Search Results for sustained-load threshold

This article summarizes the metallurgy of carbon and alloy steels, followed by discussions on their major mechanical properties, namely, static fracture toughness, dynamic fracture toughness, fatigue or sustained-load crack growth rates, and fatigue or sustained-load thresholds. It addresses fatigue crack propagation and sustained-load crack propagation, as well as the fundamental aspects of fracture in steels. The article illustrates the effects of variations in the alloy chemistry, microstructure, temperature, strain rate, and environment on various fracture toughness or crack growth rate parameters.

Corrosion fatigue refers to the phenomenon of cracking in materials under the combined actions of fatigue loading and a corrosive environment. This article focuses on the various mechanisms of corrosion fatigue, namely, hydrogen-assisted cracking, anodic dissolution, and surface energy reduction. It discusses the variables affecting corrosion fatigue. The effect of fatigue load frequency, environment, grain size, stress ratio, waveform, and temperature fatigue crack growth are also discussed.

This article begins with a discussion on the classification of hydrogen embrittlement and likely sources of hydrogen and stress. The article describes several hydrogen embrittlement test methods, including cantilever beam tests, wedge-opening load tests, contoured double-cantilever beam tests, rising step-load tests, and slow strain rate tensile tests. It also describes the interpretation of test results and how to control hydrogen embrittlement during production.

This article describes the incubation, nucleation, and propagation of stress-corrosion cracking and how to evaluate it using standard tests. It discusses constant-strain, constant-load, bending, and uniaxial tension testing and how they compare when evaluating smooth and precracked test specimens under elastic-strain, plastic-strain, and residual-stress conditions. The article provides guidance on specimen selection and preparation, strain rate, and test equipment. It also examines service and laboratory test environments and provides detailed information on how to test various steels and alloys and how to interpret test results.

In high-strength aluminum alloys, stress-corrosion cracking (SCC) is known to occur in ordinary atmospheres and aqueous environments. This article discusses the mechanisms of SCC in aluminum alloys, providing information on two main types of SCC models: those of anodic dissolution based on electrochemical theory and those that involve the stress-sorption theory of mechanical fracture. It reviews three different categories of experiments used to compare SCC performance of candidate materials for service. The categories are tests on statically loaded smooth samples, tests on statically loaded precracked samples, and tests using slowly straining samples. The article describes SCC susceptibility and ratings of SCC resistance for high-strength wrought aluminum products, such as 2xxx, 5xxx, and 7xxx series alloys, aluminum-lithium alloys, and 7xxx alloys containing copper.

This article summarizes the understanding of the mechanisms and mechanical effects of fatigue processes in highly brittle materials, with particular emphasis on ceramics. It provides a discussion on room-temperature fatigue crack growth in monolithic ceramics, transformation-toughened ceramics, and ceramic composites under cyclic compression. The cyclic damage zones ahead of tensile fatigue cracks, crack propagation under cyclic tension or tension-compression loads, and elevated-temperature fatigue crack growth in monotonic and composite ceramics, are discussed. The article presents ceramic fatigue data for fatigue crack growth testing and concludes with a discussion on life prediction for ceramics or ceramic-matrix composites.

Stress-corrosion cracking (SCC) occurs under service conditions, which can result, often without any prior warning, in catastrophic failure. Hydrogen embrittlement is distinguished from stress-corrosion cracking generally by the interactions of the specimens with applied currents. To determine the susceptibility of alloys to SCC and hydrogen embrittlement, several types of testing are available. This article describes the constant extension testing, constant load testing, constant strain-rate testing for smooth specimens and precracked or notched specimens of SCC. It provides information on the cantilever beam test, wedge-opening load test, contoured double-cantilever beam test, three-point and four-point bend tests, rising step-load test, disk-pressure test, slow strain-rate tensile test, and potentiostatic slow strain-rate tensile test for hydrogen embrittlement.

This article reviews generalized test methodologies for fatigue characterization of polymers and examines fatigue fracture mechanisms in different engineering plastics. It provides detailed micromechanistic images of crack-tip processes for a variety of semicrystalline and amorphous engineering polymers. The article describes fracture mechanics solutions and approaches to the fatigue characterization of engineering polymers when dealing with macroscale fatigue crack growth. It includes mechanistic images for high-density polyethylene, ultrahigh-molecular-weight polyethylene, nylon 6, 6, polycarbonate, and polypropylene. The article describes the micromechanisms of toughening of plastics and uses a macroscale approach of applying fracture mechanics to the fatigue life prediction of engineering polymers, building on the mechanistic concepts. It also describes the factors affecting fatigue performance of polymers.

Damage tolerance is a philosophy used for maintaining the structural safety of commercial transport aircrafts. This article describes the structural evaluations necessary to comply with the regulations contained in the Federal Air worthiness Requirements 25.571 whose guidance is given in Advisory Circular 25.571-1A from the Federal Aviation Administration. It provides an overview of the historical evolution of damage tolerance philosophy and presents a discussion of the design philosophies and a summary of the evaluation tasks for damage tolerance certification.

