Show Me: Making An Impact With Sandwiches

Research in the transportation industry is largely driven by the need to cut costs and weight and to develop structures with better impact absorption capabilities. Metal and/or composite honeycomb core materials sandwiched between metal or composite end plates have been used in high performance, high-stake aerospace structural members, and they hold a promise that sandwich structures can be made which are light, cheap, and so energy absorbent that they will revolutionize common ground transportation systems. NC A&T State University has a number of grants involving research into impact damage including impact behavior of sandwich structures. The authors feel that this research is sufficiently mature and important so that impact experiments should be added to our laboratory sequence within the department. Research investigations of these materials in our labs involve static and dynamic testing including shear testing or impact testing, and it is relatively simple to include the important attributes of impact testing as a laboratory experiment. Motivating factors for us in developing new impact experiments include: • providing our students with additional information on how to design (impact) experiments. • enhancing our current educational content on data acquisition systems for dynamic events, and • educating our graduates in an important technology for the 21 century. We feel that knowledge of impact phenomena and testing is an important modern design methodology. P ge 546.1 A laboratory experiment has been developed to highlight some of the experimental and analytical principles of impact testing. The governing equations are developed and solved in a simple fashion. Classroom learning is reinforced by impact testing of instrumented fiberglass/wood sandwich beams and subsequent processing and interpretation of laboratory data. Introduction Requirements for reduced structural weight, improved product performance, and efficiency are making composite materials increasingly attractive as primary loadcarrying structures. The move to these new applications is supported by knowledge gained through extensive composite materials experimental programs, which have shown the glass fiber-reinforced composite sandwiches to have low ultimate strains, no plastic deformation range, and no usable strength in the thickness direction this become obvious when laminates are subjected to impact. Resin matrix composites are brittle materials, and the damage caused by impact is vastly different from the damage to ductile metal structures that tend to develop visible indentations. On the other hand, brittle composite materials tend to sustain both visible and hidden damage. Such damage is usually in the form of delaminations, matrix cracks and broken internal fibers. Drop-weight impact tests are important in determining the suitability of materials systems for energy absorption and hence for transportation applications. Specimens are impacted at various drop heights by a blunted indenter (called a tup). Force-time histories are recorded in our impact tests, and specimens are tested only once. The tests begin with low drop height impacts, with the height progressively raised. Low-energy impacts usually produce little or no damage, but higher energy tests begin to cause more damage until the ultimate energy dissipation of the composite is reached. The typical force-time curve resembles a 1⁄2 cycle sine curve, and it is easily modeled as a second-order system of the impact of a rigid mass onto a linear spring, the composite specimen. Impact damage can be correlated with the onset of stiffness reduction. Quantitative relationships can easily be derived to identify damage onset and the degree of stiffness reduction. When determining impact load magnitudes, the raw data from the recorded load time histories is generally used. The load time history can indicate ply-level damage when the load history deviates from an expected linear elastic response. In the case of typical low velocity impacts on composite sandwiches, the load-time history from an elastic response, when only considering the fundamental modal response, is approximately a bell shaped curve. However, the raw-data curves are usually difficult to interpret (Figure 1a). A Fast Fourier Transform (FFT) smoothening filtering algorithm is used to aid in the interpretation of these curves. The load-time histories are FFT filtered to retain only the fundamental frequency Page 546.2 response. Damage is expected whenever the smoothened curve deviates from an approximate bell curve (Figure 1b). 0 2 4 6