Engineering topics

Johnson, W. S.

Publications and source records attributed to Johnson, W. S..

At least 55 records · Page 3

NASA NTRS1989

Elastic-plastic stress concentrations around crack-like notches in continuous fiber reinforced metal matrix composites

Continuous fiber silicon-carbide/aluminum composite laminates with slits were tested statically to failure. Five different layups were examined: (0) sub 8, (0 sub 2/ + or - 45) sub s, (0/90) sub 2s), (0/ + or - 45/90 sub s), and (+ or - 45) sub 2s. Either a 9.5 or a 19 mm slit was machined in the center of each specimen. The strain distribution ahead of the slit tip was found experimentally with a series of strain gages bonded ahead of the slit tip. A three-dimensional finite element program (PAFAC) was used to predict the strain distribution ahead of the slit tip for several layups. For all layups, except the (0) sub 8, the yielding of the metal matrix caused the fiber stress concentration factor to increase with increasing load. This is contrary to the behavior seen in homogeneous materials where yielding causes the stress concentration to drop. For the (0) sub 8 laminate, yielding of the matrix caused a decrease in the fiber stress concentration. The finite element analysis predicted these trends correctly.

Johnson, W. S.↗

NASA NTRS1989

Screening of metal matrix composites using ultrasonic C-scans

Ultrasonic C-scans can be used to find some types of defects in continuous fiber-reinforced metal matrix composites such as boron/aluminum composites. These defects are related to the fatigue behavior and fracture location of each inspected specimen. The C-scan technique determined the relative amount of defects in boron/aluminum composites. The defects were primarily identified as gaps in the fiber spacing. Those specimens with higher defect densities had shorter fatigue lives, lower fatigue endurance limits, and greater reductions in the elastic unloading modulus (that is, stiffness) because of fatigue cycling. This type of data could be used to set accept/reject levels for a composite panel based on C-scan indications.

Johnson, W. S.↗

NASA NTRS1989

Metal matrix composites: Testing, analysis, and failure modes; Proceedings of the Symposium, Sparks, NV, Apr. 25, 26, 1988

The present conference discusses the tension and compression testing of MMCs, the measurement of advanced composites' thermal expansion, plasticity theory for fiber-reinforced composites, a deformation analysis of boron/aluminum specimens by moire interferometry, strength prediction methods for MMCs, and the analysis of notched MMCs under tensile loading. Also discussed are techniques for the mechanical and thermal testing of Ti3Al/SCS-6 MMCs, damage initiation and growth in fiber-reinforced MMCs, the shear testing of MMCs, the crack growth and fracture of continuous fiber-reinforced MMCs in view of analytical and experimental results, and MMC fiber-matrix interface failures.

Johnson, W. S.↗

NASA NTRS1989

Fatigue testing and damage development in continuous fiber reinforced metal matrix composites

A general overview of the fatigue behavior of metal matrix composites (MMC) is presented. The first objective is to present experimental procedures and techniques for conducting a meaningful fatigue test to detect and quantify fatigue damage in MMC. These techniques include interpretation of stress-strain responses, acid etching of the matrix, edge replicas of the specimen under load, radiography, and micrographs of the failure surfaces. In addition, the paper will show how stiffness loss in continuous fiber reinforced metal matrix composites can be a useful parameter for detecting fatigue damage initiation and accumulation. Second, numerous examples of how fatigue damage can initiate and grow in various MMC are given. Depending on the relative fatigue behavior of the fiber and matrix, and the interface properties, the failure modes of MMC can be grouped into four categories: (1) matrix dominated, (2) fiber dominated, (3) self-similar damage growth, and (4) fiber/matrix interfacial failures. These four types of damage will be discussed and illustrated by examples with the emphasis on the fatigue of unnotched laminates.

Johnson, W. S.↗

NASA NTRS1989

Mechanisms controlling fatigue damage development in continuous fiber reinforced metal matrix composites

Damage in continuous fiber reinforced metal matrix composite materials can be quite complex since there are a number of different constituents (fiber, matrix, and the fiber/matrix interface) that can fail. Multidirectional lay-ups have an even greater number of possible damage orientations and mechanisms. Based on the simplifying assumption of equivalent constituent strain states in the absence of damage, a strain based failure criteria may be applied to determine when and where initial damage will occur. Based on the relative strain to fatigue failure of the fiber and matrix, the possible damage mechanisms of an MMC can be grouped into three categories: (1) matrix dominated, (2) fiber dominated, and (3) self-similar damage growth. A fourth type of damage development, fiber/matrix interface failure, is dependent on the relative strength of the fiber/matrix interface and the matrix yield strength. These four types of damage are discussed and illustrated by examples.

