Composite Structures

Exponent’s engineers and scientists provide technical expertise for design, analysis, manufacturing, testing, and failure investigation based on their broad backgrounds in, and practical experience and knowledge of, composite structures and advanced composite materials.

Composite structures are load-bearing elements (e.g., stiffeners, panels, shells, etc.) fabricated from materials that are typically non-metallic non-homogeneous combinations of fibers and resins. In the last several decades, composite materials have increasingly supplanted metals in many structural applications involving aircraft, aerospace, military vehicles, automobiles, civil infrastructure, medical devices, and sporting equipment. The virtues of composite structure typically include reduced weight, increased performance, and fuel economy. Advanced composite materials comprised of carbon/graphite, aramid/Kevlar, and boron fibers, for example, along with engineered resin systems, have been used as primary structure in, for instance, the Boeing 777/787, a variety of Airbus aircraft, business jets, and military aircraft such as the U.S. Air Force F-22, F-14, and the AV-8B. Cost and risk, especially when human life is involved, have impeded the use of advanced composites in commercial and military aircraft; however, these barriers are being overcome as both positive outcomes and user experiences increase. New applications for composite materials and structures emerge every day.

Our consultants are widely recognized as leaders in the failure analysis of composites. We are contributing authors to the International Civil Aviation Organization (ICAO) Manual of Aircraft Accident and Incident Investigation and active members of the Composite Materials Handbook (CMH-17). We are routinely retained to analyze composite failures in a wide variety of applications, from aircraft and satellites to civil infrastructure and sporting equipment.

We have extensive knowledge of both closed-form and numerical computational methods of analysis such as finite element analysis (FEA). Modern FEA computer programs enable nonlinear geometry and nonlinear material effects and their interactions to be considered when predicting the behavior of the structure under load. This is especially important when analyzing buckling, postbuckling, and crippling of structural elements such as edge-stiffened flat and curved panels. FEA also allows the effects of complex loads (vibration, shock, impact, compression, shear, internal pressure, environment, etc.) to be quantified when closed-form solutions are not available. Durability and damage tolerance of composite structures can be analyzed and assessed as well.

Exponent engineers and scientists have addressed questions of strength, composition, curing, physical properties and durability of composites used in piping, bath and spa, recreational, marine, automotive, and aerospace applications. We also have significant experience with the properties and performance of high performance fibers used in structural and ballistic applications.

The resin (polymer) mechanics investigations that are performed by Exponent staff focus on understanding the governing micromechanical mechanisms that control the often-complex behavior exhibited by polymer resins in different environments. Scientists and engineers use experimental characterization to assess different failure scenarios or to supply material parameters for analytical and computer-based modeling of the mechanical response in different loading environments. We perform experimental testing of the mechanical response under different conditions, including time effects, rate effects, temperature effects, and environmental effects, and have assisted clients with modeling and predictions of mechanical behavior, including creep, stress relaxation, yielding, crazing, texture development, anisotropic behavior, temperature dependence, and physical and chemical aging.

Exponent maintains complete facilities for the mechanical and environmental testing of material and structural coupon-size specimens. Custom testing of relevant large-scale composite structure is accomplished in Exponent high-bay rooms with equipment designed for both complex and high loadings.

Failure prediction in composite structure is complicated by the inherent non-homogeneity of the underlying laminated construction and the unique failure modes associated with both fiber and resin. Failure modes in composite laminate materials include those that are principally found in-plane such as fiber failure from excessive loads or cracking off-axis to the fiber in the resin, and failures found in the matrix material such as delamination between layers, crack propagation (in-plane and normal to plane), impact (damage tolerance), interlaminar tension, and transverse shear. We perform visual and microscopic examinations (including fractography) and physical and chemical analyses to identify and characterize defects, evaluate composition, and assist in determining the root cause of failures. We employ tools such as optical and scanning electron microscopy (SEM), chromatography and spectroscopy, including EDS, FTIR, GPC, and GC-MS and standard and customized mechanical testing.

Our staff consults with industrial, government, and insurance clients, as well as their outside counsels, regarding composite structures and materials used in applications such as aerospace, automotive, sporting equipment, and aerospace (control surfaces, primary and secondary structure for military and commercial civilian aircraft, helicopters, missile, and satellite systems).

We provide technical support related to product development, failure analysis, intellectual property assessment and disputes, product recall investigations, insurance investigations, and litigation. Project activities include field testing and site inspections, evaluating material composition, safety/reliability assessments, life prediction, repairability, stress analysis, assessing chemical resistance, and laboratory testing and evaluation.