Marx Rudolf, Jungwirth Franz, Walter Per-Ole
Department of Prosthetic Dentistry, Section of Dental Materials, University Hospital of the University of Technology, 52074 Aachen, Germany.
Biomed Eng Online. 2004 Nov 17;3(1):41. doi: 10.1186/1475-925X-3-41.
Slow crack growth can be described in a v (crack velocity) versus KI (stress intensity factor) diagram. Slow crack growth in ceramics is attributed to corrosion assisted stress at the crack tip or at any pre-existing defect in the ceramic. The combined effect of high stresses at the crack tip and the presence of water or body fluid molecules (reducing surface energy at the crack tip) induces crack propagation, which eventually may result in fatigue. The presence of a threshold in the stress intensity factor, below which no crack propagation occurs, has been the subject of important research in the last years. The higher this threshold, the higher the reliability of the ceramic, and consequently the longer its lifetime.
We utilize the Irwin K-field displacement relation to deduce crack tip stress intensity factors from the near crack tip profile. Cracks are initiated by indentation impressions. The threshold stress intensity factor is determined as the time limit of the tip stress intensity when the residual stresses have (nearly) disappeared.
We determined the threshold stress intensity factors for most of the all ceramic materials presently important for dental restorations in Europe. Of special significance is the finding that alumina ceramic has a threshold limit nearly identical with that of zirconia.
The intention of the present paper is to stress the point that the threshold stress intensity factor represents a more intrinsic property for a given ceramic material than the widely used toughness (bend strength or fracture toughness), which refers only to fast crack growth. Considering two ceramics with identical threshold limits, although with different critical stress intensity limits, means that both ceramics have identical starting points for slow crack growth. Fast catastrophic crack growth leading to spontaneous fatigue, however, is different. This growth starts later in those ceramic materials that have larger critical stress intensity factors.
慢裂纹扩展可以在v（裂纹速度）与KI（应力强度因子）的关系图中进行描述。陶瓷中的慢裂纹扩展归因于裂纹尖端或陶瓷中任何预先存在的缺陷处的腐蚀辅助应力。裂纹尖端的高应力与水或体液分子的存在（降低裂纹尖端的表面能）的综合作用会引发裂纹扩展，最终可能导致疲劳。应力强度因子存在一个阈值，低于该阈值裂纹不会扩展，这在过去几年一直是重要的研究课题。这个阈值越高，陶瓷的可靠性就越高，其寿命也就越长。
我们利用欧文K场位移关系从裂纹尖端附近的轮廓推导出裂纹尖端应力强度因子。通过压痕产生裂纹。阈值应力强度因子被确定为残余应力（几乎）消失时尖端应力强度的时间极限。
我们确定了目前在欧洲对牙科修复很重要的大多数全陶瓷材料的阈值应力强度因子。特别重要的发现是氧化铝陶瓷的阈值极限与氧化锆的几乎相同。
本文的目的是强调这样一个观点，即阈值应力强度因子对于给定的陶瓷材料来说，比广泛使用的韧性（弯曲强度或断裂韧性）更具内在特性，后者仅涉及快速裂纹扩展。考虑两种具有相同阈值极限但临界应力强度极限不同的陶瓷，意味着这两种陶瓷在慢裂纹扩展方面具有相同的起点。然而，导致自发疲劳的快速灾难性裂纹扩展则不同。在那些具有较大临界应力强度因子的陶瓷材料中，这种扩展开始得较晚。
