Fracture, Fatigue, Failure, and Damage Evolution, Volume 5

A finite-element-based simulation technique is being developed to predict 3-D, arbitrary, non-planar evolution of mixed-mode crack growth. The approach combines a geometrically explicit crack front re-meshing scheme, and an energy-based growth formulation to predict extension magnitudes along the crack front. The technique also leverages a new 3-D mixed-mode energy release rate decomposition using the virtual crack extension (VCE) method. The energy-based crack growth formulation, previously implemented for planar crack growth, is extended to non-planar growth situations by employing a basis-function approach to describe crack front extensions. Rather than determining point-by-point extensions, calculating a governing function alleviates numerical influences on the crack growth predictions. The simulation technique seeks to mitigate computationally biased crack growth, as found in prescribed and mesh dependent methods, for example.

The aerospace industry has experience with a range of structural failures, oftentimes due to fatigue cracks in aircraft fuselage components that are exposed to relatively high stress levels during cyclic loading effects that lead to fatigue crack initiation at material defects and near stress concentrations. These aircraft components are under complex stress states. In this study, mixed mode I/II fatigue experiments and simulations are performed for an Arcan fixture and a 6.35 mm thick Al-2024-T351 specimen, a popular aerospace alloy. Experiments were performed for Arcan loading angles that gave rise to a range of Mode I/II crack tip conditions from 0 ≤ ΔKII/ΔKI ≤ ∞. Measurements include the crack paths, loading cycles, and maximum and minimum loads for each loading angle. Simulations were performed using three-dimensional finite element analysis (3D-FEA) with 10-noded tetrahedral elements via the custom in-house FEA code, CRACK3D. While modeling the entire fixture-specimen geometry, a modified version of the virtual crack closure technique (VCCT) with automatic crack tip re-meshing and a maximum circumferential stress criterion was used to predict the direction of crack growth. Results indicate excellent agreement between experiments and simulations for the measured crack paths during the first several millimeters of crack extension.

The mixed-mode crack growth coupling mechanical and thermal loads in wood material is investigated in this numerical work. The analytical formulation the crack driving force, namely the energy release rate, is introduced by T-integral that takes into account mixed mode fracture, thermal process in orthotropic material and pressure applied on the crack lips. This new formulation is based on Nother’s theorem and the definition of the strain energy density according to Lagrangian’s and Eulerian’s configurations. Moreover, this analytical formulation is implemented in finite element software Cast3m. First of all, several numerical examples, dealing with isotropic material, are provided to illustrate the accuracy of the FEM model. Then, the crack resistance of a timber CTS (Compact Tension Specimen) is investigated to show the efficiency of the proposed approach in the case of orthotropic material.

Methods for predictive modeling and simulation of crack branching and curvilinear crack paths under cyclic out-of-phase loading conditions have been developed and implemented in a custom finite element code CRACK3D. 3D mixed-mode stress intensity factors (SIFs) for fatigue crack growth simulations with curved crack paths and curved crack fronts are determined using the 3D virtual crack closure technique (3D VCCT) and a locally structured re-meshing approach in which the local region immediately surrounding a moving crack front is automatically re-meshed with a structured mesh pattern to facilitate the 3D VCCT and maintain its accuracy. The prediction of the crack growth direction is achieved using the maximum circumferential stress criterion. Fatigue crack growth events under out-of-phase loading conditions in cruciform aluminum specimens with a central hole and an edge crack at the hole are simulated. Simulation predictions of crack branching angles and the curvilinear paths of the branched cracks agree well with experimental measurements.

Under torsional loading a circular rod of a brittle material fails along a spiral fracture surface. By notching the rod deeply enough the fracture will be constrained to a nominally flat, but localy faceted fracture surface. This faceted surface consists of a sequence of 45 degree surfaces linked to each other. Using notched PMMA rods under torsional loading, the notch depth at which the surface transitions from spiral to nominally flat is mapped out experimentally. The results show that for a notch depth/radius 20 % the surface is nominally flat. For notch depths/radius 10 % the surface is spiral.

