Lifetime-Oriented Structural Design Concepts- P8 doc

168 3 Deterioration of Materials and Structures Fig. 3.43. Electrical potential - plain specimen Fig. 3.44. Electrical potential - circular specimen The distribution of the electrical potential across the plain and circular spec- imen for different crack length is given in Figures 3.43 and 3.44. The results of both the finite element analysis and experiments for both types of specimen are represented in Figure 3.45 as a dimensionless plot of electrical resistance ratio R/R 0 against a/w where R 0 is the electrical resis- tance at the initial crack length a 0 , R is the electrical resistance at the crack length a and w is half width of the specimen at the crack height. A good agreement between experimental result and the numerical solution of evolu- tion of the electrical resistance with the crack propagation can be observed. The obtained evolution of the electrical resistance shows at the same time a high level of similarity to the measured crack propagation behaviour under cyclic fatigue load. Based on this results, it can be concluded that the method of the resistance measurement detects the appearance of the damage in the early phase, and it confirms the development of a damage evolution on the basis of microscopic crack incubation and initiation. 3.2 Experiments 169 1 1.1 1.2 1.3 1.4 1.5 1.6 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 elect. Resistance R/Ro Crack length a/w ANSYS PS Exper PS 1 1.2 1.4 1.6 1.8 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 elect. Resistance R/Ro Crack length a/w ANSYS CS Exper CS Fig. 3.45. Evolution of electrical resistance vs. crack length during fatigue - plain and circular specimen 3.2.1.1.2 Acoustic Emission The mechanisms by which metals absorb and release strain energy under stress, the modelling of which is the basis of fracture mechanics analysis, can be different and complicated. Acoustic emission is the elastic energy that is spontaneously released by materials when they undergo deformation. The stress waves which result from this sudden release of elastic energy due to micro-fracture events are of most interest to the structural engineer. These events are typically 10μm to 100μm in linear dimension. Sources of acoustic emission include many different mechanisms of defor- mation and fracture. Sources that have been identified in metals include crack growth, moving dislocations, slip, twinning, grain boundary sliding and the fracture and decohesion of inclusion. Other mechanisms fall within the definition and are detectable with acous- tic emission equipment. These include leaks and cavitation, friction (as in rotating bearings), liquefaction and solidification, solid-solid phase transfor- mation. Sometimes these sources are called secondary sources to distinguish them from the classic acoustic emission due to mechanical deformation of stressed materials. Acoustic emission examination is a nondestructive testing method with demonstrated capabilities for monitoring structural integrity, detecting leaks and incipient failures in mechanical equipment. Acoustic emission differs from most other nondestructive methods in two significant respects: • The detected energy is released from within the test object rather than being supplied by the nondestructive method, as in ultrasonics or radiography. • The acoustic emission method is capable of detecting the dynamic pro- cesses associated with the degradation of structural integrity. Acoustic emission expected in fatigue studies is primarily of the burst type. Burst type emission signals originate from sources such as intermittent 170 3 Deterioration of Materials and Structures Threshold Counts Amplitude Rise Time Decay Time Volts Duration Time Time Threshold Crossing Energy Fig. 3.46. Definition of simple waveform parameters for a burst-signal dislocation motion and crack growth in metals. A Burst signal, given in the Figure 3.46, has the following characteristic parameters: • threshold: A preset voltage level that has to be exceeded before an acoustic emission signal is detected and processed. This threshold is independent for every sensor, and must be chosen depending on the background noise. • burst: A signal whose oscillations have a rapid increase in amplitude from an initial reference level, followed by a decrease to a value close to the initial value. • hit: Total signal from the first to the last threshold crossing. • amplitude: Maximum signal amplitude within duration of the burst. • duration: The interval between the first and the last time the threshold was exceeded by the burst. • counts: The number of times the signal amplitude exceeds the preset threshold. • rise time: The time interval between the first threshold crossing and max- imum amplitude of the burst. • decay time: The time interval between the maximum amplitude of the burst and the last threshold crossing. • event: A microstructural displacement that produces elastic waves in ma- terial under load or stress, which are detected by several AE-transducer. Using time analysis the origin of acoustic emission signal can then be detected. • event counts: Counts which belong to an event. 