Short communication Refinement and characterisation of a thermodynamically stable β-[Ni(Al,Ti)]–β′-[Ni2 AlTi]–γ′-[Ni3 (Al,Ti)] metal–metal composite

Abstract

The Ni–Al–Ti system contains three intermetallic phases, γ′, β, and β′ which have complementary properties. The objective of the present investigation was to coextrude these three phases to form a metal–metal composite (MeMeC) and investigate the mechanical properties and failure mechanisms of such a material. High strengths, comparable with that of the strongest constituent phase, have been recorded for the MeMeC material. Strength is maintained at a constant level up to 500°C above which it falls progressively. The compressive strain to failure is considerably greater that that expected for monolithic β or β ′ phases. It has been shown that the interfaces between nominally single phase regions provide lines of weakness along which cracks propagate during compressive loading. Preliminary attempts to strengthen the interfaces by a 24 h heat treatment at 900°C have been carried out.     

Simultaneous sensing of the strain and points of failure in composite beams with an embedded fiber optic Michelson sensor

A fiber optic Michelson sensor was embedded in composite beams to sense the internal strain and points of failure of the composite structures. The bending deformation and matrix cracking were investigated by four-point bending tests of cross-ply composite beams with the embedded fiber optic sensor. The failure points of composite beams were detected by using both a PZT sensor and a fiber optic sensor in order to investigate the fiber optic failure signals. The failure due to matrix cracks in a composite beam was confirmed by the edge replica method. The digital processing of the fiber optic signal was carried out to determine the strains and failure points of composite beams. The failure points were observed from the processed failure signal by high-pass filtering. The initial failure strain of the composite beam was measured and processed from the fiber optic strain signal after low-pass filtering.     

On the tension test as a means of characterizing fibre composite failure mode

A model of composite tensile failure, which has been utilized previously in analysing modes of composite fracture [1], is extended to describe fracture in systems not considered previously. This model, based on tensile testing with a stiff, elongation rate controlled machine, predicts a strain concentration in the vicinity of a fibre break during such testing. The magnitude of the stress increase associated with this strain concentration is analytically predicted and compared with experimental data. The existence of this strain concentration can lead to phenomenologically different modes of composite failure. The type of failure observed depends upon the properties of the composite constituents as well as the composite parameters, fibre volume fraction (V _f), number of fibres in the composite (N) and ratio of specimen length to fibre ineffective length (L).     

Fatigue crack nucleation and growth in filled natural rubber

Rubber components subjected to fluctuating loads often fail due to nucleation and the growth of defects or cracks. The prevention of such failures depends upon an understanding of the mechanics underlying the failure process. This investigation explores the nucleation and growth of cracks in filled natural rubber. Both fatigue macro‐crack nucleation as well as fatigue crack growth experiments were conducted using simple tension and planar tension specimens, respectively. Crack nucleation as well as crack growth life prediction analysis approaches were used to correlate the experimental data. Several aspects of the fatigue process, such as failure mode and the effects of R ratio (minimum strain) on fatigue life, are also discussed. It is shown that a small positive R ratio can have a significant beneficial effect on fatigue life and crack growth rate, particularly at low strain range.     

FE analysis of the behaviour of octagonal bonded composite repair in aircraft structures

Bonded composite repair has been recognized as an efficient and economical method to extend the fatigue life of cracked aluminium components. In this work, the finite element method is applied to analyze the central crack’s behaviour repaired by a boron/epoxy composite patch. The knowledge of the stress distribution in the neighbourhood of cracks has an importance for the analysis of their repair according to the patch geometry. The effects of mechanical and geometrical properties of the patch on the variation of the stress intensity factor at the crack tip were highlighted. The obtained results show that the stress intensity factor at the repaired crack with composite patch of height 2c/3 is reduced about 5% compared to cracks repaired with octagonal patch of size c. For patch height of c/3 the reduction is about 7%. The adhesive properties must be optimised in order to increase the repair performances and to avoid the adhesive failure.     