Variations in repair parameters and techniques have a significant influence on the mechanical properties of repaired fiberglass reinforced composite materials. A common method to repair damaged fiberglass wind turbine blades is to conduct hand layup repairs after grinding out the damaged portion. In one sided repair, the ply to which the damage extends depth wise is usually ground off completely and repairs are conducted on the top surface of the next ply. The scope of this work was to observe the effect on repair fracture toughness values when repairs are conducted with this top ply ground off partially. Mixed mode I–mode II testing was carried out on repaired fiberglass composite materials to determine the crack initiation fracture toughness values of two kinds of repairs; one with the top ply intact and other with the top ply partially ground off. The results from this testing were compared to those obtained from mode I testing conducted on similar repairs.

Fracture behavior of a Ti/TiB graded material with a crack perpendicular to the gradient direction was investigated. Three-point bending experiment was conducted and full-field displacement fields were measured on both faces of the specimen, metallic and ceramic-rich surfaces, using 2D digital image correlation technique. Stress intensity factors were calculated using the displacement fields obtained on both faces of the specimen, and were compared to the effective fracture toughness determined from fracture load data. The overall fracture toughness was found to be very close to the stress intensity factor determined using the displacement field on ceramic-rich side. However, the stress intensity factors calculated on both surfaces using the displacement fields, showed different values. In addition, scanning electron microscope observations were performed to examine the fracture surface of the material. It was observed that the crack front is inclined, indicating that the fracture may have initiated in the ceramic side and followed by a very fast propagation to the metallic side.

The purpose of this research is to investigate the interaction of two adjacent cracks under cyclic loading. A series of tests at different loads and frequencies were conducted under uniaxial constant amplitude fatigue loads on API-5L grade B steel samples. Crack growth rate of two initial semi-elliptical cracks was investigated. Marker band testing technique was employed to study the crack growth rate before and after interaction. Real time optical microscopy was used to observe crack coalescence on the specimens’ surface. Moreover, SEM analysis of the fracture surface was performed for comprehensive understanding of cracks behavior.

Desiccation cracking is a phenomenon in which cracks are caused by shrinkage of drying paste material. In the present study, digital image correlation, DIC, is applied to measure the displacement field at the crack tip in a drying paste. In order to obtain magnified images around the crack tip, the control method for crack nucleation position and crack propagation direction is newly developed. Furthermore, stress intensity factors are evaluated by iterative procedure from the measured displacement field. The results show that the stress intensity factors are varied during crack propagation because of kinked crack path ant the fracture toughness depends on the paste thickness even the paste is composed of the same powder. In addition, it is found that the fracture toughness is independent of the crack propagation speed. The dependence of the fracture toughness on the paste thickness is noticeable results to analyze the fracture of desiccation cracking.

This paper reports on the fracture behavior of MWCNT-reinforced cement paste based on evidence from three-point bending tests of single-edge notched beam samples. Digital image correlation (DIC) was used to measure full-field displacements at different stages of fracture in reinforced and unreinforced samples. Strain maps extracted from displacement data were used to characterize the morphology of the fracture process zone (FPZ). The DIC principal tensile strain maps from nanoreinforced samples consistently highlighted the development of a larger FPZ prior to failure. Evidence from scanning electron microscopy analysis of fracture surfaces further supports the hypothesis that highly-dispersed and well-bonded MWCNTs contribute to toughness through crack-bridging.

The use of lightweight aluminum construction has become prevalent in marine structures and it is therefore critical that the performance of such structures in the inelastic regime be characterized. A key aspect of this is the ductile fracture behavior of panels and structural details. Here, the results of ductile fracture tests conducted on large pre-notched aluminum panels seeking to characterize this inelastic response and resulting fracture are reported. Plates fabricated from 5083-H116 and 6061-T6 panels, including a welded panel, were tested with a geometry designed to induce macro-plasticity prior to fracture. Testing was performed using a newly developed heavy steel fixture of sufficient rigidity and included the use of Digital Image Correlation (DIC) strain measurement. Through extensive data collection and analysis, the full extent and severity of plastic deformation prior to fracture were obtained. It is anticipated that this data will be of utility in the calibration and validation of numerical modeling of ductile fracture in large structures.

