W. L. Morris

Failure mechanisms of 3D woven composites in tension, compression, and bending

Abstract Observations of failure mechanisms in monotonic loading are reported for graphite/epoxy composites containing three-dimensional (3D) interlock weave reinforcement. The key phenomena are delamination and kink band formation in compression, tow rupture and pullout in tension, and combinations of these in bending. The materials exhibit great potential for damage tolerance and notch insensitivity. This is partly due to the presence of geometrical flaws that are broadly distributed in strength and space; and partly to the coarseness of the reinforcing tows, which leads to extensive debonding and reduced stress intensification around sites of failure. Rules of mixture corrected for the effects of tow irregularity suffice to estimate elastic moduli. Rough estimates of the stress at which the first failure events occur in compression or tension can be made from existing micromechanical models. Ultimate tensile failure might be modeled by regarding failed tows that are being pulled out of the composite as a cohesive zone. The characteristic length estimated for this zone, which is a direct measure of damage tolerance and notch insensitivity, has very large values of order of magnitude 0.1–0.5 m.

On the tensile failure of 3D woven composites

Abstract Tensile tests are reported for some graphite/epoxy composites with three-dimensional woven interlock reinforcement. Composite failure consists of the accumulation of discrete tow rupture events distributed over a band of damage typically 10–20 mm wide. Load—displacement data for gauges spanning the band indicate work of fracture values ranging from 0.4 to 1.1 MJ m −2 . Most of these unusually high vales derives from the ability of the composite to sustain loads near peak load (≈1 GPa) for displacements significantly beyond those at which tows have all failed. The key mechanism is very strong friction or lockup that couples sliding, broken tows to the surrounding composite. Lockup is the product of the geometrical irregularity of nominally straight tows and clamping compressive stresses generated by the through-thickness reinforcement. Lesser contributions to the work of fracture arise from plastic straightening of tows prior to their rupture and the relatively easy but prolonged pull-out of tows following failure of the lockup mechanism.

Acoustic harmonic generation due to fatigue damage in high‐strength aluminum

It is shown that acoustic second harmonic generation is a useful tool for studying surface microcrack development during fatigue of a high‐strength aluminum alloy. A fundamental (5 MHz) surface acoustic wave (SAW) was transmitted across the gauge section of flexural fatigue specimens of Al 7075‐T6. The second harmonic amplitude was determined after several increments of fatigue, as a function of external load and the amplitude of the fundamental. It was found that the second harmonic signal is at a maximum close to zero external load and increases with progressing fatigue. Harmonic generation, attributable to microcracking at the surface, has been observed as early as 10–20% of the expended fatigue life. A simple analysis to obtain a coefficient of harmonic‐generation efficiency versus applied surface stress is described. This analysis considers the effect of changes in attenuation of the fundamental and harmonic waves associated with degree of surface microcrack opening as a function of surface stress. I...

The noncontinuum crack tip deformation behavior of surface microcracks

The crack tip opening displacement (CTOD) of small surface fatigue cracks (lengths of the grain size) in Al 2219-T851 depends upon the location of a crack relative to the grain boundaries. Both CTOD and crack tip closure stress are greatest when the crack tip is a large distance from the next grain boundary in the direction of crack propagation. Contrary to behavioral trends predicted by continuum fracture mechanics, crack length has no detectable effect on the contribution of plastic deformation to CTOD. It is apparent from these observations that the region of significant plastic deformation is confined by the grain boundaries, resulting in a plastic zone size that is insensitive to crack length and to external load.

Measurement of interfacial debonding and sliding resistance in fiber reinforced intermetallics

Abstract A technique is described for measuring fiber debonding and sliding characteristics in intermetallic matrix composites. The method involves pulling a single fiber that protrudes from the composite, while measuring the applied force and relative displacements of the fiber and matrix. High resolution displacement mapping methods are used to obtain data during initial loading, where sliding progresses only partly along the fiber. Measurements from a Ti3Al/SiC composite are interpreted using the analysis of the companion paper. The results indicate a strong effect of constraint on the sliding response and provide quantitative measurements of sliding resistance, residual stresses, and debond energy. Preliminary measurements of changes in sliding resistance with cyclic loading are presented.

