Teresa J. Rinker

Pulsed infrared thermography for assessment of ultrasonic welds

Battery packs are a critical component in electric vehicles. During pack assembly, the battery cell tab and busbar are ultrasonically welded. The properties of the welds ultimately affect battery pack durability. Quality inspection of these welds is important to ensure durable battery packs. Pack failure is detrimental economically and could also pose a safety hazard, such as thermal runaway. Ultrasonic welds are commonly checked by measuring electrical resistance or auditing using destructive mechanical testing. Resistance measurements are quick, but sensitive to set-up changes. Destructive testing cannot represent the entire weld set. It is possible for a weak weld to satisfy the electrical requirement check, because only sufficient contact between the tabs and busbar is required to yield a low resistance measurement. Laboratory techniques are often not suitable for inline inspection, as they may be time-consuming, use couplant, or are only suitable for coupons. The complex surface geometry also poses difficulties for conventional nondestructive techniques. A method for inspection of ultrasonic welds is proposed using pulsed infrared thermography to identify discrepant welds in a manufacturing environment. Thermal measurements of welds were compared to electrical and mechanical measurements. The heat source distribution was calculated to obtain thermal images with high temporal and spatial resolution. All discrepant welds were readily identifiable using two thermographic techniques: pixel counting and the gradient image. A positive relationship between pixel count and mechanical strength was observed. The results demonstrate the potential of pulsed thermography for inline inspection, which can complement, or even replace, conventional electrical resistance measurements.

Damage detection using time-frequency decay-rate based features

Some automotive components, for instance a clutch housing, need to fit the assembly (dimensional constraint), and must be perfectly weight balanced for proper functioning (functional fidelity), which could be achieved by removing material from the body (relaxed geometry). While such mission critical components require 100% inspection, current non-destructive methods are often inadequate, trying to trade-off cycle time with inspection thoroughness. In this paper, a non-destructive global structural damage detection method has been demonstrated at its infancy, to be fast, accurate, and immune to inconsistent resonant frequencies which is an outcome of relaxed geometry. This method requires a traditional wide band exogenous acoustic excitation of the component being evaluated, followed by a joint time-frequency characterization of the response such that it is immune to dominant modal frequencies. A quadratic discriminant analysis on the time-frequency features successfully detected all damaged samples and falsely identified 1 among the 6 healthy samples available for testing, as damaged.

Modeling of fracture behaviours of ultrasonically welded Cu-Cu joints under different loading conditions

A lithium-ion battery pack for electric vehicles may consist of several hundreds of battery cells joined together. Each cell contains joints of multiple thin sheets of electrodes of different conductive materials such as nickel coated copper, copper and / or aluminum. These within-cell and cell-to-cell joints must withstand static and dynamic mechanical loading. Determination of their maximum loading capacity is a very important task in order to predict the life of a battery pack. The standard procedure is to apply mechanical tests, such as lap shear and pull test to each joint. This procedure is time consuming and costly. There is a strong interest nowadays in developing validated models which can predict the actual behavior of the joints under different loadings and the associated failure modes. In this paper, two finite element models are developed to predict the strength of ultrasonically welded two-, three- and four layer joints of 0.2 mm thickness copper tabs with a 1mm thickness busbar. These models have the ability to predict three modes of failure of these joints depending on the weld quality, e.g., interfacial fracture, combined interfacial-circumferential fracture, and circumferential failure. These models are experimentally validated with very good agreement between experimental and predicted results.Copyright