One of the major concerns regarding the use of lightweight materials in ship construction is the response of those materials to fire scenarios, including the residual structural performance after a fire event. This paper presents a study on creep damage evolution in 5083 marine-grade aluminum alloy and its impact on residual mechanical behavior. Samples were tested at 400 ° C and at different stress levels and times selected to achieve specified creep strain levels. Scanning electron microscopy (SEM) and high resolution optical microscopy were utilized to characterize the creep damage. The damage is primarily manifested as cavitation, grain elongation, and dynamic microstructural evolution. The cavitation morphology, orientation and grain structure evolution were investigated on three perpendicular sample surfaces (rolling, transverse, and normal faces). The competing process between cavitation and grain structure evolution were studied to develop an understanding of the creep damage mechanism. The post-fire residual strength of the Al5083 at 400 °C reveals that the cavitation counteracts the strengthening by grain elongation.

Young’s modulus and hardness of shale are important parameters for the design of hydraulic fractures and the selection of proppant. Nanoindentation has shown applicability in fine grained rocks and provides a method to measure these parameters using core fragments and drill cuttings. Nanoindentation measurements on horizontal and vertical samples can be used to quantify anisotropy. Indentation Young’s modulus correlates well with dynamic modulus obtained from acoustic velocity data on core plugs. Nanoindentation is thus a viable method to measure mechanical properties of fine grained shale. Young’s moduli obtained by nanoindentation are related to composition and porosity, e.g., total organic carbon (TOC), porosity and mineralogy. Young’s modulus showed a trends with both TOC and porosity and warrants further study to establish more robust relationships. The primary aim of this paper is to study the applicability of nanoindentation to shale and this is established by comparing nanoindentation results with standard dynamic pulse-transmission velocity measurements. Nanoindentation on Wolfcamp shale, Lyons sandstone, Sioux quartzite, Indiana limestone and pyrophyllite was performed to test whether the nanoindentation test gives representative results of the whole sample with variation in grain size. The experimental results indicate that nanoindentation measures the grain Young’s modulus due to all indentations on a single grain for coarse grained rocks and thus, resulting in incorrect Young’s modulus of rock. However, for rock with smaller grain (<indenter area), nanoindentation results are representative of multiple grains and hence correlating well with the dynamically determined Young’s modulus.

Micro-electromechanical systems enable many novel high-tech applications. Aluminum alloy thin films would be electrically favorable, but mechanical reliability forms fundamental challenges. Notably, miniaturization reveals detrimental time-dependent anelasticity in free-standing Al-alloy thin films. Yet, systematic experimental studies are lacking, perhaps due to challenges in microscale testing.

To this end, a microbeam bending methodology (with <4 με strain resolution) and nano-tensile tester (with 70 nN and <6 με resolution) have been developed for reproducible long-duration characterization of anelasticity of on-wafer 5 μm-thick Al-(1 wt%)Cu test structures under real-time in situ microscopy. Time-dependent anelasticity was indeed observed, both in bending and in tension, and a multi-mode visco-elastic model was found to described and predict the non-linear anelastic behavior between 1 and 105 s.

To gain insight in the underlying micro-mechanisms, time-dependent anelasticity was characterized under systematic variation of the grain boundary density and copper precipitate state/distribution (carefully analyzed using HRTEM/EBSD/ WAXD/EDX/SEM), yielding a wealth of information. Surprisingly, both microstructural features were excluded as the (primary) cause of the time-dependent anelasticity. Based on dynamic strain aging effects observed in nano-indentation, an underlying micromechanism responsible for time-dependent anelasticity was hypothesized.

