Toshiyuki Shimokawa1, Yoshiaki Kakuta2 and Takenori Aiyama3 formerly Tokyo Metropolitan Institute of Technology, 2Japan Aerospace Exploration Agency, 3Toyota Motor Corporation Japan
1. Introduction
A lot of polymer composite materials are being used in the structures of civil transport aircraft currently under development, such as the Boeing 787, Airbus A350, and Bombardier C Series. Their percentages of structural weight are announced at 50%, 53% and 46%, respectively. However, mostly carbon fiber/epoxy (CF/Ep) systems are being introduced into their primary structures and they are only being introduced in environments where they are not expected to encounter high temperature. On the other hand, application of carbon fiber/bismaleimide (CF/BMI) composite materials is being expanded, especially for military aircraft structures such as the airframes of the F-22 Raptor and the F-35 Lightning II Joint Strike Fighter, the jet engine nacelle skins of the F-35 as well as the thrust reverser structure of the Gulfstream G450 business jet in civil aircraft (McConnell, 2009), and is expected for structures of the next-generation supersonic transport (SST).
There are several reasons for CF/BMI system application for the structures described above. As for epoxy system composite materials, about 70°C is usually set as the design limit temperature for long term use (Brandecker & Hilgert, 1988, Fawcett et al., 1997). Meanwhile, carbon fiber/polyimide (CF/PI) system composite materials can be used for hotter structures, although they are very expensive and involve complicated processes. CF/BMI composite materials offer temperature performance and costs between those of epoxy and polyimide systems. Moreover, CF/BMI systems can be easy handled in an airframe parts manufacturing process in a way that is equivalent to that for epoxy systems.
The design limit temperature of CF/BMI composite materials for aircraft structures is supposed to be around 120°C on the basis of actual application to the mechanically loaded structures described above. If the design limit temperature is set as 120°C, it is necessary to know the detailed characteristics of static and fatigue strengths at about 150°C from the view point of the safety margin; moreover, 150°C is considered to be close to the service – limit temperature for CF/BMI composite materials. Therefore, in order to apply CF/BMI composite materials for aircraft structures that encounter medium high temperatures,
knowledge of their static and fatigue properties at 150°C is very important in comparison with that at RT.
The following reports presented the static strength of a CF/BMI composite material, G40- 800/5260, with a quasi-isotropic layup. As part of the investigation of the potential of high temperature composite materials for the next-generation SST, the authors’ group assessed open-hole (OH) tensile and compressive static strengths at RT and 120°C (Shimokawa et al., 1999-a). Hirano reported OH tensile and compressive static strengths at RT only (Hirano, 2001). Johnston and Gates investigated OH tensile static strength at from 23°C to 218°C (Johnston and Gates, 1998).
Meanwhile, the following investigations were carried out on fatigue strength or S-N relationships of CF/BMI composite materials with a quasi-isotropic stacking sequence at RT. Hirano conducted axial tension, compression, and tension-compression fatigue tests for OH specimens of a G40-800/5260 CF/BMI laminate along with CF/PI composite materials based on a small number of specimens and presented a rough estimate of S-N relationships (Hirano, 2001). For materials with a non quasi-isotropic stacking sequence, Gathercole et al. fatigue tested unnotched specimens of a T800/5245 CF/BMI laminate with [(±45, 02) 2]S layup under constant amplitude loading over a wide range of stress ratio R (=minimum stress/maximum stress), and discussed Weibull life-distributions, S-N relationships, and Goodman’s diagram based on an analysis of test results (Gathercole et al., 1994). Following this paper, Adam et al. reported the results of programmed fatigue tests of block variable loading, provided the cumulative damage fraction to failure, and discussed the applicability of a nonlinear cumulative damage rule (Adam et al., 1994). On the other hand, Tyahla and McClellan reported various kinds of test results on the durability and damage tolerance of IM6/3100 and IM6/F650 CF/BMI composite materials, including results of fatigue testing of OH specimens (Tyahla and McClellan, 1988). The layup of OH specimens was [0°(50%)/±45°(40%)/90°(10%)] and fatigue tests were carried out at only two or three stress levels for R=-1 and 10 under cold temperature dry (CTD), room temperature dry (RTD), and elevated temperature wet (ETW) conditions.
Although as described above many reports about the static and fatigue strengths of CF/BMI composite materials have been published, this chapter discusses only test results obtained for specimens with a quasi-isotropic layup, because such test results can be used for the reference or comparison data. Moreover, this chapter focuses on the high-temperature characteristics of the static and fatigue strengths.
For OH static strength at high temperatures, only data at 120°C by the authors and those of OH tensile strength by Johnston and Gates were reported. As for fatigue strength, Hirano only discussed rough S-N relationships of fatigue characteristics at RT as determined by axial tension, compression and tension-compression fatigue tests using a small number of test specimens. Under these circumstances, the authors systematically conducted static tests on NH and OH specimens and fatigue tests on OH specimens for a CF/BMI composite material with a quasi-isotropic layup at RT and high temperatures and discussed test results and the high-temperature practicality of this material (shimokawa et al., 2008).
In this chapter the authors introduce the major contents of their previous paper from a practical viewpoint. The material used was a G40-800/5260 CF/BMI composite material selected from popular CF/BMI composite materials. The objective of this study was to systematically clarify static and fatigue strength at RT and 150°C. In addition, solely static compressive tests on NH specimens were conducted at several high temperatures up to
215°C. Static tests provided static tensile and compressive strengths of NH and OH specimens. Fatigue tests under constant amplitude loading provided S-N relationships for just the OH specimens, i. e., tension, compression, and tension-compression fatigue tests. Visual and CCD microscope observation showed the fracture behavior of static and fatigue failure.
The major subjects of this chapter are as follows. (1) An offer of reference data with respect to the static and fatigue strengths at RT and high temperatures. (2) Fiber dominant and matrix (resin) dominant properties in static and fatigue strengths. (3) Open-hole and temperature dependence on static strength. (4) The dependence of the stress ratio R (R=minimum stress/maximum stress in fatigue loading) and temperature on fatigue strength and S-N relationships. (5) The strength ratio (=compression strength/ tension strength) for static and fatigue strengths. (6) The influence of load components on fatigue strength degradation, i. e., the influence of tension, compression, and tension-compression load cycles. (7) Practicality evaluation of static and fatigue strengths in the marginal high temperature region for long-term use.
