The two-dimensional (2D) delamination behavior of composite laminates under quasi-static out-of-plane opening loads has been experimentally investigated. A novel design and experimental set-up for fiber-reinforced polymer (FRP) laminated plates with a circular embedded pre-crack was developed. Increasing load-displacement curves were obtained due to the increasing crack front length during propagation. Throughout the loading process, stiffening and softening mechanisms were activated. The stretching of delaminated part of the laminates constituted the main stiffening mechanism. Once the crack started growing, a corresponding softening due to crack propagation occurred together with a secondary stiffening mechanism, fiber-bridging. These stiffness-related mechanisms were reflected in the compliance.

1. Objectives

It has been recognized that fluctuation of fluid temperature in a pipe may induce high cycle thermal fatigue in the pipe, which sometimes results in fracture of the pipe. The information about the temperature and residual stress distributions is important to prevent the high cycle thermal fatigue. It is, however, difficult to measure the inner surface temperature directly. The monitoring of the temperature on accessible outer surface is promising to solve the problem. This paper presents an inverse analysis method for estimating the temperature and residual stress distributions in the pipe from the temperature history measured on the outer surface. A regularization method is proposed for obtaining a reasonable solution.

2. Inverse Method for Estimating Temperature and Thermal Stress Distributions

Based on a mathematical analysis it was found that the relationship between the temperatures on the inner surface and on the outer surface is expressed by a reduction factor R and a phase lag Dp, which are dependent on frequency. An inverse analysis method was proposed, in which the outer surface temperature measured on the outer surface is decomposed of frequency components, and their amplitude is multiplied by a factor of 1/R and the phase is advanced by Dp. The inner surface temperature history is obtained by summing up the frequency components.

3. Regularization Method

The reduction factor R is very small for very high frequency components. Then high frequency fluctuation measured on the outer surface is exaggerated in the estimation of the inner surface temperature history. In the presence of measurement noise, incorrect high frequency components deteriorate the estimation. A regularization method is proposed to exclude the incorrect components. A maximum limit is set to the factor of 1/R. This limit is determined by considering the effect of noise to the coefficient of frequency coefficients.

4. Numerical Simulations

Numerical simulations are made to examine the applicability of the proposed method. A pipe made of stainless steel is considered. The diameter of the pipe assumed to be large compared with the pipe thickness. Is is assumed that the inner surface temperature history is made of three frequency components.

5. Results

It is found that the temperature history on the inner surface is recovered when measurement noise is not included in the measurement o the outer surface. As the noise level is increased high frequency components give rise to big fluctuation in the estimated inner surface temperature history. When the regularization method is applied reasonable estimation is made, although the high frequency components are lost. Using the temperature history the estimation of the thermal stress in the pipe is made.

6. Conclusions

This paper presents an inverse analysis method for estimating the temperature and residual stress distributions in the pipe from the temperature history measured on the outer surface. It is found that the proposed method is useful for obtaining a reasonable estimate of the inner surface temperature and thermal stresses.

The effect of the nitrogen diffusion layer formed by gas blow induction heating nitriding on the wear resistance and the fatigue properties of a titanium alloy was investigated. The nitrogen diffusion layer deteriorated the wear resistance of a titanium alloy. This is probably because hard abrasive particles worn from the diffusion layer accelerates the wear of the material. In contrast, the diffusion layer improved the fatigue properties of a titanium alloy. This is due to the high hardness of the diffusion layer, and because the layer and the substrate share the same elastic modulus, the surface stress concentration is inhibited.

Railway bridges are excited to forced vibrations during train crossing and destabilization of the ballast bed may result. Thus, instability of the rail position may occur that lead to critical states for trains and passengers, respectively. During the design process of new railway bridges that are built on high-speed railway lines in European countries a dynamic analyses of train crossing must be performed and a limit value of 3.5 m/s² of the maximum vertical bridge deck acceleration must be fulfilled according to EN 1991-2. A second limit value is the restriction of the bridge end rotations due to train crossing to a value of 6.7 ‰ according to the Austrian guideline “Dynamic calculation of railway bridges due to train crossing”.

The arising size of the bridges forced vibration amplitudes due to train crossing depends on both, train specific and bridge specific parameters. Train specific parameters represent the dynamic excitation forces and they are influenced of train speed, axle load and axle distances. The bridge specific parameters are essentially the natural frequencies and the structural damping coefficients. In cases, where the periodic excitation forces due to train crossing meets one of the natural frequencies of the bridge structure, resonant vibrations occur with unwanted large vibration amplitudes.

