Abstract
Background
Clinical failure of dental ceramics is usually reported as partial fracture of the restoration (chipping) or as catastrophic fracture of the whole structure. In contrast to metals, ceramics are linear-elastic, brittle materials exhibiting extremely low damage tolerance to failure. Well documented clinical and lab reports have shown this fracture event often occurs at loads far below their fracture strength due to intrinsic fatigue degradation via slow crack growth or cyclic fatigue mechanisms. The presence and development of surface flaws have a dominant role in damage accumulation and lifetime reduction of ceramic structures.
Aims
This ADM guidance document aims to summarize the aspects related to fatigue degradation of dental ceramics, reviewing the concepts of fatigue testing and furthermore aims to provide practical guidance to young scientists entering into fatigue related research. The description of fatigue strength is always accompanied by a clear understanding of the underlying fracture mechanisms.
1
Introduction
The mechanical performance of ceramic materials is commonly approached by measuring the fracture strength or toughness using simplified bar or disc specimens. Such methods reflect the static, inert behavior of materials at critical loads, focusing on fracture as the final event. As fracture is the rupture of the bonds, fracture strength of ceramics is known to be inversely proportional to the largest or critical flaw present in the loaded volume, as described by Griffith’s law . One can find detailed information on fracture strength and toughness in the corresponding ADM guidance documents ( www.academydentalmaterials.org ).
Any component in normal service is loaded far below its critical load either continuously or under repetitive conditions. The related mechanical phenomenon is called “fatigue”, which is often defined as the degradation (weakening) of a structural component under the influence of mechanical, chemical or biological stress – and in most cases – a combination of them.
The fatigue progression over time is shown in Fig. 1 . At certain service loads (below the fracture strength), flaws (defects, cracks) tend to grow. As the stress intensity at the crack tip increases with growing flaw size, the relation between flaw size and service life becomes exponential. Depending on the level of applied service loads, the material strength drops significantly from the inert strength and a fatigue failure is expected. However, at low service loads, fatigue (or endurance) limits may exist at a stress below which no further crack growth happens and failure will not occur no matter how many loading cycles are involved or how long a component is statically loaded (threshold value).
In dentistry, one could think of a cyclic loading scenario in a compressive or bending configuration combined with the influence of water that simulates, in vitro, the clinical conditions of mastication. Degradation of properties always occurs over time, so the fatigue parameter actually reflects the time-dependency of material performance and in the end determines the lifetime of a restoration. While inert strength measurements investigate fast fracture, fatigue investigations deal with crack initiation and the slow growth of cracks under the influence of the environment. The fast fracture criterion is termed “critical” whereas the slow growth of cracks is called “sub-critical” crack growth (SCG) .
The definition of fatigue at ambient temperatures mostly involves two major, relevant mechanisms, arising either from stress corrosion (SCG) (chemically-assisted by water) and/or from additional cyclic effects . While SCG has been demonstrated 70 years ago , in the past it was believed that there was no additional effect from cyclic loading in the fatigue behavior of brittle ceramics. Extensive research on the fatigue of metallic materials, showing that cyclic fatigue plays a dominant role, also led to insights into the damaging effect of cyclic loading for ceramics. In brief, while SCG might occur in a comparable rate independent from static, dynamic or cyclic loading, cyclic effects arise from friction and hydrolytic pressure during crack closing. Today, there is a common understanding that cyclic effects contribute to overall degradation of brittle ceramics, although to lesser extent compared to SCG .
Clinically, fatigue degradation over time is always associated with progressive surface wear (abrasion and attrition). During wear, an extended damage accumulation zone is formed on the surface with the largest defects further progressing to fatigue crack growth. A specific ADM guidance document reviewed the mechanisms involved in the intraoral wear process that controls mechanical strength degradation ( www.academydentalmaterials.org ).
This document seeks to provide an introductory guidance to the field of dental ceramics fatigue. The principles and mechanisms presented here are – within limitations – expandable to dental resin-based composites. For those readers interested in learning more about the principles behind slow crack growth, we suggest literature that provides more comprehensive coverage of the subject. For a general overview, there is an easy-to-read book recommended from Ashby and Jones on properties and applications of engineering materials. Parts D and E of this book introduce the principles of fast fracture, fracture toughness and fatigue and answers the most basic questions . Fundamental studies on glass fatigue were published by Charles and co-workers . Further reading especially on the fracture mechanics background of fatigue crack growth can be found in David Broek’s book intitled “The practical use of fracture mechanics” or in Dieter Munz and Theo Fett’s book “Ceramics” . A more recent, comprehensive review on “Fracture of Ceramics” was published by Danzer et al. . They comprehensively reviewed the concept of stress corrosion versus cyclic fatigue effects. Focusing on the aspects of ceramic fatigue related to dentistry, the book from Kelly is recommended as well as the more recent and clinically oriented review from Zhang et al. . Typical fracture modes, and fatigue mechanisms in clinical service are described and discussed. For an in-depth analysis of the fatigue responses of ceramics and constitutive models providing insights into fatigue processes the book from Suresh is highly recommended . The principles and mechanisms responsible for fatigue of resin composite can be found elsewhere .
Based on ISO and ASTM standards, fatigue of metallic materials is well described but only little guidance is available on how to perform fatigue experiments on brittle materials. A Japanese standard introduces the static bending fatigue method for fine ceramics . The ASTM-C-1368 standard is a comprehensive document describing the constant stress-rate method for evaluating slow crack growth parameters . A comparable approach on dynamic fatigue is described in the European standard EN 843-3 . The only advice related to dentistry can be found in ISO 14801 where cyclic fatigue testing of dental implants is described .
Of course, this guidance document cannot comprehensively cover all fields related to fatigue degradation, such as fracture toughness or increase of toughness with growing defects (R-curve behavior) . Also, the influence of internal stresses on toughness and strength as well as related aspects of multilayered or graded components are not addressed here. Further reading is provided by ADM guidance documents on fracture toughness and multilayered dental ceramics ( www.academydentalmaterials.org ) .
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General considerations
Damage accumulation after multiple cycles at low loads can alter the durability of ceramic parts, reducing their service life ( Fig. 1 ). This is especially true for ceramic parts operating in wet environments. Chemically-assisted crack growth (SCG) is probably the most important (and most studied, either directly or indirectly) fatigue mechanism affecting all dental ceramics (see Table 1 ). This mechanism involves the slow growth of cracks at stresses and crack tip stress intensities well below those associated with catastrophic fracture. The hydrolytic principle leading to corrosive bond rupture and cleavage in glasses and ceramics is shown in Fig. 2 .