Previous results have suggested that oxidation of nanoscale Si structures is correlated with a reduction in strength. The mechanism by which this occurs is unknown however. It is possible that the process of oxidation causes some irreversible change in the surface of the crystalline Si. It may also be possible that the change in strength is due to the presence of the oxide itself, perhaps changing the atomic or electronic distribution in some way. In this work, we allow a set of nominally identical nanoscale Si beams to oxidize over approximately five weeks while taking periodic fracture strength measurements. The oxide is then removed and strength is measured a final time. We see that after removing the oxide, strength does not recover at all, suggesting that the change is strength is indeed due to a change in the Si surface.

The advent of small-scale testing procedures coupled with scanning electron microscopy (SEM) imaging allow for high-resolution digital image correlation (DIC) studies to examine strain localization at the grain size length scale. A systematic study was performed to determine how speckle patterning parameters (speckle density and shape) affect strain resolution of DIC using SEM imaging. Strain resolution increased with increased speckle density from 23 to 58 % area fraction. Patterns with less than 23 % area fraction exhibited significant signal noise, and a loss in strain resolution due to inadequate correlation. It was also observed that when the edges of square speckles were aligned with SEM rastering directions, the noise in the

e

yy

data was double the noise in the

e

xx

data. Rotating the speckle to eliminate edge alignment with the rastering direction significantly decreased the

e

yy

strain noise. Competing optimization requirements for the correlation parameters were needed to minimize strain intensity noise or maximize spatial resolution. Application of the optimization techniques to high temperature in-situ studies of Ni-based superalloys will also be presented.

The present paper is aimed at investigating the effect of shot peening on the very-high cycle fatigue resistance of the Al-7075-T651 alloy. Pulsating bending fatigue tests (R = 0.05) were carried out on smooth samples exploring fatigue lives comprised between 10

5

and 10

8

cycles. Three peening treatments with different intensity were considered to explore different initial residual stress profiles and surface microstructural conditions. An extensive analysis of the residual stress field was carried out by measuring with the X-ray diffraction (XRD) technique the residual stress profile before and at the end of the fatigue tests, so as to investigate the onset of a stabilized residual stress field. Fatigue crack initiation sites have been investigated through scanning electron microscopy (SEM) fractography. The surface morphology modifications induced by shot peening were evaluated using an optical profilometer. The influence of surface finishing on the fatigue resistance was quantified by eliminating the surface roughness in some peened specimens through a tribofinishing treatment.

Timber structures often exhibit shear and tension perpendicular to grain. This phenomenon induces brittle failure if it is not controlled. This is particularly the case in joining zones, and even more when the beam elements are thin. These thin elements can be found for example in lattice beams. Standardized lattice timber beams appear as an efficient solution for economical, ecological and mechanical aspects. This study focuses on the mechanical behavior of notched beams. Experiments are carried out with a classic loading device and LVDT measurements as well as with the grid method which provides full-field displacement and strain measurements. Tests are conducted for various orientations of annual rings of the wood. The evolutions of strains in the zone affected by shear and tension stresses are analyzed.

NiTi (aka Nitinol) shape memory alloys can be alloyed with Nb in order facilitate a wider thermal hysteresis. The ternary alloy thermal hysteresis is nearly 100 °C, which is considerably higher than binary Nitinol. The wider hysteresis for NiTiNb makes the alloy suitable for the civil engineering operating temperature ranges. Thus the alloys are promising for integration into structures and can lead to the development of SMA hybrid composite smart structures. The wider hysteresis is attributed to the presence of second phases within the microstructure. The microstructure consists of a matrix, which undergoes the martensitic transformation and thus gives rise to shape memory behavior. A eutectic-like microstructure constituent is evident, with a substantial volume fraction of nano-precipitates dispersed throughout, and neither takes part in the transformation. Consequences of tailoring the microstructure via thermo-mechanical treatments are explored. In this work, the interaction of the non-transforming microstructural constituents with the martensitic transformation morphology is characterized using a fundamental thermo-mechanical framework. The bulk scale deformation measurement are reported as well as the meso-/micro-structure scale full-field and local deformation analyses. The technique used in this work is in-situ Digital Image Correlation (DIC). With this, the current work assesses the surface strain morphology in order to elucidate the influence of microstructure on the bulk scale shape memory responses.

