Single-fiber Push-out Tests Research Articles

Single-Fiber Push-In vs. Single-Fiber Push-Out: A Comparison Between Two Test Methods to Determine the Interfacial Properties of Brittle Matrix Composites

ABSTRACTInterfacial properties of Nicalon™-reinforced brittle matrix composites were determined from single-fiber load-controlled push-in tests using a Mechanical Properties Microprobe (Nanoindenter) and single-fiber displacement-controlled push-out tests using the Interfacial Test System (ITS). A comparison between the results from these two tests is presented along with a discussion of data analysis techniques.

Development of the tensioned push-out test for study of fibre/matrix interfaces

Finite element modelling has been used to explore the basic single-fibre push-out test and a variant in which in-plane tension is simultaneously applied. Comparison with experimental photoelastic data has shown that shear-lag based analytical models of the test are unreliable, even for the simplest cases. It has been shown that the loading geometry and, particularly, the presence of thermal residual stresses can affect the interfacial stress state, which often tends to vary significantly along the length of the fibre. For a metal-matrix composite, the peak shear stress is predicted to occur near the bottom of the fibre, which is where debonding is expected to initiate. The proportion of normal and shear stresses at the interface can be altered by the application of in-plane tension. Some previously published experimental tensioned push-out data for the SiC/Ti system have been interpreted in the light of finite element modelling, allowing an estimate to be made of the interfacial coefficient of friction.

Reaction-induced changes in interfacial and macroscopic mechanical properties of SiC monofilament-reinforced titanium

Results are presented from single fibre push-out testing, simple tension and impact three-point bending of SiC/Ti composites based on Ti-6AI-4V alloy reinforced with W-cored monofilaments having duplex C/TiB 2 coatings. Specimens were tested after various heat treatments and it is shown that a progressive increase in the interfacial shear stress for frictional sliding is observed as reaction proceeds. This increase in the resistance to frictional sliding can be correlated with a decrease in the thickness of the graphitic layer. Testing of composites subjected to the same heat treatments also revealed progressive reduction in tensile strength with increased reaction layer thickness. Monitoring of Poisson's ratios during testing was used to confirm that matrix plasticity occurred during axial loading, but inelastic behaviour was due to interfacial damage under transverse load. Consistent with this, impact energies were found to be higher under axial loading. Although some fibre pull-out was observed for axially loaded specimens without thick reaction layers, it is suggested that neither this nor any other interfacial process was contributing significantly to the work associated with impact loadings.

The tensioned push-out test for fibre-matrix interface characterisation under mixed mode loading

A novel test procedure is suggested for the exploration of interfacial mechanical properties in fibre-reinforced composites. This is based on the established single fibre push-out test, which has the advantage of being applicable to specimens which can be routinely produced from normal unidirectional long fibre composite materials. The suggested modification involves the application of equal biaxial in-plane tension while the push-out testing is carried out. This allows the interface to be subjected to various combinations of mode I and II loading, ranging from pure opening to pure shear. Hence it should be possible to obtain data characterising the resistance to interfacial debonding and disengagement over all of this range. Illustrative data are presented here for titanium-based composites, in the form of measured critical shear stresses for various applied normal stresses at the interface.

The use of single fibre pushout testing to explore interfacial mechanics in SiC monofilament-reinforced Ti—II. Application of the test to composite material

Single fibre pushout testing has been used to measure the load needed to displace a fibre, as a function of its aspect ratio. This has been done for SiC monofilaments, having duplex carbon/TiB 2 coatings, embedded in a matrix of Ti6Al4V. Wedge-shaped specimens have been used, allowing pushout of fibres with a range of aspect ratios from a single specimen. Partially pushed-out fibres have also been pushed back into the matrix. Specimens have been examined in the as-fabricated form and also after subsequent heat treatments. Analysis of the results indicates that in all cases it was the resistance to the onset of frictional sliding which was determining the pushout load. Values of the interfacial shear stress necessary for frictional sliding, τ fr, have been established, although it was not possible to measure separately the coefficient of static friction or the residual radial compressive stress. The value of τ fr was found to increase progressively on heat treating the composite. Preliminary chemical analysis work suggests that this results from an interfacial reaction, possibly one which causes the carbon layer to become reduced in thickness.

The use of single fibre pushout testing to explore interfacial mechanics in SiC monofilament-reinforced Ti—I. A photoelastic study of the test

The frozen stress photoelastic technique has been used to investigate the elastic stress distribution during single fibre pushout testing. It is shown that the axial distributions of interfacial shear stress and axial stress in the fibre are broadly consistent with the Hseuh model, based on shear lag analysis. However, the experimental shear stress distribution is more uniform than that predicted, particularly for low aspect ratio fibres, so that interfacial shear debonding strength value obtained from debonding loads on the basis of the model may be overestimates. Estimates based on a uniform shear stress along the fibre length may be preferable, particularly for low aspect ratio fibres.

Shear‐Lag Analysis of Fiber Push‐Out (Indentation) Tests for Estimating Interfacial Friction Stress in Ceramic‐Matrix Composites

A shear‐lag analysis is presented for estimating sliding friction stress at fiber‐matrix interfaces in ceramic‐matrix composites using the single‐fiber push‐out test. The analysis includes an approximate correction for the increased interfacial compression and, therefore, the interfacial friction stress arising from the transverse (Poisson) expansion of the fibers subjected to the compressive load. An exponential decrease of the interfacial shear stress along the fiber length is predicted. This result is similar to the results of a finite‐element analysis reported in the literature. The analysis also provides a basis for the experimental determination of a coefficient of interfacial friction (μ) and a residual interfacial compression (σ0). It is shown that the sliding friction stress (τf=μσ0) can be overestimated if the transverse expansion of the fibers is not taken into account.