Transient Stress Waves Research Articles

Dynamic Responses of U-Shaped Caverns under Transient Stress Waves in Deep Rock Engineering

Dynamic Responses of U-Shaped Caverns under Transient Stress Waves in Deep Rock Engineering

Application of the Lamb Wave Mode of Acoustic Emission for Monitoring Impact Damage in Plate Structures.

The impact acoustic emission (AE) of plate structures is a transient stress wave generated by local materials under impact force that contains the state information of the impacted area. If the impact causes damage, the AE from material damage will be superimposed on the impact AE. Therefore, this paper details the direct extraction of damage-induced AEs from impact AEs for the health monitoring of plate structures. The damage-induced AE was analysed based on various aspects, including the cut-off range and propagation speed characteristics of the Lamb wave mode, the correlation between the force direction and the Lamb wave mode, and the impact damage process. According to these features, the damage-induced AE wave packets were extracted and verified via impact tests on epoxy glass fibreboards. The results demonstrated the feasibility of the proposed method for determining whether an impact causes damage via the direct extraction of the damage-induced AE from the impact AE.

A new technique to measure the dynamic fracture toughness of solids

We propose and assess a new experimental technique to measure the fracture toughness of engineering materials and its sensitivity to strain rate. The proposed method is based on a ring expansion technique and it overcomes the limitations of current dynamic fracture tests, as it is not affected by transient stress wave propagation during loading and it results in spatially uniform remote stress and strain fields prior to fracture; the method is also suitable to achieve remote strain rates well in excess of 1000 s−1. We demonstrate the technique by measuring the plane-stress Mode I fracture toughness of PMMA specimens at remote strain rates ranging from 10−3 s−1 to 102 s−1. The experiments show an increase of the toughness of the material with increasing strain rate.

Thermally nonlinear generalized thermoelasticity investigation of a functionally graded thick hollow cylinder based on the finite difference method

In the present paper, the propagation and reflection of the transient displacement, temperature, and stress waves are numerically explored in a functionally graded thick-walled hollow cylinder, based on the Lord–Shulman generalized thermoelectricity theory and using the finite difference method. The motion and thermoelastic energy balance equations are first established based on the dimensionless radial displacement and temperature fields. It is assumed that the elasticity modulus, Poisson’s ratio, mass density, thermal expansion coefficient, thermal conductivity coefficient, and specific heat follow power functions of the dimensionless radius while the relaxation time is assumed to be constant within the FGM cylinder. Unlike many previous studies in which the energy balance equation was linearized by assuming small temperature rises, the present research accounts for the significant temperature variations and employs a nonlinear energy balance equation. Nondimensional forms of energy balance and motion equations are derived by introducing dimensionless quantities and discretized via the finite difference method. The Newmark method with constant average acceleration was employed to solve the highly nonlinear equations found from the finite difference method. Two different boundary conditions were used to investigate the propagation and reflection of the displacement, temperature, and stress transient waves in the radial direction of the cylinder. The results revealed the variable speed of the displacement wave and the significant role of the changes in the properties along the radius of the cylinder and the boundary conditions in altering the results.

Two dimensional stress wave propagation analysis of infinite 2D-FGM hollow cylinder

There are controversial findings on how projectile penetration into the human tissue can induce tissue damage. The evidence from recent experimental investigations has explained the related underlying mechanisms. To understand and simulate the exact biomechanical mechanisms of projectile penetration-induced tissue damage, we evaluated effects of two-dimensional (2D) transient stress wave propagation of 2D-functionally graded material (FGM, radial and circumferential inhomogeneity) in an infinite hollow cylinder. A periodic radial and circumferential material gradation fluctuating a constant value has been considered as the effective material properties, across the cylinder wall and the problem has been solved by implementing the graded finite element method with nonlinear Lagrangian shape functions. In contrast with other researches, the stress wave propagation for r and θ -inhomogeneity is different compared to the case in which the r or θ -inhomogeneity has been considered, separately. Moreover, this example resembles the bullet penetration in human tissue that produces stress wave propagation, resulting in unintentional trauma in human organs. For the case of 1D-FGM ( r – inhomogeneity), the responses are pulse-like and symmetric. Finally, due to shear waves, the θ -inhomogeneity has more visible effect on the stress wave propagation than r – inhomogeneity due to shear waves, especially when the pulse collapses.

