Double frequency piezoelectric transducer design for harmonic imaging purposes in NDT

Harmonic imaging (HI) has emerged as a very promising tool for medical imaging, although there has been little published work using this technique in ultrasonic nondestructive testing (NDT). The core of the technique, which uses nonlinear propagation effects arising in the medium due to the microstructure or the existence of defects, is the ability to design transducers capable of emitting at one frequency and receiving at twice this frequency. The transducers that have been used so far are usually double crystal configurations with coaxial geometry, and commonly using a disc surrounded by a ring. Such a geometry permits the design of broadband transducers if each transducer element is adapted to the medium with its corresponding matching layers. Nevertheless, the different geometry of the emission and reception apertures creates difficulties when resolving the images. In this work, a new transducer design with different emission and reception apertures is resented. It makes use of the traditional construction procedures used to make piezocomposite transducers and the well-known theory of the mode coupling in piezoelectric resonators when the lateral dimensions are comparable with the thickness of the piezoceramic. In this work the design, construction, and characterization of a prototype to be used in NDT of metallic materials is presented. The acoustic field is calculated using water as a propagation medium, and these theoretical predictions then are compared with the experimental measurements. The predicted acoustic performances for the case of propagation in stainless steel are shown.     

Crack trajectories under mixed mode and non-proportional loading

A novel testing procedure for mixed mode crack propagation is presented; it offers the possibility of non-proportional loading and of changing the crack trajectory during testing, features which allow non-traditional mixed mode tests which, in turn, may help in discriminating mixed mode fracture criteria. Detailed experimental stable crack trajectories and corresponding load-CMOD (or load-displacement) curves from these non-traditional tests are proposed as benchmarks for numerical programs of mixed mode crack propagation.     

Thermomechanical fatigue crack growth from laser drilled holes in single crystal nickel based superalloy

The test results undergoing thermomechanical fatigue of single crystal PWA1484 crack growth showed that the life of TMF specimens with notches (in this case laser drilled holes) is 4 times shorter than the number of cycles to failure observed on smooth gage section specimens (without holes) under the same loading conditions. Such a significant change in number of cycles to failure must be accounted in any damage tolerant turbine airfoil design system. The detailed fractographic analysis demonstrated that all cracks start crystallographically along the {1 1 1} octahedral crystallographic planes and later change to mixed mode fracture. Most of the crack propagation takes place at the low temperature portion of the cycle in the out-of-phase test; however there is noticeable damage accumulation during the high temperature compressive load portion of the cycle. Crack propagation under TMF loading conditions is considerably faster than corresponding isothermal LCF crack growth tested at the temperature and similar loading conditions of the tensile part of the TMF cycle. As results show, the applicability of the LEFM methods for single crystal TMF crack growth prediction is limited and at least should consist of mixed mode crack analysis.A new method for detecting cracks during a TMF test using induction thermography was employed. This method, coined the Active Inferred Crack Detection System (AICD), demonstrated high effectiveness in following crack progression under cyclic loading making it well suited to perform TMF crack growth testing. Using this experimental technique we also investigated the effect of secondary crystallographic orientation on crack propagation.     

The spalling of long bars as a reliable method of measuring the dynamic tensile strength of ceramics

An analysis of the spall bar test as a reliable method of determining the dynamic tensile strength of brittle materials is presented. The method is based on the propagation and reflection of elastic waves in bars. Failure occurs when compressive waves are reflected into tensile ones on reaching a free end. The study analyses the hypotheses needed to obtain the true tensile strength with this experimental technique, referring to the requirements of the material and of the experimental procedure. The analysis is complemented with numerical simulations of the testing procedure. The correct way to determine the true dynamic tensile strength of ceramic materials is outlined. Finally, the results of tests of some ceramic materials, three different aluminium oxides, an alumina reinforced with zirconia, silicon carbide and boron carbide are presented.     

