Evolution Of Strain Fields Research Articles

Analysis of Soil Response during High-Frequency Vibratory Steel Pipe Pile Driving in Soft Soil

High-frequency vibratory pile driving exhibits remarkable efficiency in soft soil, with its impact on the surrounding soil to a limited range. Studying the evolution of stress and strain fields in the soil surrounding the pile during the pile driving process is of great significance for effective control of pile driving construction and accurate prediction of the pile foundation’s bearing capacity after installation. In this study, by means of numerical simulation with consideration of the large deformations of the soil and even discontinuities induced by the pile penetration into soil, the response of the soil during the high-frequency vibratory steel pipe pile driving in silt soil was investigated. The research results show that with the increase of pile driving depth, the stress concentration zones in the soil near the pile tip continuously expanded, and the plastic strain zone mainly developed downward but little horizontally. When the vibration frequency was between 33 and 38 Hz, the pile driving efficiency was higher than that of 50 Hz. High-frequency resonance free vibratory pile driving has smaller pile–soil stresses and plastic strains compared to traditional impacting pile and static pile, and it is less prone to the formation of soil plug. Therefore, high-frequency vibratory pile driving technology exhibits good adaptability in the soft soil.

Mechanical behavior, acoustic emission and principal strain field evolution properties of layered cemented paste backfill under unconfined compression

In order to save costs, the layered filling method is adopted by mine enterprise to fill the underground goafs. To understand the mechanical behavior, damage evolution and failure modes the layered cemented paste backfill (LCPB), this study investigated the strength properties, acoustic emission (AE) characteristics and principal strain field evolution of LCPB samples with cement-to-tailings (c/t) ratio of 1:4, 1:6, 1:8, 1:10, and the height (h) of middle layer of 30 mm, 35 mm, 40 mm, 45 mm, 50 mm. The obtained results indicate that: (1) UCS of LCPB decreases with the increases of h and with the decreases of c/t ratio. The influence of c/t ratio on UCS of LCPB is in a good agreement with logarithmic function, while the variation relationship between h and UCS of LCPB satisfies the exponential function. (2) The acoustic emission signals first appeared during the transition from the compression to the elastic stage, reaching a maximum approaching the peak stress, and AE ringing counts also appeared during the post-peak stage. Internal fracture type of LCPB under external loading is a tension-shear composite fracture dominated by shear fracture. (3) The damage of LCPB occurs mainly in the middle layer and then extends to the upper and lower layer with the formation of visible localized zones. The damage of LCPB developed from significant shear damage to tensile-shear damage with the decreased of the c/t ratio, and transitioned from tensile-shear damage to complex damage dominated by shear damage and supplemented by tensile damage as h increased. Ultimately, the findings of this work can provide a reference for the design of LCPB.

Experimental investigation of temperature effects on the tensile behavior of composite‐aluminum four‐nail bolted joints

AbstractThe bolted joint is widely used to connect different composite structures in the aerospace field, and the joint is usually the weakest part, and its performance is widely influenced by temperature. This paper aims to investigate the static tensile behavior of the composite‐aluminum four‐nailed joint structure at different temperatures. Carbon fiber epoxy resin matrix composites with a layup of [(0/45)2s]2 are selected to fabricate specimens, and the test temperatures are room temperature (RT, 25 °C), 60 °C, 100 °C, and 150 °C, respectively. The surface strain field and damage evolution inside the specimens are monitored at different stages of the loading process by combining the digital image correlation (DIC) and X‐ray computed tomography (CT) techniques. During the loading process at RT, a sudden drop in load occurs after structural yielding, and the following secondary bearing is observed; however, at higher temperatures, the load continuously decreases until final failure after it reaches the peak value. Moreover, the rearrangement of stress distribution around the bolts at higher temperatures relieves the asymmetric secondary bending of the joint. The experimental findings in this study provide valuable guidance for the structural design and safety assessment for composite‐aluminum bolted joints at different temperatures.Highlights The loading process shows five stages at RT and four stages at higher temperatures. Extrusion damage led by shear cracks appears earlier at higher temperatures. Load on the upper and lower bolts is closer as the temperature increases. Higher temperatures relieve the asymmetric secondary bending of the joint.

