Takeshi Kunio

Role of Matrix Resin on Fracture Strengths of Unidirectional CFRP

As a common feature of composite materials, prominent anisotropy in mechanical prop erties is observed in unidirectional CFRP, which has higher fracture strength and stiffness along the carbon fiber strengthening component. Since it is well known that a resin matrix exhibits characteristic time and temperature dependence on mechanical behavior, that is, viscoelastic behavior, the CFRP is expected to exhibit similar behavior.

An effect of the second phase morphology on the tensile fracture characteristics of carbon steels

Abstract The fracture behavior was studied in the two phase alloy with martensitk and ferritic structure. The variation of the phase connectivity of martensitic structure makes a considerable difference in ductility of the bulk materials at room temperature. The incipient crack in the connected martensitk structure triggers cleavage cracks in the ferrite grains, while the extension of the crack in the unconnected martensitic structure is impeded by the extensive plastic flows in the ferrite. Such a marked difference in the fracture behavior can be explained from the interpretation of slip characters within the ferrite grains.

Viscoelastic analysis of residual stress in quenched thermosetting resin beams

The residual stress generated by the molding process of thermosetting resins exerts serious influences upon their mechanical properties. This residual stress is generally classified by two groups: one produced by shrinkage in the curing reaction of monomers, the other produced by the nonuniformity of the temperature distribution in the cooling process.This paper is concerned with the theoretical and experimental analysis of the generation of residual stress of the latter type, using examples of rectangular beams of thermosetting resins quenched on both the upper and lower surfaces.First, a viscoelastic model is applied to make a qualitative prediction of the residual stress in quenched beams.Second, using linear-viscoelastic theory, fundamental equations are derived for the residual stress in a viscoelastic rectangular beam, where an unsteady and nonuniform temperature distribution is assumed in the direction of depth. The theoretical values of the residual stress in rectangular beams are calculated under various quenching conditions for two resins having different viscoelastic characteristics, i.e., epoxy and unsaturated polyester.The theoretical residual-stress distributions agree fairly well with the residual stress measured experimentally at every quenching condition for both resins. The qualitative prediction that the residual stress in quenched beams is compressive in the vicinity of the upper and lower surfaces and is tensile in the inner parts is confirmed. The relaxation modulus of epoxy resin changes more greatly with time and temperature than that of unsaturated polyester resin. The theoretical and experimental analysis shows that the residual stress for the former resin is larger than that for the latter. Therefore, it is concluded that the generation of residual stress is more significant where the relaxation modulus of resin changes greatly with time and temperature.

A feasibility study of scattered-light photoelasticity in the determination of the side pressure distribution of the pressed powder bed

Abstract A method for the determination of the side pressure distribution of the pressed powder bed by the scattered-light photoelastic technique has been proposed. A cylindrical container made of photoelastic-sensitive material was used as a die, the powder in which was pressed uniaxially. The stress distribution in the wall of the container was measured by the scattered-light photoelastic method. The stress distribution at the inner surface of the container wall was obtained by extrapolation, and the pressure components of the powder bed at the powder—container interface were evaluated from the stress. The existence of a maximum of the side pressure near the surface of the powder bed is suggested.

Photoviscoelastic analysis by use of polyurethane rubber

Photoviscoelastic materials such as polyurethane rubber have a significant time- and temperature dependence mechanically as well as optically. Therefore, in order to perform the practical optical analysis of viscoelastic stress or strain, mechanical and optical characterizations of the materials are required. In other words, it is necessary to determine the time- and temperature dependent visco- and photoviscoelastic coefficients.

Residual stresses and warp generated by one-sided quench of an epoxy-resin beam

This paper discusses the warp and accompanying residual stress in a rectangular epoxy beam produced by water cooling its lower surface. First, the theoretical values of this warp and residual stress are obtained by the linear-viscoelastic theory. The specimen is then subjected to quenching. The variations in the warp are observed. After quenching, the residual stress is measured by a layer-removal method. The experimental and theoretical results are then compared and discussed.

INFLUENCE OF MATRIX RESIN ON ULTIMATE STRENGTH OF UNIDIRECTIONAL CFRP

ABSTRACT The time and temperature dependent strengths, that is, the longitudinal tensile and compressive strengths, the transverse tensile and compressive strengths and the interlaminar shear strength, of unidirectional CFRP are investigated in connection with epoxy resin of its matrix. As results, these strengths show thermo-rheologically simple behavior in the same manner as the tensile strength of epoxy resin used as the matrix of CFRP. A method for evaluation of the strengths of CFRP at any strain rate and temperature is proposed, in which the shift factor of matrix resin and the strengths of CFRP at several temperatures under a constant strain rate are used.

Photoviscoelastic analysis of thermal stress in a quenched epoxy beam

This paper reports on a procedure for photoviscoelastic analysis where the axes of principal stress, principal strain and polarization of light coincide in the presence of a large temperature change. More specially, the transient-thermal stress and strain due to stress in an epoxy beam subjected to quenching from both the upper and lower surfaces, are determined using the time-temperature-equivalent law for stress, strain and birefringence. The transient-thermal stress and strain in the beam were determined experimentally using hereditary integrations from the measurement of the transient temperature and birefringence due to the quenching of the beam. The transient thermal stress and strain were also calculated theoretically using the linear-viscoelastic theory. The experimentally determined thermal stress agrees closely with the theoretical results. The experimentally determined strain agrees qualitatively with the theoretical values. Thus, it is concluded that the photoviscoelastic technique is useful in analyzing the proposed problem.

Effect of local fibre distribution ahead of crack on the fracture toughness of FRP

A study has been made of the effect of a local fibre distribution in the vicinity of a crack on the fracture toughness Gc of glass fibre reinforced epoxy resins, which have ten kinds of fibre volume fraction including very low fraction, by means of fracture mechanics and fractography. The results obtained are summarized as follows: 1. (1) There exists a critical fibre volume fraction Vfc; below Vfc, the strengthening effect due to fibre does not affect the fracture toughness of the material. The fracture toughness of the FRP having Vf larger than Vfc increases linearly with Vf on the log-log diagram. 2. (2) A phenomenon of local pop-in at a pre-crack front precedes a global advancement of a crack front. The local pop-in occurs preferentially at some particular locations along the pre-crack front where the fibre spacing is locally wide. 3. (3) The fracture toughness of FRP depends not only on the macroscopic fibre volume fraction Vf in the material but also on the local volume fraction VfL and on the local fibre spacing Dave in the vicinity of the pre-crack front. Both increase of the VfL and decrease of the Dave have the same effect of increasing the fracture toughness of FRP. 4. (4) The scatter of fracture toughness values in a conventional fracture toughness test can be well explained by considering the effect of VfL and Dave upon the fracture toughness.

Fundamentals Of Photoviscoelastic Technique For Analysis Of Time And Temperature Dependent Stress And Strain

Polymeric materials show remarkable time and temperature dependent viscoelastic behaviors, which are quite different from elastic ones, under certain conditions, for example, at the temperature ranging from the glassy state to the rubbery. Although photoelasticity is an useful experimental method for the analysis of elastic stress, it is not applicable for analysis of time and temperature dependent viscoelastic stress and strain.