Stir zone (SZ) of AA6061-T6 double-side friction stir welding (DS-FSW) joint with a thickness of 6 mm had a poor fatigue performance compared with that of base metal (BM). The rolling process was employed to improve the fatigue performance of SZ. The results showed that rolling process increased the dislocation density in SZ and remarkably enhanced its fatigue performance. The crack initiation played a key role in the fatigue performance of SZ. It relied on the debonding of lamellar structure in SZ. The debonding of lamellar structure depended on the cracking of fine grain layer in lamellar structure.

A concept of initial cracking strength of concrete is elaborated in this study. A fracture model and associated methods for determining independent initial cracking strength and initial fracture toughness by using the three-point-bending (3-p-b) and wedge splitting (WS) concrete specimens are present. The initial cracking strength and initial fracture toughness can be simultaneously determined using a curve-fitting method from the proposed fracture model. All of the initial fracture curves can be obtained using the determined concrete materials. The initial loads of the 3-p-b and WS specimens can be predicted on the basis of the curves with ±15% ranges. Furthermore, analytical functions are used to obtain and determine the initial cracking strength and the initial fracture toughness of concretes directly. The determined values with ±15% ranges cover the most of initial loads of the 3-p-b and WS specimens.

Orthogonal experiment design together with the analysis of variance was used to examine the processing parameters (laser power, scan speed, layer thickness and hatch spacing) of selective laser melting (SLM) for superior properties of SLM parts, in which nine groups of specimens of Ti-6Al-4V were fabricated. The porosity for each group was measured and the results clarify that the influence sequence of individual parameter on the porosity is laser power > hatch spacing > layer thickness > scan speed. Ultrasonic fatigue tests (20 kHz) were conducted for the SLMed specimens in high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) regimes. The S-N data show that the fatigue strength is greatly affected by the porosity: the group with the smallest porosity percentage having the highest fatigue strength in HCF and VHCF regimes. Moreover, the observations by scanning electron microscopy revealed that fatigue cracks initiate at lack-of-fusion defects in the cases of surface and internal crack initiation.

The use of passive infrared thermography comprises great opportunities to improve understanding the fatigue damage process of crack-patched structures. Quasi-static and cyclic coupon tests are performed using metallic specimens with single-sided bonded patches and monitored with passive infrared thermography. Different test setups help to differentiate between metallic crack growth and adhesive damage on thermal images. Results show that metallic crack growth can be monitored from the patched side, also in combination with local delamination at the patch/metal interface. Thus, it is possible to analyse the overall degradation progress of the crack patched component under loading conditions and thereby to identify the driving damage mechanism of the particular repair configuration. Being able to understand the overall damage behaviour of crack patched components is essential to improve the ability of predicting its long-term behaviour.

A stress-based sectional critical plane model for multiaxial fatigue life prediction is proposed. The proposed model considers the effects of material properties and loading paths on the crack initiation and propagation behaviors. By introducing the ratio of maximum shear stress amplitude to maximum normal stress amplitude, it is divided into three sections in which the maximum normal stress plane, maximum damage plane and maximum shear stress amplitude plane are considered as the critical planes, respectively. To verify the accuracy and applicability of the proposed model, experimental data of 30CrMnSiA steel conducted by the authors and other test data of different materials from the existing literatures are utilized. For 30CrMnSiA steel, the prediction results of the proposed model demonstrate that 79.3% and 93.7% of the prediction results are within the ±2 times and ±3 times scatter band of fatigue life. For the experimental data from the existing literatures, more than 85% and 70% of the results predicted by the proposed model are within ±3 times scatter band of fatigue life for steel and aluminum alloy materials, respectively.

Droplet impingement of metallic surfaces at high impact velocities results, after some time, in erosion of the surface due to fatigue. By extending our previously published analytical model to enable the use of experimental fatigue data (S-N curves), here, for the first time, a wide range of experimental liquid droplet erosion incubation period test states for both ferrous (stainless steel AISI 316) and non-ferrous (aluminium 6061-T6) engineering metals have been investigated. To achieve this, the developed model includes additional surface hardening and a residual compressive stress state at the surface due to a water drop peening effect. As such, the interrelation of the physical and mechanical properties that follows from the model has been used to identify how changes in selected metal properties might enhance droplet impingement erosion incubation life. Model predictions for both metals, using fatigue data from S-N curves from different literature sources, showed for the droplet impact velocity range of 140 to 400 m/s an excellent agreement with results from a multi-regression equation as determined from an ASTM interlaboratory test program.