The objective of this study is to show experimentally the intermittency of the phase transformation in a shape memory alloy using a kinematical full-field measurement method. The specimen is a Cu-Al-Be single crystal with Ms = −2 °C. A uniaxial loading was applied by using a device based on gravity. In practice, a drop-by-drop device controlled by water pumps enabled us to apply a perfectly monotonic loading with very small force increments. The grid method was used to measure the strain fields on the specimen surface during the test. It is observed that the plateau which is classically obtained when the specimen transforms from austenite to martensite is actually characterized by an intermittency of the phase change. It means that the events, in terms of appearance of martensite needles and propagation, occur with an irregular alternation. The paper presents the experimental setup, the image processing and some typical results.

Increasing use of Shape Memory Alloys (SMA) for complex applications requires a robust modeling of phenomena governing their behavior. The development of micro-macro multiaxial model is relevant. Such approach relies the definition of transition scale rules, depending on the microstructure, and a description of the behavior of constituents. On the other hand, it requires experiments for identification of parameters such as enthalpies or kinetic constants and validation of the model. In this paper, in situ X-Ray Diffraction (XRD) measurements are performed during tensile tests and heating-cooling cycles. XRD permits monitoring of the average volume fraction of phases in presence. Results will be used for the validation of a multiscale and multiphased model.

Previous work has shown that, under some circumstances, failure of polymer-matrix fiber composites under bending fatigue loading can transition from compressive/local buckling failure to tensile/fiber fracture failure. For low cycle fatigue, failure tends to be compressive in nature and can be modeled based on kink band theory. For high cycle fatigue, failure tends to be tensile and can be modeled based on a fatigue damage/wear-out model. In this work, we investigate the effect of load ratio on the transition from low cycle/compressive failure to high cycle/tensile failure for a unidirectional polyester/glass fiber composite. The stiffness degradation of the beams under changing loading conditions is also investigated.

In this study, the objective was to develop, manufacture, and test hybrid nano/microcomposites with a nanoparticle reinforced matrix and demonstrate improvements to damage tolerance properties in the form of Mode-II fracture toughness and related impact damage absorption. The material employed was a woven carbon fiber/epoxy composite, with multi-wall carbon nanotubes (CNT) as a nano-scale reinforcement to the epoxy matrix. A direct-mixing process, aided by a block copolymer dispersant and sonication, was employed to produce the nanoparticle-filled epoxy matrix used in composite fabrication. Composite samples were tested as End Notched Flexure (ENF) specimens in three point bending to determine the static Mode-II fracture toughness, showing improvement of approx. 30 % for nano-reinforced composite over reference material. Certain testing and material difficulties were noted with useful implications for both the testing technique as applied to woven composite materials and the material properties of the nano-reinforced composite. Impact tests were then performed in a falling-weight drop tower to generate delamination damage in samples of hybrid and reference composite. Impact damaged specimens were imaged by ultrasonic c-scans to assess the size and internal geometry of the damage zone, showing a consistently smaller mean damage zone diameter (approx 15 %) for hybrid composite over reference material. This translated to a nominally higher Mode-II fracture toughness in the hybrid composite (approx 30 %) regardless of specific impact energy, agreeing with static Mode-II fracture toughness tests.

Adhesive joining is gaining acceptance in aerospace, automotive, and marine industries as it allows joining of similar and dissimilar materials, eliminates secondary machining processes, minimizes stress due to mechanical joining and most importantly reduces structural weight. One way of improving the performance of adhesively bonded joints is the use of nano-/micro- particle reinforcement of the adhesive or composite matrix. While considerable literature exists on nano-modified composite substrates to improve their mechanical and thermal properties, the work on nano-/micro- modified adhesively bonded joints and their fatigue behaviour is relatively limited.

