طراحی جوش، راهنمای برای مهندس پزشکی قانونی

It all starts with a vision of customer expectations. The weld design must comply with the code.

We enjoy the benefits of safe equipment, buildings, bridges and infrastructure. Engineers are trained to incorporate fail-safe designs to avoid structural failures or personal injuries. The root of welding code development is located in the history of welded items. Though today’s projects are “special” and the contract may specify “design-build” techniques, most welds are a common variety and repeated for decades. Design for code compliance is never discretionary.

The engineer who has the responsibility for design of a machine part or structural member as a weldment frequently operates under a severe handicap. Although the engineer has a mechanical background and academic training in materials and components, it is unlikely that adequate information was presented about the specific factors that enter into designs for welding. The designer needs to know how to use base materials efficiently, design for torsional resistance, design for stiffness and select the best suited weld joint for the purpose. The engineer also needs many other bits of practical information of which few are taught in schools .or found in handbooks

Practical education continues to be the answer to the success of every engineer. Education is the only way to maintain and improve our safety and security. To learn more about the welding experiences, get a code book and read it. The cost of books and education may be expensive, but the cost of ignorance is more.

Design engineers are expected to properly detail every weld to meet the project requirements and then correctly communicate with the welders. Many factors related to welding are away from engineering considerations yet become major design factors in the final product. Designers must address the welding process, inspection, welder certifications, and weld acceptance criteria. The designer must constantly think about decisions that affect production, costs, appearance, product performance and customer acceptance.

The basic types of loads are tension, compression, bending, shear and torsion. Whatever the type of load, when it is applied to a member, the member becomes stressed. The stresses cause strains, or movements within the member, the extent is governed by the modulus of elasticity of the material. Welding heat input causes loads from thermal stresses and distortion. Since a load produces stress and strain, some movement always occurs.

Loads create forces that must be carried through the structure the engineer is designing for suitable places of counteraction. The designer needs to know how to provide efficient pathways and this often requires the use of welding and joining. Loads in a welded steel design are transferred from one member to another through welds placed in weld joints. Both the type of joint and the type of weld must be specified by the designer.

The basic types of weld joints are butt, tee, corner, lap and edge. Specifying the joint does not describe the type of weld to be used. The size of the weld should always be designed with reference to the size of the thinner member. The joint cannot be made any stronger by using the thicker member for the weld size or overwelding.

Welded joint design is determined on the basis of load requirements. Variables in design and layout substantially affect costs. The designer should select the joint requiring the least amount of weld filler metal. Use the minimum root opening and included angle to reduce the amount of filler metal required. Design the joint for easy accessibility for welding. Overwelding is a common error of both design and production. Excess weld reinforcement is a common source of weldment failures. Keep the amount of welding to a minimum to reduce distortion, internal stress and the need for stress relieving and straightening.

The designer recognizes that the performance of any member of a structure is dependent on material properties and section properties. When the design is based on the efficient use of these two properties, the weldment will be functionally acceptable and use materials conservatively.

Long ago, designers worked empirically from past experience using the practical approach to welded designs. This practice became self-defeating by invariably resulting in excessively heavy designs and high fabrication costs. Today, designs are based on mathematical calculations to achieve efficient use of material properties.

Fatigue failures are identifiable and fracture analysis methods are readily available. Design applications of weld assessment models and computer simulations must fit real-world choices.

The Forensic Engineer knows to consider the published, historical requirements for welds to determine quality, uniformity or interchangeability. These documents may be rules, standards, specifications and codes. Sometimes these terms are used interchangeably.

Common definitions for these terms are:

• Rule: A prescribed guide for conduct or action. An accepted procedure, custom, or habit having the force of a regulation.
• Standard: A guide established by authority, custom or general consent as a model or example to be followed. Applies collectively to recommended practices, classifications, and methods for a welding process or applications.
• Specification: a detailed precise, explicit presentation of something or a plan or proposal of something. Clearly and accurately describes the essential technical requirement for a material, product, system or service. It indicates the procedures, methods, qualifications or equipment by which it can be determined that the requirements have been met.
• Code: A systematic statement of a body of law. Intended to be mandatory and a requirement by an authority having jurisdiction. A systematically arranged, comprehensive set of rules, standards and specifications for welding applications, published to secure uniformity and to protect the public. Established and enforced usually by a public agency. Consists of a set of conditions and requirements relating to a particular subject and indicating appropriate procedures by which it can be determined that the requirements were met.

The in-process welding and final weld must be suitable for the intended service. The Forensic Engineer knows to evaluate a weld incrementally and in its final state. The methods used to install the weld and achieve the final joint configuration often require organized sequences and procedures to avoid detrimental effects of heat on material properties. The Forensic Engineer must evaluate the appropriate rules, standards, specifications and codes when assessing a weld of interest. The expert applies fitness-for-service considerations to welds for an intended application.

All welding must meet customer expectations and comply with the code.