John H. Hemann

Trans-Ply Crack Density Detection by Acousto-Ultrasonics

The use of advanced composite materials, particularly for aerospace use, requires dimensional stability and the absence of cracks. The presence of cracks reduces mechanical characteristics and exposes the fibers and interfaces to environmental (moisture) effects. Graphite/PMR polyimide composites are candidate materials to replace metals in moderate high temperature zones (up to 316°C) in jet engines. These types of applications will introduce thermal cycling which in turn will produce trans-ply cracking unless tougher matrix materials are developed. Because these trans-ply cracks are very undesirable, NDE techniques are needed to detect the presence of trans-ply cracks. This paper will demonstrate that acousto-ultrasonics can be used to detect trans-ply cracks in graphite/PMR polyimide cross-ply and woven fabric laminates.

Thermal imaging technique to detect delaminations in CFRP-plated concrete

A thermographic technique for detecting disbonds and delaminations in concrete beams reinforced with bonded sheets of unidirectional carbon fiber reinforced plastic is described. Concrete slab test specimens are fabricated with seeded flaws of various thickness and depth from the surface. The specimens are briefly heated with a projector heat source and infrared images are captured. Digital image processing techniques are applied in order to improve image quality. Flaws are identified. Advantages and limitations of this method are discussed.

Thermoelastic stress analysis of a pultruded composite double lap joint

The use of composite materials, in particular glass/epoxy systems for structural applications has seen widespread growth. Recent examples include a bridge in Butler County, Ohio and a covered pedestrian bridge that is scheduled to be installed in Akron, Ohio. Both of these structures employ pultruded composites for the main structural members due to their high strength, light weight and the ease of manufacture into common structural shapes such as wide flanges, I-beams and box sections. The use of these shapes gives the designer the ability to use many of the same types of structural details that are common to steel design. This paper will examine the most common method of joining structural members, bolted connections. The analysis of bolted connections in composite materials has been widely reported in the literature. Analysis methods have ranged from two and three dimensional finite element analysis to more empirical methods of calculating the stress concentration factors based on experimental data. This paper will focus on the use of the thermoelastic stress analysis method to determine the stress concentration around a steel pin loaded in double shear by a pultruded glass fiber composite. Further studies were conducted to determine the time dependent material behavior on the thermoelastic stress analysis signal output. The following is a description of the theory, experimental setup, and a summary of results.

A review of failure models for ceramic matrix composite laminates under monotonic loads

A review of short-term failure models with potential for ceramic matrix composite laminates under monotonic loads is presented. Phenomenological, semi-empirical, shear-lag, fracture mechanics, damage mechanics, and statistical models for the fast fracture analysis of continuous fiber unidirectional ceramic matrix composites under monotonic loads are surveyed

Mathematical Modeling of Wave Propagation in Anisotropic Media (Mathematical Solution)

Mathematical modeling of the interaction of ultrasonic waves with anisotropic media (21 constants) is carried out. An anisotropic plate immersed in a liquid medium is acted upon by an ultrasonic incident wave of arbitrary frequency and angle of incidence. The incident acoustic harmonic plane wave originates in the upper half-space fluid. Expressions for reflection and transmission coefficients are derived and presented graphically as a function of the angle of incidence and the product of the frequency and the thickness of the plate. Also, the phase of both coefficients versus frequency are presented. All field variables can be fully specified using this mathematical modeling. This study is very general and can be easily specified to special cases of wave propagation and geometric and material configurations. It can be applied to both Rayleigh and Lamb waves. The numerical computations were done through the use of the VAXIMA/MACSYMA software package.

A Review of Failure Models for Ceramic Matrix Composite Laminates Under Monotonic Loads

Ceramic matrix composites offer significant potential for improving the performance of turbine engines. In order to achieve their potential, however, improvements in design methodology are needed. In the past most components using structural ceramic matrix composites were designed by trial and error since the emphasis of feasibility demonstration minimized the development of mathematical models. To understand the key parameters controlling response and the mechanics of failure, the development of structural failure models is required. A review of short term failure models with potential for ceramic matrix composite laminates under monotonic loads is presented. Phenomenological, semi-empirical, shear-lag, fracture mechanics, damage mechanics, and statistical models for the fast fracture analysis of continuous fiber unidirectional ceramic matrix composites under monotonic loads are surveyed.