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At least 487 records · Page 27

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NASA NTRS1988

Semi-microdroplet assay for cell adhesion molecules

A new cell-to-cell adhesion assay was devised. Using dissociated embryos of the sea urchin, this procedure involves rotating a 0.100 ml suspension of single cells with 0.100 ml of the solution to be tested in the bulb portion of a transfer pipet with the tip removed. After 1 hour of rotation at 60 rpm at 15 C, the contents of each bulb were transferred into individual wells of a 96 well flat bottom plate. After the plate was incubated for 1 hour at 15 C, black and white photographs were taken with a 35 mm camera attached to an inverted photomicroscope. Examining a proof sheet of the negatives directly allowed a rapid evaluation of suspected cell adhesion promoting factors. A ranking system was used to evaluate all samples. The assay was tested by examining the effect of specific solutions on the aggregation of single cells obtained from dissociated 23 hour embryos.

Tawa, Lawrence Shinzo↗

NASA NTRS1987

Adhesion at the interface in cured graphite fiber epoxy-amine resin composites

The effect of high temperature curing on the interface between unsized or epoxy-sized graphite fiber tow and epoxy-amine resin was examined by scanning electron microscopy of compression and freeze fractured specimens. Little or no adhesion was found between the unsized graphite fiber tows and the epoxy-amine resin on curing at 165 C for 17 hrs. Epoxy-sized graphite fibers showed a similar lack of adhesion between the fiber tows and the epoxy-amine resin at 3 and 17 hr cures, although good penetration of the resin into the sized fiber tows had occurred. Interfacial bond strengths for the composites could not be effectively measured by compression fracture of specimens.

Needles, Howard L.↗

NASA NTRS1987

Evaluation of Ti-6Al-4V surface treatments for use with a polyphenylquinoxaline adhesive

Three surface treatments for Ti-6Al-4V adherends were evaluated using a thermoplastic polymer monoether polyphenylquinoxaline, MEPPQ, which had been shown in previous studies to have good potential as a high temperature adhesive for aerospace applications. Initial results based on long term thermal exposure at 232 C (450 F) using the phosphate-fluoride (PF) and chromic acid anodized (CAA) treatments with MEPPQ adhesive were not encouraging. A significant improvement in strength retention and a change in failure mode (cohesive) at 232 C (450 F) was found for the SHA treated specimens compared to the PF and CAA treatments. Although an improvement in long term thermal durability was obtained with the SHA treatment of Ti-6Al-4V, an improved surface treatment with better long term durability is still required for aerospace applications.

Progar, Donald J.↗

NASA NTRS1988

The use of fluoroepoxy compounds as adhesives to bond fluoroplastics without any surface treatment

The use of fluoroepoxy compounds as adhesives to bond fluoroplastics was investigated. Fluoroepoxy compounds with a F-content higher than 46 percent were able to bond a highly fluorinated plastic such as Teflon PTFE (76 percent F) to give a respectable bond strength without any surface treatment. The advantage of the fluoroepoxy adhesive vanished when applied to a less fluorinated plastic such as Tefzel which contains 55 percent fluorine. Among the curing agents explored, octafluorooctamethylenediamine showed its potential as a cocuring agent for improving both the viscosity of the compound and the flexibility of the cured product.

Lee, Sheng Yen↗

NASA NTRS1988

Debonding characteristics of adhesively bonded woven Kevlar composites

The fatigue damage mechanism of an adhesively bonded joint between fabric reinforced composite adherends was investigated with cracked-lap-shear specimens. Two bonded systems were studied: fabric Kevlar 49/5208 epoxy adherends bonded together with either EC 3445 or FM-300 adhesive. For each bonded system, two specimen geometries were tested. In all specimens tested, fatigue damage occurred in the form of cyclic debonding; however, the woven Kevlar specimens gave significantly slower debond growth rates and higher fracture toughness than previously found in the nonwoven adherend specimens. The surfaces for the woven adherends were not smooth; rather, they had regular crests (high spots) and troughs (low spots) due to the weave pattern. Radiographs of the specimens and examination of their failure surfaces revealed that fiber bridging occurred between the crests of the two adherends in the debonded region. The observed improvements in debond growth resistance and static fracture toughness are attributed to this bridging.