Strain concentration degree defining ductility and catastrophic failure of 6061 aluminum alloy

The physical control of ductile deformation and failure behaviors of aluminum alloys is still open to investigation, although this is a central problem for material design and safety evaluation. In this study, 6061 aluminum alloy sheets with different pre-cut depths were uniaxially stretched, and the evolution of strain fields was monitored to reveal the relationship between the strain concentration and failure process. The average strain/strain fluctuation ratio and the strain propagation velocity were defined to quantify the strain concentration degree. The strain concentration degree is an intrinsic factor hosting deformation ductility, maximum bearing load, and failure modes. The underlying mechanism of this changing ductility and failure process is the increased strain/stress concentration degree due to increased pre-cut depth. The experimental results indicated that higher ductility resulted in a larger catastrophic failure in the 6061 aluminum alloy. A lower deformation ductility led to a more gradual macroscopic failure. Our findings suggest that various origins inducing different strain concentration degrees can cause various effects on ductility and failure modes of 6061 aluminum alloy.

Spatial heterogeneity of precursory accelerating deformation in uniaxially compressed sandstones and prediction of failure time

The power law acceleration has been validated as an effective method for predicting failure time, however, the precursory acceleration distribution in local monitoring signals is still unclear. In this paper, the spatial distributions of precursory acceleration of strains at different positions and in various size windows are reported based on surface strain fields measured from sandstone. The sandstone specimens are uniaxially compressed and a digital image correlation technique is applied to monitor the evolution of surface strain fields. The precursory acceleration of strains exhibits heterogeneities in amplitudes and durations, which are related to the evolution of strain localization in a higher strain zone. Strain rates at higher strain zones are prone to acceleration. There exists a well-defined size for monitoring and identifying the precursory acceleration of strains. In the vicinity of failure, strain accelerations in different positions and various size windows are described by a power law behavior with a constant exponent. The results show that prediction is stable and agrees well with the actual failure time.

Enhancing strength and ductility of Al-matrix composite via a dual-heterostructure strategy

Aluminum matrix composites (AMCs) often have low ductility, which has been a long-lasting issue in the last few decades. This problem arises largely from the non-deformability of reinforcement particles, which leads to premature failure of the matrix-particle interfaces. Here we propose a new microstructural design strategy for AMCs: distribute the reinforcement particles non-uniformly to form dual-heterostructured AMCs. The zones with high-density particles are recognized as the hard zones, which carry less plastic strain than the particle-free zones to prevent premature interfacial failure. A dual-heterostructured Al-matrix nanocomposite is fabricated, in which AlN nanoparticles are distributed in a dual-level hierarchy: first level heterogeneous nanoparticle distribution and second level heterogeneous zones with different grain sizes. The dual heterostructure produced a unique dual level hetero-deformation induced (HDI) strengthening and hardening to produce high strength and ductility. The dual level HDI strengthening effect has been revealed by the inflection points on the loading-unloading-reloading stress-strain curves. Furthermore, the evolution of local strain fields during the in-situ tensile deformation directly proved the occurrence of strain partitioning, in which the ductile particle free zones have carried a larger strain than the hard particle rich zones. Dispersive shear strain bands are observed for the first time in AMCs. These findings are expected to help design other metal matrix composites with superior mechanical properties.

Interaction between flaws and failure characteristics of red sandstone containing double flaws under compressive-shear loading

The failure characteristics of the fractured rock mass and the interaction between flaws were significantly affected by the flaw configurations. This study quantitatively investigated the effect of flaw geometries on the interaction between flaws under compressive-shear loading. The evolution of the surface strain field was recorded and analyzed using digital image correlation (DIC) technology, enabling the localization of crack paths and the determination of crack initiation mechanisms. Combined with DIC and scanning electron microscopy, both the macroscopic and microscopic failure characteristics of specimens with double flaws subjected to compressive-shear loading were comprehensively investigated. The results showed that the axial peak force of flawed specimen was sensitive to the flaw configuration, and the axial peak forces of flawed specimens with parallel flaws were greater than those with coplanar ones. The tensile strain was the primary factor dominating the initiation and propagation of cracks from flaw tips, whereas the cracking mechanism of edge cracks was a combination of tensile and shear strains. In addition, two novel parameters, namely independent parameter Ri and reinforced parameter Rr, were proposed to quantitatively characterize the independent effect discovered between coplanar flaws and the reinforced effect between parallel flaws, respectively. Besides, it was observed that the flaw configuration has a significant effect on rock failure characteristics, and three typical crack coalescent modes were found. The findings of this paper could facilitate a better understanding of the interaction between flaws and rock failure characteristics subjected to compressive-shear loading conditions.

