The crystallization degree in semi-crystalline thermoplastics plays an important role in determining the final properties of structural composite material (e.g. toughness, stiffness and solvent resistance). The main purpose of this work is to study different induced degrees of crystallinity in carbon fiber (CF) reinforced polyphenylene sulfide (PPS) composites, by using three different cooling rates during hot compression molding processing (51%, 58% and 62% of crystallinity). In this study, the morphology, thermal and mechanical properties of the produced laminates were investigated and compared. The results showed an increase in the storage modulus (9.8%), Young's modulus (9.2%) and ILSS (14.2%) for the lower cooling rates. Evidences of fiber/interface improvement and crystallites nucleation on the fiber reinforcement surface were also identified.

International audienceIn this paper, a non unified elasto-viscoplastic behavior model based on internal state variables, is investigated in order to describe the thermo-mechanical stress-strain fatigue response of 55NiCrMoV7 tempered martensitic steels (AISI L6). This model considers a quadratic yield criterion to define the elasticity domain. It allows the determination of two inelastic strain mechanisms resulting from two stress states which can be related to the typical continuous cyclic softening of the tempered martensitic steels. This cyclic softening is reproduced through an isotropic component (drag stress). A memory effect is also introduced to take into account the influence of the plastic strain range on the amount of the cyclic softening. The kinematic component (back stress) of the model allows the description of complex load conditions to which tool steels are subjected. Strain recovery (Baushinger effect), time recovery terms (cyclic behavior including tensile dwell times) and ratcheting effects are considered. The numerical implementation is addressed and two integration methods (explicit and implicit) of the constitutive equations are presented. Moreover, the identification methodology of the model parameters from only two sets of experimental data is presented; the coefficients of the kinematic and isotropic parts are determined successively. The main difficulty consists in reaching a good description both of the cyclic behavior for different strain rates and the ratcheting effect. Last, a validation stage of the three dimensional model is investigated from low cycle fatigue tests performed on different notched specimens

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High temperature low cycle fatigue tests have been carried out on a 55NiCrMoV8 martensitic steel in the temperature range 200° C to 550° C. At all temperatures, the material shows evidence of cyclic softening. Manson-Coffin and Basquin analysis is applied for lifetime analysis. An elasto-plastic kinematic and isotropic hardening model, taking into account the strain path history, was identified using the isothermal LCF tests. Identification methodology is presented and the model validated by comparing experimental and predicted evolution of hysteresis loops during symmetrical and non-symmetrical total strain LCF testing.

To adapt liquid reactive processes to thermoplastic composite manufacturing, synthesis kinetics of PA6 from ε-caprolactam monomer have been investigated. This reaction involves simultaneously polymerization of monomers and crystallization of growing chains. It has been studied by DSC over a wide range of isothermal and non-isothermal conditions, and TGA, DSC and WAXS experiments were performed on polymerized samples. Shape of DSC curves allowed to identify the degree of interaction between polymerization and crystallization. At high temperature or high heating rate, polymerization occurs first and crystallization is delayed because of the low supercooling. When phenomena are fully decoupled, the final properties are close to conventional PA6. For lower conditions, crystallization depends on the availability of chains and polymerization controls the synthesis time. The high mobility of the environment favors the final degree of conversion and the properties of the crystalline phase. At very low temperature, a two-step crystallization is observed, due to a critical molecular weight that is required for nucleation and the degree of conversion could be affected by entrapment of reactive species in crystals.

Tempered martensitic steels were investigated in isothermal and thermomechanical fatigue conditions and general features of their cyclic behaviour are reported. A cyclic anisothermal constitutive model with internal variables was formulated to describe their behaviour; it allows the description of the Baushinger effect, the continuous cyclic softening, the strain rate dependence and the plastic strain memorisation. When compared to experimental results, isothermal and anisothermal model predictions show good coherence. This model is helpful to understand and explain some experimental results such as the increase in strain softening when strain rate decreases in isothermal testing, as well as the relations between mean stress and temperature range or cyclic softening with strain amplitude in anisothermal conditions.

Microstructural evolutions of the 55NiCrMoV7 steel during tempering were investigated by transmission electron microscopy, scanning electron microscopy and X-ray diffraction in order to describe the main mechanisms of softening. The softening resistance is strongly associated with evolution of obstacles to the movement of dislocations (prior austenitic grain boundary, lath boundary, secondary carbides, etc.). Only the average size of carbides was found to be influenced by tempering conditions. Moreover, a strong correlation observed between the hardness measured after tempering and the average size of carbides showing that this easy test could in this case partially characterize the state of the microstructure after tempering. Performing hardness measurements at the as-quenched, tempered and annealed states, a kinetic law of tempering based on the work of Johnson, Mehl and Avrami has been proposed. This law was validated in the case of complex tempering and for other steels and can well describe the evolution of hardness during tempering.

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