3.2 Experiments 171 3.2.1.1.2.1 Location of Acoustic Emission Sources The ability to locate the sources of acoustic emission is one of the most important functions of the multichannel instrumentation system used in field application. One of the methods for detecting the emission source is the mea- surement of the time differences in reception of the stress waves at a number of sensors in an array. Depending on the sensor location linear (1D), two and three dimensional problems can be defined. 3.2.1.1.2.2 Linear Location of Acoustic Emission Sources Consider the situation where two sensors are mounted on a linear structure. Assume that an acoustic emission event occurs somewhere on the structure, and that the resulting stress waves propagate in both directions at the same velocity. Using the measurement of the time differences between hits it is possibletolocatepositionofacoustic emission source. If the time difference between the hits of both sensors is zero, it would indicate a site precisely midway between the sensors. In general, for the case of constant velocity, the source location is given by: d = 1 2 (D − VΔt) (3.6) where D is the distance between sensors, V is the constant wave velocity, Δt is the time deference and d is the distance from the first hit sensor. If the source is outside the sensor array, the time difference measurement corresponds to the time of flight betweenoutersensorpairandremains constant. 3.2.1.1.2.3 Location of Sources in Two Dimensions The case of location of sources in two dimensions requires a minimum of three sensors. The input data now include a sequence of three hits and two time difference measurements (between the first and second hit sensors and the first and third hit sensors), as can be seen in the Figure (3.47). Then: Δt 1 V = r 1 − R Δt 2 V = r 2 − R (3.7) which yields R = 1 2 D 2 1 − Δt 2 1 V 2 Δt 2 1 V + D 1 cos(Θ − Θ 1 ) (3.8) and R = 1 2 D 2 2 − Δt 2 2 V 2 Δt 2 2 V + D 2 cos(Θ 3 − Θ) (3.9) Equations (3.8) and (3.9) can be solved simultaneously to provide the location of a source in two dimensions. 172 3 Deterioration of Materials and Structures Z 1 Z 2 D 2 Sensor 1 D 1 X 1 , Y 1 X 3 , Y 3 X S , Y S Source Sensor 3 r 1 Sensor 2 X 2 , Y 2 R Θ Θ 3 Θ 1 Reference r 2 Fig. 3.47. Location of the source in two dimensions 3.2.1.1.2.4 Kaiser Effect Kaiser effect is the phenomenon that a material under load emits acoustic waves only after a primary load level is exceeded. During reloading these materials behave elastically and little or no acoustic emission will be recorded before the previous maximum stress level is achieved. This is true only for materials in which no change in microstructure, such as dislocation movement or crack initiation, can be observed. The case when acoustic emission is recorded before the previous maximum load is reached is known as felicity effect and describes the breakdown of the Kaiser effect. If we define the ratio between the load level at which the acoustic emission appears and previous maximum load level as felicity ratio, it can be used as the associated quantitative measure of the felicity effect. In the case of the Kaiser effect the value of the felicity ratio is 1. 3.2.1.1.2.5 Experimental Procedures Experiments were performed by a hydro-dynamic tension-torsion testing system (Schenck/Instron Fast Track 8800. The displacement was measured by a real time analog-built mean of 3 displacement transducers (HBM-W5TK), and the load was measured by a 160kN load cell. The fatigue load was defined as cyclic sinusoidal load in tension range. Tests were performed either with constants amplitude for the entire duration of the test, or as a block-test with the amplitude which is constant inside the block and differs between the blocks. Stress-ratio range was R=0.05 and R=0.25. Test frequency was 9Hz for plain specimen and 6Hz for circular specimen. Acoustic emission was detected on two types of specimens. The first type is a plain specimen with thickness of 5mm shown in Figure 3.48. The sec- ond type is a circular specimen with inconstant thickness and outer diameter 3.2 Experiments 173 t=5 40 22 10 26,2 50 R5 R10 85 R3 30 330 280 230 225,5 Fig. 3.48. Geometry of the plain specimen (dimension in mm) 108,169 R757,25 111,883 146 150 166 0,8 4,847 R2 R40 0,9 19 9,701 4,668 Fig. 3.49. Geometry of the circular specimen (dimension in mm) 166mm shown in Figure 3.49. All specimens were made from heat-treatable steel 42CrMo4 (No. 1.7225). Two types of AE-transducers with appropriate preamplifiers were used for the detection of acoustic emission. The first set was 4 piezoelectric transducers R15 with resonant frequency 150kHz and 4 single In-Line 40dB preamplifiers with a 100-300kHz bandpass filter. The second set was 4 wideband piezoelec- tric transducers WD with operating frequency range 100-1000kHz and 4 volt- age preamplifiers with 20/40/60dB selectable gain and 100-1200kHz bandpass filter. All acoustic emission signals were amplified with 40dB and recorded us- ing Physical Acoustics software on a two Two-Chanel-AE-Boards. The sample rate for all measurements was 10MHz. All AE-transducers were clamped to the specimen with a spring clamp. The coupling between the specimen and the transducers was made with a silicon gel. The acoustic emission threshold was set to 35dB, as a compromise between effectively avoiding background noise and cutting off low level signals dur- ing damage evolution. The threshold setting depended on the experimental conditions. 