Evaluation on the fracture toughness and strength of fiber reinforced brittle matrix composites

It is well known in the fracture mechanics community that the performance of brittle materials, such as different types of ceramics which have low fracture toughness, improves significantly when fibers are added into the material. This is because the presence of fibers deters the crack propagation. Fibers bridge the gap between two adjacent surfaces of the crack and reduce the crack tip opening displacement, thus make it harder to propagate. Several investigators have experimentally studied how the length, diameter and volume fraction of fibers affect the fracture toughness of fiber reinforced brittle matrix composite materials. However, to this date not much work has been done to develope a micro-mechanics based simplified mathematical model of fiber reinforced composites that can quantitatively explain the increase of the fracture toughness and strength of a composite with volume fraction, length and diameter of fibers, used for strengthening the composite, this is what is attempted in this paper.     

Effect of thermal properties on laser cutting of continuous glass and carbon fiber-reinforced polyamide 6 composites

The single mode fiber laser cutting of two types of continuous fiber-reinforced thermoplastic composites (TPC) with polyamide 6 (PA6) matrix and a fiber content of 60 wt% was investigated: a glass fiber-reinforced composite (PA6/GF60) and a carbon fiber-reinforced composite (PA6/CF60). Preliminary, the thermal properties of the composites were analyzed to derive the cutting mechanisms. Thermogravimetric Analysis (TGA) showed decomposition onset temperatures of 457.6°C for the PA6 matrix and 737.3°C for the carbon fibers, while no thermal decomposition of the glass fibers was detected up to 1,000°C. In consequence, the single mode fiber laser cutting of the PA6/GF60 composites required a significantly higher number of cutting cycles in conjunction with an increased heat affected zone (HAZ) as compared to the PA6/CF60 composites. Analysis of the HAZ and the mechanical properties of the laser cut composites by optical microscopy and uniaxial tensile testing showed an anisotropy and dependency on the laminate structure. However, the obtained mechanical properties of the laser cut composites were found comparable to their water jet cut counterparts.     

Mechanical properties of bio-absorbable PLA/PGA fiber-reinforced composites

This study investigated important mechanical properties, the flexural strength and flexural modulus, of polyglycolic acid (PGA) fiber-reinforced polylactic acid (PLA) composites fabricated by melt-mixing. The flexural strength and flexural modulus were estimated using three-point bending tests conducted at 37°C. Both the flexural strength and flexural modulus were increased by PGA fiber reinforcement. Viscoelastic properties were also investigated using dynamic mechanical analysis (DMA) under tensile loading. Results show that PGA fiber, which acts as the nucleate agent of PLA, restrains the molecular chains of PLA. That restraint reduces deformation at the same stress condition, thereby improving the PLA flexural properties.     

Ultimate strength of composite laminates with free-edge delamination

The present paper deals with the free edge effect of composite laminates by using a generalized quasi-three dimensional analysis and experimental verification of an analysis performed for laminates with Teflon inserted on interfaces to simulate initial free-edge delamination. We performed tensile tests for laminates [30₂/?30₂/90]_s carbon-epoxy laminates with free-edge delamination under uniaxial tension. The experiment reveals that extensional stiffness of the laminate decreases by the initiation of the delamination, and that strength of the laminate without delamination is smaller than that of the laminates with delamination. Generalized quasi-three dimensional finite element technique, which employs energy release rate and maximum stress criteria, is developed to estimate behavior of the laminate after initial delamination. The numerical result by use of this technique predicts the ultimate strength of the laminates with sufficient accuracy according as the comparison with an experimental stress-strain curve. In the experiment conducted both for the laminate with initial delamination and the laminate without initial delamination, an unexpected results were obtained that is the ultimate load of the laminate without initial delamination was lower than that of the laminate with initial delamination. We presented clear explanation on the phenomenon occurred and developed the method to predict the nonlinear behavior of the laminate with or without initial delamination.     