Strain rockburst failure characteristics and mechanism of high stress circular hard rock tunnel triggered by dynamic impact load

Strain rockburst poses a great threat to the construction safety of deep engineering, especially induced by dynamic impact loads such as blasting from the adjacent tunnel excavation. Aiming to clarify the failure characteristics and mechanism of strain rockburst induced by the dynamic impact load, a series of dynamic impact tests under different biaxial prestress conditions were conducted on the sandstone cube specimens with prefabricated circular holes by using the new biaxial Hopkinson pressure bar equipment. The real-time rockburst process and fracture characteristics on both sidewalls were captured by the high-speed camera. The test results show that the strain rockburst process can be divided into: the calm stage, slab buckling and spalling stage, rock slabs ejection stage, and V-shaped notch formation stage. Ultimately, a symmetrical V-shaped notch failure zone was formed on the sidewall, similar to the site. The strain field evolution characteristics and displacement deformation characteristics around the circular hole during the rockburst process were further explained by using digital image correlation (DIC). The failure mechanism of sidewall strain rockburst was closely related to spalling failure, characterized by dynamic tensile failure. The static stress provides the initial compressive stress field and governing the stress distribution around the tunnel. Dynamic impact load disrupts the original stress equilibrium of the surrounding rock, inducing significant alterations in internal stresses and strains within the surrounding rock, consequently inducing spalling or rockburst failure. The influences of lateral pressure on rockburst were also analyzed in detail, while the lateral pressure influences the severity of strain rockburst, which determines the dynamic load response characteristics, influence range of strain concentration zone, displacement deformation characteristics of the tunnel surrounding rock, and the severity of strain rockburst decreases with the increase of lateral pressure.

Failure mode of parallel-fractured rock-like sample with different inclinations

Parallel fractures, a kind of common geological defect in natural rock mass, show a variety of failure mechanism and increases the difficulty in the timely warning of geological disasters such as landslide. Therefore, studying the mechanical properties and failure mechanism of parallel-fractured rock mass occupies a vital position in the stability analysis and warning technology of disaster precursor. In response to this, the laboratorial efforts on micro fracture mode and macro strength prediction of parallel-fractured rock-like samples with different inclinations were made in this paper. Firstly, the parallel-fractured samples were prepared by changing the fracture parameters, and the uniaxial compression test was then carried out. According to the mechanical parameter variations of samples during axial loading, the multiple indexes affecting the uniaxial compressive strength of parallel-fractured samples was clarified, and the specific influence of fracture inclination was quantified by mathematical method. Finally, the progressive failure mechanism of samples was analyzed through strain field evolution in virtue of digital image correlation (DIC) technique, and the main failure modes of parallel-fractured samples were summarized. Meanwhile, the acoustic emission (AE) signals were collected and processed to further reveal the failure characteristics of parallel-fractured samples with different inclinations in a microscopical perspective.

Application of the bulge test combined with 3D-DIC for biaxial characterization of PLA films and defect detection

The bulge test is a versatile mechanical testing technique widely applied in materials science, engineering, and biomedical research to understand the mechanical behavior of materials and structures. In this study, PLA blown films were characterized through the bulge test, and the corresponding properties were compared to those obtained from uniaxial t!esting. To achieve more accurate and precise deformation measurements, 3D digital image correlation (3D-DIC) was used, offering superior capabilities in terms of accuracy, precision, and non-destructive testing. This technique can detect defects such as cracks, voids, and delamination in materials and structures. During the bulge testing, 3D-DIC was utilized to analyze the strain field evolution within the test sample and identify regions of reduced film thickness. Notably, the results indicated zones with strain concentration aligned parallel to the film machine direction, suggesting localized thickness reduction in those areas.

