Densification and strength evolution in solid-state sintering Part II Strength model

A compact gains strength in sintering through low-temperature interparticle bonding, followed by further strength contributions from high-temperature densification. On the other hand, thermal softening substantially reduces a compact's strength at high temperatures. Therefore, the in situ strength during sintering is determined by the competition among interparticle neck growth, densification, and thermal softening. Distortion in sintering occurs when the compact is weak. Most strength models for sintered materials are semi-empirical relations based on the sintered fractional density. These models do not include microstructure or sintering cycle parameters; thus, they do not provide guidelines for thermal cycle design to improve compact dimensional control. A strength evolution model is derived which combines sintering theories and microstructure parameters, including interparticle neck size, solid volume fraction, and particle coordination number. The model predicts sintered strength and when combined with thermal softening gives a good prediction of in situ strength. The validity of the model is verified by comparison to experimental data for sintered and in situ strength of bronze and steel powders.     

Densification and strength evolution in solid-state sintering Part I Experimental investigation

Prealloyed bronze (Cu-10Sn) powder and a mixed elemental steel (Fe-2Ni-0.9C) powder were evaluated for strength evolution during sintering. For the bronze powder, test samples were fabricated using a loose powder casting method, while the steel powder was formed by injection molding. In situ strength during sintering was measured using a bending fracture test. Primary focus was on measuring the effects of sintering temperature and time on in situ strength evolution. Sintering temperature had the most significant effect, but the strength underwent significant gains prior to densification. The results are explained by the competition among interparticler neck growth, densification, and thermal softening. Sinter strengthening is initially governed by interparticle bonding, followed by a contribution from densification at high temperatures. However, high temperatures also lead to significant strength degradation due to thermal softening. Densification is favored by the declining in situ strength associated with thermal softening at high temperatures.     

A microstructural approach to flaw size dependence of strength in engineering ceramics

In this work, microstructural effects on the flaw size dependence of ceramic strength were investigated from aspects of stress analysis in the grain just ahead of the crack tip and also R-curve behaviour. In the analysis, it was assumed that the stress averaged in one grain just ahead of the crack tip, in ceramics, might control the fracture from a flaw. A microstructurally modified fracture criterion using the averaged stress was established by introducing the R-curve due to the grain bridging effect for longer cracks. A new R-curve of an exponential type was proposed for the fracture criterion. The criterion could adequately express the central trend in the dispersal of experimental results in the strength versus flaw size relation. To explain the scatter of results, the size distribution and the crystallographic anisotropy of the grain ahead of the crack tip were examined as dominant factors. The lower bound of strength scatter was estimated from the largest grain size, and the strength dispersion was reduced by decreasing the range of grain size variation. In FEM simulations, each element was regarded as one grain with a different crystallographic orientation, which was randomly selected by using a series of quasi-uniform random numbers. It was revealed that the scatter of strength due to crystallographic variations was smaller than the strength dispersion caused by a distributed grain size.     

Flexural strength of steel fibre-reinforced cement composites

The paper considers two existing theories for the flexural strength of steel fibre-reinforced cement composites, which are based on the post-cracking strength concept and on the rule of mixtures, respectively. It is shown that the former theory has serious limitations because of the omission of the matrix strength contribution to the ultimate composite strength. The rule of mixtures is strictly valid for composites under a direct tensile state of stress and its extension to the flexural state of stress has not been theoretically verified. An expression for the flexural strength of steel fibre-reinforced cement composites has been derived, based on an assumed stress block and fundamental principles of flexural mechanics. The derived expression for flexural strength is shown to be valid to the experimental results of this investigation and to the flexural strength data available from previous research. In the experimental part of this investigation, theV _f(l/d) ratio of fibres in a wide range of cement matrices was kept constant. Variations in composite strength were achieved by different mix proportions of the matrix and by long-term curing both under marine exposure and under laboratory curing. In the experimental results from previous research, however, changes in composite strength were caused by differentV _f(l/d) ratios of fibres.     