Influence of excavation damaged zone on the dynamic response of circular cavity subjected to transient stress wave

In this study, the Hoek Brown criteria is primarily employed to calculate the distribution of excavation damaged zone (EDZ) around the cavity buried at 500 m, 1000 m, and 2000 m based on the variation of the in-situ stress and the mechanical properties of the rockmass. Then, the simulation scheme Spectral Element Method (SEM) is applied to investigate the dynamic responses of the excavation-damaged cavity that subjected to the dynamic disturbance, including peak particle velocity (PPV), particle vibration frequency (PVF), particle displacement, and dynamic stress concentration factors (DSCF). Three two-dimensional (2D) models embedding a circular hole with different shapes of EDZ are prepared to numerically study the dynamic responses of the underground cavity, finding the influencing factors, such as the shapes of EDZ, the incident direction of the dynamic disturbance, and the dynamic source frequency. The cavity that is surrounded by EDZ has resembled as the one with a series of low-velocity areas. The results indicate that the underground cavity and EDZ become a scatter to the incoming stress waves that correspondingly produce high amplitude scattered waves. The influencing factors to the dynamic response of the cavity are summarized to be the shape of EDZ, the angle of stress disturbances, and the wavelength of incoming stress waves. We also find that the amplitude of stress disturbance around the cavity is proportional to the areas of EDZ and the frequency of the incident waves. In particular, only the tensile stress concentrates around the cavity that is surrounded by EDZ.

Determination of thermoelastic stress wave propagation in nanocomposite sandwich plates reinforced by clusters of carbon nanotubes

Adding small amounts of carbon nanotubes (CNTs) into the face sheets of sandwich structures can significantly improve their thermo-mechanical responses. However, the formation of CNT clusters, especially at high volume fractions of CNTs, dramatically affects the mechanical properties of the resulted nanocomposites, which is usually ignored. In this paper, by considering the formation of CNT clusters, we have investigated transient heat transfer and stress wave propagation in polymeric sandwich plates with two nanocomposite face sheets. The face sheets were made of clusters of CNTs embedded in a polymeric matrix. The volume fractions of CNTs and their clusters were assumed to be functionally graded along the thickness of face sheets. The proposed sandwich plate was subjected to thermal and impact pressure loads. Eshelby–Mori–Tanaka’s approach was applied to evaluate the material properties of the resulted nanocomposite with components with temperature-dependent material properties. Reddy’s third-order shear deformation theory and a moving least square shape function-based mesh-free method were utilized for thermoelastic dynamic analysis. The effects of CNT cluster size, distribution, and volume fraction as well as thermal load on the thermoelastic dynamic behavior of nanocomposite sandwich plates were investigated. It was observed that the distribution and cluster size of CNTs had significant effects on the amplitude and speed of thermoelastic stress wave propagation in the nanocomposite sandwich plates.

A novel photothermal excitation cracked medium in gravitational field with two-temperature and hydrostatic initial stress

ABSTRACTA New Approach model for a theory of two temperature with finite linear opening Mode-I crack inside a semi-infinite semiconducting medium is presented. The mechanical force influence during the photothermal process with the influence of gravity and hydrostatic initial stress in two dimensions are used. The transient compressive stress wave travels along the crack from the loading region to the crack face. Using mathematical methods under the purview of different three theories (Lord–Şhulman (LS) includes one relaxation time, Green–Lindsay (GL) includes two relaxation times, and the classical dynamical coupled theory (CD)). The exact expression of the physical quantities and the two temperature coefficient ratios are obtained analytically using the harmonic wave technique and illustrated graphically. Comparisons between the results of the three theories are also introduced.

Transient thermal stress wave and vibrational analyses of a thin diamond crystal for X-ray free-electron lasers under high-repetition-rate operation.

High-brightness X-ray free-electron lasers (FELs) are perceived as fourth-generation light sources providing unprecedented capabilities for frontier scientific researches in many fields. Thin crystals are important to generate coherent seeds in the self-seeding configuration, provide precise spectral measurements, and split X-ray FEL pulses, etc. In all of these applications a high-intensity X-ray FEL pulse impinges on the thin crystal and deposits a certain amount of heat load, potentially impairing the performance. In the present paper, transient thermal stress wave and vibrational analyses as well as transient thermal analysis are carried out to address the thermomechanical issues for thin diamond crystals, especially under high-repetition-rate operation of an X-ray FEL. The material properties at elevated temperatures are considered. It is shown that, for a typical FEL pulse depositing tens of microjoules energy over a spot of tens of micrometers in radius, the stress wave emission is completed on the tens of nanoseconds scale. The amount of kinetic energy converted from a FEL pulse can reach up to ∼10 nJ depending on the layer thickness. Natural frequencies of a diamond plate are also computed. The potential vibrational amplitude is estimated as a function of frequency. Due to the decreasing heat conductivity with increasing temperature, a runaway temperature rise is predicted for high repetition rates where the temperature rises abruptly after ratcheting up to a point of trivial heat damping rate relative to heat deposition rate.

