Mahyar S. Dadkhah

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.

The Macroscopic Elasticity of 3D Woven Composites

The elastic properties of graphite/epoxy composites with three-dimensional interlock weave reinforcement have been measured over length scales somewhat greater than the characteristic length of the weave pattern. Orientation averaging models similar to those developed elsewhere over the last twenty years provide estimates of elastic constants that are in fair agreement with the experimental data. However, in-plane Youngs moduli are consistently too high and properties related to the through-thickness reinforcement show considerable scatter. Most of the discrepancies can be attributed to waviness and other geometrical irregularities in nominally straight tow segments. Much improved agreement with in-plane properties is obtained by measuring and accounting for the out-of-plane waviness of in-plane tows. Other observed distortions of in-plane tows and irregularity in through-thickness tows are very difficult to quantify experimentally. There results a significant and apparently unavoidable uncertainty in predictions of properties that depend strongly on the through-thickness reinforcement. Nevertheless, the utility of orientation averaging models in applications where the in-plane properties are paramount is clearly verified.

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.

Cracking and damage mechanisms in ceramic/metal multilayers

Abstract Investigations of cracking in multilayered ceramic/metal composites are presented. Two aspects are considered: crack renucleation across intact single metal layers and subsequent crack extension. Crack renucleation criteria are determined and compared with predictions. High-resolution strain-mapping techniques are employed to determine the surface strain fields surrounding cracks. Good agreement is found between these experimental measurements and the predictions of a small-scale yielding model. Subsequent crack progression occurs either by the extension of a dominant, nearly planar crack or by the formation of a zone of periodically spaced cracks. Both patterns are analyzed. The dominant cracking behavior is found to depend on the volume fraction and yield strength of the metal.

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.

Simple models for triaxially braided composites

Comparison of measurements and theory shows that the elastic properties of some triaxially braided glass/urethane composites are predicted as well as they can be by a simple laminate model modified to account for measured tow waviness. Any remaining discrepancies are the result of inevitable irregularity in the positioning of tows and are not grounds for more sophisticated modelling. The same modified laminate model provides estimates of the partitioning of in-plane stresses among tows of different orientation. Estimates of these local stresses correlate well with measured compressive and tensile strengths in the primary load-bearing direction. Absolute values for compressive strength are predicted quite accurately by kink band models, using independently measured values for tow misalignment angles and the critical shear stress for resin flow. Some summary design principles are presented based on the model of elasticity and observations and modelling of tensile and compressive strength.

Compression-compression fatigue of 3D woven composites

Polymer composites with 3D woven graphite fiber reinforcement (3D interlock weaves) have been tested in compression-compression fatigue under load control. As under monotonic loading, the principal mechanism of failure is kink band formation in the primary load bearing tows. Observations of kink bands and microcracking in sectioned specimens suggest that fatigue progresses by the accumulation of damage to the resin within individual tows. It is conjectured that resin damage leads to failure by lowering the critical stress for kink band formation on a single cycle. If resin damage is assumed to accumulate at a rate proportional to some power of the local axial shear stress in a misaligned tow, then a simple formula follows for the cycles to kink band formation. Under load control, only a few kink bands are required for specimen failure. Then the formula is also the basis for estimates of fatigue life. Fatigue life data and measured misalignment angles, which determine the local axial shear stress, support the fatigue model.

Fatigue crack growth and stress redistribution at interfaces

The role of the interface in redistributing stress around cracks in multilayered ceramic/metal composites is investigated. The emphasis is on the different effects of interfacial debonding or of plastic slip in the metal phase adjacent to strongly bonded interfaces. The experiments are conducted on alumina/aluminum multilayered composites. Monotonic loading precracked test pieces causes plastic shear deformation within the aluminum layer at the tip of the notch without debonding. However, interfacial debonding can be induced by cyclic loading, in accordance with a classical fatigue mechanism. Measurements of the stress around the crack demonstrate that debonding is much more effective than slip at reducing the stress ahead of the crack.

Development of micro-electro-mechanical optical scanner

Rockwell is working on the development of a micro-electromechanical optical scanner based on bimorph microactuators. This scanner is lightweight, is small, and has superior scanning performance. The scanner is a low-power (<1 W) device that has large scan angles (?20 deg) and scan rates in the range of 100 to 2000 Hz. It works for all wavelengths and offers the potential for monolithic integration with both electronics and optics for on-chip signal processing and control. The optical scanner consists of two main components—actuator and mirror— which are fabricated on a silicon cantilever beam. The actuator is comprised of a bimorph layer covered with two metal layers, which function as top and bottom electrodes. The mirror can be as large as 12 mm2 in area, is placed at the end of the cantilever beam, and is designed for maximum optical flatness. The optical efficiency of the device is very high and can exceed 90% on proper metallization of the mirror area. The scan angle is a function of beam thickness, power efficiency of the bimorph, and many other design criteria. Through many improvements in these design parameters, a scan angle greater than 20 deg is expected to be achieved with high yield.

On determining temperature dependent interfacial shear properties and bulk residual stresses in fibrous composites

Abstract The model of the preceding paper is applied to the analysis of interfacial sliding during thermal cycling in three titanium or titanium aluminide alloys reinforced by SiC fibers. The model is found to give an excellent account of the experimental measurements. By fitting the model to the data, values are obtained in all cases for the critical interfacial shear stress, τ 0 at room temperature. In two cases, values are also obtained for the bulk, axial residual stresses at room temperature, and the average of d τ 0 /d T over the interval of temperature T between room temperature and the maximum temperature attained in the thermal cycling. The residual stresses are in good agreement with other measurements. In the third case, the residual stresses cannot be determined; but, if values for them are taken from other experiments, then the same average of d τ 0 /d T can be determined. The values of τ 0 and the implied coefficient of friction are consistent in all cases with frictional sliding in graphitic layers in the carbon rich coatings on the SiC fibers.