This test is conducted by passing a pulse of ultrasonic through concrete to be tested and measuring the time taken by pulse to get through the structure. Higher velocities indicate good quality and continuity of the material, while slower velocities may indicate concrete with many cracks or voids.
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The test can also be used to evaluate the effectiveness of crack repair.[7] Ultrasonic testing is an indicative and other tests such as destructive testing must be conducted to find the structural and mechanical properties of the material.[8][9][10][11]
In India, till 2018 ultrasonic testing was conducted according to IS 13311-1992.From 2018, procedure and specification for Ultrasonic pulse velocity test is outlined in IS 516 Part 5:Non destructive testing of concrete Section 1:Ultrasonic Pulse Velocity Testing. This test indicates the quality of workmanship and to find the cracks and defects in concrete.[12][13][14][15][16][17]
Figure "Historic case of disk fracture by vibration" (Ref. 12.6.3.3-4): The early Whittle engine had a double-flow radial compressor (middle diagram). At 14,000 RPM it began to make a screaming sound. If the engine was operated for even a short time while making this sound, radial cracks would occur in the transition from the blades to the disk (bottom right diagram). Emitted sounds can still be used for verification of dangerous vibrations today, albeit in an altered form. During dangerously intense vibrations, the parts emit characteristic sonic frequencies. Frequency analysis is capable of using these sounds to detect fatigued parts.The bottom right diagram shows the damage symptoms after the blade failure. The blades of the axial turbine also fractured due to high-frequency vibrations (also see Fig. "Vibration dampers at roots of turbine rotor blades"). In order to avoid this damage, important methods for analyzing excitement possibilities were developed at the time. These include the Campbell diagram (Fig. "Development preventing vibrations"). In addition to the typical expected fractures at the blade transition to the root platform (fundamental flexural modes), dynamic fatigue fractures also occurred in unusual/unexpected areas of the turbine rotor blades, such as the center or tip of the blade. Whittle identified the cause as a thermal element that was located in the exhaust gas flow roughly one meter behind the turbine. The realization that flow disturbances can be located far behind the excited part is always startling. After the disturbance was removed, the dynamic fatigue fractures no longer occurred.
Figure "Vibration dampers at roots of turbine rotor blades" (Example "Dampening vibrations to tackle fatigue cracks"): Dynamic fatigue fractures in turbine rotor blades have been a problem from the very beginnings (Figs. "Historic case of disk fracture by vibration" and "Vibrations by remote engine copmponents") of engine development. Unfortunately, in this case of a helicopter engine (middle diagram), the available literature gives no information regarding the source of the excitement, nor does it mention the turbine stage in which the fractures occurred. The excitement evidently occured via a flow disturbance and its scope was not recognized in the development phase. It is possible that the rotor blades of the first HPT stage (bottom right diagram) were excited by the turbine stator vanes in a certain RPM range. The detailed diagram at bottom left is a schematic depiction of the dampers made of metal sheeting that were introduced as a remedy for the problem (Fig. "Influences on damping of parts").
Illustrations 12.6.3.3-13 and 12.6.3.3-14: This case occurred during the development phase of a small, low-performance turbine for a helicopter (top diagram). Without any indication of imbalances or other anomalies in operation, the integral cast turbine disk of the last stage burst at full RPM in a steady operating state. Fragments of the annulus had broken off the disk. The entire fracture surface could be seen as a combination of two fractures that ran together at an angle to one another that was tangential to the balancing ring (bottom diagram). The fracture surface analysis revealed a bow-shaped limited fracture zone in the middle of each of the annulus fragments near the balancing ring. It also showed stage 1 characteristics (cleavage cracks) that are typical for this Ni-based cast material (Fig. "Stage 1 at dynamic fatigue cracks"). A testing-technical vibrational analysis (Fig. "Experimental determinating disk vibration modes" ) with heaps of powder resulted in a distribution pattern that corresponds with the bottom left diagram. This Chladni figure showed that a three nodal diameter vibration (Fig. "Vibration models of disks") is excited exactly at full RPM by the three bearing braces of the exit housing located behind the disk.A successful provisional solution was the installation of a disturbing brace with no load-bearing function. It was only intended to dampen the vibration in the right rhythm with a disturbing pulse. The final solution was four equally-spaced supporting braces, from which no dangerous excitement was expected (middle diagram). The effectiveness of this solution was confirmed by later experience.
Figure "Fraction of turbine disk by nodal cycle vibration": This case concerns the fracture of an integral cast turbine disk from the second stage of a low-performance helicopter engine (top diagram). The fracture occurred following the initiation of several cracks at the circumference in the transition radius of a balancing ring (bottom diagram). The explanation for the damage was a nodal circle-vibration with a nodal diameter (Fig. "Vibration models of disks"). This vibration could have been excited by a neighboring disk that is connected to the damaged part by the centrical tie rod. The turbine disks of the first and third stages of the same engine type had fractured several times due to disk vibrations (Figs. "Cyclic spin test disk fatigue crack " and "Experimental determinating disk vibration modes").
Figure "Vibrations at seal membrane": In the area of the labyrinth bracket below the stator assembly of the first stage of the low-pressure turbine (top right diagram) of a large shaft-power engine, several damaging dynamic fatigue fractures occurred in the HCF range following a constructive change. These were dynamic cracks along the circumference (bottom right diagram) on the front wall of the ring duct that is formed by the seal bracket (top right detail).The constructive change connected with the damage was a wire seal ring between the stator vanes and the labyrinth bracket. The seal effect against leakage air flow from the space with higher pressure levels ahead of the wall to the ring duct merely relied on the resilient force of the disk-shaped front wall of the labyrinth bracket. After several hundred operating hours, the seal ring and the contact surfaces of the seal groove in the labyrinth bracket showed heavy fretting wear (Volume 2, Chapter 6.2). The damage cause was thought to be vibration excitement of the resilient seal wall following sufficiently advanced fretting wear. This excitement principle (hydrodynamic paradox, Ref. 12.6.3.3-11 and Fig. "Vibrations of flat seals") is based on the resilient action of the cover and the pressure drop in the leakage air flow (Bernoulli) and is viewed as an exemplary physical laboratory experiment (bottom left diagram).The damage hypothesis was confirmed with the aid of a simple demonstratory experiment. A stator assembly with a seal ring was set up and realistic pressure ratios similar to those in operation were simulated. As predicted, with sufficient wear and reduced resiliency, a strong vibration excitement of the covering plate with a corresponding pulsating leakage air flow could be observed. After the damage mechanism was understood and reproduced using original parts, the requirements for a guaranteed solution were met.
Figure "Fatigue crack at compressor stator": In a small shaft-power engine (right diagram), compressor damage occurred during a trial run in the development phase. The inlet edge of the radial compressor disk of the second stage had broken out following an HCF dynamic crack (left detail). The dynamic crack did not have any pronounced lines of rest, which indicated constant crack growth. In addition, the stator (sheet metal) located behind the damaged disk had several dynamic fatigue fractures in its vanes and fastening bolts. This damage had already occurred frequently without cracks in the compressor disk. This indicated the following damage sequence:First, the vanes of the compressor stator broke due to dynamic overstress (bottom diagram). Break-outs evidently created strong flow disturbances that acted against the airflow into the compressor disk. Here, it resulted in vibration excitement in the blade in a manner typical of the fundamental flexural modes of radial compressor blades, with crack initiation in the inlet edge area. 2ff7e9595c
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