09.11: Acoustic emission signal characteristics of damage accumulation in non‐buckling steel plate shear wall

ABSTRACTThis paper deals with acoustic emission (AE) monitoring of non‐buckling steel plate shear walls, in order to estimate damage accumulation of steel plate walls under cyclic loading. When material or any internal element in steel members develops plasticity or crack begins to form and grow rapidly, a large amount of energy will be released quickly within a short period. This would produce a series of transient stress waves, which is called AE signals. AE signals of the overall process contain useful information of irreversible accumulated damage in structural fatigue fracture progression. Continuous monitoring of low cycle fatigue plastic damage accumulation in steel plate walls is demonstrated in a corrugated‐shape steel plate wall under cyclic loading. Using the laboratory test data, the relationship between AE signals and fatigue damage accumulation in steel plate walls have been studied. It is revealed that the AE hit counts and energy can be used as one of the reference standards for judging the degree of plastic damage accumulation of material. AE damage accumulation evaluation based on AE hit and AE energy is a promising method for damage evaluation of steel hysteretic damping member. Compared with other conventional damage evaluation methods, it provides a new nondestructive testing approach to quantify the plastic damage of material, and it can be used to evaluate the member condition that are difficult to access.

Acoustic emission characteristics of reinforced concrete beams with varying percentage of tension steel reinforcement under flexural loading

Reinforced concrete (RC) flanged beam specimens were tested under incremental cyclic load till failure in flexure. Simultaneously the acoustic emissions (AE) known as transient elastic stress waves released during fracture process in the same specimens were recorded. These RC flanged beam specimens were cast with different percentage of steel reinforcement (area of steel reinforcement as a percentage of the effective area of beam cross section). Crack widths depend on tensile stress in steel reinforcement present in a RC structural member. Because crack opening is a function of tensile stress in the steel rebars, the percentage of steel in the RC members influence the AE released during fracture process. In this article, a study on damage occurred in RC flanged beam specimens having different percentage of steel reinforcement using acoustic emission testing is reported. A relation between the total AE energy released and percentage of steel in RC beams has been proposed. As the percentage of steel present in the test specimen was increased, the loading cycle number entering into the heavy damage zone in NDIS-2421 damage assessment chart also increased.

A numerical and experimental study of oblique impact of ultra-high pressure abrasive water jet

An investigation of the abrasive water jet with an emphasis on the oblique impact of abrasive particles on the target plate is performed. Ultra-high jet pressure necessitates a close examination of the phenomena featured by small spatial and temporal scales. The effect of oblique impact is assessed from both numerical and practical aspects. Numerical simulation, implemented using the commercial code LS-DYNA, allows a detailed inspection of transient stress wave propagation inside the target plate. And impact experiments facilitate a qualitative description of resultant footprints of oblique water jet. Different incident angles of abrasive particles are adopted and a comparison is thereby unfolded. The results indicate that rebound, embedding, and penetration of single abrasive particle are three representative final operation states. Adjacent to the abrasive particle, the response of the target plate to oblique impact is reflected by von Mises stress distribution and plate deformation as well. Oblique impact arouses non-symmetrical stress wave distributions and distinct unbalanced node displacements at the two sides of the abrasive particle. As for the target plate, global surface morphology is in accordance with predicted effects. The most favorable surface roughness is not associated with vertical impact, and it hinges upon the selection of standoff distance. Furthermore, variation of surface roughness with incident angle is not monotonous.

