Key Engineering Materials Vols. 569-570

Abstract: The ability of a Structural Health Monitoring (SHM) system to automatically identify damage in a composite structure is a vital requirement demanded by end-users of such systems. This paper presents the demonstration of a potential method. A composite fatigue specimen was manufactured and initially tested at 1Hz for 1000 cycles. Acoustic emission (AE) signals were recorded for complete fatigue cycles periodically in order to establish a base-line associated with undamaged specimens. The specimen was then subjected to impact damage to create barely-visible impact damage (BVID) and subjected to further fatigue cycles with acoustic emission recorded until failure. The data was subsequently analysed using a range of techniques including basic RMS signal levels and frequency-based analysis. At various stages during the test, C-scanning was used to validate the results obtained. Results demonstrated that AE is capable of detecting BVID in composite materials under fatigue loading. The proposed method has wide applicability to composite structures which are subjected to cyclic loading, such as wind turbine blades.

Abstract: A strong knowledge of the fatigue state of highly advanced carbon fiber reinforced polymer composite (CFRP) structures is essential to predict the residual life and optimize intervals of structural inspection, repairs, and/or replacements. Current techniques are based mostly in measurement of structural loads throughout the service life by electric strain gauge sensors. These sensors are affected by extreme environmental conditions and by fatigue loads in such a way that the sensors and their systems require exhaustive maintenance throughout system life.This work is focused on providing a new technique to evaluate the fatigue state of CFRP structures by means of evaluating the surface roughness variation due to fatigue damage. The surface roughness is a property that can be measured in the field by optical techniques such as speckle and could be a useful tool for structural health monitoring. The relation between surface roughness and fatigue life has been assessed on CFRP test specimens. A tensile fatigue load with an R=0.1 (T-T) and a maximum load of 60% of the material ultimate strength has been applied. The surface roughness of the specimens has been determined from the surface topography measured by a high precision confocal microscope. Results show that the surface roughness of the specimens increases with the accumulation of fatigue cycles in such a way that the roughness could be taken into account as a fatigue damage metrics for CFRP.

Abstract: This paper investigates the effect of low-frequency vibration and the related temperature field on nonlinear vibro-acoustic wave modulations. Experimental modal analysis was used to find natural frequencies and mode shapes of a composite laminate plate with seeded delamination. Temperature distribution was analyzed with a thermographic camera in the vicinity of damage for the identified vibration modes. These frequencies of these vibration modes were then used for low-frequency excitation in nonlinear acoustic tests. The correlation between the thermal field and the observed wave modulations was analyzed.

Abstract: The strain-rate sensitivity of brittle materials, such as glass, ceramics or concrete-like materials, is usually easier to be performed in compression. However, also the tensile behavior, which affects phenomena such as spalling, scabbing and fragmentation, has to be investigated to achieve an exhaustive characterization. In last decades, a lot of researchers suggested spalling test as one of the best ways to characterize dynamically brittle materials. This type of test is based on propagation and reflection of elastic waves: the fracture for spalling occurs when, in the material, the tensile stress state, obtained by the reflection on a free surface of a compressive pulse, exceeds the strength limit. These conditions are usually reached using a SHPB setup: a striker bar is launched against the input bar, which is in contact with a long bar specimen free at the opposite surface. In this work, the spalling test has been performed to investigate the dynamic tensile behavior of graphite. The apparatus is actuated by a pneumatic gas-gun (1.5 m long). Striker and input bars are made of high-strength steel 10 mm of diameter. Different striker lengths are used (100 and 80 mm) to obtain different pulse lengths and amplitudes. The input bar is 3.4 m long and is instrumented in the middle. The specimens are 200 mm long and 10 mm of diameter, instrumented at 80 from the free surface with strain-gages.

Abstract: Damage after impact often involves aeronautic structures. The aircraft can be involved in impacts during the assembly stage and operative life. Typical impacts can be related to falling tools, hailstones, debris on the take-off strip thrown against the aircraft by the rolling tyres, maintenance operations. There are two categories of damage impact: Low and High Velocity Impact (LVI, HVI). Damages coming from low velocity impacts are difficult to identify because they are often within the composite structure and the use of non-destructive testing, e.g. ultrasonic test, is not convenient. In order to prevent catastrophic events the designers must increase the safety margin and thereby the weight of the aircraft. The present study shows two different numerical procedures based on finite elements method to investigate on some damage mechanisms of a carbon fibre reinforced plastic (CFRP) structures (e.g. interface debonding, fibre or matrix cracking) and the residual strength of such structures under live loads.

