Objective surface evaluation of fiber reinforced polymer composites

The mechanical properties of advanced composites are essential for their structural performance, but the surface finish on exterior composite panels is of critical importance for customer satisfaction. This paper describes the application of wavelet texture analysis (WTA) to the task of automatically classifying the surface finish properties of two fiber reinforced polymer (FRP) composite construction types (clear resin and gel-coat) into three quality grades. Samples were imaged and wavelet multi-scale decomposition was used to create a visual texture representation of the sample, capturing image features at different scales and orientations. Principal components analysis was used to reduce the dimensionality of the texture feature vector, permitting successful classification of the samples using only the first principal component. This work extends and further validates the feasibility of this approach as the basis for automated non-contact classification of composite surface finish using image analysis.<br /

Mechanical Behavior and Microstructural Development of Low-Carbon Steel and Microcomposite Steel Reinforcement Bars Deformed under Quasi-Static and Dynamic Shear Loading

Reinforcement bars of microcomposite (MC) steel, composed of lath martensite and minor amounts of retained austenite, possess improved strength and corrosion characteristics over low-carbon (LC) steel rebar; however, their performance under shear loading has not previously been investigated at the microstructural level. In this study, LC and MC steel cylinders were compression tested, and specimens machined into a forced-shear geometry were subjected to quasi-static and dynamic shear loading to determine their shear behavior as a function of the strain and strain rate. The as-received and sheared microstructures were examined using optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). Higher-resolution microstructural examinations were performed using transmission electron microscopy (TEM). The influence of the starting microstructure on the shear behavior was found to depend strongly on the strain rate; the MC steel exhibited not only greater strain-rate sensitivity than the LC steel but also a greater resistance to shear localization with load. In both steels, despite differences in the starting microstructure, post-mortem observations were consistent with a continuous mechanism operating within adiabatic shear bands (ASBs), in which subgrains rotated into highly misoriented grains containing a high density of dislocations

Fatigue Crack Growth Rates in Six Pipeline Steels

This study presents fatigue data for six different pipeline steels, with strengths ranging from Grade B to X100. A fatigue crack growth test for full thickness pipeline samples was developed using a Middle Tension (MT) type specimen. The six steels showed similar fatigue crack growth rate (da/dN) behavior. There were only minor differences among the steels for the threshold values and most of the stable crack growth regime. Larger differences were observed in the final stages of crack growth and fatigue failure. The effect of compressive residual stresses at the outer surface of the pipeline was also examined. A Failure Assessment Diagram technique was used to evaluate the potential failures modes of the six pipeline steels, containing, as an example, an internal surface, semi-elliptic, axially oriented flaw. Mixed-mode failure was predicted for all of the steels.Copyright © 2006 by ASM

Crack Tip Opening Angle: Measurement and Modeling of Fracture Resistance in Low and High Strength Pipeline Steels

Crack tip opening angle (CTOA) is becoming one of the more widely accepted properties for characterizing fully plastic fracture. In fact, it has been recognized as a measure of the resistance of a material to fracture, in cases where there is a large degree of stable-tearing crack extension during the fracture process. This type of steady-state fracture resistance takes place when the CTOA in a material reaches a critical value, as typically occurs in low-constraint configurations. Our current research has applied the CTOA concept as an alternative or an addition to the Charpy V-notch and the drop weight tear test (DWTT) fracture energy in pipeline characterization. A test technique for direct measurement of CTOA was developed, using a modified double cantilever beam (MDCB) specimen. A digital camera and image analysis software are used to record the progression of the crack tip and to estimate CTOA using the crack edges adjacent to the crack tip. A steady-state CTOA has been successfully measured on five different strength grades of gas pipeline steel (four low strength grades and one high strength grade: X100). In addition, two-dimensional finite element models (2D FEMs) are used to demonstrate the sequence of the fracture process and the deformation mechanisms involved. The CTOA measurements and models are correlated and agree well.Copyright © 2006 by ASM