A. S. Wronski

Kinking and compressive failure in uniaxially aligned carbon fibre composite tested under superposed hydrostatic pressure

Initiation and propagation of failure in uniaxially aligned 60% volume-fraction Type III carbon fibre-epoxide compressive specimens, strained parallel to the fibre axis, was studied at atmospheric and superposed hydrostatic pressures, H, extending to 300 MN m−2. The atmospheric axial compressive strength was approximately 1.5 GN m−2 and equal to the tensile strength, but mechanisms involving shear-operated failure of the fibres must be discounted since the failure process was very pressure sensitive above H∼ 150 MN m−2. The results also could not be satisfactorily interpreted by theories involving micro-buckling of individual fibres or laminae when the matrix shear modulus controls the compressive strength. For atmospheric tests and for H<150 MN m−2 the initiation of failure was associated with transverse cracking (longitudinal splitting) which was followed by kinking. Ahead of the propagating kink band, groups of fractured fibres were observed, which is consistent with failure of these groups by buckling; this process causes composite catastrophic failure. At higher pressures splitting was suppressed, as was interlaminar cracking in doubly-notched (in-plane shear) specimens, but kinking, which became increasingly more difficult to initiate, was the precursor of the failure process. An attempt was made to analyse failure using the fracture mechanics model of Chaplin with some success for the notched specimens.

Compressive failure and kinking in uniaxially aligned glass-resin composite under superposed hydrostatic pressure

The failure process in uniaxially-aligned 60% fibre volume fraction glass fibre-epoxide compressive specimens strained parallel to the fibre axis was investigated at atmospheric and superposed hydrostatic pressures up to 300 MN m−2. The atmospheric strength was about 1.15 GN m−2 (about 20% less than the tensile) and strongly pressure dependent, rising to over 2.2 GN m−2 at 300 MM m−2 pressure, i.e. by about 30% per 100 MN m−2 of superposed pressure. The corresponding figure is 22% if the maximum shear stress and not the maximum principal compressive stress is considered. This is incompatible with atmospheric compressive failure mechanisms controlled by weakly dependent or pressure independent processes, e.g. shear of the fibres. The results also could not be satisfactorily interpreted in terms of microbuckling of individual fibres. Kinking, involving buckling of fibre bundles was proposed as the mechanism of failure propagation, but the critical stage (for this glass reinforced plastic) is suggested as being yielding of the matrix, which initially restrains surface bundles from buckling. A strong pressure dependent failure criterion, about 25% increase per 100 MN m−2, was derived by modifying the Swift-Piggott analysis of deformation of initially curved fibres. It is postulated that it is the axial compression that causes bundle curvature. Other systems, particularly carbon fibre-reinforced plastic, in which there appears to be a transition in the critical stage of failure from bundle buckling to matrix yielding with increasing superposed pressure, are also considered.

Kinking and tensile, compressive and interlaminar shear failure in carbon-fibre-reinforced plastic beams tested in flexure

The first stage of the failure process in pultruded, 60% volume fraction, type III carbon fibre-epoxide beam specimens with span-to-depth ratios of 5, 15 and 40 deformed in flexure at atmospheric pressure was the initiation of kinking by the “compression” roller. Kink growth during the non-linear part of the load-deflection curve was followed by kink propagation at peak load. Acoustic emission and load-unload tests to detect irrecoverable deflection supported direct microscopic observations of damage. Kink growth with decreasing load, increasing deflection and accompanying redistribution of stresses led to two types of failure, commonly referred to as “flexural” and “interlaminar”. In the former, tensile failure was concurrently initiated to give the characteristic tensile and compressive zones on the failure surfaces. In the latter, the growing kink initiated interlaminar cracks in resin-rich zones as it propagated (with decreasing load) towards the convex surface. Kinking was associated with triaxial compressive stresses in the contact zone of the “compressive” roller or rollers (in the case of four-point bend specimens). When hydrostatic pressure was superposed on flexure, at pressures between 150 and 300 MNm−2 depending on the type of specimen, kinking was inhibited and eventually suppressed to give tensile failures, even in the so-called interlaminar shear strength type of specimen. When non-linear deflections were not large, the maximum principal tensile stress in the beams was close to the tensile strength of the carbon-fibre-reinforced plastic (∼1.8 GN m−2).