Normalization method is a practical method for determining the J-R curves and fracture toughness of steels. There is some concern, however, about the performance of this method on steels with small strain hardening exponent and yield strength due mainly to the assumption of infinite strain hardening exponent (n). This paper intends to analytically modify the normalization method by removing this assumption and incorporating the strain hardening in calculating the blunting corrected crack length. This modification enables the normalization method to be applied to steels with small strain hardening exponent and yield strength. Experiments are undertaken to prove the underperformance of the normalization method for steels with small strain hardening exponent and yield strength and to verify the modified normalization method (CNM). A comparison of fracture toughness determined by CNM with that by the unloading compliance method and normalization method corroborates the improved accuracy of the developed CNM. It is found in the paper that the developed CNM performs very well for materials with small strain hardening exponent and yield strength and performs better for specimens with smaller thickness and in accordance with all standards. The paper concludes that the developed CNM overcomes the deficiency of the normalization method for steels with small strain hardening exponent and yield strength.

The fretting damage and wear debris on the dental implant-abutment interface (IAI) are unclear. In this study, fatigue cycle loading (FT) and chewing cycle loading (CW) test were applied to two implant systems, the fretting damage morphology and wear debris generation on the IAI were observed by a scanning electron microscope. The torque value of the central screw was measured by electronic torque tester. The fretting damage on the IAI was relatively slight and mainly plastic deformation in the FT group, which was more serious and mainly furrow wear in the CW group. Various forms of wear debris were generated. The removal torques were lower than its pre-tightening value in both groups, the decline and loss rate of the CW group was significantly higher. This study confirmed the critical roles of fretting damages and metal wear debris on the IAI in the implant-supported prosthesis.

Based on the physical phenomenon that the fatigue cracks initiate along specific slip plane, a slip plane damage based low cycle fatigue (LCF) lifetime model for the nickel- based single crystal superalloy is established. The predicted results indicate that the lifetime model can reflect the orientation effect. In addition, in order to characterize the dwell time dependence of the LCF lifetime, creep damage and compression-creep damage are introduced to the lifetime model. Finally, the lifetime predictions under LCF loading with tensile dwell time, compressive dwell time and LCF with tensile-compressive dwell time are conducted by employing the lifetime mode, respectively. The predicted lifetimes show a good agreement with the experimental data, which verifies the accuracy of the developed lifetime model in this paper.

This study investigated the fretting fatigue behavior and mechanism of 35CrMoA steel of different contact stresses under diamond and square loading paths in the form of curved surface contact. The results show that multiple crack sources will initiate on the subsurface of the specimen under the combined effect of contact stress and cyclic stress. Under low contact stress, only one crack source dominates, causing the instantaneous fracture zone to be biased to the other side of the main crack source. Under high contact stress, the crack sources in both fretting zones play a dominant role, making the shape of the instantaneous fracture zone into a nearly circular shape with better symmetry; At the beginning of the fretting fatigue, cracks only propagate in the cross-section where they form. When they propagate to a certain depth, a component that propagates in the longitudinal direction will be generated.

The present investigation is concerned with high-cycle axial fatigue testing of a 2 mm AA6060-T6 HYB butt weld produced in the solid state using AA6082 filler metal addition. The results complement the three-point bend testing and the tensile testing done in two previous studies. In this study, optical microscope and scanning electron microscope examinations have been carried out to reveal the joint macro/microstructure and document possible surface and root defects deemed to affect fatigue life. In the as-welded condition, the HYB weld suffers from surface irregularities at the weld face and “kissing” bond formation in the root region. Despite of this, the subsequent testing shows that the fatigue properties exceed those reported for comparable AA6082-T6 gas metal arc butt welds and matching those reported for corresponding high-strength laser beam and friction stir weldments.

This work aims to investigate the anisotropic fracture and energy dissipation characteristics of marbles cored along an angle of 0°, 30°, 60° and 90° with respect to interbed planes, subjected to multi-level cyclic loading conditions. Rock fatigue deformation, strength, lifetime and dissipated energy first decreases and then increases with increasing interbed orientation, they get to the minimum for sample having 30° interbed orientation. Rock stiffness degradation is significant with the increase of cyclic level and the stiffness evolution is affected by interbed structure. The incremental rate of dissipated energy becomes faster with increase of cyclic loading level and it presents an abrupt increasing trend at the last cyclic loading level. A damage evolution model was first established based on the dissipated energy to describe the two-phase damage accumulation characteristics. It suggests that the proposed model fits well to the testing data and favorably represents the non-linear characteristics of damage accumulation.

