Michael G. Jenkins

Fatigue Performance of High-Pressure Waterjet-Peened Aluminum Alloy

An experimental study of high-pressure waterjet peening on 7075-T6 aluminum alloy was conducted to investigate the effects of waterjet on high-cycle fatigue life and fatigue crack growth. Unnotched hourglass-shaped circular cross section test specimens were fatigue tested in completely reversed rotating bending (R =S min /S max = -1) to determine fatigue life behavior (S-N curves). Single-edge-notched flat tensile test specimens were tested in the tension-tension fatigue crack growth tests (R=S min /S max =0.1) to determine fatigue crack propagation behavior (da/dN versus AK). Surface characteristics and fracture surfaces were evaluated by scanning electron microscopy (SEM). Results show that waterjet peening can increase high-cycle fatigue life, delay fatigue crack initiation, and decrease the rate of fatigue crack propagation.

Residual Stress Induced by Waterjet Peening: A Finite Element Analysis

The concept of multiple droplet impacts resulting from ultra high-pressure waterjet (UHPWJ) was used to develop a mathematical model to describe the effect of interfacial pressure on the underlying workpiece material. A non-linear elastic-plastic finite element analysis (FEA) was carried out in this study using the interfacial pressure model to predict residual compressive stresses. This three-dimensional FEA model was based on quasi-static considerations to provide prediction of both magnitude and depth of residual stress fields in a 7075-T6 aluminum alloy (A17075-T6). Results of the FEA modeling were in good agreement with experimental measurements. Effects of applied pressures on the residual stress fields are also presented and discussed as a method of estimating high-pressure waterjet induced compressive stresses under varying process conditions for peening.

Elevated temperature fracture resistance of a sic whisker reinforced/polycrystalline Al2O3 matrix composite

Abstract The fracture resistance of a SiC whisker/Al2O3 matrix composite for the test temperature range of 20° to 1400°C was determined using chevron-notched and straightnotched, three-point bend specimens. Crack mouth opening displacement (CMOD) was continuously monitored throughout the fracture tests, using a laser interferometric strain gage (LISG). The crack growth resistance curves ( R- curves ) were determined from previously established relations between the CMOD compliance, effective crack length, and the load point displacement (LPD).

Development of test standards for continuous fiber ceramic composites in the United States

Standardization activities in the United States for continuous fiber-reinforced ceramic composites (CFCCs) are reviewed. This brief review focuses on the development of test standards by subcommittee C28.07 of the American Society for Testing and Materials (ASTM) on the drafting of a section of a design code for ceramic and ceramic matrix composite components as part of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, and on the development of a set of volumes on ceramic matrix composites for Military Handbook 17 on composites. The participation of the US in the international harmonization of standards for CFCCs is also reviewed.

Mechanical behavior and numerical estimation of fracture resistance of a SCS6 fiber reinforced reaction bonded S13N4 continuous fiber ceramic composite

Continuous fiber ceramic composites (CFCCs) have advantages over monolithic ceramics: Silicon Nitride composites are not well used for application because of their low fracture toughness and fracture strength, but CFCCs exhibit increased toughness for damage tolerance, and relatively high stiffness in spite of low specific weight. Thus it is important to characterize the fracture resistance and properties of new CFCCs materials. Tensile and flexural tests were carried out for mechanical properties and the fracture resistance behavior of a SCS6 fiber reinforced Si3N4 matrix CFCC was evaluated. The results indicated that CFCC composite exhibit a rising R curve behavior in flexural test. The fracture toughness was about 4.8 MPa m1/2 , which resulted in a higher value of the fracture toughness because of fiber bridging. Mechanical properties as like the elastic modulus, proportional limit and the ultimate strength in a flexural test are greater than those in a tensile test. Also a numerical modeling of failure process was accomplished for a flexural test. This numerical results provided a good simulation of the cumulative fracture process of the fiber and matrix in CFCCs.

Fabrication, Processing, and Characterization of Braided, Continuous SiC Fiber-Reinforced/CVI SiC Matrix Ceramic Composites

Abstract Three-dimensionally reinforced continuous-fiber ceramic matrix composites (CFCCs) were fabricated from preforms of braided SiC fiber (Nicalon(™)) tows that had been coated with 0.4 μm thick layer of pyrolytic graphite. A hybrid infiltration process of chemical vapor infiltration and polymer impregnation pyrolysis was used to form the matrix. The as-fabricated CFCCs were composed of ∼34 vol% fiber and ∼36 vol%matrix with ∼29 vol% residual porosity. Appropriate specimens were tested at 20° and 1000°C along the longitudinal braiding direction in uniaxial tension, compression and three and four-point flexure. Linear stress-strain responses to well-defined proportional limits (∼75 MPa) occurred for tension and flexure at both temperatures. Nonlinear stress-strain behaviour occurred beyond the proportional limit up to fracture at ultimate strengths in tension on the order of 175-200 MPa for 20°C and 100 MPa for 1000°C. SEM fractography revealed fiber pullout for the 20°C tests and evidence of brittle f...