The effect of hydrostatic pressure on the tensile properties of pultruded CFRP

The failure process in waisted tensile specimens of pultruded 60% volume fraction carbon fibre-epoxide was investigated at atmospheric and superposed hydrostatic pressures up to 300 MN m−2. The maximum principal stress at fracture decreased from ~ 2.0 GN m−2 at atmospheric pressure to ~ 1.5 GN m−2 by 200 MN m−2 superposed pressure and then remained approximately constant. These latter failures were fairly flat and no damage preceding the catastrophic fracture was detected, which indicates that composite strength is solely controlled by fibre strength. Fracture of fibres at lower pressures appeared to commence also in the range 1.5 to 1.6 GN m−2, but, as it did not result in catastrophic failure, account has to be taken of the resin and the fibre bundles. Debonding was initiated at ~ 1.2 GN m−2 at atmospheric pressure and this stress increased to ~ 1.5 GN m−2 when 150 MN m−2 superposed pressure was applied; the pressure dependence was related to that of the resin tensile strength. This process is described as the first stage, straightening and debond initiation of curved surface bundles, on our model of tensile failure. The second stage, delamination, i.e. the growth of transverse cracks leading to the detachment of these bundles, was impeded by the transverse pressure, being suppressed beyond 150 MN m−2. Only below this pressure was load redistribution between bundles possible, but, as the pressure was increased from atmospheric, it become more difficult, resulting in a decrease in the composite tensile strength and reduced fibre pull-out.

Pyramidal yield criteria for epoxides

The tensile, compressive and shear yield strengths of two epoxides were measured under superposed hydrostatic pressure extending to 300 MN m−2. For both materials, the ratio of the moduli of the tensile,σT, to compressive,σC, yield stress at atmospheric pressure was approximately 3∶4, as has been reported previously for a number of thermoplastics. Theσ2=σ3 envelope in stress space was plotted according to these two-parameter (σC andσT) yield criteria: conical, paraboloidal and pyramidal; the best correlation was with the last. The experimental tensile and compressive data for tests under pressure, however, fit slightly better two straight lines which are consistent with a three-parameter single hexagonal pyramidal yield surface. For plane stress and shear under pressure yield envelopes of these surfaces, the correlation with experimental data is again best for the pyramidal criteria, except for biaxial or triaxial tension when these resins are brittle. The third independent parameter employed in the pyramidal criterion was the equi-biaxial compressive yield stress, determined by tensile experiments under appropriate superposed hydrostatic pressure; alternatively plane strain compressive yield stress,σPC, may be used.

Industrial processing, microstructures and mechanical properties of Fe–(2–4)Mn (–0.85Mo)–(0.3–0.7)C sintered steels

Abstract The potential of PM Mn steels has been established in laboratory experiments. This paper deals with sintering of Fe–(2–4)Mn–(0.3/0.7)C, also with 0˙85%Mo addition, in an industrial pusher furnace at 1180°C in an atmosphere of 25% hydrogen plus 75% nitrogen, obtained from a cryogenic liquid, giving an inlet dew-point of −55 °C. Tensile, bend (including fatigue) and miniature Charpy specimens were sintered in flowing gases and in semiclosed containers with a getter of ferromanganese, carbon and alumina. The quenched and tem- pered state was investigated, as was sinter hardening (cooling rate of 55 K min −1), simulated for comparison with slow cooling at 10 K min −1. As there was no forma tion of oxide networks at the combination of sintering temperature and dewpoint, in accordance with the Ellingham–Richardson diagram for Mn oxidation/reduction, the use of semiclosed containers was superfluous. The quenched and tempered specimens were brittle. Sinter hardening lead to an improvement in mechanical properties. The reproducibility of tensile and TRS data was high for the sintered materials, characterised by Weibull moduli m of 12–41. All the alloy microstructures were complex and heterogeneous, consisting of, depending on the local manganese and carbon contents, the diffusive and non-diffusive transformation products (pearlite, bainite, martensite) and additionally ferrite and retained austenite. The highest mechanical properties in the entire range of compositions investigated in the furnace cooled state: yield, tensile and bend strengths of 499, 637 and 1280 MPa, respectively, with impact energy of 18 J, and tensile and bend strains of 1˙17 and 1.57%, were achieved for the Fe–2Mn–0.85Mo–0.5C alloy, marginally superior to Fe–2Mn–0.7C. For the sinter hardened Fe–4Mn–0.3C alloy yield, tensile and bend strengths were 570, 664 and 1263 MPa, respectively, at an acceptable impact energy of 14 J, with tensile and bend strains of 0.52% and 1.8%. Many of the results compare favourably with the requirements of MPIF standard 35. Mn is a more effective strengthening agent than either Ni or Cu, or their combination, though generally at reduced plasticity.