Mechanistic evaluation of fatigue cracking in asphalt pavements

Over the last several decades, significant research has been conducted to predict the fatigue cracking performance of asphalt pavements. Recently, the simplified viscoelastic continuum damage (S-VECD) model was developed as an efficient method of characterising the fatigue performance of asphalt mixtures under a wide range of loading conditions. Two important material properties that can be determined from the S-VECD model are the damage characteristic curve that defines how damage evolves in a specimen and the energy-based failure criterion that defines when the specimen fails. These two material functions are unique for a given mixture regardless of temperature, mode of loading, stress/strain amplitude and loading history. This study presents the application of the Layered Viscoelastic Crirtical Distresses (LVECD) programme to predict the fatigue performance of 18 pavement sections from different locations in the United States and Canada. The capability of the LVECD programme to capture crack initiation, crack propagation and damage in the pavement sections is investigated by comparing the simulation results with field observations. This study found reasonable agreement in trends between the damage growth throughout the pavement cross sections as predicted by the LVECD programme and the surface crack growth as evidenced by field observations.     

Evaluating High-Frequency Visco-Elastic Moduli in Asphalt Concrete

Spectral Analysis of Surface Wave (SASW) testing was used to evaluate an improvement in the source generation repeatability of surface waves when a sheet metal couplant strip was used compared to the exposed asphalt surface, plus the increase in maximum coherent frequency achieved using a wire spring impactor compared to a solenoid driven steel striker plate. Both SASW and Multichannel Analysis of Surface Waves (MASW) were used to evaluate the high-frequency visco-elastic dynamic modulus of asphalt concrete plate specimens vs. a master curve constructed from standard dynamic modulus tests. The master curve was used to temperature and frequency shift the high-frequency moduli to an 18°C, 25 Hz reference value for comparison. SASW results tended to overestimate the 25 Hz reference modulus obtained from the master curve. MASW results agreed well with the reference modulus, when shifted using the master curve equations, but overestimated the reference modulus when shifted using a power function approximation of the linear portion of the master curve.     

Composite toughness enhancement with interlaminar reinforcement

An experimental investigation of a newly proposed through-thickness reinforcement approach aimed to increase interlaminar toughness of laminated composites is presented. The approach alters conventional methods of creating three-dimensional fiber-reinforced polymer composites in that the reinforcing element is embedded into the host laminate after it has been cured. The resulting composite is shown to possess the benefits of a uniform surface quality and consolidation of the original unreinforced laminate. This technique was found to be highly effective in suppressing the damage propagation in delamination double-cantilever beam (DCB) test samples under mode I loading conditions. Pullout testing of a single reinforcing element was carried out to understand the bridging mechanics responsible for the improved interlaminar strength of reinforced laminate and stabilization and/or arrest of delamination crack propagation. The mode I interlaminar fracture of reinforced DCB samples was modeled using two-dimensional cohesive finite-element scheme to support interpretation of the experiments.     

Proof testing of ceramic materials—an analytical basis for failure prediction

An analysis is presented which permits the accurate prediction of component lifetimes after proof testing. The analysis applies to crack propagation controlled fracture but can be used as a conservative prediction when crack initiation is predominant. The analytical predictions are confirmed in a series of time-to-failure measurements.     

Validation and enhancements for the localised experimental–numerical technique

Over many years the use of composite structures in aerospace and automobile applications has been expanding. Thus, the study of weaknesses associated with composite materials has become paramount. Delamination is a fundamental concern with these structures, and mixed mode strain energy release rates are valuable information for analysing delamination cracks. The localised experimental–numerical technique (LENT), which measures local test displacement data and combines this with local finite element analysis to evaluate the mixed mode strain energy release rates, is examined via extensive experimental testing and analysis to provide validation for the technique. Additionally, a sensitivity analysis is presented to assess the influence of pixel size on the strain energy release rate results determined using LENT. Enhancements to the method are presented focusing on reducing the pixel size and improving post-processing techniques for increased accuracy. Variations in the local area analysed with LENT are also investigated. The results demonstrate that the localised experimental–numerical technique has potential for the evaluation of mixed mode strain energy release rates using localised test data.     