Johnson, W. S.↗

NASA NTRS1989

A comparison of various micromechanics models for metal matrix composites

Four micromechanics models currently used in the analysis of metal matrix composites are reviewed. The four models are the vanishing fiber diameter model, the Aboudi model, the multicell model, and discrete fiber-matrix model. Results predicted by computer programs based on each of the models are compared. Comparisons are made for the laminate properties and laminate stress-strain behavior for boron/aluminum and silicon-carbide/titanium composites. The predictions of constituent stresses are also compared. The predictions are compared to experimental data. Each of the models did a reasonably good job predicting laminate properties and stress-strain behavior. Thus, the discriminator between the models is a question of what type of results are required, the capabilities of the model and program, or the ease of operation. The choice of a model and program depends on several factors and the same program may not be the best choice for all analysis needs. The descriptions and comparisons made in this paper should aid in the choice of a model and program.

Bigelow, C. A.↗

NASA NTRS1989

Effect of a high temperature cycle on the mechanical properties of silicon carbide/titanium metal matrix composites

An investigation is conducted of the effects of the SPF/DB cycle on continuous SiC fiber-reinforced Ti-15V-3Cr-3Al-3Sn composite's matrix, fiber, and matrix-fiber interface. The fibers in question, designated SCS-6, have a carbon core and thin, carbon-rich surface. The fatigue endurance limit at 50,000 cycles for the SPF/DB specimens was 50 percent lower than for the as-fabricated material. The substantial changes in tensile strength, fatigue life, and fracture-surface appearance due to the SPF-DB cycle are explained by a difference in the failure mechanisms due to SPF/DB-induced changes in the fiber/matrix interface strength.

Naik, R. A.↗

NASA NTRS1989

Impact and residual fatigue behavior of ARALL and AS6/5245 composite material

The impact sensitivity of aramide fiber-reinforced aluminum laminates (ARALL) was investigated by testing two types of ARALL (7075 aluminum prestrained and 2024 aluminum not prestrained), via static indentation, and the results were compared to those of sheet aluminum alloys 7075-T6 and 2024-T3 and to a state of the art composite AS6/5245. It was found that the impact resistance of the two ARALL samples was inferior to that of monolithic sheet aluminum samples, although the ARALL material made with 2024-T3 aluminum was superior to that made with 7075-T6 aluminum. The impact damage resistance of ARALL materials was at least equal to that of AS6/5245, and the AS6/5245 had higher residual tension-tension fatigue strength after impact than the ARALL samples. It was also found that the prestraining of the ARALL reduced the fatigue growth of impact damage.

Johnson, W. S.↗

NASA NTRS1988

Damage initiation and growth in fiber-reinforced MMCs

On the basis of the simplifying assumption of equivalent constituent strain rates in the absence of damage, a strain-based failure criterion can be applied to the phenomena of damage-initiation and growth in fiber-reinforced MMCs to ascertain when and where the initial damage will occur. The failure modes fall into four categories, depending on the relative fatigue behavior of the reinforcing fibers and their matrix. Attention is given to experimental results illustrating current understanding of fiber-dominated damage, self-similar fatigue damage growth, and fiber/matrix interface failures, for such MMCs as B/Al alloy, SiC/Al alloy, and SiC/Ti-15-3 alloy.

Saff, C. R.↗

NASA NTRS1988

Mechanics of metal matrix composites - A program review

A brief summary is presented of an experimental program to study metal matrix composites (MMCs) for NASA vehicles of the future. The purpose of the research is to (1) develop a composite understanding of the mechanics of MMCs and their related failure modes, (2) develop a micromechanics understanding of MMCs to help develop improved MMCs, and (3) develop the appropriate life design criteria.