Orthogonal cutting of fiber-reinforced composites: A finite element analysis

Orthogonal cutting of unidirectional fiber-reinforced polymer composites was analyzed using the finite element method. A dual fracture process was used to simulate chip formation incorporating both the maximum stress and Tsai—Hill failure criteria. All aspects of the cutting tool geometry are considered in the model including the tool rake and clearance angles, nose radius and wear land, as well as friction between the tool and work material. Predictions for the cutting forces from numerical simulations are verified with experimental measurements for orthogonal trimming of unidirectional graphite/epoxy. The principal cutting force predictions agree very well with those obtained from experiments. The influence of fiber orientation and tool geometry on the fracture stress are highlighted and their effects on the material removal process in orthogonal trimming of reinforced polymers are discussed.     

Onset of matrix cracking in angle-ply ceramic matrix composites

The problem of initial damage in angle-ply [−θ_m/0_n/θ_m] and [−θ/θ] ceramic matrix composites subjected to axial tension is considered in this paper. The damage is in the form of matrix cracks that may appear in either inclined (−θ and θ lamination angle) or longitudinal layers. As follows from the analysis, if the lamination angle of the inclined layers is small, the initial failure occurs in the 0-layers of [−θ_m/0_n/θ_m] composites or in [−θ/θ] composites in the form of bridging cracks. However, if the inclined layers form a larger angle with the load direction, they fail due to tunneling cracks. It is shown that the boundary between two different modes of failure in a representative SiC/CAS composite corresponds to a lamination angle equal to 35° in the case of [−θ_m/0_n/θ_m] composites. In the case of [−θ/θ] laminates, the boundary value of the lamination angle is equal to 45°, i.e. bridging cracks form if θ<45° and tunneling cracks appear if θ>45°.     

Ultimate strength of dented steel plates under axial compressive loads

In this paper the ultimate strength characteristics of dented steel plates under axial compressive loads are investigated using the ANSYS nonlinear finite element code. The effects of shape, size (depth, diameter), and location of the dent on the ultimate strength behavior of simply supported steel plates under axial thrust are studied. A closed-form formula for predicting the ultimate compressive strength of dented steel plates are empirically derived by curve fitting based on the computed results. The results and insights developed in the present study will be useful for damage tolerant design of steel plated structures with local denting.     

On-line sintering strength of ceramic composites

The sintering of powder-based ceramic composites is often accompanied by developments of cracks or large pores. As a rule, damage development is active only at early stages of sintering, when junctures between particles are not strong enough. A macroscopic model of sintering, taking into account broken junctures between particles is put forward. A new fracture criterion for the prediction of macroscopic damage is proposed. The model follows both densification and damage development during sintering. Modeling enables the optimization of heating regimes and the geometrical structure of ceramic composites.     

Review of machining metal matrix composites

Monotonic fault progression is an important assumption for a number of prognostic models. This assumption can be violated through human intervention and self‐healing and result in non-monotonic degradation data which not only increases the uncertainty but also may cause model failure. Methods to analyze and handle non-monotonic degradation in repairable systems are practically nonexistent in the literature. In this research, we intend to consider repairable systems in which self‐healing is possible and human interventions are desirable. We presented a novel example of self-healing for fatigue cracks analyzed by acoustic emission. The aim of the present paper is to initiate a new research area on using non-monotonic measures in degradation-based prognostics. However, this research is not a review of trend analysis techniques, and therefore, there are more techniques to be considered or developed in future studies. In effect, trend analysis should be considered as an integral part of prognostics and health management. This study considers trend analysis for three classes of data, (1) prognostic parameters, (2) degradation waveform, and (3) multivariate data. A new form of crest factor is introduced for more effective waveform analysis of non-monotonic data. In addition, two algorithms are introduced to treat non-monotonic trend. The prognostic model used in this research does not produce results without treating non-monotonicity. These kinds of algorithm have promising potential to treat non-monotonicity and deal with arbitrary stationary noise in degradation data.     

Tribological behavior of metal matrix composites

The wear and friction behavior of continuous graphite fiber reinforced metal matrix composites was investigated. Composite materials were tested against 4620 steel at 54 m s^?1 at room temperature in air without lubricant. The graphite fibers studied included rayon-, pitch- and polyacrilonitrile (PAN)-based fibers. Both high modulus and high strength PAN-based fibers were examined. The fibers were incorporated into copper- and silver-based alloys by means of a liquid metal infiltration technique. The results of this study indicate that the type of graphite fiber in the composite is the most significant factor in the wear and friction behavior of metal matrix composites. In some high modulus fiber tin-bronze composites the fiber fraction influences the wear rate but not the coefficient of friction. Neither the matrix alloy nor the composite tensile strength per se correlate with the friction and wear properties; however, there are specific trends for the various matrix alloys.     

