Physical interpretation of the reference features in textural fractography of fatigue fractures

Reference features of a fatigue fracture surface are the reference texture and reference crack growth rate which are unambiguously mutually related. The reference texture is a subset of the image texture in SEM fractographs. It is expected to be common to all fractures caused by loadings in which significant events occur sufficiently regularly and frequently. The ratio of the reference and the conventional crack growth rate called reference factor is a characteristic of a particular loading. Its value may be related to the sequence of successive sizes of the cyclic plastic zone, while the mechanism of the effect of overloads follows the models of Wheeler and Willenborg. Application to a set of nine test specimens from aluminium alloy loaded by three different loading regimes is shown.

Phase and crystal orientation study of a TRansformation Induced Plasticity steel subjected to cyclic load induced fatigue

An electron backscatter diffraction (EBSD) study of the microstructure of TRIP steel during fatigue failure. Phase and crystal orientation study of a TRIP steel subjected to cyclic load induced fatigue. The relative fractions of austenite, ferrite and martensite are quantified within the strain field of a fatigue crack tip. This data is a subset of data supporting a wider study of the fatigue properties of multiphase steels used in the automotive industry. The different microstructural phases present in these steels can influence the strain life and cyclic stabilized strength of the material due to the way in which these phases accommodate the applied cyclic strain. Fully reversed strain-controlled low-cycle fatigue tests have been used to determine the mechanical fatigue performance of a dual-phase (DP) 590 and transformation induced plasticity (TRIP) 780 steel, with transmission electron microscopy (TEM) and scanning electron microscopy (SEM-EBSD) used to examine the deformed microstructures .

Numerical models of plastic zones and associated deformations for elliptical inclusions in remote elastic loading-unloading with different R-ratios

Plastic zones and associated deformations ahead of a fatigue crack are well established nowadays. In-depth plane strain elasto-plastic finite element analysis is conducted in this investigation to understand the nature of cyclic plastic deformation and damage around soft and hard elliptical inclusions. Similar to fatigue crack tip, cyclic/reverse plastic zone and monotonic plastic zone are visible for soft elliptical inclusion. In the cyclic plastic zone, low cycle fatigue is the dominant cyclic deformation mode during symmetric load cycling, while ratcheting is dominant during asymmetric load cycling. The size of cyclic plastic zone depends upon the amplitude of remote stress while, the size of monotonic plastic zone depends upon the maximum remote stress. The size of monotonic plastic zone is equal to cyclic plastic zone during symmetric load cycling. The shape and size of plastic zones also depend upon the orientation of the soft inclusion. Cyclic plastic damage progression in the cyclic plastic zone for soft (MnS) inclusion is significant, while no cyclic plastic zone is visible for hard inclusion (Al2O3).

The effect of laser power density on the fatigue life of laser-shock-peened 7050 aluminium alloy

Laser shock peening (LSP) is an innovative surface treatment method that can result in significant improvement in the fatigue life of many metallic components. The process produces very little or no surface profile modification while producing a considerably deeper compressive residual stress layer than traditional shot peening operations. The work discussed here was designed to: (a) quantify the fatigue life improvement achieved by LSP in a typical high strength aircraft aluminium alloy and (b) identify any technological risks associated with its use. It is shown that when LSP conditions are optimal for the material and specimen configuration, a —three to four times increase in fatigue life over the as-machined specimens could be achieved for a representative fighter aircraft loading spectrum when applied at a representative load level. However, if the process parameters are not optimal for the material investigated here, fatigue lives of LSP treated specimens may be reduced instead of increased due to the occurrence of internal cracking. This paper details the effect of laser power density on fatigue life of 7050-T7451 aluminium alloy by experimental and numerical analysis.

High strain mechanical loading rapidly induces tendon apoptosis : an ex vivo rat tibialis anterior model

Background: The role of apoptosis, or programmed cell death, has only recently been explored in tendon.

Objective: To investigate the development of apoptosis after high strain loading of rat tendon.