Energy dissipation via acoustic emission in ductile crack initiation

This article presents a modeling approach to estimate the energy release due to ductile crack initiation in conjunction to the energy dissipation associated with the formation and propagation of transient stress waves typically referred to as acoustic emission. To achieve this goal, a ductile fracture problem is investigated computationally using the finite element method based on a compact tension geometry under Mode I loading conditions. To quantify the energy dissipation associated with acoustic emission, a crack increment is produced given a pre-determined notch size in a 3D cohesive-based extended finite element model. The computational modeling methodology consists of defining a damage initiation state from static simulations and linking such state to a dynamic formulation used to evaluate wave propagation and related energy redistribution effects. The model relies on a custom traction separation law constructed using full field deformation measurements obtained experimentally using the digital image correlation method. The amount of energy release due to the investigated first crack increment is evaluated through three different approaches both for verification purposes and to produce an estimate of the portion of the energy that radiates away from the crack source in the form of transient waves. The results presented herein propose an upper bound for the energy dissipation associated to acoustic emission, which could assist the interpretation and implementation of relevant nondestructive evaluation methods and the further enrichment of the understanding of effects associated with fracture.

Identification of the mechanical moduli of closed-cell porous foams using transmitted acoustic waves in air and the transfer matrix method

Closed-cell cellular polymer foams are the most efficient insulating materials commercially available. Their lightweight and mechanical resistance also make them ideal materials for packaging protection in the transport industry. A new method using guided acoustic waves in air transmitted through samples of cellular foam panels, has been developed to recover their P-wave moduli and Poisson’s ratios. These parameters were recovered from measured transmission coefficients of the foams through fitting to an elastodynamic transfer matrix method (TMM). The TMM integrates both the P and shear waves propagating in the layer. It was shown by using a finite element fluid–solid (elastic) interaction and an analytic P-waves-only model, that the two types of waves should be modeled to give a more precise representation of the data. The retrieved values were validated using vibration spectroscopy and from the measured velocity of a transient mechanical stress wave propagating in thin, long rod specimens cut from the panels. The problems inherent to these simple 1D characterization methods were pointed out and solved.

Acoustic emission source modeling using a data-driven approach

The next generation of acoustics-based non-destructive evaluation for structural health monitoring applications will depend, among other reasons, on the capability to effectively characterize the transient stress wave effects related to acoustic emission (AE) generated due to activation of failure mechanisms in materials and structures. In this context, the forward problem of simulating AE is addressed herein by a combination of experimental, analytical and computational methods, which are used to form a data-driven finite element (FE) model for AE generation and associated transient elastic wave propagation. Acoustic emission is viewed for this purpose as part of the dynamic process of energy release caused by crack initiation. To this aim, full field experimental data obtained from crack initiation monitored by digital image correlation is used to construct a traction-separation law and to define damage initiation parameters. Subsequently, 3D FE simulations based on this law are performed using both a cohesive and an extended finite element modeling approach. To create a realistic computational AE source model, the transition between static and dynamic responses is evaluated. Numerically simulated AE signals from the dynamic response due to the onset of crack growth are analyzed in the context of the inverse problem of source identification and demonstrate the effects of material and geometry in crack-induced wave propagation.

The impact of solid–fluid interaction on transient stress wave propagation due to Acoustic Emissions in multi-layer plate structures

In this work the effect of a solid–fluid interaction on stress wave propagation within a composite overwrapped pressure vessel (COPV) was investigated. A modeling approach capable of adequately capturing the multi-physics nature of the solid–fluid interaction is presented, and then validated through comparison to experimental waveforms and time–frequency distributions. With a validated modeling approach, a numerical study on stress wave propagation in COPV structures was undertaken. It was found that the compressibility of the pressurizing media used during the pressure testing of COPVs has an effect on the leakage of the propagating Lamb modes, as well as alternative direct paths that may propagate within the fluid. Results regarding the effects of the most common microstructural deformation mechanisms that occur in composite materials on the detected Lamb modes, as well as on the direct fluid path arrival are presented. Finally, the impact of source depth on the detected Lamb modes and the direct fluid path arrival was investigated. It was found that information gleaned from the direct fluid path arrival may have the potential for aiding Modal Acoustic Emission practices in source mechanism identification, determination of source orientation, as well as source depth estimation.

