Keiichiro Tohgo

A Progressive Damage Mechanics in Particle-Reinforced Metal-Matrix Composites Under High Triaxial Tension

The energy approach recently proposed by Qiu and Weng (1992) is introduced to estimate the equivalent stress of the ductile matrix in Tohgo and Chou’s (1991) incremental damage theory for particulate-reinforced composites containing hard particles. In such a composite debonding of the particle-matrix interface is a significant damage process, as the damaged particles have a weakening effect while the intact particles have a reinforcing effect. In Tohgo-Chou’s theory, which describes the elastic-plastic behavior and the damage behavior of particulate-reinforced composites, it was assumed that the debonding damage is controlled by the stress of the particle and the statistical behavior of the particle-matrix interfacial strength, and that the debonded (damaged) particles are regarded as voids, resulting in an increased void concentration with deformation. On the other hand, Qiu-Weng’s energy approach provides a reasonable equivalent stress of the matrix in the porous material and particulate-reinforced composite even under a high triaxiality. The incremental damage theory developed here enables one to calculate the overall stress-strain response and damage evolution of the composite under high triaxial tension. The stress-strain relations for porous material obtained by the present incremental theory are completely consistent with that obtained by Qiu and Weng. The influence of the debonding damage on the stress-strain response is demonstrated for particulate-reinforced composites.

Temperature dependence of sliding wear behavior in SiC whisker or SiC particulate reinforced 6061 aluminum alloy composite

Abstract The effects of applied load and temperature on the dry sliding wear behavior of 6061 aluminum alloy matrix composites reinforced with SiC whiskers or SiC particulates were investigated using a hardened steel ball and a block of the composite material. The wear rate decreased as the applied load increased. However, there was a critical load above which the wear rate increased rapidly as the applied load increased. Transitions in wear mechanisms occur below and above this load. From both the wear rate and the critical applied load, it was confirmed that the wear resistance of the 6061 aluminum alloy was enhanced by reinforcing with SiC whiskers or SiC particles, both at room and elevated temperatures. However, the temperature dependence of the critical applied load was almost the same for both 6061 A1 and the composites, and the critical load decreased with temperature.

Deformation Behavior of NiTi/Polymer Shape Memory Alloy Composites – Experimental Verifications

Composites containing NiTi shape memory alloy (SMA) long-fiber, short-fibers or Ti long-fiber in a Polycarbonate (PC) matrix have been fabricated by the injection molding technique. Also, prestrained SMA long-fiber/Epoxy matrix composites have been fabricated. The fracture behavior and thermo-mechanical deformation behavior are examined; (1) Fracture behavior – uniaxial tensile tests up to fracture for SMA long-fiber and short-fiber composite (SMAC). (2) Thermomechanical deformation behavior – tensile loading–unloading tests for Pseudoelastic (PE) long-fiber/PC matrix composites. Several thermo-mechanical loading tests for Shape Memory Effect (SME) long-fiber/PC matrix and SME long-fiber/Epoxy matrix composites were used. The obtained results are as follows: (1) The stress–strain relation up to the final fracture of the Shape Memory Alloy Composites (SMACs) showed the repeated up-and-down of the stress which corresponds to the necking of the specimen, fiber fracture, and matrix fracture. The strain for the initiation of necking and the strain for the fiber or matrix fracture in the SMACs were higher than those in the Ti composite. This is attributed to the unique stress–strain relations accompanied by the stress-induced martensitic transformation of the SMA fibers. (2) The SMAC containing PE fiber and PC exhibited the pseudoelastic-like deformation under tensile loading–unloading. (3) The SMAC containing SME fiber and PC exhibited the large contraction by heating after tensile loading–unloading, but the compressive residual stress in the matrix expected in this process was not remarkable. However, compressive residual stress in the matrix may become greater by embedding prestrained fiber in the matrix.

