Dauskardt R H, Ritchie R O, Takemoto J K, Brendzel A M
Department of Materials Science and Mineral Engineering, University of California, Berkeley 94720.
J Biomed Mater Res. 1994 Jul;28(7):791-804. doi: 10.1002/jbm.820280706.
A fracture-mechanics based study has performed to characterize the fracture toughness and rates of cyclic fatigue-crack growth of incipient flaws in prosthetic heart-valve components made of pyrolytic carbon-coated graphite. Such data are required to predict the safe structural lifetime of mechanical heart-valve prostheses using damage-tolerant analysis. Unlike previous studies where fatigue-crack propagation data were obtained using through-thickness, long cracks (approximately 2-20 mm long), growing in conventional (e.g., compact-tension) samples, experiments were performed on physically small cracks (approximately 100-600 microns long), initiated on the surface of the pyrolytic-carbon coating to simulate reality. Small-crack toughness results were found to agree closely with those measured conventionally with long cracks. However, similar to well-known observations in metal fatigue, it was found that based on the usual computations of the applied (far-field) driving force in terms of the maximum stress intensity, Kmax, small fatigue cracks grew at rates that exceeded those of long cracks at the same applied stress intensity, and displayed a negative dependency on Kmax; moreover, they grew at applied stress intensities less than the fatigue threshold value, below which long cracks are presumed dormant. To resolve this apparent discrepancy, it is shown that long and small crack results can be normalized, provided growth rates are characterized in terms of the total (near-tip) stress intensity (incorporating, for example, the effect of residual stress); with this achieved, in principle, either form of data can be used for life prediction of implant devices. Inspection of the long and small crack results reveals extensive scatter inherent in both forms of growth-rate data for the pyrolytic-carbon material.
已开展了一项基于断裂力学的研究，以表征由热解碳涂层石墨制成的人工心脏瓣膜组件中初始缺陷的断裂韧性和循环疲劳裂纹扩展速率。使用损伤容限分析预测机械心脏瓣膜假体的安全结构寿命需要此类数据。与以往使用常规（如紧凑拉伸）试样中贯穿厚度的长裂纹（约2 - 20毫米长）获取疲劳裂纹扩展数据的研究不同，本次实验针对热解碳涂层表面萌生的物理小裂纹（约100 - 600微米长）进行，以模拟实际情况。发现小裂纹韧性结果与常规测量长裂纹时得到的结果非常吻合。然而，与金属疲劳中众所周知的观察结果类似，发现基于以最大应力强度Kmax表示的外加（远场）驱动力的通常计算，在相同外加应力强度下，小疲劳裂纹的扩展速率超过长裂纹，并且对Kmax呈现负相关性；此外，它们在低于疲劳阈值的外加应力强度下扩展，低于该阈值时长裂纹被认为处于休眠状态。为解决这一明显差异，结果表明，如果以总（近尖端）应力强度（例如包含残余应力的影响）来表征扩展速率，则长裂纹和小裂纹的结果可以归一化；做到这一点后，原则上两种形式的数据均可用于植入装置的寿命预测。对长裂纹和小裂纹结果的检查揭示了热解碳材料两种形式的扩展速率数据中固有的广泛离散性。
