Samson S. Lee

Acoustic Emission Monitoring of Fiber Composite Materials and Structures

A review of the current acoustic emission literature relating to fiber reinforced composite materials is presented. Summary tables which assist m the prompt delineation of the achievements in this research area are developed. Because of the qualitative character of much of the current literature, suggestions to develop quantitative AE standards are strongly recommended

Ultrasonic attenuation and velocity in AS/3501-6 graphite fiber composite

The ultrasonic group velocity and attenuation were measured as a function of frequency for longitudinal and shear waves in the Hercules epoxy matrix (3501-6) and in the principal directions of the unidirectional Hercules graphite fiber epoxy composite (AS/3501-6). Tests were conducted in the frequency ranges 0.25–14 MHz and 0.5–3 MHz for longitudinal and shear wave modes, respectively. While the attenuation increased with frequency for all wave modes, the group velocity was independent of frequency for all wave modes. In studying the effects of transducer-specimen interface couplant and pressure, it was found that for each transducer, there exists a frequency-dependent “saturation pressure” corresponding to the maximum output amplitude of the signal.

Quantitative Nondestructive Evaluation of Automotive Glass Fiber Composites

Thermographic, ultrasonic and acoustic emission nondestructive evalua tion (NDE) characterizations of flawed and unflawed SMC-R25, SMC-R50 and XMC-3 composites are conducted. For various flawed and unflawed states of the composites, these NDE characterizations are correlated with both flaw state and tensile fracture strength. Drilled-holes and V-notch flaws of various sizes result in correspondingly lower tensile strengths; however, in creases in fabrication temperature up to 177 °C and pressure up to 13.8 MPa, post-fabrication thermal exposure at 191 °C for up to 24 hours, water immer sion at 21 °C for up to 45 days and aluminum foil inclusions are found to have no significant effects on the tensile strength. For all three materials, the maximum number of thermographic color dif ferences is correlated linearly with the flaw size to specimen thickness ratio and with the tensile fracture stress for the drilled-holes and V-notch flaws. For all three materials, the ultrasonic through-thickness attenuation at 2 MHz is correlated linearly with the tensile fracture stress, independent of the material flaw state. And, for all three materials, the acoustic emission stress delay is correlated linearly with the tensile fracture stress, independent of the material flaw state. Thus, the NDE of fiber reinforced plastics (FRP) for structural integrity assessment is demonstrably feasible.

Dynamic crack propagation and arrest in orthotropic DCB fiber composite specimens

Abstract An orthotropic double cantilever beam (DCB) model is used to study dynamic crack propagation and arrest in 90° unidirectional Hercules AS/3501-6 graphite fiber epoxy composites. The dynamic fracture toughness of the composite is determined from tests performed on the long-strip specimen and DCB crack arrest experiments are conducted. By using the dynamic fracture toughness in a finite-difference solution of the DCB governing partial differential equations, a numerical solution of the crack propagation and arrest events is computed. Excellent agreement between the experimental and numerical crack arrest results are obtained.

Stress Wave Attenuation in Thin Structures by Ultrasonic Through-Transmission.

The steady-state amplitude of the output of an ultrasonic through-transmission measurement is analyzed and the result is given in closed form. Provided that the product of the input and output transduction ratios, the specimen-transducer reflection coefficient, the specimen-transducer phase-shift parameter, and the material phase velocity are known, this analysis gives a means for determining the through-thickness attenuation of an individual thin sample. Multiple stress-wave reflections are taken into account, and so signal echoes do not represent a difficulty. An example is presented for a graphite fiber epoxy composite (Hercules AS/3501-6). Thus, the technique provides a direct method for continuous or intermittent monitoring of through-thickness attenuation of plate structures which may be subject to service structural degradation.

Nondestructive Evaluation of Strength and Separation Modes in Adhesively Bonded Automotive Glass Fiber Composite Single Lap Joints

Ultrasonic and acoustic emission (AE) nondestructive evaluation (NDE) characteriza tions of flawed (undercure of adhesive and excessive mold release on the adherends prior to bonding) and unflawed (proper cure of adhesive) adhesively bonded fiber reinforced plastics (FRP) specimens are conducted. Single lap-joint specimens are considered. The specimens consist of sheet molding compound (SMC) composite adherends bonded together with a urethane adhesive.

Acoustic-ultrasonic nondestructive evaluation of double-braided nylon ropes using the stress wave factor

Abstract Acoustic-ultrasonic nondestructive evaluation (NDE) has been conducted on a new dry Samson double-braided 2-in-1, 0·64 cm ( 1 4 in ) diameter nylon rope using the stress wave factor (SWF) measurement at various rope tensions. The rope samples considered were undamaged ropes and damaged ropes with core cut, core removed and cover cut. It is observed that different types of rope samples have their own SWF versus load characteristics. A SWF model based on two competing mechanisms for stress wave conduction in ropes has been developed. The two competing mechanisms are the decreasing transducer-rope contact area with increasing rope tension and the increasing rope compaction versus increasing rope tension. This competing-mechanisms model is found useful in describing the SWF characteristics for damaged and undamaged rope samples. Thus, this study indicates that the SWF can result from combined complex mechanisms and that the proper use of the SWF as an NDE technique must be accompanied by adequate SWF modeling.

Ultrasonic wave propagation loss factor in composite in terms of constituent properties

A model is developed to relate the energy loss factors of ultrasonic longitudinal waves propagating in the principal directions of a unidirectional graphite fiber composite to the composite constituent properties. All the constituents are assumed to behave as linear viscoelastic materials with energy dissipation properties defined by loss factors. It is found that by introducing a new constituent called the interface material, the composite and constituent properties can be brought into consistency with simple series and parallel models. An expression relating the composite loss factors to the loss factors of the constituents is derived and its coefficients are evaluated.

Double cantilever shear beam model of dynamic fracture in unidirectional fibre composites

Abstract A homogeneous orthotropic double cantilever shear beam model is analysed to study Mode I dynamic fracture in unidirectional fibre composites. The analysis follows that by Freund 1 who studied dynamic fracture in homogeneous isotropic specimens. The analysed situation is the dynamic growth of a sharp crack from a blunt pre-crack with the ends of the specimen arms held at a fixed separation distance. The analysis uses an energy balance dynamic fracture criterion. Expressions for the dynamic crack extension force, the composite dynamic fracture toughness, the crack-tip speed, the crack arrest length, the crack arrest time, and the instantaneous potential and kinetic energies are derived. A dominant quantity in these results is the crack-tip bluntness parameter which is a measure of the actual pre-propagation energy in excess of the minimum pre-propagation energy for a sharp crack. The applicability of the bluntness parameter to composites as well as the general appropriateness of the shear beam model require experimental verification.

Fibre intersections in a planar randomly oriented fibre composite

Abstract The interaction of fibres in a randomly oriented fibre composite is a key factor in determining the mechanical properties of some composites. A theoretical model is presented by means of which values of the mean and the variance of fibre intersections in a planar randomly oriented fibre composite are predicted. These predictions are compared with the mean and variance obtained from a computer simulation of the fibre intersection process.