A model for the toughness of epoxy-rubber particulate composites

Epoxy resins are toughened significantly by a dispersion of rubber precipitates. Microscopic examinations of propagating cracks in epoxy-rubber composites reveal that the brittle epoxy matrix cracks, leaving ligaments of rubber attached to the two crack surfaces. The rubber particles are stretched as the crack opens and fail by tearing at large, critical extensions. This fracture mechanism is the basis of a new analytical model for toughening. An increase in toughness (G _IC) of the composite is identified with the amount of elastic energy stored in the rubber during stretching which is dissipated irreversibly (e.g. as heat) when the particles fail. The model predicts the failure strain of the particles in terms of their size. It also relates the toughness increase to the volume fraction and tearing energy of the rubber particles. Direct measurements of the tearing strains of rubber particles, and toughness data obtained from epoxy-rubber composites, are in good agreement with the model. The particle-stretching model provides a quantitative explanation, in contribution to existing qualitative theories, for the toughening of epoxy-rubber composites.     

复合材料疲劳寿命预测

在疲劳载荷作用下,复合材料的弹性模量会随着载荷循环数的增加而不断下降,而材料中的内部损伤则不断增大。为此,本文提出复合材料的疲劳模量和累积应变的概念,并由此定义出三种预测复合材料疲劳寿命的疲劳损伤模型。文中应用这三种模型对单应力水平和多应力水平下的玻璃纤维增强环氧树脂复合材料的疲劳寿命进行了估算,并同实验结果进行了比较。     

The effect of the interaction of cracks in orthotropic layered materials under compressive loading

The non-classical problem of fracture mechanics of composites compressed along the layers with interfacial cracks is analysed. The statement of the problem is based on the model of piecewise homogeneous medium, the most accurate within the framework of the mechanics of deformable bodies as applied to composites. The condition of plane strain state is examined. The layers are modelled by a transversally isotropic material (a matrix reinforced by continuous parallel fibres). The frictionless Hertzian contact of the crack faces is considered. The complex fracture mechanics problem is solved using the finite-element analysis. The shear mode of stability loss is studied. The results are obtained for the typical dispositions of cracks. It was found that the interacting crack faces, the crack length and the mutual position of cracks influence the critical strain in the composite.     

Symmetric Edge Cracks in an Orthotropic Strip Under Normal Loading

The problem of two edge cracks of finite length, situated symmetrically in an orthotropic infinite strip of finite thickness 2 h, under normal point loading has been discussed. The displacements and stresses in plane strain conditions are expressed in terms of two harmonic functions. The problem is addressed by seeking the solution of a pair of simultaneous integral equations with Cauchy type singularities solved by finite Hilbert Transform technique. For large h, analytical expression for the stress intensity factor at the crack tip is obtained.     

Failure behavior investigation of a unidirectional carbon–carbon composite

The failure behavior and morphology of a carbon–carbon composite (C–C composite) manufactured by isothermal chemical vapor infiltration was studied by three-point bending tests, polarized light microscope and scanning electron microscope, respectively. The C–C composite was reinforced by PAN-based carbon fiber aligned in only one direction. Flexural strength and modulus of the composite were 200.9 MPa and 50.5 GPa, respectively. Failure behavior of the unidirectional C–C composite can be described as three stages including brittle fracture behavior at beginning, quasi-ductile behavior finally, and fluctuation behavior between them. Two main kinds of cracks, namely cracks parallel and perpendicular to loading direction alternately resulted in deformation evolution of the composite. The strength of interfacial bonding and cracks orientation played key roles to failure behavior of C–C composite.     

A theoretical approach for the assessment of intralaminar fracture mode and toughness of multilayered composites

Analytical evaluation of fracture toughness of a multilayered composite laminate was well-established using modified crack closure integral (MCCI) approach based on test data on the failure load. For this purpose the crack initiation direction, which is treated as a branch crack direction for the theoretical prediction, is required. The crack initiation direction in a multilayered composite laminate depends on mode of failure. In the present work, a fracture parameter n ^* is introduced to predict the mode of failure in multilayered composite having a crack and is validated. Analytical relationship for the prediction of fracture toughness of multilayered composite between a base laminate and its constituent sublaminates is also arrived at. With available test data on the toughness of a set of sub-laminates, toughness of base laminate is determined and validated. The present approach is useful in evaluating the load carrying capability of composite structures with defects in the form of cracks and this information is valuable for design.     