Within the dynamic analyses of train crossing the natural frequencies of railway bridges are computed by the ratio of bending stiffness to mass per unit length. Both, the stiffness and the mass are taken from the bridge design documents. In a large part the comparison of calculated natural frequencies with frequencies determined by in-situ modal testing show a good agreement. The structural damping of bridges is defined by the Lehr´sche damping coefficient and it cannot be calculated in practical applications. Within numerical simulations of train crossing the Lehr´sche damping coefficient is always chosen according to regulations that are given in EN 1991-2. In case of steel and concrete bridges with span lengths of more than 20 m a value of 0.5 % and of 1.5 % has be chosen, respectively. It will be shown that the damping values that are determined by in-situ experimental modal testing are in most cases significant higher and that the mentioned damping values are quite conservative.

The research project “KOMET” (funded by the Austrian Federal Railways) carried out by the Austrian research company REVOTEC, the research institute AIT and the Vienna University of Technology aims to show the potential and the benefits of assessing dynamic parameters (natural frequencies and damping coefficients) of railway bridges and their non-linear behavior using forced vibration excitation. Another focus of the project is to improve the comparability of measurements carried out by different contractors by updating a current guideline.

In-situ experimental modal tests by application of the forced vibration excitation method were performed to identify the real values of natural frequencies and damping coefficients of different construction types of railway bridges. In addition, conventional dynamic measurement methods like ambient vibration measurement, time decay after train crossing and impact excitation through impulsive hammer are also applied to the tested railway bridges. At first the gained measuring results by application of different excitation methods are compared and discrepancies are discussed. At second the determined dynamic parameters by application of the forced vibration excitation method are compared with calculated values of natural frequencies and with the damping coefficients given in European codes. Limitations of the applied forced excitation method are discussed and approaches for handling these limitations are presented. Detected discrepancies of theoretical expected and in-situ measured dynamic parameters are also discussed in detail and possible reasons for the discrepancies are mentioned. As a main project result a table of realistic damping coefficients for railway bridges of different construction type is elaborated and recommendations for choosing the damping values within dynamic analyses of train crossing are specified.

The aim of the proposed forced vibration excitation method is beside the reliable determination of natural frequencies especially the reliable and reproducible determination of the structural damping coefficients, also by taking changing environmental conditions (temperature) and nonlinear effects (e.g. size of vibration amplitude) into account. In practice it is often observed, that the application of conventional monitoring methods for modal properties of structures, like ambient vibration monitoring show limits in the evaluation of realistic structural damping values. By use of the proposed forced vibration excitation method significant vertical vibration amplitudes are generated and the values of structural damping result more realistic and reproducible. From 2015 to 2017 more than 40 railway bridges of different types with ballast track were measured in summer and winter conditions by use of the forced vibration excitation method. The span length of the bridges varies from 3 to 40 m and the measured natural frequencies varies from 3 to 80 Hz.

The forced vibrations were generated by use of multiple long stroke shakers, which are electrodynamic force generators, where the output is directly proportional to the instantaneous value of the applied current. They can deliver random or transient as well as sinusoidal waveforms of excitation force. The unit employs permanent magnets and is configured such that the armature coil remains in a uniform magnetic field over the entire stroke range - assuring force linearity. Within the performed bridge measurements, a sine-sweep signal was used to excite the railway bridges and the natural frequencies were identified within the frequency domain. For determination of structural damping a manually sweep was performed within the frequency range of interest and the half power bandwidth method was applied.

The performed work within the research project turn out, that the forced vibration excitation method provides reliable and reproducible results for the natural frequencies and structural damping values of railway bridges. Particularly, the structural damping coefficients that result from the application of the half power bandwidth method to the manually generated frequency response function, results in reliable and reproducible values as base for calibrating numerical simulations of train crossing. It is shown, that the in-situ measured structural damping coefficients of framed concrete railway bridges with ballast track are up to 3 times higher than the values given in European codes.

It is concluded, that compared with conventional methods to measure modal properties of bridges, like measuring ambient vibrations, the proposed application of a forced vibration excitation method by use of multiple long stroke shakers, provide more reliable and reproducible values for natural frequencies and structural damping coefficients, respectively.

It is well-known that failure of unreinforced, pre-notched concrete beams in bending is mainly governed by the tensile strength (“ductile” behaviour) or by the fracture toughness (brittle behaviour) depending on the geometrical scale (beam size), and the value of the relative notch depth. In particular, relatively large beams made of high-strength concrete and with a small relative notch depth, show a brittle structural behaviour (unstable crack propagation); whereas relatively small beams made of low-strength concrete and with a large relative notch depth, show a relatively ductile structural behaviour (stable crack propagation). In this contribution, the damage progress, due to crack formation and propagation, in unreinforced, pre-notched concrete beam specimens, tested in three-point bending, is analysed by the Acoustic Emission (AE) and Digital Image Correlation (DIC) techniques. Beams with rectangular cross-section were considered. A relative notch depth (a/d) equal to 0.5 is assumed in all cases. The loading process was operated by controlling the vertical displacement. The specimens were instrumented by four transducers measuring the vertical displacements in correspondence to the supports and at mid-span, and the Crack Mouth Opening Displacement (CMOD). In addition, two AE transducers were located near the notch to acquire the AE signals originated by material damage. Moreover, the evolution of the deformation process was monitored around the notch by a DIC system, therefore obtaining the time evolution of strains. By analysing the acquired data, a correlation between the AE signals registered and the displacements/strains measured at several points was looked for. Furthermore, the fracture energy of each specimen was evaluated, according to RILEM recommendation, based on the measured load–deflection curves. Scale effects on fracture energy, bending strength, and AE energy per unit area were investigated.