Mall, S.↗

NASA NTRS1988

Characterization of debond growth mechanism in adhesively bonded composites under mode II static and fatigue loadings

An experimental investigation of adhesively bonded composite joint was conducted to characterize the debond growth mechanism under mode II static and fatigue loadings. For this purpose, end-notched flexure specimens of graphite/epoxy (T300/5208) adherends bonded with EC 3445 adhesive were tested. In all specimen tested, the fatigue failure occurred in the form of cyclic debonding. The present study confirmed the result of previous studies that total strain-energy-release rate is the driving parameter for cyclic debonding. Further, the debond growth resistance under cyclic loading with full shear reversal (i.e., stress ratio, R = -1) is drastically reduced in comparison to the case when subjected to cyclic shear loading with no shear reversal (i.e., R = 0.1).

Mall, S.↗

NASA NTRS1989

Avalanche in adhesion

Consider surfaces being brought into contact. It is proposed that atomic layers can collapse or avalanche together when the interfacial spacing falls below a critical distance. This causes a discontinuous drop in the adhesive binding energy. Avalanche can occur regardless of the stiffness of external supports. A simple understanding of the origin of this phenomenon is provided. A numerical calculation has been carried out for adhesion in Ni. A new wear mechanism due to avalanche is suggested.

Smith, John R.↗

NASA NTRS1990

Adhesion Between Particles And Surfaces In A Vacuum

Report discusses experimental study of adhesion between several kinds of particles and solid surfaces. Purpose of investigation to extend available data on adhesion of contaminant particles and to contribute to understanding of ways in which acceleration redistributes contaminant particles, causing increases or decreases in contamination.

Barengoltz, Jack B.↗

NASA NTRS1990

Tensile and shear strength of adhesives

This experiment is conducted in a freshman-level course: Introduction to Engineering Materials. There are no prerequisites for the course although students should have some knowledge of basic algebra. The objectives are to tension and shear test adhesives and to determine the tensile and shear properties of adhesives. Details of equipment of procedure are given.

Stibolt, Kenneth A.↗

NASA NTRS1989

Ultrasonic study of adhesive bond quality at a steel-to-rubber interface by using quadrature phase detection techniques

The quadrature phase detection technique was used to simultaneously monitor the phase and amplitude of a toneburst signal normally reflected from an adhesively bonded steel-to-rubber interface. The measured phase was found to show a positive shift for all bonded samples with respect to the disbonded state - the phase shift being larger for samples with weaker bonds, as manifested by smaller values of applied tensile loads at failure. A model calculation, which incorporates the concept of interfacial strength into the usual problem of wave propagation in multilayered media, was used to deduce a bond-quality parameter from an experimentally measured phase shift. This bond-quality parameter was found to be correlated with the tensile strength of the adhesive bonds at failure loads.

Smith, A. C.↗

NASA NTRS1989

Qualification of room-temperature-curing structural adhesives for use on JPL spacecraft

An evaluation is made of the comparative advantages of numerous room temperature-cure structural primers and adhesives applicable to spacecraft structures. The EA 9394 adhesive and BR 127 primer were chosen for use in all primary structure bonding on the Galileo spacecraft, in virtue of adequate room-temperature lap shear and peel strengths and superior mechanical properties above 200 F. EA 9394 also offers superior work life, shelf-life, and storage properties, by comparison with the EA 934 alternative.

Carpenter, Alain↗

NASA NTRS1990

Tensile adhesion testing methodology for thermally sprayed coatings

The structure of thermally sprayed coatings consists of lamellae which are oriented parallel to the substrate surface. The lamellae separate and fracture by distinctive mechanisms which are reflected in the failure morphology, and these may be described as adhesive (between the coating and substrate), cohesive (within the coating), or mixed mode. There is a large variability in the failure stress for any nominally identical group of coatings. A lower bound for the fracture toughness of alumina coatings can be calculated as 0.2 MNm exp -3/2. The coating strength values may also be treated as belonging to the statistical distribution of the Weibull function. The Weibull modulus of the coating strength varied from 1.4 to 3.8. This analysis infers that the flaw size within coatings is highly variable and that the flaws are nonuniformly dispersed. The present work focuses on the question of whether tensile adhesion tests are an appropriate testing method for thermally sprayed materials.

