DENTAL MATERIALS 24(2008)289-298 Strength(GPa) 7 orders of magnitude higher than for chemically assisted (water-enhanced)crack growth at equivalent crack-tip stress intensities 37 In fact, a number of authors have demon Y-TZP+ Al2O4 strated crack growth under cyclic conditions that arrests when the same specimens are then held statically with the same Kmax and then resume growth under resumed cycling] Crack growth rates, da/dN, are found to be a function of the cyclic stress intensity range, AK, in a power law relationship 37: where Cis a constant. The exponent, m, is in the range of 21-42 (for metals the same relationship holds with m ranging from gness(MPavm) 2to4)B37] It is also found that for small cracks($100 um; i.e. natural g 3- Strength vs toughness curves for four types of flaw size range)(1)growth rates are far in excess of that for transformation-toughened zirconia recreated with long cracks(> mm) at equivalent applied stress intensities permission from work of Swain and Rose[36]. Solid lines and (2) that crack extension occurs at stress intensities below are analytical fits to experimental data (not shown). Dashed the threshold for long-crack growth[39]. Both of these differ- line represents the critical stress for the t-m ences in short crack behavior are likely due to the incomplete transformation. Origin of the strength maximum and the R-curve toughness development related to the incomplete non-coincidence of maximum strength and maximum establishment of a steady-state transformation zone for the toughness are discussed in the text. short crack Cyclic crack growth rates can also be influenced by cycling history under variable amplitude loading; demonstrating for four different transformation-toughened ceramics, as is accelerated growth following an increase in maximum load reproduced in Fig 3[ 29, 36 (increasing Ak) and growth retardation following a reduction Another feature of this analysis by Swain and Rose is the in peak cyclic load(decreasing AK)[37. This history effect prediction that a lower toughness is achieved at mum is thought to be related to(1) the transformation zone size trength for ceramics having smaller initial flaw sizes [36]. being sensitive to the crack-tip Kmax and(2)the steady-state This prediction comes from the feature that transformation shielding becoming maximal after crack extensions of approx zone size, h, at maximum strength depends on the initial mately five times the zone width [ 37] flaw size, Ci, with ho(Gi)[2, 36]. Recall from above that this Many characteristics discussed above regarding premature zone-size/filaw-size relationship is one reason why there is failure of zirconia materials under cyclic conditions compli no characteristic R-curve for any given material. This insight cate lifetime predictions: of Swain is also consistent with experimental data with flaw sizes known to be the smallest in y-tzP alumina materials and esign data from static tests have little value when the largest in Mg-PSZ. apparent threshold for fatigue crack growth can be approx- This strength-toughness"disconnect"also creates two imately 50% of the fracture toughness, K classes of materials at either end of the toughness continuum. Purely elastic failure criteria cannot be applied(even for The very high-strength and lower-toughness materials remain non-cyclic loading sensitive to processing flaws. In contrast the high-toughness Very high crack growth exponents(m)indicate extreme sen- and lower-strength ceramics are flaw and damage tolerant. sitivity to applied stres Swain and Rose cite the example that a 20 kg Vickers indenta- Transient crack acceleration/retardation effects imply that tion of peak-strength Y-TZP 20% alumina results in a ten-fold data from constant AK cyclic loading may be inadequate rength degradation whereas a peak-strength Mg-PSZ can The role of crack size in influencing cyclic behavior is incom- withstand twice that indentation load with no strength loss pletely understood [36]. Such considerations also lead to the generalization that for materials having a Kic less than a8 MPa"the strength is These factors imply that lifetime predictions may be enor limited by flaw size while above this value strength is limited mously sensitive to assumptions made regarding the initial by stress-activated transformations 91 defect size and in-service stresses 7. Cyclic fatigue of Low temperature degradation of 3Y-TZP transformation-toughened ceramics If most aspects of the transformation toughening ability Lifetimes of transformation-toughened ceramics are found to zirconia are positive, resulting in high strength and toughne be lower under cyclic loading than under equivalent static and reduced brittleness compared to alumina, considerable loading. Crack growth rates under cyclic conditions can amount of work has been devoted to the characterization of a296 dental materials 24 (2008) 