Issues with Combining Road Elevation Spectral Models and Vehicle Vibration Response to Estimate Vehicle Dynamic Characteristics

The research presented in this paper focuses on issues associated with the development of an experimental technique to estimate the dynamic characteristics of wheeled vehicles (namely, the frequency response function) using only in‐service response data. To validate the approach and eliminate complexities associated with multi‐wheel vehicles, a single‐wheeled prototype vehicle was designed and commissioned. The vertical vibration acceleration of the prototype vehicle's sprung mass was measured during normal operation. The power spectral density function was computed and used to estimate the frequency response function of the vehicle. A number of experiments using various configurations of the single‐wheeled prototype vehicle were undertaken, along with a series of vibration table experiments to provide a comparison with the estimated frequency response functions. The results show that the best estimate of the frequency response function using the vehicle response data provides reasonable agreement with the measured laboratory experiments when the value of the slope of the spectral function is not set to the value suggested by the International Organisation for Standardisation. Another technique was further developed to estimate the value of the pavement spectral slope using only in‐service response data; however, this technique does not yield consistent and accurate estimates. Interestingly, the main resonance of the vehicle is in agreement between the vibration table and response data around the sprung mass of all three vehicle configurations when inspected using linear scales (regardless of the variation in the spectral shape of the excitation), although the additional modes (including the unsprung mass) differs for all vehicles. Copyright © 2014 John Wiley & Sons, Ltd.     

板材厚度变化对Lamb波透射系数的影响

对Lamb波与厚度变化的板材之间的相互作用进行了分析。针对S0模式Lamb波入射情况,利用混合边界元模型计算了台阶状散射体的透射系数与厚度变化的关系,并进行了实验验证。结果表明S0模式的透射系数能够很好的反映板材的厚度变化,从而为该类材料的定量无损检测提供一定的理论支撑,在实际工业中具有重要的应用意义。     

Theoretical and experimental investigations of lateral modes in 1-3piezocomposites

Materials with a periodic microstructure show resonances caused by the elastic wave Bragg diffraction. This paper presents a simple approach to describe these resonances (called lateral resonances) in 1-3 piezoelectric composite materials which have a 2-D periodicity. Our model is based on the analysis of the propagation of transverse waves in a 2-D periodic medium of infinite thickness and takes into account the periodic and interfacial boundary conditions. This model predicts the displacement field vectors and frequencies of lateral resonances from which the phase velocity of lateral waves is determined. The theoretical and experimental variations of this velocity versus the ceramic rod width to pitch ratio are compared. It is shown that the first lateral mode frequency is maximum when the ceramic volume fraction is around 0.65. Theoretical predictions of the mechanical displacement at the composite surface are compared with measurements obtained by an interferometric laser technique. A good agreement is observed, showing that lateral waves are mainly vertically polarized     

Statistical static timing analysis using symbolic event propagation

Accurate estimation of critical path delays in circuits is a challenging task, particularly when variations due to manufacturing are considered. For small circuits (such as standard cells), simulation-based characterisation is preferred for better accuracy. For large circuits, statistical timing analysis techniques are used, but these methods typically yield a pessimistic overestimate. In view of the growing size of custom cell designs, an intermediate approach is required -one that can scale to circuits of moderate size and can produce more accurate estimates than traditional static timing analysis methods. A new method is presented that combines symbolic event propagation with statistical timing analysis and thereby achieves a significant level of accuracy with acceptable computational overhead. The benefits of the new style of analysis over the ISCAS'89 benchmark circuits are demonstrated.     

Mixed Mode Fracture of Concrete under Proportional and Nonproportional Loading

A novel testing procedure for mixed mode crack propagation in concrete is presented: four point bend of notched beams under the action of two independent force actuators. In contrast to classical procedures, this method allows nonproportional loading and crack trajectory modifications by changing the action of one actuator. Different experimental crack trajectories, under mixed mode and nonproportional loading, are presented together with the corresponding curves of load-CMOD and load-displacement. The tests were performed for three homotetic specimen sizes and two mixed mode loading conditions. The results are useful for checking the accuracy of mixed mode fracture analytical and numerical models. The models should predict the crack trajectory and a complete group of experimental records of load and displacements on several control points in the specimen. This revised version was published online in July 2006 with corrections to the Cover Date.     

