13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- For the long crack tests, the Adjusted Compliance Ratio (ACR) method [2] was proposed to adjust the stress intensity factor range ( K) by considering the remote crack closure effect, and then result in a lower threshold Kth. Also the ACR method was able to quantify bulk residual stress in the remote closure, and then reduced the scatter and bias related to residual stress and sampling effects [3]. Efforts were also carried out on improving the ASTM E647 load reduction (LR) method, which is suspected to result in improper loading history effects on crack closure. These efforts include the compression precracking (CP) procedures [4], which also result in lower Kth values. For naturally nucleating small crack, the ASTM E647 (Appendix X3) introduces several experimental techniques, which are useful for measuring the growth of fatigue cracks sized on the order of 50μm or greater, and some are applicable to even smaller cracks. For small surface crack measurements, the silicon-based replication technique is at least as reliable as the traditional acetate-based replication, and it has no apparent effect on the fatigue life of the specimen. For small crack depth measurement, NRC has gained extensive experience in the use of the marker band (MB) technique to measure small crack profile on a fracture surface in a previous project. Considering all the above factors, it was decided to test the 7075-T73 hand forging material, using both compact tension (C(T)) coupons with ACR method and single edge-notch tension (SENT) coupons with MB and replication techniques. 2. Short/small crack tests The 7075-T73 hand forgings were made following the AMS-QQ-367 specifications. NRC received four forging blocks that were 2.0” thick, 2.5~3.5” wide, and 30”~72” long. A microstructural analysis showed that this alloy has more and larger particles than pores on the ST plane (short-transverse, the crack plane), and that the particle sizes range roughly from a few m to 20-30m. The grain sizes on the ST plane can reach 100m. The tests were performed under laboratory environment; room temperature, 24 ± 3ºC and relative humidity, 30 ± 25%. The X-ray diffraction measurements, on 10 sampling spots, showed that surface residual stresses were scattered from -4 to +5 ksi (-28~34 MPa), with an average residual stress of 0.5 ksi (3.4 MPa). 2.1. Fatigue crack growth tests using C(T) coupons A compact tension C(T) coupon was designed based on ASTM E647. The width of the coupon was 2.0” (5.08 mm) and the thickness was 0.25” (6.35mm). Each coupon was tested through precracking, K-decreasing, K-increasing, and constant loading following the E647 procedures. Six stress ratios (R=0.05, 0.33, 0.45, 0.60, and 0.80), were tested using a loading frequency of 5-10 Hz. The ACR method, developed by Fatigue Technology Associate (FTA), was used to estimate the effects of the remote crack closure, which refers to crack tip shielding as a result of contact in the crack wake behind the crack tip. This is in contrast to other shielding mechanisms near to the crack tip such as plasticity. The ACR method is based on the same measurement signals that are used for the 2% crack opening force method in E647 (Appendix X2), but it does demand high quality signals with less noise. The ACR stress intensity factor, ΔKACR, is calculated as described in Figure 1. In addition to the ΔKACR results, the FTA software also provides two additional methods to estimate the effective stress intensity factor ranges (Keff) ②, i.e. the ASTM 2% opening load method (ΔKeff-OP) and the 2/ method (ΔKeff-2/, partial closure correction below the opening load). Results ② Although the effective stress intensity factor range is a useful parameter, it should be noted that different definitions and calculations have been used in different applications. They may not be comparable to each other.
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