Investigation on the Fatigue Behaviors of Maraging Steel at Different Stress Ratios

Herein, the high-cycle fatigue behaviors of 18Ni maraging steel with different tensile strength in the stress ratio range from 0.1 to 0.5 are investigated, and compared with those at the stress ratio of −1. It is found that the relationship between the fatigue strength at the stress ratios of −1 and 0.1 and tensile strength is nonmonotonic, while the fatigue strength at the stress ratio of 0.5 improves as the tensile strength heightens. The tensile strength corresponding to the optimal fatigue strength state would change with the variation of the stress ratio, which is related to the alteration of key factors affecting the fatigue damage. Moreover, it is found that the Walker equation: σ_aR = σ₋₁ ⋅ [(1 − R)/2]^β gives reasonable results for the influence of stress ratio on the fatigue strength of 18Ni maraging steel.     

Hydrogen assisted intergranular cracking in steels

McMahon suggested that interface decohesion at grain-boundary carbides and precipitates is the mechanism of hydrogen assisted intergranular cracking, HAIC, in high strength steels. In general, cleavage of grain-boundary carbides, adhesion failure or interface decohesion at grain-boundary carbides and precipitates, and crack-tip shear slip along the grain boundary could be the mechanisms of HAIC. Hydrogen reduces cleavage strength, adhesion strength and the resistance to shear slip; therefore, hydrogen assists intergranular cracking. A method of identifying such mechanisms is suggested. A generalized theory of hydrogen assisted cracking is deduced. Brittle crystals cleave on their cleavage planes. Cleavage cracking of such crystals is anisotropic. When the crack-tip stress intensity factor, K, is low, the tortuous cracking process from the anisotropy results in rapidly increasing Stage-I crack growth rate with respect to K. The mechanism of the crack growth threshold, K_TH, is also discussed.     

The mechanism of the peak in strength and toughness at elevated temperatures in alumina containing a glass phase

The fracture of an alumina containing 5% by volume of glass phase has been studied over the temperature range 20 to 900° C. Peaks in fracture stress andK _Ic at elevated temperatures have been confirmed to arise from softening of the glass phase by determining the temperature dependence of the viscosity of a glass of identical composition to that occurring in the ceramic. Observations of fracture surface show glass protrusions at temperatures of the peak in strength orK _Ic indicating the viscous stretching of glass particles bridging the opposite crack surfaces and a simple model considering the energy dissipated in this process is presented. The peaks in strength andK _Ic arise from this energy dissipation rather than from blunting of the crack.     

Effect of stress ratio on fatigue behaviour in SiC particulate-reinforced aluminium alloy composite

Axial fatigue tests have been performed at three different stress ratios, R, of ?1, 0 and 0.4 using smooth specimens of an aluminium alloy composite reinforced with SiC particulates of 20 μm particle size. The effect of stress ratio on fatigue strength was studied on the basis of crack initiation, small crack growth and fracture surface analysis. The stress ratio dependence of fatigue strength that has been commonly observed in other materials was obtained, in which fatigue strength decreased with increasing stress ratio when characterized in terms of stress amplitude. At R=?1, the fatigue strength of the SiC_p/Al composite was the same as that of the unreinforced alloy, but at R= 0 and 0.4 decreased significantly, indicating a detrimental effect of tensile mean stress in the SiC_p/Al composite. The modified Goodman relation gave a fairly good estimation of the fatigue strength at 10⁷ cycles in the unreinforced alloy, but significantly unconservative estimation in the SiC_p/Al composite. At R= 0 and 0.4, cracks initiated at the interfaces between SiC particles and the matrix or due to particle cracking and then grew predominantly along the interfaces, because debonding between SiC particles and the matrix occurred easily under tensile mean stress. Such behaviour was different from that at R=?1. Therefore, it was concluded that the decrease in fatigue strength at high stress ratios and the observed stress ratio dependence in the SiC_p/Al composite were attributed to the different fracture mechanisms operated at high stress ratios.     

