J. F. Mandell

Fibre-matrix bond strength studies of glass, ceramic, and metal matrix composites

An indentation test technique for compressively loading the ends of individual fibres to produce debonding has been applied to metal, glass, and glass-ceramic matrix composites; bond strength values at debond initiation are calculated using a finite-element model. Results are correlated with composite longitudinal and interlaminar shear behaviour for carbon and Nicalon fibre-reinforced glasses and glass-ceramics including the effects of matrix modifications, processing conditions, and high-temperature oxidation embrittlement. The data indicate that significant bonding to improve off-axis and shear properties can be tolerated before the longitudinal behaviour becomes brittle. Residual stress and other mechanical bonding effects are important, but improved analyses and multiaxial interfacial failure criteria are needed to adequately interpret bond strength data in terms of composite performance.

The Extension of Crack Tip Damage Zones in Fiber Reinforced Plastic Laminates

The size and character of the damage zone at the tip of sharp notches in fiber reinforced plastic laminates have been investigated. The variables studied were the stress intensity factor, specimen size, laminate thickness, ply thickness, ply orientation, and fiber properties. The damage zone consists of subcracks parallel to the fibers of each ply, in some cases accompanied by delamination between plies. The damage zone is found to increase in extent approximately in proportion to K2 I up to fracture for notch-sensitive laminates. For notch-insensitive laminates, a point is reached where the zone spreads rapidly across the entire specimen prior to fracture. A strong dependence of damage zone size and fracture toughness on ply thickness, fiber orientation, and fiber properties is demonstrated and discussed.

Modified mircodebonding test for direct in situ fiber/matrix bond strength determination in fiber composites

This paper describes a test method for determining the in situ fiber/matrix bond strength of composites. In a sequence of steps, a compressive load is applied to individual fibers oriented normal to a polished surface, and the interface is observed microscopically between loading steps, until debonding occurs. The direct result of a test is the force to produce debonding of a particular fiber. The data are reduced to a nominal interfacial shear strength through a finite-element analysis using a simplified model of the fiber, surrounding matrix, and uniform composite properties beyond the matrix. Bond strength results for a variety of composites range from 23 MPa for E-glass/polyester to 86 MPa for T300/5208 graphite/epoxy. The results are consistent with data available from other test methods.

Stress Intensity Factors for Anisotropic Fracture Test Specimens of Several Geometries

Stress intensity factors have been obtained for single-edge-notched, double-edge-notched, and double cantilever beam fracture toughness test specimens using a two-dimensional hybrid stress model finite element analysis. The degree of anisotropy is shown to have a significant effect on the stress intensity factor in some cases, with differing effects for different specimen shapes; however, effects of anisotropy are relatively constant for varying crack lengths of a given shape. An experimental K-calilbration for the double cantilever beam specimen is in good agreement with the analytical prediction, and the effect of geometry on the applied load to cause crack propagation is accurately predicted by the analysis.

Modeling of Marine Rope Fatigue Behavior

Fatigue data for nylon and polyester marine ropes from several testing programs in Europe and the U.S. are compared and analyzed. Failures at loads above 30-40% of the new breaking strength for nylon or 60-70% for polyester can be predicted by a model based on the creep-rupture behavior of individual fibers and yams. Failures at lower loads and higher cycles usually occur by external or internal abrasion, following a steeper S-N curve trend; an abrasion model is presented that correlates well with these data. Polyester outperforms nylon under wet conditions in both regimes of be havior for a wide range ofrope samples. Additional topics discussed are thermal failures from hysteretic heating in dry ropes, and changes during cycling in rope load-extension, hysteresis, and residual properties.

Fatigue behaviour of synthetic fibres, yarns, and ropes

S-N fatigue and creep-rupture data have been obtained for nylon 6,6 single fibres, interlaced yarns, and small ropes under a variety of loading conditions. The results show a similar degradation rate at each level of structure, with no apparent influence of inter-fibre effects. Cyclic lifetimes of single fibres of nylon 6,6 as well as polyester and aramid can be predicted from a creep rupture model. Consistent with this model, the time to failure is insensitive to frequency over a broad range. For each level of structure the strain at failure is the same whether tested in simple tension or under cyclic or creep loading. Failure modes were generally similar in creep rupture and cyclic fatigue tests; no effect of a slack load on each cycle was evident either in the failure mode or specimen lifetime.

STRUCTURAL MODELING OF DOUBLE-BRAIDED SYNTHETIC FIBER ROPES

This study is primarily concerned with the performance of double-braided ropes. widely used in marine applications. The mechanical properties of such ropes are ob tained by combining their structural features and the constitutive behavior of individual rope components. Emphasis in this study is on the tensile behavior of straight ropes as well as bent ropes, either an eye splice or in a continuous loop around a bollard- like pin. In the former case, precycling and water effects on the model predictions are discussed. For new small ropes, bending rigidity is negligible, so the bending effect is considered by modifying the geometry of the rope to allow for variation of helix periods and change of cross-sectional shapes from circular to elliptical ( flattened ) . For two extreme frictional conditions considered, i.e., infinite and zero friction, predictions of small nylon rope behavior agree well with experimental results for both simple tension and tension plus bending. There is less agreement for small PET ropes, par , ticularly in bending tests.

Crack Propagation Modes in Injection Molded Fiber Reinforced Thermoplastics

Abstract : The modes of crack propagation are reported for injection molded short glass and carbon fiber reinforced thermoplastics. The matrices ranged from ductile to brittle, including Nylon 66, polycarbonate, polysulfone, poly(amide-imide), and polyphenylene sulfide; fiber contents were 30 or 40 % by weight. The main crack is found to grow in a fiber avoidance mode, bypassing regions of agglomeration of locally aligned fibers. The local mode of crack tip advance varied with matrix ductility and bond strength. The fracture toughness and fatigue resistance of each material are related to the mode of crack growth.

The effects of sea water and concentrated salt solutions on the fatigue of nylon 6,6 fibres

Cyclic fatigue and creep rupture tests have been run on high-tenacity nylon 6,6 single fibres, yarns and small ropes in air and sea water environments. Fatigue failure in each case is by a creep rupture mechanism; yarns and small ropes show the same fatigue sensitivity as do single fibres. Sea water reduces the strength by approximately 10% under most conditions. Concentrated metallic salt solutions which cause environmental stress cracking in bulk nylon do not degrade the fibres beyond the effect of plain water. Tests on oriented nylon specimens show that environmental stress crack sensitivity is greatly reduced by orientation.

Fatigue crack propagation in 0°/90° E-glass/epoxy composites

The mode of fatigue crack growth is described for a 0/90° E-glass/ epoxy laminate under cyclic tension-tension loading. Crack growth appears to occur in a stepwise fashion with the crack remaining stationary for many cycles before each step of growth, whereupon a ligament of longitudinal ply at the crack tip is broken. A simple theory is described which assumes that the ligament at the crack tip is fatigued according to the S-N curve of the unnotched material. Using an assumed stress field and cumulative damage law, the number of cycles for initial growth from a notch and the rate of crack growth thereafter are predicted, and good agreement is demonstrated with experimental data.