Cluster-based Nonuniform Transformation Field Analysis of Gra-phene nanocomposites

Graphene nanocomposites have attracted much attention in materials science due to their superior me-chanical properties. It is difficult for conventional mul-tiscale methods to provide substantial assistance to the research of such materials due to their huge computa-tional costs. Nonuniform transformation field analysis is a very effective reduced order homogenization meth-od for elastoplastic multiscale analysis. However, the reduced order model derived from this method has the shortcoming of low universality and high application threshold. Therefore, an improved reduced order model is proposed by combining the nonuniform transfor-mation field analysis with the k-means clustering algo-rithm. One can embed the required microscopic consti-tutive model into the reduced order homogenization framework without the need to derive a new reduced order model. Based on the cluster-based nonuniform transformation field analysis, the influence of the mi-croscopic plastic strain field evolution on the macro-scopic response of the material under consideration is revealed, while the mechanical properties of graphene nanocomposites are predicted. The numerical results show that the new reduced order model can accurately predict the macroscopic mechanical properties of com-posite materials, and its acceleration rate compared to the traditional finite element computations reaches103-104 .
 Keywords:graphene nanocomposites; reduced order model; multiscale methods; clustering; nonuniform transfomation field analysis

Strain Field Evolution Analysis of Brittle Shale with Initial Fractures Based on DIC

Wellbore instability mainly occurs in shale formations, and it is of great significance to deeply analyze the characteristics of shale-failure behavior to evaluate the stability of the shale surrounding the well wall during drilling. Through a uniaxial compression experiment and DIC technology, the whole strain field of shale specimens with prefabricated holes and cracks under uniaxial compression is measured. The experimental data of load, displacement and strain field are analyzed comprehensively. The results show that the fracture location and expansion path of shale are closely related to the evolution of the strain field. The evolution of the strain field directly affects the failure behavior of the rock. Under the action of load, local high strain will first appear around the initial shale defects (pores and fractures), and stress concentration will occur. With the increase of load, cracks and failures will first appear in the local high-strain zone, and the failure will spread along the region and path and eventually lead to the overall failure of the rock. The establishment of a description method for shale-failure behavior through strain-field evolution can effectively analyze the crack behavior of shale with initial defects such as holes and cracks and provide theoretical and experimental bases for the stability evaluation of the shale surrounding the well wall, including shale-strength prediction and shale-failure mechanism.

3D strain fields in a plant fibre composite during fragmentation test: Micro-CT based DIC and DVC

This paper aims to study the evolution of the 3D strain fields in a single yarn composite during fragmentation test. The internal full-field strains were obtained by coupling micro-CT scans with digital image correlation (DIC) and digital volume correlation (DVC). An ex-situ step-by-step fragmentation test was carried out on a sample made of a hemp yarn and a polymer matrix filled with metal oxide powder, playing the role of a three dimensional speckle. Macroscopic markers were also added to enable a precise geometric alignment during the 3D reconstruction. The analysis of the tomographic images showed a stepwise progression of the damage occurrence: yarn breaks appeared first, followed by internal matrix cracks and then edge cracks occurred for high strain levels. DIC method allowed to easily and quickly determine 2D strain maps in selected planes. The use of DVC was more complex and needed powerful computing systems but it gave the volumetric strains in the whole sample. DVC calculations were compared and validated by DIC results in central planes. Experimental results were also compared with finite element simulations. This study demonstrates the viability and benefits of combining DVC and micro-CT for a deeper understanding of complex 3D strains in single yarn composites.