Therefore, in this paper, the fatigue behaviour of single lap-joints (SLJs) made of S-glass/epoxy substrates were studied. Adhesives with and without glass-bubbles (microspheres) modification were evaluated. The specimens were subjected to different fatigue (tension–tension) loadings with the maximum loading level of 90 % of the bond shear strength. As expected, the fatigue life of adhesive joints in both cases, pristine and glass bubble modified adhesives, depend on the applied cyclic loadings. Inclusion of glass bubbles improves the tensile strength and the fatigue life of single-lap joints. The observations from the experimental fatigue data and its correlation with failure could be considered as valuable information for better design of composite joints and related components.

Curved composite parts are increasingly replacing metal ribs and box structures in recent civil aerospace structures and wind turbine blades. Delamination of L-shaped composite laminates occurs by interlaminar opening stresses in addition to the interlaminar shear stresses at the curved region. An experimental setup is designed to investigate dynamic delamination in L-shaped composite brackets under quasi static shear loading. The materials are unidirectional [0]

17

and cross-ply [0/90]

17

epoxy/graphite composite laminates. The load displacement curves are recorded and subsequent dynamic delamination is captured with a million fps high speed camera. The failed specimens are analyzed under a microscope. It is seen that layup differences change the failure mechanism in composites. Multiple delaminations in one load drop are observed in failure of unidirectional laminate whereas sequential delamination at each discrete load drop is seen in cross ply laminates. In the [0] laminate single delamination in the center ply followed by symmetric delamination nucleations around the two crack tips are observed. In the 0/90 cross-ply laminate, multiple load drops are recorded and delaminations start near the inner radius by peeling of 0/90 plies sequentially at each load drop. In both layups, first time observation of intersonic delamination speeds up to 2,200 m/s are made.

In the UK, water companies renew their aging water network using novel techniques and robust materials. The most common type of material used for network rehabilitation is Polyethylene (PE) pipe. A common method of jointing PE pipe is electrofusion welding. Here, electricity is used to heat a coil that melts the fitting and the host pipe of the same material. When the joint cools it forms a bond. However, premature failure of these can occur if best practice installation principles are not followed on site.

A novel experimental rig, designed to be retrofitted to an existing servo-hydraulic fatigue testing machine, has been used to cyclically pressurise PE fittings that have been created with a controlled element of ‘poor workmanship’. Extensive fatigue tests have shown the relationship between joint failure and the dynamic pressures experienced in water distribution systems. Furthermore, the effects of poor workmanship have shown to have a detrimental effect on asset integrity.

A post-failure analysis of the fittings using non-destructive ultra-sonic methods has shown the failure paths of the fittings. Additionally, a bespoke ultra-sonic rig was designed and built to monitor the crack propagation of the fittings during live dynamic tests to confirm the mode of failure.

Next Generation Nuclear Plant (NGNP) designs for very-high-temperature reactors (VHTR) employ intermediate heat exchanger (IHX) for which the material demands are extreme. Currently, Alloy 617 and Alloy 800H are considered to be among the candidate materials for the high-temperature, helium-cooled environments that are planned for these systems. The primary helium coolant is expected to operate at temperatures at or above 750 °C, and creep crack growth (CCG) of these candidate alloys is of particular concern for their reliability in VHTRs for long-term service. Using an apparatus that was designed and constructed in-house, CCG testing was conducted on compact tension specimens at temperatures up to 850 °C in controlled environments, including air and impure helium, following ASTM standard E 1457-07. Overall, our CCG testing revealed that Alloy 617 exhibits superior resistance to creep crack growth compared to Alloy 800H. Trends observed in the mechanical behavior and microstructure of the candidate alloys as a function of environment will be discussed.