Effect of composition and thermal cycling on the adhesion strength of Sn-Zn-Al solder hot-dipped on Cu substrate

Reliability losses in many electronic systems were identified with the failure of solder joints rather than device malfunctions. The adhesion strength is an important factor for assessing the reliability of the solder joints. In this work, a pull-off test was used to investigate the adhesion strength at the interface of the (100 – x)Sn-x(5Al-Zn) lead-free solders on Cu substrate as-soldered and after thermal cycling, respectively. For the (100 – x)Sn-x(5Al-Zn) solders with the x value increased up to 40 wt%, the adhesion strength decreased from 11.8 ± 1.5 to 3.3 ± 0.9 MPa. After thermal cycling (–20–120°C) for 40 cycles, the adhesion strength of 95Sn-5(5Al-Zn) and 91Sn-9(5Al-Zn) solders decreased from 11.2 ± 1.7 to 8.2 ± 1.3, 7.6 ± 0.7 to 5.0 ± 0.8 MPa, respectively. However, the adhesion strength for the solders of 80Sn-20(5Al-Zn), 70Sn-30(5Al-Zn) and 60Sn-40(5Al-Zn) increased from 5.7 ± 1.7 to 13.3 ± 1.9 MPa, 4.8 ± 2.0 to 12.2 ± 1.8 MPa, and 3.3 ± 1.5 to 16.2 ± 1.2 MPa, respectively. The formation of intermetallic compound (IMC) is proposed for the enhancement of the strength after thermal cycling in this study.     

Strength retention of self-reinforced poly-L-lactide screws and plates: anin vivo andin vitro study

Self-reinforced poly-L-lactide (SR-PLLA) screws and multilayer plates were studied for their initial mechanical shear and/or tensile strength and strength retentionin vivo andin vitro up to 48 weeks. The plates were studiedin vitro up to 68 weeks. The loss of strength was fasterin vivo thanin vitro. The screws retained more than half of their initial shear strength up to 12 weeksin vivo and over 24 weeksin vitro. By 48 weeks they had lost nearly all of their mechanical strength in both groups. The plates retained over 54% of their initial tensile and 71% of shear strength up to 24 weeksin vivo. The shear strength did not particularly diminish up to 52 weeks of follow-upin vitro whereas the tensile strength was slightly decreased after 36 weeks. After this follow-up time the loss of strength was more rapid and by 68 weeks the shear strength had decreased to 17% and the tensile strength to 0%.     

Experimental investigations for improving part strength in selective laser sintering

Selective laser sintering (SLS) is a powder-based rapid prototyping process in which parts are built by sintering of selected areas of layers of powder using laser. Nowadays, SLS is emerging as a rapid manufacturing technique, which produces functional parts in small batches, particularly in aerospace application and rapid tooling. Therefore, SLS prototypes should have sufficient strength to satisfy functional requirements. Apart from the energy density which is the combination of laser power, beam speed and hatch spacing, various other parameters like refresh rate, layer thickness and hatch pattern influence part strength. In the present work, relationship between strength and the various process parameters namely layer thickness, refresh rate, part bed temperature and hatch pattern have been investigated. Experiments are conducted based on Taguchi method using L₁₆ modified orthogonal array. Tensile specimens of polyamide (PA2200) material as per the standard ‘ASTM D638’ are fabricated on SLS machine with constant energy density and tested on a universal testing machine for tensile strength. Optimum strength conditions are obtained by maximising signal to noise (S/N) ratio and analysis of variance (ANOVA) is used to understand the significance of process variables affecting part strength. A regression model to predict part strength has been developed. Confirmation test conducted subsequently has revealed that the results are within the confidence interval.     

Carbon fibre compressive strength and its dependence on structure and morphology

The axial compressive strength of carbon fibres varies with the fibre tensile modulus and precursor material. While the development of tensile modulus and strength in carbon fibres has been the subject of numerous investigations, increasing attention is now being paid to the fibre and the composite compressive strength. In the present investigation, pitch- and PAN-based carbon fibres with wide-ranging moduli and compressive strengths were chosen for a study of fibre structure and morphology. A rayon-based carbon fibre was also included in this study. Structural parameters (L _c, L_a(0), L _a(90), orientation parameter Z, and the spacing between graphitic planes d(00, 2)) were determined from wide angle X-ray spectroscopy (WAXS). Fibre morphology was characterized using high-resolution scanning electron microscopy (HRSEM) of fractured fibre cross-sections. The mechanical properties of the fibres, including compressive strength, the structural parameters from WAXS, and the morphology determined from HRSEM are reported. The influence of structure and morphology on the fibre compressive strength is discussed. This study suggests that the width of the graphitic sheets, the crystallite size perpendicular to the fibre axis (L _c and L _a(0)), and crystal anisotropy play significant roles in accounting for the large differences in compressive strengths of various carbon fibres.     

