The effect of thermal cycling and applied stress level on post cycling – Stress free transformation behaviour in NiTi alloys

2003 ◽  
Vol 112 ◽  
pp. 815-818 ◽  
Author(s):  
N. B. Morgan ◽  
C. M. Friend
Keyword(s):  
Thermal Cycling ◽  
Applied Stress ◽  
Stress Level ◽  
Transformation Behaviour ◽  
Niti Alloys

Effect of operating parameters on functional fatigue characteristics of an Ni-Ti shape memory alloy on partial thermomechanical cycling

2022 ◽  
pp. 1045389X2110722
Author(s):  
Swaminathan Ganesan ◽  
Sampath Vedamanickam
Keyword(s):  
Yield Strength ◽  
Applied Stress ◽  
Stress Level ◽  
Crack Nucleation ◽  
Maximum Temperature ◽  
Thermomechanical Cycling ◽  
Permanent Strain ◽  
A Value ◽  
The Stability ◽  
Cycle Temperature

In this study, the influence of upper cycle temperature (maximum temperature in a cycle) and the magnitude of applied stress on the functional properties of an SMA during partial thermomechanical cycling has been studied. A near-equiatomic NiTi SMA was chosen and tested under different upper cycle temperatures (between martensite finish (Mf) and austenite finish (Af) temperatures) and stress level (below and above the yield strength of the martensite). The upper cycle temperature was varied by controlling the magnitude of the current supply. The results show that a raise in the upper cycle temperature causes the permanent strain to increase and also lowers the stability. However, decreasing the stress imposed to a value lower than the yield strength of the martensite improves cyclic stability. The upper cycle temperature was found to influence the crack nucleation, whereas the applied stress level the crack propagation during partial thermomechanical cycling of SMAs. Therefore, decreasing the upper cycle temperature as well as the magnitude of stress applied to lower than the yield stress of martensite have been found to be suitable strategies for increasing the lifespan of SMA-based actuators during partial thermomechanical cycling.

Uniaxial Ratchetting of Filled Rubber: Experiments and Damage-Coupled Hyper-Viscoelastic-Plastic Constitutive Model

2018 ◽  
Vol 85 (6) ◽  
Author(s):  
Yifu Chen ◽  
Guozheng Kang ◽  
Jianghong Yuan ◽  
Chao Yu
Keyword(s):  
Experimental Data ◽  
Constitutive Model ◽  
Applied Stress ◽  
Stress Level ◽  
Cyclic Deformation ◽  
Room Temperature ◽  
Tensile Deformation ◽  
Accumulated Damage ◽  
Filled Rubber ◽  
Cyclic Tension

A series of stress-controlled uniaxial cyclic tension-unloading tests are discussed to investigate the ratchetting of a filled rubber at room temperature. It is shown that obvious ratchetting occurs and depends apparently on the applied stress level, stress rate, and stress history. Based on the experimental observations, a damage-coupled hyper-viscoelastic-plastic constitutive model is then developed to describe the ratchetting of the filled rubber, which consists of three branches in parallel, i.e., a hyperelastic, a viscoelastic, and a plastic one. The damage is assumed to act equally on three branches and consists of two parts, i.e., the Mullins-type damage caused by the initial tensile deformation and the accumulated damage occurred during the cyclic deformation. The developed model is validated by comparing the predicted results with the experimental data.

Threshold for Stress Corrosion Cracking Initiation of Alloy X750 under Long-Term Uniaxial Constant Load Test in High-Temperature Water

CORROSION ◽  
2004 ◽  
Vol 60 (3) ◽  
pp. 229-236 ◽  
Author(s):  
M. Yamamoto ◽  
J. Kuniya ◽  
S. Uchida
Keyword(s):  
High Temperature ◽  
Crack Initiation ◽  
Stress Corrosion ◽  
Applied Stress ◽  
Stress Level ◽  
Stress Corrosion Cracking ◽  
Pure Water ◽  
Constant Load ◽  
Corrosion Cracking ◽  
Load Test

Abstract Uniaxial constant load (UCL) tests of the nickel-based alloy X750 (UNS N07550) were performed in high-temperature pure water (288°C, 8 ppm dissolved oxygen [DO]) to investigate stress corrosion cracking (SCC) fracture time and the crack initiation process. The SCC fracture was initiated at a stress level below the 0.2% offset yield stress and many small cracks were observed in the middle of the nonfractured test specimens. The distribution of the crack length for each observation time is shown by Weibull probability distributions. Crack initiation and propagation process had different behavior depending on the applied stress level and the stress intensity factor at the crack tip. SCC initiation at the minimum applied stress is discussed with respect to the grain size, which depended on the size of an initial crack.

