Advanced imaging for early prediction and characterization of zone of L ¨ uders band nucleation associated with pre-yield microstrain Srinivasan Nagarajan n , Raghu N., Venkatraman B. Quality Assurance Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India article info Article history: Received 15 June 2012 Received in revised form 25 October 2012 Accepted 26 October 2012 Available online 5 November 2012 Keywords: L¨ uders band Infrared thermography Digital image correlation Pre-yield microstrain Yield phenomena abstract The present study involves experimental investigation of thermal and strain fields associated with generation of pre-yield plastic microstrain during tensile loading of mild steel using imaging NDE techniques such as infrared thermography (IRT) and digital image correlation (DIC). A fast array based infrared detector is used to map the temperature distribution and a DIC system is used to map the strain distribution in local zones of the material. Studies reveal that dynamic thermal and strain response of the material observed before macroscopic yield point and their spatial distributions are found to have good correlation with phenomenon of pre-yield microstrain and related experimental findings. Techniques used not only offered the possibility of visualizing the thermal and strain distributions associated with pre-yield microstrain but also to predict and visualize the zone of nucleation of L ¨ uders band much ahead of upper yield point. & 2012 Elsevier B.V. All rights reserved. 1. Introduction In materials such as low carbon steel, mild steel and Al–Mg alloys, the transition from elastic to plastic deformation is characterized by a phenomenon of material instability known as L¨ uders deformation. This type of material instability manifests in the load-elongation curve as an initial high yield point followed by sudden drop in load after which load is nearly constant for small percent of strain called L ¨ uders strain. It is then followed by strain hardening. Such localized discontinuous yielding is accom- panied by band nucleation and propagation along the gauge length of the specimen. A survey of literature reveals that the phenomenon of L ¨ uders band formation and propagation have been investigated using a combination of high speed digital cameras, optical and electron microscopy [1–11] and explained based on dislocation activities. Recent advances in imaging NDE offers new experimental tools with possibilities for visualizing the band formation and its underlying effects. Infrared thermography and Digital image correlation are two such advanced imaging nondestructive and non contact techniques. One of the earliest works in this area is by Louche et al. [12], who have applied IR imaging for visualizing the dissipative heat source variation associated with L ¨ uders band propagation. Wattrisse et al. [13] have worked on image processing based on digital image correla- tion for L ¨ uders band propagation and necking. The authors have studied various band parameters like width, orientation and velocity of band front etc. Venkatraman et al. [14–16] have successfully applied thermal imaging for characterizing the process of tensile deformation in AISI 316 SS for early detection of necking and failure zones. Localized deformation by PLC bands (a similar plastic instability phenomenon with repeated locking and unlocking of dislocations) have also been successfully characterized and corre- lated to various stress levels in the stress–strain curve using IRT and DIC [17–21]. It is well established that formation of L ¨ uders band at upper yield point correspond to cluster of plastically yielded grains in the elastic regime due to the phenomenon of pre-yield micro- strain. Small plastic strain on the order of 10 4 –10 6 that usually occurs before the onset of macroyielding is called pre-yield microstrain. Pre-yield microstrain generally fluctuates on various zones of gauge length at different instants of loading depending on the stress concentrations induced by various features under applied load like geometry of specimen, presence of inclusions or precipitates, grain boundaries, orientations of grains etc. According to Cottrell [22], pre-yield microstrain below upper yield stress is caused by dislocations breaking from the impurity atmospheres (carbon, nitrogen) in the vicinity of stress concentrators resulting in pileup of dislocations on grain boundaries. When applied stress is increased, these dislocations traverse the neighboring grains resulting in cluster of yielded grains leading to nucleation of L¨ uders band. It is the unlocking of these locked dislocations that corresponds to upper yield point and their movement corre- sponds to lower yield point. In low carbon steel, this pre-yield microstrain is reported to be of the order of 30  10 6 [23] and Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.10.082 n Corresponding author. Fax: þ91 44 27480209. E-mail address: srini0497@gmail.com (S. Nagarajan). Materials Science & Engineering A 561 (2013) 203–211