due to the experimental difficulty of directly measuring filler network behavior in samples undergoing dynamic strain. This difficulty can be overcome with in situ X-ray photon correlation spectroscopy (XPCS) measurements. The contrast between the silica filler and the rubber matrix for X-ray scattering allows us to isolate the filler network behavior from the overall response of the rubber. This in situ XPCS technique probes the microscopic breakdown and reforming of the filler network structure, which are responsible for the nonlinear dependence of modulus on strain, known in the rubber science community as the Payne effect. These microscopic changes in the filler network structure have consequences for the macroscopic material performance, especially for the fuel efficiency of tire tread compounds. Here, we elucidate the behavior with in situ dynamic strain XPCS experiments on industrially relevant, vulcanized rubbers filled (13 vol %) with novel air-milled silica of ultrahigh-surface area (UHSA) (250 m2/g). The addition of a silane coupling agent to rubber containing this silica causes an unexpected and counterintuitive increase in the Payne effect and decrease in energy dissipation. For this rubber, we observe a nearly two-fold enhancement of the storage modulus and virtually equivalent loss tangent compared to a rubber containing a coupling agent and conventional silica. Interpretation of our in situ XPCS results simultaneously with interpretation of traditional dynamic mechanical analysis (DMA) strain sweep experiments reveals that the debonding or yielding of bridged bound rubber layers is key to understanding the behavior of rubber formulations containing the silane coupling agent and high-surface area silica. These results demonstrate that the combination of XPCS and DMA is a powerful method for unraveling the microscale filler response to strain which dictates the dynamic mechanical properties of reinforced soft matter composites. With this combination of techniques, we have elucidated the great promise of UHSA silica when used in concert with a silane coupling agent in filled rubber. Such composites simultaneously exhibit large moduli and low hysteresis under dynamic strain.

Reversible Cyclic Softening in HSLA Steels at Low Homologous Temperatures

Critical mechanical components employed in energy applications require certification of their mechanical response under monotonic or cyclic loading at various constant temperatures. However, components often experience combined load and temperature changes, which are more complex and time-consuming to evaluate. This work presents previously unreported reversible cyclic softening in low-alloy steel undergoing loading sequences between 293 K and 453 K (0.15 to 0.25 of the homologous temperature). Mechanical cycling at 80 pct of the yield stress results in a similar cyclic response for all temperatures, while cycling at 90 pct of the yield stress doubles the cyclic strain when preceded by cycling at 453 K. The softening can be reversed by maintaining the material at elevated temperatures and cannot be explained by dislocation recovery mechanisms or changes in the microstructure. Instead, serrated deformation at specific deformation levels suggests that Dynamic Strain Aging (DSA) contributes to softening, which is further supported by a theoretical analysis of interstitials and dislocations mean free paths at various temperatures.

On the Influence of Precipitation on the Dynamic Strain Aging in Mg-2%Nd

The Mg-2%Nd alloy, thermally treated at 520°C and quenched, exhibits plastic instabilities at intermediate temperatures between 100°C and 300°C when it is compressively tested because of the dynamic strain aging phenomenon caused by the presence of neodymium atoms in solid solution. During compression tests, there is a competition between the precipitation of Mg-Nd particles and the DSA phenomenon, which depends largely on the test temperature. Thus, below 200°C, the formation of neodymium-rich precipitates is slow, and the interaction is insignificant. Under these conditions, the alloy exhibits a normal behavior of the DSA, i.e., the critical strain decreases with the increase in the test temperature. However, for test temperatures > 200°C, the formation of neodymium-rich precipitates is faster and the evolution of the critical strain with temperature is inverted because of the decrease of neodymium in solid solution.

Experimental study on dynamic mechanical properties and damage characteristics of coral reef limestone

The coral reef limestone (CRL) in this study is characterized by well-developed pores, varied density range, and obvious growth line. To investigate the dynamic mechanical properties and damage characteristics of CRL, a series of impact experiments were conducted by employing a split Hopkinson pressure bar (SHPB) device. The effects of growth line inclination, strain rate and density on the dynamic mechanical properties of CRL were analyzed, and their damage characteristics were estimated by a quantitative analysis of the failure patterns. The results show that the stress–strain curve of CRL has a longer elastic stage and a shorter failure stage compared with porous rock-like materials, the porous properties of CRL are not obvious. The dynamic peak stress of CRL with the same density decreases gradually with the increasing growth line inclination. The strain rate effect of dynamic peak stress of CRL has a dependent on density, and the correlation between dynamic peak stress and strain rate becomes more obvious for CRL with higher density. The dynamic peak stress of CRL increases exponentially with increasing density, and density shows a much greater effect on peak dynamic stress compared with the growth line inclination. In addition, CRL exhibits distinct failure patters and fragment morphology from terrigenous rocks. CRL has an obvious fractal characteristic with fractal dimension of 2.09–2.76, which is influenced by growth line inclination, strain rate and density consistent with dynamic strength.