Microstructure and mechanical properties of sintered (2–4)Mn–(0·6–0·8)C steels

AbstractMechanical properties of 2–4% manganese PM steels were determined in tension and in bending following laboratory sintering in dry, hydrogen rich atmospheres. Youngs modulus determined by an extensometric technique was about 115 GPa; when measured by an ultrasonic method it was about 153 GPa, in accordance with the‘law of mixtures’. The microstructures, significantly devoid of oxide networks, were predominantly pearlitic, but frequently with variability for specimens similarly processed, resulting in appreciable variations in the stresses for macroscopic yielding and fracture. The majority of the experiments were conducted on 3 and 4Mn–0·6C alloys and for these R0·1 was in the range 275–500 MPa, tensile strength (TS) 300–600 MPa, and (apparent) transverse rupture strength (TRS) 640–1260 MPa. Statistical techniques were employed to analyse the data. When careful control of processing was maintained, the Weibull modulus was highest, at about 17, for TS of furnace cooled specimens, and lowest, about ...

Development of robust processing routes for powder metallurgy high speed steels

Abstract This paper reviews progress made in understanding the factors which control the supersolidus liquid phase sintering of high speed steel powders to full density. The correlation between alloy composition and sintering behaviour is discussed for a number of alloy systems. Realising that for complete densification it is necessary for sintering to take place in the liquid +γ+M6C+MC (or MX) phase region, two approaches have been developed to extend this critical phase field. This enables a scientific development of alloys that are more robust to process variations than currently sintered high speed steels of standard (for wrought materials) compositions. The new alloy systems possess wider process or sintering windows and have lower optimum sintering temperatures. The first approach relies on computer aided alloy design: vacuum sintering windows extending to 30–40 K at temperatures of 1170–1200°C have been achieved for novel Fe–C–4Cr–14Mo(–8Co)systems. The second approach involves sintering vanadium enriched high speed steels (HSSs) in nitrogen rich atmospheres. Such processing promotes the formation of MX carbonitrides in place of the more massive MC carbides. The solidus is lowered and sintering windows of ∼30 K at temperatures of 1140–1150°C have been achieved. Compared with wrought HSSs, directly sintered materials have uniform, coarser microstructures. The low levels of residual porosity achieved enable attainment of metal and wood cutting properties comparable to those achieved with wrought and hipped HSSs of similar compositions.

Supersolidus liquid phase sintering of high speed steels: Part 3: computer aided design of sinterable alloys

AbstractCalculated multicomponent phase diagrams were used to identify high speed steel (HSS) type alloys having the potential to exhibit enhanced sinter ability. The requirement was for an extensive austenite + carbide + liquid phase field. Of the six tungsten and molybdenum based systems studied, Fe–14Mo–C + 4Cr–8Co systems were potentially the most promising. Appropriate compositions were water atomised and additional alloys prepared by blending annealed powders with graphite powders. Powders were compacted to green densities of about 70% theoretical and then vacuum sintered. Sinterability was assessed in terms of sintered densities and microstructures. Alloys containing Fe–13Mo–1·3C, Fe–14Mo–4Cr–1·3C, and Fe–14Mo– 8Co–4Cr–1·4C were sintered to full density at temperatures as low as 1170°C, 70–150 K lower than for existing HSSs. Sintering windows were 20– 30 K, a significant improvement on existing HSSs. As sintered microstructures consisted of angular M6 C carbides dispersed in martensitic matrixes, w...

The tensile properties of pultruded GRP tested under superposed hydrostatic pressure

The failure mechanisms in waisted tensile specimens of pultruded 60% volume fraction glass fibre-epoxide were investigated at atmospheric and superposed hydrostatic pressures extending to 350 MN m−2. The maximum principal stress at fracture decreased from ∼1.7 GN m−2 at atmospheric pressure to ∼1.3 GN m−2 at 250 MN m−2 superposed pressure and remained approximately constant at higher pressures, as had been observed with carbon fibre reinforced plastic (CFRP) and a nickel-matrix carbon fibre composite. In the high-pressure region the failure surfaces were fairly flat, consistent with the fracture process being solely controlled by fibre strength. Pre-failure damage, in particular debonding, was initiated at ∼0.95 GN m−2 at atmospheric pressure and this stress rose to ∼1.2 GN m−2 at 300 MN m−2 superposed pressure, i.e. by about 9% per 100 MN m−2. Unlike the pressure dependence in CFRP, this contrasts with the pressure dependence of the resin tensile strength, about 25% per 100 MN m−2, but can be associated with that of the fibre bundle/resin debonding stress, about 12% per 100 MN m−2 superposed pressure. Consistent with this interpretation, glass fibres of the failure surfaces were resin-free, again in contrast to CFRP.