Relationship between ductile crack initiation and void volume fraction

Abstract An investigation on ductile crack initiation in structural steel has been made, based on the concept of Gursons yield function for porous material. First, the condition of ductile crack initiation in the uniform stress field has been investigated. The condition of ductile crack initiation under various stress triaxiality obtained from the tests on axisymmetric notched tensile specimens is well expressed by the condition of constant void volume fraction analytically obtained from Gursons model. This result means that the condition of constant void volume fraction may be used as the criterion of ductile crack initiation. Secondly, the behavior of void growth and ductile crack initiation in the area near the notch tip under mode I and mode II loading has been investigated. Under mode I loading, the increase in void volume fraction around the notch with an increase in applied load agrees well with the behavior of porous material predicted by the finite element analysis based on Gursons yield function, and the ductile crack initiation can be predicted by the concept of critical void volume fraction as in the case of uniform stress-strain field given above. The same criterion is not applicable to the crack initiation under mode II loading and further study is needed.

Fatigue Crack Initiation and Growth under Mixed Mode Loading in Aluminum Alloys 2017-T3 and 7075-T6

Abstract Fatigue crack initiation and growth characteristics under mixed mode loading have been investigated on aluminum alloys 2017-T3 and 7075-T6, using a newly developed apparatus for mixed mode loading tests. In 2017-T3, the fatigue crack initiation and growth characteristics from a precrack under mixed mode loading are divided into three regions—shear mode growth, tensile mode growth and no growth—on the ΔK I - ΔK II plane. The shear mode growth is observed in the region expressed approximately by ΔK II > 3MPa√m and ΔK II / ΔK I > 1.6. In 7075-T6, the condition of shear mode crack initiation is expressed by ΔK II > 8 MPa√m and ΔK II / ΔK I > 1.6, and continuous crack growth in shear mode is observed only in the case of ΔK I / ΔK II , ≅ 0. The threshold condition of fatigue crack growth in tensile mode is described by the maximum tensile stress criterion, which is given by Δσ θmax √2π r ≅ 1.6MPa√m, in both aluminum alloys. The direction of shear mode crack growth approaches the plane in which K I decreases and K II increases towards the maximum with crack growth. da / dN - ΔK II relations of the curved cracks growing in shear mode under mixed mode loading agree well with the da / dN - ΔK II relation of a straight crack under pure mode II loading.

Load carrying capacity of a broken ellipsoidal inhomogeneity

In particle or short-fiber reinforced composites, cracking of the reinforcements is a significant damage mode because the broken reinforcements lose load carrying capacity. This paper deals with elastic stress distribution and load carrying capacity of intact and broken ellipsoidal inhomogeneities. Axisymmetric and three dimensional finite element analyses have been carried out on intact and broken ellipsoidal inhomogeneities in an infinite body under uniaxial tension and under pure shear. For the intact inhomogeneity, as well known as Eshelbys solution [18], the stress distribution is uniform in the inhomogeneity and nonuniform in the surrounding matrix. On the other hand, for the broken inhomogeneity, the stress in the region near the crack surface is considerably released and the stress distribution becomes more complex. The average stress in the inhomogeneity represents its load carrying capacity, and the difference between the average stresses of the intact and broken inhomogeneities indicates the loss of load carrying capacity due to cracking damage. The load carrying capacity of the broken inhomogeneity is expressed in terms of the average stress of the intact inhomogeneity and some coefficients. The coefficients are given as functions of an aspect ratio for a variety of combinations of the elastic moduli of inhomogeneity and matrix. It is found that a broken inhomogeneity with high aspect ratio maintains higher load carrying capacity than one with low aspect ratio.