Hydrogen damage is a term used to designate a number of processes in metals by which the load-carrying capacity of the metal is reduced due to the presence of hydrogen. This article introduces the general forms of hydrogen damage and provides an overview of the different types of hydrogen damage in all the major commercial alloy systems. It covers the broader topic of hydrogen damage, which can be quite complex and technical in nature. The article focuses on failure analysis where hydrogen embrittlement of a steel component is suspected. It provides practical advice for the failure analysis practitioner or for someone who is contemplating procurement of a cost-effective failure analysis of commodity-grade components suspected of hydrogen embrittlement. Some prevention strategies for design and manufacturing problem-induced hydrogen embrittlement are also provided.

This article provides an overview of the classification of hydrogen damage. Some specific types of the damage are hydrogen embrittlement, hydrogen-induced blistering, cracking from precipitation of internal hydrogen, hydrogen attack, and cracking from hydride formation. The article focuses on the types of hydrogen embrittlement that occur in all the major commercial metal and alloy systems, including stainless steels, nickel-base alloys, aluminum and aluminum alloys, titanium and titanium alloys, copper and copper alloys, and transition and refractory metals. The specific types of hydrogen embrittlement discussed include internal reversible hydrogen embrittlement, hydrogen environment embrittlement, and hydrogen reaction embrittlement. The article describes preservice and early-service fractures of commodity-grade steel components suspected of hydrogen embrittlement. Some prevention strategies for design and manufacturing problem-induced hydrogen embrittlement are also reviewed.

This article provides a review of fatigue test methodologies and an overview of general fatigue behavior, fatigue crack initiation and fatigue crack propagation of advanced engineering plastics. It also describes the factors affecting fatigue performance of polymers and concludes with information on fractography, a useful tool in failure analysis.

Stress-corrosion cracking (SCC) is a phenomenon in which time-dependent crack growth occurs when the necessary electrochemical, mechanical, and metallurgical conditions exist. This article provides an overview of the environmental phenomenon, mechanisms, and controlling parameters of SCC. It describes the phenomenological and mechanistic aspects of the initiation and propagation of SCC. The article includes a phenomenological description of crack initiation and propagation that describes well-established experimental evidence and observations of stress corrosion. Discussions on mechanisms describe the physical process involved in crack initiation and propagation. The article also includes information on dissolution models and mechanical fracture models.

Hydrogen damage is a form of environmentally assisted failure that results from the combined action of hydrogen and residual or applied tensile stress. This article classifies the various forms of hydrogen damage and summarizes the theories that seek to explain these types of degradation. It reviews hydrogen degradation in specific ferrous and nonferrous alloys, namely, iron-base alloys, nickel alloys, aluminum alloys, copper alloys, titanium alloys, zirconium alloys, and vanadium, niobium, tantalum, and their alloys. An outline of hydrogen damage in intermetallic compounds is also provided.

The approaches to spectrum life prediction in components can be classified into two types, namely, history-based methods, using the life-fraction rule or other damage rules, and postservice evaluation methods. This article discusses the variables affecting the material crack growth rate behavior and those essential elements in making spectrum crack growth life prediction. It provides information on life assessment for bulk creep damage.

This article provides ASTM standard definitions for fatigue and describes the approaches that are used to design finite or infinite life, used in a complementary sense in fatigue design. It explains four distinct phases of fatigue: nucleation, structurally dependent crack propagation, crack propagation, and final instability. The article discusses the significant role that fatigue plays in industrial design applications.

This article discusses the basic approach for predicting the corrosion-fatigue life of structural components. It describes two types of tests that are normally used in combination: cycles-to-failure tests, which focus on crack initiation, and crack propagation tests, which focus on crack growth rates under cyclic load. The article examines corrosion-fatigue cracking along with the effects of cracking due to stress corrosion and hydrogen embrittlement, which often occur together. It explains how test parameters such as loading and environmental conditions impact crack growth mechanisms and data interpretation.

This article describes the types, mechanism, and typical test methods along with their configurations for the evaluation of hydrogen embrittlement, stress-corrosion cracking, and corrosion fatigue with an emphasis on fracture mechanics methodologies for metals. An overview on the environmentally assisted crack growth of polymers is also included. The article details the evaluation of nanoscale environmental effects and indentation-induced cohesive cracking. It also provides information on scanning probe microscopy.

Brittle materials, such as ceramics, intermetallics, and graphites, are increasingly being used in the fabrication of lightweight components. This article focuses on the design methodologies and characterization of certain material properties. It describes the fundamental concepts and models associated with performing time-independent and time-dependent reliability analyses for brittle materials exhibiting scatter in ultimate strength. The article discusses the two-parameter and three-parameter Weibull distribution for representing the underlying probability density function for tensile strength. It reviews life prediction reliability models used for predicting the life of a component with complex geometry and loading. The article outlines reliability algorithms and presents several applications to illustrate the utilization of these reliability algorithms in structural applications.

This article discusses several examples of fatigue load histories that intentionally create artificial fracture-surface markings during testing such that they are measurable by post-test quantitative fractography (QF). It reviews a number of methods for providing fatigue fracture-surface markers to aid QF of fatigue crack growth (FCG). These methods are based on load changes, including reordering the basic load histories and/or adding loads to them. The article also provides some guidelines for obtaining recognizable FCG markers for a variety of load histories and crack-growth regimes for coupons, components, and, particularly, full-scale fatigue tests.