No abstract

Adhesive bonding of structural aircraft materials depends upon a thorough understanding of factors that affect bond strength and durability. It is well known that surface preparation is critical to adhesive bond performance. However, little information is available regarding the effect of surface preparation procedures on adhesive bonding of high-temperature polyimide composites. Mechanical treatments such as hand sanding are commonly used for composite materials, and in general are known to improve the wettability of the surface as well as effect contaminant removal. Less is known about the influence of mechanical treatments on the surface chemistry. This article discusses the effects of mechanical surface preparation on the adhesive bond strength, durability, and failure mode of several high-temperature polyimide-based composites and adhesives.

Impact damage in laminates of four different composite systems was investigated. These composite systems included two thermoplastic-based composites (PEEK and Torlon) and two epoxy-based composites (toughened and brittle BMIs). At low impact ve locities, PEEK (polyetheretherketone) and Torlon composites showed significantly less matrix cracking and delamination and greater strength retention capability than the two epoxy-based composites. However, beyond a certain threshold velocity, the thermoplastic composites experienced a sudden drop in flexural strength, more so than the epoxy-based composites.Between the two epoxy-based systems, the toughened BMI composite exhibited much better impact resistance properties.

Load-frequency effect on fatigue life of a thermoplastic and toughened BMI composite was investigated. Experimental results were obtained using three frequencies at two load levels with three types of laminates, each containing a center hole. Besides fatigue life, the temperature history near the hole and the specimen stiffness were monitored. In addition, the hysteresis loop was recorded at various stages of the fatigue test to observe the variation in viscoelastic property of the specimen. From the fatigue test results, it was found that fatigue life of matrix-dominated thermoplastic composite laminates decreased as load frequency increased. This behavior is opposite to that of epoxy-based fiber composites. For fiber-dominated thermoplastic laminates under tension-compression load, very low load-frequencies (less than 0.4 Hz) could significantly lower their fatigue life. Analytical models were developed to account for the load-frequency effect on fatigue life of thermoplastic composites. Temperature rise near the hole was included in the model to account for temperature effect.

No abstract