Analysis of strictly bound modes in photonic crystal fibers by use of a source-model technique

We describe a source-model technique for the analysis of the strictly bound modes propagating in photonic crystal fibers that have a finite photonic bandgap crystal cladding and are surrounded by an air jacket. In this model the field is simulated by a superposition of fields of fictitious electric and magnetic current filaments, suitably placed near the media interfaces of the fiber. A simple point-matching procedure is subsequently used to enforce the continuity conditions across the interfaces, leading to a homogeneous matrix equation. Nontrivial solutions to this equation yield the mode field patterns and propagation constants. As an example, we analyze a hollow-core photonic crystal fiber. Symmetry characteristics of the modes are discussed and exploited to reduce the computational burden.     

Smoothing artificial stress concentrations in voxel-based models of textile composites

Voxel meshing is an effective method to discretise the internal architectures of multi-phase materials. Spurious stresses are however introduced in the vicinity of a multi-material interface due to the stepped, block-like representation of smooth boundaries. A stress averaging technique is presented to eliminate artificial mesh-imposed stress concentrations. The effect of the local averaging domain size, averaging weight function, and mesh dependence is explored. The voxel finite element method with stress averaging is then further developed to study progressive damage propagation and failure analysis of composites. An additional control, based on the failure plane angle of each element, is included to ensure propagation of damage in the direction dictated by the physics of the process rather than mesh artefacts. It is found that the stress averaging technique is an effective way to alleviate local stress concentrations and can ensure correct damage and failure mode prediction when compared to a conformal mesh.     

Bending strength of delaminated aerospace composites

Buckling-driven delamination is considered among the most critical failure modes in composite laminates. This paper examines the propagation of delaminations in a beam under pure bending. A pre-developed analytical model to predict the critical buckling moment of a thin sub-laminate is extended to account for propagation prediction, using mixed-mode fracture analysis. Fractography analysis is performed to distinguish between mode I and mode II contributions to the final failure of specimens. Comparison between experimental results and analysis shows agreement to within 5 per cent in static propagation moment for two different materials. It is concluded that static fracture is almost entirely driven by mode II effects. This result was unexpected because it arises from a buckling mode that opens the delamination. For this reason, and because of the excellent repeatability of the experiments, the method of testing may be a promising means of establishing the critical value of mode II fracture toughness, G(IIC), of the material. Fatigue testing on similar samples showed that buckled delamination resulted in a fatigue threshold that was over 80 per cent lower than the static propagation moment. Such an outcome highlights the significance of predicting snap-buckling moment and subsequent propagation for design purposes.     

Terminal ballistic effect on the crack growth assessment of a helicopter rotor drive

An experimental impact test of a 7.62 NATO projectile perforating a helicopter tail rotor shaft is briefly presented. With the aim to reproduce the operational worst case, the damage occurs during the flight, the impacted specimen is subsequently tested with a multiaxial testing machine. This procedure allows the evaluation of the capability of the shaft to withstand a recovery mission, which is simulated by applying a torsional fatigue spectrum to the shaft. The nucleation and propagation of fatigue cracks have been monitored during the test.Finite element analyses have been carried out to simulate both impact and crack propagation. The state of stress is multiaxial: it is induced by torque, residual stress field and shape of the damage. Mixed mode propagation has hence been considered by means of an equivalent SIF. Numerical results are in agreement with experimental data.     

A detailed study of crack propagation in cement-based fibre composite beams under bending

Multiple cracking of beams made of cement-based composite was extensively studied by means of a suitable non-contact measurement technique. Images of a grid-pattern bonded onto the surface of a tested specimen were captured by a digital camera. Image processing by means of an appropriate software provided the displacement field within the zone under investigation. Cracks were revealed by discontinuities in the displacement field. Results are presented for three specimens tested in three- or four-point bending. Location, actual length and width are easily and precisely determined for each of the cracks within the zone under investigation. High sensitivity, early crack detection and no need to anticipate crack location are among the main features of this full-field measurement method. Crack propagation is derived from crack diagrams obtained at successive loading levels. Initial length, spacing, opening, propagation and profile of cracks are discussed in terms of mode of loading and bond properties of the material.