Fracture Analysis

Deformation and Fracture

Magnitude and nature of stresses have developed in materials make significant effect on root cause of failure. Combination of environmental effects and materials defects could intensify the failure progress. Mechanical testing such as tensile test, micro and nano hardness, impact, fatigue, and any additional customized testing could identify defect and weakness in materials.  Standard tests and results analysis are carried out in compliance with ASTM recommendations and equivalent international codes to determine materials properties and matching materials specifications for specified requirements in design.Static and dynamic loadings provide various surface fracture orientation and texture due to plastic deformation, fatigue, impact, physical defects, geometrical stress concentration, and imperfections in materials. Mechanical testing such as tensile, bending, fracture mechanics, fatigue, hardness, and impact provide data for analysis and identification of cause of fracture failure. Knowledge based understanding of fracture mechanisms in various materials in micro and macro scales and application of fundamentals of engineering mechanics support failures analysis. Effects of fatigue stress, time dependent loading, and environment on the crack-growth rate assist in evaluation and estimation of long-life strength and life estimation of structural components.

 

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Evaluation of Fatigue Fracture

The early stages of failure analysis include the collection of background information and the selection of appropriate samples for laboratory testing.  Additional steps should include site inspection, a timeline history of the failure, material specifications, review of maintenance and repair records, number of past failures for the same component and any material substitutions made.  A visual examination of the failed part or structure, as well as non-destructive testing of the component, with extensive photographic documentation should be performed first.  The failed parts selected for laboratory testing and analysis should be carefully stored or protected during transport to prevent any damage to the fracture surfaces from humidity, dust, and dirt.

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Creep Deformation and Rupture

The creep phenomenon is a time-dependent deformation of materials under loading.  Metals show primarily the creep behavior at elevated temperatures above 0.4TM, where TM is the melting point in Kelvin scale.  The primary consideration in design of products for application at high temperatures, such as the aircraft gas turbine engines, power plant boilers, nuclear reactors, pressure vessels, etc, is the time dependent deformation. Exposure to dynamic stresses, corrosive gases, and atmosphere at high temperatures will intensify rupture of parts.

Fracture surfaces of the creep ruptured parts will provide evidence as microvoid coalescence, intergranular separation, and elongated features due to exposure to stresses in a considerable time. Effect of oxidation or diffusion gases would make fractography difficult.

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Fractography

Fracture surface and metallographic examination of cross section of fracture provide a significant amount of information on the root cause of failure, materials defects, and type of stresses.  Ductile fracture, mostly due to static overloading will appear as sheared dimples and elongated lips similar to a cup and cone mode.

A macroscopic visual examination of the fracture surface and external surfaces of the part begins the investigation and will be followed by microscopic examinations. An optical stereo microscope examination at magnifications of 50X or less will help to reveal fracture surface details, confirm fracture initiation locations and mode of failure, and reveal possible evidence of surface damage at the locations of fatigue crack initiation.  There are differences observed in fatigue fracture surface appearances caused by the magnitude of the applied stress and the remaining cross sectional area when the fracture passes through each area.  The main differences are observed by macroscopic visual fractography. Fatigue fracture surfaces typically show two distinct regions: the fatigue crack initiation and propagation region and the final overload region.  In the final overload region, the presence of slanted 45 degree shear zones and their elongated fibrous dimple structure, or brittle cleavage features are indicative of rapid loading conditions.

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