Thermal stresses in a laminate composite with infinite row of parallel cracks normal to the interfaces

Under the assumption of plane strain, some linear thermoelastic problems concerning a laminate composite containing an infinite row of parallel cracks situated normal to the bond lines are solved by the use of integral transform technique. The thermal stresses are caused by a uniform heat flow disturbed by the presence of the cracks and the interfaces. The problem is reduced to that of solving a Fredholm integral equation of the second kind which is solved numerically by the use of Gaussian quadrature formulas. The effect of various quantities of physical interest on the stress intensity factor is shown graphically.Calculations are also carried out for the stress intensity factor for a laminate composite with a crack normal to the interfaces. This is accomplished by taking the limiting case of a laminate composite with an infinite row of parallel cracks normal to the interfaces.     

Characterising the Correlations of Failure Events: A 2‐D Block‐and‐Springs Model

Abstract: To mimic observations from acoustic emission experiments for random systems, we used a block‐and‐springs model to investigate the effect that increasing strain has on the locations of microscopic failure events leading to macroscopic failure across the sample. Model results show that failure events, which are initially located randomly throughout the sample, begin to cluster as stress build‐up near earlier failure events. At failure, the system‐wide fracture network was found to have a fractal dimension, D_f ≈ 1.29. To quantify the observed clustering, we applied a number of different measures of this space‐time behaviour: (i) the stress–strain curve; (ii) the total number of broken bonds and the average energy released by the broken bonds, (iii) the number distribution of cracks with s broken bonds, N(s), and the number distribution of cracks with s broken bonds or more, N(≥s), both of which follow power‐laws agreeing with earlier predictions; and (iv) the number–number and energy–energy correlations at time t between a failure event at position (x′, y ′) and a failure event at (x′ + x, y ′ + y). Our results quantify the short‐range clustering, exhibiting quantitatively and qualitatively different behaviour from the long‐range clustering at failure; our results also show that the energy released outpaces the number of broken bonds.     

A microdebonding study of the high-temperature oxidation embrittlement of a cross-ply glass-ceramic/SiC composite

A constant cross-section specimen with adhesively bonded tabs has been used for an investigation of the high-temperature tensile behavior of a cross-plied glass-ceramic-matrix composite consisting of CAS-II reinforced with Nicalon SiC fiber. Oxidation of the exposed interfaces along matrix cracks in 0 ° plies lowers the composite failure strain at 800 °C to the 0 ° ply matrix-cracking strain. Scanning electron microscopy and microdebonding analysis of the fracture surfaces indicate that the embrittlement process is the result of oxidation of the carbon-rich interphase as the matrix crack encounters 0 ° ply fibers, the interphase subsequently fuses with a higher bond strength and the crack grows through the fibers. Planar cracks grow inwards from the surface, covering the entire fracture surface given enough time (or sufficient strain). Degradation of the fibers does not appear to contribute to the embrittlement. Transverse plies crack at a lower strain than does the matrix in the 0 ° plies. However, it appears that oxygen does not enter 90 ° ply cracks in sufficient quantity to produce oxidation embrittlement, at least up to the 0 ° matrix-cracking strain. The strain to crack the 90 ° plies does not decrease significantly at high temperatures despite the fact that the cracks are primarily in the fiber/matrix interphase as they grow across the 90 ° plies.     