The elastic energy released by micro-cracking yields to macroscopic fracture whose mechanical vibrations are converted into electromagnetic (EM) oscillations over a wide range of frequencies, from few Hz to MHz, and even up to microwaves. As regards Acoustic Emission (AE), the classical monitoring techniques allow an observation over a range of frequencies up to hundreds of kHz. In this paper the authors investigate if, during compression tests on brittle materials, which involve catastrophic fractures, it is possible to identify in the stressed materials mechanical oscillations in a frequency range higher than that characteristic of the AE and comprised between MHz and THz. This excited state of matter could be a precursor of subsequent resonance phenomena of nuclei able to produce neutron bursts, especially in the presence of sudden catastrophic fractures. This phenomenon has been also very recently argued from a theoretical physical point of view by Widom et al

.

In this investigation experimental evidences emerge by means of a confocal sensor able to measure the resonance frequency of the specimen. The basic idea is to use a laser light focused onto a spot of the specimens surface subjected to mechanical compression. A photo-detector measures the intensity of the reflected light and then gives the frequency variation that is proportional to the vibration frequency of the spot particles.

The electromagnetic (EM) signals detected during failure of brittle materials are analogous to the anomalous radiation of geoelectromagnetic waves observed before major earthquakes, reinforcing the idea that the EM effect can be applied as a forecasting tool for seismic events.

Moreover, it has been argued (Widom-Swain-Srivastava) that the elastic energy released by micro-crack production eventually yields to forming macroscopic fractures, whose mechanical vibrations are converted into electromagnetic oscillations over a wide range of frequencies, from few Hz to MHz, and even up to microwaves.

This excited state of the matter could be a precursor of subsequent resonance phenomena of nuclei able to produce neutron bursts in the presence of stress-drops or sudden catastrophic fractures. As a matter of fact, neutron emissions (NE) measured at seismic areas exceed the usual neutron background up to three orders of magnitude in correspondence to rather appreciable earthquakes.

In this work the Authors measure the EM pulses generated during micro-cracking of rock specimens by a dedicated loop antenna sensitive up to MHz.

Taking into account the relationship between EM, NE and seismic activity, it will be possible to set up a sort of alarm system that could be at the base of a warning network.

The few non-visual methodologies make use of wired devices. Systems based on wireless transmission should be cost efficient and adaptive to different structures. The Acoustic Emission (AE) technique is an innovative monitoring method useful to detect damage, as well as to evaluate the evolution and the location of cracks. This paper shows the capability of a new data processing system based on a wireless AE equipment, very useful to long term monitoring of steel, concrete, and masonry structures. To this purpose, computer-based procedures, including an improved AE source location based on the Akaike algorithm, are implemented. These procedures are performed by automatic AE data processing and are used to evaluate the AE results in steel structures monitored during fatigue loading condition. In the most critical cases, or in some cases requiring long in situ observation periods, the AE monitoring method is fine tuned for a telematic procedure of processing AE data clouds to increase the safety of structures and infrastructural networks.

Some of the most significant civil and historical structures, such as bridges, viaducts, dams and ancient monumental buildings, may collapse below the critical loading conditions due to fatigue effects. In the present paper, damage level evolution during fatigue tests of rock specimens will be evaluated by Acoustic Emission technique (AE). The fundamental parameters of AE will be analyzed in order to interpret the damage level evolution. The relationships between the cumulative AE counts and the damage accumulation during fatigue experiments will be considered for different kinds of iron-rich rocks (granite, basalt, magnetite). The achievement of critical conditions will be recognized through synthetic parameters based on AE, such as the

b

-value of the Gutenberg-Richter law and considering the increase in low-frequency components during the damage accumulation.

In this paper the crack propagation process was monitored by two different experimental techniques. Acoustic emissions sensors were placed on the sample in order to monitor the evolution of the acoustical events during the test; at the same time the change in temperature was monitored by thermography. Test were run on aluminum samples (Al 5068). Specimens were previously cracked, by cutting notches having known sizes and geometry. Successively, X-Ray diffractometry analysis were performed in order to establish the given initial stress state of each sample. Specimen were then subjected to mechanical tests. During these tests the crack propagation was continuously monitored and recorded by both techniques. Data obtained, in terms of number of hits, amplitude signals and maps’ temperature, were critically compared in order to assess the capability of each technique in following the evolution of the damage process.