Mechanisms of compressive failure in 3D composites

Abstract Angle interlock woven polymer matrix composites have been studied under uniaxial monotonic compression. With considerable variations arising from the geometry of the reinforcement and the degree of constraint in the test, the materials were found to be macroscopically ductile, with compressive strains to failure occasionally exceeding 15%. In contrast, some tests on stiched laminates showed brittle behavior, with essentially no load bearing capacity beyond the strain for peak load (∼1%). The mechanisms of failure in the woven composites were determined by a combination of optical microscopy (both in situ and of sectioned specimens), moire interferometry, stereoscopy, and digital image comparison. In all cases, the central failure event was kink band formation in the primary load bearing, axial tows. Various characteristics of the reinforcement geometry were observed to influence kink band formation, including initial misalignment of the load bearing tows and intersections of load bearing tows and through-thickness reinforcing tows. Such geometrical characteristics acted as flaws, tending to lower macroscopic stiffness and strength, but promoting the broad distribution of damage throughout the specimen and averting catastrophic failure. Guidelines for achieving the optimum compromise between strength and damage tolerance may be inferred from these observations.

Growth rate models for short surface cracks in AI 2219-T851

Rates of fatigue propagation of short Mode I surface cracks in Al 2219-T851 are measured as a function of crack length and of the location of the surface crack tips relative to the grain boundaries. The measured rates are then compared to values predicted from crack growth models. The crack growth rate is modeled with an underlying assumption that slip responsible for early propagation does not extend in significant amounts beyond the next grain boundary in the direction of crack propagation. Two models that contain this assumption are combined: 1) cessation of propagation into a new grain until a mature plastic zone is developed; 2) retardation of propagation by crack closure stress, with closure stress calculated from the location of a crack tip relative to the grain boundary. The transition from short to long crack growth behavior is also discussed.

The Effect of Grain Size on Fatigue Growth of Short Cracks

The influence of alloy grain size on growth rates of surface cracks 20 to 500 μm in length was studied in Al 7075-T6 specimens prepared in 12 and 130 μn grain sizes. Grain boundaries temporarily interrupt the propagation of cracks shorter than several grain diameters in length. Linear elastic fracture mechanics is inadequate to describe resulting average growth rates which must instead be characterized as a function of cyclic stress amplitude, σa, and alloy grain size as well as stress intensity range, σK. These observations are rationalized using two models, one that relates crack closure stress to alloy grain size, and a second that relates the development of microplasticity in a new grain in the crack path to grain size. In addition, growth rates were found to be faster in fully reversed loading than in tension-tension loading, especially in the large grained material. Evidence is presented to demonstrate that this is a consequence of the fatigue induced development of a compressive residual surface stress during tension-tension loading. These complex effects, and the role of grain size in determining short crack growth, are discussed.

A high accuracy automated strain-field mapper

The design and capabilities of a computer-automated high-spatial-resolution displacement-measurement system are described. The system is used to determine the relative displacement fields generated by thermal or mechanical loads by comparing a pair of SEM or optical micrographs, one recorded before the load is applied and the other afterwards. The displacements are measured by cross-correlation analysis of the relative positions of visible surface texture on the micrographs. Displacement accuracy on a specimen surface is ±60 Ă for optical microscopy, and ±10 Ă for scanning electron microscopy. Both in-plane or out-of-plane deformation can be characterized depending on the angle at which the specimen is viewed. This instrument has the potential of quantifying surface deformation over submicron gage lengths and will be an invaluable tool in experimental micromechanics.

Fatigue crack initiation and early propagation in Al 2219-T851

Fatigue crack initiation in Al 2219-T851 for fully reversed loading(R = σ/σmax =−1) parallel to the material rolling direction is found to occur at intermetallic inclusions at the specimen surface. The inclusions are not involved in crack initiation for fatigue perpendicular to the rolling direction, and for this orientation crack initiation is at grain boundaries and specimens have an increased fatigue life. Except for fatigue at low peak stress, multiple numbers of microcracks are formed and for selected failed specimens the number of cracks has been determined as a function of crack length. Such crack length distribution measurements show that there is significant retardation of microcracks by interaction with grain boundaries. Furthermore it is found that the coalescence of microcracks provides a mechanism for cracking to “jump“ grain boundaries and reduce fatigue lifetime. The effect of relative humidity on this process is to increase the observed mean crack length, and decrease the number of crack initiations apparently due to weakening of the matrix-intermetallic interface at potential initiation sites. The overall result is that no significant dependence of fatigue life on relative humidity is found.