J. E. Ritter

Predicting lifetimes of materials and material structures

The mechanical strength of brittle materials under stress is of prime importance in applications where allowable design stress, lifetime, and reliability are critical issues. A proper analysis should enable an engineer to select an allowable design stress that will permit a brittle component to function for the expected lifetime with an acceptable low probability of failure. It is the purpose of this paper to provide the background for assessing the mechanical reliability of brittle materials under tensile strength.

Use of the microindentation technique for determining interfacial fracture energy

The microindentation technique was used to determine the interfacial fracture energy of epoxy coatings on soda‐lime glass substrates. An analytical model was developed for calculating fracture energy based on indenter load versus debond crack size measurements. Finite‐element analysis was used to determine the relative amounts of opening and shear loadings at the debond crack tip. The calculated fracture energies are compared to values determined by the double‐cantilever‐beam technique and the four‐point flexure‐beam technique.

Appraisal of biaxial strength testing

Abstract Three biaxial strength tests (ring-on-ring, piston-on-3 ball, and ball-on-ring) were evaluated using finite element analysis. Although in all three tests some uncertainties exist regarding the calculation of fracture stresses from the analytical equation, if fracture occurs within the loading ring, the ring-on-ring loading is thought to give the most accurate measure of strength. In addition, it was found that specimen shape (square vs. circular) had no effect on the stress distribution within the supports and that the stress at the edge of the specimen was less than 10% of the maximum stress for an overhang greater than about 40%. Fracture strength measurements on soda-lime glass gave support to these finite element results.

Measurement of adhesion of thin polymer coatings by indentation

A microindentation technique was developed to measure the adhesive shear strength of thin polymer coatings on glass substrates. Indentation‐induced debonding of the coating was observed to occur under three different conditions: Type I was with the deformations underneath the indenter being essentially elastic; Type II was with the deformations underneath the indenter being plastic; and Type III was after the indenter penetrated the substrate. Stress analyses to calculate the interfacial shear stress for the indentation‐induced debonding of thin coatings are presented for the three types of debonding. All three stress analyses are based upon the linear, elastic analysis of the contact stresses arising from indentation of a soft coating on a rigid substrate. These analyses provide a basis for using controlled indentation debonding as a quantitative measure of the interfacial shear strength of thin coatings to rigid substrates.

Analysis of fatigue data for lifetime predictions for ceramic materials

Accuracy in data analysis is of utmost importance because lifetime predictions are extremely sensitive to experimental uncertainty in the crack growth parameters. The limitations of the conventional data reduction techniques used for analysing static and dynamic fatigue data are reviewed and new, statistical methods of data reduction that offer advantages over the conventional techniques are discussed.

Moisture-assisted crack growth at epoxy–glass interfaces

The double cleavage drilled compression (DCDC) test was used to measure the critical energy release rate, moisture-assisted crack growth, and fatigue threshold for epoxy–glass interfaces bonded with and without a silane coupling agent. The DCDC specimen consists of two glass beams (either soda-lime or fused silica) bonded together with an epoxy adhesive. A through-the-thickness hole is drilled in the centre of the specimen. In the DCDC test compressive loading causes tensile stresses to develop at the poles of the drilled hole. Cracks then nucleate in the epoxy–glass interface, extend from the poles, and propagate axially along the interface in primarily mode I loading. The resistance to moisture-assisted crack growth at untreated epoxy–glass interfaces is significantly less than that in monolithic glass specimens. However, the resistance to moisture-assisted crack growth at silane bonded epoxy–glass interfaces can be comparable with or greater than that in monolithic glass. Silane bonding of epoxy to glass is more effective with fused silica than soda-lime glass, with the fatigue limit of silane bonded epoxy–fused silica interfaces being about 2.5 times greater than that for silane bonded epoxy–soda-lime glass. These results are discussed in terms of possible interfacial crack growth mechanisms.

Critique of test methods for lifetime predictions

Failure predictions for ceramics depend on the experimental parameters that measure the strength distribution (m and sigma 0) and the time-dependency of strength (n). These parameters can be determined by measuring strength as a function of stressing rate in a test environment that simulates the service environment. To minimize the uncertainty in these experimental parameters, at least 30 samples per stressing rate should be tested over a stressing rate range of at least 3 orders of magnitude. The uncertainty in these experimental parameters can be taken into account in design calculations by the use of appropriate safety factors. Thus, through well designed experiments coupled with a reliability analysis, rational design decisions can be made that ensure the successful use of ceramics in demanding structural applications.

Erosion damage in structural ceramics

Abstract The erosion damage (material removal and strength degradation) in a variety of structural ceramics (Al 2 O 3 , SiC, Si 3 N 4 and MgO) as a function of kinetic energy of the impacting particle is compared. Based on this data an erosion damage model is developed by assuming that the kinetic energy of the impacting particle goes into grain boundary cracking and the subsequent grain fall-out creates a hemispherical pit with an annular crack of about a grain diameter in size. The model predicts that, as kinetic energy increases, larger pits are formed; however, the ratio of the annular crack to pit size decreases, causing the stress intensity factor for this pit-crack defect to become insensitive to the size of the pit. Thus, strength degradation is predicted to level off at high kinetic energies. The dependence of the erosive wear data on the kinetic energy of the impacting particle and the grain size and toughness of the target material agreed well with the model, as well as the strength degradation results of sintered Al 2 O 3 . However, the post-erosion strength of sintered SiC showed an increase at high impacting kinetic energies. This discrepancy was thought to be related to the fact that, in SiC, relatively shallow, rather than hemispherical, pits were formed on erosion.

Application of fracture‐mechanics theory to fatigue failure of optical glass fibers

The fatigue behavior of optical glass fibers was determined in air at 23°C and 55% relative humidity by the dynamic‐fatigue test technique in which strength is measured as a function of stressing rate. The good correlation found between the fatigue test data and fracture‐mechanics theory indicates that failure is controlled by slow crack growth of preexisting flaws and that fracture‐mechanics theory can be used in making failure predictions for optical glass fibers.

Effect of microstructure on the erosion and impact damage of sintered silicon nitride

The erosion rates and impact damage of two sintered silicon nitride materials with identical compositions but different microstructures were determined as a function of impacting particle (SiC) kinetic energy and temperature (25–1000° C) using a slinger-type erosion apparatus. The coarse-grained silicon nitride had significantly better resistance to impact damage than the fine-grained material. Crack-microstructure interactions were characterized using scanning electron microscopy and showed that crack-bridging was an important toughening mechanism in the coarse-grained material. Post-impact strength data were significantly less than those predicted from the indentation-strength data, due to impact flaws linking up prior to fracture. Consistent with its greater fracture resistance, the erosion rate of the coarse-grained material was less than that of the fine-grained material for erosion at 25 deg, and was independent of erosion temperature.