Influence of thermal gradients on stress state of veneered restorations

ObjectivesTo assess transient and residual stresses within the porcelain of veneered restorations (zirconia and metal) as a result of cooling rate and porcelain thickness. MethodsPorcelain-on-zirconia (PZ) and porcelain-fused-to-metal (PFM) crowns were fabricated with 1 or 2mm of porcelain. Thermocouples were attached both internally and externally to the crowns to record transient temperatures. For fast cooling, the furnace was opened after the holding time and switched off. Slow cooling was accomplished by opening the furnace at 50°C below the glass transition temperature (Tg) of the material. An axially symmetric FEA model simulated thermal stresses. Time-dependent temperature equations from thermocouple readings were set as boundary conditions. Framework materials and the porcelain below Tg were considered to behave elastically. Visco-elastic behavior was assumed for porcelain above the Tg modeling properties as dependent on cooling rate. ResultsDifferences in residual stress were found for fast and slow cooled PZ and PFM crowns. Significant transient stress waves were observed within the porcelain when fast cooling through Tg. They are believed to be related to non-uniform volumetric changes originated from thermal gradients. Results were confirmed by modeling and physical testing of crowns containing a defect. SignificanceResidual stresses do not distinguish PZ from PFM. High magnitude transient stresses observed within the porcelain during fast cooling may explain clinical fractures involving internal defects. Stress waves may also originate internal micro-cracking which could grow under function. Therefore, slow cooling, especially for all-ceramic crowns with thick porcelain, is important to prevent thermal gradients and high-magnitude transient stresses.

Extracting Acoustic Emission Signal Feature of Grinding Processing

Acoustic Emission (AE) may be defined as a transient elastic stress wave generated by the rapid release of strain energy in local area of material. To overcome the limitation of some traditional techniques, the AE technique, which provides high sensitivity and responding speed, were developed in the present paper. AE signature is usually difficult to be extracted and characterized in grinding process of 1Cr18Ni9Ti coatings due to their high hardness, great ductility, inhomogeneous structure and irregular surface with lots of hard points and pores. In this paper, AE signal of stationary grinding status before wheel-workpiece contact was characterized first, then AE signal of the grinding process was analyzed using root-mean-square (RMS) and power spectrum method. Results showed that before the contact occurred, the grinding signal is stable, with low amplitude and frequency ranging all frequency channels and no peak signal. However, when contact occurred, the RMS and spectrum of AE signal increased obviously and the bandwidth varied exquisitely between 100 KHz and 300 KHz. The real contact time between wheel and workpiece was about 0.5 to 1 ms.

Detection and Analysis of Defect in Steel Tube Using Vibration Impact Acoustic Emission (VIAE) Method

Currently the tube inspection technologies are normally invasive and time consuming, and therefore, an effective and non-invasive tube inspection method is required. This paper presents the research works on the development of non-invasive and non-destructive defect detection system and analysis for the ASTM A179 seamless steel tubes. Vibration excitation using impact hammer method is used to generate transient stress waves in steel tubes. Specifically, three steel tubes were used including a good steel tube and two steel tubes with through-hole artificial defect at different locations. An acoustic emission technique is utilised to detect and capture the stress waves propagation in the steel tubes. The variations of the stress waves propagated in the different condition of the steel tubes were successfully characterized and differentiated using three conventional statistical features namely root mean square (r.m.s), crest factor and kurtosis. The acquired experimental results show that the newly proposed Vibration Impact Acoustic Emission (VIAE) method is capable of detecting the presence of defect in the steel tubes

Microstructural level response of HMX–Estane polymer-bonded explosive under effects of transient stress waves

The effect of transient stress waves on the microstructure of HMX–Estane, a polymer-bonded explosive (PBX), is studied. Calculations carried out concern microstructures with HMX grain sizes on the order of 200 μm and grain volume fractions in the range of 0.50–0.82. The microstructural samples analysed have an aspect ratio of 5:1 (15×3 mm), allowing the transient wave propagation process resulting from normal impact to be resolved. Boundary loading is effected by the imposition of impact face velocities of 50–200 m s −1 . Different levels of grain–binder interface strength are considered. The analysis uses a recently developed cohesive finite element framework that accounts for coupled thermal–mechanical processes involving deformation, heat generation and conduction, failure in the forms of microcracks in both bulk constituents and along grain/matrix interfaces, and frictional heating along crack faces. Results show that the overall wave speed through the microstructures depends on both the grain volume fraction and interface bonding strength between the constituents and that the distance traversed by the stress wave before the initiation of frictional dissipation is independent of the grain volume fraction but increases with impact velocity. Energy dissipated per unit volume owing to fracture is highest near the impact surface and deceases to zero at the stress wavefront. On the other hand, the peak temperature rises are noted to occur approximately 2–3 mm from the impact surface. Scaling laws are developed for the maximum dissipation rate and the highest temperature rise as functions of impact velocity, grain volume fraction and grain–binder interfacial bonding strength.