Multi-Sensor Approach to Non-Destructive Evaluation of Damage Around Circular Holes in Composite Laminates

2005 ◽  
Author(s):  
G. P. Tandon ◽  
R. Y. Kim
Keyword(s):  
Carbon Fiber ◽  
Applied Stress ◽  
Stress Level ◽  
Failure Modes ◽  
Matrix Cracking ◽  
Strain Gages ◽  
Fiber Breakage ◽  
Electric Resistance ◽  
Damage Growth ◽  
Hole Edge

One of the more formidable problems in composite research is the study of delamination and other failure modes in the vicinity of a circular hole in a laminate, e.g., a circular cut-out in a structure. In this problem, the singularity varies around the periphery of the hole as well as through the thickness of the laminate. Under tensile loading, the early failure modes in this problem consist of transverse cracks in various layers, so that delamination occurs only after other damage is precipitated, followed by fiber breakage leading to failure. A literature review of past work clearly shows that mechanical testing with simultaneous AE monitoring is a fruitful technique to study damage accumulation in composite systems. The acoustic-ultrasonic (AU) testing combines the high sensitivity of ultrasonics to internal damage and the method of acoustic emission technique to characterize elastic waves. As damage accumulates in the specimen along the wave path, the net internal damping increases and changes the wave parameters such as peak amplitude, duration, etc. accordingly. Additionally, a range of experimental results over the last decade has further shown that the mechanical deformation and electric resistance of carbon fiber reinforced polymers are coupled, so that the material is inherently a sensor of its own damage state. The monitoring of electric resistance and capacitance changes, linked to the modifications of the conduction paths in the composite, allows the detection of damage growth. It seems logical that a natural extension of these different approaches is the determination of damage mode, e.g., fiber breakage, matrix cracking or delamination, and damage size and position, based on combined measurements from these techniques. These multiple techniques will serve a two-fold purpose, namely, enable comparison as well as complement each other in case of incomplete damage mapping from one set of sensors For this study, we will consider carbon fiber-reinforced toughened bismaleimide, (IM7/5250-4) quasi-isotropic laminate coupons 12” long, 4” wide with hole at the center under tension. Figure 1 shows the damage which occurs around a 0.75” hole in a [45/0/-45/90]s graphite epoxy laminate obtained by radiography after unloading the test specimen from an applied stress of 50 Ksi. The failure stress for this laminate was 56.4 Ksi. Damage in the form of ply cracks in the 90, 45, and −45 plies and delamination around hole edges is clearly evident. The radiograph taken after unloading from a 50 Ksi stress level clearly shows the location and extent of damage, but contains no specific information about the sequence and the timing of damage events. Figure 2 shows stress-strain curves obtained from strain gages mounted at various distances away from the hole edge along with the far-field value. The stress-strain curves provide useful information regarding the initiation as well as the growth of the damage, as evidenced by jump in strain levels and onset of nonlinearity. Damage initiation is first picked up by the strain gage which is mounted closest to the hole edge at a stress level of 21 Ksi. Subsequently, other strain gages begin to sense damage growth as the applied stress level increases. The strain gage data provides useful information regarding initiation, growth and severity of damage, but it is difficult to assign specific damage modes and their location to the measurements. This example clearly demonstrates the needs, with the associated benefits, of the multiple sensor approach. In this work, three different hole sizes (0.25”, 0.5” and 0.75”) will be investigated. This example problem will enable us to examine the combined effects of cut-outs, matrix cracking, delamination and fiber breakage on the ability of various NDE techniques to assess damage. The development and growth of damage in the composite laminate with a hole under compression will be markedly different than in tension. Under compression, the major damage modes are fiber buckling and delamination, and will also be investigated.

scholarly journals The Nature of the β→α″ Transformation in Ti-Nb Alloys

2020 ◽  
Vol 321 ◽  
pp. 11052
Author(s):  
E.M. Hildyard ◽  
L.D. Connor ◽  
N. Martin ◽  
D. Rugg ◽  
H.J. Stone ◽  
...  
Keyword(s):  
Martensitic Transformation ◽  
Shape Memory ◽  
Stress Level ◽  
Total Stress ◽  
Synchrotron Diffraction ◽  
New Approach ◽  
Transformation Behaviour ◽  
Shape Memory Behaviour ◽  
Start Temperature

The martensitic transformation in Ti-Nb alloys can result in superelastic and shape memory behaviour but significant discrepancies exist between the transformation conditions reported for a given composition. To elucidate the reasons for these variations, in situ synchrotron diffraction experiments have been performed on Ti-24Nb (at.%) in two different microstructural conditions. Markedly different transformation behaviour was observed between these two conditions, including fully reversible superelastic behaviour below the apparent martensitic start temperature. These results could not be rationalised using thermally based transformation descriptions. As such, a new approach is introduced based upon the total stress level in a material.

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