Strengthening Mechanisms in Bulk FeCoCrNiAl x (x=0, 0.5, 1.0) High-Entropy Alloys Fabricated by Laser Melting Deposition

Herein, FeCoCrNiAl _x (x = 0, 0.5, 1.0) high-entropy alloys (HEAs) are fabricated by the laser melting deposition (LMD) technique. With the increase of Al content, the LMD-ed microstructure transitions from a single face-centered cubic (FCC) phase to a dual-phase structure containing a small amount of body-centered cubic (BCC) phase (5.3%), and the proportion of the final BCC phase increases significantly to 98.2%. In addition to the compression tests, four strengthening models are used to evaluate the theoretical strength of the three alloys. The addition of Al element as grain refiner can improve the ultimate compressive strength of HEAs; however, the yield strength and plasticity do not improve, as theoretically expected. The FCC phase with more slip systems leads to higher plasticity in the LMD-ed FeCoCrNi HEA but results in lower yield strength. The LMD-ed FeCoCrNiAl_0.5 HEA exhibits the best combination of strength and plasticity. Therefore, to meet the required service requirements, the content of Al in the FeCoCrNiAl _x HEA should be carefully controlled under the premise of considering the actual working conditions.     

Residual strength of unidirectional fibre-metal laminates based on JC toughness of C(T) and SE(B) specimens: comparison with M(T) test results

Fibre‐metal laminates (FMLs) are structural composites designed with the aim of producing very low fatigue crack‐propagation rate, damage‐tolerant and high‐strength materials, if compared to aeronautical Al alloys. Their application in aeronautical structures demands a deep knowledge of a wide set of mechanical properties and technological values, including both fracture toughness and residual strength. The residual strength of FMLs have been traditionally determined by using wide centre‐cracked tension panels M(T). The use of this geometry requires large quantities of material and heavy laboratory facilities. In this work, fracture toughness ( J_C) of some unidirectional FMLs laminates was measured using a recently proposed methodology for critical fracture toughness evaluation on compact tension C(T) and single‐edge bend SE(B) specimens. Additionally, residual strength values of wider M(T) specimens with different widths (W from 150 to 200 mm) and several crack to width ratios (2a/W) were experimentally obtained. Some experimental residual strength values of M(T) specimens (W from 150 to 400 mm and different 2a/W ratios) of Arall were also obtained from the bibliography. Based on J_C results from C(T) and SE(B) specimens, and either using or not using crack‐tip plasticity corrections, the residual strengths of the M(T) specimens were predicted and compared to the experimental ones. The results showed good agreement, especially when crack‐tip plasticity corrections were applied.     

Evaluation of fracture parameters of concrete from bending test using inverse analysis approach

In this study, an inverse analysis approach is developed to obtain the fracture parameters of concrete, including stress–crack opening relationship, cracking and tensile strength as well as fracture energy, from the results of a three-point bending test. Using this approach, the effects of coarse aggregate size (5–10, 10–16, 16–20 and 20–25 mm) and matrix strength (compressive strength of 40 and 80 MPa, respectively) on the fracture parameters are evaluated. For normal strength concrete, coarse aggregate size and cement matrix strength significantly influence the shape of σ–w curve. For a given total aggregate content, small aggregate size leads to a high tensile strength and a sharp post-peak stress drop. The smaller the coarse aggregate, the steeper is the post-peak σ–w curve. By contrast, in high strength concrete, a similar σ–w relationship is obtained for various aggregate sizes. The post-peak stress drop for high strength concrete is more abrupt than that for normal strength concrete. Also, the smaller the coarse aggregate size, the higher is the flexural strength. For both normal and high strength concrete, fracture energy and characteristic length are found to increase with increase of coarse aggregate size.     

Ultimate tensile strength of loblolly pine strands using stochastic finite element method

The purpose of the study was to predict the failure strength of different orientation of wood strands from different growth ring positions under tension loading. Stochastic models were constructed to account for the uncertainty of material properties. The Tsai–Hill criteria were used to predict the ultimate tensile strength (UTS). The UTS results from experimental testing were used to validate the results from models. The difference of UTS between experimental and SFEM ranged from 0.09 to 11.09%. Different stress distributions were found for different orientation strand models, whereas uniform stress distribution was found for homogeneous models. The magnitude of the stress distribution was greater for strands from the growth ring number 11–20. Sensitivity analysis showed that grain orientation and growth ring number influenced the UTS of strands. UTS of strands from growth ring number 1–10 showed strength indexes (X _t, Y _t, and S) as dominant factor, whereas UTS of strands from growth ring number 11–20 showed both strength indexes and stress components (σ ₁, σ ₂, and τ ₁₂) as dominant factors.     