ASTM Standards for Monolithic and Composite Advanced Ceramics: Industrial, Governmental and Academic Cooperation

Monolithic and composite advanced ceramics have reached sufficient levels of material development to warrant serious consideration for applications in advanced heat engines. These applications require optimum material behavior with physical and mechanical property reproducibility, component reliability, and well-defined methods of data treatment and materials analysis. As new materials are introduced into the market place, these issues are best dealt with via standard methods. The primary standards writing organization in the U.S. is the American Society for Testing and Materials (ASTM), a private, nonprofit corporation which relies upon the voluntary cooperation of industry, government, and academe to develop standards by consensus. ASTM Committee C28 “Advanced Ceramics” has been active since 1986 and currently has 16 standards “on the books” with 17 standards in the balloting process. Overviews of the five subcommittees of C28 are presented. Accomplishments to date are discussed, as well as future activities, including a brief summary of joint cooperative efforts with international standards formulating organizations.© 1996 ASME

Onset of Cumulative Damage (First Matrix Cracking) and the Effects of Test Parameters on the Tensile Behavior of A Continuous Fibre-Reinforced Ceramic Composite (CFCC)

Continuous-fibre ceramic matrix composites (CFCCs) comprise a recently-introduced subset of ceramic materials and are finding numerous potential applications in the aerospace and energy generation industries. The result of the micromechanical interaction of fibers, fiber coatings, matrix, and their interfaces give CFCCs much greater resistance to catastrophic failure than monolithic ceramics while retaining the high-temperature strengths, corrosion/erosion resistances, high stiffnesses, and low densities characteristic of ceramic materials. As a result, CFCCs have the potential to overcome the inherent brittleness of monolithic advanced ceramics which is often cited as a major limitation preventing these materials from becoming widely accepted in modern designs [1].

Observations of inelastic deformation during high-temperature fatigue and failure of a polycrystalline silicon nitride

Abstract The effects of rate-dependent inelastic deformation were observed during tensile static, dynamic, and cyclic fatigue testing of a hot-isotatically pressed, monolithic, polycrystalline silicon nitride in ambient air at temperatures of 1150, 1260 and 1370°C. Constant stresses in static fatigue ranged from 50 to 300 MPa. Stress rates in dynamic fatigue ranged from 10 −4 to 10 1 MPa/s. Waveforms in cyclic fatigue included trapezoid, triangle and sine at a stress ratio, R , of 0.1, frequencies of 0.1 and 10 Hz, and maximum stresses ranging from 75 to 325 MPa. At 1150°C, all fatigue results showed a similar slow crack growth failure mechanism with no separate cyclic fatigue mechanism. However, at 1260 and 1370°C the failure mechanism was multi-faceted. For both temperatures, the failure in static fatigue was dominated by the accumulation of diffusion-controlled creep cavities. In dynamic fatigue, the inelastic deformation exhibited by non-linear stress-strain curves supported the conclusion of failure by slow crack growth alone at high stress rates and by a mixture of slow crack growth and creep damage at low stress rates. Under cyclic loading, ‘enhanced’ times to failure, attributed to viscous rate effects, occurred regardless of waveform or frequency. The stress rate was related to the stress at the onset to non-linearity, σ 0 , indicating a quasi-endurance limit below which cyclic fatigue had little ‘enhancing’ effect. Creep compliance relations indicated non-linear viscoelastic (or viscoplastic) behavior, leading to speculation on the role of this behavior in producing the ‘enhanced’ cyclic fatigue resistance.

Ceramic matrix technologies for composites and their widespread enabling supporting implementation

Advanced materials such as ceramic matrix composites (CMCs) are an enabling technology for the implementation of many advanced engineering designs (for example, advanced, low emission, low fuel consumption gas turbines). However, an often overlooked critical aspect of this enabling technology role for CMCs are the enabling supporting technologies of standards, design codes and data bases that provide the tools to allow engineers to benchmark, design with and analyse CMCs for the advanced designs. In this paper, the current states of standards, design codes and data bases for CMCs are reviewed in light of their role as supporting enabling technologies. Potential advances and impediments to continued developments of these enabling supporting technologies are discussed.