Methods: The right tibialis anterior tendons of three rats were prepared for mechanical loading, and left tendons were prepared identically as non-loaded controls. Tendon was loaded with 20% strain for six hours using a 1 Hz longitudinal sine wave signal. The following were used to assess apoptosis: (a) a monoclonal mouse antibody (F7-26) to label single stranded DNA breaks; (b) a rabbit polyclonal antibody that specifically recognises the cleaved form of caspase-3.

Results: Light microscopy confirmed that the high strain protocol induced a stretch overload injury. Control tendons showed little or no staining with the F7-26 antibody, but the loaded tendons displayed numerous apoptotic cells. The percentage of apoptotic cells (20%) in the loaded tendon was significantly greater than in the control tendon (1%) (p = 0.000). The labelled cells colocalised with abnormal nuclear morphology, including nuclear fragmentation. The staining against cleaved caspase-3 was positive in loaded tendons only, and localised both to nucleus and cytoplasm.

Conclusion: This experiment extends knowledge of human tendon apoptosis by showing that apoptosis can occur in response to short term, high strain mechanical loading. This is the first report of mechanical loading of intact tendon causing excessive apoptosis.

Influences of steady and cyclic die rotation on the compression of aluminium

A series of experiments are reported for compression of an aluminum cylinder with monotonic and cyclic die rotation. When the die is monotonically rotated, a higher angular velocity or a lower compression speed of the tool leads to a greater load reduction in comparison of that seen with a stationary die. The test results also show that cyclic die rotation causes a cyclic fluctuation in the load-displacement curve. During the die deceleration phase, the compression load increases until it reaches the level obtained in conventional compression with stationary dies. However, the compression load is observed to reduce to levels lower than those obtained in monotonic rotating compression tests during the die acceleration phase. The frequency of rotating direction change seems to affect the position of load peaks only, not the amplitude of the peaks.

The effect of prestrain and bake hardening on the low-cycle fatigue properties of TRIP steel

The scope of this study was to examine the effects of plane strain prestrain, induced via cold-rolling, and subsequent automotive paint bake hardening cycle on both tensile and fatigue properties of a hot rolled TRIP780 multiphase steel. Strain-life data has been generated for as-received (0% prestrain), 10% and 20% prestrained samples, in both baked and unbaked conditions. Cold rolling  increased the number of strain reversals to failure at high cyclic strain amplitudes with no effect at low strain amplitudes. Bake hardening increased the number of reversals to failure at high cyclic strain amplitudes. The prestrained material exhibited partial cyclic softening, with some residual strength increase. The residual strength increase was attributed to the austenite to martensite transformation that occurred during the prestraining process.

Microstructural analysis of pre-strained TRIP steel after fatigue testing

The widespread introduction of multiphase sheet steels in the automotive industry has led to considerable interest in the fatigue properties of these materials. The different microstructural phases within matelials such as TRIP steels can influence the fatigue behaviour due to the manner in which the cyclic strain is accommodated within these phases. In this study fully reversed straincontrolled fatigue tests were perfonnrmed on a commercially-produced uncoated TRIP 780 steel both in the as-received and 20 % prestrained condition. The pre-strained TRIP steel showed significant cyclic softening at higher strain amplitudes, whereas some initial work hardening was observed at lower strain amplitudes before cyclic softening. The cyclic stabilised strength of the pre-strained TRIP steel was independent of strain amplitude, while the cyclic stabilised strength of the as-received TRIP steel increased with strain amplitude. Transmission Electron Microscopy TEM was used to examine the effect of the cyclic deformation on the microstructure of the different conditions, with the differences in fatigue behaviour explained based on the differences in the deformation structure formed within the steel (i.e. dislocation density and sub-structure and microband formation).