Fracture criterion for kinking cracks in a tri-material adhesively bonded joint under mixed mode loading

The fracture behavior of a composite/adhesive/steel bonded joint was investigated by using double cantilever beam specimens. A starter crack is embedded at the steel/adhesive interface by inserting Teflon tape. The composite adherend is a random carbon fiber reinforced vinyl ester resin composite while the other adherend is cold rolled steel. The adhesive is a one-part epoxy that is heat cured. The Fernlund-Spelt mixed mode loading fixture was employed to generate five different mode mixities. Due to the dissimilar adherends, crack turning into the adhesive (or crack kinking) associated with joint failure, was observed. The bulk fracture toughness of the adhesive was measured separately by using standard compact tension specimens. The strain energy release rates for kinking cracks at the critical loads were calculated by a commercial finite element analysis software ABAQUS in conjunction with the virtual crack closure technique. Two fracture criteria related to strain energy release rates were examined. These are (1) maximum energy release rate criterion (G_max) and, (2) mode I facture criterion (G_II = 0). They are shown to be equivalent in this study. That is, crack kinking takes place at the angle close to maximum G or G_I (also minimum G_II, with a value that is approximately zero). The average value of G_IC obtained from bulk adhesive tests using compact tension specimens is shown to be an accurate indicator of the mode I fracture toughness of the kinking cracks within the adhesive layer. It is concluded that the crack in tri-material adhesively bonded joint tends to initiate into the adhesive along a path that promotes failure in pure mode I, locally.     

Two bonded strips containing an infinite row of interface cracks

The plane strain problem of two bonded dissimilar isotropic elastic strips is considered. It is assumed that the composite strip contains an infinite row of interface cracks located symmetrically on the centerline. The case in which each of the cracks is opened out by the same constant pressure is discussed in detail and numerical results are reported for quantities of practical interest.     

The thermo-mechanical behaviour of Cu-Cr in-situ composite

The mechanical response of in-situ copper-chromium composite was modelled using a deformation mechanism map approach. The stresses in each phase were predicted as a function of temperature and strain rate. From this the ratio of phase stresses, and hence the degree of load transfer, was obtained. The extent of load transfer and the predicted deformation modes of the two phases were then related to the expected failure mechanisms of the composite. Three modes of composite failure were predicted. Copper-chromium in-situ composites, and pure copper, were produced by a casting and swaging route. The mechanical properties were then characterised experimentally by tensile testing. Mechanical tests confirmed that the composite showed significantly higher strength than the unreinforced material. The observed failure modes of the composite were compared with the predictions.     

Probabilistic initial failure strength of hybrid and non-hybrid laminates

This paper deals with the initial failure of unidirectional hybrid laminates and [±θ/90]_s non-hybrid laminates. The initial failure strength or strain at the failure of low elongation layers is analysed by the statistical approach based upon the weakest link model. First ply failure is adopted as a criterion for the initial failure of the laminate. An expression for determining the first ply failure strength has been derived and this expression can be reduced to a volumetric relation as a special case. It is also shown that the initial failure strength or strain is greater in composites composed of low elongation and high elongation materials than in the pure low elongation composite. This is the result of the “size effect”, that is the failure probability is lower in the composite with the smaller size of low elongation material. A good agreement between the theoretical and experimental results is achieved. On leave from the Institute of Interdisciplinary Research, The University of Tokyo, Japan.     

Fracture strength assessment and aging signs detection in human cortical bone using an X-FEM multiple scale approach

We present a multiple scale approach for modeling multiple crack growth in human cortical bone under tension. The Haversian microstructure, a four phase composite, is discretized by a classical finite element method fed with the morphological and mechanical characteristics, experimentally measured, to mimic human bone heterogeneity at the micro scale. The fracture strength of human bone, exhibiting aging signs, is investigated through tensional percolation simulations in statistical microstructures. The cracks are initiated at the micro scale at locations where a critical elastic-damage strain-driven criterion is met. The cracks, modeled by the eXtended Finite Element Method, are then grown until complete failure when a critical stress intensity factor criterion is attained. The model provides the fracture strength and the global response at the material scale and the stress–strain fields at the microscopic level. The model creates a constitutive law at the material scale and emphasizes the influence of the microstructure on bone failure and fracture risk assessment. These results are validated against experiments.