Silicon Carbide Whisker/Alumina Matrix Composites: Effect of Whisker Surface Treatment on Fracture Toughness

The fracture toughness of a 30 vol% SiC whisker/Al₂O₃ matrix composite was evaluated as a function of whisker surface chemistry. Two types of SiC whiskers (Silar-SC-9 and Tateho-SCW-1-S) were investigated. Modification of the whisker surface chemistry was achieved by subjecting the whiskers to thermal treatments under controlled atmospheres. Whisker surface chemistry, as determined by X-ray photoelectron spectroscopy, was correlated to the fracture toughness of the composites.     

Microstructure and Fracture Toughness of Hot-Pressed Silicon Carbide Reinforced with Silicon Carbide Whisker

The effects of β-SiC whisker addition on the microstructural evolution and fracture toughness ( K _IC) of hot-pressed SiC were investigated. Most of the whiskers added disappeared during the densifcation process by transformation into the α-phase. The remaining whiskers acted as nuclei for grain growth, resulting in the formation of large tabular grains around the whiskers. The tabular grains around the whiskers were believed to be formed because of the extreme anisotropy of the interfacial energy between α- and β-SiC. The K _IC of the material was improved significantly by the whisker addition. The increase in the K _IC was attributed to crack bridging followed by grain pullout as a result of the formation of tabular grains in a fine matrix.     

High-Temperature Fracture Toughness of Sapphire

The fracture toughness of sapphire with crack propagation parallel to the basal plane was measured from 1200° to 1500°C. Fracture surfaces of near c -axis fibers tested in tension were used to determine the values. The toughness was constant and equal to 1.4 ± 0.1 Mpa^.m^1/2 over the entire temperature range.     

Fracture Toughness of Adhesive Joints

To define the influence of the processing variables on the resistance of epoxy joints to brittle crack extension during short loading times, the fracture toughness, g_ic, of the joints was measured as a function of the following variables:

1. Hardener type (TEPA vs. HHPA)

2. Ratio of hardener to resin content

3. Post-cure temperature

and 4. Joint geometry (thickness and width)

It was found that the toughness of the TEPA hardened system varied by a factor of four-to-one as the ratio of hardener to resin content and post-cure temperature varied within what might be considered reasonable limits for manufacturing. The toughness of the HHPA hardened system varied only over the middle half of this same range.

For both systems, toughness increased with joint thickness over the range of 2 to 50 mils.     

Fracture Toughness of Metaphosphate Glasses

The fracture toughnesses and Young's elastic moduli of several metaphosphate glasses were measured by the indentation technique. The results show that the normal glasses lie along a K_Ic vs E line between (0.4, 40) and (0.7, 60) MPa-m¹¹² and GPa, respectively. The abnormal ZnO and MgO metaphosphates appear to be tougher and are above the line of the normal glasses.     

Micro-Indentation Fracture Toughness Evaluation with a Novel Microscopy

A novel microscopy is developed for evaluating the fracture toughness of thin ceramic sheets using the micro-indentation method. This microscopy overcomes the theoretical limit of measurement resolution associated with optical microscopes. Using a modified optical microscope, the crack lengths produced by micro-indentation on the surface of an as-fired ceramic sheet can now be measured. Experimental results on an 8 mol% yttria-stabilized zirconia ceramic substrate show that the accuracy of the fracture toughness evaluation is satisfactory. This micro-indentation fracture toughness evaluation method is efficient and economical for thin ceramic substrate surfaces, either polished or as-fired.     

Influence of Thermal Conductivity and Fracture Toughness on the Thermal Shock Resistance of Alumina—Silicon–Carbide–Whisker Composites

The thermal shock resistance (indentation–quench method), fracture toughness, and thermal conductivity of three alumina–silicon–carbide–whisker composites and alumina have been investigated. A new procedure for the evaluation of thermal conductivity data is suggested, and higher room-temperature thermal conductivity than that reported in the literature is determined for silicon carbide whiskers. The ranking of the materials according to thermal shock resistance is consistent with the ranking according to fracture toughness but disagrees with the ranking according to thermal conductivity. This finding supports the analytically obtained result that, in defining thermal shock resistance, fracture toughness is more important than thermal conductivity.     