Cyclic deformation of advanced high-strength steels : mechanical behavior and microstructural analysis

The fatigue properties of multiphase steels are an important consideration in the automotive industry. The different microstructural phases present in these steels can influence the strain life and cyclic stabilized strength of the material due to the way in which these phases accommodate the applied cyclic strain. Fully reversed strain-controlled low-cycle fatigue tests have been used to determine the mechanical fatigue performance of a dual-phase (DP) 590 and transformation-induced plasticity (TRIP) 780 steel, with transmission electron microscopy (TEM) used to examine the deformed microstructures. It is shown that the higher strain life and cyclic stabilized strength of the TRIP steel can be attributed to an increased yield strength. Despite the presence of significant levels of retained austenite in the TRIP steel, both steels exhibited similar cyclic softening behavior at a range of strain amplitudes due to comparable ferrite volume fractions and yielding characteristics. Both steels formed low-energy dislocation structures in the ferrite during cyclic straining.

Role of microstructure in the low cycle fatigue of multi-phase steels

The low cycle fatigue (LCF) behaviour of several commercially-produced multiphase steels was studied; including dual-phase (DP) and transformation induced plasticity (TRIP). In addition, a novel TRIP980 hybrid microstructure was examined that consisted of coarse ferrite grains along with low temperature bainite regions interspersed with retained austenite. Fully reversed strain controlled fatigue tests were conducted on the different steels to determine the cyclic stress response and strain to failure. The effects of the cyclic deformation on the microstructures were analysed using electron backscattered diffraction (EBSD) and X-ray diffraction (XRD). Results showed that the initial cyclic hardening behaviour and low cyclic softening ratio observed in the TRIP steels was not necessarily due to austenite to martensite transformation. Differences between the austenite transformation behaviour of the conventional and novel hybrid TRIP microstructures was related to the different surrounding phases and the size of the retained austenite.

Understanding in-vivo abrasion fatigue or common suture materials used in arthroscopic and open shoulder surgery

In orthopaedic surgery the reattachment of tendon to bone requires suture materials that have stable and durable properties to allow healing at the tendon-bone interface. Failure rates of this type of surgery can be as high as 25%. While the tissue suture interface is a weak link, proportions of these failures are caused by in-vivo abrasion of the suture with bone and suture anchor materials. Abrasion of the suture material results from the movement of the suture through the eyelet by the surgeon during surgery, or with limb movement after surgery as the suture is not rigidly restrained within the eyelet. During movement the suture is subjected to bending and frictional forces that can lead to fatigue induced failure. This paper investigates the mechanism of bending abrasion fatigue induced failure of number two grade braided sheath only and braided sheath/multifilament core sutures. Sutures were oscillated over a stainless steel wire at low frequency under load in a dry state to simulate the bending and frictional forces between suture and eyelet. Failure mechanism was determined by video microscopy of the suture during abrasion combined with optical microscopy analysis of partially and fully abraded sutures. Braided only structures had high friction loading on the small number of fibres at the abrasion interface. This caused rapid single fibre breakages that accumulate to cause suture failure. The addition of ultra-high molecular weight polyethylene core fibres to a braided suture distributed the applied load across multiple fibres at the abrasion interface. This improved abrasion resistance by 15-20 times that of braided sheath alone.

Effect of martensite volume fraction on low cycle fatigue behaviour of dual phase steels: experimental and microstructural investigation

The low cycle fatigue (LCF) behaviour of a dual phase (DP) steel with different martensite volume fractions has been investigated, with particular focus on fatigue life, cyclic hardening/softening behaviour and microstructural evolution. DP steels with martensite volume fractions between 13% and 88% were produced and their monotonic and cyclic deformation behaviours evaluated. The LCF life has been examined in depth and compared with published literature. It has been concluded that, once normalised for plastic strain amplitude, the fatigue life was found to be significantly reduced by an increase in the martensite volume fraction. All alloys were observed to show some initial cyclic hardening followed by cyclic softening. Clear sub-cell formation occurred in ferrite grains irrespective of the martensite volume fraction, and it is suggested that this cell formation and martensite softening are responsible for the cyclic softening behaviour.