174 3 Deterioration of Materials and Structures 67 60 40 4 3 2 1 x 117 Fig. 3.50. The position of AE-transducers on the plain specimen 90 98 4 2 1 3 Fig. 3.51. The position of AE-transducers on the circular specimen The position of AE-transducers on the plain and circular specimen is given in Figures 3.50 and 3.51 respectively. The position of AE-transducers on the plain specimen was given in such a way, that transducers 1 and 4 are so- called guard transducers with the function to eliminate signals originating outside the specimen test section from the recorded data. Thus, extraneous signals such as those emanating from load-chain noise or from servo-valves and hydraulic pump were avoided without loss of data. The AE-transducer 2 and 3 were used as measuring sensors. In the case of the circular specimen all 4 transducers are measuring sensors. 3.2.1.1.2.6 Experimental Results Acoustic emission recorded during the test is represented using acoustic emission events counts per cycle over the whole stress range and cumulative acoustic emission event counts during fatigue damage. For all experiments, the load was in the range, which leads to high cycle fatigue with brittle damage. This type of load and the brittle damage behaviour lead to the significant 3.2 Experiments 175 0 200 400 600 800 1000 1200 1400 0 200000 400000 600000 800000 1e+06 1.2e+06 Event Counts Cycles PS16 PS17 PS20 PS22 PS24 PS29 PS31 Fig. 3.52. Acoustic emission event count rate during fatigue - plain specimen 0 1000 2000 3000 4000 5000 6000 7000 8000 0 50000 100000 150000 200000 Event Counts Cycles CS02 CS05 Fig. 3.53. Acoustic emission event count rate during fatigue - circular specimen increase in the acoustic emission output, as the crack advances towards final failure. The rate of acoustic emission in the form of acoustic emission events counts per cycle over the whole stress range is given in the Figures 3.52 and 3.53. When the load was in the elastic range, the low acoustic emission output was evident during initial cycles, due mostly to microscopic dislocation dynamics. This stage was followed by a dead period with almost no acoustic emission. In this stage of fatigue damage accumulation results in the long crack-initiation lifetime. Low energy dislocation motion, which generates acoustic emission 176 3 Deterioration of Materials and Structures 0 10000 20000 30000 40000 50000 0 200000 400000 600000 800000 1e+06 1.2e+06 Total Event Counts Cycles PS16 PS17 PS20 PS22 PS24 PS29 PS31 Fig. 3.54. Acoustic emission total event counts during fatigue - plain specimen 0 1e+06 2e+06 3e+06 4e+06 5e+06 6e+06 7e+06 8e+06 9e+06 1e+07 1.1e+07 0 50000 100000 150000 200000 Total Event Counts Cycles CS02 PS05 Fig. 3.55. Acoustic emission total event counts during fatigue - circular specimen waves, is frequently under background noise and is relatively hard to de- tect. Only discrete acoustic emission events counts represent the existence of acoustic emission and consequential evolution of fatigue damage. The crack propagation, which is connected with the high rate of the acoustic emission, occurs in the third stage. The release of the elastic energy due to the crack propagation has a significant level and the detection of the acoustic emission is not influenced by the background noise as in the second stage of the fatigue. Evolution of cumulative acoustic emission event counts during fatigue dam- age is given in the Figures 3.54 and 3.55 and represent a cumulative fatigue 3.2 Experiments 177 0 500 1000 1500 2000 2500 168 128 60 0 Total Event Counts Location [mm] PS16 0 500 1000 1500 2000 2500 168 128 60 0 Total Event Counts Location [mm] PS20 0 500 1000 1500 2000 2500 168 128 60 0 Total Event Counts Location [mm] PS29 0 100 200 300 400 168 128 60 0 Total Event Counts Location [mm] PS31 Fig. 3.56. The location of the origin of acoustic emission for the plain specimen Fig. 3.57. The location of the origin of acoustic emission for the circular specimen damage process. This process can be divided into the same three stages as indicated for the rate of acoustic emission: (i) microscopic dislocation dynam- ics; (ii) microscopic crack incubation and initiation; (iii) macroscopic crack propagation. The location of the origin of acoustic emission was computed using time difference measurement methods described earlier. The obtained distance rep- resents the distance between origin of acoustic emission and AE-transducers. The location of the origin of acoustic emission for the plain specimen is given [...]... Hursit Ibuk u A great number of concrete structures is exposed to cyclic mechanical loading scenarios Therefore, the reliability of such structures depends among other influences also on the degree of structural degradation due to fatigue loading In order to estimate the state of a structure it is necessary to know the development of the degradation of the material properties during its lifetime However, . this sudden release of elastic energy due to micro-fracture events are of most interest to the structural engineer. These events are typically 10μm to 100μm in linear dimension. Sources of acoustic. emission examination is a nondestructive testing method with demonstrated capabilities for monitoring structural integrity, detecting leaks and incipient failures in mechanical equipment. Acoustic emission. emission method is capable of detecting the dynamic pro- cesses associated with the degradation of structural integrity. Acoustic emission expected in fatigue studies is primarily of the burst type. Burst