Gary L. Farley

Crushing Characteristics of Continuous Fiber-Reinforced Composite Tubes

Composite tubes can be reinforced with continuous fibers. When such tubes are subjected to crushing loads, the response is complex and depends on interaction between the different mechanisms that control the crushing process. The modes of crush ing and their controlling mechanisms are described. Also, the resulting crushing process and its efficiency are addressed.

Energy absorption of composite materials

This paper presents results of a study on the energy absorption characteristics of selected composite material systems and compares the results with aluminum. Com posite compression tube specimens were fabricated with both tape and woven fabric prepreg using graphite/epoxy (Gr/E), Kevlar® epoxy (K/E) and glass/epoxy (Gl/E). Chamfering and notching one end of the composite tube specimen reduced the peak load at initial failure without altering the sustained crushing load, and prevented catastrophic failure. Static compression and vertical impact tests were performed on 128 tubes. The results varied significantly as a function of material type and ply orien tation. In general, the Gr/E tubes absorbed more energy than the Gl/E or K/E tubes for the same ply orientation. The [0/±15] Gr/E tubes absorbed more energy than the aluminum tubes. Gr/E and Gl/E tubes failed in a brittle mode and had negligible post crushing integrity, whereas the K/E tubes failed in an accordian buckling mode similar to the aluminum tubes. The energy absorption and post crushing integrity of hybrid composite tubes were not significantly better than that of the single material tubes.

Effect of specimen geometry on the energy absorption capability of composite materials

Static crushing tests were conducted on graphite and Kevlar1 reinforced epoxy tubes to examine the influence of specimen geometry on the energy absorption capability of com posite materials. Tube inside diameter to wall thickness (D/t) ratio was determined to sig nificantly affect the energy absorption capability of composite materials. As D/t ratio decreases, the energy absorption capability increases nonlinearly. The energy absorption capability of K/E tubes was found to be geometrically scalable but energy absorption of Gr/E tubes was not geometrically scalable.

Effect of Fiber and Matrix Maximum Strain on the Energy Absorption of Composite Materials

Static crushing tests were conducted on graphite composite tubes to examine the in fluence of fiber and matrix maximum strain at failure on the energy absorption capability of graphite reinforced composite material. Fiber and matrix maximum strain at failure were determined to significantly effect energy absorption. The higher strain at failure composite material system, AS-4/5245, exhibited superior energy absorption capability compared to AS-4/934, T300/5245 or T300/934 composite material. Results of this in vestigation suggest that to achieve maximum energy from a composite material a matrix material that has a higher strain at failure than the fiber reinforcement should be used.

The effects of crushing speed on the energy-absorption capability of composite tubes

The energy-absorption capability as a function of crushing speed was determined for Thornel 300-Fiberite 934 (Gr-E) and Kevlar-49-Fiberite 934 (K-E) composite material. Circular cross section tube specimens were crushed at speeds ranging from 0.01 m/sec to 12 m/sec. Ply orientations of the tube specimens were [0/± θ]2 and [± θ]2 where θ = 15, 45, and 75 degrees. Based upon the results of these tests the energy-absorption capability of Gr-E and K-E was determined to be a function of crushing speed. The magnitude of the effects of crushing speed on energy-absorption capability was determined to be a function of the mechanisms that control the crushing process. The effects of crushing speed on the energy-absorption capability is related to whether the mechanical response of the crushing mechanism that controls the crushing process is a function of strain rate. Energy-absorption capability of Gr-E and K-E tubes ranged between 0 and 35 percent and 20 and 45 percent, respectively depending upon ply orientation. The crushing modes based upon exterior appearance of the crushed tubes were unchanged as a result of changing crushing speed for either material. However, the interlaminar crushing behavior of the Gr-E specimens changed with crushing speed.

Prediction of the Energy-Absorption Capability of Composite Tubes

A method of predicting the crash-related energy-absorption capability of composite tubes is presented. The method is based upon a phenomenological model of the crushing process exhibited by continuous-fiber-reinforced tubes. A finite element method is used to model the crushing process. The analysis is compared with experiments on Kevlar-epoxy and graphite-epoxy tubes. Reasonable agreement is obtained between the analysis and experiment.

An Evaluation of the Iosipescu Specimen for Composite Materials Shear Property Measurement

A detailed evaluation of the suitability of the Iosipescu specimen tested in the modified Wyoming fixture is presented. A linear finite element model of the specimen is used to assess the uniformity of the shear stress field in the vicinity of the notch, and demonstrate the effect of the nonuniform stress field upon strain gage measurements used for the determination of composite shear moduli. Based upon test results from graphite epoxy laminates, the proximity of the load introduction point to the test section and the material orthotropy greatly influence the individual gage readings, however, shear modu lus determination is not significantly affected by the lack of pure shear. Correction factors are needed to allow for the nonuniformity of the strain field and the use of the average shear stress in the shear modulus evaluation. The correction factors are determined for the region occupied by the strain gage rosette. A comparison of the strain gage readings from one surface of a specimen with corresponding data from moire interferometry on the opposite face documented an extreme sensitivity of some fiber orientations to eccentric loading which induced twisting and spurious shear stress-strain curves. The discovery of specimen twisting explains the apparently inconsistent shear property data found in the literature. Recommendations for improving the reliability and accuracy of the shear modu lus values are made, and the implications for shear strength measurement discussed.

Analogy for the effect of material and geometrical variables on energy-absorption capability of composite tubes

Simplified procedures for determining the qualitative effect a variable has on structural response of a composite tube are very useful in both preliminary design as well as in providing insight into the general response. An analysis procedure is pre sented that can be used to determine the qualitative change in the sustained crushing load due to a change in specimen material properties or geometry. The analysis procedure is similar in form to the equation for the buckling load of a column on an elastic foun dation.

Compression Response of Thick Layer Composite Laminates with Through-the-Thickness Reinforcement

Compression and compression-after-impact (CAI) tests were conducted on seven different AS4-3501-6 [0/90] 0.64-cm thick composite laminates. Four of the seven laminates had through-the-thickness (TTT) reinforcement fibers. Two TTT reinforcement methods, stitching and integral weaving, and two reinforcement fibers, Kevlar and carbon, were used. The remaining three laminates were made without TTT reinforcements and were tested to establish a baseline for comparison with the laminates having TTT rein forcement. Six of the seven laminates consisted of nine thick layers whereas the seventh material was composed of 46 thin plies. The use of thick-layer material has the potential for reducing structural part cost because of the reduced part count (layers of material).

Crushing characteristics of composite tubes with near-elliptical' cross sections

An experimental investigation was conducted to determine whether the energy-absorption capability of near-elliptical cross-section composite tubular specimens is a function of included angle. Each half of the near-elliptical cross-section tube is a seg ment of a circle. The included angle is the angle created by radial lines extending from the center of the circular segment to the ends of the circular segment. Graphite- and Kevlar- reinforced epoxy material was used to fabricate specimens. Tube ply orientation was [±45] N where the number of ±45 pairs was 2, 4, 6, and 12. Tube internal diameters were 2.54, 3.81, and 7.62 cm, and included angles were 180, 160, 135, 115, and 90 degrees. Based upon the test results from these tubes, energy-absorption capability increased between 10 and 30 percent as included angle decreased between 180 and 90 degrees for the materials evaluated. Energy-absorption capability was a decreasing nonlinear function of the ratio of tube internal diameter to wall thickness.