Fracture Toughness of Spray-Dried Powder Compacts

The strengths and fracture toughness values were measured for alumina powder compacts containing two different binder systems. Diametral compression was used to measure both the tensile strength and the fracture toughness (through-thickness notch). This methodology was very useful in linking processing parameters, such as binder choice and compaction stress, to the quality of the green bodies. Observations of the compact structure before and after fracture showed that the binders segregated to the region between the spray-dried granules. The presence of the excess binder in this region was linked to both the failure mode and the creation of secondary cracks.     

Short-Crack Fracture Toughness of Silicon Carbide

The fracture toughness of four different silicon carbides was measured using single-edge precracked beam (SEPB) and indentation/strength techniques. Two were development grades with similar microstructures and chemistries, and yet exhibited different fracture modes. The grade that exhibited a predominantly intergranular fracture had an SEPB fracture toughness (6.4 MPa√m) 88% higher than the one that showed primarily a transgranular fracture (3.4 MPa√m). The higher fracture toughness was associated with a modest increase in average strength (25%), although there was a significant increase in the Weibull modulus (11–32). Fracture toughness at short crack lengths was assessed by an indentation method that used fracture strengths, crack lengths at fracture, and a new method of estimating the constant δ that characterizes the residual driving force of the plastic zones based on the stable growth of the indentation cracks from the initial ( c ₀) to the instability ( c ^*) lengths. The results showed a rising crack-growth-resistance behavior for the grade exhibiting intergranular fracture, while the grade showing transgranular fracture had a flat crack-growth resistance. Tests on two commercial grades of silicon carbide showed similar behaviors associated with the respective fracture modes.     

Fracture Toughness of Chemically Vapor-Deposited Diamond

The fracture toughness of chemically vapor-deposited diamond is estimated by a Vickers indentation method. Freestanding diamond films of 400-μm thickness are produced with plasma-enhanced chemical vapor deposition and highly polished for indentation testing. Indentation testing was performed with a microhardness tester using a load range of 5 to 8 N. The average fracture toughness is estimated as 5.3 ± 1.3 MPA · m^1/2.     

The Fracture Toughness of Adhesive-Bonded Joints

Fracture mechanics is related to adhesion theory and the testing of adhesive-bonded joints in the lap-shear configuration. The complexity of the stress field necessitates the strain energy release rate approach, which is followed to derive the relation for a lap-shear sample: G_c = P_c ²/4b (dC/da). G_c is the fracture toughness (critical strain energy release rate), P_c is the breaking or crack instability load, a and b are crack lengths and widths, respectively, and C is the sample compliance for the Tap-shear sample with a crack of these dimensions at each loading edge. It was found that G_c ranged from 1.18 to 1.42 with an average value of 1.34 in.-lb./in.² for epoxy bonded aluminum strips (EPON 934 and Alcald 2024-T3). Evidence, in the form of photoelastic stress patterns, suggesting that crack extension occurs in the opening mode in lap-shear samples is presented and discussed.     

Dynamic Fracture Toughness of Piezoelectric Ceramics

Both dynamic and static three‐point bending fracture testings of 32 piezoelectric ceramic samples were performed in four different poling directions. A modified split Hopkinson pressure bar method with a polyvinylidene fluoride (PVDF)‐based gauge was utilized for the dynamic experiments. The loading rate greatly influenced the fracture toughness in two ways: the dynamic fracture toughness values were much higher than those of the static fracture toughness, and unlike the static fracture toughness, the influence of poling direction on the dynamic fracture toughness was not obvious.     

