Rachel Briggs’ critique of “antirealist” accounts of scientific law— including my own perspectivalist best-system account—is part of a project meant to show that Humean conceptions of scientific law are more problematic than has been commonly realized. Indeed, her argument provides a new challenge to the Humean, a thoroughly epistemic version of David Lewis’ “big, bad bug” for Humeanism. Still, I will argue, the antirealist (perspectivalist and expressivist) accounts she criticizes have the resources to withstand the challenge and come out stronger for it. Attention to epistemic possibilities, I argue, shows a number of advantages to a perspectivalist account of scientific law.

AbstractThe process for certification of composite structures for civilian, CIVIL, and military, MIL, aircraft has been evolving for about 40 years. The technology base for the certification process is now mature enough to support the process. This article describes how the certification process has been tailored to the specific sensitivities of the polymeric matrix material component in the composite structures. Matrix sensitivities strongly influence the certification process: the material allowables, the need for environmental compensation load factors in the static test demonstration, the use of load enhancement factors and environmental conditions in the fatigue and damage tolerance testing, and material and process management for composite structures. The evolution of material and design values for composites is presented with suggestions for improving the cost and schedule efficiency for essential database supporting the airframe certification.

The damage tolerance of composite structures is intimately linked to the morphology and extent of flaws or damage. Impacts are a commonly occurring source of threat to composites that can produce a "seed" damage state which inherently controls the subsequent durability and damage tolerance of the affected structure. Transverse impact to composites is of particular concern due to the possibility of exciting damage modes that are difficult, or even not possible, to visually detect from the exterior (impact-side) surface. Some examples of such damage are delamination, backside-only fiber failure, debonding of internal substructure (e.g., stringers and stiffeners, doublers, joints), and crushing and separation of sandwich panel core. Impact damage is highly dependent upon the nature of the threat and conditions associated with the impact event. This chapter will provide an overview of impact damage threats that are common to composite aircraft structures and describe the relationship of these threat sources to the damage that is seeded. An historical overview of the impact damage tolerance methodologies developed by military aircraft programs, and subsequently widely adopted across the composite structures community, is also provided.

Recent composite technology research and development efforts have focused on discontinuous carbon fiber/epoxy molding systems derived from chopped aerospace-grade unidirectional tape prepreg. Although the average elastic modulus of this material has been shown to be as high as that of the continuous tape quasi-isotropic benchmark, experimental measurement by means of strain gage or extensometer has shown variation as high as 20%. Digital Image Correlation can be used successfully to obtain a full-field strain measurement, and it shows that a highly non-uniform strain distribution exists on the surface of the specimen, with distinct peaks and valleys. This pattern of alternating regions of high and low strain gradients, and which exhibit a characteristic shape and size, can be described in terms of Random Representative Volume Element (RRVE). The RRVE proposed here exhibits random elastic properties, which are assigned based on stochastic distributions. This approach leads to the analysis method proposed here, which is designed to compensate for the fact that traditional methods cannot capture the experimentally observed variation in modulus within a specimen and among different specimens. The method utilizes a randomization process to generate statistical distributions of fractions and orientations of chips within the RRVE, and then applies Classical Laminated Plate Theory to an equivalent quasi-isotropic tape laminate to calculate its average elastic properties. Validation of this method is shown as it applies to a finite element model that discretizes the structure in multiple RRVEs, whose properties are generated independently of the neighboring ones, and then are solved simultaneously. The approach generates accurate predictions of the strain distribution on the surface of the specimen.

John F. Halpin Oakland University Rochester, MI 48309 248 37

Measurements of the distribution of residual stress with depth from the surface in laser peened coupons were made in a high-strength aluminum alloy. Residual stresses were measured using slitting (also known as the crack compliance method). Measurements were made on several coupons to: compare laser peening (LP) and shot peening residual stresses; ascertain the influence of LP parameters on residual stress; determine whether tensile residual stress existed outside the peened area; assess the variation of residual stress with in-plane position relative to the layout of the laser spots used for peening; and, determine the importance of a uniform spatial distribution of laser energy within the spot. Residual stress 0.1 mm from the surface due to LP and shot peening were comparable and the depth of the compressive stress for LP was far greater than for shot peening. Variations of most LP parameters did not significantly alter residual stress at shallow depths. Residual stresses adjacent to the peened area were found to be compressive. Decreased levels of surface residual stress were found when laser spots had a non-uniform distribution of laser intensity.