Strain field evolution and crack coalescence mechanism of composite strength rock-like specimens with sawtooth interface

The mechanical behaviors of composite rock masses containing a complex interface widely distributed in nature are significantly different from those of monolithic rock masses. The objective of this study is to perform a series of uniaxial compression tests to study the influence of sawtooth interface parameters on the fracture behavior of rock-like composite specimens. Meantimes, the acoustic emission (AE) and digital image correlation (DIC) technique were used to classify damage stages and record strain field evolution, respectively. The results of this study demonstrate that stress–strain curves of composite rock-like specimens with different sawtooth interfaces display characteristics of plastic failure, friction failure characteristics and two different brittle failure characteristics in the post-peak phase. The peak strength of the specimen first decreases and then increases as the inclination of the interface increases, achieving the maximum value of 28.29 MPa at the inclined interface of 90°. The strain concentration zone is mainly generated and expanded around the interface. The parameters of the sawtooth have a significant influence on the type and location of cracks developed along the interface. In addition, there is a strong relationship between the mode of failure of the specimen and the strength of the rock layer and the inclination of the interface. According to the relative positional relationship between the coalescence path of the specimen and the interface, the failure modes of the specimen are classified as single weak rock layer failure (S-AB-0, S-AB-15), oblique failure through rock layer interface (S-AB-30, S-AB-45), slip failure along the interface (S-AB-60, S-AB-75) and axisymmetric failure along the interface (S-AB-90). This result is of great importance for a thorough understanding of the crack coalescence mechanism and strain field evolution of rock-like composite specimens.

Experimental study of thermomechanical behaviour of Gum Metal during cyclic tensile loadings: the quantitative contribution of IRT and DIC

ABSTRACT Thermomechanical behaviour of Gum Metal (Ti–23Nb–0.7Ta–2.0Zr–1.2O, at.%) under cyclic tension was experimentally investigated using infrared thermography and digital image correlation. The thermomechanical characteristics of particular stages of the subsequent loading-unloading cycles of Gum Metal were identified, i.e. (I) the linear, elastic loading accompanied by the temperature drop, (II) the nonlinear super-elastic loading related to the temperature growth, (III) the transient stage (at which both the superelastic-like behaviour and the plastic one are present simultaneously) and the temperature starts growing fast, (IV) the plastic deformation with a significant growth of temperature, (V) the superelastic-like unloading accompanied by a fast drop in temperature, (VI) the transient unloading with a slower decrease in temperature and (VII) the elastic unloading, with a slight increase in temperature. Thermoelastic effect in Gum Metal during both loading and unloading was analysed in each tensile cycle. Finally, the evolution of strain and temperature fields just before unloading in each cycle was discussed and a comparison of the fields at selected stages of cycles 12 and 24 was presented. The results of this work enabled us to identify the non-dissipative processes of elastic and superelastic-like deformations as well as the dissipative process of plastic deformation.

Experimental Study on Mechanical Properties and Deterioration Mechanism of Red Sandstone from the Panjiatuo Landslide under Action of Acidic Drying−Wetting Cycles

Due to frequent water level fluctuations and complex hydrochemical environments, rock slopes in reservoir areas progressively deteriorate and become unstable. This study investigated the coupling effect of drying−wetting cycles and acidic solutions on the physical and mechanical properties, strain field evolution, failure mode, and micro-mechanism of red sandstone using a series of laboratory experiments (wave velocity tests, uniaxial compression tests, the digital image correlation method, scanning electron microscopy, and X-ray diffraction). The results showed that with increasing drying−wetting cycles, the mass, P-wave velocity, elastic modulus, and uniaxial compressive strength decreased monotonically, while the water absorption and apparent strain in the strain localization band increased. Moreover, the failure mode transitioned gradually from tensile failure to shear failure or tensile-shear composite failure. The decrease in the solution pH values aggravated the changes in the physical and mechanical parameters and contributed to an increase in the secondary cracks and the occurrence of shear behavior. In addition, the coupling effect of drying−wetting cycles and acidic solutions accelerated the worsening of the microstructure and the dissolution of minerals, resulting in a loose structure with well-developed pores and fissures. These changes provide a favorable explanation for the mechanical property deterioration of red sandstone subjected to acidic drying−wetting cycles.