Polymeric foams have good capacity of absorbing energy in compression, but are brittle in tension. Linear Elastic fracture Mechanics is successfully applied to assess the integrity of structures with polymeric foams. The fracture toughness represents an important parameter. The different approaches to estimate the fracture toughness of polymeric foams are reviewed, analytical and numerical micromechanical models and experimental investigations. Focus is given on the parameters influencing the fracture toughness of polymeric foams like specimen type, solid material, density, loading speed, size effect and temperature. Data on mixed mode loading and dynamic fracture toughness are also presented. The last part of the paper presents some results to increase the fractured toughness by reinforcing of polymeric foams.

In the present investigation temperature dependence fatigue strength behaviour of Inconel 825 super alloys is investigated. Based on the experimental results different S-N models have been derived and suitable model for the prediction of fatigue strength have been proposed. An inverse power and exponential relation between fatigue strength and absolute temperature is demonstrated. The proposed models are used to predict the fatigue life using well known Palmgren-Miner rule. Based on high to low and low to high load steps test data sets under identical test conditions, Miner rule based statistical damage constant is stochastically modeled for fatigue life prediction at different level of probability and validated. The modeling process combines a probabilistic fatigue damage accumulation and a stress-life-temperature relation technique.

This paper presents a study on the effect of microstructure on the fatigue crack growth rate (FCGR) in advanced normalised-rolled (NR) and thermomechanical control process (TMCP) S355 steels in the Paris Region of the da/dN vs. ΔK log-log plot. The environments of study were air and seawater (SW), under constant amplitude sinewave fatigue loading. Discussions were based mainly on the comparison between the crack path in the TMCP and NR steels. Fundamentally, three phenomena: crack-tip diversion, crack-front bifurcation and metal crumb formation were observed to influence the rate of fatigue crack growth (FCG). The prevalence of these phenomena appears to be a function of the nature of the material microstructure, environment and crack-tip loading conditions. The three factors appear to retard the crack growth by reducing or re-distributing the effective driving force at the main active crack tip. A crack path containing extensively the three phenomena was observed to offer strong resistance to FCG. Increase in the FCGR was observed with decrease in the crack-tip diversion angle, branched-crack length and metal crumbs formed. In SW, the degree of the electrochemical dissolution of the microplastic zone (or crack-tip blunting) appears to be an additional factor influencing crack growth in ferrite-pearlite (α-P) steel. This study, generally tends to present microstructural features that strongly influenced FCGR in α-P steels in the Paris Region both in air and SW. This work is very important in the design of fatigue resistant steel.

Electron Beam Melting (EBM) is one of a few additive manufacturing technologies capable of making full-density functional metallic parts realized from raw materials in the form of powders. The ability of direct fabrications of metallic parts can accelerate product designs and developments in a wide range of metallic-part applications, especially for complex components, which are difficult to make by conventional manufacturing means. To capitalize on these benefits, it must be shown that the mechanical performances of parts produced by EBM can meet design requirements. In this research an intensive mechanical characterization aimed at determining static and fatigue performance of the alloy Ti6Al4V processed by EBM has been performed. The effect of both postprocessing treatments (HIP and surface finish) on the mechanical behavior was evaluated by mechanical testing, microstructural study, computed tomography analysis and fracture surface investigation.

Low-cycle fatigue testing of a lead-free solder (InnoLot) based on Sn-3.8Ag-0.7Cu (SAC387) with three simultaneous additions of bismuth, nickel and antimony was conducted using miniature-sized fatigue specimens at different temperatures and strain amplitudes. The experiments show a decline of the load capacity of the solder alloy with the number of loading cycles. The fatigue life of the solder is also decreased by the level of imposed temperature. The modified Coffin-Manson and Morrow models were used to analyze the behavior under fatigue and predict lifetime. The parameters in the two fatigue models which were determined by considering different temperatures and total strain amplitudes. Compared to other reference lead-free solders, the InnoLot solder shows much better fatigue strength. The better fatigue strength is found to result from the effect of BiNiSb elements. Also, lifetime predictions were made with both models for the solder alloy under different conditions.

The fatigue behaviour of notched and unnotched specimens produced by additively manufactured Inconel 718 are analysed in the as-built and heat-treated conditions. The surfaces display high roughness and defects acting as fatigue initiation sites. In the as-built condition, fine sub-grains were found, while in the heat-treated state, the sub-grains were removed and the dislocation density recovered. SN-curves are predicted based on tensile properties, hardness and defects obtained by fractography, using the √area-method.