Thermal residual stresses in thermoplastic CFRP-steel laminates: Modification and influence on fatigue life

Thermal residual stresses (TRS) in hybrid materials and structures occur by the mismatch of thermal expansion of different materials. Especially when combining metals with carbon fiber reinforced plastics (CFRP), a significant level of internal stresses can be reached. High processing temperatures and high stiffness of the constituents are also responsible for high stress levels. Laminates of thermoplastic CFRP (unidirectional carbon fiber reinforced polyamide 6) and stainless steel foils are a suitable material system to examine the TRS in detail. Since TRSs in the steel fraction are of tensile nature, these superpose to externally applied loads, resulting in higher efforts for the material and thus reduced lifetimes under cyclic fatigue loading. Therefore, a reduction of TRS is desired. Two methods for TRS reduction were applied, and its influence on fatigue lifetime was investigated. Firstly, specimens were stretched by a preloading to reduce TRS by yielding of the metal. Secondly, non-symmetric laminates were gradually cooled down after consolidation to compensate TRS formation by non-symmetric shrinkage. While preloading of materials and structures is known for TRS modification, the gradually cooling is not established, yet. Both modification principles were numerically investigated before experimental validation. A significant increase of lifetime was reached by TRS reduction.

Prediction of the spring-in of cylindrically orthotropic media and cross-ply laminates

The equation for prediction of the spring-in angle of a cylindrically orthotropic segment is shown to be independent of all material properties except for the anisotropic coefficients of thermal expansion and a stress-free state is insured for the corresponding unconstrained deformation. In contrast, the complete cylindrical geometry is shown to provide constraint to thermal deformation and thereby induce thermal residual stresses in the form of a moment. The method of superposition is demonstrated whereby traction-free conditions yield stress-free cylindrical elements with corresponding angular displacements at the element free boundaries. The first derivation of the spring-in equation is attributed to Radford, in contrast to the widely accepted view that the equation was first developed by Spencer et al. Finite-element methods, combined with the superposition approach, further validate the accuracy of the Radford equation for cylindrically orthotropic segments and explore its limitations for multiaxial composite laminates.

A Comparative Study of Mechanical Properties, Thermal Conductivity, Residual Stresses, and Wear Resistance of Aluminum-Alumina Composites Obtained by Squeeze Casting and Powder Metallurgy

Squeeze casting and powder metallurgy techniques were employed to fabricate AlSi12/Al2O3 composites, which are lightweight structural materials with potential applications in the automotive industry. The impact of the processing route on the material properties was studied. Comparative analyses were conducted for the Vickers hardness, flexural strength, fracture toughness, thermal conductivity, thermal residual stresses, and frictional wear. Our results show that the squeeze cast composite exhibits superior properties to those obtained using powder metallurgy.

Effect of residual stresses and cold-straightening on the compressive resistance of solid round steel columns

The objective of this research is to study the effects of residual stresses and cold-straightening on the compressive resistance of solid round steel columns. Thermal residual stresses in selected solid round sizes were determined from experimental study, finite element analysis, and previous research. In the experimental investigation, classical boring-out method using water-jet technology was applied on four samples with different diameters. Finite element models were constructed for the determination of thermal reidual stresses for columns with 12 different diameters. The results were then compared with results obtained from a recent study on the predictionof symmetrical residual stresses in solid rounds using X-ray diffraction method. For the non-symmetrical residual stresses arising from cold-straightening, the equation developed by Nitta and Thurlimann was adopted in the finite element modeling to study the effect of non-symmetrical residual stresses on the compressive resistance of solid round steel columns. The Finite Element Analysis has been conducted on different bar diameter (1.5 inch to 12 inch diameter) and length, as well as initial out-of-straightness.

Effect of residual stresses and cold-straightening on the compressive resistance of solid round steel columns

The objective of this research is to study the effects of residual stresses and cold-straightening on the compressive resistance of solid round steel columns. Thermal residual stresses in selected solid round sizes were determined from experimental study, finite element analysis, and previous research. In the experimental investigation, classical boring-out method using water-jet technology was applied on four samples with different diameters. Finite element models were constructed for the determination of thermal reidual stresses for columns with 12 different diameters. The results were then compared with results obtained from a recent study on the predictionof symmetrical residual stresses in solid rounds using X-ray diffraction method. For the non-symmetrical residual stresses arising from cold-straightening, the equation developed by Nitta and Thurlimann was adopted in the finite element modeling to study the effect of non-symmetrical residual stresses on the compressive resistance of solid round steel columns. The Finite Element Analysis has been conducted on different bar diameter (1.5 inch to 12 inch diameter) and length, as well as initial out-of-straightness.