Experimental characterization of cyclic behaviour of pure lead: Temperature sensitivity and strain-rate effects

Proton beam pulses with an energy of 20 GeV/c collide with a pure-lead based target installed in the neutron Time-Of-Flight facility (n_TOF) at the European Laboratory for Particle Physics (CERN). The interaction between the proton beam and lead produces neutrons via spallation mechanism and results in a rapid temperature increase and propagation of stress waves. To evaluate the material response in such challenging conditions, a reliable thermo-mechanical characterization is necessary for the calibration of an appropriate constitutive model for pure lead that is valid under cyclic plasticity and high temperature. In this work, the experimental bases for the development of such constitutive material description are lied. Starting with metallurgical characterization, the typical grain size of the material was initially investigated as well as any variations in the metallurgical features. The grains appeared to have an equivalent size ranging from 2 to 6 mm. Then, static tensile tests were conducted at room temperature and different strain-rates from 10−1 to 10−4 s−1. The obtained results were crucial for optimizing the specimen geometry and test setup for the subsequent cyclic tests. Tension-compression cyclic tests were performed at different strain amplitudes from 0.1 to 1.5%, and at three different temperatures (room temperature, 90 °C and 150 °C). The strain amplitudes were controlled by an extensometer and the strain field evolution during the test was recorded by means of 2D DIC.

In-plane shear experimental method and mechanical behavior of ceramic matrix mini-composites

In-plane shear mechanical experiments of ceramic matrix mini-composites (CMMC) were realized in this paper by overcoming the difficulties of material preparation, specimen design, gripping and loading, deformation measurement, etc. The in-plane shear stress-strain responses of different matrix volume fractions were obtained based on the method. The stress-strain response of CMMC was strongly non-linear, and its elastic modulus and strength were positively correlated to the matrix volume fraction. The main factors affecting the digital image correlation (DIC) based micro-region shear deformation measurement are analyzed quantitatively, and the corresponding solutions are discussed. The DIC calculation time model is established, and the accurate estimation of DIC processing time is realized. The shear strain field evolution clearly captured the matrix crack initiation and propagation.

Energy evolution and crack development characteristics of sandstone under freeze-thaw cycles by digital image correlation.

Freeze-thaw erosion is the main reason for rock mass instability in cold regions and poses major threats to public safety. In this study, the stress threshold, energy, and strain field evolution of sandstone and the variation in stress intensity factor of fractures in various stress fields were all investigated after freeze-thaw cycles by uniaxial compression tests and digital image correlation technology. The results show that the elastic modulus, crack initiation stress, and peak stress all fell by 97%, 92.5%, and 89.9%, respectively, as the number of freeze-thaw cycles approaches 80. Elastic energy's storage capacity also dropped from 0.85 to 0.17. Sandstone's strain was increased by freeze-thaw erosion, which also improved ductility and shortened the cracking time. The stress intensity factor at the crack tip was positively correlated with the tip inclination angle and negatively correlated with the number of freeze-thaw cycles. This study provides a useful reference for understanding the stability of rock masses and the characteristics of crack derivation in cold regions.

Combined effects of fault geometry and roadway cross-section shape on the collapse behaviors of twin roadways: An experimental investigation

Natural faults in key areas can significantly affect the stability of underground twin roadways. To assess the combined effect of fault geometry and roadway cross-section shape on the collapse behaviors of twin roadways, a series of uniaxial compression tests were carried out on the double-hole specimens using pre-embedded joints to represent faults. In the experiments, the digital image correlation (DIC) and acoustic emission (AE) techniques were used to monitor the strain field evolution and internal crack development in the specimens. The test results showed that the strain localization zones were mainly generated near the roadways and joint, and the initiation locations and patterns of the induced cracks were directly affected by the roadway cross-section shape and joint inclination (α). According to the collapse paths, the collapse modes of the twin roadways can be divided into three categories for different α: joint lap collapse (α = 0°), quasi-integrity collapse (α = 90°), and through-joint collapse (with α ranging from 30° to 60°). In general, the circular twin roadways were found to be more stable than the square twin roadways. Moreover, as α increased from 0° to 90°, the collapse stress was observed to first decrease to its minimum at α = 45° and then increase. Finally, the detailed stress distributions around the roadways and joint were numerically analyzed to supplement the experimental results. The results from this work can be of great engineering significance for the design and construction of similar underground roadways in jointed rock masses to achieve structural stability and safety.