The seismic events that struck central Italy in 2016 caused severe damage to a wide range of buildings and infrastructures. Masonry buildings were particularly affected even for low values of peak ground acceleration. In this paper, the data recorded by three masonry buildings belonging to the Seismic Observatory of Structures (OSS) network are used to detect their seismic damage by means of Spectral Entropy (SE). However, entropy measures are sensitive to the energy inserted in the system, since an input of energy can bring to a more deterministic behavior of the structure and thus to a reduction of the entropy indicator. When non-stationary time series are used to evaluate the presence of damage (e.g. mild seismicity) the entropy of the system could be underestimated, leading to misleading results. For these reasons, in this paper an indicator based on the SE is proposed to assess the occurrence of damage also in presence of mild seismicity.

A Digital Image Correlation (DIC) based method is proposed to characterize Mode I fatigue delamination onset and propagation in laminated composites. With the help of DIC, the displacement field around a delamination crack is obtained and further processed to determine the position of the crack tip. With this method the delamination length can be measured automatically in each cycle with a precision on the order of few hundreds of micrometers. The fatigue delamination onset life is then determined by detecting the increase of the delamination length, and the fatigue delamination propagation rate is calculated. The proposed method produces more conservative fatigue life measurements in comparison with the compliance increase method in ASTM D6115.

This paper describes 18 tensile tests performed on welded specimens made of 3 stainless steel grades: EN 1.4307 (304L) and EN 1.4404 (316L) austenitic grades and EN 1.4062 duplex grade. For each grade, 3 tests were carried out parallel to the weld causing shear stresses (in the weld throat plane, parallel to the weld throat axis) and 3 tests along the transverse direction, perpendicular to the weld, causing a combination of normal (perpendicular to the weld throat plane) and shear (in the weld throat plane, perpendicular to the weld throat axis) stresses. The digital image correlation (DIC) technique was used to measure the fracture surface. Based on these experiments, an assessment of the current design rules was made.

The E39 coastal highway route in Norway is envisioned a future without ferries. A submerged floating tunnel built in concrete has been suggested as a means of crossing wide and deep fjords. Blast loading against this type of structure could have disastrous consequences, and potentially cause the collapse of the entire structure. To investigate the response of tubular concrete structures subjected to blast loading, standard off-the-shelf unreinforced concrete pipes were tested using live explosives. A plastic explosive was used to generate the load, and the tests were filmed by two synchronised high-speed cameras. Three pressure sensors equidistant from the charge position logged the pressure. Further, three different positions for the charges were used to investigate the effect of charge position. The charge size was varied for each position to find the amount of explosives needed to breach the pipe. It was found that a contact charge detonated from the outside requires almost twice the explosive amount to breach the pipe than a contact charge detonated from the inside, suggesting a significant confinement effect. Numerical simulations using finite elements produced good qualitative results.

In recent decades, polymer-carbon nanotube (CNTs) composite materials have attracted much attention for their potential applications in unique lightweight materials with distinctly superior mechanical properties. For structural applications, high strength can be delivered and conveniently tuned in nanocomposites by guaranteeing a good load-transfer at the CNT/polymer interfaces which is conventionally achieved with the help of complex chemical functionalization approaches.

In this paper, the mechanical properties of Nickel-coated carbon nanotube (Ni-CNT) reinforced Polyamic acid- nanocomposite (Ni-CNTs /PAA) and (CNTs/PAA) arepresented in terms of material rigidity and damping capacity. The Ni nanoparticles on the CNTs outer walls, help to induce an interlocking mechanism at the CNTs/matrix interface. A significant Elastic modulus increase is thus observed for this type of samples. On the other hand, the Ni-CNTs/PAA nanocomposite, decreases its damping capacity when compared with bare CNTs/PAA nanocomposite. It is worth noting that the recorded rigidity increase and damping decrease is reached despite the significantly smaller amount of CNTs (40% lower weight fraction) contained in the Ni-coated samples. The investigated nanocomposite types were in fact conceived with the same total weight, thus with the same amount of filler without taking into consideration the higher weight of Ni nanoparticles. The results here reported proof the concept that when metal nanoparticles coat the CNTs outer walls, an important improvement of the CNTs-matrix load transfer can be reached without the need to undergo into complex CNTs functionalization procedures. This is an advantage since chemical functionalizations typically have the drawback of introducing defects on the CNT walls.