This work describes the development of subsize specimen fatigue crack growth rate test techniques using these miniature bar specimens. What region are we in initially and at failure? We will use our prior failure knowledge to explain and correlate observed facts from fatigue tests. We will now build upon our knowledge of failure mechanisms and study material failures caused by cyclic loading or fatigue. The part has been modeled as a column with through the thickness edge crack as shown in Figure 5. Since three different materials were tested, it is possible to draw conclusions about the limitations of using these extremely small specimens in terms of material yield and ultimate strength and other standard tensile properties. Figure 6 shows the data collected and shows the signals from the notch corner and tip — compare to Figure 2 to understand the corresponding ultrasonic image area. We have also developed testing protocols for custom sample designs, components, and fasteners.
The fatigue crack propagation behavior of many materials can be divided into three regions as shown in the image. The graph of Cyclic Stress is plotted against the Crack Growth Rate, with stress intensity being the controlling variable. The notch was 3mm deep leaving a 12mm ligament at the outset of the experiment. The diffraction mechanism at the local crack tip level is highly dependent on the local orientation of the tip and, as the crack tip propagates, there are times when no signal will be detected. Calculate if necessary and include the raw a vs.
C and n values are the values you determined from the laboratory but use the expression for Δ K that is appropriate for your machine part. . It was clear that given the likely vibrations during the fatigue testing the ability of the probe to remain attached to the specimen would be an issue. The useful life of these components may be governed by the rate of subcritical crack propagation. The graphs include both dadN vs. Paris Law and threshold values are provided.
See the Appendix for help with the use of your experimental result in your analysis. It is possible to set these parameters to measure growth rate even at the fast fracture period at the end. Create another plot showing a vs. Comment of what region the Paris law covers on a time basis e. Figure 4 shows the test items to include the probe, the reference specimen and two test specimens. Fatigue crack growth predictions with data from varying R-test were generally slightly more conservative compared to predictions using data obtained from conventional testing.
The information presented in this paper was collected in a laboratory fatigue test and work is now ongoing to generate the instrumentation to measure the growth rate of a natural crack initiated during the simulated fatigue testing of a full size component. You are to determine how many days this part will last until it fails catastrophically. The fracture surface microstructures of the subsize and standard size specimens are compared to show the similarities of the crack growth mechanisms in the two specimen configurations. Figure 1 shows the design of the test specimen and Figure 2 shows the ultrasonic phased array technique setup. This time you are assigned to evaluate a part that has been subjected to cyclic loading. The criteria was to ensure that the component failed within 20 minutes leading to a stress range between 5kN to 0. The Fracture Toughness laboratory examined the phenomenon of materials separating or fragmenting into two or more pieces under slow loading rate condition.
This laboratory builds directly on the Fracture Toughness laboratory you completed earlier. For other components the crack growth life might be a substantial portion of the total life of the assembly. This work describes the development of subsize specimen fatigue crack growth rate test techniques using these miniature bar specimens. It can be seen and noted clearly on the video that the nature of the crack tip growth leads to moments in time when the crack tip cannot be seen — ie no signal is generated by the crack tip to be detected in the pulse-echo mode. The test conditions typically are R-ratio, waveform type, frequency, K-gradient, test temperature, and environment. Appendix Using experimental fatigue crack growth data to predict failure.
Currently the crack length is measured either by compliance changes of the sample using a clip gauge or by the potential drop method. The key was to establish this recording frame rate such that it was within the speed of the crack growth and setting the ultrasonic instrumentation fast enough to capture the fast fracture point at failure. First uses the Δ K max value you determined from the experiment to determine K Ic. The test specimen was 290mm long, 29mm wide and 15mm deep. The real-time measurement of the crack size allows establishment of the growth rate. Would this equation generally be applicable over the entire crack growth curve? Figure 15 Graph showing the growth of the fatigue crack measured using the position of the crack tip from the phased array ultrasonic image. This work describes the development of subsize specimen fatigue crack growth rate test techniques using these miniature bar specimens.
The lab also has a high precision inspection microscope for measuring dimensions of specimens before testing and crack surfaces after testing. They excel at working with clients to develop custom test procedures for their unique requirements. A standard fatigue test specimen with a starter notch was fabricated. Figure 2 also shows two geometric features of the notch which can be clearly identified in the ultrasonic signal data in Results. Our fracture mechanics lab has 29 servo-hydraulic test frames providing loads up to 110,000 lbf. N data in your report a is the crack length and N is number of cycles. Explain your answer in detail.
With the critical crack size known, we have to integrate the Paris law to determine the number of cycles, N f, until failure. Small powerful rare earth magnets were selected to generate sufficient force to keep the probe attached to the specimen during the test. The equation for the stress intensity factor still holds, therefore, you can calculate a critfor your machine part. Since three different materials were tested, it is possible to draw conclusions about the limitations of using these extremely small specimens in terms of material yield and ultimate strength and other standard tensile properties. In addition, include the Fatigue Precrack Data Report Form if available. Since three different materials were tested, it is possible to draw conclusions about the limitations of using these extremely small specimens in terms of material yield and ultimate strength and other standard tensile properties. This exploratory project was aimed at optimising the instrumentation which will monitor a known crack in the component online live during a test.
These tests are conducted to examine and evaluate the behavior, susceptibility, and extent of resistance of certain materials to sharp-notch tension, tear, axial fatigue, strain-controlled fatigue, surface crack tension, creep crack, and residual strain. Results The video of the test showing the growth and failure due to a fatigue crack can be found. You need to determine the critical size of the crack, a crit, that will cause failure. Fatigue Crack Growth Testing characterizes the rate at which a preexisting crack will grow under varying stress states. These facts led to a design approach that required the damage tolerance analysis to assume a material flaw exists in the worst orientation and at the most undesirable location. It can characterize more ductile, tougher materials that exhibit stable crack growth before failure, which may prevent valid results being obtained from E399 testing. It also points out factors effecting your experimental result that must be addressed in your report.