A criterion for splitting crack initiation in unidirectional fiber-reinforced composites

Unidirectional fiber-remforced composites exhibit a high degree of anisot ropy in strength. Crack initiation from a pre-existing crack almost always occurs along the fiber direction; it is known as the splitting crack. One criterion for the splitting crack initi ation under mixed mode condition is proposed. In this criterion, the splitting crack initia tion is characterized by a unique interaction curve on the Kf σ-Kf τ plane, where Kf σ and K f τ are the tensile and shear stress intensity factors along the fiber direction. Fracture toughness tests under mixed mode loading are systematically carried out on two graphite-epoxy systems: AS4/3501-6 and IM7/85517. For both composites, the condi tion of the splitting crack initiation can be characterized by a vertical line of Kf σ = con stant on the K f σ-Kf τ plane. This means that the tensile stress intensity factor along the fiber direction is responsible for the splitting crack initiation. Furthermore, it is found that under the conditions of Kf σ = 0 or Kf σ < 0, damage occurs at locations away from the tip of the pre-existing crack before crack initiation and that the fracture toughness under such conditions is high.

The influence of debonding damage on fracture toughness and crack-tip field in glass-particle-reinforced Nylon 66 composites

This paper deals with the influence of the debonding damage between particles and matrix on fracture toughness and crack-tip field of particle-reinforced composites. Tensile strength and fracture toughness were examined on seven kinds of glass-particle-reinforced Nylon 66 composites in which the volume fraction of glass particles and the interface treatment between particles and matrix were changed. Although the main damage mode is the debonding damage between particles and matrix in both interface-treated and untreated composites, in the interface-treated composite the debonding damage is hard to occur because of high interfacial strength. The interface-treated composites are superior in tensile strength, and inferior in fracture toughness to the interface-untreated composite. In order to explain the influence of the debonding damage on the fracture toughness, numerical analysis of a crack-tip field was carried out on both composites by using a finite-element method which was developed on the basis of an incremental damage theory of particle-reinforced composites. The damage development around a crack-tip depends on the interfacial strength between particle and matrix and the particle volume fraction. It is found that the debonding damage reduces the stress level around the crack-tip and acts as the toughening mechanism. The mechanical performance of particle-reinforced composites is obtained as the results of the competitive effects of the intact hard particles and the debonding damage.

Incremental damage theory and its application to glass-particle-reinforced nylon 66 composites

This paper deals with influence of particle volume fraction and debonding damage between particles and matrix on the stress-strain response in particle-reinforced ductile matrix composites. Tensile tests are carried out on seven kinds of glass-particle-reinforced nylon 66 composites, which are different in a particle volume fraction and treatment of interface between the particles and matrix. The stress-strain response of the composites depends on both the particle volume fraction and the interface treatment. Youngs modulus and Poissons ratio are characterized by only the particle volume fraction, while tensile strength depends on both the particle volume fraction and interface treatment. With increasing particle volume fraction, the tensile strength increases first and then becomes constant in the interface-treated composites, and decreases in the interface-untreated composites. Numerical analyses of the stress-strain response and damage behavior of the composites are carried out based on an incremental damage theory which describes the plasticity of the matrix and the debonding damage. The stress-strain relations of the interface treated composites are characterized only by influence of particle volume fraction while those of the interface-untreated composites are explained by considering the particle volume fraction and interfacial debonding.

Fatigue Behavior of Unidirectional Jute Spun Yarn Reinforced PLA

Abstract Natural fiber reinforced composites, which can be carbon-neutral materials, have been investigated for use as alternative materials to glass-fiber reinforced plastics (GFRP). The fatigue properties of natural fiber reinforced plastics are, however, not well known. In this study, uniaxial tensile fatigue tests of unidirectional jute spun yarn reinforced polylactic acid (PLA) were conducted in order to clarify the fatigue strength. The damage and fracture morphology of composite specimens were observed to elucidate the fatigue mechanism. Results show that the fatigue strength decreases concomitantly with increasing number of cycles. The fatigue strength at 106 cycles was 55% of the ultimate strength, which is an almost identical percentage to that of GFRP. The fatigue failure of composite specimens was probably caused by the breakage of jute filaments at the tips of fatigue cracks in PLA. This implies that the fatigue strength of the composite was strongly affected by the fatigue properties of PLA.