Abstract: The mechanical properties of geopolymers can be obtained through different kinds of experimental tests: this paper is focused on the compressive strength (i.e. in a direction parallel to the loading axis) for the case of uniaxial compression. The compressive strength of such materials is traditionally characterized by the 28^th-day value, but their strength is expected to increase in time at a continuously decreasing rate. The knowledge of the strength vs. time law is of importance when a structure is subjected to a certain type of loading at a later age. In this work inorganic polymers from activated metakaolin (alumina silicate inorganic polymers, obtained from alkali activation of powders containing SiO₂+Al₂O₃ > 80%_wt) are reported. In order to improve their compressive strength a percentage of polyethylene glycol has been added, thus obtaining a hybrid (organic-inorganic) geopolymer. Many factors can influence significantly the compressive strength of such materials e.g. w/c ration, aggregate content, water curing period, polyethylene/glycol ratio. Afterwards experimental compressive tests (performed in a Zwick-Roell^® testing machine) have been carried out varying the polyethylene/glycol ratio and the main dimensions of the samples.

Abstract: Glass fibre reinforced composites (GFRC), used in the manufacture of wind turbine blades, can suffer unpredictable, post-manufacturing, in-situ structural failure. The economic cost of remediation of such blade failures is extremely high, both on land and offshore. We suggest using zero-time-to-echo (ZTE) magnetic resonance imaging (MRI) as a method for characterising glass fibre reinforced composites. Initially, we demonstrate that carbon-13 Nuclear Magnetic Resonance (NMR) spectra acquired at 800MHz provide finger-print like information and that there is sufficient hydrogen-1 NMR signal available to conduct MRI. This work explores the efficacy of using zero timetoecho (ZTE) magnetic resonance imaging (MRI) to detect the rapidly decaying Hydrogen-1 (¹H) NMR signal from a representative sample. A 400MHz surface resonator was developed made of a 20mm diameter loop formed with 1.5mm thick silver wire and designed with variable tuning and matching in order to investigate the ¹H-MRI signal at 9.4T static magnetic field strength. The thickness of the GFRC was determined from the MRI data to be 3.51±0.28mm while the physical measurement using a caliper device resulted in a measurement of 3.45mm. Hence, a high spatial resolution accuracy is provided by ZTE MRI. Very short T₂* (<20μs) of the material led to stronger blurring artefacts for the blade material compared to heat shrink used for insulating the silver wire of the detector. ¹H images of the blade material were acquired demonstrating that ZTE is a suitable technique for acquiring image data from glass fibre materials. Work is on-going in studying the relaxation time parameters and chemical frequency shifts for materials with and without structural weaknesses in order to improve the predictive power of the technique. In conclusion, ZTE-MRI can provide useful information about the mechanical properties of glass fibre reinforced composites.

Abstract: This paper describes a Remote Collaborative Visual Inspection System for integrated management by linking digital movie captured by a running vehicle and online road drawings. The newly developed road condition assessment system consists of commercially available on-board high-resolution video cameras during the visual inspection of road pavements and road appurtenances and a Web connection system. The system enables users to select a road section in a road register on an on-screen map and visually observe not only pavements but also road facilities, slopes, the state of vegetation, and road-occupying structures. The system, therefore, can be expected to help reduce visual detection failures compared with conventional visual observation from moving vehicles and make highly objective evaluation possible through observation by two or more persons. By using image data, the system also provides basic data that can be used for not only maintenance but also road planning.

Abstract: Base isolators, like lead rubber bearings, high damping rubber bearings or friction pendulum systems are extensively used in practice in many seismically active regions to protect structures against earthquake forces. The present paper reports the results obtained from the experimental study aimed to determine the effectiveness of the Polymeric Bearings in suppressing structural vibrations during dynamic excitations and therefore to minimize structural damage. The response of the analyzed two-storey structure model both fixed and supported by the Polymeric Bearings under different earthquake excitations was studied. The reduction in lateral response due to seismic events was measured by comparing the peak accelerations of the two-storey steel structure models – with and without base isolation system. The results of the study have shown that the use of the Polymeric Bearings leads to significant improvement in dynamic properties by suppressing structural vibrations. The results have indicated that the Polymeric Bearings can be successfully used to reduce damage of structures during seismic events.

Abstract: This paper proposes a symptom-based reliability analysis method for deteriorating reinforced concrete (RC) structures on the basis of monitored data. The structural flexural resistance due to reinforcement corrosion is selected as a symptom which reflects the deteriorating structural performance. The symptom reliability and remaining service life are then estimated from the Weibull model for the structural flexural resistance development. The results for the numerical study example show the proposed approach is capable of prediction the remaining service life for the deteriorating RC structures subjected to the reinforcement corrosion.