Kiyoshi Takahashi

Fracture Behavior of Unidirectional Commingled-Yarn-Based Carbon Fiber/Polyamide 6 Composite under Three-Point Bending

Flexure-response and fracture behavior of unidirectional commingled-yarn-based carbon fiber (CF)/polyamide (PA) 6 composite were investigated using three-point edgewise and flatwise bend tests. Difference of the experimental flexure data from that expected was attributed to poorly bonded fiber/matrix interfaces and deviated fiber bundle orientations. For the fracture test, four kinds of notch direction were adopted: edgewise notches parallel (L) and transverse (T) to the major direction of fiber bundles, and flatwise notches parallel (ZT) and perpendicular (ZT) to this direction. Strength and absorbed fracture energy for L and ZL specimens exhibited high sensitivity to notching. ZL specimens exhibited the lowest values of the critical stress intensity factor, K c which were slightly superior to those of unfilled PA6 matrix. T specimens showed the largest values of the strain energy release rate Gc which were mostly occupied by the plastic fracture energy: the large plastic energy coincided with generation of the crushing-mode failure on the compression side. Enlargement of the compression area was analyzed by means of the rigidity reduction resulting from the fracture progression.

Fracture Roughness Evolution During Mode I Dynamic Crack Propagation in Brittle Materials

The evolution of roughness RMS during dynamic fracture in thin plates of epoxy (Araldite B) was studied in relation to important fracture parameters such as crack velocity a, acceleration and deceleration ä and mode I stress intensity factors ( KID ). Dual-focus high speed photography was carried out to evaluate values of KID by the caustic ( KID ( C ) ) and photoelastic ( KID ( P ) ) methods simultaneously. A specially designed jig applied successive tensile loadings to specimens which resulted in cyclic change of a and ä. The changes of a, RMS and KID are shown to be qualitatively similar with respect to the value of a. Quantitatively, a typical result showed that the position of the first maximum value of a came 7 mm behind the first loading axis, while that of KID ( P ), RMS and KID, ( C ) appeared on the fracture surface 7 mm, 15 mm, and 25 mm behind the axis, respectively. The agreement between KID ( C ) and KID ( P ) was acceptable except for data around the first loading axis. Discussions are given on the uniqueness problem of a and KID.

Interlaminar fracture toughness degradation of radiation-damaged GFRP and CFRP composites

Degradation properties of epoxy matrix GFRP and CFRP composites irradiated by high-energy electrons were evaluated by interlaminar fracture toughness measurements in Mode I (GIc) and in mixed mode (Mode I+II, Gmc). The results were then compared with microscopic observations of fracture surfaces and dynamic viscoelastic properties to explain the radiation-induced degradation mechanisms. The sensitivity of radiation-induced degradation was much more pronounced in GIc than in Gmc, or in the interlaminar shear strength. Interlaminar fracture toughness measurements were found necessary for proper evaluation of the radiation-induced degradation of FRP composites. For GFRP, a significant decrease in GIc was found. Debonding of glass fibers and epoxy matrix (or degradation of silane coupling agents) plays an important role in degradation, in addition to resin degradation. Thus, the improvement of the radiation resistance of fiber-resin interfaces as well the matrix itself is of supreme importance in order to inc...

An analysis of mode I interlaminar deformation ahead of crack front in composite DCB specimens

An analytical model was employed to study mode I interlaminar deformation in double cantilever beam specimens of unidirectional carbon fiber/epoxy laminates. The crack opening displacements experimentally determined by moiré interferometry were compared with the model to evaluate the interlaminar tensile modulus and strain distribution ahead of the crack front. The findings showed that the modulus makes an important contribution to increasing the value of crack opening displacement, and that the strain distribution was dependent on both the modulus and thickness of the specimen. Also, a specific correlation was recognized between the modulus and the maximum strain evaluated at the crack tip.

Effects of Crack Velocity and Acceleration on Dynamic Stress Intensity Factor of Epoxy.

Dynamic crack propagation in epoxy specimens was studied using the method of caustics in combination with a Cranz Schardin high speed camera. Single-edge-cracked tensile speciments were fractured under pin-loading conditions so that cracks could undergo acceleration, deceleration and re acceleration stages in one fracture process. Dynamic stress intensity factor KID, crack velocity a and acceleration a were evaluated in the course of crack propagation to examine the effects of a and a on KID, Results showed that a and a strongly affected the values of KID, and for a constant a the decelerating crack had a larger value of KID than the accelerating or re-accelerating crack. Also, it was found that KID could be expressed as two parametric functions of a and a for epoxy specimens.

Measurement of Displacement Fields around Crack Tip in Brittle and Ductile Polymers.

Moire interferometry was utilized to study fracture behavior of polymer specimens under mode I loading. Displacement fields around a crack tip were experimentally determined for two kind specimens of brittle and ductile polymers. The results were compared with the displacements given by linear fracture mechanics or HRR fields to study their applicability to the polymers. Examinations were also made on the effect of material properties and the influence of load applied to the specimens.

Velocity Dependent Dynamic Fracture Toughness of Araldite B Simultaneously Determined by Caustic and Photoelastic Methods

Using the method of caustics and the photoelastic method, dual focus high speed photography was attempted to simultaneously evaluate values of dynamic toughness K ID for Araldite B. Specimen geometries were so chosen that fracture acceleration and deceleration occurred in one specimen to study the dependence of K ID on the crack velocity \( \dot a \) . It is concluded that curves of K ID (\( \dot a \) ) determined by both methods exhibit hysteresis similarly with higher K ID values in the deceleration area than those in the acceleration area.