Search Results for cracks

This article describes the test techniques that are available for monitoring crack initiation and crack growth and for obtaining information on fatigue damage in test specimens. These techniques include optical methods, the compliance method, electric potential measurement, and gel electrode imaging methods. The article discusses the magnetic techniques that are primarily used as inspection techniques for detecting fatigue cracks in structural components. It details the principles and operation procedures of the liquid penetrant methods, positron annihilation techniques, acoustic emission techniques, ultrasonic methods, eddy current techniques, infrared techniques, exoelectron methods, and gamma radiography. The article explains the microscopy methods used to determine fatigue crack initiation and propagation. These include electron microscopy, scanning tunneling microscopy, atomic force microscopy, and scanning acoustic microscopy. The article also reviews the X-ray diffraction technique used for determining the compositional changes, strain changes, and residual stress evaluation during the fatigue process.

This article defines different types of small cracks and identifies the different factors that influence small-crack behavior. Appropriate analysis techniques, including both rigorous scientific and practical engineering treatments, are briefly described. Important material data issues are addressed, including increased scatter in small-crack data and recommended small-crack test methods. The article highlights the applications where small cracks may be particularly important.

This article begins with an overview of the various aspects of infrared pulse thermography used to detect disbondments, delaminations, and generalized corrosion. It describes the distinctive phases of the pulse thermographic process and the key components that are required to perform active thermography. The components include an excitation source, a thermographic camera, and a computer with software that controls the instrumentation, acquires data, and displays the results. The article discusses the process and experimental setup of sonic thermography used for crack detection.

This article summarizes the aspects of crack shape and irregularity that are relevant to fatigue and fracture of surface cracks. It discusses the nature of three-dimensional surface cracks and variables that influence crack shape. These variables include the grain size, residual stresses, texture, loading mode, environment, and crack coalescence. Measurement of crack shapes or aspect ratios during fatigue crack growth can be performed by a number of techniques. The article describes the estimation of the stress-intensity factor for arbitrarily-shaped cracks and failure prediction methods for arbitrarily-shaped flaws.

This article commences with a discussion on the characteristics of stress-corrosion cracking (SCC) and describes crack initiation and propagation during SCC. It reviews the various mechanisms of SCC and addresses electrochemical and stress-sorption theories. The article explains the SCC, which occurs due to welding, metalworking process, and stress concentration, including options for investigation and corrective measures. It describes the sources of stresses in service and the effect of composition and metal structure on the susceptibility of SCC. The article provides information on specific ions and substances, service environments, and preservice environments responsible for SCC. It details the analysis of SCC failures, which include on-site examination, sampling, observation of fracture surface characteristics, macroscopic examination, microscopic examination, chemical analysis, metallographic analysis, and simulated-service tests. It provides case studies for the analysis of SCC service failures and their occurrence in steels, stainless steels, and commercial alloys of aluminum, copper, magnesium, and titanium.

This article describes the types of closure mechanisms, including plasticity-induced, roughness-induced, oxide-induced, and fretting-debris-induced. It discusses test techniques used to establish a valid threshold value for aluminum alloy crack growth thresholds, steel crack growth thresholds, and titanium alloy crack growth thresholds. The near-threshold behavior of nickel-base superalloys is described. The article briefly reviews the threshold results for fiber-reinforced, whisker-reinforced, and particulate-re-inforced metal-matrix alloys. It explains the near-threshold fatigue crack propagation in welded joints and the fundamental considerations for modeling threshold behavior. The article concludes with a discussion on the effects of thresholds in the engineering design process.

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.

While there are many fracture mechanisms that can lead to the failure of a plastic component, environmental stress cracking (ESC) is recognized as one of the leading causes of plastic failure. This article focuses on unpacking the basic concepts of ESC to provide the engineer with a better understanding of how to evaluate and prevent it. It then presents factors that affect and contribute to the susceptibility of plastic to ESC: material factors, chemical factors, stress, and environmental factors. The article includes the collection of background information to understand the circumstances surrounding the failure, a fractographic evaluation to assess the cracking, and analytical testing to evaluate the material, design, manufacturing, and environmental factors.