High-Temperature Fracture Toughness of Duplex Microstructures

The temperature dependence of the fracture toughness of ceramics exhibiting duplex microstructures was studied relative to their single-phase constituents using two test methods: bend testing of chevron-notched beams, and the indentation-crack-length technique. The two materials systems studied were Al₂O₃: c -ZrO₂(Y) and A1₂O₃:Y₃A1_SO₁₂ (YAG), and the testing temperature ranged from room temperature to 1200°C. The study showed that in both systems the duplex materials showed higher toughness values than their single-phase constituents above 800°C. This result was attributed to the contribution of low-energy interphase boundaries to the overall composite toughness. Indentation crack length measurements gave toughness values and trends comparable to those determined by the chevronnotched beam method. By comparing the results of the two test methods it was possible to demonstrate that the indentation calibration constant (ξ) shows no significant temperature or material dependence. For the zirconia-containing materials, however, indentation at elevated temperatures is accompanied by significant localized plasticity, which suppressed the radial cracking. Under such conditions, some caution is warranted, since localized plasticity can lead to an overestimation of the fracture toughness.     

The Fracture Toughness of Adhesive-Bonded Joints

Fracture mechanics is related to adhesion theory and the testing of adhesive-bonded joints in the lap-shear configuration. The complexity of the stress field necessitates the strain energy release rate approach, which is followed to derive the relation for a lap-shear sample: G_c = P_c²/4b (dC/da). G_c is the fracture toughness (critical strain energy release rate), P_c is the breaking or crack instability load, a and b are crack lengths and widths, respectively, and C is the sample compliance for the Tap-shear sample with a crack of these dimensions at each loading edge. It was found that G_c ranged from 1.18 to 1.42 with an average value of 1.34 in.-lb./in.² for epoxy bonded aluminum strips (EPON 934 and Alcald 2024-T3). Evidence, in the form of photoelastic stress patterns, suggesting that crack extension occurs in the opening mode in lap-shear samples is presented and discussed.     

Mixed-Mode Fracture Toughness of Ceramic Materials

An experimental technique whereby pure mode I, mode II, and combined mode I-mode II fracture toughness values of ceramic materials can be determined using four-point bend specimens containing sharp, through-thickness precracks is discussed. In this method, notched and fatigue-precracked specimens of brittle solids are subjected to combined mode I-mode II and pure mode II fracture under asymmetric four-point bend loading and to pure mode I under symmetric bend loading. A detailed finite element analysis of the test specimen is performed to obtain stress intensity factor calibrations for a wide range of loading states. The effectiveness of this method to provide reproducible combined mode I-mode II fracture toughness values is demonstrated with experimental results obtained for a polycrystalline Al₂O₃. Multiaxial fracture mechanics of the Al₂O₃ ceramic in combined modes I, II, and III are also described in conjunction with the recent experimental study of Suresh and Tschegg (1987). While the mode II fracture toughness of the alumina ceramic is comparable to the mode I fracture toughness K _Ic, the mode III fracture initiation toughness is 2.3 times higher than K _Ic. The predictions of fracture toughness and crack path based on various mixed-mode fracture theories are critically examined in the context of experimental observations, and possible effects of fracture abrasion on the apparent mixed-mode fracture resistance are highlighted. The significance and implications of the experimental methods used in this study are evaluated in the light of available techniques for multiaxial fracture testing of brittle solids.     

Effective Fracture Toughness of Microcracked Materials

Explicit analytical formulas are derived for the stress intensity factors at the tips of a main crack and of a microcrack for the two-dimensional case of a collinear microcrack. This configuration is used to derive an estimate of the toughness degradation due to microcracks linking up with an advancing main crack. The implications of this estimate for theoretical predictions of the toughening due to stress-induced micro-cracking are discussed.     

Fracture Toughness of Ceramics Measured by a Chevron-Notch Diametral-Compression Test

A diametral-compression test that features disk specimens with chevron notches was developed and applied to measure fracture toughness (K_lc) of three ceramics. Fracture–toughness values calculated from the fracture loads and a compliance analysis were in good agreement with values measured by other fracture–mechanics techniques. Advantages of the proposed test include simple specimen geometry, minimal requirements for fixtures, and consequent potential for application at elevated temperatures.