289–298 Fig. 3 – Strength vs. toughness curves for four types of transformation-toughened zirconia recreated with permission from work of Swain and Rose [36]. Solid lines are analytical fits to experimental data (not shown). Dashed line represents the critical stress for the t→m transformation. Origin of the strength maximum and the non-coincidence of maximum strength and maximum toughness are discussed in the text. for four different transformation-toughened ceramics, as is reproduced in Fig. 3 [29,36]. Another feature of this analysis by Swain and Rose is the prediction that a lower toughness is achieved at maximum strength for ceramics having smaller initial flaw sizes [36]. This prediction comes from the feature that transformation zone size, h, at maximum strength depends on the initial flaw size, ci, with h ∝ (ci) [2,36]. Recall from above that this zone-size/flaw-size relationship is one reason why there is no characteristic R-curve for any given material. This insight of Swain is also consistent with experimental data with flaw sizes known to be the smallest in Y-TZP alumina materials and largest in Mg-PSZ. This strength-toughness “disconnect” also creates two classes of materials at either end of the toughness continuum. The very high-strength and lower-toughness materials remain sensitive to processing flaws. In contrast the high-toughness and lower-strength ceramics are flaw and damage tolerant. Swain and Rose cite the example that a 20 kg Vickers indenta￾tion of peak-strength Y-TZP 20% alumina results in a ten-fold strength degradation whereas a peak-strength Mg-PSZ can withstand twice that indentation load with no strength loss [36]. Such considerations also lead to the generalization that for materials having a KIC less than ≈8 MPam1/2 the strength is limited by flaw size while above this value strength is limited by stress-activated transformations [9]. 7. Cyclic fatigue of transformation-toughened ceramics Lifetimes of transformation-toughened ceramics are found to be lower under cyclic loading than under equivalent static loading. Crack growth rates under cyclic conditions can be 7 orders of magnitude higher than for chemically assisted (water-enhanced) crack growth at equivalent crack-tip stress intensities [37]. In fact, a number of authors have demon￾strated crack growth under cyclic conditions that arrests when the same specimens are then held statically with the same Kmax and then resume growth under resumed cycling [37–39]. Crack growth rates, da/dN, are found to be a function of the cyclic stress intensity range, K, in a power law relationship [37]: da dN = C(K) m (13) where C is a constant. The exponent, m, is in the range of 21–42 (for metals the same relationship holds with m ranging from 2 to 4) [37]. It is also found that for small cracks (≤100m; i.e. natural flaw size range) (1) growth rates are far in excess of that for long cracks (≥3mm) at equivalent applied stress intensities and (2) that crack extension occurs at stress intensities below the threshold for long-crack growth [39]. Both of these differ￾ences in short crack behavior are likely due to the incomplete R-curve toughness development related to the incomplete establishment of a steady-state transformation zone for the short crack. Cyclic crack growth rates can also be influenced by cycling history under variable amplitude loading; demonstrating accelerated growth following an increase in maximum load (increasing K) and growth retardation following a reduction in peak cyclic load (decreasing K) [37]. This history effect is thought to be related to (1) the transformation zone size being sensitive to the crack-tip Kmax and (2) the steady-state shielding becoming maximal after crack extensions of approx￾imately five times the zone width [37]. Many characteristics discussed above regarding premature failure of zirconia materials under cyclic conditions compli￾cate lifetime predictions: • Design data from static tests have little value when the apparent threshold for fatigue crack growth can be approx￾imately 50% of the fracture toughness, Kc; • Purely elastic failure criteria cannot be applied (even for non-cyclic loading); • Very high crack growth exponents (m) indicate extreme sen￾sitivity to applied stress; • Transient crack acceleration/retardation effects imply that data from constant K cyclic loading may be inadequate; • The role of crack size in influencing cyclic behavior is incom￾pletely understood. These factors imply that lifetime predictions may be enor￾mously sensitive to assumptions made regarding the initial defect size and in-service stresses. 8. Low temperature degradation of 3Y-TZP If most aspects of the transformation toughening ability of zirconia are positive, resulting in high strength and toughness and reduced brittleness compared to alumina, considerable amount of work has been devoted to the characterization of a