Berndt, Christopher C.↗

NASA NTRS1991

Measuring Adhesion And Friction Forces

Cavendish balance adapted to new purpose. Apparatus developed which measures forces of adhesion and friction between specimens of solid materials in vacuum at temperatures from ambient to 900 degrees C. Intended primarily for use in studying adhesion properties of ceramics and metals, including silicon carbide, aluminum oxide, and iron-base amorphous alloys.

Miyoshi, Kazuhisa↗

NASA NTRS1991

Grit Blasting Scribes Coats For Tests Of Adhesion

Grit-blasting technique for cutting line gaps in paints, hard coats, lubricants, and other coating films undergoing development. Line gaps cut in chevron patterns, groups of parallel lines, or other prescribed patterns, in preparation for testing adhesions of coats to substrates by attempting to peel patterned areas off with adhesive tapes. Damage to substrate reduced.

Novak, Howard L.↗

NASA NTRS1990

A phenomenological model of coating/substrate adhesion and interfacial bimetallic peeling stress in composite mirrors

Adhesion and interfacial stress between metal films and structural composite material substrates is discussed. A theoretical and conceptual basis for selecting coating materials for composite mirror substrates is described. A phenomenological model that interrelates cohesive tensile strength of thin film coatings and interfacial peeling stresses is presented. The model serves as a basis in determining gradiated materials response and compatibility of composite substrate and coating combinations. Parametric evaluation of material properties and geometrical factors such as coating thickness are used to determine the threshold stress levels for maintaining adhesion at the different interfaces.

Mcelroy, Paul M.↗

NASA NTRS1990

Uses of Auger and x ray photoelectron spectroscopy in the study of adhesion and friction

Three studies are described characterizing the possible contributions of surface science to tribology. These include surface contamination formed by the interaction of a surface with the environment, contaminants obtained with diffusion of compounds, and surface chemical changes resulting from selective thermal evaporation. Surface analytical tools such as Auger electron spectroscopy (AES) and x ray photoelectron spectroscopy (XPS) incorporated directly into adhesion and friction systems are primarily used to define the nature of tribological surfaces before and after tribological experimentation and to characterize the mechanism of solid-to-solid interaction. Emphasis is on fundamental studies involving the role of surfaces in controlling the adhesion and friction properties of materials emerging as a result of the surface analyses. The materials which were studied include metals and ceramics such as elemental metals, amorphous alloys (metallic glasses), and silicon-based ceramics.

Miyoshi, Kazuhisa↗

NASA NTRS1990

Friction and wear of TPS fibers: A study of the adhesion and friction of high modulus fibers

The adhesional and frictional forces between filaments in a woven fabric or felt, strongly influenced the processability of the fiber and the mechanical durability of the final product. Even though the contact loads between fibers are low, the area of contact is extremely small giving rise to very high stresses; principally shear stresses. One consequence of these strong adhesional and frictional forces is the resistance of fibers to slide past each other during weaving or when processed into nonwoven mats or felts. Furthermore, the interfiber frictional forces may cause surface damage and thereby reduce the fiber strength. Once formed into fabrics, flexural handling and manipulation of the material again causes individual filaments to rub against each other resulting in modulus, brittle fibers such as those used in thermal protection systems (TPS). The adhesion and friction of organic fibers, notably polyethylene terephthalate (PET) fibers, have been extensively studied, but there has been very little work reported on high modulus inorganic fibers. An extensive study was made of the adhesion and friction of flame drawn silica fibers in order to develop experimental techniques and a scientific basis for data interpretation. Subsequently, these methods were applied to fibers of interest in TPS materials.

Bascom, Willard D.↗

NASA NTRS1990

Processable polyimide adhesive and matrix composite resin

A high temperature polyimide composition prepared by reacting 4,4'-isophthaloyldiphthalic anhydride with metaphenylenediamine is employed to prepare matrix resins, adhesives, films, coatings, moldings, and laminates, especially those showing enhanced flow with retention of mechanical and adhesive properties. It can be used in the aerospace industry, for example, in joining metals to metals or metals to composite structures. One area of application is in the manufacture of lighter and stronger aircraft and spacecraft structures.

Pratt, J. Richard↗