Johnson, W. S.↗

NASA NTRS1988

Fatigue damage criteria - Matrix, fibers and interfaces of continuous fiber reinforced metal matrix composites

Continuous fiber reinforced metal matrix composites (MMC) are projected for use in high temperature, stiffness critical parts that will be subjected to cyclic loadings. Depending on the relative fatigue behavior of the fiber and matrix, and the interface properties, the failure modes of MMC can be grouped into four catagories: (1) matrix dominated, (2) fiber dominated, (3) self-similar damage growth, and (4) fiber/matrix interfacial failures. These four types of damage are discussed and illustrated by examples. The emphasis is on the fatigue of unnotched laminates.

Johnson, W. S.↗

NASA NTRS1988

Fatigue testing and damage development in continuous fiber reinforced metal matrix composites

Johnson, W. S.↗

NASA NTRS1988

Adhesively bonded joints: Testing, analysis, and design; Proceedings of the International Symposium, Baltimore, MD, Sept. 10-12, 1986

The papers contained in this volume provide an overview of the state of the art of testing, analysis, and design of adhesively bonded joints. The papers are grouped in the following sections: mechanical testing, stress analysis, failure mechanisms, and design and durability. Specific topics discussed include determination and verification of elastic parameters for adhesives; factors influencing elastic stresses in double cantilever beam specimens; special mixed finite elements for interfacial stress analysis of adhesively bonded joints; debonding characteristics of adhesively bonded woven Kevlar composites; and environmental durability of adhesively bonded joints.

Johnson, W. S.↗

NASA NTRS1988

A comparison of pure mode I and mixed mode I-III cracking of an adhesive containing an open knit cloth carrier

Static and fatigue tests were carried out on two commercial modified epoxy film adhesives with a wide open knit polyester carrier in order to compare crack resistance in mode I and mixed mode I-III loading. The carrier cloth is found to have a significant influence on the cracking behavior of the adhesives. The open air net carrier used in this study separates from the adhesive in mode I cracking but shreds during mixed-mode crack extension. This decreases the opening mode toughness but increases the mixed-mode toughness as compared with results obtained earlier using a heavier knit carrier. The results suggest that the type of carrier may have a far larger influence on crack resistance than is generally recognized.

Ripling, E. J.↗

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 NTRS1987

Stress analysis of the cracked-lap-shear specimen - An ASTM round-robin

This ASTM Round Robin was conducted to evaluate the state of the art in stress analysis of adhesively bonded joint specimens. Specifically, the participants were asked to calculate the strain-energy-release rate for two different geometry cracked lap shear (CLS) specimens at four different debond lengths. The various analytical techniques consisted of 2- and 3-dimensional finite element analysis, beam theory, plate theory, and a combination of beam theory and finite element analysis. The results were examined in terms of the total strain-energy-release rate and the mode I to mode II ratio as a function of debond length for each specimen geometry. These results basically clustered into two groups: geometric linear or geometric nonlinear analysis. The geometric nonlinear analysis is required to properly analyze the CLS specimens. The 3-D finite element analysis gave indications of edge closure plus some mode III loading. Each participant described his analytical technique and results. Nine laboratories participated.

Johnson, W. S.↗

NASA NTRS1987

Metal matrix composites - Their time to shine?

Basic types of metal matrix composite (MMC) systems (namely, the particulates-, the whisker/flakes-, and the continuous-fiber-reinforced) are discussed together with the advantages and the disadvantages of each system. Special consideration is given to the new MMC systems under development that meet the needs of aerospace applications and to the properties of stiffness and thermal expansion of these systems. As a family of structural materials, MMCs have great potential for missile airframe applications.

Johnson, W. S.↗

NASA NTRS1987

Fatigue damage accumulation in various metal matrix composites

The purpose of this paper is to review some of the latest understanding of the fatigue behavior of continuous fiber reinforced metal matrix composites. The emphasis is on the development of an understanding of different fatigue damage mechanisms and why and how they occur. The fatigue failure modes in continuous fiber reinforced metal matrix composites are controlled by the three constituents of the system: fiber, matrix, and fiber/matrix interface. The relative strains to fatigue failure of the fiber and matrix will determine the failure mode. Several examples of matrix, fiber, and self-similar damage growth dominated fatigue damage are given for several metal matrix composite systems. Composite analysis, failure modes, and damage modeling are discussed. Boron/aluminum, silicon-carbide/aluminum, FP/aluminum, and borsic/titanium metal matrix composites are discussed.

Johnson, W. S.↗