Search Results

Fatigue analysis using finite element models of a full vehicle body structure subjected to proving ground durability loads is a very complex task. The current paper presents an analytical procedure for fatigue life predictions of full body structures based on a time-domain approach. The paper addresses those situations where this kind of analysis is necessary. It also discusses the major factors (e.g., stress equivalencing procedure, cycle counting method, event lumping and load interactions) which affect fatigue life predictions in the procedure. A comparison study is conducted which explores the combination of these factors favorable for realistic fatigue life prediction. The concepts are demonstrated using a body system model of production size.

Hardware benchmarking of body overall stiffness and joint stiffness of the best-in-class competitive vehicles is a common practice in the automobile industry. However, this process does not provide design insights of competitive body structures, which relate stiffness performance to key architectural designs. To overcome this drawback, a CAD body-in-prime model of a competitive vehicle is developed using laser/optical scanning technology and a corresponding CAE model is built based on the CAD data. A deep-dive structural efficiency study is conducted using this model and “pros” and “cons” of the architectural design of each individual joint and each section of major load-carrying members of this body structure are identified. This analytical benchmarking (or reverse engineering) process enables a company to adopt best-in-class design practices and achieve competitive advantages in vehicle designs.

Traditionally, vehicle durability cracks have been treated to be local problems as a result of poor designs of notches, welds, holes, corners or reinforcements. The problems were usually found and fixed at a late design stage which often resulted in weight and cost penalties for a vehicle program. However, in many instances, the local problems mentioned above are simply the consequence of a poor global design. The global problems can generally be grouped into three categories: stress induced fatigue problems due to excessive global stresses as a result of body structural discontinuities, load induced fatigue problems due to excessive loads input to a body as a result of suspension designs, and vibration induced fatigue problems due to unfavorable structural resonance. The current paper presents a CAE analysis process which can be used at the upfront design stage to assess vehicle durability performances from a global design point of view.

This paper benchmarks some methods of nondestructive testing for zero and high mileage spot weld quality/integrity and degradation evaluation (pin holes, voids, cracks, fatigue, corrosion, etc.). The methods include X-ray radiography, ultrasonic imaging, ultrasonic pulse/ echo, pulsed infrared or thermography, and laser/TV holographic interferometry imaging. The advantages and limitations of each method are provided with descriptive principles and real test examples. It is found that X-ray radiography combined with ultrasonic echo technique is the most favorable one considering time and cost for the current zero and high mileage spot weld evaluation.

There are two distinct classes of body CAE models (detailed and concept models) that can be used to support vehicle body design and development. A detailed finite element model achieves computational accuracy by precisely simulating component geometries and assembly interfaces. On the other hand, a concept model simulates stiffness behavior of joints and major load-carrying members (e.g., pillars, rails, rockers, etc.) in a body structure. The former is quite useful for conducting trade-off studies when detailed design drawings become available. The latter is valuable for up-front design direction studies prior to detailed design evolution. In concept models, major load-carrying members are universally represented by cross sectional properties (e.g., area, moments of inertia and torsion constant). The key difference between various kinds of concept models is the representation of body joints.

Joint stiffness is a major contributor to the vehicle body overall bending and torsional stiffness which in turn affects the vehicle NVH performance. Each joint consists of spot welds which function as load paths between adjacent sheet metal. Spot welds tend to lose structural integrity as a result of fatigue, loosening, aging, wear and corrosion of parts as a vehicle accumulates mileage. Experimental methods are used to identify potential degradation mechanisms associated with a spot weld. A CAE model which simulates a vehicle body joint generically is used to determine the effects of each individual degradation mode of a spot weld on joint stiffness. A real life B-pillar to roof joint CAE model of a production vehicle is then employed to examine the significance of weld distribution on joint stiffness degradation. The knowledge derived from this study can be used as a guidance in designing vehicle body structures with respect to spot weld distribution.

Traditionally, door seals are designed to achieve good wind noise performance, water leakage and door closing effort in a vehicle design and development process. However, very little is known concerning the effect of door seal design on vehicle squeak and rattle performance. An earlier research work at Ford indicates a strong correlation between the diagonal distortions of body closure openings (in a low frequency range 0 - 50 Hz) and overall squeak and rattle performance. Another research at Ford reveals that relative accelerations between door latch and striker in a low frequency region (0 - 50 Hz) correlate well with door chucking performance. The findings of this research work enable engineers to assess squeak and rattle and door chucking performance using vehicle low frequency NVH CAE models at a very early design stage.

Due to several indeterminate factors, the assessment of the durability performance of a vehicle body is traditionally accomplished using test methods. An analytical fatigue life prediction method (four-step durability process) that relies mainly on numerical techniques is described in this paper. The four steps comprising this process include the identification of high stress regions, recognizing the critical load types, determining the critical road events and calculation of fatigue life. In addition to utilizing a general purpose finite element analysis software for the application of the Inertia Relief technique and a previously developed fatigue analysis program, two customized programs have been developed to streamline the process into an integrated, user-friendly tool. The process is demonstrated using a full body, finite element model.