Relations between fatigue strength and other mechanical properties of metallic materials

The relations between fatigue strength and other mechanical properties especially the tensile strength of metallic materials are reviewed. After analyzing the numerous fatigue data available, the qualitative or quantitative relations between fatigue strength and hardness, strength (tensile strength and yield strength) and toughness (static toughness and impact toughness) are established. Among these relations, the general relation between fatigue strength σ_w and tensile strength σ_b, σ_w = σ_b(C ? P ? σ_b), where C and P are parameters, (hereafter, the general fatigue formula) can well predict the fatigue strength with increasing the tensile strength in a wide range for many materials such as conventional metallic materials, newly developed materials and engineering components. On the basis of the experimental results of many materials, the fatigue damage mechanism, especially for high‐strength steels, is proposed. It is suggested that the general fatigue formula can provide a new clue to predict the fatigue strength and design the materials by adjusting material parameters P and C adequately.     

Joining of silicon nitride ceramics by hot pressing

The joining of hot-pressed silicon nitride ceramics, containing Al₂O₃ and Y₂O₃ as sintering aids, has been carried out in a nitrogen atmosphere. Uniaxial pressure was applied at high temperature during the joining process. Polyethylene was used as a joining agent. Joining strength was measured by four-point bending tests. The effects of joining conditions such as temperature (from 1400 to 1600°C), joining pressure (from 0.1 to 40 MPa), holding time (from 0.5 to 8 h) and surface roughness (R _max) of the joining couple (about 0.12, 0.22 and 1.2m) on the joining strength were examined. The joining strength was increased with increases in joining temperature, joining pressure and holding time. Larger surface roughness caused lower joining strength. The higher joining strength was attributed to a larger true contact area. The area was increased through plastic deformation of the joined couple at elevated temperatures. The highest joining strength attained was 567 MPa at room temperature, which was about half the value of the average flexural strength of the original body. The high temperature strength measured at 1200° C did not differ very much from the room-temperature value.     

Mechanical properties of friction welded 6063 aluminum alloy and austenitic stainless steel

Friction welding of dissimilar metal combination of aluminum alloy and austenitic stainless steel was examined to investigate the effect of welding conditions on mechanical properties of the dissimilar metal combination. The welded joints were produced by varying forge pressure (F _g), friction pressure (F _r), and burn-off length (B). The joints were subjected to mechanical testing methods such as the tension, notch Charpy impact tests. The tensile strength and toughness decrease with an increase in friction pressure. The tensile strength decreases with an increase in burn-off length at a low forge pressure while tensile strength increases with an increase in burn-off length at a high forge pressure. The tensile failure of the welded joint occurred in aluminum alloy just away from interface in the thermo-mechanically affected zone indicates good joint strength at the condition of low friction pressure, high forge pressure, and high burn-off length. The maximum tensile strength was observed with low friction pressure and high forge pressure. The tensile strength of dissimilar joint is approximately equal to tensile strength of 6063 aluminum alloys at the condition of low friction pressure, high forge pressure, and high burn-off length. The tensile and impact failure of joints was examined under scanning electron microscope and failure modes were discussed.     

Relationship between hardness and tensile properties in various single structured steels

Abstract

An attempt has been made to establish a relationship between hardness and tensile properties for various single structured steels: ferrite, pearlite, bainite, and martensite. It is found that the proportionality constant A _Y of hardness to yield strength changes from 5.79 to 3.17 and is highest for the ferrite steel and lowest for the tempered martensitic steels. A less pronounced change was found in the proportionality constant A _T of hardness to tensile strength (from 3.97 to 2.72). A dependence on microstructure of the proportionality constant at 8% strain A _0.08 was found as well. This difference in A was found to be attributable mostly to the effect of different work hardening behaviours owing to different microstructures. Regression analysis shows that hardness can be expressed as a function of accessible material parameters such as composition, grain size, and transformation temperatures for various single structured steels within a certain degree of accuracy.