Drilling of hybrid aluminium matrix composites

This paper presents the influence of cutting parameters on thrust force, surface finish, and burr formation in drilling Al2219/15SiCp and Al2219/15SiCp-3Gr composites. The composites were fabricated using the liquid metallurgy method. The tools used were commercially available carbide and coated carbide drills. The results revealed that feed rate had a major influence on thrust force, surface roughness, and exit burr formation. Graphitic composites exhibit lesser thrust force, burr height, and higher surface roughness when compared to the other material. The reduced thrust force and burr height is attributed to the solid lubricating property of the graphite particles. The higher surface roughness value for Al2219/15SiCp-3Gr composite is due to the pullout of graphite from the surface. The chips formed when machining graphitic composites are more discontinuous when compared to SiCp reinforced composites and hence advantageous.     

Experimental investigations of damage analysis in drilling of woven glass fiber-reinforced plastic composites

This paper presented a new comprehensive approach to select cutting parameters for damage factor in drilling of glass fiber-reinforced polymer (GFRP) composite material. The influence of drilling on surface quality of woven GFRP plastic composite material was investigated experimentally. Drilling tests were carried out using carbide drills of 8 mm in diameter at 50, 70, and 90 m/min cutting speeds and at 0.06, 0.12, and 0.18 mm/rev feed rates. Damage factor was investigated based on hole entrance and exit. Analysis of variance (ANOVA) test was applied to the experimental results. The compared values were employed by Duncan test to identify which groups were significantly different from other groups.     

A statistical approach to torsional strength of brittle solids under hydrostatic pressure

The fracture mechanics concept and statistical treatment are used to explain the effect of hydrostatic pressure on the torsional strength of brittle solids which are supposed to contain numerous flaws. Predictions of the mean values of torsional strength for solid and hollow cylinders of chalk under hydrostatic pressure are made from material properties determined in torsion test at atmospheric pressure. Good agreement is found with experiment.     

Fretting wear of Al-Si alloy matrix composites

Ball-against-disk type fretting wear tests for Al-Si alloy matrix composites in contact with bearing steel were conducted in wet air to investigate the effects of relative slip amplitude on friction and wear of the composites. In the larger range of relative slip amplitude, the Al-Si alloy-impregnated graphite composite (ALGR-MMC) shows lower friction coefficients than those of alumina short fiber-reinforced composite (ASFR-MMC) and hollow silica particle-reinforced composite (HSPR-MMC). Although the wear rate of the ALGR-MMC is higher than that of the ASFR-MMC and HSPR-MMC, the composite hardly causes damage to the mating material due to adhesion of compacted films of graphite powder and Al-Si alloy wear particles.     

Multiscale model of micro curing residual stress evolution in carbon fiber-reinforced thermoset polymer composites

Frontiers of Mechanical Engineering - In this study, the micro curing residual stresses of carbon fiber-reinforced thermoset polymer (CFRP) composites are evaluated using a multiscale modeling...     

A dynamic punch method to quantify the dynamic shear strength of brittle solids

Shear strength is an important material parameter for brittle solids. This parameter has been extensively used in material failure models. Although a few methods have been proposed to quantify this parameter under the static loading condition, there is no such a method available to measure it under dynamic loading conditions. This paper presents a punch shear device to measure the dynamic shear strength of brittle solids. In this method, a split Hopkinson pressure bar system (SHPB) is used to exert the dynamic load to a thin disc sample, which is placed in a specially designed holder to minimize the bending stress induced by punching. The sample holder also allows the punch head to load the sample directly and in combination with momentum-trap technique in SHPB, it enables soft recovery of the rock plug and rock ring produced by the punching test. The flexibility and applicability of this method is demonstrated by the application of an isotropic and fine-grained sandstone. Within the theoretical framework of the classical Mohr-Coulomb failure model, the obtained dynamic shear strengths are consistent with the dynamic tensile strengths for the same rock from the literature.